2.23: Photosynthesis Summary - Biology

2.23: Photosynthesis Summary - Biology

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

What is photosynthesis?

The process of using the energy in sunlight to make food (glucose). Is it really as simple as that? Of course not. As you have seen, photosynthesis includes many steps all conveniently condensed into one simple equation. In the five concepts describing photosynthesis, this process has been presented in an introductory fashion. Obviously, much more details could have been included, though those are beyond the scope of these concepts.



The following 10 points summarize photosynthesis.

  • 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2
  • Autotrophs store chemical energy in carbohydrate food molecules they build themselves. Most autotrophs make their "food" through photosynthesis using the energy of the sun.
  • Photosynthesis occurs in the chloroplast, an organelle specific to plant cells.
  • The light reactions of photosynthesis occur in the thylakoid membranes of the chloroplast.
  • Electron carrier molecules are arranged in electron transport chains that produce ATP and NADPH, which temporarily store chemical energy.
  • The light reactions capture energy from sunlight, which they change to chemical energy that is stored in molecules of NADPH and ATP.
  • The light reactions also release oxygen gas as a waste product.
  • The reactions of the Calvin cycle add carbon (from carbon dioxide in the atmosphere) to a simple five-carbon molecule called RuBP.
  • The Calvin cycle reactions use chemical energy from NADPH and ATP that were produced in the light reactions.
  • The final product of the Calvin cycle is glucose.


  • What is photosynthesis?

The process of using the energy in sunlight to make food (glucose). But of course it is much more complex than that simple statement. Photosynthesis is a multistep biochemical pathway that uses the energy in sunlight to fix carbon dioxide, transferring the energy into carbohydrates, and releasing oxygen in the process.

  • What is NADPH?

Nicotinamide adenine dinucleotide phosphate, an energy carrier molecule produced in the light reactions of photosynthesis. NADPH is the reduced form of the electron acceptor NADP+. At the end of the light reactions, the energy from sunlight is transferred to NADP+, producing NADPH. This energy in NADPH is then used in the Calvin cycle.

  • Where do the protons used in the light reactions come from?

The protons used in the light reactions come from photolysis, the splitting of water, in which H2O molecules are broken into hydrogen ions, electrons, and oxygen atoms. In addition, the energy from sunlight is used to pump protons into the thylakoid lumen during the first electron transport chain, forming a chemiosmotic gradient.

  • How do you distinguish between the Calvin cycle and the Krebs cycle?

The Calvin cycle is part of the light-independent reactions of photosynthesis. The Calvin cycle uses ATP and NADPH. The Krebs cycle is part of cellular respiration. This cycle makes ATP and NAPH.

  • Do photosynthesis and cellular respiration occur at the same time in a plant?

Yes. Photosynthesis occurs in the chloroplasts, whereas cellular respiration occurs in the mitochondria. Photosynthesis makes glucose and oxygen, which are then used as the starting products for cellular respiration. Cellular respiration makes carbon dioxide and water (and ATP), which are the starting products (together with sunlight) for photosynthesis.

Common Misconceptions

  • A common student misconception is that plants photosynthesize only during daylight and conduct cellular respiration only at night. Some teaching literature even states this. Though it is true the light reactions can only occur when the sun is out, cellular respiration occurs continuously in plants, not just at night.
  • The “dark reactions” of photosynthesis are a misnomer that often leads students to believe that photosynthetic carbon fixation occurs at night. This is not true. It is preferable to use the term Calvin cycle or light-independent reactions instead of dark reactions.
  • Though the final product of photosynthesis is glucose, the glucose is conveniently stored as starch. Starch is approximated as (C6H10O5)n, where n is in the thousands. Starch is formed by the condensation of thousands of glucose molecules.

Explore More

Use this resource to answer the questions that follow.

  • Avoid Misconceptions When Teaching About Plants at
  1. Why is it more appropriate to say chloroplasts, rather than chlorophyll, are necessary for photosynthesis?
  2. Why is much more than six water molecules necessary for photosynthesis?
  3. Do plants absorb any green light? Explain your answer.

What is photosynthesis in biology

What is photosynthesis in biology:- photosynthesis is an anabolic process in which complex carbohydrates are synthesized by green parts with the help of water and carbon dioxide in the presence of light. During this process green plant trap solar energy and convert it into chemical energy . This energy is stored in the form of adenosine triphosphate (ATP) are reduced nicotinamide adenine dinucleotide phosphate (NADPH). plants utilize this energy in the reduction of carbon dioxide by which carbohydrates are formed. Hence this process is also called carbon assimilation.

The photosynthesis can be defined as Photosynthesis is an anabolic process in which complex carbohydrates are synthesizes from simple substance like carbon di oxide and water in the presence of light by chlorophyllous cells of the plants and oxygen is a by products.

Earlier the mechanism of photosynthesis was represent by the following simple equation-

6CO2+6H2O → C6H12O6 + 6O2 ↑

Chapter Summary

The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, the evolution of photosynthesis allowed living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today.

Only certain organisms (photoautotrophs), can perform photosynthesis they require the presence of chlorophyll, a specialized pigment that absorbs certain wavelengths of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a byproduct into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm.

8.2 The Light-Dependent Reactions of Photosynthesis

The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a and then to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of hydrogen ions. The hydrogen ions flow through ATP synthase during chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing carrier for the light-independent reactions.

8.3 Using Light Energy to Make Organic Molecules

Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO2 from the atmosphere. An enzyme, RuBisCO, catalyzes a reaction with CO2 and another organic compound, RuBP. After three cycles, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which is then ready to react with more CO2. Photosynthesis forms an energy cycle with the process of cellular respiration. Because plants contain both chloroplasts and mitochondria, they rely upon both photosynthesis and respiration for their ability to function in both the light and dark, and to be able to interconvert essential metabolites.

8.2 The Light-Dependent Reaction of Photosynthesis

In this section, you will explore the following questions:

  • How do plants absorb energy from sunlight?
  • What are the differences between short and long wavelengths of light? What wavelengths are used in photosynthesis?
  • How and where does photosynthesis occur within a plant?

Connection for AP ® Courses

Photosynthesis consists of two stages: the light-dependent reactions and the light-independent reactions or Calvin cycle. The light-dependent reactions occur when light is available. The overall equation for photosynthesis shows that is it a redox reaction carbon dioxide is reduced and water is oxidized to produce oxygen:

The light-dependent reactions occur in the thylakoid membranes of chloroplasts, whereas the Calvin cycle occurs in the stroma of chloroplasts. Embedded in the thylakoid membranes are two photosystems (PS I and PS II), which are complexes of pigments that capture solar energy. Chlorophylls a and b absorb violet, blue, and red wavelengths from the visible light spectrum and reflect green. The carotenoid pigments absorb violet-blue-green light and reflect yellow-to-orange light. Environmental factors such as day length and temperature influence which pigments predominant at certain times of the year. Although the two photosystems run simultaneously, it is easier to explore them separately. Let’s begin with photosystem II.

A photon of light strikes the antenna pigments of PS II to initiate photosynthesis. In the noncyclic pathway, PS II captures photons at a slightly higher energy level than PS I. (Remember that shorter wavelengths of light carry more energy.) The absorbed energy travels to the reaction center of the antenna pigment that contains chlorophyll a and boosts chlorophyll a electrons to a higher energy level. The electrons are accepted by a primary electron acceptor protein and then pass to the electron transport chain also embedded in the thylakoid membrane. The energy absorbed in PS II is enough to oxidize (split) water, releasing oxygen into the atmosphere the electrons released from the oxidation of water replace the electrons that were boosted from the reaction center chlorophyll. As the electrons from the reaction center chlorophyll pass through the series of electron carrier proteins, hydrogen ions (H + ) are pumped across the membrane via chemiosmosis into the interior of the thylakoid. (If this sounds familiar, it should. We studied chemiosmosis in our exploration of cellular respiration in Cellular Respiration.) This action builds up a high concentration of H+ ions, and as they flow through ATP synthase, molecules of ATP are formed. These molecules of ATP will be used to provide free energy for the synthesis of carbohydrate in the Calvin cycle, the second stage of photosynthesis. The electron transport chain connects PS II and PS I. Similar to the events occurring in PS II, this second photosystem absorbs a second photon of light, resulting in the formation of a molecule of NADPH from NADP+. The energy carried in NADPH also is used to power the chemical reactions of the Calvin cycle.

Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 2 of the AP ® Biology Curriculum Framework, as shown in the table. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.A Growth, reproduction and maintenance of living systems require free energy and matter.
Essential Knowledge 2.A.2 The light-independent reactions of photosynthesis in eukaryotes involve a series of reactions that capture free energy present in light.
Science Practice 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Science Practice 3.1 The student can pose scientific questions.
Learning Objective 2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy.
Essential Knowledge 2.A.2 The light-independent reactions of photosynthesis in eukaryotes involve a series of reactions that capture free energy present in light.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy.
Big Idea 4 Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A Interactions within biological systems lead to complex properties.
Essential Knowledge 4.A.2 Chloroplasts are specialized organelles that capture energy through photosynthesis.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 4.4 The student is able to make a prediction about the interactions of subcellular organelles.
Essential Knowledge 4.A.2 Chloroplasts are specialized organelles that capture energy through photosynthesis.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 4.5 The student is able to construct explanations based on scientific evidence as to how interactions of subcellular structures provide essential functions.
Essential Knowledge 4.A.2 Chloroplasts are specialized organelles that capture energy through photosynthesis.
Science Practice 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Learning Objective 4.6 The student is able to use representations and models to analyze situations qualitatively to describe how interactions of subcellular structures, which possess specialized functions, provide essential functions.

Teacher Support

This section deals with the first half of photosynthesis. These reactions capture light energy and store it in chemicals for short periods of time to fuel the second half of photosynthesis. This is also where free oxygen can be released, but carbon dioxide is not captured or fixed.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.5][APLO 2.16][APLO 2.18][APLO 1.9][APLO 1.32][APLO 4.14][APLO 2.2][APLO 2.3][APLO 2.23][APLO 1.15][APLO 1.29]

How can light be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules (Figure 8.9). However, autotrophs only use a few specific components of sunlight.

What Is Light Energy?

Teacher Support

Everybody knows what a rainbow is, but some students may not be able to connect it to actual light sources. Obtain some way of refracting light, such as a prism, and use it to separate the components of several light sources, such as an older, incandescent light bulb, a new fluorescent type of light bulb and actual sunlight.

When discussing the electromagnetic spectrum, include the fact that when someone sets a radio station to its number, such as 92.1 or 1450 on the dial, they are really setting the radio to the specific wavelength of spectrum used by the station.

The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which portion is therefore referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength , the distance between consecutive points of a wave. A single wave is measured from two consecutive points, such as from crest to crest or from trough to trough (Figure 8.10).

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure 8.11). The difference between wavelengths relates to the amount of energy carried by them.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum (Figure 8.11) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms.

Absorption of Light

Teacher Support

Stress the differences in the amount of energy at each wavelength, and the usefulness of the wavelengths for energy capture. Discuss what is in a “grow light” (artificial light source for plants grown indoors).

Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, called bleaching. So retinal pigments can only “see” (absorb) 700 nm to 400 nm light, which is therefore called visible light. For the same reasons, plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm plant physiologists refer to this range for plants as photosynthetically active radiation.

The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light portion of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy (Figure 8.12).

Understanding Pigments

Teacher Support

Concentrate on the types and functions of chlorophylls and carotenoids that are found in leaves. Discuss how all of them are always there even though they are not visible in the summer. They are visible in the fall.

Ask the class what color coats people tend to wear in the summer and in the winter. Discuss why they do this.

Different kinds of pigments exist, and each absorbs only certain wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae each class has multiple types of pigment molecules. There are five major chlorophylls: a, b, c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher plant chloroplasts and will be the focus of the following discussion.

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the absorption spectrum . The graph in Figure 8.13 shows the absorption spectra for chlorophyll a, chlorophyll b, and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.

Many photosynthetic organisms have a mixture of pigments using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation (Figure 8.14).

When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a spectrophotometer can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases.

How Light-Dependent Reactions Work

Teacher Support

Photosystems I and II can be confusing. Obtain diagrams of both systems and use them to go through the steps of the pathways. Discuss why some plants use the cyclic form of the systems and some the linear form. Discuss why oxygen is released during one pathway, but not the other.

The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in Figure 8.15. Protein complexes and pigment molecules work together to produce NADPH and ATP.

The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem , two types of which are found embedded in the thylakoid membrane, photosystem II (PSII) and photosystem I (PSI) (Figure 8.16). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).

Both photosystems have the same basic structure a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex , which passes energy from sunlight to the reaction center it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.

Visual Connection

The reaction center contains a pair of chlorophyll a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation they can actually give up an electron in a process called a photoact . It is at this step in the reaction center, this step in photosynthesis, that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate.PSII and PSI are two major components of the photosynthetic electron transport chain , which also includes the cytochrome complex . The cytochrome complex, an enzyme composed of two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.

The reaction center of PSII (called P680 ) delivers its high-energy electrons, one at the time, to the primary electron acceptor , and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to PSI. P680’s missing electron is replaced by extracting a low-energy electron from water thus, water is split and PSII is re-reduced after every photoact. Splitting one H2O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one molecule of diatomic O2 gas. About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration.

As electrons move through the proteins that reside between PSII and PSI, they lose energy. That energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used to synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called P700 ). P700 is oxidized and sends a high-energy electron to NADP + to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP + into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to fine tune that ratio to exactly match the chloroplast’s constantly changing energy needs.

Generating an Energy Carrier: ATP

Teacher Support

Discuss the similarities between ATP production in the light dependent reactions and in cellular respiration.

As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.

To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP (Figure 8.16). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure.

Link to Learning

Visit this site and click through the animation to view the process of photosynthesis within a leaf.

  1. Electrons from PS I cause the reduction of NADPH to ext^+! .
  2. Electrons from PSII cause the reduction of ext^+ to NADPH.
  3. Electrons from PS I cause the reduction of ext^+! to NADPH.
  4. Electrons are gained which causes the oxidation of ext^+! .

Everyday Connection for AP® Courses

If the stomata were sealed, what would happen to oxygen ( ext_2) and carbon dioxide ( ext_2) levels in a photosynthesizing leaf?

  1. ext_2 levels would increase and ext_2 levels would decrease.
  2. ext_2 levels would increase and ext_2 levels would decrease.
  3. ext_2 and ext_2 levels would both decrease.
  4. ext_2 and ext_2 levels would both increase.

Science Practice Connection for AP® Courses

Your teacher has set up 3 demonstrations. Each setup includes a clear dialysis bag filled with starch solutions at three different concentrations: 1%, 25%, and 60%. Dialysis tubing is semipermeable, as it contains pores that permit the passage of small ions and molecules, like water but will not permit the passage of larger molecules, like proteins. In this way, the dialysis bag models a semipermeable cell membrane.

Think About It

On a hot, dry day, plants close their stomata to conserve water. Predict the impact of this on photosynthesis and justify your prediction.

Teacher Support

Materials: 3 dialysis bags, 3 medium-sized beakers, stock starch solution, distilled water, iodine dropper bottle, thread, balance or scale

Preparation: To prepare the percent starch solutions, determine the volume of the solution you wish to use (e.g. 100 ml) and add the mass of solute, in grams, equivalent to the desired percent concentration:

Percent solution = [Mass of solute (g) / Volume of solution (ml)] x 100

Label each dialysis bag with one of the three concentrations. Then, moisten the dialysis tubing to make it easier to open. Fill each tube with the corresponding solution about three-quarters of the way fill in order to leave space to tie off the top of the bag. Tie the tops of the bags tightly with standard thread. Do not place the bags in the iodine solution yet, as they will first be weighed in front of the students.

With students present, explain to them the details of the setup and show them how iodine is used as an indicator for the presence of starch. Then fill each beaker about three-quarters of the way with distilled water. Add 3-8 drops of iodine, based on the strength of your iodine, and stir so that the solution is yellow in color. Then, weigh each dialysis bag and record the weights on a chart visible to the class. Immerse the dialysis bags in the iodine solution for 45 min to 1 hour. Then, remove the bags and carefully rinse them under a gentle tap. Weigh each bag and place the results on the board.

Results: Starch molecules cannot pass through the dialysis tubing. However, the iodine solution can pass from the beaker into the dialysis bag. This turns the starch solution from colorless to purple. The amount of iodine that diffuses into the bag is related to the concentration of each solution. As the starch concentration increases, more iodine solution will diffuse into the bag, causing the bag to increase in weight.

The Think About It question is an application of Learning Objective 4.4 and Science Practice 6.4 because students are making a prediction about how interactions of cellular organelles and structures affect the rate of photosynthesis.

Possible answer:

As an Amazon Associate we earn from qualifying purchases.

Want to cite, share, or modify this book? This book is Creative Commons Attribution License 4.0 and you must attribute OpenStax.

    If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:

  • Use the information below to generate a citation. We recommend using a citation tool such as this one.
    • Authors: Julianne Zedalis, John Eggebrecht
    • Publisher/website: OpenStax
    • Book title: Biology for AP® Courses
    • Publication date: Mar 8, 2018
    • Location: Houston, Texas
    • Book URL:
    • Section URL:

    © Jan 12, 2021 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License 4.0 license. The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.

    Photosynthesis In Higher Plants Notes, Revision, Summary, Important Formulae

    Here are complete Photosynthesis In Higher Plants important notes and summary. This summarizes most important formulae, concepts, in form of notes of Photosynthesis In Higher Plants which you can read for NEET preparation.

    The important notes of Biology for Photosynthesis In Higher Plants pdf download are available here for free.

    Aspirants of National Eligibility cum Entrance Test should also solve all past year papers of NEET.

    Notes Photosynthesis ICSE Class 10 Biology

    Students should refer to Photosynthesis ICSE Class 10 Biology notes provided below designed based on the latest syllabus and examination pattern issued by ICSE. These revision notes are really useful and will help you to learn all the important and difficult topics. These notes will also be very useful if you use them to revise just before your Biology Exams. Refer to more ICSE Class 10 Biology Notes for better preparation.

    Quick Review
    ➢ Photosynthesis is the process in which green plants or their parts produce complex carbon containing compounds with the help of inorganic raw materials such as CO2 and water in the presence of sunlight. O2 is Liberated as the bi-product.

    The whole of the oxygen liberated comes from water, by a process called photolysis of water.
    ➢ This is an anabolic, endergonic (requires energy) & oxidation-reduction process. Photosynthesis is the only process by which solar energy is converted into chemical energy.
    ➢ A chloroplast is an oval, minute, double membrane organelle present in green plants. It contains a green pigment called chlorophyll which absorbs light energy for photosynthesis. Internally it contains a matrix or stroma and the thylakoids. At some places the thylakoids are arranged themselves to form a stalk of coins like structure called grana.
    Each stalk of thylakoid sacs are connected by a structure known as stromal lamellae. The chloroplast mainly occurs in chlorenchymatous cells of leaves.
    ➢ Stomata are the structures mainly responsible for the gaseous exchange in the process of photosynthesis, respiration and transpiration.
    ➢ Stomata open in light when guard cells are turgid and close when they are flaccid. These turgor changes leading to the opening and closing of stomata.
    ➢ According to Malate K+ ions pump hypothesis, the turgor changes that open and close the stomata are due to the reversible absorption and loss of potassium ions ( K + ).
    ➢ The stomata open when guard cells absorb K+ ions from surrounding epidermal cells. The intake of K + ions by guard cells takes place through H + and K + ion exchange. The H + ions appear in the guard cell due to dissociation of maleic acid. The maleic acid dissociate into malate anions and H + ions in the guard cells.

    ➢ During night, the K+ ion concentration decreases in the guard cells. This lowers the osmotic pressure of the guard cells which results in the closing of stomata due to exosmosis.
    ➢ Mechanism of photosynthesis : Photosynthesis consists of two stages – The light phase and the dark phase. In the light phase, light is absorbed and used by chlorophyll. This, therefore, is called the photochemical reaction or Hill’s reaction. It can be described in two steps – Photolysis and formation of assimilatory powers.
    ➢ Photolysis is the process of splitting of water in the presence of light. Water splits into H + and OH – ions. H + ions go to NADP+ while OH – ions split into water, oxygen and electron in the presence of Z complex enzymes, Mn +² and Cl – ions.

    4 H2O → 4 H+ + 4OH –
    4OH → 2H2O + O2 ↑ + 4e –
    Mn +² , Cl – , Z complex

    ➢ Assimilatory powers [ATP and NADPH] are formed in non-cyclic photophosphorylation. These assimilatory powers are used in dark reaction. The assimilatory powers are for fixation of CO2 into glucose.
    ➢ Photophosphorylation is the process in which ADP ( Adenosine Diphosphate ) is converted into ATP ( Adenosine Triphosphate ) by the addition of one phosphate ( Pi ) group, i.e inorganic phosphate, utilizing the energy from photons.
    ADP + Pi ATP
    ➢ Dark reaction (Biosynthetic phase) or Blackman reaction involves the fixation and reduction of CO2 resulting in the formation of carbohydrates.
    ➢ The dark phase occurs in the stroma of the chloroplast where all the enzyme necessary for CO2 fixation and synthesis of sugar and starch are located.
    ➢ RuBP (Ribulose bisphosphate) acts as a primary acceptor of atmospheric Co2 in the beginning of this phase and by the utilization of ATP and NADPH (products of light reaction) glucose is synthesized and RuBP is regenerated. The overall reaction of this phase is

    6 RuBP + 6 CO2 + 18 ATP + 12e – → 6 RuBP + C6 H12O6 + 18 ADP + 18 Pi + 12 NADP + 6H2O
    The hydrogen ions combine with CO2 to form glucose.
    ➢ Glucose produced during photosynthesis is soluble in water & consists of small molecules. As soon as glucose is produced. It is converted into starch by the process of polymerisation. The starch is insoluble in water. It is used for the storage of food in the plant cells.
    ➢ Adaptation of a leaf for photosynthesis – Large surface area to maximize light harvesting.

    – Thinness of leaves to reduce distance for CO2 to diffuse through the leaf and to ensure light penetrates into the middle of the leaf.
    – Arrangement of chloroplast on the upper surface of leaves so as to receive maximum amount of light.
    – Presence of more stomata to allow rapid exchange of gases ( CO2 and O2 ).

    Different Experiments and Factors Affecting Photosynthesis

    Quick Review
    Test for photosynthesis : Leaf is killed in boiling water ( 5-10 minutes ), dried, decolourised in warm spirit, moistened and dipped in iodine solution, Blue colour indicates starch.
    CO2 is necessary for photosynthesis/Moll’s half experiment : One half of destarched leaf is inserted in air tight wide mouthed bottle having small quantity of KOH ( for absorption of CO2) and illuminated. Starch test after one hour indicates absence in inserted half and presence in outer half ( where CO2 is available ).
    Light is necessary for photosynthesis : Intact leaf of a destarched plant is fitted in Ganong’s light screen with a designed cut in its lid. The same is exposed to light for some time and then tested for starch. Only the design through which light falls on the leaf becomes blue coloured.
    Chlorophyll is essential for photosynthesis : Illuminated variegated leaf of Coleus/Photos is tested for starch. Only those areas turn blue which had chlorophyll.

    Factors affecting Photosynthesis

    External Factors
    Light : Photosynthesis is successfully accomplished in the visible light (380 – 760 nm wave-length) of the spectrum. Rate of photosynthesis is maximum in red light, average in blue light and minimum in green light. A moderate light intensity is favourable for high rate of photosynthesis.
    Carbon dioxide : 0.03 % CO2 is present in atmosphere. Increase in CO2 concentration upto 0.9% increases the rate of photosynthesis but concentration above 0.9% is harmful and decreases the rate of photosynthesis.
    Temperature : Generally the photosynthesis increases with an increases in temperature in the range of 10-35°C. Beyond the 35°C the rate of photosynthesis decreases.
    Water : About 1% total water absorbed is used in photosynthesis. Water deficiency reduces the rate of photosynthesis.

    Internal Factors
    Chlorophyll : It is essential for photosynthesis to occur. The rate of photosynthesis per unit of chlorophyll decreases with the age of leaf.
    Accumulation of end product of photosynthesis – The rate of photosynthesis falls with the accumulation of food synthesized by photosynthesis.

    Know the Terms

    Photo-oxidation : The reaction of a substance with oxygen under the influence of light.
    Accessory pigments : These are the pigments other than chlorophyll present in photosystems which help in capture light and pass it to the photocentres.
    ➢ CAM plants : These refers to the succulents and some other plants which show crassulacean acid metabolism.
    Chemosynthesis : The process of synthesis of food in which the organisms use chemical reactions to obtain energy from inorganic compounds.
    Photocentre : It is a primary pigment molecule in the photosystem where energy is used to derive of chemical reactions.
    Photorespiration : It is highly wasteful, light dependent, utilization of oxygen and release of carbon-dioxide without release of energy.
    Photosystem : It is light harvesting system present in the thylakoids of chloroplast.
    ➢ Photosynthetically Active Radiation ( PAR ) – It is visible spectrum of light between 400 nm to 700 nm.
    Photolysis : It is splitting of water molecules in the chloroplast in presence of light.
    Translocation : It is the process of movement of food from leaves to roots via phloem.
    Autotrophs : Organisms capable of synthesizing their required nutrients from simple inorganic compounds in the presence of solar energy and chlorophyll.
    Heterotrophs : An organism that cannot synthesize its own food and must feed on nutrients manufactured by autotrophs.
    Carotenoids : A group of yellow, orange and red pigments found mostly in plastids.
    Fluorescence : Emission of light, usually visible, of wave-length different from that absorbed from irradiated materials or from impact of electrons.
    Phosphorescence : It is delayed emission of long wave radiations by irradiate substances which continues for sometime after removal of irradiation source.
    Reaction Centres : It is chlorophyll molecule which converts light energy into chemical energy by bringing about electrical charge separation. Thus chlorophyll molecules act as reaction centres.
    Phytol : It is a carbon chain which is attached to porphyrin ring of chlorophyll like a tail. Chemically phytol is C20H39OH.
    Solarisation : Destruction of chlorophyll due to high light intensity is called solarisation.
    Compensation point : The light intensity at which rate of photosynthesis is equal to the rate of respiration ( in morning and evening ).

    Autotrophic Nutrition in Plants

    We know that all living organisms consume some form of nutrients to sustain life. Animals consume plants or other animals. Plants consume carbon dioxide and water from the environment to produce food.

    Therefore, the process of taking in a source of energy (food) from outside the body of an organism to inside is known as nutrition.

    Do you know what mode of nutrition is carried out in plants? Plants have an autotrophic mode of nutrition. The term ‘autotrophic’ is derived from the Greek word ‘Auto’ meaning self and ‘troph’ meaning nutrition.

    In this mode of nutrition, plants prepare or synthesize their own food with the help of inorganic raw materials. Thus, they are known as autotrophs.

    Let us explore how plants prepare their own food.


    Energy is essential for all life processes. All living organisms require nutrition. What is the ultimate source of nutrition on Earth?

    The sun is the ultimate source of energy on Earth. Energy from the sun is captured by plants and converted into usable form. Thus, the origin of all foods is the food prepared by plants. This food is also consumed by animals.

    Autotrophs such as green plants and some bacteria prepare or synthesize their own food. They are capable of trapping solar energy with the help of a green pigment called chlorophyll. This trapped solar energy is then converted into chemical energy of food using CO2 and H2O.

    Photosynthesis is the process by which chlorophyll-containing cells present in leaves synthesize food in the form of carbohydrates by using carbon dioxide, water and sunlight.

    Therefore, the raw materials required for photosynthesis are CO2 and H2O and the products formed are carbohydrates and O2. Hence, the process can be represented as:

    Let us discuss the raw materials required for photosynthesis.

    How are raw materials consumed by plants?

    The raw materials required for photosynthesis are CO2 and H2O and the products formed are carbohydrates and O2.

    Entry of raw materials
    Plants obtain water through their roots. Water is then transported to all plant parts with the help of the xylem.
    Exchange of gases (entry of CO2 and release of O2) occurs through the stomata.

    Stomata are tiny pores present mainly on the surface of leaves. They are also present on the surface of young stems and roots.

    Stomata consist of a stomatal opening or stoma, which is surrounded by two distinct epidermal cells known as guard cells. The opposing inner walls of the guard cells are thick and inelastic. The remaining walls are thin and elastic.

    Theories behind Stomatal Opening and Closing

    How do plants control the opening and closing of the stomata?

    Two phases of photosynthesis

    The process of photosynthesis occurs in two phases – Photochemical Phase and Biosynthetic Phase.

    Photochemical Phase – A series of chemical reactions take place in the presence of light, as light behaves as catalyst is called as photochemical phase. The light reactions take place in thylakoids of the chloroplasts.

    Light reactions – As the name suggests, this reaction takes place in the presence of light. Light energy is absorbed by chlorophyll molecules and is utilized for splitting water molecules into hydrogen and oxygen. Additionally in this phase, assimilatory power in the form of ATP and NADPH2 are produced. Light reactions occur in the membranes of thylakoids.

    Events occurring during light reactions:
    Absorption of light energy by chlorophyll molecules
    Splitting of water molecules into hydrogen and oxygen atoms
    Formation of ATP and NADPH2

    Reactions involved in Photolysis:-

    Biosynthetic Phase – It includes the reactions that are not dependent on light (but may happen during day time as well). It results in synthesis of carbohydrates, or ‘food’, using the energy produced through light reactions.

    Dark reactions – This reaction does not require direct light and occurs in the stroma of chloroplasts. During this phase, ATP and NADPH2 (formed during light reactions) are utilized for the reduction of CO2 to carbohydrates (food).

    Event occurring during dark reaction: Reduction of CO2 to form carbohydrates Transformation of glucose molecules to 1 mole of Starch is called polymerisation.

    Some interesting facts:
    Do you know that the total amount of O2 produced by an acre of trees per year is equal to the amount consumed by around 18 people annually!
    One tree produces nearly 260 pounds of O2 annually.
    Hydrogen is a clean fuel. Some green algae such as Chlamydomonas reinhardtii are being cultured to convert water into O2 and H2. This mass production of hydrogen could prove to be beneficial, but is still under research.

    End Results of Products Of Photosynthesis
    • Glucose: Simple glucose is utilised by plants in the following ways:
    • For consumption by plant cells
    • For storage as insoluble starch
    • For conversion into sucrose
    • For synthesis of fats, proteins, etc
    • Water: It can be re-utilised in the continuance of photosynthesis.
    • Oxygen: Some of it is used in respiration of leaves and rest diffuses out.

    Global warming

    Do you know that global warming can be reduced by growing more plants?

    Green plants, as we know, utilize CO2 and water to produce food and in the process, release O2 gas. Thus, green plants help in reducing the amount of CO2 in the atmosphere. CO2 is a green house gas, which is one of the reasons for global warming.

    Chloroplast- The site of Photosynthesis

    Site of Photosynthesis
    • Mesophyll cells in green leaves have large number of chloroplasts, which are the site of photosynthesis.
    • Chloroplast consists of the grana and the stroma lamellae (forming a membranous system) and the fluid stroma.
    • Membrane system of chloroplast − traps light, and synthesises ATP and NADPH (site of light-dependent reaction of photosynthesis)
    • Stroma − CO2 is incorporated into the plant by enzymatic reactions, leading to the synthesis of sugar (site of light-independent reaction of photosynthesis)
    • Chloroplasts are aligned along the walls of mesophyll cells so as to get optimum light.

    Pigments Involved in Photosynthesis
    • An absorption spectrum is the graph plotted against the fraction of light absorbed by the pigment.
    • An action spectrum is the rate of a physiological activity plotted against the wavelength of light.
    • The similarity of the action spectrum of photosynthesis and the absorption spectrum of chlorophyll tells us that chlorophylls are the most important pigments in the process.

    (A) Absorption spectrum of chlorophylls a, b, and carotenoids
    (B) Action spectrum of photosynthesis
    (C) Action spectrum superimposed on absorption spectrum of chlorophyll a
    4 types of pigments may be present in leaves: • Chlorophyll a (blue-green)
    Chlorophyll b (yellow-green)
    • Xanthophylls (yellow)
    Carotenoids (yellow to yellow-orange)
    Chlorophyll a is the main pigment in photosynthesis.
    In VIBGYOR spectra, chlorophyll a shows maximum absorption, and hence, the rate of photosynthesis is the highest at the blue and red regions.
    Accessory pigments: Chlorophyll b, xanthophylls and carotenoids
    Absorb a wider range of light, and transfer the energy to chlorophyll a
    Protect chlorophyll a from photo-oxidation

    Factors Affecting Rate of Photosynthesis

    Photosynthesis is influenced by internal (plant) factors and external factors.
    • Internal factors: Number, size and orientation of leaves, mesophyll cells and chloroplasts, internal carbon dioxide concentration and the amount of chlorophyll.
    • External factors: Availability of sunlight, temperature, carbon dioxide concentration and water.
    • Law of Limiting Factors − If a chemical process is affected by more than one factor, then its rate will be determined by factor which is nearest to its minimal value (factor which directly affects the process if its quantity is changed).
    This law of limiting factor was given by Blackman (1905).
    • Light
    • Incident light ∝ CO2 fixation rate but at higher light intensities, the rate does not increase further as other factors become limiting
    Light is rarely a limiting factor (with exception of the shade plants or plants of dense forest) because light saturation occurs at 10% of the full sunlight.
    Beyond a point, if incident light is increased, then it leads to decrease in photosynthesis due to breakdown of chlorophyll.

    • CO2 Concentration
    • Major limiting factor
    • Usually low in atmosphere (0.03 − 0.04%)
    • Up to 0.05% − increases rate of CO2 fixation
    • > 0.05% − damaging effect
    • Though both C3 and C4 show increase in rate of photosynthesis at high light intensities accompanied by high CO2 concentration. The saturation point for C3 is obtained at higher concentrations as compared to C4. Therefore, CO2 concentration is more of a limiting factor for C3 plants.
    • Increased CO2 concentration is beneficial for greenhouse crops such as tomatoes and bell pepper.

    • Temperature
    • Dark reactions are more sensitive to an increase in temperature.
    • C4 plants respond more to an increase in temperature and show higher rate of photosynthesis as compared to C3 plants.
    • Adaptations according to habitat also affect temperature optimum for photosynthesis. Tropical plants have higher temperature optimum compared to plants growing in temperate climates.

    • Water
    • Water stress causes stomata to close and hence, less CO2 is available.
    • Water stress causes the leaves to wilt, thereby reducing their surface area and metabolic activity as well.

    Experiments Related to Photosynthesis

    We know that raw materials are utilized by plants to prepare food. Do plants prepare food at all times? Are there any essential conditions required for photosynthesis?

    1. Sunlight is essential for photosynthesis

    Place a healthy green potted plant in a dark room for 1-2 days. This is done to ensure that the plant consumes all its reserve food and the leaves do not contain any starch. Then, cover a portion of a leaf of this plant on both sides with two uniform pieces of black paper, fixed in position with two paper clips.

    Now, expose this plant to bright light. After a few hours, remove the leaf, decolourize it with alcohol, and test the presence of food (starch) with iodine solution.

    You will observe that the portion of the leaf covered with black paper does not show any presence of starch (food).

    Explanation of the activity:

    The food prepared by plants (carbohydrates) through the process of photosynthesis is stored as starch. This starch reacts with the iodine solution to change to blue-black colour. Only those portions of the leaf that were exposed to sunlight could photosynthesise and hence, change to blue-black colour when tested with iodine.

    2. Chlorophyll is essential for photosynthesis

    Place a variegated plant (i.e., a plant which has both green and non-green areas, for e.g., croton or money plant) in a dark room for 2 – 3 days. This is done to ensure that all the reserve food (starch) is utilized.

    Place this plant in sunlight for six hours to allow photosynthesis to take place. Then, pluck a leaf from this plant and trace the green areas on a sheet of paper.

    Now, decolourize the leaf using alcohol and dip it in a dilute solution of iodine for a few minutes. Wash this leaf with water and compare it with the tracings of the leaf done earlier. It will be observed that only the green areas of the leaf could photosynthesize.

    The leaf is treated with alcohol so that it loses its green colour (chlorophyll pigment) and blue-black colour (in presence of starch) obtained after treatment with iodine.

    The green parts of a variegated leaf contain chlorophyll. Therefore, only these parts could photosynthesize and manufacture food. Thus, the change in colour was observed only in these parts.

    3 CO2 is essential for photosynthesis

    Select two healthy potted plants of nearly the same size and label them as A and B. Place them in a dark room for 2-3 days. Then, place two glass plates under both the plants. Place a watch-glass containing potassium hydroxide besides pot A. Cover both the plants by inverting separate bell jars over them. Potassium hydroxide, as we know, is used to absorb CO2. Therefore, CO2 is not available for plant A.

    Now, seal the bottom of the jars to the glass plates with the help of Vaseline. This prevents the entry of CO2 into the set up. Then, place the plants under sunlight for 2 – 3 hours. Test one leaf each from both plants for the presence of starch, using alcohol and iodine (as explained in the previous activity). It will be observed that plant B has a higher amount of starch as compared to plant A

    Explanation of the activity:

    This happens because potassium hydroxide present besides plant A absorbs all the CO2. Therefore, plant A is not able to photosynthesize and manufacture food. Hence, the amount of starch present in plant B is higher than plant A.

    Photosynthesis in a laboratory

    Place an aquatic plant (hydrilla) in a beaker filled with water. Cover the plant with a transparent funnel. Then, invert a test tube over the open end of the funnel.

    While inverting the test tube, make sure it does not contain any air bubbles. Place this apparatus in sunlight and observe the changes.

    It will be observed that after sometime, air bubbles (O2) emerge in the test tube.

    Importance of Photosynthesis and Carbon Cycle

    Adaptations in Leaves for Photosynthesis

    Leaves often exhibit a number of adaptations to increase the rate of photosynthesis. Some of them are as follows:

    Large surface area for maximum sunlight absorption
    Leaf arrangement at correct angle for maximum light
    Transparent cuticle and upper epidermis to allow free entry of light
    Large number of stomata for maximum exchange of gases
    Thin leaves to reduce the distance between cells involved in rapid transportation
    Chloroplasts concentrated at the upper layer to absorb light quickly
    Extensive vein system for rapid transportation

    Importance of Photosynthesis
    • Provides food- Photosynthesis is the basis for production of food by the autotrophs, i.e. plants. All other organisms are directly or indirectly dependent on the food produced by plants for their survival.
    • Provides oxygen- Oxygen is produced during photosynthesis which is the life supporting gas. All organisms are dependent on the oxygen to sustain their life.

    Carbon Cycle
    Carbon cycle is a series of chemical reactions in which carbon as a chemical element gets consumed by living organisms and again gets restored in the atmosphere by various means.

    The steps involved in carbon cycle are:
    • Autotrophs use carbon dioxide to produce carbohydrates through photosynthesis.
    • These carbohydrates keep travelling from one to another organism through food chain.
    • Plants and animals respire by oxidising carbohydrates to produce energy.
    • In the process of decomposition, the bacteria break down the inorganic matter to release carbon dioxide back into atmosphere.
    • Combustion also releases the carbon dioxide stored in fuels back to the atmosphere.

    Photosynthesis Notes Bi

    6. THE FOODS MADE BY AUTOTROPHS ARE stored in various Organic Compounds, primarily CARBOHYDRATES, including a SIX-CARBON SUGAR called GLUCOSE.

    7. Plants, algae, and some prokaryotes (Bacteria) are all types of Autotrophs.

    8. Only 10 percent of the Earth’s 40 million species are Autotrophs.

    9. Without Autotrophs, all other living things would DIE. Without PRODUCERS you cannot have CONSUMERS.

    10. Autotrophs not only make Food for their own use, but STORE a great deal of Food for use by other organisms (CONSUMERS).

    11. Most Autotrophs use ENERGY from the SUN to make their food, but there are other organisms deep in the ocean that obtain Energy from INORGANIC COMPOUNDS. (CHEMOSYNTHESIS)

    12. Organisms that CANNOT Make their own food are called HETEROTROPHS OR CONSUMERS.

    13. Heterotrophs include animals, fungi, and many unicellular organisms, they stay alive by EATING AUTOTROPHS or other HETEROTROPHS.

    14. Because Heterotrophs must consume other organisms to get Energy, they are called CONSUMERS.

    15. Only part of the energy from the Sun is Used by Autotrophs to make Food, and only part of that Energy can be passed on to other Consumers. A Great Deal of the Energy is LOST as HEAT.

    16. Enough Energy is passed from Autotroph to Heterotroph to give the Heterotroph the Energy it needs.

    17. Photosynthesis involves a COMPLEX SERIES of Chemical Reactions, in which the PRODUCT of One Reaction is Consumed in the Next Reaction.

    18. A Series of Reactions linked in this way is referred to as a BIOCHEMICAL PATHWAY. (Figure 6-1)

    19. Autotrophs use biochemical pathways of photosynthesis to manufacture organic compounds from Carbon Dioxide, CO2, and Water. During this conversion, molecular OXYGEN, O2, is Released.

    20. Some of the energy stored in organic Compounds is Released by Cells in another set of Biochemical Pathways, Known as CELLULAR RESPIRATION. (Chapter 7)

    21. Both Autotrophs and Heterotrophs Perform Cellular Respiration.

    22. During Cellular Respiration in Most Organisms, Organic Compounds are Combined with O2 to Produce ADENOSINE TRIPHOSPHATE or ATP, Yielding CO2 and Water as Waste Products.

    23. The PRODUCTS of Photosynthesis, ORGANIC COMPOUNDS and O2, are the REACTANTS used in CELLULAR RESPIRATION.



    1. In Plants, the INITIAL REACTIONS in Photosynthesis are known as the LIGHT REACTIONS.

    3. A Photosynthetic Cell contains anywhere from ONE to Several Thousands Chloroplasts.

    4. A Chloroplasts is surrounded by TWO MEMBRANES. The INNER Membrane is Folded into many Layers. (Figure 6-2)

    5. A Chloroplasts Inner Membrane layers fuse along the edges to Form THYLAKOIDS.


    7. Each Thylakoid is a closed Compartment surrounded by a Central Space. THE THYLAKOIDS ARE SURROUNDED BY A GEL-LIKE MATRIX (SOLUTION) CALLED THE STROMA. (Figure 6-2)

    8.THE NEATLY FOLDED THYLAKOIDS THAT RESEMBLE STACKS OF PANCAKES ARE CALLED GRANA. The Thylakoids are Interconnected and are Layered on top of one another to form the STACKS of Grana.

    9. Each Chloroplasts may contain hundreds or more Grana.

    10. Hundreds of Chlorophyll Molecules and other Pigments in the Grana are organized into PHOTOSYSTEMS.


    1. LIGHT is made of Particles called PHOTONS that move in WAVES.

    2. The Distance between peaks of the waves is called WAVELENGTH.

    3. Different Wavelengths of Light Carry different amounts of Energy.

    5. You can separate White Light into its component colors by passing the light through a PRISM.

    6. The resulting array of colors, ranging from red at one end to violet at the other is called the VISIBLE SPECTRUM.

    7. Each Color of Light has different Wavelengths, and a Different Energy.

    8. When light strikes an object, its component colors can be Reflected, Transmitted, or Absorbed by an object.

    9. An Object that ABSORBS ALL COLORS appears BLACK.


    11. Objects or Organisms vary in Color because of their specific combination of Pigments.

    12. WAVELENGTHS that are REFLECTED by Pigments are SEEN as the object’s COLOR.


    3. The TWO most common Types of Chlorophylls are designated Chlorophyll a and Chlorophyll b.

    4. A Slight difference in molecular structure between Chlorophyll a and Chlorophyll b causes the Two molecules to Absorb different colors of light.

    5. Chlorophyll’s ABSORB VIOLET, BLUE AND RED LIGHT. These are the Wavelengths of Light that Photosynthesis Occurs. (Figure 6-4)

    6 Chlorophyll a ABSORBS LESS BLUE Light but MORE RED Light than Chlorophyll b Absorbs.

    7. ONLY Chlorophyll a is DIRECTLY INVOLVED in the LIGHT REACTIONS of Photosynthesis. Chlorophyll b ASSISTS Chlorophyll a in Capturing Light Energy and is called an ACCESSORY PIGMENT.

    8. By Absorbing colors Chlorophyll a CANNOT Absorb, the Accessory Pigments enable Plants to Capture MORE of the Energy in Light

    9. Chlorophylls REFLECT and TRANSMIT GREEN LIGHT, causing Plants to appear GREEN.

    10. Another group of Accessory Pigments found in the Thylakoid Membranes, called the CAROTENOIDS, INCLUDES YELLOW, RED, AND ORANGE PIGMENTS THAT COLOR CARROTS, BANANAS, SQUASH, FLOWERS AND AUTUMN LEAVES.

    11. The Carotenoids in Green Leaves are usually masked by Chlorophylls until Autumn when Chlorophylls break down.


    1. Photosynthesis is the process that provides energy for almost all Life.

    2. During Photosynthesis, Autotrophs use the Sun’s Energy to make Carbohydrate Molecules from Water and Carbon Dioxide, Releasing Oxygen as a Byproduct.


    6CO2 + 6H2O + LIGHT C6H12O2 + 6O2
    4. In this equation the Six-Carbon Sugar GLUCOSE and Oxygen are the Products.

    5. The Energy Stored in Glucose and other Carbohydrates can be used later to produce ATP during Cellular Respiration.

    STAGE 1 – CALLED THE LIGHT DEPENDENT REACTIONS. Energy is Capture from Sunlight. Water is Split into Hydrogen Ions, Electrons, and Oxygen (O2). The O2 Diffuses out of the Chloroplasts (Byproduct).

    STAGE 2 – The Light Energy is Converted to Chemical Energy, which is Temporarily Stored in ATP and NADPH.

    STAGE 3 – CALLED THE CALVIN CYCLE. The Chemical Energy Stored in ATP and NADPH powers the formation of Organic Compounds (Sugars), Using Carbon Dioxide, CO2.

    1. The Chlorophylls and Carotenoids are grouped in Cluster of a Few Hundred Pigment Molecules in the Thylakoid Membranes.

    2. Each Cluster of Pigment Molecules is referred to as a PHOTOSYSTEM. There are Two Types of Photosystems known as PHOTOSYSTEM I AND PHOTOSYSTEM II.

    3. Photosystem I and Photosystem II are similar in terms of pigments, but they have Different Roles in the Light reactions.

    4. The Light Reactions BEGIN when Accessory Pigment molecules of BOTH Photosystems Absorb Light.

    5. By Absorbing Light, those Molecules Acquire some of the Energy that was carried by the Light Waves.

    6. In each Photosystem, the Acquired Energy is Passed to other Pigment Molecules until it reaches a Specific Pair of CHLOROPHYLL a Molecules.

    7. The Events occur from this point on can be Divided into 5 STEPS. (Refer to Figure 6-5)

    STEP 1 – Light Energy Forces Electrons to enter a Higher Energy Level in the TWO Chlorophyll a Molecules of Photosystem II. These Energized Electrons are said to be “EXCITED”.

    STEP 2 – The Excited Electrons have enough Energy to Leave Chlorophyll a Molecules. Because they have lost Electrons, the Chlorophyll a Molecules have undergone an OXIDATION REACTION (lost of Electrons). Each Oxidation Reaction must be accompanied by a REDUCTION REACTION (some substance must Accept the Electrons). The Substance is a Molecule in the Thylakoid Membrane Known as a PRIMARY ELECTRON ACCEPTOR.

    STEP 3 – The Primary Electron Acceptor then Donates (gives) the Electrons to the First of a Series of Molecules located in the Thylakoid. This Series of Molecules is called an ELECTRON TRANSPORT CHAIN, because it Transfers Electrons from One Molecule to the Next in Series. As the Electrons are pass from molecule to molecule, they LOSE most of the Energy they acquired when they were Excited. The Energy they LOSE is Harnessed to Move Protons into the Thylakoid.

    STEP 4 – At the same time Light is Absorbed by Photosystem II, Light is also Absorbed by Photosystem I. Electrons move from a Pair of Chlorophyll a Molecules in Photosystem I to another Primary electron Acceptor. The electrons that are LOST by these Chlorophyll a Molecules are REPLACED by the Electrons that have passed through the electron Transport Chain from Photosystem II.

    STEP 5 – The Primary Electron Acceptor of Photosystem I donates Electrons to different Electron Transport Chain. This Chain brings Electrons to the side of the Thylakoid Membrane that FACES THE STROMA. There Electrons COMBINE with a PROTON and NADP+. NADP+ is an Organic Molecule that ACCEPTS Electrons during REDOX Reactions. This reaction causes NADP+ to be Reduced to NADPH.


    2. If the electrons were NOT Replaced, both Electron Transport Chains would STOP, and Photosynthesis would NOT Occur.

    3. The Replacement Electrons are provided by WATER MOLECULES. Enzymes (RuBP carboxylase or Rubisco) inside the Thylakoid SPLITS Water Molecules into PROTONS, ELECTRONS, AND OXYGEN.

    4. For Every TWO Molecules of Water that are Split, FOUR Electrons become available to Replace those lost by Chlorophyll Molecules in Photosystem II.

    5. The PROTONS that are produced are left inside the Thylakoid, while Oxygen Diffuses out of the Chloroplasts and can Leave The Plant.

    6. OXYGEN can be regarded as a Byproduct of the Light Reaction – it is NOT Needed for Photosynthesis.

    7. The Oxygen that results from Photosynthesis is ESSENTIAL for Cellular Respiration in most organisms, including Plants.

    8. The photochemical splitting of water in the light-dependent reactions of photosynthesis, catalyzed by a specific enzyme is called Photolysis.

    9. The enzyme that speeds up this reaction, called RuBP carboxylase (Rubisco), about 20-50% of the protein content in chloroplast, and it may be one of the most abundant proteins in the biosphere.

    2. Chemiosmosis Relies on a CONCENTRATED GRADIENT of Protons Across the Thylakoid Membrane.

    3. Protons are Produced from the Breakdown of Water Molecules, Other Protons are Pumped into the Thylakoid from the Stroma during Photosystem II.

    4. Both these mechanisms act to build up a Concentration Gradient of Protons. The Concentration of Protons is HIGHER in the Thylakoid than in the Stroma.

    5. The Concentration Gradient Represents Potential Energy. The energy is Harnessed by a Protein called ATP SYNTHASE, which is located in the Thylakoid Membrane.

    6. ATP Synthase makes ATP by ADDING a PHOSPHATE GROUP to ADENOSINE DIPHOSPHATE, OR ADP. By Catalyzing the Synthesis of ATP from ADP, ATP Synthase functions as an Enzyme.

    7. ATP Synthase Converts Potential Energy of the Protons Concentrated Gradient into Chemical Energy of ATP.

    8. Together, NADPH and ATP Provide Energy for the Second Set of Reactions in Photosynthesis.


    The Second Set of reactions in photosynthesis involves a biochemical pathway known as the CALVIN CYCLE. This pathway produces Organic Compounds, using the energy stored in ATP and NADPH during the Light Reactions. The Calvin Cycle is named after Melvin Calvin (1911-1997), the American scientist who worked out the details of the pathway.

    OBJECTIVES: Summarized the main events of the Calvin Cycle. Describe what happens to the compounds made in the Calvin Cycle. Distinguish between C3, C4, and CAM Plants. Explain how environmental factors influence photosynthesis.


    1. In the Calvin Cycle, Carbon Atoms From CO2 are Bonded, or “FIXED”, into Organic Compounds.

    2. The incorporation of CO2 into Organic Compounds is referred to as CARBON FIXATION.

    3. The Calvin Cycle has THREE Major Steps, Which OCCUR within the STROMA of the Chloroplasts. (Figure 6-8)

    STEP 1 – CO2 Diffuses into the Stroma from the surrounding Cytosol. An Enzyme combines a CO2 Molecule with a FIVE CARBON CARBOHYDRATE CALLED RuBP (ribulose bisphosphate). The PRODUCT is a Six-Carbon Molecule that Splits into a Pair of Three-Carbon Molecules known as PGA (3-phosphoglycerate).

    STEP 2 – PGA is Converted into another Three-Carbon Molecule, PGAL, in a Two Part Process:

    A. Each PGA Molecule Receives a Phosphate Group from a molecule of ATP – forming ADP

    B. The resulting compound then Receives a Proton from NADPH (forming NADP+) and Releases a Phosphate Group, Producing PGAL.

    In addition to PGAL, these Reactions produce ADP, NADP+, and Phosphate. These Three Products can be used again in the Light Reactions to Synthesis additional Molecules of ATP and NADPH.

    STEP 3 – Most of the PGAL is Converted back into RuBP in a series of reaction to Return to Step 1 and allow the Calvin Cycle to Continue. However, SOME PGAL Molecules LEAVE the Calvin Cycle and can be used by the Plant Cell to Make other Organic Compounds.


    1. Each Turn of the Calvin Cycle Fixes One CO2 Molecule. Since PGAL is a Three-Carbon Compound, it takes Three Turns of the Cycle to Produce each Molecule of PGAL.

    2. For Each Turn of the Cycle TWO ATP, and TWO NADPH Molecules are used in Step 2, and ONE ATP Molecule used in Step 3.

    3. THREE Turns of the Calvin Cycle uses NINE Molecules of ATP and SIX Molecules of NADPH.

    4. The Simplest OVERALL Equation for Photosynthesis, including both Light Reactions and the Calvin Cycle, can be written as:

    6CO2 + 6H20 + LIGHT ENERGY C6H12O6 + 6O2

    1. The Calvin Cycle is the MOST Common Pathway for Carbon Fixation. Plant Species that fix Carbon EXCLUSIVELY through the Calvin Cycle are known as C3 PLANTS.

    2. Other Plant Species Fix Carbon through alternative Pathways and then Release it to enter the Calvin Cycle.

    3. These alternative pathways are generally found in plants that evolved in HOT, DRY Climates.

    5. Stomata are the major passageway through which CO2 Enters and O2 Leaves a Plant.

    6. When a plant’s Stomata are partly CLOSED, the level of CO2 FALLS (Used in Calvin Cycle), and the Level of O2 RISES (as Light reactions Split Water Molecules).

    7. A LOW CO2 and HIGH O2 Level inhibits Carbon Fixing by the Calvin Cycle. Plants with alternative pathways of Carbon fixing have Evolved ways to deal with this problem.

    8. C4 PLANTS – Allows certain plants to fix CO2 into FOUR-Carbon Compounds. During the Hottest part of the day, C4 plants have their Stomata Partially Closed. C4 plants include corn, sugar cane and crabgrass. Such plants Lose only about Half as much Water as C3 plants when producing the same amount of Carbohydrate.

    9. THE CAM PATHWAY – Cactus, pineapples have different adaptations to Hot, Dry Climates. They Fix Carbon through a pathway called CAM. Plants that use the CAM Pathway Open their Stomata at NIGHT and Close during the DAY, the opposite of what other plants do. At NIGHT, CAM Plants take in CO2 and fix into Organic Compounds. During the DAY, CO2 is released from these Compounds and enters the Calvin Cycle. Because CAM Plants have their Stomata open at night, they grow very Slowly, But they lose LESS Water than C3 or C4 Plants.

    1. The Rate at which a plant can carry out photosynthesis is affected by the PLANT’S ENVIRONMENT.


    3. LIGHT INTENSITY – One of the most Important, As Light Intensity INCREASES, the Rate of Photosynthesis Initially INCREASES and then Levels Off to a Plateau.

    4. CO2 LEVELS AROUND THE PLANT – Increasing the level of CO2 Stimulates Photosynthesis until the rate reaches a Plateau.

    5. TEMPERATURE – RAISING the Temperature ACCELERATES the Chemical Reactions involved in Photosynthesis. The rate of Photosynthesis Increase as Temperature Increases. The rate of Photosynthesis generally PEAKS at a certain Temperature, and Photosynthesis begins to Decrease when the Temperature is further Increased. (Figure 6-10b)

    #99 Photosynthesis overview

    Photosynthesis is a series of reactions in which energy transferred as light is transformed to chemical energy.

    Energy from light is trapped by chlorophyll, and this energy is then used to

    • split apart the strong bonds in water molecules to release hydrogen
    • produce ATP
    • reduce a substance called NADP.

    NADP stands for nicotinamide adenine dinucleotide phosphate, which - like NAD - is a coenzyme.
    The ATP and reduced NADP are then used to add hydrogen to carbon dioxide , to produce carbohydrate molecules such as glucose . These complex organic molecules contain some of the energy that was originally in the light. The oxygen from the split water molecules is a waste product, and is released into the air.

    Factors affecting the rate of photosynthesis

    • Light Intensity – the more the better (unless it’s going to damage the plant)
    • Compensation Point – The ratio between photosynthesis and respiration within a plant, if they’re equal or more respiration takes place the plant cannot grow.
    • Carbon Dioxide – This can be the most common limiting factor with plants wanting as much as possible. The atmosphere is only 0.03% CO2.
    • Temperature – since photosynthesis is a biochemical process with enzymes, temperature will affect this.
    • Water – is needed during photosynthesis during stage 1.

    IGCSE Biology

    The most common experiment for this is using pond weed, which is placed under water then factors are varied:
    A lamp is moved further from the plant
    Baking powder is added to the water (increasing CO2)
    A white leaved plant is tested against a green leaved plant (green has more chlorophyll).

    The gas it gives off- being the products of photosynthesis- is counted as bubbles or measured by downwards displacement. This shows the speed of photosynthesis under different conditions.
    Iodine can be used to test the production of starch.


    What organism is investigated when looking at starch production?

    When I did it it was just with a green leaf

    Plants because they store energy as starch, but animals and fungi store energy in glycogen.

    Good luck to everyone on Tuesday btw :)


    u need to add in more on iodine tests. u can check p.109-110 of the edexcel 2009 edition

    Watch the video: Photosynthesis - GCSE Biology 9-1 (September 2022).


  1. Gokinos

    but this is great!

  2. Akirisar

    I can suggest to come on a site, with an information large quantity on a theme interesting you.

  3. Diktilar

    Today, I signed up specifically to join the discussion.

  4. Tibault

    Just what you need. Good topic, I will participate. Together we can come to the right answer.

Write a message