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What would happen if we inject a chloroplast organelle into an animal cell?
Will the animal cell destroy it? Or is it possible that the chloroplast will somehow survive, and even replicate? Could there be photosynthesis in such a cell, or will some of the necessary mechanisms be missing?
To answer your bigger question:
Yes, most of this is possible - under some conditions -, and animals and animal cells can acquire chloroplasts, and use them.
E.g.: see Elysia chlorotica whose cells actively take up chloroplasts and use them, and keep them alive (though not replicating). - Though some genes of algae are also contained in the Elysia chlorotica genome - which may be considered as partial replication.
Also there are salamanders that have replicating algae within them (since embryogenesis) - even algae (with chloroplasts) within animal cells - though here the algae might be rather understood as symbionts or "cell types", and the animal cells don't have the chloroplasts by themselves.
Chloroplasts - Show Me the Green
Chloroplasts are the food producers of the cell. The organelles are only found in plant cells and some protists such as algae. Animal cells do not have chloroplasts. Chloroplasts work to convert light energy of the Sun into sugars that can be used by cells. The entire process is called photosynthesis and it all depends on the little green chlorophyll molecules in each chloroplast.
Plants are the basis of all life on Earth. They are classified as the producers of the world. In the process of photosynthesis, plants create sugars and release oxygen (O2). The oxygen released by the chloroplasts is the same oxygen you breathe every day. Mitochondria work in the opposite direction. They use oxygen in the process of releasing chemical energy from sugars.
Plant Cell Vs. Animal Cell Similarities
Both plant and animal cells are eukaryotic in nature, having a well-defined membrane-bound nucleus.
It is present in both cell types. The nucleus carries most of the genetic material in the chromosomes, which carry the genetic information in the form of DNA (deoxyribonucleic acid).
It is a semi-permeable or selectively-permeable membrane that encloses the contents of a cell, allowing only selected molecules to enter the cell and blocking out the others.
They act as the powerhouse of the cell, converting food into energy. Animal cells have more number of mitochondria, as they are the only source of energy. They also contain a small amount of DNA.
Endoplasmic Reticulum (ER)
These membrane-bound organelles consist of a series of sac-like structures that help in the production of proteins and lipids, and their transportation to the Golgi apparatus. Rough ER helps in transporting proteins and smooth ER aids in the production of lipid.
They act as sites, where proteins synthesize from amino acids. Some ribosomes are attached to the endoplasmic reticulum, while others float freely in the cytoplasm.
It is a flattened sac-like structure which receives and processes proteins from the endoplasmic reticulum, and transports them to various locations within the cell or sends them out of the cell.
Both prokaryotic and eukaryotic cells have a plasma membrane (Figure 6), a phospholipid bilayer with embedded proteins, that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane. We will cover the plasma membrane in more detail in a later unit but here is an overview of this cell surface structure.
Figure 6. The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.
The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus) (Figure 7). Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.
Figure 7. Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit “micrograph”: modification of work by Louisa Howard)
Chloroplast Function Key Points
- Chloroplasts are chlorophyll-containing organelles found in plants, algae, and cyanobacteria. Photosynthesis occurs in chloroplasts.
- Chlorophyll is a green photosynthetic pigment within the chloroplast grana that absorbs light energy for photosynthesis.
- Chloroplasts are found in plant leaves surrounded by guard cells. These cells open and close tiny pores allowing for the gas exchange needed for photosynthesis.
- Photosynthesis occurs in two stages: the light reaction stage and the dark reaction stage.
- ATP and NADPH are produced in the light reaction stage which occurs within chloroplast grana.
- In the dark reaction stage or Calvin cycle, ATP and NADPH produced during the light reaction stage are used to generated sugar. This stage occurs in plant stroma.
Cooper, Geoffrey M. "Chloroplasts and Other Plastids." The Cell: A Molecular Approach, 2nd ed., Sunderland: Sinauer Associates, 2000,
The Cell Wall
The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose, a polysaccharide comprised of glucose units. When you bite into a raw vegetable, like celery, it crunches. That&rsquos because you are tearing the rigid cell walls of the celery cells with your teeth.
Figure (PageIndex<1>): Cellulose: Cellulose is a long chain of &beta-glucose molecules connected by a 1-4 linkage. The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule.
Chloroplasts in an animal cell - Biology
why is the chloroplast absent in animal cell
Chloroplast is present in plant cell because plants are autotrophs, they prepare their own food through photosynthesis which occurs in chloroplasts. But in Animals, its not required as they are dependent on either plants or on other organisms for food.
Anush Manuel answered this
it is because the plants need to cook their food and wants to have colour , but in animal cell food is not cooked and they dont want to havegreen colour in them
Devanshi D Dash answered this
No it does mot have chloroplast, if it had choloroplast it would be green, plant cells are green because of the choloroplast, and animal cells are not green.
Even plants being wutotrophs needs the chloroplast to trap the sunlight for photosynthesis whereas animals are heterotophs which dont need any chloroplast.
Electron Micrographs of Cell Organelles | Zoology
In this article we will discuss about:- 1. The Electron Micrograph of Mitochondria 2. The Electron Micrograph of Golgi Complex 3. The Electron Micrograph of Endoplasmic Reticulum 4. The Electron Micrograph of Lysosomes 5. The Electron Micrograph of Plastids 6. The Electron Micrograph of Nucleus.
- The Electron Micrograph of Mitochondria
- The Electron Micrograph of Golgi Complex
- The Electron Micrograph of Endoplasmic Reticulum
- The Electron Micrograph of Lysosomes
- The Electron Micrograph of Plastids
- The Electron Micrograph of Nucleus
1. The Electron Micrograph of Mitochondria:
It is an electron micrograph of cell’s largest and most important organelle – the mitochondria and is characterized by the following features (Fig. 7 & 8):
(1) The name mitochondria was given by Benda (1898) and their ma n function was brought to light by Kingsbury (1912).
(2) Each mitochondria in section appears as sausage or cup or bowl shaped structure lined by double membranes. Theoretically, the membrane is similar n structure and chemical composition to plasma membrane.
(3) Two membranes are separated by a 6-8 mm wide fluid filled space called peri-mitochondrial space.
(4) The inner membrane is projected into the central cavity as finger like outgrowths- the cristae.
(5) Numerous small, rounded & stalked particles – The oxysomes or F1 or ATPare are attached to the inner surface of inner membrane.
(6) The central cavity is filled with matrix which theoretically possesses circular DNA 55 s ribosomes and respiratory enzymes.
(7) The main function of mitochondria is to synthesize chemical energy- ATP from glucose as substrate.
(8) From one molecule of glucose 38 ATP molecules (40%) are synthesized and the rest of the energy (60%) goes as heat.
2. The Electron Micrograph of Golgi Complex:
It is the electron micrograph of Golgi complex along with its line drawing and is characterized by the following features (Fig.9 & 10):
(1) It was discovered by Camillio Golgi (1898) and was named after his name.
(2) The Golgi complex, as is visible in electron microphotograph, is a stack (bundle) of hollow tubules, which in actual form are hollow flattened sacks arranged above each other. On either side certain large globular vesicles and smaller vacuoles are also visible.
(3) Each tubule or lamella is lined by membrane, which is theoretically similar to plasma membrane in structure and chemical composition.
(4) The Golgi complex is more prominent and well developed in secretory cells and absent in RBC of mammals and prokaryotic cells.
(5) Its main function is to glycolise the proteins which are synthesized by ribosomes i.e., It converts these inert proteins into glycoprotein’s to act as hormones, enzymes and co­enzymes.
(6) It also helps in the formation of lysosomes and acrosome of sperms.
3. The Electron Micrograph of Endoplasmic Reticulum:
It is an electron micrograph of endoplasmic reticulum and is characterized by following features (Fig. 11 & 12):
(1) It was discovered and named by Porter (1948).
(2) It is made up of large number of interconnected and branched tubules, long, flattened and sac-like cisternae and hollow approximately rounded vesicles present all over in the cytoplasm forming a continuous system.
(3) Each tubule, cisternae or vesicle is made up of membrane, which is theoretically similar to plasma membrane in structure and chemical composition.
(4) Some cisternae and tubules bear small, dark, rounded and granular structures, ribosomes, along their surface. This endoplasmic reticulum is called rough or granular E.R. The endoplasmic reticulum without ribosomes is called smooth or agranular ER.
(5) The main function of rough endoplasmic reticulum is protein synthesis.
(6) The main functions of smooth endoplasmic reticulum are:
(b) Synthesis of lipids & cholesterol
(c) To mobilize Ca+++ and Mg++ ions and (1) Glycogenolysis.
(7) It is absent in R.B.C. of mammals and prokaryotic cells.
(8) Both types of reticulum provide mechanical support, transport with in the cell, conduction of nerve and electric impulses and formation of nuclear membrane at the time of cell division.
4. The Electron Micrograph of Lysosomes:
This is the electron micrograph of Lysosome, and is characterized by following features.
These are also called Suicide bags or Death bags of the cell (Fig. 13 &14):
(1) They were discovered by de Duve (1954).
(2) They are spherical or irregular membrane bound vesicles filled with digestive enzymes.
(3) The Lysosomes in a cell occur in three forms viz., primary lysosome, secondary lysosome and residual body.
(4) The primary lysosomes are nascent lysosomes which are in a dormant stage the secondary lysosome are those which have fused with phagocytic vesicles and has released their enzyme contents into the vesicle. This is also called phagosome. The residual body is one which has completed its digestive function and is ready to be thrown out of the cell.
(5) They develop from Golgi complex.
(6) Besides digestion, their other function is autophagic digestion during extreme starvation or extreme toxicities.
They also promote:
(d) Defence against disease, bacteria and viruses and
(7) These are absent in mammalian RBC, Prokaryotic cells and most plant cells.
5. The Electron Micrograph of Plastids:
This is an electron-micrograph of plastid or chloroplast, which is an integral component of all green plant leaves and is characterized by following features (Fig. 15 & 16):
(1) They may be spheroidal, ovoid, stellate or collar shaped and differ in size and number in different cells.
(2) Each chloroplast is a sac-like structure, which is made up of double membranes separated from one another by periplastidial space.
(3) Two types of double membranous lamellae are embedded in the stroma or matrix filled cavity:
(a) Smaller flattened disc-shaped lamellae – The thylakoids, placed one above the other in a stack – the grana.
(b) Larger tubular lamellae between grana called lamellae or frets which connect adjacent granna.
(4) The Inner surface between the two membranes of a thylakoid bear countless granular chlorophyll particles the Ouantasomes.
(5) The plastids also have their own circular DNA 55 s – Ribosomes and RNA
(6) The main function of chloroplast or plastid is to synthesize carbohydrate molecules from CO2 + H2O using light energy.
6. The Electron Micrograph of Nucleus:
This is an electron micrograph of nucleus. (Fig. 17 & 18):
(1) Nucleus was discovered by Brown (1831).
(2) It is a characteristic entity of almost all eukaryotic cells except mammalian RBCs.
(3) The nucleus is generally one but may also be two, four or many.
(4) Each nucleus is surrounded by double nuclear membranes perforated by numerous nuclear pores. Each nuclear membrane is just like unit membrane. Inside, there is present a large darkly stained nucleolus and a network of chromatin threads.
(5) The nucleolus is responsible for all the ribosomal RNA synthesis and chromatin (DNA) is responsible for controlling all the metabolic activities of cell as well as for all hereditary activities.
(6) The chromatin threads are made up of double helical DNA molecule which are the carrier of heredity units- the genes.
BIO 140 - Human Biology I - Textbook
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By the end of this section, you will be able to:
- Describe the structure of eukaryotic cells
- Compare animal cells with plant cells
- State the role of the plasma membrane
- Summarize the functions of the major cell organelles
Have you ever heard the phrase &ldquoform follows function?&rdquo It&rsquos a philosophy practiced in many industries. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should be built with several elevator banks a hospital should be built so that its emergency room is easily accessible.
Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells ( Figure 1). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others and 3) several, rod-shaped chromosomes. Because a eukaryotic cell&rsquos nucleus is surrounded by a membrane, it is often said to have a &ldquotrue nucleus.&rdquo The word &ldquoorganelle&rdquo means &ldquolittle organ,&rdquo and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions.
At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let&rsquos first examine two important components of the cell: the plasma membrane and the cytoplasm .
Figure 1: These figures show the major organelles and other cell components of (a) a typical animal cell and (b) a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole&mdashstructures not found in animal cells. Plant cells do not have lysosomes or centrosomes.
The Plasma Membrane
Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 2), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane.
Figure 2: The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.
The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus) (Figure 3). Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.
Figure 3: Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit "micrograph": modification of work by Louisa Howard)
The cytoplasm is the entire region of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). It is made up of organelles suspended in the gel-like cytosol , the cytoskeleton, and various chemicals (Figure 1). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there, too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.
Typically, the nucleus is the most prominent organelle in a cell (Figure 1). The nucleus (plural = nuclei) houses the cell&rsquos DNA and directs the synthesis of ribosomes and proteins. Let&rsquos look at it in more detail (Figure 4).
Figure 4: The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.
The Nuclear Envelope
The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 4). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.
The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.
Chromatin and Chromosomes
To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nuclei of its body&rsquos cells. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins are attached to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin (Figure 5) chromatin describes the material that makes up the chromosomes both when condensed and decondensed.
Figure 5: (a) This image shows various levels of the organization of chromatin (DNA and protein). (b) This image shows paired chromosomes. (credit b: modification of work by NIH scale-bar data from Matt Russell)
We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.
Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope (Figure 1). Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small (Figure 6). Ribosomes receive their &ldquoorders&rdquo for protein synthesis from the nucleus where the DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.
Figure 6 Ribosomes are made up of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins.
Because proteins synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), ribosomes are found in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.
Mitochondria (singular = mitochondrion) are often called the &ldquopowerhouses&rdquo or &ldquoenergy factories&rdquo of a cell because they are responsible for making adenosine triphosphate (ATP), the cell&rsquos main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.
In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don&rsquot get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.
Mitochondria are oval-shaped, double membrane organelles (Figure 7) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.
Figure 7. This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane. (credit: modification of work by Matthew Britton scale-bar data from Matt Russel
Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water.) For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.
Vesicles and Vacuoles
Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them: The membranes of vesicles can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components.
Animal Cells versus Plant Cells
At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles, but there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells also have centrioles associated with the MTOC: a complex called the centrosome. Animal cells each have a centrosome and lysosomes, whereas plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole, whereas animal cells do not.
The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other (Figure 8). Each centriole is a cylinder of nine triplets of microtubules.
Figure 8. The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder made up of nine triplets of microtubules. Nontubulin proteins (indicated by the green lines) hold the microtubule triplets together.
The centrosome (the organelle where all microtubules originate) replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the exact function of the centrioles in cell division isn&rsquot clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division.
Animal cells have another set of organelles not found in plant cells: lysosomes. The lysosomes are the cell&rsquos &ldquogarbage disposal.&rdquo In plant cells, the digestive processes take place in vacuoles. Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.
The Cell Wall
If you examine Figure 1b, the diagram of a plant cell, you will see a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose (Figure 9), a polysaccharide made up of glucose units. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That&rsquos because you are tearing the rigid cell walls of the celery cells with your teeth.
Figure 9. Cellulose is a long chain of &beta-glucose molecules connected by a 1-4 linkage. The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule.
Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food.
Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast&rsquos inner membrane is a set of interconnected and stacked fluid-filled membrane sacs called thylakoids (Figure 10). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.
Figure 10. The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome.
The chloroplasts contain a green pigment called chlorophyll , which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle.
We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.
Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = &ldquowithin&rdquo) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. We have already mentioned that microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine.
Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just as mitochondria and chloroplasts do. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.
The Central Vacuole
Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 1b, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell&rsquos concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That&rsquos because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.
The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm.
Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleus&rsquos nucleolus is the site of ribosome assembly. Ribosomes are either found in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration they are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.
Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells.
Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm.
Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleolus within the nucleus is the site for ribosome assembly. Ribosomes are found in the cytoplasm or are attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria perform cellular respiration and produce ATP. Peroxisomes break down fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.
Animal cells also have a centrosome and lysosomes. The centrosome has two bodies, the centrioles, with an unknown role in cell division. Lysosomes are the digestive organelles of animal cells.
Plant cells have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole expands, enlarging the cell without the need to produce more cytoplasm.
The endomembrane system includes the nuclear envelope, the endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport membrane lipids and proteins.
The cytoskeleton has three different types of protein elements. Microfilaments provide rigidity and shape to the cell, and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural elements of centrioles, flagella, and cilia.
Animal cells communicate through their extracellular matrices and are connected to each other by tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other by plasmodesmata.