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Energy released during the production of ATP?

Energy released during the production of ATP?


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When glucose is used during aerobic and anaerobic exercise, how much energy is expended or required?

During aerobic exercise:

$C_6H_{12}O_6 + 6 O_2 o ATP + H_2O + 6 CO_2$ + energy

During anaerobic exercise:

$C_6H_{12}O_6 + 6 O_2 o 2 C_3H_6O_3 + 2 ATP$ + energy

For each of the reactions above what is the value of the '+ energy' part?


This is difficult to answer exactly since the thermodynamics of cellular metabolism are not well understood. These are spontaneous reactions, so there is certainly a loss of Gibbs' energy $Delta G < 0$; this energy corresponds to your "energy" term on the product side of the reactions. For the anaerobic case (glycolysis) a balanced reaction formula is

Glucose + 2 ADP + 2 Pi $ ightarrow$ 2 Lactate + 2 ATP + 2 H2O

Details like phosphate (Pi) and water is important here. Now, if we know the free energies $G$ of each substrate and product, we can calculate the $Delta G$ as the sum of the products' energies minus the sum of the reactants. Unfortunately, this energy $G$ is hard to get at. It depends on a number of things, like the molecule structure, concentration, temperature and the pH and ion strength of the solution. These things are not exactly known, so the answer will be uncertain. This is actually an active area of research, and sophisticated methods have been developed for calculating $G$. One of the best are from a groyp at the Weizmann institute in Israel, and is available at http://equilibrator.weizmann.ac.il

If we assume all concentrations to be 1mM, this method gives $Delta G = -118.7$ for the glycolytic reaction above. This is the energy lost (mainly as heat) in the reaction. For comparison with energy captures by ATP, we can break up the reaction into two parts and calculate

Glucose $ ightarrow$ 2 Lactate $quadquadquadquadquad Delta G = -206.7$

2 ADP + 2 Pi $ ightarrow$ 2 ATP + 2 H2O $quadquadDelta G = 87.0$

which sums to -118.7, as expected. This tells us the fraction of the Gibb's energy captured as ATP is 87/206.7 = 42%, while 58% is lost as "heat". So glycolysis has an efficiency of 42%. (For comparison, a combustion engine is somewhere at 15--20%.)

The aerobic case is more difficult, because the stoichiometry is not even fixed: the number of ATP molecules actually obtained from 1 molecule of glucose depends on the efficiency of the respiratory chain, on cytosolic NADH oxidation, and otherthings. But let's say we get 30 ATP. Then

Glucose + 30 ADP + 30 Pi + 6 O2 $ ightarrow$ 6 CO2 + 30 ATP + 36 H2O

has $Delta G = -1608$, which can be broken up into

Glucose + 6 O2 $ ightarrow$ 6 CO2 + 6 H2O $quadquadDelta G = -2913$

30 ADP + 30 Pi$ ightarrow$ 30 ATP + 30 H2O $quadquadDelta G = 1305$

So here 1305/2913 = 45% of the energy is captured. So in this sense, glycolysis and respiration are similar in efficiency. Varying the number of ATP changes this value; try it! Of course, glycolysis can only partially oxidize glucose (into lactate), while respiration achieves complete oxidation to CO2, extracting much more energy.

Again, these values depends heavily on concentrations of metabolites inside cells, which are not well known. Try changing them at the equilibrator web site and note the effects! Unfortunately, many biochemistry textbooks present values of $Delta G$ assuming reactants at 1M (!) which is completely irrelevant to actual conditions in living cells.


In biology, anaerobic respiration is a way for an organism to produce usable energy, in the form of adenosine triphosphate, or ATP, without the involvement of oxygen; it is respiration without oxygen. This process is mainly used by prokayotic organisms (bacteria) that live in environments devoid of oxygen. Although oxygen is not used, the process is still called respiration because the basic three steps of respiration are all used, namely glycolysis, the citric acid cycle, and the respiratory chain, or electron transport chain. It is the use of the third and final step that defines the process as respiration. In order for the electron transport chain to function, a final electron acceptor must be present to take the electron away from the system after it is used. In aerobic orgainisms, this final electron acceptor is oxygen. Oxygen is a highly electronegative atom and therefore is an excellent candidate for the job. In anaerobes, the chain still functions, but oxygen is not used as the final electron acceptor. Other less electronegative substances such as sulfate (SO4), nitrate (NO3), and sulfur (S) are used. Oftentimes, anaerobic organisms are obligate anaerobes, meaning they can only respire using anaerobic compounds and can actually die in the presence of oxygen.

Anaerobic respiration is not the same as fermentation, which does not use either the citric acid cycle or the respiratory chain (electron transport chain) and therefore, cannot be classified as respiration.

Aerobic Respiration

Aerobic respiration requires oxygen in order to generate energy (ATP). Although carbohydrates, fats, and proteins can all be processed and consumed as reactant, it is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion in order to be fully oxidized by the Krebs cycle. The product of this process is energy in the form of ATP (Adenosine Triphosphate), by substrate-level phosphorylation, NADH and FADH2.

Simplified reaction: C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) ΔG = -2880 kJ per mole of C6H12O6

The negative ΔG indicates that the products of the chemical process store less energy than the reactants and the reaction can happen spontaneously; In other words, without an input of energy

The reducing potential of NADH and FADH2 is converted to more ATP through an electron transport chain with oxygen as the "terminal electron acceptor". Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. This works by the energy released in the consumption of pyruvate being used to create a chemiosmotic potential by pumping protons across a membrane. This potential is then used to drive ATP synthase and produce ATP from ADP. Biology textbooks often state that 38 ATP molecules can be made per oxidised glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system). However, this maximum yield is never quite reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into the mitochondrial matrix and current estimates range around 29 to 30 ATP per glucose.

Aerobic metabolism is 19 times more efficient than anaerobic metabolism (which yields 2 mol ATP per 1 mol glucose). They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells.

Thus,more energy is released during Aerobic Respiration than Anaerobic Respiration.


Energy released during the production of ATP? - Biology

Catabolic pathways are controlled by enzymes, proteins, electron carriers, and pumps that ensure that the remaining reactions can proceed.

Learning Objectives

Explain how catabolic pathways are controlled

Key Takeaways

Key Points

  • Glycolysis, the citric acid cycle, and the electron transport chain are catabolic pathways that bring forth non-reversible reactions.
  • Glycolysis control begins with hexokinase, which catalyzes the phosphorylation of glucose its product is glucose-6- phosphate, which accumulates when phosphofructokinase is inhibited.
  • The citric acid cycle is controlled through the enzymes that break down the reactions that make the first two molecules of NADH.
  • The rate of electron transport through the electron transport chain is affected by the levels of ADP and ATP, whereas specific enzymes of the electron transport chain are unaffected by feedback inhibition.

Key Terms

  • phosphofructokinase: any of a group of kinase enzymes that convert fructose phosphates to biphosphate
  • glycolysis: the cellular metabolic pathway of the simple sugar glucose to yield pyruvic acid and ATP as an energy source
  • kinase: any of a group of enzymes that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific target molecules (substrates) the process is termed phosphorylation

Control of Catabolic Pathways

Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the electron transport chain tend to catalyze non-reversible reactions. In other words, if the initial reaction takes place, the pathway is committed to proceeding with the remaining reactions. Whether a particular enzyme activity is released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP).

Glycolysis

The control of glycolysis begins with the first enzyme in the pathway, hexokinase. This enzyme catalyzes the phosphorylation of glucose, which helps to prepare the compound for cleavage in a later step. The presence of the negatively-charged phosphate in the molecule also prevents the sugar from leaving the cell. When hexokinase is inhibited, glucose diffuses out of the cell and does not become a substrate for the respiration pathways in that tissue. The product of the hexokinase reaction is glucose-6-phosphate, which accumulates when a later enzyme, phosphofructokinase, is inhibited.

Glycolysis: The glycolysis pathway is primarily regulated at the three key enzymatic steps (1, 2, and 7) as indicated. Note that the first two steps that are regulated occur early in the pathway and involve hydrolysis of ATP.

Phosphofructokinase is the main enzyme controlled in glycolysis. High levels of ATP, citrate, or a lower, more acidic pH decrease the enzyme’s activity. An increase in citrate concentration can occur because of a blockage in the citric acid cycle. Fermentation, with its production of organic acids like lactic acid, frequently accounts for the increased acidity in a cell however, the products of fermentation do not typically accumulate in cells.

The last step in glycolysis is catalyzed by pyruvate kinase. The pyruvate produced can proceed to be catabolized or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, the enzyme is inhibited. The enzyme’s activity is increased when fructose-1,6-bisphosphate levels increase. (Recall that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis. ) The regulation of pyruvate kinase involves phosphorylation, resulting in a less-active enzyme. Dephosphorylation by a phosphatase reactivates it. Pyruvate kinase is also regulated by ATP (a negative allosteric effect).

If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase. If either acetyl groups or NADH accumulate, there is less need for the reaction and the rate decreases. Pyruvate dehydrogenase is also regulated by phosphorylation: a kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated.

Citric Acid Cycle

The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH. These enzymes are isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease. When more ATP is needed, as reflected in rising ADP levels, the rate increases. α-Ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA, a subsequent intermediate in the cycle, causing a decrease in activity. A decrease in the rate of operation of the pathway at this point is not necessarily negative as the increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.

Citric Acid Cycle: Enzymes, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, catalyze the reactions that make the first two molecules of NADH in the citric acid cycle. Rates of the reaction decrease when sufficient ATP and NADH levels are reached.

Electron Transport Chain

Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases: ATP begins to build up in the cell. This change in the relative concentration of ADP to ATP triggers the cell to slow down the electron transport chain.

Electron Chain Transport: Levels of ADP and ATP affect the rate of electron transport through this type of chain transport.


How do you calculate ATP from glycolysis?

Outcomes of Glycolysis. Glycolysis starts with one molecule of glucose and ends with two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH.

Likewise, why is ATP 38 or 36? Calculations giving 36-38 ATP per glucose are based on the assumption that oxidation of NADH produces 3 ATP and oxidation of UQH2 (FADH2, Succinate) produces 2 ATP. They translocate protons outward across the inner mitochondrial membrane, and the resulting proton gradient is used by the ATP synthase to produce ATP.

Keeping this in consideration, how is 36 ATP produced?

Cellular respiration produces 36 total ATP per molecule of glucose across three stages. Breaking the bonds between carbons in the glucose molecule releases energy. There are also high energy electrons captured in the form of 2 NADH (electron carriers) which will be utilized later in the electron transport chain.

Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. Biology textbooks often state that 38 ATP molecules can be made per oxidized glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system).


3.5.2 Respiration (A-level only)

Opportunities for skills development

Glycolysis is the first stage of anaerobic and aerobic respiration. It occurs in the cytoplasm and is an anaerobic process.

Glycolysis involves the following stages:

  • phosphorylation of glucose to glucose phosphate, using ATP
  • production of triose phosphate
  • oxidation of triose phosphate to pyruvate with a net gain of ATP and reduced NAD.

If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD. The oxidised NAD produced in this way can be used in further glycolysis.

If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport.

Aerobic respiration in such detail as to show that:

  • pyruvate is oxidised to acetate, producing reduced NAD in the process
  • acetate combines with coenzyme A in the link reaction to produce acetylcoenzyme A
  • acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle
  • in a series of oxidation-reduction reactions, the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost
  • synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory)
  • other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.

Students could use a redox indicator to investigate dehydrogenase activity.

Required practical 9: Investigation into the effect of a named variable on the rate of respiration of cultures of single-celled organisms.


Structure of an ATP Molecule

The ATP molecule consists of a purine base, pentose sugar and phosphate group. The purine base, adenine is attached to 1′ carbon atom of ribose, which is a pentose sugar. The three phosphate groups are attached to the pentose sugar at the position of 5′ carbon atom. Following is the structure of the ATP molecule with its constituents.

The energy is stored in the P-P-P or the phosphate bond which is released when the bond is broken and ATP converts into ADP through the process of hydrolysis which is also known as dephosphorylation.

In the above reaction, ADP is Adenosine Di-Phosphate and Pi is inorganic phosphate. The reaction shows the breaking of ATP and the release of energy. The reaction can also be reversed and ADP can be converted to ATP but it will require the same amount of energy which is released during the process i.e. 30.6 KJ. This process is known as condensation or phosphorylation. This takes place because the ATP molecule is a very unstable and gets hydrolyzed very soon. The bonds between the phosphate group in the ATP molecule is weaker than the ADP molecule. Hence, we can say that the presence of one phosphate group can make a difference in the energy production and consumption of the molecule.


How does glycolysis produce ATP?

Glycolysis produces energy through the form of ATP. ATP is created directly from glycolysis through the process of substrate-level phosphorylation (SLP) and indirectly by oxidative phosporylation (OP). The three stages of glycolysis are phosphorylation of glucose to glucose-6-phosphate (G6P) which requires ATP, production of triose phosphate (TP) and oxidation of TP to pyruvate, which yields 2 reduced NAD molecules (NADH) and 4 ATP per glucose. 2 ATP molecules were used in the first stage so net ATP gain is 2 ATP. This is substrate-level phosphorylation. Indirectly, ATP is produced through oxidative phosphorylation. This requires the electron transport chain (ETC) and ATP synthase found on the inner mitochondrial membrane. Briefly, in the ETC the protons from reduced NAD are pumped across the membrane, using the transfer of electrons as an energy source, (from the inner mitochondrial matrix to the intermembrane space) to produce a chemiosmotic gradient. This chemiosmotic gradient provides the energy to drive formation of ATP from ADP and inorganic phosphate within the ATP synthase enzyme.


Electron Transport Chain of Photosynthesis | Plants

The light-driven reaction of photosynthesis also called light reaction (Hill reaction), referred to as electron transport chain, were first propounded by Robert Hill in 1939. The electron transport chain of photosynthesis is initiated by absorption of light by photosystem II (P68o).

When P680 absorbs light, it is excited and its electrons are transferred to an electron acceptor molecule. Therefore, P680 becomes a strong oxidising agent, and splits a molecule of water to release oxygen. This light-dependent splitting of water molecule is called photolysis.

However, manganese, calcium and chloride ions play important roles in photolysis of water. After photolysis of water, electrons are generated, which are then passed to the oxidised P680. Now, the electron deficient P680 (as it had already transferred its electrons to an acceptor molecule) is able to restore its electrons from the water molecule.

After accepting electron from the excited P680, the primary electron acceptor is reduced. The primary electron acceptor in plants is pheophytin. The reduced acceptor which is a strong reducing agent, now donates its electrons to the downstream components of the electron transport chain.

Photosystem I (PS I):

Similar to photosystem II (P680), photosystem I (P700) is excited on absorption of light and gets oxidised, and transfers its electrons to the primary electron acceptor (pheophytin), which, in turn gets reduced. While the oxidised P700 draws electrons from photosystem II, the reduced electron acceptor of photosystem I, transfers electrons to ferredoxin and ferredoxin-NADP reductase to reduce NADP to NADPH2.

NADPH2 is a powerful reducing agent, and is utilised in the reduction of CO2 to carbohydrates in the carbon reaction of photosynthesis. The reduction of CO2 to carbohydrates requires energy in the form of ATP, produced through electron transport chain. Process of ATP formation from ADP in the presence of light in chloroplasts is called photophosphorylation.

The Light Reaction (Hill Reaction):

The light reaction is thought to be responsible for the production of a ‘reducing power’ and oxygen from water as a result of light energy. This is as follows: The light energy, after absorption by chlorophyll, splits H2O.

(i) The (H) combines with an unidentified compound (probably ferredoxin) and is passed from this to NADP.

(ii) The NADPH2 can cause the reduction of phosphoglyceric acid……. Phosphoglyceraldehyde, together with some ATP production.

(iii) The (OH) forms H2O and oxygen:

The light reaction gives rise to two very important productions:

(i) A reducing agent NADPH2 and

(ii) An energy rich compound ATP.

These two products of the light reaction are utilized in the dark phase of photosynthesis.

The energy transformations in photosynthesis are as follow:

(i) The radiant energy of an absorbed quantum is transformed into the energy of an activated pigment molecule

(pigment molecule or activated pigment)

(ii) Now the activated pigment removes an electron from the hydroxyl ion derived from the water molecule. The (OH) represents the ‘free radical’. These are uncharged, but highly reactive forms.

(iii) The free radicals react in many ways the release of oxygen and formation of free radicals of hydrogen takes place.

(iv) The H + ions from water, together with the electron attached to the pigment are transferred to certain molecules, which then carry the reducing power to other reactions.

(v)Another reaction is the recombination of the split products of water into the water molecules itself.

This reaction is strongly energy-releasing. The chloroplast puts this reaction to work by causing it to synthesize energy-rich ATP from a precursor molecule ADP and inorganic phosphate

(6) The energy of the ATP can now be used, in the reduction of CO2 to sugar by the reducing power (NADP.H) generated in the light reaction.

This way, the radiant energy has been converted to the chemical energy of the sugar molecule by passing through a photo-activated pigment, photolyzed water fragments, and ATP. The main function of light energy in photosynthesis is to produce ATP through a complex of reactions called photophosphorylation.

The subsequent reactions leading to the formation of sugar from CO2 can proceed entirely in darkness.

Photophosphorylation:

Photosynthetic phosphorylation:

With the discovery that CO2 can be assimilated in isolated chloroplasts, this came into existence that the chloroplast must contain the enzymes necessary for this assimilation and must be able to produce the ATP (adenosine tri-phosphate) essential for the formation of the main photosynthesis products.

Arnon and his co-workers (1954) demonstrated that the isolated chloroplasts can produce ATP in the presence of light. They gave the name to this process photosynthetic phosphorylation.

This was revealed for the first time that mitochondria are not the only cytoplasmic particles that produce ATP. ATP formation in chloroplasts differs from that in mitochondria in that it is free from respiratory oxidations. During this process the light energy is being converted to ATP. In other words, there is a conversion to light energy of chemical energy.

ATP is only one of the necessary requirements for the reduction of carbon dioxide to the carbohydrate level. A reductant must be formed in photosynthesis that will provide the hydrogens or electrons for this reduction. Arnon (1951) demonstrated that isolated chloroplasts are capable of reducing pyridine nucleotides in light.

The photochemical reaction and an enzyme system are capable of utilizing the reduced pyridine nucleotide as soon as this was formed, Arnon (1957) found that NADP. H2 is the reduced pyridine nucleotide in photosynthesis.

In the presence of H2O. ADP (adenosine di-phosphate) and orthophosphate (P), substrate amounts of NADP (nicotinamide adenine dinucleotide phosphate) were reduced, accompanied by the evolution of oxygen.

The equation is as follow:

As shown by the equation the evolution of one molecule of oxygen is accompanied by the reduction of two molecules of NADP and esterification of two molecules of orthophosphate. Together, ATP and NADPH2 provide the energy requirements for CO2 assimilation. Arnon gave name to this power assimilatory power (i.e., ATP + NADPH2).

According to Arnon (1967), in bacterial photosynthesis NADH2 is utilized of NADPH2.

In the late 1950’s the reduction of NADP + was thought to be associated with a soluble protein factor found in chloroplasts. Arnon et al. (1957) observed that this protein reduced NADP + accompanied by the evolution of oxygen. They termed it the ‘NADP reducing factor.’

Thereafter the NADP reducing factor was purified and called photosynthetic pyridine nucleotide reductase (PPNR), since its catalytic activity was only apparent when chloroplasts were kept in light.

Tagawa and Arnon (1962) recognized that PPNR is one of a family of nonhemenonflavin, iron-containing proteins that is universally present in chloroplasts. These proteins were given a generic name ferredoxin.

When ferredoxin was not discovered, NADP was thought to be the terminal electron acceptor of the photosynthetic light reaction. Arnon (1967) revealed that illuminated chlorophyll reacts directly with ferredoxin and not with NADP + .

The exposition of chlorophyll to light causes a flow of electrons to ferredoxin. Now the reduced ferredoxin causes the reduction of NADP + in an enzyme catalyzed reaction that is independent of light. In other words, ferredoxin is termed as terminal electron acceptor of the photosynthetic light reaction.

The reduction of NADP takes place by ferredoxin. Under normal condition, in photosynthesis ferredoxin reduced by the acceptance of an electron is immediately reoxidized by NADP + . The reduction of NADP by ferredoxin is catalyzed by ferredoxin-NADP reductase. This shows that the mechanism of NADP + reduction in photosynthesis completes in three steps.

(i) Photochemical reduction of ferredoxin

(ii) Reoxidation of ferredoxin by ferredoxin NADP + reductase and

(iii) Reoxidation of ferredoxin-NADP + reductase by NADP + .

According to Arnon there are two types of photophosphorylation:

(i) Non-cyclic photophosphorylation and

(ii) Cyclic photophosphorylation.

Non-Cyclic Photophosphorylation:

This is a result of an interaction of photosystem I (PSI) and photosystem II (PSII). In non-cyclic photophosphorylation, the electron is not returned to the chlorophyll molecule, but is taken up by NADP ± which thereafter reduces to NADPH. Here the electron that returns to the chlorophyll molecule is derived from an outside source which is water.

In this process oxygen is released and both NADPH2 − and ATP are formed. In green plants and many photosynthetic bacteria, however, illumination is known to produce also NADPH2 − which provides hydrogen for the reduction of carbon dioxide in the day.

The electron lost by the excited chlorophyll is accepted by NADP along with a proton resulting in the formation of NADPH2. The light energy is now stored in the NADPH2 molecule. The proton required for the reduction of NADP is released from the dissociation of water molecule by photolysis into hydrogen H ± and hydroxyl ions OH.

The hydroxyl ions react to produce water and molecular oxygen.

The reaction is as follows:

Here the hydroxyl ion also releases an electron that is accepted by the cytochromes of the chloroplast. In turn, the cytochrome donates this electron to the chlorophyll molecule, which already lost an electron earlier. The energy released during this transfer of electron from the cytochrome is utilized in the formation of ATP by the photophosphorylation of ADP.

In water molecule hydrogen is strongly bound to oxygen and this can be cleaved only by the use of energy. This energy is supplied by light. This way, in non-cyclic photophosphorylation light energy takes part in two processes, i.e., the activation of chlorophyll molecule and photolysis (cleavage) of water.

In non-cyclic photophosphorylation one molecule of NADPH2 and one molecule of ATP are produced by the activation of chlorophyll molecule by a photon, while in cyclic photophosphorylation two molecules of ATP are produced for each photon absorbed by chlorophyll.

The overall reaction of photophosphorylation is as follows:

Cyclic Photophosphorylation:

When non-cyclic photophosphorylation is stopped under certain conditions, cyclic photophosphorylation takes place. The non-cyclic photophosphorylation can be stopped by illuminating isolated chloroplasts with light of wavelength greater than 680 nm.

By this way, only photosystem I (PS I) is activated, as it has a maximum absorption at 700 nm, and photosystem II (PS II), which absorbs at 680 nm, remains inactivated.

Due to inactivation of PS II, the electron flow from water to NADP is stopped, and also CO2 fixation is retarded.

When CO2 fixation stops, electrons are not removed from reduced NADPH. Thus, NADPH will not be oxidised and NADP will not be available as an electron acceptor.

Under above-mentioned conditions, cyclic-photophosphorylation occurs.

During cyclic-photophosphorylation, electrons from photosystem I (PS I) are not passed to NADP from the electron acceptor, as NADP is not available in oxidised state to receive electrons.

Hence, the electrons are transferred back to P700.

This type of movement of electrons from an electron acceptor to P700 result in the formation of ATP from ADP, and the process is called cyclic photophosphorylation.

During cyclic photophosphorylation oxygen is not released, as there is no photolysis of water and NADPH2 is also not produced.

In cyclic photophosphorylation the excited electron lost by the chlorophyll is returned to it through vitamin K or FMN (flavin mononucleotide) and cytochromes. The chlorophyll molecule on losing an electron assumes a positive charge and subsequently the electron is transferred to a second acceptor.

This second acceptor is a group of substances collectively known as cytochrome system. All the members of cytochrome system are variants of cytochrome. Ultimately these cytochromes transfer the electron to the chlorophyll molecule from where it was lost initially.

The electromagnetic energy of the light is utilized in the formation of ATP. This means that light energy is being converted into chemical energy. Here the electron after leaving a chlorophyll travels in a cyclic way and ultimately returns to the same molecule from which it initiated, and therefore, this process has been termed by Arnon as cyclic photophosphorylation.

The final electron acceptor and the initial electron donor is the same substance—the chlorophyll. No outside material takes part in the process. During cyclic photophosphorylation, one electron and two ATP molecules are formed.

One ATP molecule is being formed when the electron travels from the cofactor (i.e., vitamin K or FMN) to the cytochromes while the other when it travels from the cytochromes back to the chlorophyll molecule.

Here the light energy is being converted into chemical energy.

In nature both processes of photophosphorylation proceed simultaneously. In green plants the non-cyclic electron transfer is essential for the production of NADPH2 and ATP.

The oxygen is evolved during the process. The cyclic electron transfer fulfils the requirement of the low yield of ATP during non-cyclic process. This way, the complete light phase of photophosphorylation produces ATP and NADPH2 and oxygen is evolved.

NADPH2 is a biological reductant that brings about the reduction of carbon dioxide to carbohydrates in the dark phase of photosynthesis. Here both NADPH2 and ATP provide energy for reduction. The assimilatory power of the cell is constituted by these two components. The energy of these components is derived from visible part of sunlight.

In the dark phase of photosynthesis the energy that is stored in NADPH2 and ATP, is being transferred to the molecules of organic substances and stored there in the form of chemical energy.

During photosynthesis the electromagnetic energy of visible light is being converted into chemical energy. Now this energy is utilized by living cells as the driving force for various vital activities. This act of the conversion of energy is brought about by the photosynthetic cells of green plants or photosynthetic bacteria.

Here the solar energy is trapped by the chlorophyll apparatus. As soon as the light energy is being transformed into chemical energy, it may be used in the formation of carbohydrates, protein synthesis and other important vital activities.

The living are so designed that they can use only chemical energy for various metabolic activities. The light energy cannot be directly used for these vital activities. The light reaction of the higher plants takes place in the grana of the chloroplasts.


Formation of ATP in Anaerobic Cells | Bioenergetics

In this article we will discuss about the formation of ATP in anaerobic cells.

We know that the formation of ATP in the respiratory chain is because it is the most common process in living organisms. The strict anaerobes, i.e. species which can live only in the absence of oxygen (in the ground or mud, for example) are comparatively rare.

On the contrary, there are some facultative anaerobes, organisms which preferentially utilize oxygen to oxidize their nutri­tive substances when oxygen is present but which can also live in absence of oxygen, then drawing the energy they require from fermentation processes.

This capacity of conserving the energy of food in anaerobiosis (production of ATP during fermentation) is found not only in some microorganisms but also in numerous more advanced organisms, and the formation of ATP during the fermentation of glucose is a process of energy conservation which has reached man as we will see below, the anaerobic degradation of glucose precedes its complete oxidation by the Krebs cycle and the respiratory chain.

Various substances (sugars, notably glucose, fatty acids, amino acids) can be utilized by bacteria in anaerobiosis, which also helps in identifying or classifying these bacteria. Furthermore, these bacteria — even when they utilize glucose for example — can also be distinguished according to the mode of fermenta­tion some degrade glucose into ethanol, others into acetone, butanol, butyric acid, propionic acid or lactic acid.

But in a large number of cases, glucose is split into 2 fragments, of which one is oxidized by the other, and this oxidation- reduction process liberates energy, a part of which is stored in the form of ATP. We will study here only the fermentation of glucose to lactic acid, a mechanism found not only in certain bacteria (e.g., of the genus Lactobacillus), but also in higher animals (anaerobic glycolysis).

Energy Yield of Lactic Fermentation:

The transformation of glucose into lactic acid corresponds to the following overall equation:

In reality, in living cells, this cleavage of glucose implies a dozen of reactions and first the utilization of 2 ATP for the formation of glucose-6-phosphate and fructose-1,6-bisphosphate, then the formation of 4 ATP, which gives the fol­lowing overall equation:

On the basis of ∆G0 values and admitting that the formation of one ATP requires at least 7 kcal/mole, one can calculate a minimum energy conservation efficiency of about 27% (2 x 7 x 100/52) with reference to the energy liberated by the cleavage of glucose into lactic acid but if this yield is expressed with reference to the 686 kcal which can be liberated by the sudden oxidation of glucose, then it would be 14 X 100/686 # 2%, i.e. a very low yield compared to the 40% obtained in aerobiosis when glycolysis is followed by the Krebs cycles and by electron transfer in the respiratory chain.

It is quite clear that in anaerobiosis, only a very small part of the total energy of the glucose molecule can be “recovered” and consequently, to accomplish an identical work, anaerobic cells must consume much more (up to 20 times more) glucose per unit time than the aerobic cells.

Glycolysis therefore appears to be a more primitive, less elaborate means of storing energy in the form of ATP this is further confirmed by the fact that the enzymes (about ten) catalysing the various steps are in free state in the soluble part of the cellular cytoplasm, and not grouped in pluri-enzymatic systems of complex structure like the enzymes catalyzing the reactions of respiration and photosynthesis in mitochondria and chloroplasts.

Mechanisms of Energy Conservation. Substrate-Related Phosphorylation:

Figure 4-35 which summarizes the various steps of glycolysis, one observes that paradoxically the first steps of this process (which must lead to the formation of ATP) consume ATP: these steps are the phosphorylation of glucose and that of fructose-6-phosphate.

But then, for each C3 fragment there are 2 steps of phosphorylation of ADP into ATP and therefore, 4 ATP are formed as one molecule of glucose gives 2 C3 fragments. Finally, 2 ATP are consumed and 4 ATP formed, which is conform to the gain of 2 ATP indicated by the overall reaction given in the previous paragraph. We will now see the mechanisms by which these ATP molecules are formed during the glycolysis.

In addition to phosphorylations which take place along the respiratory chain and to photosynthetic phosphorylations, there are substrate-related phosphorylations two examples are given below.

A. Formation of ATP during the oxidation of glyceraldehyde 3-phosphate into 3-phosphoglyceric acid:

The oxidation of an aldehyde to acid:

is an exergonic reaction (∆G0 # -7 kcal/mole). In the cells, this energy is not necessarily dissipated as heat it can be utilized for the endergonic reaction of ATP formation:

The energy produced by the oxidation of aldehyde is then stored in the ATP formed and one can write an overall reaction, the free energy change of which is practically nil:

R-CHO + ADP + Pi → R-COOH + 2H + ATP (∆G0#0)

This is a process involving 2 enzymes — a dehydrogenase, then a kinase — with intermediate formation of an acyl-phosphate, a high energy potential compound (see table) which can liberate energy of the order of 10 kcal/mole during its hydrolysis.

This common intermediate is, on the one hand, the oxidation product of 3-phosphoglyceraldehyde (first reaction), and on the other hand, the donor of the phosphate group to ADP to form ATP (second reaction), as shown by figure 3-10. The free energy of hydrolysis of this compound is considerable because of the high electron density of the anhydride bond be­tween the phosphate and the carboxyl, and of the proximity of the 2 phosphate groups with their double negative charge.

One can see that during the dehydrogenation of 3-phosphoglyceraldehyde there is formation of NADH + H + . When the respiratory chain functions, the transfer of electrons from NADH + H + to oxygen enables – via the shuttles of the reducing power – the formation of 3 ATP, and since one glucose molecule gives 2 C3 fragments, 6 additional ATP can be formed when glycolysis is followed by the aerobic phase, but not during the strict anaerobic glycolysis.

B. Formation of ATP during the conversion of 2-phosphoglyceric acid into pyruvic acid:

It was mentioned above that — in glycolysis as well as in the respiratory chain — energy is stored in the form of ATP during the oxidation-reduction reactions. But, on examining 2-phosphoglyceric acid and pyruvic acid, it is not evident that the passage from one of these compounds to the other implies such a process.

In fact, the study of the detailed mechanism of this transformation (fig. 4-29) shows a displacement of electrons within the molecule which can be regarded as an intramolecular oxidation-reduction. There is here also, forma­tion of an intermediate compound — phosphoenol-pyruvic acid — having a high energetic potential due to high electron concentration around the carbon 2, partly due to the formation of a double bond (2 pairs of electrons) and also to a decrease of distances and bond angles around this carbon.

This compound has a free energy of hydrolysis of the order of 12.8 kcal/mole (see table) so that, on yielding its phosphate group, it allows the formation of one molecule of ATP (see fig. 3-11).

Let us consider the second step of this transformation: since the hydrolysis of phosphoenolpyruvate has a AG0 # -12.8 kcal/mole and the formation of ATP, a ∆G0 # + 7 kcal/mole, the ∆G0 of the reaction phosphoenol-pyruvate + ADP → pyruvate + ATP is of the order of -5 to – 6 kcal/mole if we take an average value of-5.45 kcal/mole and refer to the table below, we find that this value corresponds to a log K = + 4, i.e. K = 10 000 as we know, this value means that the equilibrium of the reaction will be reached when practically all the phosphoenolpyruvate and ADP will be transformed, in other words, this reaction is almost total.

In practice, one often adds phosphoenolpyruvate and pyruvate-kinase as ATP generating mixture in cell-free systems while studying in vitro endergonic reac­tions (synthesis of proteins for example) which could no longer take place if ATP were hydrolysed to ADP and happened to be lacking in the reaction mixture.


7.1 Energy in Living Systems

By the end of this section, you will be able to do the following:

  • Discuss the importance of electrons in the transfer of energy in living systems
  • Explain how ATP is used by cells as an energy source

Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions, which occur at the same time. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions .

Electrons and Energy

The removal of an electron from a molecule (oxidizing it), results in a decrease in potential energy in the oxidized compound. However, the electron (sometimes as part of a hydrogen atom) does not remain unbonded in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound). The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of high-energy electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.

Electron Carriers

In living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy electrons between compounds in biochemical pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) (Figure 7.2) is derived from vitamin B3, niacin. NAD + is the oxidized form of the molecule NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). Note that if a compound has an “H” on it, it is generally reduced (e.g., NADH is the reduced form of NAD).

NAD + can accept electrons from an organic molecule according to the general equation:

When electrons are added to a compound, it is reduced. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD + is reduced to NADH. When electrons are removed from a compound, it is oxidized. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD + is an oxidizing agent, and RH is oxidized to R.

Similarly, flavin adenine dinucleotide (FAD + ) is derived from vitamin B2, also called riboflavin. Its reduced form is FADH2. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD + and FAD + are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis in plants.

ATP in Living Systems

A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the “energy currency” of the cell, and, like currency, this versatile compound can be used to fill any energy need of the cell. How? It functions similarly to a rechargeable battery.

When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The energy is used to do work by the cell, usually when the released phosphate binds to another molecule, thereby activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients.

ATP Structure and Function

At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group (Figure 7.3). Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine diphosphate (ADP) the addition of a third phosphate group forms adenosine triphosphate (ATP).

The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation , releases energy.

Energy from ATP

Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed, and the resulting hydrogen atom (H + ) and a hydroxyl group (OH - ), or hydroxide, are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (Pi), and the release of free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water, which was broken down into its hydrogen atom and hydroxyl group (hydroxide) during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP.

Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In nearly every living thing on Earth, the energy comes from the metabolism of glucose, fructose, or galactose, all isomers with the chemical formula C6H12O6 but different molecular configurations. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.

Phosphorylation

Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows one of the substrates (such as ATP) and reactants to more readily react with each other in reactions involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (

P). This is illustrated by the following generic reaction, in which A and B represent two different substrates:

When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism.

Substrate Phosphorylation

ATP is generated through two mechanisms during the breakdown of glucose. A few ATP molecules are generated (that is, regenerated from ADP) as a direct result of the chemical reactions that occur in the catabolic pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the free energy of the reaction is used to add the third phosphate to an available ADP molecule, producing ATP (Figure 7.4). This very direct method of phosphorylation is called substrate-level phosphorylation .

Oxidative Phosphorylation

Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process, chemiosmosis, which takes place in mitochondria (Figure 7.5) within a eukaryotic cell or the plasma membrane of a prokaryotic cell. Chemiosmosis , a process of ATP production in cellular metabolism, is used to generate 90 percent of the ATP made during glucose catabolism and is also the method used in the light reactions of photosynthesis to harness the energy of sunlight. The production of ATP using the process of chemiosmosis is called oxidative phosphorylation because of the involvement of oxygen in the process.

Career Connection

Mitochondrial Disease Physician

What happens when the critical reactions of cellular respiration do not proceed correctly? This may happen in mitochondrial diseases, which are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation but not the other steps of respiration. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial diseases, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disorders.


Energy released during the production of ATP? - Biology

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

7. Biochemical Pathways-Photosynthesis

Ultimately, the energy to power all organisms comes from the sun. An important molecule in the process of harvesting sunlight is chlorophyll, a green pigment that absorbs light energy. Through photosynthesis, light energy is transformed to chemical-bond energy in the form of ATP. ATP is then used to produce complex organic molecules, such as glucose. It is from these organic molecules that organisms obtain energy through the process of cellular respiration. Recall from chapter 4 that, in algae and the leaves of green plants, photosynthesis occurs in cells that contain organelles called chloroplasts. Chloroplasts have two distinct regions within them: the grana and the stroma. Grana consist of stacks of individual membranous sacs, called thylakoids, that contain chlorophyll. The stroma are the spaces between membranes (figure 7.2).

FIGURE 7.2. The Structure of a Chloroplast, the Site of Photosynthesis

Plant cells contain chloroplasts that enable them to store light energy as chemical energy. It is the chloroplasts that contain chlorophyll and that are the site of photosynthesis. The chlorophyll molecules are actually located within membranous sacs called thylakoids. A stack of thylakoids is known as a granum.

The following equation summarizes the chemical reactions photosynthetic organisms use to make ATP and organic molecules:

There are three distinct events in the photosynthetic pathway:

1. Light-capturing events. In eukaryotic cells, photosynthesis takes place within chloroplasts. Each chloroplast is surrounded by membranes and contains chlorophyll, along with other photosynthetic pigments. Chlorophyll and the other pigments absorb specific wavelengths of light. When specific amounts of light are absorbed by the photosynthetic pigments, electrons become “excited.” With this added energy, these excited electrons can enter into the chemical reactions responsible for the production of ATP. These reactions take place within the grana of the chloroplast.

2. Light-dependent reactions. Light-dependent reactions use the excited electrons produced by the light-capturing

Solution to Global Energy Crisis Found in Photosynthesis?

The most important chemical reaction on Earth, photosynthesis, is thought to have been around about 3 billion years. There has been plenty of time for this metabolic process to evolve into a highly efficient method of capturing light energy. Terrestrial and aquatic plants and algae are little solar cells that convert light into usable energy. They use this energy to manufacture organic molecules from carbon dioxide and water.

Photosynthetic organisms capture an estimated 10 times the global energy used by humans annually. Scientists and inventors have long recognized the value in being able to develop materials that mimic the light-capturing events of photosynthesis. The overall efficiency of photosynthesis is between 3-6% of total solar radiation that reaches the earth. Recently the National Energy Renewable Laboratory (NREL) verified that new organic-based photovoltaic solar cells have demonstrated 6% efficiency. They are constructed of a new family of photo-active polymers—polycarbazoles. Developers see their achievement as a major breakthrough and are hoping to develop solar cells with efficiencies in excess of 10%.

These cells have the ability to capture light energy and, at the same time, be used in many a variety of situations. Flexible plastic, leaf-like sheets can be attached to cell phones, clothing, awnings, roofs, toys, and windows to provide power to many kinds of electronic devices.

events. Light-dependent reactions are also known as light reactions. During these reactions, excited electrons from the light-capturing events are used to produce ATP. As a by-product, hydrogen and oxygen are also produced. The oxygen from the water is released to the environment as O2 molecules. The hydrogens are transferred to the electron carrier coenzyme NADP + to produce NADPH. (NADP + is similar to NAD + , which was discussed in chapter 5.) These reactions also take place in the grana of the chloroplast. However, the NADPH and ATP leave the grana and enter the stroma, where the light- independent reactions take place.

3. Light-independent reactions. These reactions are also known as dark reactions, because light is not needed for them to occur. During these reactions, ATP and NADPH from the light-dependent reactions are used to attach CO2 to a 5-carbon molecule, already present in the cell, to manufacture new, larger organic molecules. Ultimately, glucose (C6H12O6) is produced. These light-independent reactions take place in the stroma in either the light or dark, as long as ATP and NADPH are available from the light-dependent stage. When the ATP and NADPH give up their energy and hydrogens, they turn back into ADP and NADP + . The ADP and the NADP + are recycled back to the light-dependent reactions to be used over again.

The process of photosynthesis can be summarized as follows. During the lightcapturing events, light energy is captured by chlorophyll and other pigments, resulting in excited electrons. The energy of these excited electrons is used during the light-dependent reactions to disassociate water molecules into hydrogen and oxygen, and the oxygen is released. Also during the light-dependent reactions, ATP is produced and NADP + picks up hydrogen released from water to form NADPH. During the light-independent reactions, ATP and NADPH are used to help combine carbon dioxide with a 5-carbon molecule, so that ultimately organic molecules, such as glucose, are produced (figure 7.3).

FIGURE 7.3. Photosynthesis: Overview

Photosynthesis is a complex biochemical pathway in plants, algae, and certain bacteria. This illustrates the three parts of the process: (a) the light-capturing events, (b) the light-dependent reactions, and (c) the light-independent reactions. The end products of the light-dependent reactions, NADPH and ATP, are necessary to run the light-independent reactions and are regenerated as NADP + , ADP, and P. Water and carbon dioxide are supplied from the environment. Oxygen is released to the environment and sugar is manufactured for use by the plant.

3. Photosynthesis is a biochemical pathway that involves three kinds of activities. Name these and explain how they are related to each other.

4. Which cellular organelle is involved in the process of photosynthesis?


Energy released during the production of ATP? - Biology

What is the term for metabolic pathways that release stored energy by breaking down complex molecules? anabolic pathways catabolic pathways fermentation pathways thermodynamic pathways bioenergetic pathways

What is the term used for the metabolic pathway in which glucose (C6H12O6) is degraded to carbon dioxide (CO2) and water? cellular respiration glycolysis fermentation citric acid cycle oxidative phosphorylation

Which of the following statements is (are) correct about an oxidation-reduction (or) redox reactions? The molecule that is reduced gains electrons. The molecule that is oxidized loses electrons. The molecule that is reduced loses electrons. The molecule that is oxidized gains electrons. Both A and B are correct.

Which statement is not correct with regard to redox (oxidation-reduction) reactions? A molecule is reduced if it loses electrons. A molecule is oxidized if it loses electrons. An electron donor is called a reducing agent. An electron acceptor is called an oxidizing agent. Oxidation and reduction always go together.

A molecule is reduced if it loses electrons.

The molecule that functions as the reducing agent (electron donor) in a redox or oxidation-reduction reaction gains electrons and gains energy. loses electrons and loses energy. gains electrons and loses energy. loses electrons and gains energy. neither gains nor loses electrons, but gains or loses energy.

loses electrons and loses energy.

When electrons move closer to a more electronegative atom, what happens? Energy is released. Energy is consumed. The more electronegative atom is reduced. The more electronegative atom is oxidized. A and C are correct.

Why does the oxidation of organic compounds by molecular oxygen to produce CO2 and water release free energy? The covalent bonds in organic molecules are higher energy bonds than those in water and carbon dioxide. Electrons are being moved from atoms that have a lower affinity for electrons (such as carbon) to atoms with a higher affinity for electrons (such as oxygen). The oxidation of organic compounds can be used to make ATP. The electrons have a higher potential energy when associated with water and CO2 than they do in organic compounds. The covalent bond in O2 is unstable and easily broken by electrons from organic molecules.

Electrons are being moved from atoms that have a lower affinity for electrons (such as carbon) to atoms with a higher affinity for electrons (such as oxygen).

Which of the following statements about NAD+ is false? NAD+ is reduced to NADH during both glycolysis and the citric acid cycle. NAD+ has more chemical energy than NADH. NAD+ is reduced by the action of dehydrogenases. NAD+ can receive electrons for use in oxidative phosphorylation. In the absence of NAD+, glycolysis cannot functio

NAD+ has more chemical energy than NADH.

Where does glycolysis takes place? mitochondrial matrix mitochondrial outer membrane mitochondrial inner membrane mitochondrial intermembrane space cytosol (liquid portion of the cytoplasm)

cytosol (liquid portion of the cytoplasm)

The ATP made directly during glycolysis is generated by substrate-level phosphorylation. electron transport. photophosphorylation. chemiosmosis. oxidation of NADH to NAD+.

The oxygen consumed during cellular respiration is involved directly in which process or event? glycolysis accepting electrons at the end of the electron transport chain resulting in the production of water the citric acid cycle the oxidation of pyruvate to acetyl CoA the phosphorylation of ADP to form ATP

accepting electrons at the end of the electron transport chain resulting in the production of water

Which process in eukaryotic cells will proceed normally whether oxygen (O2) is present or absent? electron transport glycolysis the citric acid cycle oxidative phosphorylation chemiosmosis

Which of the following statements about glycolysis is false? Glycolysis has steps involving oxidation-reduction reactions. The enzymes of glycolysis are located in the cytosol of the cell. Glycolysis can operate in the complete absence of O2. The end products of glycolysis are CO2 and H2O Glycolysis makes ATP exclusively through substrate-level phosphorylation.

The end products of glycolysis are CO2 and H2O

During glycolysis, when glucose is catabolized to pyruvate, most of the energy of glucose is transferred to ADP, forming ATP. transferred directly to ATP. retained in the pyruvate. stored in the NADH produced. used to phosphorylate fructose to form fructose-6-phosphate.

In addition to ATP, what are the end products of glycolysis? CO2 and H2O CO2 and pyruvate NADH and pyruvate CO2 and NADH H2O, FADH2, and citrate

Starting with one molecule of glucose, the "net" products of glycolysis are 2 NAD+, 2 H+, 2 pyruvate, 2 ATP, and 2 H2O. 2 NADH, 2 H+, 2 pyruvate, 2 ATP, and 2 H2O. 2 FADH2, 2 pyruvate, 4 ATP, and 2 H2O. 6 CO2, 6 H2O, 2 ATP, and 2 pyruvate. 6 CO2, 6 H2O, 36 ATP, and 2 citrate.

2 NADH, 2 H+, 2 pyruvate, 2 ATP, and 2 H2O.

A molecule that is phosphorylated has an increased chemical reactivity it is primed to participate in a chemical reaction. has a decreased chemical reactivity it is less likely to provide energy for cellular work. has been oxidized as a result of a redox reaction involving the gain of an inorganic phosphate. has been reduced as a result of a redox reaction involving the loss of an inorganic phosphate. has less energy than before its phosphorylation and therefore less energy for cellular work.

has an increased chemical reactivity it is primed to participate in a chemical reaction.

Which kind of metabolic poison would most directly interfere with glycolysis? An agent that reacts with oxygen and depletes its concentration in the cell An agent that binds to pyruvate and inactivates it An agent that closely mimics the structure of glucose but is not metabolized An agent that reacts with NADH and oxidizes it to NAD+ An agent that blocks the passage of electrons along the electron transport chain

An agent that closely mimics the structure of glucose but is not metabolized

During cellular respiration, acetyl CoA accumulates in which location? cytosol mitochondrial outer membrane mitochondrial inner membrane mitochondrial intermembrane space mitochondrial matrix

How many carbon atoms are fed into the citric acid cycle as a result of the oxidation of one molecule of pyruvate? 2 4 6 8 10

All of the following are functions of the citric acid (i. e., Krebs) cycle except production of ATP. production of NADH. production of FADH2. release of carbon dioxide. adding electrons and protons to oxygen, forming water.

adding electrons and protons to oxygen, forming water.

Carbon dioxide (CO2) is released during which of the following stages of cellular respiration? glycolysis and the oxidation of pyruvate to acetyl CoA oxidation of pyruvate to acetyl CoA in the citric acid (Krebs) cycle the citric acid cycle and oxidative phosphorylation oxidative phosphorylation and fermentation fermentation and glycolysis

oxidation of pyruvate to acetyl CoA in the citric acid (Krebs) cycle

For each molecule of glucose that is metabolized by glycolysis and the citric acid cycle, what is the total number of NADH + FADH2 molecules produced? 4 5 6 10 12

Cellular respiration produces the most chemical energy in the form of ATP from which of the following? substrate-level phosphorylation oxidative phosphorylation converting oxygen to ATP transferring electrons from organic molecules to pyruvate generating carbon dioxide and oxygen in the electron transport chain

During aerobic (oxygen present) respiration, electrons travel downhill in which sequence? food _ citric acid cycle _ ATP _ NAD+ food _ NADH _ electron transport chain _ oxygen glucose _ pyruvate _ ATP _ oxygen glucose _ ATP _ electron transport chain _ NADH food _ glycolysis _ citric acid cycle _ NADH _ ATP

food _ NADH _ electron transport chain _ oxygen

Where do the catabolic products of fatty acid breakdown enter into the citric acid cycle? pyruvate malate or fumarate acetyl CoA Ñ-ketoglutarate succinyl CoA

Where are the proteins of the electron transport chain located? cytosol mitochondrial outer membrane mitochondrial inner membrane mitochondrial intermembrane space mitochondrial matrix

mitochondrial inner membrane

During aerobic respiration, which of the following directly donates electrons to the electron transport chain at the lowest energy level? NAD+ NADH ATP ADP + Pi FADH2

The primary role of oxygen in cellular respiration is to yield energy in the form of ATP as it is passed down the respiratory chain. act as a terminal acceptor for electrons in electron transport combine with carbon, forming CO2. combine with lactate, forming pyruvate. catalyze the reactions of glycolysis.

act as a terminal acceptor for electrons in electron transport

During oxidative phosphorylation, H2O is formed. Where does the oxygen for the synthesis of the water come from? carbon dioxide (CO2) glucose (C6H12O6) molecular oxygen (O2) pyruvate (C3H3O3) lactate (C3H5O3)

Which metabolic process is most closely associated with intracellular membranes? substrate-level phosphorylation oxidative phosphorylation glycolysis the citric acid cycle alcohol fermentation

Energy released by the electron transport chain is used to pump H+ ions into which location? cytosol mitochondrial outer membrane mitochondrial inner membrane mitochondrial intermembrane space mitochondrial matrix

mitochondrial intermembrane space

During aerobic cellular respiration, a proton gradient in mitochondria is generated by ________ and used primarily for ________. the electron transport chain ATP synthesis the electron transport chain substrate-level phosphorylation glycolysis production of H2O fermentation NAD+ reduction diffusion of protons ATP synthesis

the electron transport chain ATP synthesis

The direct energy source that drives ATP synthesis during respiratory oxidative phosphorylation is oxidation of glucose to CO2 and water. the thermodynamically favorable flow of electrons from NADH to the mitochondrial electron transport carriers. the final transfer of electrons to oxygen. the difference in H+ concentrations on opposite sides of the inner mitochondrial membrane. the thermodynamically favorable transfer of phosphate from glycolysis and the citric acid cycle intermediate molecules of ADP.

the difference in H+ concentrations on opposite sides of the inner mitochondrial membrane.

When hydrogen ions are pumped from the mitochondrial matrix across the inner membrane and into the intermembrane space, the result is the formation of ATP. reduction of NAD+. restoration of the Na+/K+ balance across the membrane. creation of a proton gradient. lowering of pH in the mitochondrial matrix.

creation of a proton gradient.

Where is ATP synthase located in the mitochondrion? cytosol electron transport chain outer membrane inner membrane mitochondrial matrix

Which process could be compared to how rushing steam turns a water wheel? the citric acid cycle ATP synthase activity formation of NADH in glycolysis oxidative phosphorylation the electron transport system

How many molecules of carbon dioxide (CO2) would be released from the complete aerobic respiration of a molecule of sucrose (C12H22 O11), a disaccharide? 2 3 6 12 38

Each time a molecule of glucose (C6H12O6) is completely oxidized via aerobic respiration, how many oxygen molecules (O2) are required? 1 2 6 12 38

Which of the following produces the most ATP when glucose (C6H12O6) is completely oxidized to carbon dioxide (CO2) and water? glycolysis fermentation oxidation of pyruvate to acetyl CoA citric acid cycle oxidative phosphorylation

Approximately how many molecules of ATP are produced from the complete oxidation of two molecules of glucose (C6H12O6) in cellular respiration? 2 4 15 38 76

Assume a mitochondrion contains 58 NADH and 19 FADH2. If each of the 77 dinucleotides were used, approximately how many ATP molecules could be generated as a result of oxidative phosphorylation (chemiosmosis)? 36 77 173 212 1102

Which of the following occurs in the cytosol of the cell? glycolysis and fermentation fermentation and chemiosmosis oxidation of pyruvate to acetyl CoA citric acid cycle oxidative phosphorylation

glycolysis and fermentation

Which metabolic pathway is common to both cellular respiration and fermentation? the oxidation of pyruvate to acetyl CoA the citric acid cycle oxidative phosphorylation glycolysis chemiosmosis

The ATP made during fermentation is generated by which of the following? the electron transport chain substrate-level phosphorylation chemiosmosis oxidative phosphorylation aerobic respiration

Muscle cells in oxygen deprivation convert pyruvate to ________, and in this step gain ________. lactate ATP alcohol CO2 alcohol ATP ATP NADH2 lactate NAD+

Phosphofructokinase is an important control enzyme in the regulation of cellular respiration. Which of the following statements concerning phosphofructokinase is not true? It is activated by AMP (derived from ADP). It is inhibited by ATP. It is activated by citrate, an intermediate of the citric acid cycle. It specifically catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, an early step of glycolysis. It is an allosteric enzyme.

NOT: It is inhibited by ATP. It is an allosteric enzyme.

Phosphofructokinase is an allosteric enzyme that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, an early step of glycolysis. In the presence of oxygen, an increase in the amount ATP in a cell would be expected to inhibit the enzyme and thus slow the rates of glycolysis and the citric acid cycle. activate the enzyme and thus slow the rates of glycolysis and the citric acid cycle. inhibit the enzyme and thus increase the rates of glycolysis and the citric acid cycle. activate the enzyme and increase the rates of glycolysis and the citric acid cycle. inhibit the enzyme and thus increase the rate of glycolysis and the concentration of citrate.

inhibit the enzyme and thus slow the rates of glycolysis and the citric acid cycle.

Pyruvate is formed on the inner mitochondrial membrane. in the mitochondrial matrix. on the outer mitochondrial membrane. in the nucleus. in the cytosol.

The immediate energy source that drives ATP synthesis by ATP synthase during oxidative phosphorylation is the oxidation of glucose and other organic compounds. the flow of electrons down the electron transport chain. the affinity of oxygen for electrons. the H+ concentration gradient across the inner mitochondrial membrane. the transfer of phosphate to ADP.

the H+ concentration gradient across the inner mitochondrial membrane.

Which metabolic pathway is common to both fermentation and cellular respiration? the citric acid cycle the electron transport chain glycolysis synthesis of acetyl CoA from pyruvate reduction of pyruvate to lactate

The final electron acceptor of the electron transport chain that functions in oxidative phosphorylation is oxygen. water. NAD+. pyruvate. ADP.

When electrons flow along the electron transport chains of mitochondria, which of the following changes occurs? The pH of the matrix increases. ATP synthase pumps protons by active transport. The electrons gain free energy. The cytochromes phosphorylate ADP to form ATP. NAD+ is oxidized.

The pH of the matrix increases.

Which of the following is a true distinction between fermentation and cellular respiration? Only respiration oxidizes glucose. NADH is oxidized by the electron transport chain in respiration only. Fermentation, but not respiration, is an example of a catabolic pathway. Substrate-level phosphorylation is unique to fermentation. NAD+ functions as an oxidizing agent only in respiration.

NADH is oxidized by the electron transport chain in respiration only.

Most CO2 from catabolism is released during glycolysis. the citric acid cycle. lactate fermentation. electron transport. oxidative phosphorylation.