C3. Aerobic Coupling of Oxidation and ATP Synthesis - Biology

A quick glance reveals that we have taken glucose a small fraction along the way of oxidizing every carbon in it to CO2 and H2O. The complete oxidation happens under aerobic condition when the glycolytic pathway is followed by the Kreb's cycle. Pyruvate formed in glycolysis enters the mitochondrial matrix, and get oxidatively decarboxylated to a 2C molecule, acetylCoA by the enzyme pyruvate dehydrogenase.

Acetyl CoA then enters the Kreb's cycle (also called the tricarboxylic acid (TCA) cycle. It is shown below.

C2. Anerobic Coupling of Oxidation and ATP Synthesis

Our main goal is to understand how oxidation reactions can lead to ATP synthesis. First let us consider ATP production under anaerobic condition, such as which often occurs during the fight or flight response. You know how terribly you feel when you run a 100 m dash. Your muscles feel horribly due to lactic acid buildup, and you know you can't seem to get enough dioxygen into your body. Under these conditions, a pathway called glycolysis (which you studied in biology) is active. In this pathway, glucose, a 6 carbon hexose, is converted to two, 3C molecules - pyruvate. A detail description is shown below.

Since we most interested in energy transduction at this point, let's consider just two important step in glycolysis that directly lead to ATP synthesis. Only one oxidative step is found in this pathway, namely the oxidative phosphorylation of the 3C glycolytic intermediate glyceraldehyde-3-phosphate, to 1,3-bisphosphglyercate, a mixed anhydride (see link below for mechanism). The oxidizing agent is NAD+ and the phosphorylating agent is NOT ATP but rather Pi. The enzyme is named glyceraldehyde-3-phosphate dehydrogenase. It contains an active site Cys, which helps explain how the enzyme can be inactivated with a stoichiometric amounts of iodoacetamide. A general base in the enzyme abstracts an H+ from Cys, which attacks the carbonyl C of the glyceraldehyde, forming a tetrahedral intermediate. Instead of the expected reaction (which would be the protonation of the alkoxide in an overall nucleophilic addition reaction at the aldehyde), a hydride leaves from the former carbonyl C to NAD+ in an oxidation step. Notice, this is a two electron oxidation reaction similar to seen in alcohol dehydrogenase. An acyl-thioester intermediate has formed, much like the acyl intermediate that formed in Ser proteases. Next inorganic phosphorous, Pi, attacks the carbonyl C of the intermediate in a nucleophilic substitution reaction to form the mixed anhydride product, 1,3-bisphoshphoglycerate. Although we have formed a mixed anhydride, we cleaved a sulfur ester, which is destabilized with respect to its hydrolysis products (since the reactant, the thioester, is not stabilized by resonance to the extent of regular esters owing to the poor donation of electrons from the larger S to the carbonyl-like C.) In the next step, catalyzed by the enzyme phosphoglycerate kinase, ADP acts an a nucleophile which attacks the mixed anhydride of the 1,3-bisphosphoglyerate to form ATP. Note that the enzyme is name for the reverse reaction. We have coupled oxidation of an organic molecule (glyceraldehyde-3-phosphate) to phosphorylation of ADP through the formation of a "high" energy mixed anhydride, 1,3-bisphosphoglycerate.

The linkage between oxidation of glyceraldehyde-3-phosphate and the phosphorylation of ADP by 1,3-bisphosphoglycerate can be artificially uncoupled by adding arsenate, which has a similar structure as phosphate. The arsenate can form a mixed anhydride at C1 of glyceraldehyde-3-phosphate, but since the bridging O-As bond is longer and not as strong as in the mixed anhydride, it is easily hydrolyzed. This prevents subsequent transfer of phosphate to ADP to form ATP.

Jmol : Updated Glyceraldehyde-3-phosphate dehydrogenase (NAD) Jmol14 (Java) | JSMol (HTML5)

Under anaerobic conditions, glucose is metabolized through glycolysis which converts it to two molecules of pyruvate. Only one oxidation step has been performed when glyceraldehyde 3-phospate is oxidized to 1,3-bisphosphoglycerate. To regenerate NAD+ so glycolysis can continue, pyruvate is reduced to lactate, catalyzed by lactate dehydrogenae. These reactions take place in the cytoplasm of cells actively engaged in anaerobic oxidation of glucose (muscle cells for examples during sprints). Note that the enzyme is named for the reverse reaction, the oxidation of lactate by NAD+.

Lactate in the muscle can go by way of the blood to the liver (where NAD+ is not depleted) and be converted back to pyruvate and eventually back to glucose through a pathway called gluconeogenisis. The liver can export the glucose into the blood from where it can be taken up by the muscle for ATP production. This cycle is called the Cori cycle.

C3. Aerobic Coupling of Oxidation and ATP Synthesis

A quick glance reveals that we have taken glucose a small fraction along the way of oxidizing every carbon in it to CO2 and H2O. The complete oxidation happens under aerobic condition when the glycolytic pathway is followed by the Kreb's cycle. Pyruvate formed in glycolysis enters the mitochondrial matrix, and get oxidatively decarboxylated to a 2C molecule, acetylCoA by the enzyme pyruvate dehydrogenase.

Acetyl CoA then enters the Kreb's cycle (also called the tricarboxylic acid (TCA) cycle. It is shown below.

Archived version of full Chapter 8C: ATP and Oxidative Phosphorylation

Biochemistry Online by Henry Jakubowski is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

10.3: ATP and Oxidative Phosphorylation

  • Contributed by Henry Jakubowski
  • Professor (Chemistry) at College of St. Benedict/St. John's University
  • explain reasons for the strongly exergonic hydrolysis of carboxylic acid anhydrides, phosphoric acid anhydrides, mixed anhydrides, and analogous structures and give approximate values for the &DeltaG0 of hydrolysis of them
  • identify from Lewis structures molecules whose hydrolytic cleavage are strongly exergonic
  • explain how the exergonic cleavage of phophoanhydride bonds in ATP can be coupled to the endergonic synthesis of macromolecules like proteins
  • draw mechanisms to show how oxidation and phosphorylation reactions are coupled in anaerobic metabolism through the productions of a mixed anhydride catalyzed by the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase
  • explain how arsenate can double oxidation and phosphorlyation reactions in glycolysis
  • explain how NAD+ can be regenerated from NADH in anaeroboic condition to allow glycolysis to continue
  • explain the general flow of electrons from NADH to dioxgen through a series of mobile and membrane protein bound electron acceptors in electron transport in the mitochondria inner member.
  • explain with picture diagrams how oxidation and phosphorylation reactions (to produce ATP) are coupled in aerobic metabolism through the generation and collapse of a proton gradient in the mitochondria
  • draw pictures diagrams explaining the structure of F1F0ATPase in the inner mitochondria member and explain using the picture how ATP synthesis is coupled to protein gradient collapse
  • write an equation for the electrochemical potential and use it to calculate the available &DeltaG0 for ATP production on proton gradient collapse, given typical values for &DeltapH and &DeltaE across the membrane

Biological oxidation reactions serve two functions:

  1. Oxidation of organic molecules can produce new molecules with different properties (e.g., an increase in solubility is observed on hydroxylation of aromatic substrates by cytochrome P450) and Likewise, amino acids can be oxidized to produce neurotransmitters.
  2. Most biological oxidation reactions occur, however, to produce energy to drive thermodynamically unfavored biological processes such as protein and nucleic acid synthesis, or motility.

Chemical potential energy is not just released in biological oxidation reactions. Rather, it is transduced into a more useful form of chemical energy in the molecule ATP (adenosine triphosphate). This chapter will discuss the properties that make ATP so useful biologically, and how exergonic biological oxidation reactions are coupled to the synthesis of ATP.

Membrane-Associated Energy Transduction in Bacteria and Archaea


Fermentations are anaerobic redox processes in which ATP is usually generated by substrate-level phosphorylation . In some special cases, partial reactions of fermentative pathways are catalyzed by membrane-residing enzymes, and the free-energy change of the reaction is coupled to the generation of an electrochemical-ion gradient. Examples are the biotin-dependent methylmalonyl-CoA decarboxylase of Propionigenium modestum, the oxaloacetate decarboxylase involved in citrate fermentation, or the glutaconyl-CoA decarboxylase involved in glutamate fermentation they generate a ΔμNa + . By means of H + /Na + exchange transporters the sodium gradient can be coupled to a proton gradient, or the ΔμNa + can directly drive ATP synthesis by a Na + -translocating ATP synthetase. In methanogenic archaea methyl-group transfer from an N atom to an SH group drives a sodium pump and generates also a ΔμNa + . The above examples demonstrate that in anaerobic fermentations also non-redox reactions can play an important role in membrane associated energy transduction.

C3. Aerobic Coupling of Oxidation and ATP Synthesis - Biology

Cells couple the exergonic reaction of ATP hydrolysis with endergonic reactions to harness the energy within the bonds of ATP.

Learning Objectives

Explain the role of ATP as the currency of cellular energy

Key Takeaways

Key Points

  • Adenosine triphosphate is composed of the nitrogenous base adenine, the five-carbon sugar ribose, and three phosphate groups.
  • ATP is hydrolyzed to ADP in the reaction ATP+H2O→ADP+Pi+ free energy the calculated ∆G for the hydrolysis of 1 mole of ATP is -57 kJ/mol.
  • ADP is combined with a phosphate to form ATP in the reaction ADP+Pi+free energy→ATP+H2O.
  • The energy released from the hydrolysis of ATP into ADP is used to perform cellular work, usually by coupling the exergonic reaction of ATP hydrolysis with endergonic reactions.
  • Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane while phosphorylation drives the endergonic reaction.

Key Terms

  • energy coupling: Energy coupling occurs when the energy produced by one reaction or system is used to drive another reaction or system.
  • endergonic: Describing a reaction that absorbs (heat) energy from its environment.
  • exergonic: Describing a reaction that releases energy (heat) into its environment.
  • free energy: Gibbs free energy is a thermodynamic potential that measures the useful or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure (isothermal, isobaric).
  • hydrolysis: A chemical process of decomposition involving the splitting of a bond by the addition of water.

ATP: Adenosine Triphosphate

Adenosine triphosphate (ATP) is the energy currency for cellular processes. ATP provides the energy for both energy-consuming endergonic reactions and energy-releasing exergonic reactions, which require a small input of activation energy. When the chemical bonds within ATP are broken, energy is released and can be harnessed for cellular work. The more bonds in a molecule, the more potential energy it contains. Because the bond in ATP is so easily broken and reformed, ATP is like a rechargeable battery that powers cellular process ranging from DNA replication to protein synthesis.

Molecular Structure

Adenosine triphosphate (ATP) is comprised of the molecule adenosine bound to three phosphate groups. Adenosine is a nucleoside consisting of the nitrogenous base adenine and the five-carbon sugar ribose. The three phosphate groups, in order of closest to furthest from the ribose sugar, are labeled alpha, beta, and gamma. Together, these chemical groups constitute an energy powerhouse. The two bonds between the phosphates are equal high-energy bonds (phosphoanhydride bonds) that, when broken, release sufficient energy to power a variety of cellular reactions and processes. The bond between the beta and gamma phosphate is considered “high-energy” because when the bond breaks, the products [adenosine diphosphate (ADP) and one inorganic phosphate group (Pi)] have a lower free energy than the reactants (ATP and a water molecule). ATP breakdown into ADP and Pi is called hydrolysis because it consumes a water molecule (hydro-, meaning “water”, and lysis, meaning “separation”).

Adenosine Triphosphate (ATP): ATP is the primary energy currency of the cell. It has an adenosine backbone with three phosphate groups attached.

ATP Hydrolysis and Synthesis

ATP is hydrolyzed into ADP in the following reaction:

Like most chemical reactions, the hydrolysis of ATP to ADP is reversible. The reverse reaction combines ADP + Pi to regenerate ATP from ADP. Since ATP hydrolysis releases energy, ATP synthesis must require an input of free energy.

ADP is combined with a phosphate to form ATP in the following reaction:

ATP and Energy Coupling

Exactly how much free energy (∆G) is released with the hydrolysis of ATP, and how is that free energy used to do cellular work? The calculated ∆G for the hydrolysis of one mole of ATP into ADP and Pi is −7.3 kcal/mole (−30.5 kJ/mol). However, this is only true under standard conditions, and the ∆G for the hydrolysis of one mole of ATP in a living cell is almost double the value at standard conditions: 14 kcal/mol (−57 kJ/mol).

ATP is a highly unstable molecule. Unless quickly used to perform work, ATP spontaneously dissociates into ADP + Pi, and the free energy released during this process is lost as heat. To harness the energy within the bonds of ATP, cells use a strategy called energy coupling.

Energy Coupling in Sodium-Potassium Pumps

Energy Coupling: Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane.

Cells couple the exergonic reaction of ATP hydrolysis with the endergonic reactions of cellular processes. For example, transmembrane ion pumps in nerve cells use the energy from ATP to pump ions across the cell membrane and generate an action potential. The sodium-potassium pump (Na + /K + pump) drives sodium out of the cell and potassium into the cell. When ATP is hydrolyzed, it transfers its gamma phosphate to the pump protein in a process called phosphorylation. The Na + /K + pump gains the free energy and undergoes a conformational change, allowing it to release three Na + to the outside of the cell. Two extracellular K + ions bind to the protein, causing the protein to change shape again and discharge the phosphate. By donating free energy to the Na + /K + pump, phosphorylation drives the endergonic reaction.

Energy Coupling in Metabolism

During cellular metabolic reactions, or the synthesis and breakdown of nutrients, certain molecules must be altered slightly in their conformation to become substrates for the next step in the reaction series. In the very first steps of cellular respiration, glucose is broken down through the process of glycolysis. ATP is required for the phosphorylation of glucose, creating a high-energy but unstable intermediate. This phosphorylation reaction causes a conformational change that allows enzymes to convert the phosphorylated glucose molecule to the phosphorylated sugar fructose. Fructose is a necessary intermediate for glycolysis to move forward. In this example, the exergonic reaction of ATP hydrolysis is coupled with the endergonic reaction of converting glucose for use in the metabolic pathway.

Mitochondria oxidize substrates to generate the ATP that fuels muscle contraction and locomotion. This review focuses on three steps in oxidative phosphorylation that have independent roles in setting the overall mitochondrial ATP flux and thereby have direct impact on locomotion. The first is the electron transport chain, which sets the pace for oxidation. New studies indicate that the electron transport chain capacity per mitochondria declines with age and disease, but can be revived by both acute and chronic treatments. The resulting higher ATP production is reflected in improved muscle power output and locomotory performance. The second step is the coupling of ATP supply from O2 uptake (mitochondrial coupling efficiency). Treatments that elevate mitochondrial coupling raise both exercise efficiency and the capacity for sustained exercise in both young and old muscle. The final step is ATP synthesis itself, which is under dynamic control at multiple sites to provide the 50-fold range of ATP flux between resting muscle and exercise at the mitochondrial capacity. Thus, malleability at sites in these subsystems of oxidative phosphorylation has an impact on ATP flux, with direct effects on exercise performance. Interventions are emerging that target these three independent subsystems to provide many paths to improve ATP flux and elevate the muscle performance lost to inactivity, age or disease.

Mitochondria are the powerhouses of biological tissues. In muscles, they link oxidation of substrates to phosphorylation that generates ATP. The contractile fibers then use ATP to generate force and motion. The role of mitochondria as the terminal sink for O2 in the respiratory system that sets the limit to maximum O2 uptake at the muscle level is well established (Weibel et al., 1991). A causal pathway linking this oxidation to the phosphorylation that generates ATP to fuel muscle force production and exercise performance is also clear. However, less clear is the direct role that mitochondria play in setting the limits to exercise performance. A few studies have made this connection in human subjects using exercise-training experiments, which are well known for raising the capacity for ATP supply (Jubrias et al., 2001) and mitochondrial volume density (Hoppeler et al., 1985). These studies show that a direct increase in muscle performance results from the greater energy supply capacity after endurance training. Thus mitochondria provide the bridge between the pathways that delivery oxygen and the synthesis of ATP that fuels muscle contraction and exercise performance.

One path to elevate ATP supply to raise exercise performance is to increase the mitochondrial content of muscle, as is typically found in endurance training of young subjects (Hoppeler et al., 1985). A second path is to improve the capacity for ATP generation per mitochondrion by targeting the processes underlying ATP supply. One such mechanism is the coupling of oxidation to phosphorylation (mitochondrial coupling efficiency), which can be improved to elevate ATP generation per O2 uptake. An example of this improvement is the acute effect of dietary nitrate on mitochondrial coupling efficiency, with direct effects on exercise efficiency in humans (Jones, 2014 Larsen et al., 2007). Free nitrate is released in the muscle cell by drinking beetroot juice, and acts via the nitrous oxide pathway to elevate mitochondrial ATP synthesis per O2 uptake (often expressed divided by 2 to yield the biochemical term of P/O). After drinking beetroot juice, the human subjects showing the largest increase in P/O had a correspondingly elevated exercise efficiency (leg power output per O2 uptake) on a cycle ergometer (Larsen et al., 2007). This link between mitochondrial and exercise efficiencies is the predicted response based on the thermodynamic connection between these processes (Whipp and Wasserman, 1969). Thus it is possible to adjust the underlying processes in oxidative phosphorylation to improve exercise performance.

A second example of the malleability of oxidative phosphorylation comes from the rapid effect of an antioxidant targeted to the mitochondrion, SS-31, on the capacity of mitochondria to generate ATP. A 1 h infusion of SS-31 in old mice raised P/O by 50% but doubled the phosphorylation capacity (ATPmax) in vivo in hindlimb muscles (Siegel et al., 2013). This greater rise in ATPmax than in P/O implies not only improvement in mitochondrial coupling efficiency but also a rapid rise in the capacity for electron transport chain (ETC) flux (O2 uptake capacity). The mechanism for this increase in the ETC flux capacity is thought to be stabilization of cardiolipin on the inner mitochondrial membrane, thereby restoring a key bridge in electron flow through the ETC (Birk et al., 2014 Szeto, 2014). These treatments demonstrate that there are many sites in oxidative phosphorylation that are potential targets for treatment to improve mitochondrial function. What is remarkable about beetroot juice and SS-31 is their speed of action in raising ATP supply capacity and exercise performance (1 h!). In contrast, many months of exercise training are needed to achieve the same goal (Jubrias et al., 2001). Thus multiple sites in oxidative phosphorylation have the potential to elevate energy supply and exercise performance – very rapidly with some treatments – without increasing the mitochondrial pool.

In this review I evaluate the three major steps in oxidative phosphorylation for malleability (Fig. 1) (Nicholls and Ferguson, 2002) that can improve exercise performance. The first step is the ETC, which oxidizes NADH to pump H + to generate a proton motive force across the inner mitochondrial membrane. The second step uses the proton motive force to drive phosphorylation via the F1F0-ATP synthase. The third step involves short-circuiting the H + gradient via a number of processes that leak H + through the inner mitochondrial membrane, thereby circumventing phosphorylation. My goal is to evaluate how each step contributes to ATP production and thereby has impact on exercise performance. The exciting implication of these insights is that mitochondria have multiple sites of malleability that provide targets for optimizing function to improve ATP flux and elevate muscle exercise performance in both healthy and diseased states.

ATP synthase

Figure 4. The structure of the ATP synthase. The ATP synthase is composed of two subunits: F0 and F1. Each of these subunits is composed of multiple proteins. The F0 subunit is composed of integral membrane proteins: 1 copy of protein a 2 copies of protein b and between 12-14 copies of protein c, which forms the channel for protons to flow across the membrane. The F1 subunit is composed of five distinct proteins: 3 copies of alpha , 3 copies of beta , 1 copy each of gamma , delta and epsilon . As protons flow across the membrane, the c subunits and the attached delta and epsilon subunits rotate. The remainder of the subunits are stationary. As protons enter, the ring formed by the c subunits and the stalk (composed of the delta and epsilon subunits) rotate. This rotation induces conformational changes in the beta subunits, catalyzing the synthesis of ATP.

The PMF is used to drive ATP synthesis via the ATP synthase, a protein complex that converts the energy of the proton gradient into chemical bonds. The ATP synthase has two distinct subunits: the transmembrane F0 subunit, which contains a protein channel for the flow of protons and the F1 subunit, which protrudes into the matrix space and catalyzes the synthesis of ATP from ADP and inorganic phosphate (Figure 4). A portion of the F1 subunit termed the stalk links the two subunits. As protons flow through the channel in the F0 subunit, they cause the embedded stalk to rotate in the stationary F1 subunit, thereby converting the energy of the electrochemical gradient into mechanical energy. As the stalk rotates in one direction, it induces conformational changes in the proteins of the F1 subunit, which, in turn, catalyze the synthesis of ATP - thereby converting the mechanical energy of stalk rotation to chemical bond energy. Approximately three protons must pass through the ATP synthase complex for one ATP molecule to be synthesized.

The Role of Carbohydrate Response Element–Binding Protein in the Development of Liver Diseases

4.3 Liver Tumors

The Warburg effect with aerobic glycolysis efficiently produces ATP synthesis and consequently promotes cell proliferation by reprogramming metabolism to increase glucose uptake and stimulating lactate production. 65 High-proliferating cancer cells use increased fatty acid synthesis to support the rate of cell division. Tong et al. reported that suppression of ChREBP in HCT116 colorectal cancer cells and HepG2 hepatoblastoma cells resulted in diminished aerobic glycolysis, de novo lipogenesis, and nucleotide biosynthesis, but stimulated mitochondrial respiration. 66 This suggests that suppression of ChREBP serves as a metabolic switch from aerobic glycolysis to oxidative phosphorylation. Suppression of ChREBP caused p53 activation and cell cycle arrest. Therefore, ChREBP plays an important role in redirecting glucose metabolism to anabolic pathways and suppressing p53 activity. 66

In the ChREBP activation mechanism, upregulation of ChREBP is seen in human breast cancer and human metastatic prostate cancer. 67–69 UDP-GlcNAc in the hexosamine biosynthesis pathway is a substrate for O-GlcNAcylation. Elevation of O-GlcNAcylation levels is observed in several kinds of cancers, such as breast, lung, colorectal, liver, bladder, prostate, chronic lymphocytic leukemia (CCL), and pancreatic cancers. 70 ChREBP is O-GlcNAcylated by O-GlcNAc transferase (OGT), resulting in increased ChREBP transactivity. 24,71,72 Thus, ChREBP is implicated in tumor cell proliferation.

The relationship between ChREBP activation and cell proliferation is also seen in the pathogenesis of liver carcinomas. 66,73,74 In human hepatocellular carcinomas with poor outcome, mRNA levels of ChREBP and other lipogenic genes are elevated. 73 Moreover, advanced glycation end products, which are increased in diabetic conditions, promote ChREBP expression and cell proliferation in liver cancer cells by increasing the reactive oxygen species. 74 Therefore, ChREBP suppression may be beneficial for the prevention of HCC proliferation.

C3. Aerobic Coupling of Oxidation and ATP Synthesis - Biology


Note: a review of basic chemistry is recommended prior to covering energy conversion reactions

Photosynthesis and Cellular (Aerobic) Respiration

These two processes have many things in common.
1. Both processes occur in organelles that descended from formerly free-living bacteria (endosymbiont theory): chloroplasts and mitochondria.
2. The organelles where these processes occur have complex internal membrane systems that are essential to the processes.
3. These processes rely on existing molecules in cells to carry out the energy conversion reactions: electron holders (NAD + , NADP + ), ADP and ATP, miscellaneous sugars, etc.
4. Both processes involve synthesis of ATP via a proton (H + ) concentration gradient.
5. Photosynthesis and respiration are essentially the reverse of each other. Photosynthesis starts with CO 2 and reduces it to sugar reduction requires energy, which is obtained from light. Respiration starts with sugar and oxidizes it to CO 2 oxidation releases energy which is collected as ATP. In order to get electrons to reduce CO2, photosynthesis oxidizes H2O to O2. Respiration uses the electrons obtained from the oxidation of sugar to reduce O2 to H2O.

------ energy input from light ----->
6 CO 2 + 12 H 2 O ------------------------------------------ C 6 H 12 O 6 + 6 O 2 + 6 H 2 O
<---- energy output as ATP -------

Photosynthesis History and Light

1772: Joseph Priestly showed that a spring of mint could "restore" contaminated air, i.e. release O2

1779: Jan Ingen-Housz showed that green plant parts and light were required to oxygenate air

1882: T. W. Engelmann demonstrated which parts of the visible light spectrum were required in order for plants to release oxygen

1930s: C. B. van Niel proposed that the O2 released by plants in the light comes from H2O (confirmed in 1941 by Ruben and Kamen)

Collection of light energy for photosynthesis requires pigments that can absorb blue and red light: chlorophyll a, chlorophyll b, and carotenoids (carotenes and xanthophylls). The pigments must be organized into photosystems. There are two: PSII and PSI. Each photosystem consists of a reaction center containing 2 chl a molecules and an antenna complex of accessory pigments (chl b, carotenoids) and more chl a. The two types of photosystems are connected to each other via a chain of electron carrier molecules. The photosystems + electron carriers = Z-scheme.

1804: N. T. de Saussure found that approximately equal volumes of CO2 and O2 are exchanged during photosynthesis. (He also found that the weight that plants gained during photosynthesis could not be accounted for by the weight difference between CO2 and O2 he attributed the difference to the involvement of water as a reactant in photosynthesis.)

1864: Julius von Sachs demonstrated that only the parts of leaves exposed to light synthesized starch

1946-1953: Calvin, Benson, Bassham, and their associates used 14 CO2, Chlorella (unicellular green alga), and paper chromatography to elucidate how plants reduce CO2 to sugar: the Calvin cycle

Tying Things Together

1950s: the molecules that connect the water oxidation and carbon reduction processes were discovered: NADP + /NADPH, ADP/ATP

Photosynthesis as a Chloroplast Event

Each chloroplast is bounded by a double membrane. Inside the chloroplasts are stacks of membrane sacks. Each sack is a thylakoid. The Z-scheme is located in the thylakoid membranes. The small volume aqueous compartment inside the thylakoid membrane is called the lumen (your textbook calls the lumen the "thylakoid space"). The large volume aqueous area outside of the thylakoid, but still within the chloroplast, is the stroma. The reduction of CO2 to sugar occurs in the stroma.

Photosynthesis occurs in two sets of reactions that are linked by electron carrier molecules (NADP + /NADPH) and ADP/ATP. The two reactions go by several names. I'll be sticking to light reactions and Calvin cycle for what your textbook calls the light-dependent reactions and light-independent reactions, respectively. (I am not fond of the term "light-independent reactions" for the Calvin cycle because at least five of the enzymes of the Calvin cycle are light activated. Also, use of the term "Calvin cycle" recognizes the Nobel Laureate who figured out the process.)

Light energy is used to excite an electron in one of the reaction center chl a molecules in PSII. The excited electron leaves PSII and travels to PSI via the electron carriers of the Z-scheme. At PSI, the electron comes to rest at the PSI reaction center. The electron gets re-excited by more light energy, leaves PSI, and travels to NADP + . Once two electrons reach NADP + , it is reduced to NADPH. NADP + /NADPH are located in the stroma.

How does PSII replace the electrons that keep leaving? By splitting water in a process called photolysis:
2 H 2 O ---------> O 2 + 4 H + + 4 e -

Photolysis occurs on the lumen side of the thylakoid membrane. The protons (H + ) that get released from water are trapped in the small lumen space by the thylakoid membrane. As the electrons move from PSII to PSI, more protons (H + ) are moved from the stroma to the lumen. Eventually, you get a pretty sizeable H + gradient between the lumen and the stroma. There is an enzyme that lets the chloroplasts harvest the energy of the gradient by converting ADP to ATP in the process of photophosphorylation. The ATP is in the stroma.

2. The Calvin Cycle
There are three phases to the Calvin cycle: carboxylation, reduction, and regeneration

A. Carboxylation
Carbon dioxide and RuBP (a C5 sugar) are combined to give two molecules of PGA (a C3 acid).
The enzyme that catalyzes this reaction is abbreviated rubisco.

B. Reduction
This phase uses the 2/3 of the ATP and all of the NADPH produced during the light reactions.
Each PGA molecule is reduced to GA3P (a C3 sugar).

C. Regeneration
This phase uses the last of the ATP to turn a bunch of GA3P molecules into a bunch of RuBP molecules so more carbon dioxide can be brought in.

D. What the abbreviations stand for:
rubisco = RuBP carboxylase and oxygenase
RuBP = ribulose-1,5-bisphosphate
PGA = 3-phosphoglyceric acid
GA3P = glyceraldehyde-3-phosphate (also known as 3-phosphoglyceraldehyde or PGald)
ATP = adenosine triphosphate
ADP = adenosine diphosphate
NADP + /NADPH = nicotinamide adenine dinucleotide phosphate (oxidized/reduced)

Once you have lots of GA3P, it can be used to make glucose, fructose, sucrose, and starch.
(The reactions of respiration, besides providing a means to change the stored calories of sugars into useable energy, let a cell start the process of converting carbons from carbohydrates into a variety of molecules: amino acids, nucleotides, pigments, hormones, etc.)

Photosynthesis as a Leaf Event

Besides looking at photosynthesis as a chloroplast event, you need to remember that it is also a leaf event.

Recall that a leaf is composed of three tissues:
1. epidermis
holes for gas exchange called stomata (guard cells open and close the holes)
covered by a wax layer called cuticle
2. vascular tissue
xylem + phloem together in a vascular bundle (vein)
3. mesophyll (ground tissue)
tightly packed layer of cells = palisade mesophyll
loose cell layer with lots of air spaces = spongy mesophyll
(a review of leaf anatomy is recommended)

Most plants open their stomata during the day (light) so CO 2 enters the leaf for photosynthesis. Downside: water evaporates out of the stomata whenever they are open. Evaporation is fastest when the temperatures are highest, which would also be during the day. The stomata close at night when photosynthesis is not going on (no need to let in CO 2 ).

Some plants have a system that lets them open their stomata at night to collect and store CO2. During the day, they can close their stomata to conserve water but still do photosynthesis. These plants are known as CAM plants . CAM == Crassulacean acid metabolism. CAM was first discovered in members of the Crassulaceae family. CAM has since been found in many angiosperm families (both monocots and dicots), a seedless vascular plant, and a gymnosperm.

CAM plants grow in arid habitats: deserts, alpine regions, as epiphytes. CAM plants have at least some succulence (water storing). Two CAM plants are important from the money end of things: pineapple and orchids.

PEP = phosphoenolpyruvate, a C3 acid.
CO 2 can be attached to PEP by the enzyme PEP carboxylase.

At night, the stomata are open. Starch is broken down to produce PEP. PEP combines with CO 2 to form a C4 acid. This C4 acid is stored in the vacuole. During the day, the stomata close. The C4 acid is broken down to release CO 2 and a C3 acid. The C3 acid is converted back to starch. The CO 2 enters the Calvin cycle.

CAM is estimated to occur in

10% of plant species. C3 photosynthesis (where the only carbon reactions are the Calvin cycle ones) occurs in

89% of species. The remaining

1% do C4 photosynthesis. Although C4 accounts for only a fraction of the photosynthesis it attracts a lot of study because (1) it is a highly efficient form of photosynthesis and (2) it accounts for the high productivity of such major crops as corn, sugar cane, sorghum, and millet.

rubisco = RuBP carboxylase and oxygenase

The CO 2 lost because of the oxygenase reaction is called photorespiration. It is a problem under conditions of high temperature, high light intensity, and low water. Under these conditions, a C3 plant might lose 50% of its carbon via photorespiration.

How can you decrease photorespiration? Keep rubisco away from O 2 . Some plants do this by engaging in C4 photosynthesis .

C4 plants have a distinctive leaf anatomy. There is a prominent ring of cells around the vascular bundles = the bundle sheath. The mesophyll cells form a ring that is tightly appressed to the bundle sheath cells. Kranz anatomy .

In a mesophyll cell, CO 2 and PEP combine to form a C4 acid. The C4 acid is sent to a bundle sheath cell. In the bundle sheath cell, the CO 2 is released from the C4 acid and enters the Calvin cycle. The C3 acid that remains goes back to the mesophyll cell, is made back into PEP, and is ready to carry more CO 2 . (Rubisco is located only in the bundle sheath cells.) So, a CO 2 shuttle system delivers CO 2 to rubisco the leaf anatomy keeps O 2 away from the bundle sheaths. Result ==> no photorespiration.

Downside to C4: the CO 2 shuttle is not a free ride. It adds 2 ATP to the standard 3 ATP (for the Calvin cycle) needed per CO 2 . So C4 is only cost effective for plants in an environment where photorespiration is a common problem.

Photosynthesis as a Whole Plant Event

One final thing to remember about photosynthesis is that it is a whole plant event. The roots need to take in essential elements from the soil. Many of the elements that plants require have some role in photosynthesis: sulfur, magnesium, iron, manganese, chlorine, nitrogen, copper, phosphorus. Potassium is needed to open the stomata to let in CO 2 . There needs to be an adequate water supply coming in from the roots to keep the stomata open. Under conditions of water stress, the stomata will close (at least partly, if not completely).

Cellular (Aerobic) Respiration

The overall reaction for cellular (aerobic) respiration is essentially photosynthesis in reverse. Basically, respiration involves oxidizing sugar carbons to CO 2 . Once again, an electron transport system is used to create a hydrogen ion (proton) gradient that is used to make ATP. The final depository for the electrons is O 2 , which is reduced to water.

Respiration is strongly identified with the mitochondria, however, the starting reactions are in the cytosol. If oxygen is available, the mitochondria get involved with respiration, and the internal structure of the mitochondria is critical to the process.

Two membranes: outer membrane and inner membrane
Small volume aqueous space: intermembrane space
Large volume aqueous space: matrix

The carbon oxidation takes place in two sets of reactions, glycolysis in the cytosol and the Krebs cycle in the matrix of the mitochondria. Both glycolysis and the Krebs cycle occur in steps. Stepwise oxidation is important because it allows the cell to:
1. dissipate energy that is released as heat
2. generate intermediates ==> steps to start making amino acids, N-bases, other sugars for cell wall and nucleic acids, fatty acids, chlorophyll, anthocyanins, hormones, alkaloids, essential oils, etc.

Glycolysis oxidizes glucose or fructose (C6 sugars) to pyruvate (C3 acid). Some of the energy released during the oxidation is captured as ATP. The electrons released by the oxidation are held by NADH. Pyruvate is an organic acid. If the carbon is organic, it is at least partly reduced. Therefore, there are still calories available in pyruvate.

The Krebs Cycle
The Krebs cycle finishes the carbon oxidation process. All of the carbons in pyruvate are oxidized to CO 2 . It is a cyclic process, with the pyruvate carbons getting attached to an existing cycle molecule prior to oxidation. Once the pyruvate carbons are completely oxidized, the cycle molecule needs to be regenerated. A little bit of ATP is made during the Krebs cycle, but mostly it is an oxidation process. Most of the electrons are held by NADH, with a few held by FADH2.

Electron Transport Chain (ETC)
The electrons from NADH and FADH2 are passed to a system of electron carriers in the inner mitochondrial membrane, eventually reaching O 2 which is then reduced to H 2 O. As the electrons pass through the carriers, protons (H + ) are moved from the matrix to the intermembrane space. This establishes a proton gradient. When the protons pass through a channel protein back to the matrix, ATP is synthesized. How much ATP gets made? It depends on who is doing the counting. Most plants that oxidize one glucose molecule completely to CO 2 will get between 30 and 38 ATP from glycolysis, Krebs cycle, and ETC. The majority of that ATP (85-90%) comes from the ETC. The high end of ATP yield (35-38 ATP) represents about 40% of the energy that was available in a molecule of glucose. The remaining energy is given off as heat.

Some plants deliberately have an ATP yield in the mid-teens. Their electrons take an alternative route in the ETC and don't build as large as proton gradient by the time they get to O 2 . Because the ATP yield is down, these plants release more heat. These plants are called thermogenic . They can raise the temperature in their microenvironment as much as 10 o C. This temperature increase volatilizes scents to attract pollinators more efficiently. The Titan Arum you saw at the end of "The Birds and the Bees" video is thermogenic.

In the absence of oxygen, plants can still carry out glycolysis, but any reactions involving the mitochondria are not available. Without oxygen and the ETC, where can NADH dump off its electrons? Enter fermentation: a set of reactions that lets NADH unload electrons so glycolysis can continue. Most plants use alcohol (ethanol) fermentation. The most likely anaerobic scenario for plants ==> roots in water-logged soil.

Some plants (hydrophytes) are specialized to have their roots always in water, like anchored aquatic plants and emerged plants (water lilies, cattails, etc.) These plants have large air spaces in the ground tissue of their leaves and stems. These air spaces serve as passage ways to get O 2 to the roots.

What do photosynthesis and respiration have in common? In what ways are they essentially the reverse of each other?

Be able to describe the structure of a chloroplast: thylakoid (thylakoid membrane + lumen), stroma. Which set of photosynthetic reactions occurs in the thylakoid? Which in the stroma?

How are the light reactions and the Calvin cycle reactions linked? ==> NADP + /NADPH, ATP/ADP

light reactions:
photosystems, Z-scheme, electron carrier molecules, photolysis, H + gradient, photophosphorylation

Calvin Cycle
three phases of the Calvin cycle: carboxylation, reduction, and regeneration
What is the sugar that is produced as a result of the carbon reduction process? What are the possible fates of the sugar once a plant has accumulated a lot of it?

Besides looking at photosynthesis as a chloroplast event, you need to remember that it is also a leaf event. Inside of a leaf are three tissues: epidermis, vascular tissue, mesophyll (ground tissue). Be able to relate the leaf tissues to their roles (leading or supporting) in photosynthesis.

What are CAM plants? How do they manage to do photosynthesis if their stomata are closed during the day for water conservation? In what habitats do you find CAM plants?

CAM is estimated to occur in

10% of plant species. C3 photosynthesis (where the only carbon reactions are the Calvin cycle ones) occurs in

89% of species. The remaining

1% do C4 photosynthesis. Why do C4 plants attract so much attention? What is photorespiration?
What is rubisco? How is it involved in photosynthesis? How is it involved in photorespiration?

C4 plants have Kranz anatomy. How does Kranz anatomy relate to C4 photosynthesis?

Be able to give at least two examples each of C3 plants, C4 plants, and CAM plants.

How is the root system important to photosynthesis?

Mitochondria structure: outer membrane, inner membrane, intermembrane space, matrix

carbon oxidation: glycolysis in the cytosol and the Krebs cycle in the mitochondrial matrix. Be able to briefly state what happens as a result of these two processes.

Why is stepwise oxidation of sugar important?
What is the importance of generating carbon intermediates during glycolysis and the Krebs cycle?

Electron Transport Chain (ETC)
Be able to briefly state what happens as a result of this process.

From a cellular point of view, what is the function of fermentation reactions?

ATP and Biological Energy, Cellular Metabolism and Fermentation, and Photosynthesis (from the Online Biology Book by M.J. Farabee at Estrella Mountain Community College)

Why Study Photosynthesis (from Devens Gust at Arizona State University)

Energy and Movement, Photosynthesis, and Respiration (from Steven Wolf at California State University Stanislaus)

Watch the video: ATP synthase in action (December 2021).