AP Biology 3.5-3.6: Photosynthesis and Respiration

Glycolysis

Glycolysis is the anaerobic pathway that converts one six-carbon glucose molecule into two three-carbon pyruvate molecules. It occurs in the cytoplasm of both prokaryotic and eukaryotic cells and does not directly require oxygen.

Where does glycolysis occur, and why is it considered anaerobic?

Glycolysis occurs in the cytoplasm. It is considered anaerobic because oxygen is not directly used in any of its reactions.

What are the two broad phases of glycolysis?

The energy-investment phase uses 2 ATP to phosphorylate and prepare glucose for cleavage. The energy-payoff phase oxidizes the resulting three-carbon molecules and produces ATP and NADH.

What is the net chemical output of glycolysis per glucose molecule?

Glycolysis produces 2 pyruvate, 2 NADH, and a net gain of 2 ATP per glucose. Four ATP are formed, but 2 ATP are consumed during the investment phase.

What is the overall ATP accounting for glycolysis?

Two ATP are invested early, and four ATP are generated later by substrate-level phosphorylation. Therefore, the net ATP gain is $4-2=2$ ATP per glucose.

How does phosphorylation help retain glucose inside a cell?

Adding a negatively charged phosphate converts glucose into glucose-6-phosphate, which cannot readily cross the hydrophobic interior of the plasma membrane. This also prevents it from continuing to exit through glucose transporters.

GLUT proteins

GLUT proteins are integral membrane transport proteins that facilitate the diffusion of glucose down its concentration gradient. Glucose can also enter some cells by secondary active transport against its concentration gradient.

What reaction does hexokinase catalyze in glycolysis?

Hexokinase transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. This is the first energy-investment reaction.

What is the role of the isomerase in the second step of glycolysis?

An isomerase converts glucose-6-phosphate into fructose-6-phosphate. This rearrangement positions the functional groups so the six-carbon intermediate can later be split into two three-carbon molecules.

Why is phosphofructokinase important in glycolysis?

Phosphofructokinase transfers a phosphate from ATP to fructose-6-phosphate, forming fructose-1,6-bisphosphate. It is a major rate-limiting enzyme and is inhibited when ATP is abundant, helping match pathway activity to cellular energy demand.

How does ATP regulate phosphofructokinase?

High ATP concentration inhibits phosphofructokinase, whereas high ADP concentration activates it. This negative-feedback regulation slows glucose catabolism when the cell already has sufficient ATP.

What happens when fructose-1,6-bisphosphate is cleaved during glycolysis?

Aldolase splits fructose-1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. An isomerase then converts the dihydroxyacetone phosphate into a second glyceraldehyde-3-phosphate.

Why do the reactions of glycolysis after cleavage occur twice per glucose?

The six-carbon glucose derivative is split into two three-carbon molecules. Both molecules are converted into glyceraldehyde-3-phosphate and proceed through the energy-payoff phase.

What occurs during the oxidation of glyceraldehyde-3-phosphate?

Glyceraldehyde-3-phosphate is oxidized, and $NAD^+$ accepts the resulting high-energy electrons to form NADH. An inorganic phosphate is added without using another ATP, producing 1,3-bisphosphoglycerate.

Substrate-level phosphorylation

Substrate-level phosphorylation is the direct transfer of a high-energy phosphate group from a metabolic intermediate to ADP, forming ATP. Glycolysis uses this mechanism in the reactions catalyzed by phosphoglycerate kinase and pyruvate kinase.

How is ATP produced in the seventh and tenth steps of glycolysis?

In the seventh step, 1,3-bisphosphoglycerate transfers a phosphate to ADP, forming ATP and 3-phosphoglycerate. In the tenth step, phosphoenolpyruvate transfers a phosphate to ADP, forming ATP and pyruvate.

What are the roles of phosphoglycerate mutase and enolase in glycolysis?

Phosphoglycerate mutase moves the phosphate group of 3-phosphoglycerate from carbon 3 to carbon 2. Enolase removes water from 2-phosphoglycerate, forming phosphoenolpyruvate (PEP), which has a high-energy phosphate bond.

Why must NADH be converted back to $NAD^+$ for glycolysis to continue?

The oxidation of glyceraldehyde-3-phosphate requires oxidized $NAD^+$. If $NAD^+$ is depleted because NADH cannot be reoxidized, the energy-payoff phase slows or stops.

How can cells regenerate $NAD^+$ when oxygen is unavailable?

Fermentation provides an alternative pathway that oxidizes NADH back to $NAD^+$. This allows glycolysis to continue, although fermentation itself does not produce the additional ATP associated with aerobic respiration.

What happens to glycolysis in mature mammalian red blood cells?

Mature mammalian red blood cells lack mitochondria, so glycolysis is their only source of ATP. If glycolysis stops, they cannot maintain essential ion pumps such as the sodium-potassium pump and eventually die.

Why is pyruvate kinase considered a rate-limiting enzyme?

Pyruvate kinase catalyzes the final ATP-producing step of glycolysis. If insufficient pyruvate kinase is available, the pathway can proceed through earlier reactions but the final ATP-forming reaction is impaired.

What is the net redox change in glycolysis?

Two molecules of $NAD^+$ are reduced to 2 NADH per glucose because two glyceraldehyde-3-phosphate molecules are oxidized. Thus, glycolysis transfers some of glucose's chemical energy to reduced electron carriers.

What happens to pyruvate when oxygen is available in a eukaryotic cell?

Pyruvate is transported into the mitochondrial matrix and converted into acetyl-CoA. This links glycolysis to the citric acid cycle and aerobic respiration.

Pyruvate oxidation

Pyruvate oxidation is the three-step conversion of pyruvate into acetyl-CoA. It removes a carboxyl group as $CO_2$, oxidizes the remaining two-carbon group while reducing $NAD^+$ to NADH, and transfers the acetyl group to coenzyme A.

What are the products of pyruvate oxidation per pyruvate and per glucose?

Each pyruvate yields 1 acetyl-CoA, 1 $CO_2$, and 1 NADH. Because one glucose produces two pyruvates, pyruvate oxidation per glucose yields 2 acetyl-CoA, 2 $CO_2$, and 2 NADH.

Coenzyme A (CoA)

CoA is a carrier molecule that binds and activates acetyl groups, forming acetyl-CoA. It is derived from vitamin B5, pantothenic acid.

Why is pyruvate oxidation an oxidation-reduction process?

The two-carbon portion of pyruvate loses electrons as it is oxidized to an acetyl group. $NAD^+$ accepts those electrons and is reduced to NADH.

Citric acid cycle

The citric acid cycle, also called the Krebs cycle or TCA cycle, is a cyclic pathway in the mitochondrial matrix that oxidizes acetyl groups. It regenerates oxaloacetate and produces reduced electron carriers, $CO_2$, and a small amount of ATP or GTP.

How does the citric acid cycle differ structurally from glycolysis?

Glycolysis is a linear pathway that begins with glucose and ends with pyruvate. The citric acid cycle is a closed loop in which the final reactions regenerate the molecule, oxaloacetate, required for the first reaction.

How does acetyl-CoA enter the citric acid cycle?

The two-carbon acetyl group from acetyl-CoA combines with four-carbon oxaloacetate to form six-carbon citrate. CoA is released and can later carry another acetyl group.

What are the products of one turn of the citric acid cycle?

Each acetyl-CoA entering the cycle produces 2 $CO_2$, 3 NADH, 1 $FADH_2$, and 1 ATP or GTP. The cycle itself does not directly consume oxygen.

What are the citric acid cycle products per glucose molecule?

Because one glucose produces two acetyl-CoA, two turns yield 4 $CO_2$, 6 NADH, 2 $FADH_2$, and 2 ATP or GTP.

Why is the citric acid cycle considered an aerobic pathway even though it does not directly use oxygen?

The cycle requires $NAD^+$ and FAD to accept electrons. These carriers must be regenerated by transferring electrons to later stages of aerobic respiration, where oxygen ultimately accepts electrons; without this regeneration, the cycle cannot continue.

Where do the carbon atoms released as $CO_2$ in glucose oxidation originate?

Two carbons are released during pyruvate oxidation, and four more are released during two turns of the citric acid cycle. All six carbons originally present in glucose are eventually released as $CO_2$, although the acetyl carbons are not necessarily released on their first cycle turn.

What is the role of NADH and $FADH_2$ in cellular respiration?

NADH and $FADH_2$ carry high-energy electrons from glycolysis, pyruvate oxidation, and the citric acid cycle to the electron transport chain. Electron transfer there is coupled to ATP production.

Why does the citric acid cycle produce $FADH_2$ rather than NADH during succinate oxidation?

The electrons released when succinate is oxidized have insufficient energy to reduce $NAD^+$ but can reduce FAD. The resulting $FADH_2$ transfers its electrons directly to the electron transport chain because the enzyme is embedded in the inner mitochondrial membrane.

How is ATP or GTP formed in the citric acid cycle?

A high-energy bond formed during the conversion of succinyl-CoA to succinate drives substrate-level phosphorylation. Depending on the tissue, the product is ATP or GTP; GTP is energetically equivalent to ATP but is used more selectively, especially in protein synthesis.

What types of reactions occur in the citric acid cycle?

The cycle includes oxidation-reduction, decarboxylation, dehydration, and hydration reactions. These reactions progressively oxidize the acetyl group while regenerating oxaloacetate.

How is the citric acid cycle regulated by cellular energy status?

High ATP and NADH generally inhibit key oxidative steps, while ADP stimulates activity when energy demand is high. ATP also inhibits the initial condensation of acetyl-CoA with oxaloacetate.

Why is the citric acid cycle described as amphibolic?

It is amphibolic because it serves both catabolic and anabolic roles. Although it oxidizes acetyl groups to harvest energy, several intermediates can also be diverted to synthesize compounds such as nonessential amino acids.

Photosynthesis

Photosynthesis is a set of biological processes in which organisms containing light-absorbing pigments convert light energy into chemical energy. In oxygenic photosynthesis, the stored chemical energy is commonly incorporated into carbohydrates.

Which organisms commonly perform oxygenic photosynthesis?

Most plants, algae, and cyanobacteria perform oxygenic photosynthesis. These organisms use light energy and release oxygen as a byproduct of splitting water.

What is the relationship between photosynthesis and cellular respiration?

Photosynthesis stores light energy in chemical bonds of organic molecules such as carbohydrates. Cells can later extract that stored energy through cellular respiration.

What is oxygenic photosynthesis, and where does its oxygen come from?

Oxygenic photosynthesis is photosynthesis that produces molecular oxygen. The oxygen is released when water is split during the light-dependent reactions.

Anoxygenic photosynthesis

Anoxygenic photosynthesis is photosynthesis that does not release oxygen. Some bacteria use substances such as hydrogen sulfide instead of water as an electron source and may release sulfur as a byproduct.

What role do photosynthetic pigments and reaction centers play?

Photosynthetic pigments or chromophores absorb light energy. The absorbed energy is directed to reaction centers, where it initiates the electron-transfer processes that convert light energy into a usable chemical form.

What types of molecules commonly store chemical energy produced by photosynthesis?

Photosynthetic organisms store energy in carbon-containing organic compounds, especially carbohydrates such as sugars, starches, and cellulose. The energy is stored in the compounds' chemical bonds.

Why is photosynthesis important to Earth's biosphere?

Photosynthesis supplies much of the biological energy supporting complex life and maintains a large portion of atmospheric oxygen. It also converts inorganic carbon into energy-rich organic compounds.

How can some archaea perform photosynthesis without fixing carbon or producing oxygen?

Some archaea use retinal-based pigments to absorb light and generate a proton gradient across their cell membrane. Proton movement through membrane proteins can then drive ATP synthesis without water splitting or oxygen release.