This lesson stays at the CLEP level: big-picture pathways, locations, and inputs/outputs — not an enzyme-by-enzyme biochemistry tour. On the CLEP exam, molecular and cellular biology accounts for roughly one-third of the questions, so knowing these energy pathways cold is high-value.
Cellular respiration breaks down glucose to capture energy in ATP. Aerobic respiration (with oxygen) has three stages:
| Stage | Location | Key inputs | Key outputs |
|---|---|---|---|
| Glycolysis | Cytoplasm (cytosol) | Glucose | 2 pyruvate, 2 ATP (net), 2 NADH |
| Krebs (citric acid) cycle | Mitochondrial matrix | Pyruvate derivatives | CO₂, ATP, NADH, FADH₂ |
| Electron transport chain (ETC) | Inner mitochondrial membrane | NADH, FADH₂, O₂ | Most of the ATP, water |
Glycolysis does not require oxygen and happens in the cytoplasm. The Krebs cycle and the ETC occur inside the mitochondrion. In the ETC, electrons pass down a chain of carriers, driving the production of most of the cell's ATP; oxygen is the final electron acceptor, combining with electrons and hydrogen to form water.
Approximate net ATP yield of aerobic respiration: about 36–38 ATP per glucose. For the CLEP exam, remember the order of magnitude ("mid-30s"), not a precise number.
When oxygen runs short, the ETC stalls and NADH cannot be unloaded. Fermentation regenerates NAD⁺ so that glycolysis (and its small 2-ATP yield) can keep running.
Key point: fermentation's main purpose is regenerating NAD⁺, not producing large amounts of ATP.
Photosynthesis captures light energy to build sugar. It occurs in the chloroplast.
[GRAPH: Chloroplast cross-section — stacked thylakoid membranes (grana) suspended in a fluid stroma, enclosed by a double membrane. Light reactions occur in the thylakoid membranes; the Calvin cycle occurs in the stroma.]
| Phase | Location | Inputs | Outputs |
|---|---|---|---|
| Light-dependent reactions | Thylakoid membranes | Light, H₂O | O₂, ATP, NADPH |
| Calvin cycle (light-independent) | Stroma | CO₂, ATP, NADPH | Sugar (G3P → glucose) |
In the light reactions, water is split, releasing O₂ as a byproduct and providing electrons; ATP and NADPH are produced. The Calvin cycle then uses CO₂, ATP, and NADPH to fix carbon into sugar. The Calvin cycle needs the products of the light reactions, so in the dark it quickly halts once ATP/NADPH run out.
Overall: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂
Respiration and photosynthesis are near-opposites: the products of one are largely the reactants of the other.
This cycling of carbon, oxygen, and energy links producers and consumers across the biosphere.
1. B — Cytoplasm (cytosol). Fix: Glycolysis is the one stage that occurs outside the mitochondrion. - A: Confuses glycolysis with the Krebs cycle, which is in the matrix. - C: Names the ETC location, not glycolysis. - D: A chloroplast location — wrong organelle entirely. - E: The nucleus houses DNA, not glucose breakdown. - Fix rule: "Glyco-lysis = cyto-plasm; the mitochondrion handles the later stages."
2. C — Electron transport chain / inner mitochondrial membrane. Fix: The ETC is embedded in the inner (cristae) membrane where it pumps protons. - A: Glycolysis is cytoplasmic, not membrane-bound. - B: The Krebs cycle is in the matrix, not the cytoplasm. - D: The Krebs cycle is in the matrix, not the outer membrane. - E: Glycolysis is cytoplasmic, not in the matrix. - Fix rule: Match each stage to its home: cytoplasm → matrix → inner membrane.
3. A — About 36–38 ATP. Fix: Full aerobic breakdown captures energy in the mid-30s of ATP per glucose. - B: 2 ATP is the net from glycolysis alone. - C: 4 ATP is glycolysis's gross (before subtracting the 2 invested). - D: An arbitrary middle value with no basis. - E: Far too high; no pathway yields ~100 ATP per glucose. - Fix rule: Aerobic ≈ "mid-30s"; anaerobic glycolysis alone = 2.
4. E — Lactic acid fermentation. Fix: Without oxygen, fermentation regenerates NAD⁺ so glycolysis's ATP output continues. - A: The Krebs cycle needs oxygen downstream and is not the rescue step. - B: The ETC requires oxygen and is exactly what stalls. - C: The Calvin cycle is photosynthesis, not respiration. - D: Oxidative phosphorylation is the oxygen-dependent step that shuts down. - Fix rule: No O₂ in muscle → lactic acid fermentation keeps glycolysis going.
5. D — Ethanol and carbon dioxide. Fix: Yeast perform alcoholic fermentation, producing ethanol plus CO₂ (which leavens bread). - A: Lactic acid is the animal-muscle form, not yeast. - B: Glucose and oxygen are photosynthesis products, not fermentation. - C: Water and CO₂ are aerobic respiration outputs. - E: Oxygen and NADPH belong to photosynthesis's light reactions. - Fix rule: Yeast → alcohol + CO₂; muscle → lactic acid.
6. E — Oxygen. Fix: Splitting water (photolysis) releases O₂ as a byproduct in the light reactions. - A: CO₂ is a reactant of the Calvin cycle, not from splitting water. - B: Glucose forms later in the stroma, not from water splitting. - C: NADPH is made in the light reactions but is not a gas. - D: ATP is made in the light reactions but is not a gas. - Fix rule: The O₂ you breathe comes from split water, not from CO₂.
7. C — Sugar. Fix: The Calvin cycle fixes CO₂ using ATP and NADPH to build carbohydrate (G3P → glucose). - A: Oxygen is a light-reaction byproduct, not a Calvin-cycle product. - B: Water is a reactant/byproduct elsewhere, not the cycle's product. - D: Chlorophyll is a pigment, not synthesized by the Calvin cycle. - E: Light energy is an input to the light reactions, not a product. - Fix rule: Calvin cycle = the "sugar factory" of the chloroplast.
8. A — Light reactions continue briefly; the Calvin cycle halts. Fix: The Calvin cycle needs CO₂; without it, carbon fixation stops while light reactions run until ATP/NADPH back up. - B: Reversed — the Calvin cycle is the phase that depends on CO₂. - C: The light reactions do not require CO₂, so they do not stop immediately. - D: Missing CO₂ must eventually stall the whole process. - E: Fermentation is unrelated to a photosynthesizing plant cell here. - Fix rule: Remove a reactant → the phase that uses it halts first.
9. B — The products of one are largely the reactants of the other. Fix: Photosynthesis makes glucose + O₂; respiration consumes them and returns CO₂ + H₂O. - A: Photosynthesis occurs in plants/algae, not animal cells. - C: Only photosynthesis releases O₂; respiration consumes it. - D: Only respiration consumes glucose; photosynthesis makes it. - E: Both processes are enzyme-driven. - Fix rule: The two pathways are chemical mirror images that feed each other.
10. D — Final electron acceptor, forming water. Fix: O₂ accepts spent electrons at the chain's end, combining with H⁺ to form water and keeping the chain flowing. - A: NADH donates electrons at the start; oxygen does not. - B: Splitting to release CO₂ describes neither oxygen nor the ETC. - C: ADP phosphorylation depends on the proton gradient and ATP synthase. - D is correct. - E: Carbon fixation is photosynthesis, not the ETC. - Fix rule: In the ETC, O₂ is the last stop — accept electrons, make water.
11. B — The oxygenated culture yields far more ATP. Fix: Aerobic respiration (~36–38 ATP) vastly outproduces anaerobic glycolysis (2 ATP). - A: Reversed; anaerobic conditions yield far less. - C: They cannot be identical when one uses the full ETC. - D: Both still make some ATP; glycolysis runs either way. - E: Oxygen availability is the key driver of ATP yield here. - Fix rule: Oxygen present → high ATP; oxygen absent → only glycolysis's 2.
12. C — Incorrect; fermentation regenerates NAD⁺. Fix: Fermentation yields no new ATP itself; its role is recycling NAD⁺ so glycolysis's small ATP output continues. - A: Aerobic respiration far outproduces fermentation. - B: 36 ATP is the aerobic figure, not fermentation's. - C is correct. - D: Glycolysis still nets 2 ATP, so "no ATP at all" overstates it. - E: Fermentation is specifically the anaerobic (no-oxygen) route. - Fix rule: Fermentation's job is NAD⁺ recycling, not ATP mass-production.
1. B — Cytoplasm (cytosol). Fix: Glycolysis is the one stage that occurs outside the mitochondrion. - A: Confuses glycolysis with the Krebs cycle, which is in the matrix. - C: Names the ETC location, not glycolysis. - D: A chloroplast location — wrong organelle entirely. - E: The nucleus houses DNA, not glucose breakdown. - Fix rule: "Glyco-lysis = cyto-plasm; the mitochondrion handles the later stages."
2. C — Electron transport chain / inner mitochondrial membrane. Fix: The ETC is embedded in the inner (cristae) membrane where it pumps protons. - A: Glycolysis is cytoplasmic, not membrane-bound. - B: The Krebs cycle is in the matrix, not the cytoplasm. - D: The Krebs cycle is in the matrix, not the outer membrane. - E: Glycolysis is cytoplasmic, not in the matrix. - Fix rule: Match each stage to its home: cytoplasm → matrix → inner membrane.
3. A — About 36–38 ATP. Fix: Full aerobic breakdown captures energy in the mid-30s of ATP per glucose. - B: 2 ATP is the net from glycolysis alone. - C: 4 ATP is glycolysis's gross (before subtracting the 2 invested). - D: An arbitrary middle value with no basis. - E: Far too high; no pathway yields ~100 ATP per glucose. - Fix rule: Aerobic ≈ "mid-30s"; anaerobic glycolysis alone = 2.
4. E — Lactic acid fermentation. Fix: Without oxygen, fermentation regenerates NAD⁺ so glycolysis's ATP output continues. - A: The Krebs cycle needs oxygen downstream and is not the rescue step. - B: The ETC requires oxygen and is exactly what stalls. - C: The Calvin cycle is photosynthesis, not respiration. - D: Oxidative phosphorylation is the oxygen-dependent step that shuts down. - Fix rule: No O₂ in muscle → lactic acid fermentation keeps glycolysis going.
5. D — Ethanol and carbon dioxide. Fix: Yeast perform alcoholic fermentation, producing ethanol plus CO₂ (which leavens bread). - A: Lactic acid is the animal-muscle form, not yeast. - B: Glucose and oxygen are photosynthesis products, not fermentation. - C: Water and CO₂ are aerobic respiration outputs. - E: Oxygen and NADPH belong to photosynthesis's light reactions. - Fix rule: Yeast → alcohol + CO₂; muscle → lactic acid.
6. E — Oxygen. Fix: Splitting water (photolysis) releases O₂ as a byproduct in the light reactions. - A: CO₂ is a reactant of the Calvin cycle, not from splitting water. - B: Glucose forms later in the stroma, not from water splitting. - C: NADPH is made in the light reactions but is not a gas. - D: ATP is made in the light reactions but is not a gas. - Fix rule: The O₂ you breathe comes from split water, not from CO₂.
7. C — Sugar. Fix: The Calvin cycle fixes CO₂ using ATP and NADPH to build carbohydrate (G3P → glucose). - A: Oxygen is a light-reaction byproduct, not a Calvin-cycle product. - B: Water is a reactant/byproduct elsewhere, not the cycle's product. - D: Chlorophyll is a pigment, not synthesized by the Calvin cycle. - E: Light energy is an input to the light reactions, not a product. - Fix rule: Calvin cycle = the "sugar factory" of the chloroplast.
8. A — Light reactions continue briefly; the Calvin cycle halts. Fix: The Calvin cycle needs CO₂; without it, carbon fixation stops while light reactions run until ATP/NADPH back up. - B: Reversed — the Calvin cycle is the phase that depends on CO₂. - C: The light reactions do not require CO₂, so they do not stop immediately. - D: Missing CO₂ must eventually stall the whole process. - E: Fermentation is unrelated to a photosynthesizing plant cell here. - Fix rule: Remove a reactant → the phase that uses it halts first.
9. B — The products of one are largely the reactants of the other. Fix: Photosynthesis makes glucose + O₂; respiration consumes them and returns CO₂ + H₂O. - A: Photosynthesis occurs in plants/algae, not animal cells. - C: Only photosynthesis releases O₂; respiration consumes it. - D: Only respiration consumes glucose; photosynthesis makes it. - E: Both processes are enzyme-driven. - Fix rule: The two pathways are chemical mirror images that feed each other.
10. D — Final electron acceptor, forming water. Fix: O₂ accepts spent electrons at the chain's end, combining with H⁺ to form water and keeping the chain flowing. - A: NADH donates electrons at the start; oxygen does not. - B: Splitting to release CO₂ describes neither oxygen nor the ETC. - C: ADP phosphorylation depends on the proton gradient and ATP synthase. - D is correct. - E: Carbon fixation is photosynthesis, not the ETC. - Fix rule: In the ETC, O₂ is the last stop — accept electrons, make water.
11. B — The oxygenated culture yields far more ATP. Fix: Aerobic respiration (~36–38 ATP) vastly outproduces anaerobic glycolysis (2 ATP). - A: Reversed; anaerobic conditions yield far less. - C: They cannot be identical when one uses the full ETC. - D: Both still make some ATP; glycolysis runs either way. - E: Oxygen availability is the key driver of ATP yield here. - Fix rule: Oxygen present → high ATP; oxygen absent → only glycolysis's 2.
12. C — Incorrect; fermentation regenerates NAD⁺. Fix: Fermentation yields no new ATP itself; its role is recycling NAD⁺ so glycolysis's small ATP output continues. - A: Aerobic respiration far outproduces fermentation. - B: 36 ATP is the aerobic figure, not fermentation's. - C is correct. - D: Glycolysis still nets 2 ATP, so "no ATP at all" overstates it. - E: Fermentation is specifically the anaerobic (no-oxygen) route. - Fix rule: Fermentation's job is NAD⁺ recycling, not ATP mass-production.