Cellular Energetics — AP Biology
1. Enzyme Kinetics and Regulation ★★☆☆☆ ⏱ 5 min
Cellular energetics is the study of how cells capture, transform, store, and use energy to carry out biological functions. It centers on energy coupling, where exergonic (energy-releasing) reactions drive endergonic (energy-requiring) reactions, with ATP acting as the universal cellular energy currency. This unit makes up 12-16% of your total AP Biology exam score, making it one of the highest-yield topics.
The relationship between reaction rate and substrate concentration is described by the Michaelis-Menten equation:
v = \frac{V_{max}[S]}{K_m + [S]}
- $v$ = instantaneous reaction rate
- $V_{max}$ = maximum reaction rate, achieved when all enzyme active sites are saturated
- $[S]$ = substrate concentration
- $K_m$ = Michaelis constant, substrate concentration at ½ $V_{max}$; lower $K_m$ = higher enzyme-substrate affinity
Factors that alter enzyme activity include temperature (human enzymes optimal at ~37°C, higher temperatures cause denaturation), pH (most optimal 6-8, except stomach pepsin optimal at pH 2), substrate concentration, and enzyme concentration.
- **Competitive inhibition**: Inhibitor binds the active site, competing with substrate; increases $K_m$, does not change $V_{max}$
- **Non-competitive inhibition**: Inhibitor binds an allosteric site, changing active site shape; lowers $V_{max}$, does not change $K_m$
- **Feedback inhibition**: End product of a pathway inhibits the first enzyme in the pathway to prevent wasteful overproduction
2. Aerobic Cellular Respiration ★★★☆☆ ⏱ 6 min
Aerobic cellular respiration breaks down glucose to release stored energy, producing ATP for cellular work. It has four core steps (glycolysis, pyruvate oxidation, citric acid cycle, oxidative phosphorylation) with a total net yield of 30-32 ATP per glucose molecule.
**Glycolysis** occurs in the cytoplasm and does not require oxygen, so it is used by both aerobic and anaerobic organisms. One 6-carbon glucose is split into two 3-carbon pyruvate molecules. Net outputs per glucose: 2 ATP (substrate-level phosphorylation), 2 NADH, 2 H₂O.
**Pyruvate oxidation** (intermediate step) occurs in the mitochondrial matrix. Each pyruvate is converted to acetyl-CoA, releasing 1 CO₂ and producing 1 NADH per pyruvate (2 total per glucose).
**Citric Acid Cycle (Krebs Cycle)** also occurs in the mitochondrial matrix, oxidizing acetyl-CoA to CO₂. Net outputs per glucose (two cycle turns): 2 ATP (substrate-level phosphorylation), 6 NADH, 2 FADH₂, 4 CO₂.
**Oxidative Phosphorylation** occurs on the inner mitochondrial membrane, producing ~90% of ATP from aerobic respiration. NADH and FADH₂ donate electrons to the ETC, which uses released energy to pump H⁺ into the intermembrane space, creating a proton gradient. H⁺ flows through ATP synthase to power ATP production via chemiosmosis. Modern AP values are 2.5 ATP per NADH and 1.5 ATP per FADH₂, with oxygen as the final electron acceptor.
3. Anaerobic Fermentation ★★☆☆☆ ⏱ 3 min
Fermentation is an anaerobic pathway that operates when oxygen is not available to act as the final electron acceptor for the ETC. The only purpose of fermentation is to regenerate $NAD^+$, which is required for glycolysis to continue producing small amounts of ATP. No additional ATP is produced in fermentation beyond the 2 net ATP from glycolysis.
- **Lactic acid fermentation**: Pyruvate is reduced by NADH to form lactate, regenerating NAD⁺. No CO₂ is produced. Occurs in human muscle during intense exercise and in lactic acid bacteria used to make yogurt and cheese.
- **Alcohol fermentation**: Pyruvate is converted to acetaldehyde (releasing 1 CO₂ per pyruvate), then acetaldehyde is reduced by NADH to ethanol, regenerating NAD⁺. Used by yeast in bread, beer, and wine production.
4. Photosynthesis ★★★☆☆ ⏱ 4 min
Photosynthesis converts light energy into chemical energy stored in glucose, carried out by plants, algae, and cyanobacteria. The overall balanced reaction is:
6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2
All reactions occur in chloroplasts: light-dependent reactions take place on the thylakoid membranes, and the Calvin cycle (light-independent reactions) occurs in the stroma.
In **light-dependent reactions**, chlorophyll pigments in photosystems II and I absorb light energy to excite electrons. Excited electrons move down an ETC, releasing energy to pump H⁺ into the thylakoid lumen to create a proton gradient. ATP is produced via chemiosmosis (photophosphorylation). Water is split to replace electrons lost from photosystem II, releasing O₂ as a waste product. NADP⁺ is the final electron acceptor, reduced to NADPH. Outputs: ATP, NADPH, O₂.
The **Calvin cycle** uses ATP and NADPH from the light reactions to fix CO₂ into glucose, with three phases: (1) Carbon fixation: CO₂ combines with the 5-carbon RuBP, catalyzed by the enzyme RuBisCO, forming 3-carbon molecules; (2) Reduction: ATP and NADPH convert 3-carbon molecules into G3P, the precursor for glucose; (3) RuBP regeneration: 5 of every 6 G3P are used to rebuild RuBP to repeat the cycle. Producing 1 glucose requires 6 CO₂, 18 ATP, and 12 NADPH.
Common Pitfalls
Why: Students confuse gross ATP produced in the first steps of glycolysis (4) with net ATP (2, since 2 ATP are used to initiate glycolysis)
Why: Students take the "light-independent" label literally, forgetting the cycle relies on ATP and NADPH only produced when light is available
Why: Both pathways use ETCs, leading to confusion
Why: Older textbooks use these estimates, but the AP Biology CED now uses modern values
Why: Students assume CO₂ is split to release oxygen, but this is incorrect