📚Study Guide: Cellular Energetics
Unit 3: Cellular Energetics
Cellular energetics is the engine of life, encompassing the processes by which cells capture, store, and release energy. This unit covers photosynthesis and cellular respiration in exhaustive detail, requiring students to understand not just the inputs and outputs of each stage but the spatial organization within chloroplasts and mitochondria that makes these processes possible. Photosynthesis converts light energy into chemical energy stored in glucose, while cellular respiration breaks down glucose to produce ATP, the universal energy currency of the cell. The AP exam frequently tests the interdependence of these processes, asking students to trace carbon, energy, and electron carriers between them. Understanding chemiosmosis—the process by which proton gradients drive ATP synthesis—is absolutely critical. Students must also grasp anaerobic processes like fermentation, which allow cells to continue ATP production in the absence of oxygen. Experimental design is heavily emphasized, particularly investigations using chromatography to separate photosynthetic pigments or respirometers to measure oxygen consumption. Mastery of redox reactions, NAD+/NADH, FAD/FADH2, and the role of oxygen as the final electron acceptor will separate top-scoring students from the rest.
Key Concepts
- Photosynthesis Overview: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2. Occurs in chloroplasts. Light-dependent reactions in thylakoids produce ATP and NADPH. Calvin cycle in stroma uses ATP and NADPH to fix CO2 into glucose.
- Cellular Respiration Overview: C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP. Occurs in mitochondria. Glycolysis in cytoplasm produces 2 ATP and 2 NADH. Pyruvate oxidation produces 2 NADH. Krebs cycle produces 2 ATP, 6 NADH, 2 FADH2. Oxidative phosphorylation (ETC and chemiosmosis) produces ~26-28 ATP.
- Chemiosmosis: In both photosynthesis and cellular respiration, electrons passing through electron transport chains pump protons (H+) across a membrane, creating a proton-motive force. ATP synthase uses this gradient to phosphorylate ADP into ATP.
- Anaerobic Respiration and Fermentation: When oxygen is absent, cells use fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. Lactic acid fermentation occurs in human muscle cells; alcoholic fermentation occurs in yeast.
- Redox Reactions: Oxidation is the loss of electrons; reduction is the gain of electrons. NAD+ is reduced to NADH; FAD is reduced to FADH2. Oxygen is the final electron acceptor in aerobic respiration.
Vocabulary
- ATP (Adenosine Triphosphate): The main energy currency of cells; consists of adenine, ribose, and three phosphate groups. Energy is released when the terminal phosphate bond is hydrolyzed.
- NAD+/NADH: Nicotinamide adenine dinucleotide; an electron carrier that accepts electrons (becomes NADH) during catabolic reactions and donates them to the electron transport chain.
- Calvin Cycle: The light-independent reactions of photosynthesis; fixes CO2 into organic molecules using ATP and NADPH produced in the light reactions.
- Glycolysis: The breakdown of glucose into two molecules of pyruvate, occurring in the cytoplasm with a net gain of 2 ATP.
- Oxidative Phosphorylation: The production of ATP using energy derived from the transfer of electrons in the electron transport chain and the resulting proton gradient.
- Proton-Motive Force: The potential energy stored in the form of a proton (H+) gradient across a membrane, used by ATP synthase to generate ATP.
Processes and Diagrams to Know
- Photosystems I and II: Know the Z-scheme of noncyclic photophosphorylation: PSII absorbs light at 680nm, splits water, passes electrons to PSI via electron transport chain. PSI absorbs at 700nm and reduces NADP+ to NADPH.
- Mitochondrial Structure: Label outer membrane, inner membrane (cristae), intermembrane space, and matrix. ETC occurs on inner membrane; Krebs cycle in matrix.
- Chromatography: Paper chromatography separates pigments by solubility and molecular weight. Rf value = distance pigment traveled / distance solvent traveled.
Experimental Designs
- Respirometer Experiment: Measure oxygen consumption by germinating seeds at different temperatures. Controls: non-germinating seeds, glass beads. Calculate rate by measuring change in gas volume.
- Pigment Chromatography: Extract pigments from spinach leaves and separate using a solvent front. Calculate Rf values and identify chlorophyll a, chlorophyll b, and carotenoids.
Common Mistakes
- Forgetting Water Splitting: In photosynthesis, water is split (photolysis) to replace electrons lost from PSII. Oxygen is a byproduct of this reaction, not of CO2 fixation.
- Confusing NADH and NADPH: NADH is primarily used in cellular respiration; NADPH is primarily used in photosynthesis (Calvin cycle). They are not interchangeable.
- Counting ATP Incorrectly: Glycolysis produces 4 ATP but uses 2, so net gain is 2 ATP. The Krebs cycle produces 2 GTP (equivalent to ATP) per glucose.
- Thinking Fermentation Produces ATP: Fermentation does NOT produce additional ATP. It only regenerates NAD+ so glycolysis can continue producing 2 ATP per glucose.
AP Exam Strategies
- Trace Carbon and Energy: When asked about connections between photosynthesis and respiration, explicitly trace: CO2 fixed in Calvin cycle becomes glucose, which is broken down in glycolysis, releasing CO2 in pyruvate oxidation and Krebs cycle.
- Draw the Proton Gradient: In chemiosmosis questions, sketch or describe the higher concentration of H+ in the thylakoid lumen or intermembrane space and ATP synthase allowing H+ to flow back.
- Distinguish Aerobic vs. Anaerobic: Clearly state that aerobic respiration yields ~30-32 ATP while anaerobic yields only 2 ATP (from glycolysis).
- Explain Experimental Results: If given respirometry data, explain that increased temperature increases kinetic energy and enzyme activity up to a point (denaturation).
Real-World Applications
- Biofuels: Algae and cyanobacteria are engineered to optimize photosynthetic efficiency for biodiesel production.
- Exercise Physiology: Understanding lactic acid fermentation explains muscle fatigue during intense exercise when oxygen delivery is insufficient.
- Agriculture: C4 and CAM photosynthetic pathways (though in Unit 8) are adaptations to hot, arid conditions; Unit 3 provides the foundation for understanding these variations.