Unit 2 of AP Biology delves into the fascinating world of cellular energetics, exploring how cells harvest and utilize energy to power life's processes. This comprehensive guide provides detailed notes covering key concepts, ensuring you're well-prepared for the exam.
I. Introduction to Energy and Metabolism
A. Energy and its Transformations:
- Energy: The capacity to do work. This can manifest in various forms, including kinetic (energy of motion) and potential (stored energy).
- Thermodynamics: The study of energy transformations. Understanding the First Law (conservation of energy) and the Second Law (entropy increases in a closed system) is crucial.
- Free Energy (Gibbs Free Energy, ΔG): The energy available to do work. A negative ΔG indicates a spontaneous reaction (exergonic), while a positive ΔG indicates a non-spontaneous reaction (endergonic).
- Enzymes: Biological catalysts that speed up reaction rates by lowering the activation energy. They are highly specific and crucial for metabolic processes.
B. Metabolic Pathways:
- Catabolism: Metabolic pathways that break down complex molecules into simpler ones, releasing energy (exergonic). Examples include cellular respiration and fermentation.
- Anabolism: Metabolic pathways that build complex molecules from simpler ones, requiring energy input (endergonic). Examples include protein synthesis and photosynthesis.
- Coupled Reactions: The pairing of an exergonic reaction with an endergonic reaction, using the energy released from the former to drive the latter. ATP hydrolysis is a common example.
II. Cellular Respiration
A. Overview:
Cellular respiration is a catabolic pathway that breaks down glucose to produce ATP, the cell's primary energy currency. The process occurs in four main stages: glycolysis, pyruvate oxidation, the Krebs cycle (citric acid cycle), and oxidative phosphorylation (electron transport chain and chemiosmosis).
B. Glycolysis:
- Occurs in the cytoplasm.
- Anaerobic process (does not require oxygen).
- Net production: 2 ATP, 2 NADH, and 2 pyruvate molecules per glucose molecule.
C. Pyruvate Oxidation:
- Occurs in the mitochondrial matrix.
- Pyruvate is converted to acetyl-CoA, producing NADH and releasing CO2.
D. Krebs Cycle (Citric Acid Cycle):
- Occurs in the mitochondrial matrix.
- Acetyl-CoA is oxidized completely, producing ATP, NADH, FADH2, and releasing CO2.
E. Oxidative Phosphorylation:
- Occurs in the inner mitochondrial membrane.
- Electron Transport Chain (ETC): Electrons from NADH and FADH2 are passed down a series of protein complexes, releasing energy used to pump protons (H+) across the membrane, creating a proton gradient.
- Chemiosmosis: Protons flow back across the membrane through ATP synthase, driving ATP synthesis. This is called chemiosmosis because the energy is derived from a chemical gradient.
- Oxygen's Role: Oxygen acts as the final electron acceptor in the ETC, forming water. Without oxygen, the ETC would halt, and ATP production would drastically decrease.
III. Fermentation
Fermentation is an anaerobic process that regenerates NAD+ from NADH, allowing glycolysis to continue in the absence of oxygen. Two main types exist:
- Lactic Acid Fermentation: Pyruvate is reduced to lactate. Occurs in muscle cells during strenuous exercise.
- Alcoholic Fermentation: Pyruvate is converted to acetaldehyde, then to ethanol. Used by yeast in bread making and alcoholic beverage production.
IV. Photosynthesis
Photosynthesis is the process by which plants and some other organisms convert light energy into chemical energy in the form of glucose. It occurs in two main stages:
- Light-dependent Reactions: Occur in the thylakoid membranes of chloroplasts. Light energy is absorbed by chlorophyll, driving electron transport and ATP and NADPH synthesis. Oxygen is released as a byproduct.
- Light-independent Reactions (Calvin Cycle): Occur in the stroma of chloroplasts. ATP and NADPH from the light-dependent reactions are used to fix carbon dioxide and synthesize glucose.
V. Connecting Cellular Respiration and Photosynthesis
Cellular respiration and photosynthesis are interconnected processes. The products of photosynthesis (glucose and oxygen) are the reactants of cellular respiration, and the products of cellular respiration (carbon dioxide and water) are the reactants of photosynthesis. This cyclical relationship sustains life on Earth.
This detailed outline provides a solid foundation for understanding AP Biology Unit 2. Remember to supplement these notes with textbook readings, practice problems, and further research to solidify your knowledge. Good luck!
