Unit 3 of AP Biology delves into the fascinating world of cellular energetics, focusing on how cells acquire, store, and utilize energy. This is a crucial unit, as it underpins many biological processes. These notes will provide a comprehensive overview, touching upon key concepts and processes you'll need to master for the AP exam.
I. Introduction to Cellular Respiration and Fermentation
Cellular respiration is the process by which cells break down glucose to produce ATP, the primary energy currency of the cell. This process occurs in three main stages:
A. Glycolysis
- Location: Cytoplasm
- Input: Glucose (6-carbon sugar)
- Output: 2 pyruvate (3-carbon molecules), 2 ATP (net gain), 2 NADH
- Anaerobic process: Does not require oxygen.
- Key enzymes: Hexokinase, Phosphofructokinase, Pyruvate Kinase (these are important regulatory points).
B. Pyruvate Oxidation
- Location: Mitochondrial matrix
- Input: 2 pyruvate molecules from glycolysis
- Output: 2 Acetyl-CoA, 2 NADH, 2 CO2
- Transition step: Links glycolysis to the Krebs cycle.
C. Krebs Cycle (Citric Acid Cycle)
- Location: Mitochondrial matrix
- Input: 2 Acetyl-CoA
- Output: 6 NADH, 2 FADH2, 2 ATP, 4 CO2
- Cyclic process: Regenerates oxaloacetate to continue the cycle.
D. Oxidative Phosphorylation (Electron Transport Chain & Chemiosmosis)
- Location: Inner mitochondrial membrane
- Input: NADH and FADH2 from previous stages
- Output: ~32-34 ATP, H2O
- Electron carriers: Ubiquinone (Coenzyme Q), Cytochromes
- Chemiosmosis: Utilizes the proton gradient established across the inner mitochondrial membrane to drive ATP synthesis via ATP synthase.
E. Fermentation
- Anaerobic process: Occurs in the absence of oxygen.
- Two main types: Lactic acid fermentation (in animals and some bacteria) and alcoholic fermentation (in yeast and some bacteria).
- Purpose: Regenerates NAD+ to allow glycolysis to continue.
II. Photosynthesis
Photosynthesis is the process by which plants and other photosynthetic organisms convert light energy into chemical energy in the form of glucose. This process occurs in two main stages:
A. Light-Dependent Reactions
- Location: Thylakoid membranes within chloroplasts
- Input: Light energy, H2O
- Output: ATP, NADPH, O2 (oxygen is a byproduct)
- Photosystems: Photosystem II (PSII) and Photosystem I (PSI) are key protein complexes that absorb light energy and initiate electron transport.
- Electron transport chain: Similar to the ETC in cellular respiration, but uses light energy to drive electron flow.
B. Light-Independent Reactions (Calvin Cycle)
- Location: Stroma of chloroplasts
- Input: ATP, NADPH, CO2
- Output: Glucose (G3P)
- Carbon fixation: The initial step where CO2 is incorporated into an organic molecule.
- RuBisCO: The enzyme responsible for carbon fixation.
III. Regulation of Cellular Respiration and Photosynthesis
Both cellular respiration and photosynthesis are tightly regulated processes. Key factors influencing their rates include:
- Substrate availability: The amount of glucose (respiration) or CO2 (photosynthesis) available.
- Enzyme activity: Regulation of enzyme activity through allosteric regulation and feedback inhibition.
- Environmental factors: Temperature, light intensity (photosynthesis), oxygen availability (respiration).
IV. Connections and Comparisons Between Cellular Respiration and Photosynthesis
Cellular respiration and photosynthesis are essentially reverse processes. The products of one are the reactants of the other. Understanding their interconnectedness is vital for a comprehensive understanding of energy flow in ecosystems.
This is a condensed overview of AP Bio Unit 3. For a deeper understanding, refer to your textbook, class notes, and practice problems. Remember to focus on the connections between different processes and the regulation of these metabolic pathways. Good luck with your studies!
