AP Bio Unit 3
Cellular Respiration
Introduction to Cellular Respiration
Cellular respiration is the biochemical process by which cells convert glucose and oxygen into energy (ATP), carbon dioxide, and water.
The process involves two main parts: aerobic respiration (with oxygen) and anaerobic respiration (without oxygen).
Role of Electron Carriers
Electrons are delivered by NADH and FADH₂, which are passed to a series of electron acceptors that move toward the terminal electron acceptor, which is oxygen (O₂) in aerobic conditions.
In photosynthesis, the terminal electron acceptor is NADP⁺.
Aerobic prokaryotes use oxygen as a terminal electron acceptor, while anaerobic prokaryotes use other molecules.
Formation of Proton Gradient
The transfer of electrons is accompanied by the formation of a proton gradient across the inner mitochondrial membrane or the internal membrane of chloroplasts.
This creates a high concentration of protons on one side of the membrane, which is separated from a region of lower proton concentration.
In prokaryotes, the passage of electrons also results in the movement of protons across the plasma membrane.
ATP Formation via Chemiosmosis
Protons flow back through ATP synthase by a mechanism known as chemiosmosis, which drives the formation of ATP from ADP and inorganic phosphate.
This process has two key names:
Oxidative phosphorylation occurs in cellular respiration.
Photophosphorylation occurs in photosynthesis.
Oxidative Phosphorylation Defined
Oxidative phosphorylation is the process in which ATP is synthesized as a result of the transfer of electrons from NADH or FADH₂ to O₂ via a series of electron carriers, occurring primarily in the mitochondria.
It serves as the major source of ATP in aerobic organisms.
Decoupling Oxidative Phosphorylation
Decoupling oxidative phosphorylation from electron transport leads to the generation of heat, which can be beneficial for endothermic organisms to regulate body temperature.
Summary of Cellular Respiration Processes
Glycolysis
Process: Glycolysis takes place in the cytoplasm.
Inputs and Outputs:
In: Glucose
Out: 2 Pyruvates, 2 ATP (4 produced, 2 net), 2 NADH
Nature: Anaerobic
Link Reaction and Krebs Cycle
Location: Takes place in the matrix of aerobic mitochondria.
For each molecule of glucose:
Inputs: 2 Pyruvates, 2 ADP + Pi, 2 NAD⁺, 2 FAD
Outputs: 6 CO₂, 2 ATP, 8 NADH, 2 FADH₂
The Krebs cycle is crucial for the oxidation of pyruvate and the release of CO₂.
Electron Transport Chain (ETC)
Location: Inner mitochondrial membrane
Inputs: 10 NADH, 2 FADH₂, 32 ADP + P
Outputs: 6 O₂, 32 ATP, and 6 H₂O
Nature: Aerobic; it is the final step in the aerobic respiration pathway.
Core Metabolic Pathways
Core metabolic pathways such as glycolysis and oxidative phosphorylation are conserved across all domains of life, including Archaea, Bacteria, and Eukarya.
Aerobic Respiration Overview
Cell respiration in eukaryotes involves a series of enzyme-catalyzed reactions that release energy from biological macromolecules.
Main stages of aerobic respiration include:
Glycolysis: An anaerobic process that occurs in the cytoplasm.
Link Reaction: Converts pyruvate into Acetyl CoA in mitochondria; it occurs before the Krebs Cycle.
Krebs Cycle: Further oxidation of Acetyl CoA occurs, releasing CO₂, ATP, and electron carriers.
Electron Transport Chain: Utilizes electron carriers to produce ATP.
Photosynthesis
Summary of Photosynthesis
Photosynthesis captures solar energy and converts it into chemical energy (sugars), initially evolved in prokaryotic organisms such as cyanobacteria.
Equation for Photosynthesis:
NADPH is the reduced form of NADP⁺, which serves as an electron carrier in anabolic reactions, including lipid and nucleic acid synthesis.
Light-Dependent Reactions
These occur in the thylakoid membranes of chloroplasts. They generate ATP and NADPH from solar energy.
Water is split in photosystem II (PSII), releasing O₂ as a byproduct. Energy from light boosts electrons to higher energy states.
An electron transport chain connects PSII and PSI (Photosystem I), resulting in a proton gradient that drives ATP synthesis via ATP synthase.
Light-Independent Reactions (Calvin Cycle)
The Calvin cycle occurs in the stroma, synthesizing carbohydrates from CO₂ using the ATP and NADPH produced in the light-dependent reactions.
Key Points: 3 RuBP molecules are recycled, and the captured energy is used to reduce carbon dioxide into organic products.
Enzymes and Their Role in Metabolism
Enzyme Structure and Function
Enzymes act as biological catalysts that speed up reactions by lowering the activation energy.
The enzyme's active site must have a compatible shape and charge for substrate binding.
Enzymes are: reusable, specific, affected by pH and temperature, and can denature if these conditions change drastically.
Types of Enzyme Activity Modulation
Competitive Inhibitors: Bind to the active site, competing with the substrate.
Non-competitive Inhibitors: Bind to allosteric sites, changing the enzyme shape and function.
Influences on Enzyme Activity
Temperature and pH: Enzyme activity can increase with temperature, up to an optimal point, after which denaturation may occur. Similar effects are observed with pH.
Concentration effects: The presence and concentration of substrates/products can influence the enzyme's efficiency.
Fermentation
Anaerobic Fermentation Processes
Fermentation allows glycolysis to continue in the absence of oxygen, producing either lactic acid or ethanol as byproducts.
Pathways for fermentation:
Lactic Acid Fermentation: Occurs in muscles under anaerobic conditions:
Alcohol Fermentation: Occurs in yeast:
Key Takeaways on Cellular Respiration
Cellular respiration is vital for energy production in living organisms, with aerobic respiration yielding significantly more ATP than anaerobic processes.
Both processes (respiration and fermentation) are characteristic across all life, accentuating their importance in biological systems.