Question
describe (a) the process and (b) the function of cellular respiration
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(a) The process of cellular respiration
Overview
- Cellular respiration is the set of metabolic pathways that cells use to extract energy from organic molecules (most often glucose) and convert it into usable chemical energy (ATP). In eukaryotes it occurs in the cytoplasm (early steps) and mitochondria (later steps); in prokaryotes equivalent steps occur in the cytosol and across the plasma membrane.
Overall chemical equation (for complete aerobic oxidation of glucose)
- C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat)
Major stages (with location and main products per glucose)
1. Glycolysis (cytoplasm)
- One 6‑carbon glucose → two 3‑carbon pyruvate.
- Net products: 2 ATP (substrate‑level phosphorylation), 2 NADH, 2 pyruvate.
- Operates anaerobically or aerobically (pyruvate fate depends on O2).
2. Pyruvate oxidation (mitochondrial matrix in eukaryotes)
- Each pyruvate → acetyl‑CoA + CO2; reduces NAD+ → NADH.
- Per glucose: 2 acetyl‑CoA, 2 CO2, 2 NADH.
3. Citric acid cycle (Krebs cycle; mitochondrial matrix)
- Each acetyl‑CoA enters cycle and is oxidized to CO2.
- Per glucose: 4 CO2, 2 ATP (as GTP), 6 NADH, 2 FADH2 (from both acetyl‑CoA).
4. Oxidative phosphorylation (inner mitochondrial membrane)
- Electron transport chain (ETC): electrons from NADH and FADH2 pass through complexes I–IV, driving protons from the matrix into the intermembrane space and creating a proton gradient.
- Oxygen is the final electron acceptor, forming water.
- Chemiosmosis: protons flow back through ATP synthase, driving synthesis of ATP from ADP + Pi.
- Typical ATP yield from oxidative phosphorylation is variable; whole‑cellular ATP yield per glucose is roughly 30–32 ATP in modern estimates (older texts list 36–38); exact number depends on shuttle systems and membrane proton leak.
Alternatives / anaerobic conditions
- If oxygen is absent, cells use fermentation (e.g., lactic acid or alcoholic fermentation) to regenerate NAD+ from NADH so glycolysis can continue; fermentation yields far less ATP (2 ATP per glucose) and produces lactate or ethanol + CO2.
- Some organisms use anaerobic respiration with alternative terminal electron acceptors (e.g., nitrate, sulfate).
Other biochemical details
- Two modes of ATP generation: substrate‑level phosphorylation (glycolysis, citric acid cycle) and oxidative phosphorylation (ETC + chemiosmosis).
- Reducing equivalents (NADH, FADH2) shuttle electrons to the ETC; the proton motive force stores energy used by ATP synthase.
(b) The function of cellular respiration
Primary functions
- Energy production: convert chemical energy stored in food molecules into ATP, the cell’s main immediate energy currency, used to power endergonic processes such as biosynthesis, active transport, muscle contraction, and signal transduction.
- Maintain redox balance: regenerate oxidized cofactors (NAD+, FAD) required for continued operation of catabolic pathways.
Secondary and additional functions
- Provide metabolic intermediates: many intermediates of glycolysis and the citric acid cycle are precursors for biosynthesis (amino acids, nucleotides, lipids, heme, etc.).
- Heat production: some of the energy released is dissipated as heat, important for thermoregulation (e.g., brown adipose tissue uncoupling proteins).
- Control of reactive oxygen species (ROS) and signaling: the ETC is a source of ROS that can act in signaling or cause oxidative stress; cells have regulatory and repair mechanisms tied to respiration.
- Flexibility and adaptation: allows cells to adapt to oxygen availability and different substrates, supporting survival in variable environments.
Summary
- Cellular respiration is the stepwise oxidation of fuels (mostly glucose) to CO2 and H2O, capturing released free energy in ATP. It couples catabolism to the cell’s energy and biosynthetic needs and is essential for growth, maintenance, and activity of living cells.
Overview
- Cellular respiration is the set of metabolic pathways that cells use to extract energy from organic molecules (most often glucose) and convert it into usable chemical energy (ATP). In eukaryotes it occurs in the cytoplasm (early steps) and mitochondria (later steps); in prokaryotes equivalent steps occur in the cytosol and across the plasma membrane.
Overall chemical equation (for complete aerobic oxidation of glucose)
- C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat)
Major stages (with location and main products per glucose)
1. Glycolysis (cytoplasm)
- One 6‑carbon glucose → two 3‑carbon pyruvate.
- Net products: 2 ATP (substrate‑level phosphorylation), 2 NADH, 2 pyruvate.
- Operates anaerobically or aerobically (pyruvate fate depends on O2).
2. Pyruvate oxidation (mitochondrial matrix in eukaryotes)
- Each pyruvate → acetyl‑CoA + CO2; reduces NAD+ → NADH.
- Per glucose: 2 acetyl‑CoA, 2 CO2, 2 NADH.
3. Citric acid cycle (Krebs cycle; mitochondrial matrix)
- Each acetyl‑CoA enters cycle and is oxidized to CO2.
- Per glucose: 4 CO2, 2 ATP (as GTP), 6 NADH, 2 FADH2 (from both acetyl‑CoA).
4. Oxidative phosphorylation (inner mitochondrial membrane)
- Electron transport chain (ETC): electrons from NADH and FADH2 pass through complexes I–IV, driving protons from the matrix into the intermembrane space and creating a proton gradient.
- Oxygen is the final electron acceptor, forming water.
- Chemiosmosis: protons flow back through ATP synthase, driving synthesis of ATP from ADP + Pi.
- Typical ATP yield from oxidative phosphorylation is variable; whole‑cellular ATP yield per glucose is roughly 30–32 ATP in modern estimates (older texts list 36–38); exact number depends on shuttle systems and membrane proton leak.
Alternatives / anaerobic conditions
- If oxygen is absent, cells use fermentation (e.g., lactic acid or alcoholic fermentation) to regenerate NAD+ from NADH so glycolysis can continue; fermentation yields far less ATP (2 ATP per glucose) and produces lactate or ethanol + CO2.
- Some organisms use anaerobic respiration with alternative terminal electron acceptors (e.g., nitrate, sulfate).
Other biochemical details
- Two modes of ATP generation: substrate‑level phosphorylation (glycolysis, citric acid cycle) and oxidative phosphorylation (ETC + chemiosmosis).
- Reducing equivalents (NADH, FADH2) shuttle electrons to the ETC; the proton motive force stores energy used by ATP synthase.
(b) The function of cellular respiration
Primary functions
- Energy production: convert chemical energy stored in food molecules into ATP, the cell’s main immediate energy currency, used to power endergonic processes such as biosynthesis, active transport, muscle contraction, and signal transduction.
- Maintain redox balance: regenerate oxidized cofactors (NAD+, FAD) required for continued operation of catabolic pathways.
Secondary and additional functions
- Provide metabolic intermediates: many intermediates of glycolysis and the citric acid cycle are precursors for biosynthesis (amino acids, nucleotides, lipids, heme, etc.).
- Heat production: some of the energy released is dissipated as heat, important for thermoregulation (e.g., brown adipose tissue uncoupling proteins).
- Control of reactive oxygen species (ROS) and signaling: the ETC is a source of ROS that can act in signaling or cause oxidative stress; cells have regulatory and repair mechanisms tied to respiration.
- Flexibility and adaptation: allows cells to adapt to oxygen availability and different substrates, supporting survival in variable environments.
Summary
- Cellular respiration is the stepwise oxidation of fuels (mostly glucose) to CO2 and H2O, capturing released free energy in ATP. It couples catabolism to the cell’s energy and biosynthetic needs and is essential for growth, maintenance, and activity of living cells.
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