Respiration: MDCAT Biology notes

Cellular respiration and ATP

Bioenergetics is the quantitative study of energy relationships in living systems. Metabolism is the sum of all chemical reactions in a cell. Cellular respiration is the oxidation of food, mainly glucose, to release energy, which is stored in ATP, the cell's energy currency. Breaking the terminal phosphate bond of ATP releases about 7.3 kcal/mol. Cells do not use the energy of glucose all at once, because such a large release would be lost as heat. Instead, it is released in small steps and captured as ATP.

Coenzymes carry hydrogen and electrons: $\mathrm{NAD^+}$ and FAD in respiration, and NADP (which has three phosphate groups) in photosynthesis.

Stage 1: Glycolysis (cytoplasm)

Glycolysis is the oldest and most primitive respiratory pathway. It needs no oxygen and happens in both aerobic and anaerobic respiration. One glucose (6C) is oxidized to two pyruvate (3C) molecules.

• Preparatory phase: 2 ATP are used to phosphorylate glucose. Glucose-6-phosphate is isomerized to fructose-6-phosphate. Fructose-1,6-bisphosphate splits into G3P and DHAP, and DHAP is isomerized to G3P.
• Oxidative (payoff) phase: each G3P is oxidized, making 2 NADH in total and 4 ATP by substrate-level phosphorylation. The last step forms pyruvate.
• Net gain: 2 ATP and 2 NADH per glucose.

Stage 2: Pyruvate oxidation and Krebs cycle (mitochondrial matrix)

With oxygen present, pyruvate enters the mitochondrion. It is decarboxylated and oxidized to acetyl CoA, releasing $\mathrm{CO_2}$ and forming NADH. Acetyl CoA is the common entry point for carbohydrates, fats and proteins.

In the Krebs (citric acid) cycle, acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Oxaloacetate is regenerated at the end, with malate being oxidized by $\mathrm{NAD^+}$ in the last step. The enzymes are in the matrix. Each acetyl CoA yields 3 NADH, 1 $\mathrm{FADH_2}$, 1 ATP (as GTP) and 2 $\mathrm{CO_2}$.

Stage 3: Electron transport chain (inner mitochondrial membrane)

• NADH and $\mathrm{FADH_2}$ pass electrons to coenzyme Q and then along the cytochromes.
• Each carrier is first reduced (it accepts electrons) and then oxidized (it passes them on).
• Molecular oxygen is the final electron acceptor and forms water. Reducing one $\mathrm{O_2}$ takes 4 electrons, which come from 2 NADH.
• The energy released pumps protons, and ATP synthase makes ATP. This is oxidative phosphorylation, explained by Mitchell's chemiosmosis, which combines electrochemical and osmotic events.
• End products: ATP and water. Each NADH gives about 3 ATP and each $\mathrm{FADH_2}$ about 2.

Anaerobic respiration (fermentation)

Fermentation gives only the 2 ATP of glycolysis, about 2% of the energy in glucose. Alcoholic fermentation is used in baking (the $\mathrm{CO_2}$ raises dough). Lactic acid build-up in muscle causes fatigue, pain and cramps.

Key formulas

$$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy\ (36\text{–}38\ ATP)}$$

• Eukaryotes: about 36 ATP per glucose. Prokaryotes: 38 ATP, because no ATP is spent moving NADH into mitochondria.
• Yeast: $\mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + 2\ ATP}$

Common MDCAT traps

• Glycolysis uses 2 ATP and makes 4, so the net is 2.
• Oxygen is the final electron acceptor, not $\mathrm{NAD^+}$ or $\mathrm{NADP^+}$.
• Yeast gives ethanol and $\mathrm{CO_2}$. Muscle gives lactate only, with no $\mathrm{CO_2}$.
• Carriers are reduced first, then oxidized.
• High energy consumption is not a consequence of muscle fermentation. Cramps, pain and tiredness are.

Quick revision

• Glycolysis happens in the cytoplasm; the Krebs cycle in the matrix; the ETC on the inner membrane (cristae).
• The end product of glycolysis is pyruvate.
• Acetyl CoA combines with oxaloacetate to form citrate.
• Aerobic end products: $\mathrm{CO_2}$, water and energy.
• Chemiosmosis makes ATP during the ETC.