Energy and Respiration Overview

Overview

This lecture covers CIE Topic 12: Energy and Respiration, focusing on key biochemical processes that provide energy to organisms, including ATP structure, aerobic and anaerobic respiration, and plant adaptations for respiration under low-oxygen conditions.

Energy in Living Organisms

• All organisms require energy for metabolic processes like active transport, movement, and building polymers.
• Energy is obtained from breaking bonds in molecules such as glucose and used to form new bonds in biosynthesis.
• Energy cannot be created or destroyed, only transformed.

ATP: Structure and Function

• ATP (adenosine triphosphate) is an immediate energy source for cellular processes.
• ATP is a nucleotide derivative made of adenine (nitrogenous base), ribose (pentose sugar), and three inorganic phosphates.
• Energy is released when the bond between phosphates is broken (hydrolysis), forming ADP and Pi.
• ATP formation is a condensation reaction (ADP + Pi → ATP + H₂O) catalyzed by ATP synthase.
• Phosphorylation adds a phosphate, making molecules more reactive.

Respiratory Substrates and Energy Yield

• Carbohydrates, lipids, proteins, and alcohol can serve as respiratory substrates with different energy values per gram.
• Lipids provide the highest energy yield; proteins and carbohydrates provide less.

Glycolysis

• Glycolysis occurs in the cytoplasm and does not require oxygen.
• Glucose is phosphorylated using 2 ATP, forming fructose 1,6-bisphosphate, then split into two triose phosphate (TP) molecules.
• TPs are oxidized, forming two pyruvate, two reduced NAD (NADH), and a net gain of 2 ATP (4 produced, 2 used).

Link Reaction and Krebs Cycle

• Pyruvate is transported into the mitochondrial matrix via active transport.
• Link reaction: Pyruvate oxidized (loses hydrogen and CO₂) to acetate, forms reduced NAD, and combines with coenzyme A to form acetyl CoA.
• Krebs cycle: Acetyl CoA combines with oxaloacetate to form citrate; multiple redox reactions produce reduced NAD, reduced FAD, ATP, and CO₂.
• Per glucose: 6 reduced NAD, 2 reduced FAD, 2 ATP, 4 CO₂ produced from two cycles.

Oxidative Phosphorylation

• Occurs on the inner mitochondrial membrane (cristae).
• Reduced NAD and FAD donate hydrogen, splitting into protons and electrons.
• Electrons travel along the electron transfer chain, releasing energy to pump protons, creating an electrochemical gradient.
• Protons return via ATP synthase, powering ATP production.
• Oxygen acts as the final electron acceptor, forming water.

Anaerobic Respiration

• Occurs in cytoplasm when oxygen is absent; only glycolysis proceeds.
• Pyruvate is reduced to lactate (animals) or ethanol and CO₂ (plants/microbes) to regenerate NAD.
• Only 2 ATP produced per glucose anaerobically; lactate buildup causes muscle fatigue and requires oxygen to be cleared.

Rice Plant Adaptations

• Develop aerenchyma (air-filled spaces) for gas exchange in flooded soils.
• Perform ethanol fermentation in roots under anaerobic conditions to produce ATP.
• Allocate resources to above-ground parts to maintain photosynthesis and growth.

Key Terms & Definitions

• ATP (Adenosine Triphosphate) — Energy currency molecule with adenine, ribose, and three phosphate groups.
• Phosphorylation — Addition of a phosphate group to a molecule.
• Glycolysis — Cytoplasmic process breaking glucose into pyruvate, producing ATP and NADH.
• Link Reaction — Conversion of pyruvate to acetyl CoA in mitochondria, producing NADH and CO₂.
• Krebs Cycle — Series of reactions in mitochondrial matrix, generating ATP, NADH, FADH₂, and CO₂.
• Oxidative Phosphorylation — ATP production via electron transport chain and chemiosmosis.
• Reduced NAD/FAD (NADH/FADH₂) — Coenzymes carrying hydrogen/electrons for ATP synthesis.
• Anaerobic Respiration — Energy production without oxygen, yielding lactate or ethanol.
• Aerenchyma — Air-filled plant tissue aiding gas exchange in waterlogged conditions.

Action Items / Next Steps

• Review energy yields of different respiratory substrates.
• Memorize key stages and locations of aerobic respiration.
• Practice diagrams for glycolysis, link reaction, Krebs cycle, and oxidative phosphorylation.
• Study rice plant adaptations for anaerobic conditions.
• Prepare for practical respirometer experiments and RQ calculations.