Edexcel International A Level Biology Respiration, Muscles and the Internal Environment Exam Questions

About Edexcel International A Level Biology Respiration, Muscles and the Internal Environment

This topic is part of Unit 5 (Respiration, Internal Environment, Coordination and Gene Technology) of Edexcel International A Level Biology. It covers aerobic respiration in detail, from glycolysis to the Krebs cycle and oxidative phosphorylation, and anaerobic respiration with lactate. You study skeletal muscle structure, the sliding filament theory and fast and slow twitch fibres. The topic then explains how heart rate and ventilation are controlled during exercise, and how homeostasis uses negative feedback, including thermoregulation by the hypothalamus and osmoregulation by the kidney and ADH. Respirometer and spirometer practicals, and data on exercise, are frequently examined.

What you need to know

• Aerobic respiration Glycolysis in the cytoplasm forms pyruvate; the link reaction and Krebs cycle in the matrix produce CO2, reduced NAD and reduced FAD; oxidative phosphorylation on the inner mitochondrial membrane makes most ATP by chemiosmosis.
• Anaerobic respiration Pyruvate is reduced to lactate, regenerating NAD so glycolysis continues. Lactate is converted back to pyruvate or glucose in the liver after exercise.
• Muscles Muscles work in antagonistic pairs. In contraction, Ca2+ binds troponin, tropomyosin moves, myosin heads bind actin and ATP is used for the power stroke and detachment. Slow twitch fibres have many mitochondria and myoglobin.
• Heart and ventilation control The SAN sets heart rhythm; the cardiovascular control centre adjusts rate. A spirometer measures tidal volume, vital capacity and oxygen consumption; the ventilation centre responds to rising CO2.
• Thermoregulation The hypothalamus detects blood temperature changes and triggers vasodilation, sweating or shivering and vasoconstriction through negative feedback.
• Kidney and osmoregulation Ultrafiltration in the glomerulus is followed by selective reabsorption in the proximal convoluted tubule. The loop of Henle creates a salt gradient, and ADH increases water reabsorption from the collecting duct.

Common mistakes in Respiration, Muscles and the Internal Environment

• Saying the Krebs cycle produces most ATP; most ATP comes from oxidative phosphorylation.
• Writing that muscles contract when actin and myosin shorten; the filaments slide past each other.
• Describing vasodilation as blood vessels moving closer to the skin; arterioles widen so more blood flows near the surface.
• Saying ADH makes urine more dilute; it makes urine more concentrated.
• Forgetting that oxygen is the final electron acceptor in the electron transport chain.

Try three Respiration, Muscles and the Internal Environment exam-style questions