OxfordAQA International A Level Biology 3.4.4 Skeletal Muscles as Effectors Exam Questions

About OxfordAQA International A Level Biology 3.4.4 Skeletal Muscles as Effectors

Skeletal muscles as effectors in Unit 4 explains how muscles contract. You need to describe the gross and microscopic structure of skeletal muscle and the ultrastructure of a myofibril, then use the sliding filament theory to explain contraction, including the roles of actin, myosin, calcium ions, tropomyosin and ATP. You should be able to interpret diagrams and electron micrographs of myofibrils in different states of contraction. You also need to explain antagonistic muscle pairs, the roles of ATP and phosphocreatine in supplying energy, and the structure, location and properties of slow and fast muscle fibres. It links to synapses, respiration and homeostasis.

What you need to know

• Muscle structure Skeletal muscle is made of muscle fibres containing many myofibrils. Myofibrils are divided into sarcomeres with thick myosin filaments and thin actin filaments, giving dark A bands and light I bands, with the H zone in the middle and Z lines at each end.
• Starting contraction An action potential spreads into the fibre and calcium ions are released from the sarcoplasmic reticulum. Calcium ions cause tropomyosin to move, exposing binding sites on actin.
• Cross-bridge cycle Myosin heads bind to actin to form actomyosin cross-bridges, then pivot, pulling the actin filaments towards the centre of the sarcomere. ATP binding detaches the myosin head, and hydrolysis of ATP returns the head to its original position.
• Changes in the sarcomere During contraction the sarcomere, I band and H zone shorten, but the A band stays the same length because myosin filaments do not shorten.
• Energy supply ATP is the immediate energy source. Phosphocreatine regenerates ATP rapidly by giving its phosphate to ADP, supplying energy for short, intense bursts of activity.
• Slow and fast fibres Slow fibres contract slowly for long periods, respire aerobically and have many mitochondria, much myoglobin and a good blood supply. Fast fibres contract quickly but tire quickly, rely more on anaerobic respiration and have more glycogen and phosphocreatine.

Common mistakes in 3.4.4 Skeletal Muscles as Effectors

• Saying the actin and myosin filaments shorten. They slide past each other; the filaments stay the same length.
• Saying ATP is needed for the myosin head to bind to actin. ATP is needed to detach the head and to re-cock it.
• Saying the A band shortens during contraction. The A band stays the same; the I band and H zone shorten.
• Saying muscles push. Muscles can only pull, so they work in antagonistic pairs against an incompressible skeleton.

Try three 3.4.4 Skeletal Muscles as Effectors exam-style questions