Step 1. Collagen is a structural protein — its physiological role is to provide tensile strength and structural support in connective tissue such as skin, bone, tendons and cartilage.

Step 2. The protein 'responsible for muscle contraction' is Myosin (working together with actin) — a contractile protein of muscle fibres.

Step 3. Immunoglobulin IgG is an antibody; its physiological role is immune defence — recognising and binding foreign antigens/pathogens.

Step 4. The protein 'significant in respiration' is Haemoglobin — it binds and transports oxygen in the blood.

Step 5. Fibrinogen's physiological role is blood clotting (coagulation) — it is converted to fibrin to form a blood clot.

Match each named protein (or blank) to its standard, well-esta …

• Naming actin alone without myosin (or vice versa) for muscle contraction — both work together, though myosin is the motor protein most directly 'responsible' for the contractile power stroke. …