Skeleton and joints, structure and movement

The skeleton is the internal framework of bones that supports the body and allows movement, working together with muscles, tendons and ligaments at each joint.

A joint is where two or more bones meet, and its structure decides how much and in which directions the bones can move. Exam questions often use the elbow or knee and test whether you can explain movement as the pulling action of muscles across a joint, so it helps to know both the parts of a joint and how a pair of muscles works together.

Functions of the skeleton

• Support, holds the body upright and gives it shape
• Protection, for example, the skull protects the brain and the ribcage protects the heart and lungs
• Movement, bones act as levers, with muscles attached to allow movement
• Production of blood cells, carried out by bone marrow inside certain bones
• Mineral storage, bones store minerals such as calcium and phosphorus

Parts and functions

How structure suits function

Movement at a joint depends on muscles pulling on bones across it, and on the joint being shaped and lubricated for the motion it must allow.

Because the mark scheme rewards the link between a feature and its role, learn each part with the reason it is there.

• Muscles can only pull by contracting, so joints are moved by antagonistic pairs, one muscle contracts to move the bone one way while the other relaxes, then the roles reverse.
• A hinge joint has flat, cylindrical bone surfaces that restrict movement to one plane, giving stability for actions like walking.
• A ball-and-socket joint has a rounded head in a cup-shaped socket, allowing movement in almost any direction, such as swinging the arm.
• Cartilage covers the bone ends and absorbs shock while reducing friction as the bones move.
• Synovial fluid lubricates the joint, and ligaments hold the bones together so the joint stays stable during movement.

Related processes

Movement is produced by the muscular system acting on the skeleton. At the elbow, the biceps and triceps form an antagonistic pair: the biceps contracts and pulls on the forearm bone through its tendon to bend (flex) the arm, while the triceps relaxes; to straighten (extend) the arm, the triceps contracts and the biceps relaxes.

A bone acts as a lever, the joint as a fulcrum and the muscle provides the effort, so a small movement of the muscle produces a larger movement of the bone.

These movements are controlled by the nervous system: nerve impulses travel from the brain or spinal cord along motor neurons to the muscles, telling them when to contract. This is why the skeleton and joints link closely to the topics of coordination and response, and to the neuron and reflex arc, where the same muscles act as effectors.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): The diagram shows the human arm at the elbow, with the biceps and triceps muscles. (a) Name the type of joint at the elbow. [1 mark] (b) Describe how the biceps and triceps work together to bend the arm. [3 marks] (c) State the function of the tendon. [1 mark] (d) Explain how cartilage and synovial fluid make movement at the joint easier. [2 marks]

Model answer: (a) A hinge joint. (b) The biceps contracts and the triceps relaxes; the biceps pulls on the forearm bone so the arm bends (flexes) at the elbow.

(c) The tendon attaches the muscle to the bone and transmits the force of contraction. (d) Cartilage covers the bone ends and reduces friction and absorbs shock, while synovial fluid lubricates the joint, so the bones move smoothly with little friction or wear.

Marking note: in part (b) you must say which muscle relaxes as well as which contracts; naming only the contracting muscle scores part marks.

Source:SRC-DSKP-EN

Frequently asked questions