support and movement

When plant cells take up lots of water by osmosis they become rigid, like a blown up balloon. We say they are turgid. This provides a lot of support to plants. When a plant dries out the cells lose their turgidity, which is why the leaves start to wilt.


Most of the largest plants we see, such as trees, have wood in them. Wood provides a lot of support and prevents branches from breaking or from wilting if they dry out. Lignin is the substance produced in the cells of woody plants that gives them this structure.

The skeleton of a mammal is made of bone and cartilage. Bone is a lot stronger than cartilage, because it needs to provide support for the body and a firm structure for muscles to attach to so that they can move parts of the body. Cartilage is the softer substance found in your ears, in your nose and between the joints. Cartilage does not contain as much calcium as bone, so it is more flexible which is why you can bend your ears. It is also smoother, which is why it is found in joints to stop the bones from rubbing together.

You need to know the names and positions of the following parts of the arm: the ulna, radius, humerus, scapula, tendons, biceps an triceps. The two bones in the forearm are the radius and the ulna. The ulna is the one that forms the elbow. The humerus is the bone in the upper arm. The scapulas are also known as the shoulder blades and are the plates located on your upper back. The biceps and the triceps are found either side of the humerus. When the biceps contracts it pulls on the bones in the forearm and bends the arm. When the triceps contracts it pulls on the part of the ulna that sticks out to make the elbow and this straightens the arm.

The contraction of these muscles produces a turning effect, with the elbow acting as the pivot. You may remember from your physics that the ?moment? (or turning force) is the force multiplied by its perpendicular distance (distance at right angles) from the pivot. This means that there is a bigger moment if the force is not acting near the pivot. This is why door handles are not near the hinge. The design of the human arm has the muscles attached very close to the pivot. Therefore they need to produce very large forces in order to move the arm. The arm could lift much heavier weights if the muscle was attached near the hand, but of course it would get in the way. You might be asked to make some moments calculations to do with the arm.

A joint occurs wherever two bones meet. Not all joints can move (for instance the various bones in your skull are fixed in place). A synovial joint allows the movement of two bones without the bones rubbing together and causing damage. The bones have a layer of cartilage at the ends and synovial fluid in the gap between the bones helps reduce friction, a bit like oil in a motor.