Support and Movement

Living things need support to hold their shape against gravity, and many need movement to find food, escape danger and reproduce. In CSEC Integrated Science this topic looks at how support and movement are achieved in plants and in humans, and at the importance of good posture and exercise.

Support in plants

Plants do not have bones. A young, non-woody plant is held up mainly by turgor pressure. When plant cells take in water by osmosis, the water pushes the cell contents against the cell wall, making each cell firm (turgid). Millions of turgid cells together keep stems and leaves rigid and upright.

If a plant loses too much water (for example in dry soil or hot sun), the cells lose water and become flaccid, the turgor pressure drops, and the plant wilts. Watering restores turgor and the plant becomes firm again.

Larger plants and trees also have extra support from:

• Lignin, a strong, woody material deposited in the walls of xylem vessels, which strengthens the stem;
• the arrangement of strengthening tissue towards the outside of the stem, which resists bending.

Support and movement in humans — the skeleton

Humans have an internal bony framework, the skeleton, which has several functions:

• Support — holds the body upright and gives it shape.
• Protection — the skull protects the brain, the ribcage protects the heart and lungs, the backbone protects the spinal cord.
• Movement — bones act as levers that muscles pull on.
• Production of blood cells in the marrow of some bones.
• Storage of minerals such as calcium and phosphate.

Joints

A joint is where two or more bones meet. Movable (synovial) joints allow movement and have features that reduce friction and cushion the bones:

• Cartilage covers the ends of the bones, providing a smooth surface and absorbing shock.
• Synovial fluid lubricates the joint.
• Ligaments hold the bones together.

Two important types to know:

• Hinge joint (elbow, knee) — allows movement in one plane, like a door hinge.
• Ball-and-socket joint (shoulder, hip) — allows movement in all directions.

How muscles produce movement

Muscles can only pull (contract); they cannot push. So they work in antagonistic pairs — one muscle reverses the action of the other. At the elbow:

• To bend (flex) the arm, the biceps contracts and the triceps relaxes.
• To straighten (extend) the arm, the triceps contracts and the biceps relaxes.

Muscles are attached to bones by tendons, which are tough and inelastic so that the pull of the muscle is transferred directly to the bone. The bone acts as a lever and the joint as the pivot.

The structure of a bone

Bone is a living tissue, not just a dry framework. A typical long bone (such as the femur in the thigh) has:

• a hard outer layer (compact bone) that gives strength;
• a lighter spongy bone towards the ends, which reduces weight while staying strong;
• a central cavity containing bone marrow, where red blood cells are made;
• a covering membrane and a supply of blood vessels and nerves, showing it is alive and can grow and repair.

Bone is hard because the living cells lay down minerals — mainly calcium and phosphate — around themselves. This is why a diet lacking calcium or vitamin D leads to weak, soft bones (rickets in children). The combination of mineral hardness and a living, slightly flexible framework lets bone resist both crushing and bending.

Bones as levers

When a muscle pulls on a bone, the bone turns about a joint, which acts as a pivot (fulcrum). This is exactly how a lever works, and it is why a small movement of a muscle near the joint can produce a large, fast movement at the end of a limb. For example, when the biceps contracts only a few centimetres, the hand at the far end of the forearm moves a much greater distance. The trade-off is that the muscle must pull with a large force to move a smaller load at speed — the body is built for rapid movement rather than for lifting with the least effort. Recognising the limb as a lever system (effort = muscle, load = the weight moved, pivot = joint) links this topic to the physics of moments.

Comparing support in plants and animals

It is worth seeing the two systems side by side:

This contrast explains why a plant wilts on a hot day but an animal does not, and why animals can move quickly while plants cannot.

Posture, exercise and health

Good posture means holding the body so that the least strain is placed on muscles and ligaments. Poor posture — slouching, lifting heavy loads incorrectly, carrying an unbalanced bag — can cause back pain and long-term damage to the spine. You should lift with the back straight and the knees bent, using the strong leg muscles.

Exercise keeps the support and movement system healthy:

• it strengthens muscles and improves their tone;
• it keeps joints flexible and mobile;
• it strengthens bones, helping to prevent weakening (such as osteoporosis later in life);
• it improves the supply of blood, and therefore oxygen and glucose, to the muscles.

Lack of exercise leads to weak muscles, stiff joints and poor posture.

Common exam mistakes

• Saying muscles "push" a bone back — muscles only pull; the antagonistic partner produces the opposite movement.
• Confusing tendons (muscle to bone) with ligaments (bone to bone).
• Forgetting that plant support comes mainly from turgor pressure (water), not from any skeleton.
• Listing skeleton functions but giving only one; aim to give support, protection, movement and one other.

Key terms to remember

• Turgor pressure — the pressure of water pushing the cell contents against the wall, supporting non-woody plants.
• Flaccid / wilting — what happens when plant cells lose water and turgor drops.
• Lignin — the woody material in xylem that gives extra support to stems.
• Skeleton (endoskeleton) — the internal bony framework that supports and protects the body.
• Joint — where bones meet; hinge (one plane) or ball-and-socket (all directions).
• Cartilage — smooth, shock-absorbing tissue at the ends of bones.
• Ligament — connects bone to bone; tendon — connects muscle to bone.
• Antagonistic pair — two muscles (e.g. biceps and triceps) that produce opposite movements.

Quick recap

• Plants are supported mainly by turgor pressure; loss of water makes cells flaccid and the plant wilts. Lignin in xylem gives extra strength.
• The human skeleton supports and protects the body, allows movement, makes blood cells and stores minerals.
• Joints (hinge and ball-and-socket) contain cartilage, synovial fluid and ligaments to allow smooth movement.
• Muscles work in antagonistic pairs (e.g. biceps and triceps) and pull on bones via tendons.
• Good posture and regular exercise keep muscles, joints and bones healthy.