Support in plants and animals: structural principles

Learning objectives

By the end of this lesson, you should be able to:

• explain why organisms need support
• distinguish support from movement
• describe how plants remain upright
• explain the roles of the cell wall, turgor and supporting tissues
• compare hydrostatic, exoskeletal and endoskeletal support

Why support is necessary

Support maintains an organism's shape, protects delicate structures and helps keep body parts in positions where they can function. In plants, support helps leaves receive light and keeps flowers and fruits suitably positioned. In animals, support maintains body form, protects internal organs and provides attachment points for muscles.

Support and movement are related but not identical. A structure may support an organism without producing movement. Movement usually requires muscles or other contractile structures acting against a supporting framework.

Support in plants

Plants do not have bones. Their support comes from the properties of cell walls, the pressure of cell contents against cell walls, and specialised tissues.

Cell walls

Plant cell walls contain cellulose microfibrils. Cellulose gives the wall tensile strength and helps prevent cells from bursting when water enters. The walls of neighbouring cells fit together to form a supporting framework throughout the plant.

Turgor pressure

When a plant cell absorbs water by osmosis, its vacuole enlarges and the cell contents press against the cell wall. The resulting internal pressure is called turgor pressure. A turgid cell is firm and helps support soft stems and leaves. When a plant loses too much water, cells become flaccid; the tissues lose firmness and the plant may wilt.

Turgor is especially important in non-woody plants. It is not the only source of support: woody plants also rely on strengthened cell walls and lignified tissues.

Supporting tissues

Collenchyma is often found beneath the epidermis of young stems and in leaf stalks. Its flexible walls allow growing parts to bend without easily breaking. Sclerenchyma fibres and sclereids provide greater rigidity. Xylem is specialised for water transport, but lignin in its walls also helps prevent collapse and supports the plant.

How plant form helps support

Roots anchor plants in the soil and resist pulling forces. Stems hold leaves and flowers above the ground. Branching spreads leaves into the light. In grasses and other plants, narrow flexible stems may bend in wind and recover, reducing the chance of snapping. In woody plants, secondary growth and lignified tissues increase strength.

Support systems in animals

Animals use three broad structural arrangements.

1. Hydrostatic skeleton: support is provided by fluid under pressure within a body cavity, against which muscles act. Earthworms use this arrangement. Circular and longitudinal muscles change body shape and help produce movement.
2. Exoskeleton: a hard supporting covering outside the body. Arthropods such as insects and crabs have a chitinous exoskeleton. It protects the body and provides surfaces for muscle attachment. Growth requires moulting because the rigid exoskeleton cannot expand continuously.
3. Endoskeleton: an internal framework of bone and/or cartilage, as in vertebrates. It supports the body, protects organs and provides attachment points for muscles. It can grow with the organism.

Comparing plant and animal support

Plant support relies heavily on cell walls, water pressure and strengthened tissues. Animal support commonly relies on a hydrostatic skeleton, an exoskeleton or an endoskeleton, with muscles acting against the supporting structure. Both systems maintain shape and resist mechanical forces, but they are built from different materials and are adapted to different patterns of growth and movement.

WASSCE examination focus

When asked how a young herbaceous plant is supported, include turgor pressure and collenchyma. For woody stems, mention lignified xylem and sclerenchyma. Do not confuse collenchyma with sclerenchyma: collenchyma is living and flexible, while mature sclerenchyma cells are usually dead and heavily lignified.

Quick check

1. Define turgor pressure.
2. Why does a plant wilt when it loses water?
3. Name the tissue that gives flexible support to young stems.
4. State one advantage and one limitation of an arthropod exoskeleton.
5. Give an example of an animal with a hydrostatic skeleton.

Answers: 1. Pressure exerted by the cell contents against the cell wall when the cell is turgid. 2. Cells lose turgor and tissues become flaccid. 3. Collenchyma. 4. It protects the body and provides muscle attachment; it must be moulted for growth. 5. Earthworm.