Specialised Structures & Osmosis

LEAVES

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Leaves come in many shapes and sizes. Even on the same plant, leaves can show variation in their shape and size dependent on their position on the plant. Leaves lower down on the plant, or those on plants found in low light environments (i.e. on the forest floor), tend to be bigger in order to maximise their exposure to sunlight.

STOMATA

The process of photosynthesis requires a regular supply of carbon dioxide and water. Water is obtained from the roots. Carbon dioxide diffuses into the leaf from the atmosphere through tiny pores called stomata. Stomata are usually only present on the underside of the leaf as they are also a major route for water loss (in the form of water vapour). Stomata are surrounded by two guard cells.When water is plentiful, guard cells take in water from surrounding cells by the process of osmosis, causing them to swell. The side of the guard cell facing the stoma has a thickened wall. When they swell, the thickened wall prevents that side of the guard cell from stretching. This causes the guard cell to bend, resulting in the stoma opening.

Guard cells take in water from surrounding cells by the process of osmosis, causing them to swell. The side of the guard cell facing the stoma has a thickened wall. When they swell, the thickened wall prevents that side of the guard cell from stretching. This causes the guard cell to bend, resulting in the stoma opening.

Guard cells lose water to surrounding cells by the process of osmosis, causing them to become flacid. This causes the thickened walls to come closer together closing the stomatal pore preventing further water vapour from escaping.

ROOTS

Roots have three primary functions: to absorb water, to anchor the plant in the soil, to store food (in the form of starch). Meristematic cells in the root tip divide rapidly producing new cells. These cells elongate as their vacuoles fill with water before differentiating and maturing. The root tip is protected by a slime-producing protective layer called a root cap. The slime lubricates the root tip as it pushes through the soil. Root hairs increase the surface area of the root, increasing its ability to absorb water. Xylem cells form a star-shaped area in the middle of the root. These cells are surrounded by phloem cells. Xylem and phloem tissue is encased in a ring of endodermal cells. The outermost layers consist of cortex tissue surrounded by epidermis.

OSMOSIS

The cell membrane of plant cells allow certain small particles to pass through them and not others. They are partially (or selectively) permeable. Osmosis is the name for the process whereby water moves from an area of high concentration to an area of low concentration through a selectivity-permeable membrane. Osmosis is vitally important to plants. Plants gain water by osmosis through their roots, and it is osmosis that moves water into plant cells, making them turgid or stiff, and thus able to hold the plant upright.

Specialised Structures - Leaves

Specialised Structures - Stomata

Specialised Structures - Roots

Osmosis