Year 1 · Organs and cells · Section 2.3

Tissues

Cells

Organisms consist of one or more cells, the basic building blocks of life. Most cells are so small that you need a microscope to see them. Cells have different shapes: some are rounded or cube-shaped, and some have long extensions. A human body contains roughly 30 trillion cells.

Human tissues

Cells with a similar structure and function form a tissue. Examples include:

• Bone tissue: Contains bone cells.
• Muscle tissue: Found in muscles.
• Nervous tissue: Found in the brain and nerves, including nerves supplying the heart.

Organs such as the heart usually contain several tissue types.

Extracellular matrix

Many tissues contain extracellular matrix: material between the cells. Its properties vary. It may be soft or fluid-rich, or hard and mineral-rich, as in bone.

The matrix can provide support. Some cells connect with other cells through extensions.

Plant tissues

Plant organs are also made of tissues. Examples include:

• Epidermis: A protective layer of cells on the upper and lower surfaces of a leaf.
• Stomata: Small openings in the epidermis, often mainly on the underside of a leaf.

Stomata allow carbon dioxide to enter and oxygen and water vapour to leave. Opening and closing them helps regulate water loss. When water loss is too high, stomata may close.

Annual rings

The cambium is a thin layer of dividing cells beneath a tree's bark. It produces new wood towards the inside of the trunk.

In many temperate trees, spring growth produces larger cells and lighter wood. Later growth produces smaller cells and darker wood. Growth slows or stops during autumn and winter.

Together, early and late wood form one annual ring. The oldest wood is near the centre of the trunk. Counting rings near the base can help estimate a tree's age.