Tissues

1. The idea of division of labour

In multicellular organisms like plants and animals, billions of cells share the work of the body. Instead of every cell doing everything, groups of cells take up specific jobs — this is called division of labour. A group of cells that are similar in structure and origin, and which together perform a particular function, is called a tissue. Blood, phloem and muscle are all examples of tissues. Because each tissue does one job very well, the organism becomes far more efficient than a single cell could ever be.

2. Why plants and animals have different tissues

Plants are stationary (fixed in one place), so most of their body is made of supportive, dead tissue that needs little energy to maintain. Plants also keep growing throughout life only in certain regions. Animals move around in search of food, mates and shelter, so they need more living tissue and consume more energy. Growth in animals is uniform (the whole body grows together), while in plants growth is limited to special dividing regions. This is why plant and animal tissues are organised very differently.

3. Plant tissues — Meristematic tissue

Meristematic tissue is made of cells that keep dividing throughout the plant’s life, helping it grow. These cells are small, have thin cell walls, dense cytoplasm, prominent nuclei and no vacuoles. Depending on where they are found, meristems are of three kinds. Apical meristem is at the tips of roots and shoots and increases their length. Lateral meristem (cambium) is on the sides of stems and roots and increases their girth (thickness). Intercalary meristem is found at the base of leaves or internodes (as in grasses) and helps in length growth there.

4. Plant tissues — Permanent tissue

When meristematic cells lose their power to divide and take up a fixed shape, size and function, they become permanent tissue. This process is called differentiation. Permanent tissues are of two types: simple (one cell type) and complex (more than one cell type).

Simple permanent tissues: Parenchyma has loosely packed living cells with thin walls; it stores food and helps in photosynthesis when it contains chlorophyll (then called chlorenchyma). In aquatic plants, parenchyma with large air spaces is called aerenchyma and helps the plant float. Collenchyma has cells thickened at the corners; it gives flexibility so plant parts can bend without breaking (found below the epidermis in stems and leaf stalks). Sclerenchyma is made of long, dead cells with thick lignified walls; it makes the plant hard and stiff (the husk of a coconut is made of sclerenchyma).

Protective tissue: The outermost layer is the epidermis, usually a single layer of cells that protects all parts of the plant. It often has a waterproof coating of cutin. Small pores called stomata in the leaf epidermis, each bordered by two kidney-shaped guard cells, allow exchange of gases and loss of water vapour (transpiration). In older woody stems, a tissue called cork forms; its dead cells have a substance called suberin that makes them impervious to water and gases.

5. Complex permanent tissues — Xylem and Phloem

Complex tissues are made of more than one type of cell working together; they make up the vascular bundles (the transport system of the plant). Xylem conducts water and minerals from roots to the rest of the plant. It is made of tracheids, vessels, xylem parenchyma and xylem fibres. Most xylem cells are dead with thick walls. Phloem conducts food (sugars) made in leaves to all other parts. It is made of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Phloem cells are living (except phloem fibres), and transport in phloem moves in both directions.

6. Animal tissues — Epithelial tissue

Epithelial tissue covers and lines body surfaces and forms the boundary that separates body systems. Its cells are tightly packed with little material between them, forming a continuous protective sheet. Types include: squamous (flat, thin — lining of mouth, blood vessels and air sacs); cuboidal (cube-shaped — lining of kidney tubules); columnar (tall, pillar-like — inner lining of the intestine); ciliated columnar (with hair-like cilia — lining of the respiratory tract, pushing out dust and mucus); and glandular epithelium which folds inward to form glands that secrete substances.

7. Animal tissues — Connective tissue

Connective tissue binds, supports and connects different organs. Its cells are loosely spaced in a jelly-like, fluid or solid matrix. Blood is a fluid connective tissue with a liquid matrix called plasma carrying red blood cells, white blood cells and platelets; it transports gases, food and wastes. Bone is strong and hard; it forms the framework that supports the body and protects organs. Cartilage is smooth and flexible (in nose, ear and joints). Tendons connect muscles to bones (tough, inelastic), while ligaments connect bone to bone (elastic). Areolar tissue fills spaces inside organs, and adipose tissue stores fat below the skin.

8. Animal tissues — Muscular tissue

Muscular tissue is made of long cells (muscle fibres) that can contract and relax to cause movement. Striated (skeletal/voluntary) muscles are attached to bones, show light and dark bands, and work under our will (e.g. arm muscles). Smooth (unstriated/involuntary) muscles are spindle-shaped, have no bands and work on their own (e.g. food pipe, blood vessels). Cardiac muscle is found only in the heart; it is involuntary but striated, and contracts and relaxes rhythmically throughout life without tiring.

9. Animal tissues — Nervous tissue

Nervous tissue is highly specialised to receive and conduct messages quickly. Its cells are called neurons (nerve cells). A neuron has a cell body with a nucleus, branched dendrites that receive signals, and a long thread-like axon that carries the signal away. A single nerve cell may be up to a metre long. Many nerve fibres bound together form a nerve. The brain, spinal cord and nerves are all made of nervous tissue, allowing the body to respond to stimuli almost instantly.