Tissues Notes

In unicellular organisms a single cell performs all functions. In multicellular organisms, groups of cells take up specific jobs — this is division of labour. Cells specialised for one function are grouped together as a tissue, which makes the work more efficient.

In plants, since they are fixed and much of their body is made of dead cells that still provide mechanical strength, a lot of supportive tissue is present. Plants grow mainly at certain points (tips of roots and shoots).

Animals move around, so they use more energy and have more muscular tissue; their growth is uniform over the body, and living tissue makes up most of the body.

Meristematic tissue consists of actively dividing cells. The cells are small, thin-walled, with dense cytoplasm, a large nucleus, and no or very small vacuoles.

By position: apical meristem is at the tips of roots and shoots and increases length (primary growth); lateral meristem (cambium) lies along the sides and increases girth (secondary growth); intercalary meristem lies at the base of leaves or internodes (as in grasses) and helps in regrowth after grazing.

As the cells produced by meristems mature and take up a permanent shape, size, and function, they form permanent tissue — this process is called differentiation.

Simple permanent tissues have one type of cell. Parenchyma: living, thin-walled cells with intercellular spaces; stores food, and when it contains chlorophyll (chlorenchyma) it does photosynthesis; aerenchyma in aquatic plants has large air spaces that help floating. Collenchyma: living cells thickened at the corners; provides flexibility to stems and leaf stalks so they can bend without breaking. Sclerenchyma: dead cells with thick lignified walls and no living contents; makes plant parts hard and stiff (e.g. husk of coconut, seed coats).

Complex permanent tissues have more than one type of cell working together for conduction. Xylem carries water and dissolved minerals upward from roots to the rest of the plant; it is made of tracheids, vessels, xylem parenchyma (living) and xylem fibres, and most of it is dead. Phloem carries food (made in leaves) to all parts; it is made of sieve tubes, companion cells, phloem parenchyma, and phloem fibres, and is mostly living.

The outermost protective layer of the plant body is the epidermis, often coated with a waxy cutin to reduce water loss; in older woody stems and roots this is replaced by cork, whose dead cells have suberin and make the surface impervious to gases and water.

Epithelial tissue is a covering or lining tissue; its cells are tightly packed with little material between them, resting on a basement membrane. Squamous epithelium (flat cells) lines the mouth, blood vessels, and air sacs and allows diffusion; cuboidal epithelium lines kidney tubules and glands; columnar epithelium lines the intestine and may bear cilia (ciliated columnar) to move mucus, as in the respiratory tract; glandular epithelium forms glands.

Connective tissue joins, supports, and packs organs, and its cells are loosely spaced in a matrix. Blood has a fluid matrix (plasma) with red cells, white cells, and platelets, and transports gases, food, wastes, and hormones. Bone is a hard connective tissue with a matrix of calcium and phosphorus; cartilage is softer and smooths joint surfaces (also in nose and ear). Ligaments connect bone to bone; tendons connect muscle to bone. Areolar tissue fills spaces inside organs; adipose tissue stores fat below the skin and around organs.

Muscular tissue has elongated 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 are under our control. Smooth (unstriated, involuntary) muscles are found in the walls of the stomach, intestine, blood vessels, and are not under our control. Cardiac muscle is found only in the heart, is striated but involuntary, and contracts rhythmically without tiring throughout life.

Nervous tissue is made of neurons. A neuron has a cell body with a nucleus and cytoplasm, from which fine branches (dendrites) and a single long fibre (axon) arise. Neurons are highly specialised to receive stimuli and conduct impulses very rapidly from one part of the body to another; nerves are bundles of axons.

Together, muscular and nervous tissue let animals respond quickly to changes and move — abilities that plants, with their different lifestyle, do not need in the same way.