Class 9 Science Chapter 3: Tissues in Action — Notes, Examples & Practice

A tissue is a group of cells, similar in structure (or working together), that perform a particular function. Blood, bone, xylem and the lining of the stomach are tissues.

Tissues combine into organs (a leaf, the heart), organs into organ systems (the digestive system), and organ systems into an organism. This division of labour makes large, complex bodies possible.

Plants and animals organise tissues differently. Plants are fixed in one place; many of their tissues are dead and supportive, and growth happens only in certain regions. Animals move about and use more energy, so most of their tissues are living, and growth is more uniform.

Meristematic tissue is made of actively dividing cells. The cells are small, thin-walled, packed closely without spaces, with a large nucleus, dense cytoplasm and few or no vacuoles — everything is geared to making new cells.

Apical meristem at the tips of roots and shoots makes the plant longer. Lateral meristem (cambium), in a ring in stems and roots, makes them thicker — that is how a teak tree trunk widens year after year. Intercalary meristem at the base of leaves or at nodes (as in grasses and bamboo) lets those parts grow back after being cut or grazed.

As new cells mature they lose the ability to divide and become specialised: this is differentiation, which produces permanent tissues.

Simple permanent tissues are made of one kind of cell. Parenchyma has living, thin-walled, loosely packed cells with spaces between them; it stores food and, when it contains chloroplasts, it is called chlorenchyma (photosynthesis). In water plants, parenchyma with large air spaces (aerenchyma) helps them float.

Collenchyma has living, elongated cells thickened at the corners; it gives flexible support, which is why leaf stalks and young stems bend without breaking.

Sclerenchyma has dead cells with walls thickened by lignin, often with no spaces; it gives strength and hardness — the husk of a coconut, the fibres of jute, the gritty texture of a pear. The epidermis is the outer protective layer; it often secretes a waxy cuticle to cut water loss and has stomata for gas exchange and transpiration. In older stems the epidermis is replaced by cork, with dead cells containing suberin.

Complex tissues contain more than one type of cell working together. Xylem and phloem together form vascular bundles, the plant's transport system.

Xylem carries water and minerals upward from roots to leaves. It has tracheids and vessels (dead, lignified tubes), xylem parenchyma (living, stores food) and xylem fibres (support). Water moves in one direction only — up.

Phloem carries food (sugars) made in the leaves to all parts of the plant — up or down. It has sieve tubes (living cells with perforated end walls), companion cells, phloem parenchyma and phloem fibres (the only dead part). Removing a ring of bark (girdling or debarking) cuts the phloem, so the roots starve and the tree eventually dies.

Epithelial tissue covers the body and lines organs and cavities. Its cells fit tightly with little space between them, resting on a basement membrane. Squamous epithelium (flat cells) lines blood vessels and lung air sacs, where thinness allows quick exchange; stratified squamous (many layers) forms skin and protects against wear. Cuboidal epithelium lines kidney tubules and glands; columnar epithelium lines the intestine for absorption, and ciliated columnar epithelium in the windpipe sweeps mucus away.

Connective tissue binds, supports and transports. Its cells are spread out in a matrix. Blood has a fluid matrix (plasma) with red cells, white cells and platelets. Bone has a hard matrix of calcium and phosphorus. Cartilage has a flexible matrix (nose tip, ear, ends of bones). Areolar tissue fills spaces between organs. Adipose tissue stores fat and insulates.

Tendons connect muscle to bone — strong but not very elastic. Ligaments connect bone to bone at joints — strong and elastic, allowing movement at hinge, ball-and-socket and pivot joints.

Muscle tissue is made of long cells (muscle fibres) with contractile proteins that shorten to cause movement. Skeletal (striated) muscle is attached to bones and under our control (voluntary); its cells are long, cylindrical, unbranched and multinucleate, with light and dark bands.

Smooth muscle (unstriated) lines the stomach, intestine, blood vessels and iris; it is involuntary, with spindle-shaped cells and one nucleus each. Cardiac muscle in the heart is involuntary and contracts rhythmically all life long; its cells are cylindrical, branched and uninucleate.

Nervous tissue in the brain, spinal cord and nerves is made of neurons. A neuron has a cell body with a nucleus, short branched dendrites that receive signals, and a long axon that carries the signal away. Neurons can be very long — up to about a metre — and carry messages as electrical impulses.