Tissues

In a multicellular organism, not every cell does every job. Instead, cells with a similar structure group together and take up one particular function. A tissue is defined as a group of cells that are similar in structure and are organised together to perform a specific function. This grouping of cells into tissues is called division of labour, and it allows an organism to work more efficiently, just as different departments in a school (teaching, sports, library) each focus on one job.

Plants and animals show different patterns of tissues because their needs are different. Plants are stationary (they do not move from place to place), so a large part of a plant's body is made of dead, supportive tissue that requires no energy for maintenance. Growth in plants also occurs only in certain fixed regions, such as the tips of roots and shoots. Animals, on the other hand, move about in search of food, mates and shelter, so most animal tissues are living, and growth is more uniform throughout the body since energy expenditure for movement is high and repair of worn-out tissue is constant.

Broadly, plant tissues are divided into meristematic tissue (actively dividing cells) and permanent tissue (cells that have stopped dividing and taken up a fixed role). Permanent tissue is further of two kinds: simple (made of one type of cell, e.g. parenchyma) and complex (made of more than one type of cell working together, e.g. xylem). Animal tissues are grouped into four main types: epithelial, connective, muscular and nervous tissue.

The growth of plants occurs only in certain specific regions, unlike in animals where growth is more generalised. This is because plant growth takes place mainly by cell division at specific places, called meristematic tissue. Cells of meristematic tissue are packed closely together, have a dense cytoplasm, a thin cellulose cell wall and a prominent nucleus. They usually lack vacuoles, since the cell is not yet storing anything and is fully focused on dividing.

Based on the region where they are present, meristematic tissue is of three types. Apical meristem is present at the growing tips of stems and roots and increases the length of the plant. Lateral meristem (also called cambium) is present in the lateral sides of the stem or root, such as in the bark and vascular bundles, and increases the girth (thickness) of the plant. Intercalary meristem is present at the base of leaves or internodes on either side of the older permanent tissue; it is commonly seen in grasses, which is why grass grows back quickly even after being cut or grazed.

Once a meristematic cell has divided and reached its final position in the plant body, it stops dividing further and takes up a specific role such as storage, support or transport. This process of a cell losing the ability to divide and becoming specialised is called differentiation, and the resulting tissue is called permanent tissue.

Simple permanent tissue is made up of only one type of cell, and all the cells perform the same function together. There are three kinds. Parenchyma is the most common tissue; its cells are loosely packed, thin-walled and living, and it mainly stores food. Parenchyma containing chlorophyll and carrying out photosynthesis is called chlorenchyma, while parenchyma with large air cavities that gives buoyancy to aquatic plants and helps them float is called aerenchyma. Collenchyma occurs in the leaf stalk and below the epidermis in stems; its cells are living, elongated and have unevenly thickened corners, which gives mechanical support and flexibility, allowing plant parts to bend without breaking (for example, in the stem of a young plant). Sclerenchyma is the tissue that makes a plant hard and stiff; its cells are dead, long, narrow and thick-walled due to deposition of a substance called lignin. Sclerenchyma is present in the husk of coconut, the hard covering of nut shells and seeds, in stems, in the veins of leaves, and around vascular bundles.

Epidermis is a protective tissue that forms the outermost covering of the plant, usually made of a single layer of flattened cells, often covered with a waxy, waterproof layer called the cuticle that reduces water loss.

Complex permanent tissue is made up of more than one type of cell, and all of them work together as a unit to transport substances through the plant. There are two kinds. Xylem transports water and dissolved minerals from the roots upward to the rest of the plant, and it is made of four elements: tracheids, vessels, xylem parenchyma and xylem fibres. Except for xylem parenchyma, the other three are dead cells, and their thick lignified walls also give mechanical strength to the plant. Phloem transports food (prepared during photosynthesis) from the leaves to all other parts of the plant, and it is made of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Except for phloem fibres, all the other phloem elements are living cells.

Epithelial tissue forms a covering or lining for the body, body cavities and hollow organs. Its cells are tightly packed with very little intercellular space, and it rests on a thin membrane called the basement membrane. Based on shape and function, epithelial tissue is of several types. Squamous epithelium is made of flat, thin cells fitted together like tiles, found lining the mouth, the walls of blood vessels and the alveoli of the lungs, where its thinness allows easy diffusion of substances. Cuboidal epithelium is made of cube-like cells, forming the lining of kidney tubules and the ducts of salivary glands, and it provides mechanical support. Columnar epithelium has tall, pillar-like cells and is found in the lining of the intestine, where it helps in absorption and secretion. When columnar (or cuboidal) cells bear fine hair-like projections called cilia, they are called ciliated epithelium; it is found in the trachea (windpipe), where the cilia move together to push out mucus and trapped dust particles. Some epithelial cells become specialised for secretion and form glandular epithelium, such as in sweat glands and salivary glands.

Connective tissue connects and supports different parts of the body. Unlike epithelial tissue, its cells are loosely spaced and embedded in a matrix, which may be jelly-like, fluid, dense or rigid depending on the function of the tissue. Blood is a fluid connective tissue containing plasma (matrix), red blood cells, white blood cells and platelets, and it transports gases, digested food, hormones and waste through the body. Bone forms the framework that supports the body; it has a hard matrix made of calcium and collagen. Tendons connect muscles to bones and are fibrous, strong but not very elastic. Ligaments connect one bone to another and are very elastic, allowing movement at joints. Cartilage has a smooth matrix that smoothens bone surfaces at joints and is found in the nose, ear, trachea and larynx. Areolar connective tissue fills the space inside organs, supports internal organs and helps in repair of tissues. Adipose tissue stores fat and is found below the skin and between internal organs, acting as an insulator.

Muscular tissue is made of elongated cells, also called muscle fibres, and is responsible for all forms of movement in our body. Muscle cells contain special proteins called contractile proteins, which contract and relax to cause movement. There are three types of muscular tissue. Striated (skeletal) muscle is attached to bones and helps in voluntary movements of the body (movements under our control), such as moving the arms or legs; its cells are long, cylindrical, unbranched and multinucleate, and show alternate light and dark bands. Unstriated (smooth) muscle is found in the walls of internal organs such as the stomach, intestine, blood vessels and the iris of the eye, and it causes involuntary movements (not under our control), such as movement of food through the alimentary canal; its cells are long, spindle-shaped, with a single nucleus and no bands. Cardiac muscle is the muscle of the heart; its cells are cylindrical, branched and uninucleate, and are joined end to end by special junctions called intercalated discs, which help the muscle contract as a single unit rhythmically and tirelessly throughout life without getting fatigued; it works involuntarily.

Nervous tissue is made of cells called neurons that are specialised for receiving stimuli and conducting nerve impulses rapidly from one part of the body to another. A neuron has three main parts: the cell body (cyton), containing the nucleus and most of the cytoplasm; short, branched projections called dendrites that receive the impulse; and a single, long projection called the axon that transmits the impulse away from the cell body towards another neuron or an organ. The junction between the axon of one neuron and the dendrite of the next is called a synapse, across which the impulse is passed on. Many neurons joined together form a nerve, and together nervous tissue allows the brain to control and coordinate the working of all other tissues and organs in the body.