Tissues

A tissue is a group of cells that are similar in structure and work together to do a specific job. Multicellular organisms use the division of labour: instead of every cell doing every job (as a single-celled Amoeba must), different groups of cells specialise — some form a covering, some conduct materials, some contract to cause movement. This specialisation is efficient, but it means cells lose some of their flexibility; a muscle cell cannot suddenly start photosynthesising.

Plants and animals both use tissues, but their tissues are organised quite differently, and the reason comes down to how each group of organisms lives. Most animals move about looking for food, a mate or safety, so they need energy quickly and repeatedly — this calls for tissues capable of fast, repeated contraction and repair, and animal organs mostly use fully-grown, mature cells throughout their body. Plants, in contrast, are fixed in one place. They don't need tissues for locomotion, but they do need to keep growing for as long as they live, in specific regions — at the tips of roots and shoots, and in girth. So plants keep a permanently young, dividing tissue at these growing points, while the rest of the plant body is made of mature, non-dividing tissue. This single difference in lifestyle explains why plant tissues are classified first by whether they can still divide (meristematic vs permanent), while animal tissues are classified by the specific job they do (epithelial, connective, muscular, nervous).

Meristematic tissue is the actively dividing tissue found only at specific growing regions of a plant. Its cells are small, densely packed with no intercellular spaces, have a large nucleus, and are typically isodiametric (roughly equal in all dimensions) with thin cellulose walls and dense cytoplasm — there is very little or no vacuole, because the cell's energy goes into division, not storage.

Based on location, meristematic tissue is of three kinds. Apical meristem is present at the growing tips of stems and roots and increases the length of the plant (this is why roots and shoots grow longer). Lateral meristem (also called the cambium) is present along the sides of stems and roots — for example the vascular cambium and cork cambium — and increases the girth (thickness) of the plant, which is how tree trunks widen with age. Intercalary meristem is found at the base of leaves or internodes (for example in grasses) and helps the leaf or internode elongate; it eventually gets converted into permanent tissue.

Cells produced by meristematic tissue divide and grow for a while, then take up a specific role, losing their ability to divide — this is how meristematic tissue gives rise to all the permanent tissues of the plant.

When a meristematic cell stops dividing and takes up a fixed shape, size and function, it becomes permanent (or differentiated) tissue. Simple permanent tissue is made of only one type of cell.

Parenchyma is the most basic, unspecialised permanent tissue — living cells, usually loosely packed with large intercellular spaces, thin cellulose walls, and a large central vacuole. It forms the bulk of leaves, fruits, seeds and the soft parts of the plant, and its main job is storage of food and water. Where parenchyma contains chlorophyll and carries out photosynthesis, it is specially called chlorenchyma (found in the leaf mesophyll); where it stores air, mostly in aquatic plants, it is called aerenchyma, whose large air spaces give buoyancy.

Collenchyma gives flexible mechanical support — think of how a young stem or leaf stalk can bend without snapping. Its cells are living, elongated, and irregularly thickened at the corners with cellulose and pectin; they have little or no intercellular space. Collenchyma occurs in the stem below the epidermis, in leaf stalks, and allows bending in stems and leaves without breaking, while permitting some growth as the organ elongates.

Sclerenchyma gives rigid, permanent strength. Its cells are dead at maturity — long and narrow, with thick walls hardened by lignin, leaving no internal space (no protoplasm) at all. Sclerenchyma makes tissue hard and stiff, and is found in the husk of a coconut, the covering of seeds and nuts, and around the vascular bundles of stems.

A protective tissue also belongs here: the epidermis is the outermost single layer of cells covering the whole plant body, secreting a waxy, waterproof cuticle on aerial parts to check water loss, and containing stomata (with guard cells) on leaves for gas exchange. In older, woody stems and roots, the epidermis is replaced by several layers of dead, compactly arranged cork cells, made impervious to gases and water by a chemical called suberin in their walls — this is how cork acts as a protective tissue against infection and water loss.

Complex permanent tissues are made of more than one type of cell, all working together as a single conducting unit. Plants have two complex tissues, together called vascular tissue, arranged in strands called vascular bundles.

Xylem conducts water and dissolved minerals from the roots upward to the rest of the plant, and also gives mechanical strength. It is made of four elements: tracheids and vessels are both long, tube-like, dead cells (vessels have their end walls dissolved, forming a continuous pipe; tracheids retain tapering end walls with pits) — these two carry water and provide support; xylem parenchyma is living, stores food and helps sideways conduction; xylem fibres are dead, thick-walled cells that mainly support the tissue. Because tracheids and vessels are dead with no cytoplasm blocking the lumen, water and minerals move through them very efficiently in one direction, from roots to leaves.

Phloem transports the food made by photosynthesis, mainly from the leaves to the storage organs and to every growing part of the plant — this is why phloem transport is called translocation and can move material in more than one direction depending on need. Phloem also has four elements: sieve tubes are living, tube-like cells with perforated end walls (sieve plates) but no nucleus at maturity; companion cells are living, nucleated cells attached to sieve tubes that control their activity; phloem parenchyma stores food; phloem fibres are dead, and give mechanical support. Except for phloem fibres, all phloem elements are living, which fits their job of actively moving food rather than merely acting as passive pipes.

Epithelial tissue is the covering or lining tissue of the animal body — it covers the outer surface of the body and lines internal cavities and organs. Its cells are tightly packed in one or more layers with very little intercellular material, sitting on a thin basement membrane that separates them from the tissue below, so it forms a protective, selectively permeable barrier that also allows exchange of materials.

Types are named by cell shape and arrangement. Squamous epithelium has flat, thin, tile-like cells fitted together like tiles on a floor; it lines the mouth, oesophagus, blood vessels and alveoli of the lungs, where a thin barrier helps diffusion. Cuboidal epithelium has cube-shaped cells and lines kidney tubules and the ducts of glands, often helping absorption and secretion. Columnar epithelium has tall, pillar-like cells and lines the gut, where its height helps absorption and secretion. Where columnar (or cuboidal) cells bear hair-like cilia, it is called ciliated epithelium; it lines the respiratory tract, where the beating cilia sweep out dust and mucus. Where an epithelial layer's cells are specialised to secrete substances, groups of them fold inward to form glandular epithelium, making up glands such as the salivary glands (secreting saliva) — glands can be unicellular (single secretory cells) or multicellular.

Connective tissue is the most abundant and widely distributed tissue in the body — it binds, supports and connects other tissues and organs. Unlike epithelium, its cells are loosely spaced and embedded in an intercellular matrix, which can be jelly-like, fibrous, or solid, and it is this matrix that gives each type of connective tissue its particular property.

Areolar connective tissue fills the space inside organs, supports internal organs, and helps repair tissue damage; it lies between the skin and muscles, and around blood vessels and nerves. Adipose tissue stores fat in large cells, and is found below the skin and between internal organs, acting as an insulator against temperature change and as a store of energy. Skeletal connective tissues give the body its framework: bone has a hard matrix of calcium and phosphorus compounds, forms the framework supporting the body, and works with muscles to bring about movement; cartilage has a matrix that is solid but flexible (smoothens bone surfaces at joints), found in the nose, ear, trachea and between vertebrae. Ligaments (which connect bone to bone) and tendons (which connect muscle to bone) are dense, fibrous connective tissues — ligaments are elastic and have great strength for flexibility at joints, tendons are strong but not so elastic.

Blood is also a connective tissue: it has a fluid matrix called plasma, in which red blood cells, white blood cells and platelets are suspended; it transports gases, digested food, hormones and waste to different parts of the body, connecting every organ to every other.

Muscular tissue is made of elongated cells, called muscle fibres, which contain special contractile proteins that shorten and relax to produce movement. There are three types.

Striated (skeletal) muscle is attached to bones and helps in voluntary body movement — the cells are long, cylindrical, unbranched, and multinucleate, showing alternate light and dark bands (hence 'striated'); it is under voluntary control and tires quickly. Unstriated (smooth) muscle is found in the walls of internal organs such as the alimentary canal, iris of the eye, ureters, bronchi and blood vessels; its cells are long, spindle-shaped (tapering at both ends) and uninucleate, with no striation, and it works involuntarily — you cannot consciously start or stop your gut moving food along, and it does not fatigue quickly. Cardiac muscle is the tissue of the heart wall; its cells are cylindrical, branched, and uninucleate, joined end to end at junctions called intercalated discs, and it contracts rhythmically and involuntarily throughout life without ever fatiguing.

Nervous tissue is made to receive stimuli and conduct impulses rapidly from one part of the body to another, coordinating the body's responses. Its basic unit is the neuron (nerve cell), which has three main parts: the cell body (cyton), containing the nucleus and most of the cytoplasm; short, branched dendrites that receive impulses and carry them toward the cell body; and one long axon that carries the impulse away from the cell body to another neuron, a muscle, or a gland, ending in fine branches. The junction between the axon tip of one neuron and the dendrite of the next is called a synapse, where the impulse is passed on chemically.

Many nerve fibres bundled together with connective tissue make up a nerve. Neurons, together with the brain and spinal cord (the central nervous system), give animals a fast, coordinated way to sense and respond to their surroundings — far faster than the chemical (hormonal) coordination used by the rest of the body, though the two systems work together.