Tissues

In Amoeba one cell does movement, food, gases, respiration and excretion. A multicellular body has millions of cells, and most become skilled at one or two jobs. Such a cluster is a tissue. Blood, phloem and muscle are examples.

A plant stays fixed, so most of its tissues give support. A dead cell can still give strength and needs less upkeep. Animals move for food and shelter and spend more energy. Most of their tissues are living. Growth in a plant is limited to some regions. Growth in an animal is more even, so dividing and non-dividing regions are not marked off.

Apical meristem is at the tips of the stem and the root and it increases length. Lateral meristem, the cambium, increases the girth of the stem or the root. Intercalary meristem is at the base of the leaves or on the internode, on either side of the node, in a twig.

New cells are at first like the meristem. As they grow they take the form of other tissues. These cells are very active: dense cytoplasm, thin cellulose walls, a prominent nucleus, and no vacuoles. A vacuole is for storage and turgidity in a mature cell. A dividing cell needs that space for cytoplasm.

A meristem cell takes a fixed form, size and job and stops dividing. That process is differentiation. Permanent tissues form this way. A permanent tissue of one kind of cell is called simple.

Parenchyma is the packing tissue. The cells are living, relatively unspecialised, thin-walled and loosely arranged, so large spaces lie between them. It supports and stores food. When it has chlorophyll it is called chlorenchyma and it photosynthesises. In an aquatic plant large air cavities give buoyancy. That parenchyma is aerenchyma. The husk of a coconut is sclerenchyma.

Collenchyma lets the plant bend without breaking. It gives this flexibility in the leaf and the stem, and it also gives mechanical support. It is found in the leaf stalk, below the epidermis. The cells are living, elongated and irregularly thickened at the corners. There is very little space between them.

Sclerenchyma makes the plant hard and stiff. The cells are dead, long and narrow. The wall is thickened by lignin. Lignin acts like cement and hardens the wall. Often no space is left inside. It occurs in the stem, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts.

The epidermis of aerial parts secretes a waxy, water-resistant layer on the outside. It protects against water loss, mechanical injury and parasitic fungi. A desert plant has a thick coating of cutin on the outer surface. The epidermis of the root bears long hairs that increase the surface for absorption.

The small pores in the leaf are stomata. Two kidney-shaped guard cells enclose them. Gases are exchanged here and transpiration also happens here. Photosynthesis needs carbon dioxide. In an older stem a secondary meristem replaces the epidermis with cork. Cork cells are dead and packed tight. Suberin in the wall makes them impervious to gases and water.

A simple tissue is made of one kind of cell. A complex tissue has more than one kind, and they share one job. Xylem and phloem are like that. Together they make a vascular bundle. This tissue let complex plants survive on land.

Xylem has four parts: tracheids, vessels, xylem parenchyma and xylem fibres. The walls are thick and many cells are dead. Tracheids and vessels are tubular, so water and minerals travel vertically. Parenchyma stores food and helps to conduct water sideways. Fibres are mainly for support.

Phloem has four parts: sieve tubes, companion cells, phloem fibres and phloem parenchyma. The wall of a sieve tube is perforated. Food can move in both directions. Food made in the leaves is carried to the other parts. Except for the phloem fibres, the rest of the phloem is living.

An animal has four tissues: epithelial, connective, muscular and nervous. Blood is connective and muscle is muscular tissue. Epithelium is the covering and protective tissue. It covers organs and cavities and keeps systems apart. Skin, the lining of the mouth, blood vessels, lung alveoli and kidney tubules are all epithelium.

The cells form a tightly packed sheet. There is very little cement between them and almost no intercellular space. Anything entering or leaving the body crosses at least one epithelium. A fibrous basement membrane separates it from the tissue below.

Blood vessels and lung alveoli have simple squamous epithelium. The cells are extremely thin and flat. The lining of the mouth and the oesophagus is also squamous. Skin is arranged in many layers so that it can stand wear. That is stratified squamous epithelium.

The inner lining of the intestine is columnar. The tall cells help absorption and secretion. In the respiratory tract these cells have cilia, which push mucus forward. Kidney tubules and the ducts of salivary glands are made of cuboidal epithelium and give mechanical support. Sometimes the epithelium folds inward and forms a multicellular gland. That is glandular epithelium. Skin is stratified squamous, the intestine is columnar.

Cells of connective tissue are loosely spaced and sunk in an intercellular matrix. The matrix may be jelly-like, fluid, dense or rigid. The matrix of blood is the fluid plasma. Red blood cells, white blood cells and platelets float in it. Plasma contains proteins, salts and hormones. Blood carries gases, digested food, hormones and wastes.

The frame of bone supports the body, anchors the muscles and holds the organs. It is strong and non-flexible. The hard matrix is made of calcium and phosphorus compounds. A ligament joins bone to bone. It is very elastic and has very little matrix. A tendon joins muscle to bone. It is fibrous, with great strength and limited flexibility.

Cartilage has widely spaced cells. The solid matrix is made of proteins and sugars. It smooths bone surfaces at joints. It is also in the nose, ear, trachea and larynx. The ear can be folded. The bone of the arm cannot.

Areolar tissue lies between the skin and the muscles, around blood vessels and nerves, and in the bone marrow. It fills space inside organs, supports them and helps repair of tissues. Fat is stored in adipose tissue below the skin and between organs. The cells are filled with fat globules. This tissue also acts as an insulator. A drop of blood on a slide shows the cells. That look is not a numbered activity.

Muscular tissue is made of long cells called muscle fibres. They contain contractile proteins that contract and relax, so there is movement. The muscle of the limbs is voluntary. We move it when we wish and stop it when we wish. It is mostly skeletal muscle attached to bone. Staining shows light and dark bands, so it is called striated. The cell is long, cylindrical, unbranched and multinucleate.

Movement of food in the alimentary canal and the contraction of a blood vessel are involuntary. Smooth, or unstriated, muscle does this. The same muscle is in the iris of the eye, the ureters and the bronchi of the lungs. The cell is long, with pointed ends, spindle-shaped and uninucleate. There are no bands. Heart muscle contracts in rhythm throughout life. The cell is cylindrical, branched and uninucleate. The figure marks bands, but this is not voluntary striated muscle. Cardiac muscle is involuntary.

Every cell can answer a stimulus. Nervous tissue is specialised for this and carries the message very fast. The brain, the spinal cord and the nerves are made of this tissue. The cell is called a neuron.

It has a cell body with a nucleus and cytoplasm. Usually there is one long axon and many short branched dendrites. A single neuron may be up to a metre long. Many nerve fibres bound by connective tissue make a nerve. A nerve impulse moves a muscle when we wish. Nerve and muscle together let an animal move quickly in answer to a stimulus.