WBCS Preliminary (Biology): Tissues

1. A group of cells of the same type or of a mixed type having a common origin and performing similar functions are called tissues.
2. Plant tissues are of two main types –Meristematic Tissues (Meristems) and Permanent.
3. Meristematic tissues are composed of cells that divide continuously. As the cells of this tissue are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles. Found in growing tips of root and shoot. The main function of meristematic tissue is to continuously form a number of new cells and help in growth.
4. Permanent tissues are derived from meristematic tissues. They have lost the power of dividing, having attained their definite form and size. In their earlier stages the cells are more or less similar in structure but slowly they become specialized and form permanent tissues. They can be classified into simple and complex tissues.
5. Parenchyma, collenchyma and sclerenchyma are three types of simple tissues. Xylem and phloem are types of complex tissues.
6. Parenchyma, a type of permanent tissue, consists of relatively unspecialised cells with thin cell walls. They are live cells. They are usually loosely packed,so that large spaces between cells (intercellular spaces) are found in this tissue. This tissue provides support to plants and also stores food. In some situations, it contains chlorophyll and performs photosynthesis,and then it is called chlorenchyma.In aquatic plants, large air cavities are present in parenchyma to give buoyancy to the plants to help them float. Such a parenchyma type is called aerenchyma.
7. The flexibility in plants is due to another permanent tissue, collenchyma. It allows easy bending in various parts of a plant (leaf, stem) without breaking. It also provides mechanical support to plants.
8. Yet another type of permanent tissue is sclerenchyma. It is the tissue which makes the plant hard and stiff. The cells of this tissue are dead.
9. Complex tissues are made of more than one type of cells. All these cells coordinate to perform a common function. Xylem and phloem are examples of such complex tissues. They are both conducting tissues and constitute a vascular bundle.
10. Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. The cells have thick walls, and many of them are dead cells. Tracheids and vessels are tubular structures. This allows them to transport water and minerals vertically. The parenchyma stores food and helps in the sideways conduction of water. Fibres are mainly supportive in function.
11. Phloem is made up of four types of elements: sieve tubes, companion cells, phloem fibres and the phloem parenchyma. Sieve tubes are tubular cells with perforated walls. Phloem is unlike xylem in that materials can move in both directions in it. Phloem transports food from leaves to other parts of the plant. Except for phloem fibres, phloem cells are living cells.
12. Animal tissues can be epithelial, connective, muscular and nervous tissue.
13. The covering or protective tissues in the animal body are epithelial tissues. Epithelium covers most organs and cavities within the body. Depending on shape and function, epithelial tissue is classified as squamous, cuboidal, columnar, ciliated and glandular.
14. The different types of connective tissues in our body include areolar tissue, adipose tissue, bone, tendon, ligament, cartilage and blood.
15. Muscular tissue consists of elongated cells, also called muscle fibres. This tissue is responsible for movement in our body. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement. Striated, unstriated and cardiac are three types of muscle tissues.
16. The brain, spinal cord and nerves are all composed of nervous tissue. Each nerve cell is called a neuron. These are highly specialized cells. They have the ability to receive stimulus from within or outside and send impulses to different parts of the body. Each cell consists of three parts, the cyton or the cell body, the dendrons which are short processes arising from the cyton and further branch into thin dendrites and the axon which is a single long cylindrical process forming fine branches terminally. Dendrites receive impulses and the axon takes impulses away from the cell body.