Tissues
CBSE Class 9 · Science · Notes and practice questions
The chapter explains why multicellular organisms need tissues and how plant and animal tissues are specialised, covering meristematic and permanent plant tissues and the four animal tissue types.
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A single-celled organism can carry out all life processes within one cell. In multicellular organisms, different functions are divided among groups of cells, and such a group is called a tissue. This chapter compares plant and animal tissues. Plants need long-distance transport and standing support while animals need movement and quick responses, so the same principle of division of labour produces very different arrangements of cells in the two kingdoms.
Plant growth stays confined to meristems, small regions of actively dividing cells. The daughter cells produced by meristems later differentiate into permanent tissues, which normally cannot divide. Simple permanent tissues are of one cell type: thin-walled parenchyma for packing and storage, collenchyma thickened at the corners for flexible support, and sclerenchyma with thick lignified walls for mechanical strength. The protective epidermis covers the outer surface. Xylem and phloem are complex permanent tissues made of more than one cell type, and together they form the conducting system of the plant.
Animal tissues are grouped into four classes according to function. Epithelial tissue lines and covers surfaces. Connective tissue binds, supports and protects, and its cells lie in a matrix; blood, bone and cartilage all belong to this class. Muscular tissue contracts to produce movement and includes skeletal, smooth and cardiac fibres. Nervous tissue forms the brain, spinal cord and nerves and transmits impulses through neurons. In every case structure matches function, so exam questions often ask you to identify a tissue from its location or from a short description.
Key terms
- Meristematic tissue
- Tissue whose cells divide actively and produce new cells. The cells are thin-walled, have dense cytoplasm and a prominent nucleus, and usually lack intercellular spaces. Apical meristem increases length at root and shoot tips, lateral meristem increases girth.
- Permanent tissue
- Tissue formed when meristematic cells lose the ability to divide after differentiation. It may be simple, with one kind of cell, or complex, with several kinds of cells performing a common function. Epidermis, xylem and phloem are permanent tissues.
- Parenchyma
- Living simple permanent tissue with thin-walled cells and distinct intercellular spaces. It stores food and water, and when it contains chloroplasts it can photosynthesise.
- Collenchyma
- Living simple permanent tissue whose cell walls are thickened unevenly at the corners. It gives flexible support to young stems and leaf stalks so organs can bend without breaking.
- Sclerenchyma
- Usually dead simple permanent tissue with uniformly thickened lignified walls and no intercellular spaces. It gives mechanical strength and makes hard parts such as husks, shells and the gritty cells of fruits.
- Epidermis
- Outermost protective layer of cells around plant organs. It is transparent and often has a waxy cuticle that reduces water loss; stomata in leaf epidermis allow gas exchange and root epidermis produces root hairs.
- Xylem
- Complex permanent tissue that conducts water and minerals from the root upward. Its main conducting elements are tracheids and vessels; most of the cells are dead, except xylem parenchyma, and lignified walls give mechanical support.
- Phloem
- Complex permanent tissue that transports food made by leaves to all parts of the plant. Sieve tubes conduct food and companion cells keep the sieve tubes alive; movement in phloem can take place in both directions.
- Animal tissues
- The four principal animal tissues are epithelial, connective, muscular and nervous. Epithelial tissue forms tightly packed coverings and linings; connective tissue keeps organs in place with a nonliving matrix; muscle cells shorten to cause movement; neurons of nervous tissue receive and send impulses.
Practice questions with answers
1. What is a tissue? Give one reason why multicellular organisms need tissues.
A tissue is a group of similar cells that perform a common function together. Multicellular organisms have many functions to carry out, and different groups of cells can specialise in different functions. This division of labour is not needed in a unicellular organism because a single cell carries out every process.
2. At the tip of a growing root, which plant tissue is found and what kind of growth does it cause?
The tissue is meristematic tissue, specifically the apical meristem. These thin-walled cells divide actively and produce new cells, and their differentiation lengthens the root.
3. State two differences between xylem and phloem.
Xylem carries water and minerals, while phloem carries food. Xylem is composed mainly of dead tracheids and vessels, whereas phloem is composed of living sieve tubes and companion cells.
4. A plant tissue is made of living cells whose walls are thickened at the corners. Name the tissue and give one location.
It is collenchyma. It provides flexible support and is found in young stems, leaf stalks and the ribs of leaves.
5. Why are sclerenchyma cells strong but usually dead?
Sclerenchyma cells have heavily thickened walls with a uniform deposit of lignin. The lignified wall is so hard that it gives mechanical strength to the plant, and the cell contents usually disappear as the cell matures; the rigid dead walls alone are enough for support.
6. Name the protective tissue that covers plant roots and leaves. How is it adapted?
It is the epidermis. The leaf epidermis has stomata for gaseous exchange, the root epidermis forms root hairs to absorb more water, and in aerial parts a waxy cuticle helps reduce water loss.
7. Identify the animal tissue that covers surfaces and lines cavities and has no intercellular spaces.
It is epithelial tissue. Its cells are packed tightly and rest on a basement membrane, which protects the underlying organs and controls absorption, secretion and exchange of materials.
8. Why is blood counted as a connective tissue?
Blood has a fluid matrix called plasma and blood cells suspended in it, just as other connective tissues have cells dispersed in a matrix. It connects different parts of the body by transporting oxygen, nutrients, hormones and wastes, so it is grouped under connective tissue.
9. State one distinguishing feature of each of the three types of muscle.
Striated muscle fibres are long, multinucleated, show striations and are under voluntary control; smooth muscle fibres are spindle-shaped, have no striations and work involuntarily; cardiac muscle fibres are branched, striated and contract involuntarily throughout life.
10. Describe the structure of a neuron and state the direction in which an impulse travels through it.
A neuron has a cell body containing the nucleus, several short dendrites, and one long axon. Dendrites receive messages from other neurons and pass them to the cell body, while the axon carries the impulse forward to the next neuron or to a muscle.
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