What is a tissue? Levels of organisation
A tissue is a group of cells that look alike and work together to do one job. In a one-celled amoeba, one cell does everything. In big organisms, work is shared: this is called division of labour.
Levels from small to big:
- Cell – e.g. a muscle cell
- Tissue – many muscle cells = muscle tissue
- Organ – different tissues together, e.g. heart, leaf
- Organ system – organs working together, e.g. digestive system
- Organism – the whole living being
Plants and animals have different tissues: plants stand still, so many of their tissues are dead and give strength; animals move, so most of their tissues are living and use more energy. Plants grow in certain zones only; animals grow more evenly.
Plant tissues: meristematic and permanent, xylem and phloem
Meristematic tissue (dividing tissue)
Young cells with thin walls, dense cytoplasm, a large nucleus and almost no vacuole. They keep dividing.
- Apical meristem: at tips of root and shoot – makes the plant longer.
- Lateral meristem (cambium): along the sides – makes stem and root thicker.
- Intercalary meristem: at the base of leaves or near nodes (like in grass) – helps regrow.
Permanent tissue
Cells that stopped dividing and took up a fixed job (this change is called differentiation).
Simple permanent tissues (one type of cell):
- Parenchyma: living, loose cells with spaces; stores food. With chlorophyll it is chlorenchyma; with big air spaces in water plants it is aerenchyma (helps float).
- Collenchyma: living, walls thick at corners; gives bendiness, e.g. leaf stalks.
- Sclerenchyma: dead cells with thick lignin walls; makes parts hard, e.g. coconut husk.
- Protective tissue: epidermis (outer layer, may have waxy cutin; has stomata for gas exchange and water loss) and cork (dead, with suberin, on old trees).
Complex permanent tissues (more than one cell type) – the plant's pipes:
| Xylem | Phloem |
|---|---|
| Carries water and minerals | Carries food (sugar) |
| One way: root → leaves | Both ways |
| Tracheids, vessels, xylem parenchyma, xylem fibres | Sieve tubes, companion cells, phloem parenchyma, phloem fibres |
| Mostly dead cells (only parenchyma living) | Mostly living (only fibres dead) |
Try it at home
Put a white flower or a celery stalk in water mixed with food colour. After a few hours, cut the stem across and find the coloured dots – the xylem.
Animal tissues: epithelial, connective, muscular, nervous
1. Epithelial tissue (covering)
Cells packed tightly with no gaps, on a thin base membrane. Covers the body and lines organs.
- Squamous: flat, thin – lines blood vessels, lung sacs; stacked in skin (stratified squamous).
- Cuboidal: cube-shaped – kidney tubules, glands.
- Columnar: tall – lines the intestine, helps absorb.
- Ciliated: with tiny hairs – windpipe, pushes mucus out.
2. Connective tissue (joining, supporting)
Cells spread out in a matrix (jelly, liquid or hard filling).
- Blood: liquid matrix (plasma) with RBCs, WBCs and platelets; carries gases, food, hormones.
- Bone: hard matrix of calcium and phosphorus; frame of the body.
- Cartilage: smooth and bendy; ear, nose tip, ends of bones at joints.
- Ligament: joins bone to bone; strong and a little stretchy.
- Tendon: joins muscle to bone; very strong, not stretchy.
- Areolar tissue: fills space between skin and muscles, around organs.
- Adipose tissue: stores fat, keeps us warm.
3. Muscular tissue (movement)
Long cells (muscle fibres) that contract and relax.
- Striated (skeletal, voluntary): stripes, many nuclei; arms and legs; we control them.
- Smooth (unstriated, involuntary): spindle-shaped; gut, blood vessels, iris.
- Cardiac: branched, only in the heart; works all life without tiring.
4. Nervous tissue (messages)
Made of neurons. Each has a cell body with a nucleus, short branches called dendrites and one long axon. Neurons carry electrical messages quickly. Found in brain, spinal cord and nerves.
The musculoskeletal system and how to care for it
Bones, cartilage, joints, ligaments, tendons and skeletal muscles together form the musculoskeletal system. It gives shape, protects soft organs (skull protects brain, ribs protect heart and lungs), stores calcium, makes blood cells in bone marrow and lets us move.
How we move: a muscle can only pull. When the biceps contracts, it pulls the forearm bone through a tendon and the arm bends at the elbow joint. Muscles work in pairs (biceps bends, triceps straightens).
Care tips
- Posture: sit with a straight back, keep the screen at eye level, carry the school bag on both shoulders.
- Lifting: bend the knees, not the back.
- Exercise: at least 60 minutes of play or yoga daily; stretching keeps joints flexible.
- Food: calcium (milk, curd, ragi, green leaves), protein (dal, eggs, paneer) and vitamin D (morning sunlight) keep bones strong.
- Safety: warm up before sports, use helmets and knee pads, rest an injured joint.
Weak bones can lead to rickets in children (lack of vitamin D) and osteoporosis in old age.
Key formulas and definitions
- Tissue = group of similar cells doing one job
- Cell → Tissue → Organ → Organ system → Organism
- Meristematic: apical (length), lateral (thickness), intercalary (regrowth)
- Simple permanent: parenchyma, collenchyma, sclerenchyma; protective: epidermis, cork
- Xylem: water + minerals, one way up; Phloem: food, both ways
- Animal tissues: Epithelial, Connective, Muscular, Nervous
- Ligament: bone–bone; Tendon: muscle–bone
- Muscles: striated (voluntary), smooth (involuntary), cardiac (heart)
Worked examples
1. Grass grows back quickly after it is cut by a lawn mower. Which tissue helps?
Intercalary meristem at the base of the leaves or nodes keeps dividing, so the grass regrows.
2. Which tissue makes a coconut husk hard, and are its cells alive?
Sclerenchyma. Its cells are dead, with thick lignin walls.
3. Why do lotus and water hyacinth float?
They have aerenchyma – parenchyma with large air spaces – which makes them light and buoyant.
4. A player tears the tissue that joins the thigh bone to the shin bone at the knee. Which tissue is it?
Ligament, because it joins bone to bone.
5. Name the tissue: it lines the windpipe and has tiny hairs that push dust and mucus out.
Ciliated columnar epithelium.
6. Why is blood called a connective tissue even though it is a liquid?
It has cells (RBC, WBC, platelets) spread in a matrix (plasma), and it connects all parts of the body by carrying oxygen, food, hormones and waste.
Common mistakes
- Writing that tendon joins bone to bone. Tendon joins muscle to bone; ligament joins bone to bone.
- Saying xylem carries food. Xylem carries water and minerals; phloem carries food.
- Thinking all permanent tissues are dead. Parenchyma and collenchyma are living; sclerenchyma is dead.
- Calling blood 'not a tissue'. Blood is a fluid connective tissue.