Tissues in Action Class 9 Notes and Important Questions

These Tissues in Action Class 9 notes explain how groups of cells team up to do one job, first in plants and then in animals. You will go through the chapter in order, learn the key terms, and finish with important questions, model answers and the mistakes students most often make.

What is a tissue? Division of labour

A tissue is a group of cells, often similar in structure, that work together to perform a specific function. It is one step above the single cell in the biological hierarchy.

In a unicellular organism like Amoeba, one cell does every life process. In multicellular organisms such as humans and plants, these tasks are shared among different groups of cells. This sharing is called division of labour, and it makes the whole organism more efficient.

• Animals: muscle tissue is specialised for movement; nervous tissue carries messages.
• Plants: xylem transports water and phloem transports food.

Think of a school: one teacher cannot teach every subject, run the library and coach sports at once. Splitting the work gets more done, and better.

Why plant and animal tissues are different

Dead cells with thick walls give strength without the energy cost of keeping living cytoplasm. That suits a plant that cannot move.

Meristematic tissue: where plants grow

Growth in plants happens only in certain regions that contain actively dividing cells, called meristematic tissue. Meristematic cells are small, have thin walls, dense cytoplasm with many organelles, and a large, prominent nucleus. Vacuoles are generally absent, because these cells are busy dividing and do not need large storage spaces.

Simple permanent tissues

When meristematic cells lose the ability to divide and take on a permanent shape and function, they differentiate into permanent tissues.

Epidermis (protective tissue): the outermost layer, a single layer of tightly packed flat cells. It protects against mechanical injury, water loss and parasites. Many plants have a waxy layer of cutin on it, called the cuticle; in dry habitats a thick cuticle reduces water loss. In roots, epidermal cells form root hairs, which increase the surface area for absorbing water and minerals.

Leaf epidermis has small pores called stomata for gaseous exchange and transpiration (evaporation of water vapour). Transpiration creates a transpiration pull in the xylem that moves water upward, and it helps remove waste.

Complex permanent tissues: xylem and phloem

Complex tissues have more than one type of cell working as a unit.

• Xylem carries water and minerals upward from the roots. It is made of tracheids, vessels, xylem parenchyma and xylem fibres. Most xylem cells, except parenchyma, are dead with thick walls, so xylem also gives mechanical support.
• Phloem carries food (sugars) from leaves to other parts. It is mostly living cells: sieve tubes, companion cells, phloem parenchyma and phloem fibres. Sieve tubes have perforated walls. Phloem parenchyma stores food and substances like resin, tannins and latex.

Animal tissues: epithelial and connective

Epithelial tissue covers the body and lines internal organs and cavities. Its cells are tightly packed with almost no spaces, forming a continuous sheet that controls exchange of materials and protects against germs and water loss.

• Squamous (thin, flat): rapid diffusion in blood vessels and lungs.
• Stratified squamous (many layers): protects skin from wear and tear.
• Columnar and cuboidal: absorption and secretion.
• Sensory epithelium: receptor cells with cilia for smell, taste, sound and balance, in the nostrils, taste buds and inner ear.

Connective tissue supports, binds and connects organs. Its cells are loosely spaced in an intercellular matrix, and the matrix decides the job.

Platelets in blood help clotting at an injury so that too much blood is not lost.

Muscles, nerves and joints

Muscular tissue is made of long cells called muscle fibres.

• Skeletal: voluntary, striated, attached to bones.
• Smooth: involuntary, spindle-shaped, in organs like the stomach.
• Cardiac: only in the heart; branched, involuntary, works throughout life without fatigue.

Nervous tissue is made of neurons that receive and pass on electrical impulses. A neuron has a cell body (with the nucleus), dendrites (receive signals) and a long axon (carries the message to the axon terminals).

Muscles pull on bones through tendons. The backbone is a series of small bones called vertebrae, with a cartilage disc between each pair for cushioning and bending. The adult skeleton is about 12–15 per cent of body weight. The rib cage has 12 pairs of ribs and protects the heart and lungs.

Remember this

• Tissue = group of cells working together for one function.
• Apical: length. Lateral: girth. Intercalary: regrowth.
• Xylem: water up, mostly dead cells. Phloem: food, mostly living cells.
• Sclerenchyma is dead; parenchyma and collenchyma are living.
• Ligament: bone to bone. Tendon: muscle to bone.

Important questions with answers

1. How does forming tissues lead to division of labour? Similar cells group to do one function, so the body carries out complex processes more efficiently than if every cell did every task.

2. Why do plant tissues contain more dead cells than animal tissues? Plants are stationary and need strength to stay upright. Dead, thick-walled cells give rigidity without the energy cost of living cytoplasm.

3. Why are vacuoles generally absent in meristematic cells? Vacuoles store materials and give turgidity in mature cells. Meristematic cells are actively dividing and do not need large storage.

4. Why do plants in dry habitats have a thick cuticle? The thick cutin layer reduces water loss through transpiration, helping the plant survive.

5. What are the functions of stomata? They allow gaseous exchange and transpiration. Transpiration creates a pull that helps move water up the xylem.

6. Differentiate xylem and phloem by the state of their cells. Xylem is mostly dead cells (tracheids, vessels, fibres) for transport and support. Phloem is mostly living cells (sieve tubes, companion cells) for food transport.

7. Where is simple squamous epithelium found, and why? In blood vessels and lung alveoli, because its thin, flat cells allow rapid diffusion.

8. How do cardiac muscles differ from skeletal muscles? Cardiac muscles are involuntary and branched and work rhythmically without fatigue; skeletal muscles are voluntary and unbranched.

9. Why are skull joints fixed? They form a rigid structure that protects the brain, eyes and ears.

Common mistakes to avoid

• Swapping ligament and tendon.
• Writing that sclerenchyma is living, or collenchyma is dead.
• Saying phloem carries water. Xylem carries water and minerals; phloem carries food.
• Mixing up lateral meristem (girth) with apical meristem (length).

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