Notes
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Tissues and division of labour
A tissue is a group of similar cells that work together to perform a specific function. Tissues form organs, organs form organ systems, and organ systems make up an organism. The division of labour among tissues increases the efficiency of the body.
Plant growth and meristems
- Apical meristem: at root and shoot tips. Increases the length of roots and stems.
- Lateral meristem: a ring of cells in the stem, also called the cambium. Increases the girth (thickness) of the stem. Annual rings in a tree's trunk show its growth; counting them estimates the tree's age.
- Intercalary meristem: at the base of leaves and nodes (the base of internodes). Helps grasses regrow after grazing or cutting.
Meristematic cells are small, have thin walls, a large nucleus, dense cytoplasm and few or no vacuoles. Some remain meristematic; others lose the ability to divide and become permanent tissues. This is called differentiation.
Simple permanent tissues
- Epidermis (protective): outermost single layer of tightly packed cells, covered by a waxy cuticle. Contains stomata for gas exchange and transpiration. Root hairs increase surface area for absorbing water and minerals.
- Parenchyma (supporting): living cells with thin walls and intercellular spaces. Stores food, and in green parts performs photosynthesis. In aquatic plants, air spaces help them float.
- Collenchyma (supporting): living cells with unevenly thickened corners from pectin. Gives flexibility to stems and leaves.
- Sclerenchyma (supporting): dead cells with thick, lignified walls. Gives hardness and strength, for example in coconut husk and walnut shell.
Complex permanent tissues (conducting)
- Xylem: transports water and minerals from roots to other parts. Made of tracheids, vessels, xylem parenchyma (living) and xylem fibres (sclerenchyma). Tracheids and vessels are dead, tubular cells.
- Phloem: transports food from leaves to other parts. Made of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Mostly living cells. Companion cells regulate the sieve tubes.
Tissue systems: dermal tissue system (outer covering), ground tissue system (parenchyma, collenchyma and sclerenchyma), vascular tissue system (xylem and phloem).
Animal tissues: epithelial and connective
- Epithelial tissue: closely packed cells with very little space between them. Forms the outer covering (skin) and lines internal organs. Types: single flat layers for exchange (lung and blood vessels), many layers for protection (skin, mouth), cuboidal or columnar cells for secretion (salivary, sweat and stomach lining), receptor cells for sensing (nostrils, taste buds), and tall pillar-like cells with hair-like structures for absorption (small intestine).
- Connective tissue: connects and supports organs. Blood is a fluid connective tissue (plasma matrix). Bone has a rigid matrix of calcium and phosphorus compounds. Cartilage has a soft, jelly-like matrix that gives flexibility and cushioning. Tendons join muscle to bone; ligaments join bone to bone.
Blood and its components
- Plasma is about 55% of total blood volume; formed elements are about 45%.
- RBCs contain haemoglobin, an iron-rich protein that gives blood its red colour. They live for about 4 months.
- Platelets help blood clot at an injury.
- WBCs collect at infected areas, causing pus formation and inflammation.
Muscular tissue
- Skeletal muscle: voluntary; long, cylindrical, multinucleate fibres with light and dark bands (striated). Attached to bones.
- Smooth muscle: involuntary; spindle-shaped, single nucleus, not striated. Found in the stomach and intestines, for slow, continuous movements like digestion.
- Cardiac muscle: involuntary; found only in the heart; branched cells with a single nucleus and faint striations. Works rhythmically without tiring.
Nervous tissue
Neurons (nerve cells) receive, process and transmit messages. A neuron has a cell body (with nucleus), dendrites that receive signals, and a long axon that carries messages to axon terminals, which pass them to other cells.
Joints and the musculoskeletal system
- Ball and socket joint: rounded bone fits into a shallow hollow. Allows movement in many directions (shoulder, hip).
- Hinge joint: bends in one direction, like a door hinge (elbow, knee).
- Pivot joint: one bone turns within a ring of another (neck, allowing the head to turn).
- Fixed joint: bones do not move (skull, protecting the brain).
The musculoskeletal system is made of bones, muscles, joints, cartilage, tendons and ligaments. Muscles pull on bones through tendons to produce movement under the control of the nervous system.
Skeletal system and rib cage
The skeleton includes the skull, the vertebral column (made of vertebrae with cartilage discs) and the rib cage. Twelve pairs of ribs protect the heart and lungs; their flexible cartilage lets the chest expand and contract during breathing.
Key terms at a glance
Meristem: region of actively dividing cells. Differentiation: the process by which meristematic cells become specialised permanent tissues. Stomata: pores in the epidermis. Internode: the stem between two nodes.
Numericals
Worked solutions for the body weight estimates in Activity 3.4.
ESTIMATE THE WEIGHT OF BONES
Given: total body weight = 60 kg. Bone mass is about 12 to 15% of body weight for adults.
Weight of bones = total body weight x bone percentage / 100
Step 1. At 12%: 60 x 12 / 100 = 7.2 kg.
Step 2. At 15%: 60 x 15 / 100 = 9 kg.
Answer: bones weigh about 7.2 to 9 kg.
ESTIMATE THE WEIGHT OF MUSCLES
Given: total body weight = 60 kg. Adult males have about 40 to 50% muscle (figure varies with age, sex and body type).
Weight of muscles = total body weight x muscle percentage / 100
Step 1. At 40%: 60 x 40 / 100 = 24 kg.
Step 2. At 50%: 60 x 50 / 100 = 30 kg.
Answer: muscles weigh about 24 to 30 kg.
CHECK THE ESTIMATE
Step 1. Bones (9 kg at 15%) + muscles (30 kg at 50%) = 39 kg, which is 65% of body weight.
Step 2. Compare: the rest is organs, fat, skin, blood and water. The estimate is reasonable because it is below the total weight.
NCERT Q&A
Answers to the Pause and Ponder questions and the Revise, Reflect, Refine exercises.
Pause and Ponder
PAUSE AND PONDER 1
Why are fibres of coconut husk hard and brittle, while the leaf stalks of coriander are soft and flexible?
Answer: Coconut husk fibres are sclerenchyma, with thick, lignified walls that make them hard and strong. Coriander leaf stalks are mainly collenchyma, whose cells have pectin-thickened corners that allow bending without breaking.
PAUSE AND PONDER 2
Why is a thick cuticle advantageous for a plant in the desert but disadvantageous for a plant living underwater?
Answer: In the desert, a thick cuticle reduces water loss by transpiration. Underwater, the plant needs to take in dissolved gases and water through its surface, and a thick cuticle would block this.
PAUSE AND PONDER 3
How does water reach the leaves of tall trees, and how does food move from leaves to other parts?
Answer: Water evaporates from the leaves through stomata (transpiration), creating a pull that draws a continuous column of water up the xylem vessels and tracheids. The dead xylem cells form open tubes for this flow. Food made in leaves moves through the sieve tubes of phloem, where companion cells regulate loading and unloading of sugars.
PAUSE AND PONDER 4
What will happen if there were no stomata in the epidermis of the stem or leaves?
Answer: Gas exchange (intake of carbon dioxide and release of oxygen) would be reduced, and transpiration would be greatly reduced. Photosynthesis would slow down, and the transpiration pull that moves water up the xylem would be weaker.
PAUSE AND PONDER 5
Look at the dance poses in Fig. 3.17. Which joints are involved, and what movement does each joint allow?
Answer: Ball and socket joints (shoulders and hips) allow movement in many directions and circular movements. Hinge joints (elbows and knees) allow bending in one direction. Pivot joints (neck and forearm) allow rotation. Fixed joints in the skull do not move.
Revise, Reflect, Refine
QUESTION 1
Meristematic tissues divide repeatedly. Which property of their cells allows this?
Answer: (iii). They have thin walls, dense cytoplasm and a large prominent nucleus, which suit rapid and continuous cell division.
QUESTION 2
If a plant cannot transport food from leaves to roots, which tissue is malfunctioning?
Answer: (ii) Phloem. Phloem carries food from leaves to other parts.
QUESTION 3
Why are the epithelial tissues lining an animal's internal organs usually only one or a few cells thick?
Answer: (iii). Thin layers allow quick exchange of materials, such as in the lungs and blood vessels.
QUESTION 4
Compare the ankle, knee and hip positions in the straight-leg jump and the normal jump (Fig. 3.21).
Answer: In the straight-leg jump, the knees and ankles are kept stiff and straight, so the jump relies more on the hips and leg muscles. In the normal jump, the knees and ankles bend naturally, so the joints absorb impact and store energy, and the hips move in a similar pattern.
QUESTION 5
Which type of joint is involved when you bend your knees and ankles?
Answer: (ii) Hinge. Knee and ankle joints bend in one direction.
QUESTION 6
Choose the correct option for each case (A to D).
Answer: A: (iii), assertion true, reason false, since thin layers (not multiple layers) are suited to gas exchange. B: (i), cardiac muscle has many mitochondria and a blood supply, which explains its endurance. C: (iv), the assertion is false because tendons connect muscle to bone; the reason is true. D: (iii), hinge joints move in one plane; the reason about sliding in all directions is false.
QUESTION 7
Analyse the graph of the teak tree's diameter and annual rings with age, and identify the tissue responsible for girth.
Answer: The diameter increases steadily with age, and the number of annual rings equals the age (one ring per year). (i) Girth increases each year as the stem grows wider. (ii) The number of annual rings is equal to the age of the tree. (iii) The lateral meristem (cambium), a ring of cells just inside the bark, is responsible for girth.
QUESTION 8
An elephant has debarked a tree. Which functions are hampered, which tissue would be affected further, and what assumptions are being made?
Answer: (i) Food transport from leaves to roots is hampered, because the bark contains phloem. (ii) The lateral meristem (cambium) under the bark would be damaged, which affects girth growth. (iii) Food transport and growth would be affected if the tissues beneath the bark were badly damaged. (iv) Assumptions include that the tree is a dicot with secondary growth, and that the damage is only to the bark; the answers change if the cambium is also damaged.
QUESTION 9
A mango sapling's stem bends during monsoon winds without breaking. Which tissue is responsible? What if it were replaced with sclerenchyma?
Answer: Collenchyma gives this flexibility. If it were replaced by sclerenchyma, the stem would be rigid and would be more likely to break in strong winds.
QUESTION 10
In Sohan's sugarcane cuttings, type B sprouted and grew, while type A did not. What explains this and what should be kept the same?
Answer: (i) Type B cuttings included nodes, where buds and meristematic tissue are located, so they could grow. (ii) Type B had nodes and buds; type A did not. (iii) Observation: sprouting and increase in length over the weeks. (iv) Keep the length, thickness, age and origin of cuttings, and the water, light and soil conditions the same.
QUESTION 11
Rohan says a tissue is a group of similar cells performing similar functions. Rajiv says this is true only for simple tissues. Who is right?
Answer: Rajiv is right. Complex tissues such as xylem and phloem are made of different cell types working together, so the definition for simple tissues does not apply directly.
QUESTION 12
Which tissue gives coconut husk fibres their strength? Why could living parenchyma not do the same?
Answer: Sclerenchyma gives this strength, because its dead cells have thick, lignified walls. Living parenchyma has thin cell walls and cannot provide the same hardness and strength.
QUESTION 13
Vibha says meristematic cells are located only at root and shoot apices. What do you think? What question can Neha ask Vibha?
Answer: The statement is incorrect. Intercalary meristems are at nodes and the base of internodes, and lateral meristems (cambium) form rings in stems. Neha can ask: "Where does a plant's girth increase come from?" or "Where do grasses regrow after grazing?"
QUESTION 14
A plant cell and an animal cell are of the same size. Which will have a larger vacuole? What assumptions are you making?
Answer: (i) The plant cell has a large central vacuole that can occupy most of the cell. (ii) Assumptions: both are mature cells, and the vacuole size is typical for each type.
QUESTION 15
A textbook states that each plant tissue performs only one specific function. What questions would you ask, and what examples would you use?
Answer: The statement is incorrect. Ask: Does one tissue ever do more than one job? Examples: xylem conducts water and gives support; phloem conducts food and is partly supportive; parenchyma stores food and photosynthesises; the epidermis protects and, in roots, absorbs water.