Tissues in Action — Detailed Notes
A tissue is a group of cells similar in structure that work together to perform a specific function. In multicellular organisms there is a hierarchy: cells → tissue → organ → organ system → organism. Different tissues bring about division of labour, raising the body’s efficiency.
3.1 Why plant and animal tissues differ
Plants are mostly fixed and need support — their cells have a rigid cell wall. Animals move, so their wall-less cells are flexible. Plants and animals also differ in nutrition (plants make food by photosynthesis; animals digest food) and in growth patterns, so their tissues differ in structure and function.
3.2 Plant tissues — Meristematic tissue (growth)
Meristematic tissue = actively dividing cells. Features: small cells, thin walls, dense cytoplasm, large prominent nucleus, no (or tiny) vacuoles, tightly packed (no intercellular spaces) — all suited to rapid division. Three types:
• Apical meristem — at root and shoot tips → growth in length (the onion-root experiment shows roots grow only from the tip).
• Lateral meristem — a ring in the stem → growth in girth/thickness (produces annual growth rings; also cork cambium forms the bark).
• Intercalary meristem — at the base of internodes/just above nodes (e.g. in grasses) → regrowth after cutting/grazing.
By differentiation, meristematic cells lose the ability to divide and become specialised permanent tissue.
3.2.4 Permanent tissues
Protective — Epidermis: outermost single layer of flat cells with a waxy cuticle (cutin) to reduce water loss; bears root hairs (absorption) and stomata in leaves (gas exchange + transpiration → transpiration pull).
Simple permanent (supporting) tissues (one cell type): Parenchyma — living, thin-walled, loosely packed; stores food, does photosynthesis (chlorenchyma), and forms air spaces (aerenchyma) that help aquatic plants float. Collenchyma — living cells with corners thickened by pectin; gives flexibility (bending stems/tendrils without breaking). Sclerenchyma — thick walls with lignin, mostly dead; gives hardness/strength (coconut husk, nut shells).
Complex permanent (conducting) tissues (many cell types): Xylem — carries water & minerals upward and gives strength; made of tracheids, vessels, xylem parenchyma (only living part) and xylem fibres (mostly dead/sclerenchymatous). Phloem — carries food (mostly living); made of sieve tubes, companion cells, phloem parenchyma and phloem fibres.
Three tissue systems: dermal (protective outer), ground (parenchyma/collenchyma/sclerenchyma), and vascular (xylem + phloem).
3.3 Animal tissues (four types)
Epithelial — covers the body and lines organs; closely packed cells; types by function: thin flat (exchange — lungs/blood vessels), many-layered (protection — skin), gland cells (secretion), receptor cells with cilia (sensory), tall columnar (absorption — small intestine).
Connective — connects/supports; has a matrix. Blood (fluid matrix = plasma; RBCs with haemoglobin carry O₂, WBCs fight infection, platelets clot). Bone (hard matrix with calcium & phosphorus — strength/support). Cartilage (soft jelly-like matrix — flexibility, cushions bone ends). Tendon connects muscle to bone; ligament connects bone to bone (limits movement, prevents dislocation).
Muscular — Skeletal (voluntary): long, cylindrical, unbranched, multinucleate, striated. Smooth (involuntary): spindle-shaped, single nucleus, no striations (stomach, intestine). Cardiac: cylindrical, branched, single nucleus, faint striations — works tirelessly in the heart.
Nervous — made of neurons; parts: cell body (nucleus), dendrites (receive signals), axon (carries message) ending in axon terminals. Controls and coordinates the body.
3.4–3.6 Musculoskeletal & skeletal system, joints
Muscles pull on bones (via tendons) to cause movement at joints, under the control of the nervous system. Bones ≈12–15% of body weight. Joints: ball-and-socket (shoulder/hip — all-direction movement), hinge (elbow, knee — one plane), pivot (neck/skull — rotation side to side), fixed (skull bones — no movement). The skeletal system (skull, vertebral column with cartilage discs, rib cage of 12 pairs of ribs) supports and protects organs; the rib cage moves to help breathing.
Totipotency (F. C. Steward, carrot phloem cells, 1958): some mature plant cells can de-differentiate, divide and re-differentiate into a whole plant — the basis of plant tissue culture.
Key Terms
tissue, meristematic (apical/lateral/intercalary), differentiation, epidermis/cuticle/stomata, parenchyma/collenchyma/sclerenchyma, xylem/phloem, dermal/ground/vascular systems, epithelial/connective/muscular/nervous tissue, matrix, tendon/ligament, neuron/axon/dendrite, joints, totipotency.
Revise, Reflect, Refine — All NCERT Exercise Answers
1Meristematic tissues divide repeatedly. Which property of their cells allows this?
(iii) They have thin walls, dense cytoplasm and a large prominent nucleus (and little or no vacuole, tightly packed). These features suit continuous, rapid cell division. Thick walls, large vacuoles or being already differentiated would all prevent division.
2If a plant cannot transport food from leaves to roots, which tissue is malfunctioning?
(ii) Phloem. Phloem conducts food (sugars) made in the leaves to the rest of the plant. Xylem carries water/minerals, epidermis protects, and sclerenchyma supports — none of these transport food.
3Why are epithelial tissues lining internal organs usually only one or a few cells thick?
(iii) To allow quick exchange of materials across them. A thin lining lets gases, nutrients and liquids diffuse rapidly (e.g. gas exchange in the lung alveoli, absorption in the intestine). A thick layer would slow diffusion.
4Straight-leg jump (stiff knees/ankles) vs normal jump (bend knees/ankles): how did ankle, knee and hip positions differ?
In the normal jump the ankle, knee and hip bend (flex) and then straighten (extend), letting the muscles store and release force smoothly and absorb the landing. In the straight-leg jump these joints stay stiff/extended, so little force is generated and the landing is jarring (shock is not absorbed). This shows joints and muscles must work together for efficient, safe movement.
5Which type of joint is involved when you bend your knees and ankles?
(ii) Hinge joint. Like a door hinge, the knee (and ankle) bends and straightens mainly in one plane / one direction.
6Assertion–Reason (choose: i both true & R explains A; ii both true, R not explanation; iii A true R false; iv A false R true):
A. A: Epithelium is well-suited for gas exchange in lungs; R: it consists of multiple layers of tall cells that slow diffusion → (iii) A is true but R is false (exchange epithelium is a single thin flat layer, which speeds diffusion). B. A: Cardiac muscle contracts continuously without fatigue; R: cardiac cells have many mitochondria and rich blood supply → (i) both true and R correctly explains A. C. A: Tendons connect bone to bone and allow joint movement; R: tendons are tough tissue transmitting force from muscle to bone → (iv) A is false (tendons connect muscle to bone; bone-to-bone is a ligament) but R is true. D. A: In a hinge joint movement is primarily in one plane; R: bone ends are shaped to allow sliding in all directions → (iii) A is true but R is false (a hinge allows one plane, not all directions).
7Graph of tree age vs diameter and number of annual rings (Table 3.7). (i) interpret (ii) relation of diameter to rings (iii) tissue responsible for girth & its location.
(i) As age increases, the stem diameter increases — a roughly straight-line (positive) relationship — showing the tree keeps adding girth each year. (ii) The number of annual rings equals the age in years, and more rings go with greater diameter — one ring is added per year. (iii) The lateral meristem (vascular cambium), located as a ring along the circumference of the stem, is responsible for the increase in girth.
8A tree severely debarked by an elephant. (i) function(s) hampered (ii) tissue affected by further trunk damage (iii) function hampered if tissues beneath bark badly damaged (iv) assumptions.
(i) The bark contains phloem, so transport of food from leaves to roots is hampered (and protection against water loss/microbes is lost). (ii) Further damage would affect the vascular cambium (lateral meristem) and xylem beneath the bark. (iii) If the xylem is damaged, upward transport of water and minerals stops and the tree loses mechanical support. (iv) Assumption: debarking removed phloem/cambium but the xylem was initially intact; if instead the xylem was already cut, water transport would fail immediately and the tree would die faster.
9A young mango stem bends flexibly in monsoon winds without breaking. Which tissue gives this flexibility? What if it were replaced by sclerenchyma?
Collenchyma gives the young stem its flexibility (its pectin-thickened corners allow bending without breaking). If replaced by sclerenchyma (thick, lignified, rigid, dead cells), the stem would become hard and brittle — it would resist bending and could snap/break in strong winds instead of bending, which is harmful for a growing plant.
10Sohan grows sugarcane from cuttings; type ‘B’ sprouted, type ‘A’ did not. (i) why B grew but A didn’t (ii) difference in B (iii) observation/measurement made (iv) parameters kept same.
(i) Type ‘B’ cuttings contained a node with intercalary/axillary meristematic (dividing) cells, which can sprout new shoots; type ‘A’ (only internode, no node) lacked such dividing cells. (ii) Presence of a node (with a bud/meristem) in B. (iii) Observation: whether shoots sprouted and their growth/length over the weeks. (iv) Keep soil, water, light, temperature, cutting length and planting depth the same for both for a fair comparison.
11Rohan: ‘a tissue is a group of similar cells doing similar functions’; Rajiv: ‘true for simple tissues but different for complex tissues’. Explain.
Both are partly right. For simple tissues (parenchyma, collenchyma, sclerenchyma) the statement holds — they are made of one type of cell. But complex tissues (xylem, phloem) are made of several different cell types (e.g. xylem = tracheids, vessels, xylem parenchyma, fibres) that together perform one overall function (conduction). So a tissue is better defined as a group of cells with a common origin and function, not necessarily identical cells.
12Coconut-husk fibres are tough and used for mats. Which tissue gives this strength? Why can’t living parenchyma do the same?
Sclerenchyma provides the strength — its cells have thick, lignin-rich walls and are dead, making them hard, tough and fibrous. Parenchyma has thin cell walls and is living with large intercellular spaces, so it is soft and cannot withstand the mechanical stress and wear that mats require.
13Vibha says ‘meristematic cells are located only at root and shoot apices’. Is this correct? What question can Neha ask?
The statement is incorrect — apical meristem is at the tips, but lateral meristem lies along the stem (girth) and intercalary meristem at the nodes/internode bases (e.g. grasses). Neha could ask: “If meristems are only at the tips, then how does a tree trunk grow thicker, and how does grass grow back after being cut?” — prompting Vibha to recognise lateral and intercalary meristems.
14A plant cell and an animal cell are of the same size. (i) Which has the larger vacuole? Why? (ii) assumptions.
(i) The plant cell has the larger vacuole — a mature plant cell usually has one big central vacuole filled with cell sap that stores water/minerals and maintains turgor; animal cells have only small, temporary vacuoles. (ii) Assumption: the plant cell is a mature cell (a young/meristematic plant cell has little or no vacuole, so the comparison assumes maturity).
15A textbook says ‘each plant tissue performs only one specific function’. What questions would you ask to test this, and which tissues would you examine?
Questions: “Does any single tissue do more than one job?”, “What all functions does parenchyma perform?”, “Does xylem only conduct water, or also give support?” Examples: parenchyma stores food, does photosynthesis (chlorenchyma) and floats aquatic plants (aerenchyma); xylem both conducts water and gives mechanical strength; epidermis protects and (via stomata) enables gas exchange/transpiration. These show the statement is incorrect — many tissues are multifunctional.