Chapter 3
Tissues in Action
All questions solved — Activities 3.1–3.5, “Think as a Scientist”, the 5 Pause & Ponder, and every “Revise, Reflect, Refine” exercise — with step-wise working and original labelled diagrams.
In-text Questions
Activities · “Think as a Scientist” · Pause & Ponder
Onion roots in two jars. On day 3 the root tips of Jar B are cut. What trend appears in the data, and what do you infer?
- Jar A: roots keep growing in length every day.
- Jar B: roots stop growing once the tips are cut on day 3.
Inference: roots grow only from their tips, which contain continuously dividing cells. This growth zone is the apical meristem (also at shoot tips), responsible for increase in length.
Why do you think the cells of meristematic tissue lack vacuoles?
Meristematic cells are busy dividing continuously. A large vacuole would fill the cell with water/cell sap and leave little room for the dense cytoplasm and organelles needed for division. By having little or no vacuole, the cell stays small and packed with cytoplasm, so it can keep dividing rapidly. (Vacuoles develop later, when the cell differentiates into a permanent storage cell.)
Which tissue helps you move? Which lets you sense heat/cold? Which allows oxygen into the blood? Which holds the body together?
| Action | Tissue responsible |
|---|---|
| Movement (clench fist) | Muscular tissue |
| Sensing heat/cold (control & response) | Nervous tissue |
| Oxygen entering the blood (in lungs) | Epithelial tissue (thin lining of alveoli) |
| Holding the body together / support | Connective tissue |
Everyday experiences with blood: a cut clots, an infection turns red & swollen, running reddens the face. What causes blood to clot, and explain these.
- Clotting: caused by platelets, which gather at the injury and help seal it.
- Red colour of blood: due to haemoglobin, an iron-rich protein in RBCs (which live ~4 months).
- Infection turns red/swollen, may form pus: White Blood Cells collect at the infected area to fight germs, causing inflammation and pus.
- Running reddens the face: muscles need more oxygen, so breathing and blood flow increase, making the face look red.
Identify the connective tissue from each action and state its function.
| Action / feel | Function | Tissue |
|---|---|---|
| Elbow — hard & rigid | Strength, support, protection | Bone |
| Ear/nose — soft, flexible, regains shape | Flexibility & cushioning ends of bones | Cartilage |
| Forearm muscle moves the fingers | Connects muscle to bone → movement | Tendon |
| Knee does not bend beyond a limit | Connects bone to bone; gives stability, limits movement | Ligament |
Bone has a rigid matrix (calcium + phosphorus); cartilage has a soft, jelly-like matrix.
Estimate the weight of your bones and muscles from your body weight. (Bone ≈ 12–15% for all adults; muscle ≈ 40–50% in males, ≈ 30–40% in females.)
- Bone weight = body weight × bone % : 50 kg × 13% = 6.5 kg
- Muscle weight (say 40%) = body weight × muscle % : 50 kg × 40% = 20 kg
- Together bones + muscles ≈ 6.5 + 20 = 26.5 kg — over half your weight.
Use your own weight in the same steps. Bone and muscle mass differ between people because of age, gender, activity level, nutrition and body type — and together they make up a large share of total body weight.
Observe what movements each body part can make (Table 3.5). Why can some parts move many ways while others move only one way?
| Body part | Movement | Joint type |
|---|---|---|
| Shoulder | All directions (rotates, raises, circular) | Ball & socket |
| Elbow, Knee | Bending in one direction | Hinge |
| Neck | Side-to-side turning | Pivot |
| Wrist, Fingers, Toes | Bending + some side movement | Hinge / gliding |
The difference is due to the type of joint: a ball-and-socket joint allows movement in all directions, while a hinge joint allows movement in only one plane.
F. C. Steward grew whole carrot plants from single phloem cells (Table 3.6 results). (a) What do you conclude about phloem cells? (b) Which combination gives highest/lowest biomass & why? (c) Same with animal cells? (d) Two commercial applications.
(a) Mature carrot phloem cells are totipotent — even a fully specialised cell can dedifferentiate, divide and redifferentiate to grow into a complete new plant under the right conditions.
(b) Highest biomass: light + air + liquid medium (the 20% increase) — air gives oxygen for respiration and the liquid medium lets nutrients reach cells freely. Lowest: the combination lacking air (or on solid medium) — without enough oxygen/nutrient contact, cell growth is reduced.
(c) No — ordinary mature animal cells are not totipotent in this way, so you would not normally get a whole animal from a single body cell using the same method.
(d) Tissue culture to mass-produce identical, disease-free crop plants (micropropagation); and producing valuable plant chemicals/medicines, or conserving rare and endangered plants.
Coconut husk fibres are hard and brittle, while coriander leaf stalks are soft and flexible. Find out the reason.
The hardness comes from the type of supporting tissue. Coconut husk is full of sclerenchyma — dead cells with thick walls hardened by lignin, making it tough, rigid and brittle. Coriander stalks are made mainly of collenchyma (and parenchyma) — living cells with pectin-thickened corners that give flexibility, so they bend easily and stay soft.
Why is a thick cuticle advantageous for a desert plant but disadvantageous for a plant living underwater?
Desert plant — advantage: a thick waxy cuticle greatly reduces water loss by transpiration, which is vital where water is scarce and the air is hot and dry.
Underwater plant — disadvantage: water is everywhere, so saving water is unnecessary. A thick cuticle would instead block the exchange of gases (CO₂, O₂) and absorption of dissolved minerals through the surface, harming photosynthesis and survival.
Water travels up against gravity through xylem. How do the ‘dead’ xylem cells work with the living leaf cells to keep water moving?
The dead xylem cells (tracheids and vessels) lose their contents and join end-to-end to form continuous, hollow, thick-walled pipes — perfect, low-resistance channels for water.
The living leaf cells do the pulling: as water evaporates from the leaves through the stomata (transpiration), it creates a suction or “transpiration pull.” Because water molecules stick together (cohesion), this pull is transmitted all the way down the xylem column, lifting water from the roots up to the top — against gravity. So dead pipes + a living pump (transpiration) work together.
What would happen if there were no stomata in the epidermis of the stem or leaves?
- No gaseous exchange: CO₂ couldn’t enter for photosynthesis and O₂ couldn’t be released — so food production would fall.
- No transpiration: the transpiration pull that lifts water up the xylem would be lost, so water and mineral transport would be poor.
- The plant would struggle to cool itself and to remove some wastes, and would eventually weaken and may die.
In the dance poses (Fig. 3.17), identify the joints involved and the movement each allows.
| Joint | Where | Movement allowed |
|---|---|---|
| Ball & socket | Shoulder, hip | Movement in all directions (raising arms, lifting leg) |
| Hinge | Elbow, knee, ankle | Bending/straightening in one plane |
| Pivot | Neck (head) | Turning side to side |
| Gliding/hinge | Wrist, fingers | Bending and small sliding (graceful hand poses) |
Classical poses combine many of these — e.g. raised arms use ball-and-socket shoulders, bent knees use hinge joints, and head tilts use the pivot joint.
Revise, Reflect, Refine
End-of-chapter exercise — all 15 questions
Which property lets meristematic cells divide repeatedly?
- (i) Thick walls for protection.
- (ii) Large vacuoles that store nutrients.
- (iii) Thin walls, dense cytoplasm and a large prominent nucleus.
- (iv) They are functionally differentiated cells.
Thin walls, dense cytoplasm and a large nucleus are exactly the features that allow rapid, continuous division. Thick walls (i) and large vacuoles (ii) belong to permanent cells, and (iv) is the opposite — meristematic cells are undifferentiated.
If a plant cannot transport food from leaves to roots, which tissue is malfunctioning?
- (i) Xylem
- (ii) Phloem
- (iii) Epidermis
- (iv) Sclerenchyma
Phloem (its sieve tubes) carries food from the leaves to the rest of the plant. Xylem only carries water and minerals upward, so the faulty tissue here must be the phloem.
Why are the epithelial tissues lining internal organs usually only one or a few cells thick?
- (i) To store food efficiently.
- (ii) To provide maximum strength.
- (iii) To allow quick exchange of materials across them.
- (iv) To reduce friction.
A thin lining means substances (gases, nutrients) have a very short distance to travel, so diffusion and exchange happen quickly — exactly what is needed in places like the lungs and intestine.
Straight-leg jump (knees & ankles stiff) vs normal jump (knees & ankles bent). How did your ankle, knee and hip positions differ?
Straight-leg jump
- Ankle, knee, hip stay straight/stiff (little bending)
- Hard landing — force jolts straight through the joints
Normal jump
- Ankle, knee and hip bend (flex) naturally
- Bending absorbs shock and stores/releases energy → softer, safer landing and a higher jump
The bent (flexed) joints of the normal jump cushion the impact, while the stiff joints of the straight-leg jump transmit the full force to the body.
Which joint is involved when you bend your knees and ankles?
- (i) Ball and socket
- (ii) Hinge
- (iii) Pivot
Knees and ankles bend back-and-forth in a single plane, just like a door hinge.
Choose: (i) both true, R explains A · (ii) both true, R not the explanation · (iii) A true, R false · (iv) A false, R true.
| Case | Verdict | Why |
|---|---|---|
| A — Epithelium suits gas exchange; R: it has many layers of tall cells that slow diffusion | (iii) | A true, but R false — the exchange epithelium is a single thin flat layer that speeds diffusion, not many tall layers. |
| B — Cardiac muscle never fatigues; R: many mitochondria + rich blood supply | (i) | Both true and R correctly explains A — plenty of energy and oxygen keep it beating tirelessly. |
| C — Tendons connect bone to bone; R: tendons transmit force from muscle to bone | (iv) | A false (tendons join muscle to bone; bone-to-bone is a ligament), R true. |
| D — Hinge joint moves in one plane; R: bone ends allow sliding in all directions | (iii) | A true, R false — a hinge does not allow all-direction movement. |
Plot age (x) vs diameter and number of annual rings (y) from Table 3.7. (i) Interpret the diameter trend. (ii) Relation between diameter and rings. (iii) Which tissue causes girth and where?
(i) The diameter of the stem increases steadily (roughly linearly) with age — the tree keeps growing in girth year after year.
(ii) The number of annual rings equals the age in years, and the diameter grows along with it. So more rings → older tree → greater diameter; counting the rings tells the tree’s age.
(iii) The lateral meristem (vascular cambium) is responsible for girth. It lies in a ring along the circumference of the stem, adding new cells inward and outward.
A tree is severely debarked by an elephant. (i) Which functions are hampered? (ii) Which tissue is affected by further trunk damage? (iii) What if tissues beneath the bark are severely damaged? (iv) Your assumptions?
(i) The bark contains the phloem, so transport of food from leaves to the rest of the tree is hampered; the tree also loses its protective covering.
(ii) Deeper damage would affect the vascular cambium (lateral meristem) and the xylem lying beneath the bark.
(iii) If these are severely damaged, water and mineral transport (xylem) stops and growth in girth (cambium) ceases — the tree may dry up and die.
(iv) Assumptions: that the bark includes phloem + protective cork, and that the damage is partial (not a complete ring around the trunk). If instead the trunk is fully girdled all around, the phloem is completely cut, no food reaches the roots, and the tree will certainly die.
A mango sapling’s stem bends in monsoon winds without breaking. Which tissue gives this flexibility? Predict the effect if it were replaced by sclerenchyma.
Collenchyma is living tissue with pectin-thickened corners that gives flexibility, letting the young stem bend in the wind and spring back.
If replaced by sclerenchyma: sclerenchyma is hard, rigid and dead (lignified). The stem would become stiff and brittle — instead of bending in strong wind it would likely snap or crack, and the young plant could not flex and grow freely.
Sugarcane cuttings: type B sprouted, type A did not. (i) Why could B grow but not A? (ii) The difference in B? (iii) What was measured? (iv) Parameters to keep same for fairness.
(i) & (ii) Type B contained a node (with a bud and intercalary/lateral meristematic tissue), which has dividing cells able to sprout into a new plant. Type A was a piece of internode with no node/bud, so it had no meristematic tissue and could not grow.
(iii) The measurement was whether the cuttings sprouted — counting how many produced new shoots (and the length/number of shoots) after a few weeks.
(iv) Keep the same: length/size of cuttings, sugarcane variety, soil, water, sunlight, temperature, and planting depth — so the only difference is node vs no-node.
Rohan: “A tissue is a group of similar cells performing similar functions.” Rajiv: true for simple tissues but different for complex tissues. Explain.
Rajiv is right. Rohan’s definition fits simple permanent tissues (parenchyma, collenchyma, sclerenchyma), which are made of one type of similar cell.
But complex tissues like xylem and phloem are made of several different kinds of cells (e.g. xylem = tracheids, vessels, xylem parenchyma, xylem fibres) that work together for one common function. So a fuller definition is: a tissue is a group of cells — similar or different — with a common origin, working together for a specific function.
Coconut husk fibres make tough mats. Which tissue gives this strength? Why can’t living parenchyma do the same?
Sclerenchyma cells have thick walls hardened with lignin and are usually dead, making them very strong, tough and fibrous — ideal for mats and ropes.
Parenchyma cannot: it is made of living cells with thin walls, loosely packed with air spaces, and has no lignin. It is soft and meant for storage/photosynthesis, so it offers no mechanical strength.
Vibha: “Meristematic cells are located only at the root and shoot apices.” Is she right? What question could Neha ask to show this is incorrect?
Vibha is incorrect. Apical meristems are at the tips, but there are also lateral meristems (along the stem’s circumference, for girth) and intercalary meristems (at the nodes/base of internodes, e.g. in grasses).
Neha could ask: “If meristems are only at the tips, then how do tree trunks grow thicker in girth, and how does grass regrow after being mowed or grazed?” These can only be explained by lateral and intercalary meristems located away from the apices.
A plant cell and an animal cell are the same size. (i) Which has a larger vacuole? Give reasons. (ii) What assumptions are you making?
(i) The plant cell has the larger vacuole. A mature plant cell has one large central vacuole filled with cell sap (storing water, minerals, wastes and maintaining turgor). Animal cells have only small, scattered vacuoles, or none.
(ii) Assumptions: that we are comparing a mature plant cell (young/meristematic plant cells have little or no vacuole — which would change the answer), and a typical animal cell. We also assume both are normal, healthy cells.
A textbook states: “Each plant tissue performs only one specific function.” What questions would you ask to test this, and which tissues would you examine?
Question to ask: “Does any single tissue carry out more than one function?” Then check examples:
- Parenchyma — stores food and does photosynthesis (chlorenchyma) and helps aquatic plants float (aerenchyma).
- Xylem — transports water and gives mechanical support.
- Epidermis — protects and (through stomata) allows gas exchange and transpiration.
These examples show the statement is incorrect — many plant tissues are multifunctional.