Chap 3 : Tissues in Action

Think It Over (Page 28)

Q – How is the study of cells and tissues significant for understanding the life processes and human welfare? A – Studying cells and tissues helps scientists understand the natural biological processes that control how plants and animals grow, develop, and survive. By understanding these intricate processes, researchers can replicate and modify them for human welfare, such as developing cures for diseases, repairing damaged organs, and improving agricultural crops.

Q – How are tissues in plants and animals different, and why? A – Plants are fixed in one place and do not move around, so their cells have rigid cell walls and supportive tissues that keep them upright and strong. Animals move from place to place to find food and shelter, so their cells lack rigid cell walls, allowing their bodies to be flexible for locomotion. Additionally, plants have specific conducting tissues to make food using solar energy (photosynthesis), while animals have specialised tissues to digest food eaten from outside sources.

Q – How is the division of labour at various levels of organisation in multicellular organisms correlated with their structure and function? A – In multicellular organisms, similar cells group together to form tissues, different tissues form organs, and organs form organ systems. Each group of cells is built in a specific way to perform a particular job efficiently, which is called division of labour. For example, muscle cells are structured to contract and cause movement, while xylem tissue in plants is structured like tubes to transport water. This division of labour increases the efficiency of the body and allows complex organisms to carry out all necessary life processes smoothly.

Activity 3.1: Let us design experiments (Pages 29–30)

Q – What trend do you observe in the data you recorded in Table 3.1 regarding the length of onion roots in Jar A and Jar B? A – The roots of the onion bulb in Jar A keep growing longer every day. In Jar B, the roots grow for the first three days, but once their tips are cut on day 3, they completely stop growing from day 4 onwards.

Q – Are your observations similar to those presented in the graphical representation (Fig. 3.2)? What do you infer? A – Yes, our observations match the graph shown in Fig. 3.2. We infer that roots grow only from their tips. This happens because the root tips contain actively dividing cells called apical meristem. When the root tips of Jar B were cut off, the actively dividing cells were removed, causing root growth to stop entirely.

In-Text Questions: Plant Tissues (Pages 31–33)

Q – Why do you think that the cells of meristematic tissues lack vacuoles? A – Meristematic cells are actively and continuously dividing to produce new cells for plant growth. Vacuoles are normally used by mature cells to store food, water, and waste, or to provide rigidity. Because meristematic cells divide so rapidly, they need dense cytoplasm and thin cell walls rather than rigid storage sacs like vacuoles.

Q – Examine the Transverse Section (T.S.) of a sunflower stem under a microscope (Fig. 3.7). What do you observe? Are all the cells similar in shape and size? How many different types of tissues can you identify? What differences do you notice among them? What might be the reason for the presence of different types of cells and tissues? A – When observing the stem under a microscope, we see distinct layers and groups of cells.

• All the cells are not similar; they differ greatly in shape, size, and wall thickness.

• We can identify several types of tissues: epidermis, collenchyma, parenchyma, sclerenchyma, phloem, lateral meristem, and xylem.
• The differences among them include thin or thick walls, presence of waxy coatings, and tubular or round cell arrangements.
• The reason for these different types of cells and tissues is the division of labour in the plant. Each tissue is specialised to perform a specific function, such as outer protection, mechanical support, or transporting food and water.

Q – What protects plants from mechanical injury, water loss, harmful microorganisms and extreme environmental conditions? A – The outermost protective layer of the plant, called the epidermis, protects all plant parts. The epidermal cells secrete a waxy, water-resistant layer called the cuticle. This cuticle prevents water loss, protects against mechanical injuries, and stops harmful parasites and microbes from invading the plant.

Q – What keeps a plant upright? Why does a fresh twig bend but a dry twig break? Why are seed coats hard and how do aquatic plants float? A – Supporting tissues (simple permanent tissues) give mechanical strength to keep the plant upright.

• A fresh twig bends without breaking because it contains collenchyma tissue, which consists of living cells with pectin deposited at the corners to provide flexibility along with support.
• A dry twig breaks easily because it has lost moisture and consists of sclerenchyma tissue, which is hard, rigid, and brittle.
• Seed coats are hard because they are made of dead sclerenchyma cells that have thick walls cemented with a tough chemical called lignin.
• Aquatic plants float because they have a specialised type of parenchyma tissue that contains large air spaces, making the plant light and buoyant.

Q – How does water reach the leaves of tall trees? How does food prepared in leaves reach other parts of the plant? A – Water and minerals are transported upward from the roots to the leaves through a tubular conducting tissue called xylem. The food prepared by green leaves during photosynthesis is transported to all other parts of the plant through another specialised conducting tissue called phloem.

Pause and Ponder (Page 33)

Q – 1. You may have noticed that fibres of coconut husk are hard and brittle, whereas the leaf stalks of coriander are soft and flexible. Find out the reason. A – Coconut husk fibres are hard and brittle because they are made of dead sclerenchyma tissue. The cell walls of sclerenchyma are heavily thickened with a hard chemical substance called lignin. On the other hand, coriander leaf stalks are soft and flexible because they contain collenchyma tissue. Collenchyma consists of living cells with pectin depositions at the corners, which allows plant parts to bend easily without breaking.

Pause and Ponder (Page 34)

Q – 2. Why do you think that a thick cuticle on the outer wall of epidermis is advantageous for a plant living in the desert but disadvantageous for a plant living underwater? A – In a desert, water is scarce, so a thick waxy cuticle is advantageous because it acts as a waterproof coating that prevents vital water from evaporating out of the plant. For an underwater plant, however, a thick waterproof cuticle would be disadvantageous because it would block the plant’s surface from absorbing dissolved gases, minerals, and water directly from its watery surroundings.

Q – 3. Once water is absorbed by plant roots, it has to travel against gravity through xylem. How do the ‘dead’ cells of the xylem work together with the living cells of leaves at the top to keep the water moving? A – The living epidermal cells of leaves contain tiny pores called stomata. Water continuously evaporates into the atmosphere through these stomata in a process called transpiration. This evaporation creates a strong suction force, known as a transpiration pull. This pull acts like a drinking straw, sucking water upward against gravity through the tubular, dead, pipe-like cells of the xylem from the roots all the way to the top leaves.

Q – 4. What do you think will happen if there were no stomata in the epidermis of the stem or leaves? A – If there were no stomata, the plant would not be able to exchange carbon dioxide and oxygen with the air, meaning it could not perform photosynthesis to make food or breathe properly. Additionally, without stomata, transpiration could not occur. Without transpiration, there would be no upward suction pull to lift water and essential minerals from the roots to the rest of the plant.

In-Text Questions: Animal Tissues (Pages 34 & 37)

Q – Which tissue helps you move? Which tissue enables you to sense heat or cold? Which tissue allows oxygen to enter the blood? Which tissue holds the body together so that the skin does not fall off?

A –

• Muscular tissue attached to our bones contracts and relaxes to help our body move.
• Nervous tissue (made of neurons) receives sensory messages and enables us to sense heat, cold, and pain.
• Epithelial tissue made of a single thin layer of flat cells lines our lungs and blood vessels, allowing oxygen to quickly diffuse into the blood.
• Connective tissue binds, supports, and connects different organs, holding the body together so that the skin stays firmly attached.

Q – What causes blood to clot when you get a cut? (From Activity 3.2 / Table 3.3) A – Special cell fragments in the blood called platelets quickly gather at the site of the cut or injury and form a blood clot to stop the bleeding.

Q – Why does a skin infection turn red, swell, and form pus? (From Activity 3.2 / Table 3.3) A – When germs enter through a skin cut, White Blood Cells (WBCs) gather at the infected area to fight and destroy the invading microbes. This defense reaction causes inflammation, which leads to increased blood flow (redness), swelling, and the formation of pus.

Q – Why do you breathe faster and why does your face turn red when you exercise or run? (From Activity 3.2 / Table 3.3) A – When you run or exercise, your working muscles need more energy and extra oxygen. To supply this demand, your breathing rate speeds up to take in more oxygen, and your heart pumps faster. This increased blood flow rushing through your blood vessels makes your face appear red.

Q – Identify the connective tissues for the following actions from Activity 3.3 (Table 3.4): (a) Touch your elbow gently (a hard and rigid structure that gives strength, support and protection). (b) Press and fold your ear or nose (a soft and flexible structure that retains shape again and cushions bone ends). (c) Touch your forearm muscles and wiggle your fingers (connects muscle to bone to bring about movement). (d) Sit on a chair and move your leg upwards till your knee allows (connects bone to bone, provides stability and prevents dislocation). A – The identified connective tissues are:

• (a) Bone
• (b) Cartilage
• (c) Tendon
• (d) Ligament

In-Text Questions: The Musculoskeletal System (Pages 38–39)

Q – Have you wondered how much of your body weight comes from bones? A – On an average, the adult human skeleton makes up about 12 to 15 per cent of the total body weight.

Q – Discuss why do bone and muscle mass differ between individuals, and how do they contribute to the overall body weight? (Activity 3.4) A – Bone and muscle mass differ from person to person depending on age, gender, physical lifestyle, diet, and body composition. For example, on average, adult males have about 40 to 50 per cent muscle mass, while adult females have roughly 30 to 40 per cent muscle mass. Together, bones (~12–15%) and muscles make up more than half of an individual’s total body weight, giving the body its shape, strength, and weight.

Q – Why do some parts of our body move easily in many directions, while others move only in a single direction? So, what actually causes the bones to move? (Activity 3.5) A – Different body parts move differently because of the specific types of joints present between the bones. For example, a ball and socket joint allows movement in all directions, whereas a hinge joint allows movement in only one direction. Bones cannot move on their own. Muscles attached to bones by strong tendons contract and pull on the bones, which acts as the force that creates movement at the joints.

Pause and Ponder (Page 40)

Q – 5. Look at the picture given below (Fig. 3.17). Carefully observe the various poses of classical and folk dances of India. Can you identify which joints are involved? Also, what type of movement each joint allows? A – When performing Indian classical and folk dances, dancers use several joints:

• Ball and socket joints (Shoulder and Hip joints): These allow free movement in all directions—forward, backward, sideways, and full circular rotation of the arms and legs.
• Hinge joints (Elbow and Knee joints): These act like door hinges, allowing the arms and legs to bend and straighten in only one direction.
• Pivot joint (Neck joint): This connects the skull to the backbone and allows the head to turn gracefully from side to side or nod up and down.
• Fixed joints (Skull bones): These hold the bones of the head firmly together without moving, protecting the brain during vigorous dance steps.

Think as a Scientist: Totipotency (Page 42)

Q – (a) What do you conclude about the characteristics of phloem cells of carrot? A – We conclude that mature phloem cells of carrot have a special property called totipotency. This means a single mature cell can regain its ability to divide (dedifferentiate) and grow into an entire, complete new plant when given nutrients and growth chemicals under laboratory conditions.

Q – (b) In which of the three combinations in Table 3.6 would you obtain the highest and lowest biomass? What could be the possible reason(s) for this observation?

A –

• Highest biomass: Obtained in the second combination (with light, air, and liquid medium + nutrients), which gave a 20% increase in cell weight.
• Lowest biomass: Obtained in the combinations where air was absent or when a solid agar medium was used without proper air stirring, resulting in reduced cell weight.
• Possible reasons: Plant cells need oxygen from the air for respiration, light for healthy growth, and a liquid medium that can be stirred so that nutrients easily reach every dividing cell.

Q – (c) Will you get the same results if you culture animal cells instead of carrot cells? A – No, you will not get the same results. Mature, specialised animal cells generally do not show complete totipotency like plant cells. If you culture a single mature animal cell, it cannot divide and regenerate into a complete, whole animal on its own.

Q – (d) Think and mention any two commercial applications of the study above. A – Two important commercial applications of plant tissue culture are:

1. Micropropagation: Rapidly growing thousands of disease-free, identical plants (like bananas or orchids) from tiny plant pieces in a laboratory.
2. Crop Improvement: Developing new plant varieties that produce higher yields, possess better nutrients, or are resistant to diseases and pests.

Revise, Reflect, Refine (Pages 44–46)

Q – 1. Meristematic tissues divide repeatedly. What property of their cells allows them to do this?

(i) They have thick walls for protection.  (ii) They contain large vacuoles that store nutrients.

(iii) They have thin walls, dense cytoplasm and large prominent nucleus.

(iv) They are functionally differentiated cells.

A – (iii) They have thin walls, dense cytoplasm and large prominent nucleus.

Q – 2. If a plant is unable to transport food from leaves to roots which tissue is malfunctioning?

(i) Xylem                           (ii) Phloem                      (iii) Epidermis                              (iv) Sclerenchyma

A – (ii) Phloem

Q – 3. Why are the epithelial tissues that line an animal’s 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 – (iii) To allow quick exchange of materials across them.

Q – 4. You can perform these two jumps (Fig. 3.21): Straight-leg jump – keep knees and ankles stiff. Normal jump – bend knees and ankles naturally. How did your ankle, knee and hip positions differ between the two jumps? A – In the straight-leg jump, the hinge joints at the knees and ankles are locked straight and stiff, making the landing hard and jarring because the bones absorb all the shock directly. In the normal jump, the hinge joints of the knees and ankles bend smoothly, and the ball and socket joints of the hips flex. This bending acts like a natural spring, allowing the muscles and cartilage to cushion the impact safely.

Q – 5. Which type of joint is involved when you bend your knees and ankles?

(i) Ball and socket                     (ii) Hinge                           (iii) Pivot

A – (ii) Hinge

Q – 6. In each of the following cases (A, B, C and D), choose the correct option as given below:

(i) Both (A) and (R) are true, and (R) is the correct explanation of (A).

(ii) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

(iii) (A) is true, but (R) is false.

(iv) (A) is false, but (R) is true.

1. Assertion: Epithelium is well-suited for gas exchange in the lungs. Reason: It consists of multiple layers of tall cells that slow down diffusion. A – (iii) (A) is true, but (R) is false. (The reason is false because lung epithelium consists of a single layer of thin, flat cells to allow rapid diffusion, not multiple layers of tall cells).
2. Assertion: Cardiac muscle can contract continuously without fatigue. Reason: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply.

A – (i) Both (A) and (R) are true, and (R) is the correct explanation of (A).

1. Assertion: Tendons connect bone to bone and allow joint movement. Reason: Tendons are made of tough connective tissue that transmits force from muscle to bone. A – (iv) (A) is false, but (R) is true. (The assertion is false because ligaments connect bone to bone, whereas tendons connect muscles to bones).
2. Assertion: In a hinge joint, movement occurs primarily in one plane. Reason: The bone ends are shaped to allow sliding in all directions. A – (iii) (A) is true, but (R) is false. (The reason is false because hinge joints only allow bending in one direction like a door, not sliding in all directions).

Q – 7. Plot a graph between the age of a tree (in years) on the x-axis and the diameter of the tree (in cm) along with the number of annual rings formed over time on the y-axis, using the data given in Table 3.7. (i) Analyse the graph in terms of the diameter of the stem over time and share the interpretation. (ii) What is the relation between the diameter of the teak tree to the annual rings formed? (iii) Which specialised tissue is responsible for the girth of the stem and where is it located? A – (When plotted on graph paper, the line moves upward steadily, showing that both tree diameter and annual rings increase as the tree grows older from 5 to 40 years).

• (i) As the age of the teak tree increases, the diameter of its stem increases steadily and continuously over time.
• (ii) There is a direct relationship: each year, exactly one new annual ring is added to the trunk. As the number of annual rings increases from 5 to 40, the tree’s diameter also increases proportionately from 4 cm to 40 cm.
• (iii) The lateral meristem is responsible for increasing the girth (thickness) of the stem. It is located in a concentric ring inside the stem along its circumference.

Q – 8. In a forest, it was observed that one of the trees was severely debarked by an elephant to meet its food requirements, as the bark is a rich source of nutrients (Fig. 3.22). Based on your learning, answer the following:

(i) Which function(s) of the tree is/are hampered by debarking?

(ii) Which plant tissue would be affected by further damage to the tree trunk even after debarking?

(iii) Which function of the tree would be hampered if the tissues beneath the bark were severely damaged?

(iv) What assumptions are you making to answer the questions above? How would the answer change if your assumptions are also changed?

A –

• (i) Debarking strips away the protective cork layer and the food-conducting phloem tissue. This leaves the tree unprotected from water loss, infections, and pests, and stops food from travelling from the leaves down to feed the roots.
• (ii) Further damage deeper into the trunk would destroy the xylem tissue (part of the vascular tissue system).
• (iii) If the underlying xylem tissue is damaged, the tree will be unable to transport water and minerals upward from the roots to the leaves. Without water, the leaves cannot make food, and the tree will wither and die.
• (iv) We assume that the elephant peeled the bark all the way around the trunk in a complete ring and cut deep into the conducting tissues. If we assume instead that the elephant only peeled a small, shallow patch on one side, some phloem and xylem would remain intact, allowing the tree to survive and slowly heal using its lateral meristems.

Q – 9. Aamrapali observed that a young mango sapling’s stem bends flexibly during monsoon winds and does not break. Which tissue is responsible for this flexibility? Predict and provide your explanation of the impact if the existing tissue was replaced by sclerenchyma. A – Collenchyma tissue is responsible for allowing the young stem to bend flexibly without breaking. If collenchyma were replaced by sclerenchyma, the stem would lose its flexibility and become hard, woody, and brittle. During strong monsoon winds, the rigid stem would not be able to bend; instead, it would snap and break easily.

Q – 10. Sohan designed an experiment for the regeneration of sugarcane, where he used cuttings to grow sugarcane. He used two types of cuttings, type ‘A’ and type ‘B’ (Fig. 3.23). After a few weeks, type ‘B’ cuttings sprouted and developed into sugarcane plants, whereas the type ‘A’ cuttings did not sprout.

(i) Why were the type ‘B’ cuttings able to grow as sugarcane but type ‘A’ could not?

(ii) What difference was present in type ‘B’ compared to type ‘A’?

(iii) What observation or measurement was made to determine whether this change had an effect?

(iv) What parameters should be kept the same for both types of cuttings to ensure a fair comparison?

A –

• (i) Type ‘B’ cuttings grew because they contained a node with actively dividing meristematic cells (intercalary meristem/buds) capable of producing new shoots and roots. Type ‘A’ did not sprout because it was cut only from the smooth internode region, which lacks dividing growth cells.
• (ii) The structural difference was that Type ‘B’ contained a node (the ring-like joint where leaves and buds arise), whereas Type ‘A’ consisted only of an internode (the straight section between nodes).
• (iii) The observation made was checking whether new green shoots and roots sprouted from the cuttings after a few weeks.
• (iv) To ensure a fair comparison, parameters such as soil type, amount of water, sunlight exposure, temperature, and cutting size must be kept exactly the same for both setups.

Q – 11. During the discussion in class, Rohan gives a statement that, “A tissue is a group of similar cells performing similar functions”. But Rajiv counter argues that, “this is true in case of simple tissues but little different in case of complex tissues”. Provide your explanation in view of the discussion in class. A – Rajiv is correct in his observation. A simple permanent tissue (like parenchyma or collenchyma) is made up of only one single type of identical cells that look alike and perform the same function. However, a complex permanent tissue (like xylem or phloem) is made up of more than one type of structurally different cells. For example, xylem contains tracheids, vessels, fibres, and parenchyma. Despite looking different, all these diverse cells coordinate and work together as a team to perform a single common function, which is transporting water.

Q – 12. Coconut husk fibres are used for mats which are tough and fibrous. Which tissue has structural features suitable for providing this strength? Explain why living parenchyma couldn’t serve the same purpose. A – Sclerenchyma tissue has the structural features required for making tough mats. Its cells are long, dead, and have thick walls cemented with a rigid chemical called lignin, which gives them high mechanical strength and durability. Living parenchyma tissue cannot serve this purpose because its cells have thin walls, are soft, loosely packed with intercellular spaces, and are designed for food storage or floating rather than withstanding heavy mechanical friction and pulling.

Q – 13. Vibha claims to her friend Neha that, “Meristematic cells are located only at the root and shoot apices”. What do you think about this statement? What question can Neha ask Vibha to help her understand further if the statement is incorrect? A – Vibha’s statement is incorrect because meristematic tissues are present in three different growth regions of a plant, not just at the tips. To help her understand, Neha can ask: “If dividing cells are located only at the root and shoot tips to increase height, then which tissue causes tree trunks to grow wider in girth over time (lateral meristem), and how do lawn grasses regenerate and grow new leaves after being mowed from the middle (intercalary meristem)?”

Q – 14. A plant cell and an animal cell are of the same size.

(i) Which cell will have a larger vacuole? Give reasons.

(ii) What assumptions are you making to answer the question above?

A –

• (i) The plant cell will have a much larger vacuole. Mature plant cells contain a prominent central vacuole that fills most of the cell’s volume to store water, cell sap, and nutrients, and to keep the cell firm and rigid. In contrast, animal cells either lack vacuoles entirely or have very small, temporary ones.
• (ii) We assume that the plant cell being observed is a mature permanent cell (like a parenchyma cell) rather than an actively dividing meristematic cell. This is because meristematic plant cells lack vacuoles to allow continuous and rapid cell division.

Q – 15. A textbook states, “Each plant tissue performs only one specific function”. What questions would you ask to critically examine the correctness of this statement? What examples of tissues would you take to find out the answers to these questions?

A –

• Questions to ask: “Can a single plant tissue perform more than one job simultaneously? For instance, can a food-storing tissue also perform photosynthesis or help a plant float? Can a water-transporting tissue also provide mechanical strength to the tree?”
• Examples to examine:

1. Parenchyma: While its main function is storing food, in green leaves it contains chlorophyll and performs photosynthesis (chlorenchyma), and in aquatic plants it forms large air spaces to provide buoyancy for floating (aerenchyma).
2. Xylem: While its primary function is transporting water and minerals, its thick-walled tracheids, vessels, and fibres also act as a structural skeleton that provides mechanical strength and support to keep the plant upright. These examples prove that a single plant tissue can perform multiple functions!