Page 28 – Think It Over
Q1: How is the study of cells and tissues significant for understanding life processes and human welfare?
Ans: The study of cells and tissues is important because cells are the basic units of life and in multicellular organisms they group together to form tissues that perform specific functions. This division of labour makes life processes more efficient, as different tissues carry out different activities such as movement, transport, and coordination. By studying cells and tissues, we understand how the body is organized and how various functions are performed, which helps in understanding life processes and contributes to human welfare.
Q2: How are tissues in plants and animals different, and why?
Ans: Plant tissues differ from animal tissues because plants are stationary and need support, while animals require movement. Hence, plant tissues are often rigid (like xylem), whereas animal tissues are more flexible and specialised for movement.
Q3: How is the division of labour at various levels of organisation in multicellular organisms correlated with their structure and function?
Ans: In multicellular organisms, there is division of labour at different levels of organisation. Cells specialise to perform specific functions and similar cells form tissues. Tissues combine to form organs, and organs work together in organ systems. Each level has a structure suited to its function, which helps in efficient functioning of the organism.
Page 33 – Pause and Ponder
Q1. 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.
Ans. Coconut husk fibres are hard and brittle because they are composed mainly of sclerenchyma tissue. Sclerenchyma cells have thick walls due to deposition of lignin – a hard substance that makes cells dead, rigid and very strong, giving coconut husk its hard, brittle nature. On the other hand, leaf stalks of coriander are soft and flexible because they contain collenchyma tissue. Collenchyma cells are living cells with unevenly thickened corners due to pectin (a flexible chemical, similar to rubber), which provides flexibility and allows the stalks to bend without breaking.
Page 34 – Pause and Ponder
Q2. 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?
Ans. For a desert plant, a thick cuticle is advantageous because it acts as a waterproof layer that greatly reduces water loss through transpiration. Since water is very scarce in desert conditions, minimising water loss is essential for the plant’s survival. For a plant living underwater, a thick cuticle is disadvantageous because such plants need to absorb water, minerals and dissolved gases directly from the surrounding water through their entire surface. A thick cuticle would block this absorption and gaseous exchange, harming the plant’s survival.
Q3. 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?
Ans. Xylem cells (tracheids and vessels) are dead and hollow, forming a continuous pipeline from roots to leaves. Their dead, tube-like structure offers very little resistance to water flow. The living cells of leaves carry out transpiration – they lose water vapour through stomata into the air. This creates a transpiration pull (a suction-like force), which draws water upward through the xylem tubes against gravity. So the dead xylem cells act as passive pipes and the living leaf cells create the suction force that keeps water moving upward continuously.
Q4. What do you think will happen if there were no stomata in the epidermis of the stem or leaves?
Ans. If there were no stomata, the following problems would occur:
- Gaseous exchange would stop – carbon dioxide needed for photosynthesis cannot enter and oxygen produced cannot exit, so photosynthesis will fail.
- Transpiration will stop – there will be no transpiration pull in the xylem, so water and mineral transport from roots to leaves will be severely affected.
- Waste gases produced by the plant cannot be eliminated, causing toxin build-up.
- Overall, the plant’s growth and survival will be severely compromised.
Page 40 – Pause and Ponder
Q5: Look at the picture given below. Carefully observe the various poses of classical and folk dances of India. Can you identify joints involved? Also, what type of movement each allows?
Ans:
- Shoulder joint – Ball and socket joint – allows arms to move forward, backward, sideways and in circular motion (seen in outstretched arms)
- Elbow joint – Hinge joint – allows bending and straightening of arm in one direction
- Hip joint – Ball and socket joint – allows legs to move in many directions including circular movements
- Knee joint – Hinge joint – allows bending and straightening of leg
- Wrist and ankle – Partial rotation and bending – allow graceful hand and foot movements
- Neck joint – Pivot joint – allows the characteristic side-to-side head movements common in Indian classical dance
Revise, Reflect, Refine
Q1. 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.
Ans. (iii) They have thin walls, dense cytoplasm and large prominent nucleus.
Thin cell walls allow the cell to divide without much resistance. Dense cytoplasm with many organelles provides all the machinery needed for rapid and continuous cell division. A large prominent nucleus provides genetic instructions for division. Vacuoles are absent in meristematic cells (not large). Thick walls and functional differentiation are characteristics of permanent tissues, not meristematic ones.
Q2. If a plant is unable to transport food from leaves to roots which tissue is malfunctioning?
(i) Xylem
(ii) Phloem
(iii) Epidermis
(iv) Sclerenchyma
Ans. (ii) Phloem.
Phloem is the conducting tissue that transports food (mainly sugars prepared by photosynthesis in leaves) to all other parts of the plant including roots and stems. Xylem transports water and minerals from roots upward. Epidermis is a protective tissue. Sclerenchyma provides strength. So if food transport from leaves to roots is affected, it is the phloem that is malfunctioning.
Q3. 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.
Ans. (iii) To allow quick exchange of materials across them.
Internal organs like the lungs, blood vessels and intestine require rapid exchange of gases, nutrients and other materials between the organ and the blood. Being only one or a few cells thick minimises the distance materials have to travel, enabling quick and efficient diffusion. A thick layer would slow down this exchange greatly.
Q4. 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?
Ans. In the straight-leg jump, the knees and ankles are kept stiff, so only the hip joint is used for the jumping motion. Landing is very hard, painful and causes greater impact on the body because the joints cannot absorb shock.
In the normal jump, the knees and ankles bend naturally. During takeoff and landing, the knee and ankle joints act as shock absorbers – cartilage at joints cushions the impact. The hip joint also participates in bending. This makes the jump smoother, more efficient and reduces stress on the bones and joints. The normal jump demonstrates why joints and cartilage are so important for movement and protection.
Q5. Which type of joint is involved when you bend your knees and ankles?
(i) Ball and socket
(ii) Hinge
(iii) Pivot
Ans. (ii) Hinge joint.
The knee and ankle joints are hinge joints. Like a door hinge, they allow movement in one direction only – bending (flexion) and straightening (extension). They do not allow sideways or circular movements. This is similar to the elbow joint, which is also a hinge joint.
Q6 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.
(A). Assertion: Epithelium is well-suited for gas exchange in the lungs.
Reason: It consists of multiple layers of tall cells that slow down diffusion.
Ans. (iii) Assertion (A) is true, but Reason (R) is false.
The assertion is correct – the epithelium lining the lungs (alveoli) is indeed well-suited for gas exchange. However, the reason given is incorrect. The epithelium in the lungs actually consists of a single layer of very thin, flat cells (squamous epithelium), NOT multiple layers of tall cells. This single thin layer allows rapid diffusion of oxygen and carbon dioxide between air and blood. Multiple layers would actually slow down gas exchange.
(B). Assertion: Cardiac muscle can contract continuously without fatigue.
Reason: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply.
Ans. (i) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Cardiac muscles are found only in the heart and they beat tirelessly and rhythmically throughout life without ever getting tired. This is because cardiac muscle cells have an exceptionally high number of mitochondria – these are the ‘energy factories’ of the cell that produce ATP (energy). Additionally, the heart muscle has a very rich blood supply (coronary arteries), ensuring constant delivery of oxygen and nutrients needed for energy production. Together, these features allow cardiac muscles to work continuously.
(C) . 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.
Ans. (iv) Assertion (A) is false, but Reason (R) is true.
The assertion is incorrect – tendons do NOT connect bone to bone. Tendons connect muscle to bone. It is ligaments that connect bone to bone and help prevent dislocation at joints. The reason, however, is correct – tendons are made of tough connective tissue and their function is to transmit the pulling force generated by muscle contraction to the bone, resulting in movement at a joint.
(D). Assertion: In a hinge joint, movement occurs primarily in one plane.
Reason: The bone ends are shaped to allow sliding in all directions.
Ans. (iii) Assertion (A) is true, but Reason (R) is false.
The assertion is correct – hinge joints like the elbow and knee allow movement in one plane only (bending and straightening). However, the reason is incorrect. The reason describes a ball and socket joint, not a hinge joint. In hinge joints, the bone ends are shaped like a hinge (interlocking ridges and grooves) that restrict movement to one direction only. It is the ball and socket joint where the rounded top of one bone fits into a hollow, allowing movement in multiple directions.
Q7. 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 the 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?
Ans.
(i) Graph Analysis: The diameter of the stem increases steadily with age. From age 5 to 10 years, growth is gradual (4 to 8 cm). Between 10 and 20 years, there is a rapid increase (8 to 24 cm). After 20 years, growth continues but at a more moderate rate. This shows that the tree grows in girth most rapidly in its younger years, then the rate of growth gradually stabilises. The diameter increases continuously as long as the tree is alive.
(ii) Relation between diameter and annual rings: The number of annual rings equals the age of the tree in years (e.g., 5 years = 5 rings, 40 years = 40 rings). Each year, one new ring is formed. The diameter at breast height (DBH) also increases with age. So, both increase with age – but not in the same proportion. Diameter growth is rapid in early years then slows. This means annual rings are not all equal in thickness – early rings may be wider (more growth) and later rings may be narrower.
(iii) The specialised tissue responsible for girth (increase in diameter/thickness of stem) is the lateral meristem. It is located along the circumference (periphery) of the stem, arranged in a ring. Cells of the lateral meristem divide and add new cells inward and outward in a concentric manner, causing the stem to become thicker over time.
Q8. 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?
Ans.
(i) Debarking removes the bark which includes the epidermis and the phloem (outer bark). Functions hampered:
- Protection – the epidermis/bark protects the tree from mechanical injury, water loss, and entry of microorganisms. Without it, the tree is exposed to infections and drying.
- Food transport – the phloem present in the bark transports food from leaves to roots. Removal of bark interrupts phloem, so roots cannot receive food (sugars) from leaves, eventually killing the roots.
(ii) After debarking, further damage to the tree trunk would affect the xylem (wood). Xylem is located beneath the phloem/bark and forms the woody part of the trunk. Additionally, if the lateral meristem (cambium) is damaged, the tree would lose its ability to grow in girth and regenerate new xylem and phloem layers.
(iii) If the tissues beneath the bark (mainly xylem) were severely damaged, the transport of water and minerals from roots to leaves would be hampered. Without water and minerals reaching leaves, photosynthesis would fail, and the tree would eventually die.
(iv) Assumption: The debarking is complete (removes all bark including phloem) and occurs around the entire circumference of the tree trunk (ring-barking). If the assumption changes – if only partial debarking occurred, leaving some phloem intact on one side – some food transport may continue, allowing the tree to partially survive longer. Similarly, if the lateral meristem (cambium) beneath the bark is intact, the tree may regenerate new bark over time.
Q9. 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.
Ans. The tissue responsible for the flexibility of the young mango sapling’s stem is collenchyma. Collenchyma consists of living cells with unevenly thickened corners due to pectin (a chemical that provides rubber-like flexibility). This tissue allows the stem to bend without breaking when subjected to wind.
If collenchyma was replaced by sclerenchyma, the stem would become hard, brittle and rigid because sclerenchyma cells have thick walls with lignin deposition that makes them dead and inflexible. Instead of bending, the stem would snap and break under the force of monsoon winds. The plant would have no ability to withstand mechanical stress. This shows why different tissues have different compositions suited to their specific functions – collenchyma for flexible support in young plants, sclerenchyma for hard, permanent support in mature parts.
Q10. 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?
Ans.
(i) Type ‘B’ cuttings were able to grow because they contained nodes – the regions of the stem that have intercalary meristem. Intercalary meristem is present at the base of internodes or just above nodes. This meristematic tissue can actively divide and give rise to new shoots. Type ‘A’ cuttings likely did not contain nodes, so they lacked meristematic tissue and had no ability to regenerate new growth.
(ii) The difference in type ‘B’ compared to type ‘A’ was the presence of nodes (which contain meristematic/intercalary meristem tissue). Type ‘B’ cuttings had at least one node that could give rise to new buds and eventually develop into a full plant. Type ‘A’ cuttings may have only had internodal sections without any node.
(iii) The observation made was whether the cuttings sprouted (produced new buds or shoots) and eventually developed into complete sugarcane plants. Measurement of the length/growth of new shoots over a few weeks was likely recorded to confirm the effect.
(iv) To ensure a fair comparison, the following parameters should be kept the same for both types of cuttings:
- Same species of sugarcane
- Same length and size of cuttings
- Same soil type and pot/container
- Same amount of water given
- Same sunlight exposure
- Same temperature and environmental conditions
- Same time period of observation
- Same number of cuttings for each type (replication)
Q11. 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.
Ans. Both Rohan and Rajiv are partially correct.
Rohan’s statement is true for simple permanent tissues like parenchyma, collenchyma and sclerenchyma – in these, all cells are of the same type and perform the same function (e.g., all parenchyma cells store food).
Rajiv’s counter-argument is correct for complex permanent tissues. Xylem is a complex tissue made of four different types of cells – tracheids, vessels, xylem parenchyma and xylem fibres. Each type of cell in xylem has a different structure and performs a different function (e.g., tracheids and vessels conduct water; xylem parenchyma stores food; xylem fibres provide support). Similarly, phloem has sieve tubes, companion cells, phloem parenchyma and phloem fibres with different structures and roles. However, all these different cells work together as a team to achieve the overall function of the complex tissue (water transport in xylem, food transport in phloem).
So a better definition would be: A tissue is a group of cells (similar or different in structure) that are organised and work together to perform one or more specific functions for the organism.
Q12. 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.
Ans. The tissue responsible for the toughness and strength of coconut husk fibres is sclerenchyma. Sclerenchyma cells have very thick cell walls due to heavy deposition of lignin – a hard, rigid chemical. These cells are dead at maturity, with the cell wall being extremely hard and strong. This gives the fibres their characteristic toughness, hardness and fibrous nature, making them suitable for making mats and ropes.
Living parenchyma cells cannot serve this purpose because:
- They have thin cell walls with no lignin deposition, making them soft and fragile
- They are living cells filled with watery cytoplasm, which would make them soft and easily damaged
- They cannot provide the rigidity, hardness or tensile strength needed for tough fibrous products
In contrast, the dead, lignin-rich sclerenchyma cells form a tough structural material that can withstand pulling forces and wear.
Q13. 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?
Ans. Vibha’s statement is incorrect. Meristematic cells are NOT located only at the root and shoot apices. While apical meristem is present at root and shoot tips, there are two more types of meristematic tissue:
- Lateral meristem – located along the circumference of stems; responsible for increase in girth/diameter
- Intercalary meristem – located at the base of internodes or just above nodes; responsible for regrowth after cutting or grazing
Neha can ask Vibha the following questions to help her understand:
- “If meristematic cells are only at root and shoot tips, how does a grass plant grow back after being mowed or grazed by animals?” (This would direct Vibha to think about intercalary meristem.)
- “If meristematic cells are only at tips, how does the trunk of a tree become thicker and wider as it grows older?” (This would direct Vibha to think about lateral meristem.)
Q14. 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?
Ans.
(i) The plant cell will have a larger vacuole compared to the animal cell. In plant cells, vacuoles are large and prominent, often occupying 60-90% of the cell volume. They play important roles in storing water (maintaining turgidity and firmness), storing nutrients and waste products, and maintaining the shape of the cell. In animal cells, vacuoles are either absent or very small and temporary (used for transport or digestion). Therefore, even if both cells are the same size, the plant cell’s vacuole will be significantly larger.
(ii) Assumptions being made:
- We are comparing a typical mature plant cell (like parenchyma) with a typical animal cell – not a specialised plant cell like a meristematic cell (which has very small or no vacuoles)
- Both cells are in a healthy, normal physiological state
- The plant cell is a non-meristematic, permanent tissue cell
- The animal cell is not a specialised secretory cell that might have larger vacuoles
If the assumption changes (e.g., if we compare a meristematic plant cell with an animal cell), the answer might be different, as meristematic cells lack vacuoles.
Q15. 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?
Ans. The statement is an oversimplification and is not entirely correct. To critically examine it, one would ask the following questions:
- Does parenchyma perform only one function? – Parenchyma primarily stores food, but it also performs photosynthesis in green parts (chlorenchyma), and in aquatic plants, specialised parenchyma (aerenchyma) forms air spaces to help plants float. So parenchyma performs multiple functions – this challenges the statement.
- Does epidermis perform only one function? – Epidermis primarily provides protection, but it also helps in transpiration (through stomata), gaseous exchange, absorption (root hair cells absorb water), and elimination of waste. So epidermis performs multiple functions.
- Does xylem perform only one function? – Xylem transports water and minerals, but xylem fibres also provide mechanical strength to the plant. So xylem has dual roles.
- Does phloem perform only one function? – Phloem transports food, but phloem parenchyma stores food, tannins, resins and latex; phloem fibres provide support. So phloem also performs multiple functions.
Conclusion: The textbook statement is incorrect. While each tissue has a primary specialised function, many plant tissues perform more than one function. The examples of parenchyma, epidermis, xylem and phloem demonstrate this clearly.