Chapter 3 : Tissues in Action

Intext questions and answers

Think It Over

1. How is the study of cells and tissues significant for understanding the life processes and human welfare? Life begins when a single cell divides itself several times to give rise to a large number of cells, which gradually form organs like the skin, muscles, bones, and nerves. Understanding these intricate natural biological processes that govern growth and development allows researchers to replicate and modify them for human welfare. For example, scientists use stem cells from the bone marrow of a healthy person to treat patients with blood cancers like Leukemia or disorders like Thalassemia.

2. How are tissues in plants and animals different, and why? Plant and animal tissues differ largely because of their mobility and their mode of nutrition. Plants are fixed in one place, so they need supporting tissues with rigid cell walls to provide strength and keep them upright. Animals, on the other hand, move from place to place; therefore, they lack a rigid cell wall, giving their cells the flexibility needed for locomotion. Additionally, plants have specific tissues adapted for synthesizing food via photosynthesis, whereas animals have tissues designed for digesting food obtained from various sources. The tissues responsible for their distinct growth patterns also differ structurally and functionally.

3. How is the division of labour at various levels of organisation in multicellular organisms correlated with their structure and function? In multicellular organisms, cells of a similar type group together to form a tissue, tissues form organs, and organs form organ systems. This structural hierarchy leads to a division of labour, which increases the body’s efficiency to carry out complex life processes. Because of this, specific structures are correlated with specific functions: for instance, muscle tissue structurally enables movement, while nervous tissue carries messages.

Pause and Ponder

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. The fibres of a coconut husk are made up of sclerenchyma tissue. Sclerenchyma cells have thick walls due to the deposition of lignin, which makes them hard, brittle, and strong. In contrast, the leaf stalks of coriander contain collenchyma tissue, which consists of living cells with unevenly thickened corners due to pectin. Pectin provides flexibility, allowing plant parts to bend easily without breaking.

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? The cuticle is a waxy layer of cutin covering the epidermis that reduces water loss. A thick cuticle is highly advantageous for a desert plant as it prevents excessive evaporation and transpiration in dry habitats. However, an underwater plant does not need to conserve water. A thick cuticle would be disadvantageous as it would block the necessary exchange of dissolved gases and nutrients directly across the epidermis from the surrounding water.

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? The living epidermal cells in the leaves contain pores called stomata. Water vapour evaporates through these stomata in a process known as transpiration. This evaporation creates a “transpiration pull” at the top of the plant, which draws water upwards against gravity through the thick-walled, tubular dead cells (tracheids and vessels) of the xylem.

4. What do you think will happen if there were no stomata in the epidermis of the stem or leaves? If a plant had no stomata, gaseous exchange essential for photosynthesis and respiration could not occur. Furthermore, transpiration would stop, meaning the plant could not create the transpiration pull needed to transport water and dissolved minerals from the roots up through the xylem. Elimination of wastes would also be severely hampered.

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?

The poses involve multiple joints working together:

• Shoulder Joint: A ball and socket joint that allows free movement of the arm forward, backward, sideways, and in a circular motion.
• Elbow and Knee Joints: Hinge joints that allow bending and straightening in one direction only.
• Neck Joint: A pivot joint that connects the skull to the backbone, allowing the head to move freely from side to side.

Think as a Scientist: From one cell to an organism (Table 3.6)

(a) What do you conclude about the characteristics of phloem cells of carrot? Mature phloem cells of a carrot retain the unique ability to dedifferentiate (regain the ability to divide) and form a mass of unspecialised cells. When given the right conditions, they can divide and redifferentiate to develop into a complete new plant—a property known as totipotency.

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

• Highest Biomass: The highest biomass (a 20% increase in fresh weight) is obtained when the cells are exposed to light, air, and a liquid medium containing nutrients. This is because light enables photosynthesis, air provides necessary gases for respiration, and the liquid medium (when stirred) allows single cells to easily shear off and divide freely.
• Lowest Biomass: The biomass is reduced when either air is restricted (using a solid medium instead of liquid) or when light is absent.

(c) Will you get the same results if you culture animal cells instead of carrot cells?

No. Highly specialized mature animal cells generally lack the natural totipotency seen in mature plant cells and cannot dedifferentiate and regenerate into an entire complex organism under these simple culture conditions.

(d) Think and mention any two commercial applications of the study above.

1. Micropropagation: Rapid mass multiplication of identical, high-yielding crop plants.
2. Disease Resistance: Generating disease-free or genetically modified disease-resistant plant varieties for modern agriculture.

Revise, Reflect, Refine

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.

• Correct Answer: (iii) They have thin walls, dense cytoplasm and large prominent nucleus.

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

• Correct Answer: (ii) Phloem.

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.

• Correct Answer: (iii) To allow quick exchange of materials across them. (A single layer of thin, flat cells helps in the rapid diffusion of liquids and gases ).

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? During the straight-leg jump, the hinge joints in the knees and ankles are kept locked, which limits movement to a single stiff plane. In a normal jump, the hinge joints at the knee and ankle naturally bend, allowing the bones to slide and absorb the shock upon landing while providing upward spring force.

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

(i) Ball and socket

(ii) Hinge

(iii) Pivot

• Correct Answer: (ii) Hinge.

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.

• 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.
  • Answer: (iii) (A) is true, but (R) is false. The epithelium lining the lungs consists of a single layer of thin, flat cells specifically to help in rapid diffusion of gases, not multiple tall layers.
• B. Assertion: Cardiac muscle can contract continuously without fatigue. Reason: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply.
  • Answer: (i) Both (A) and (R) are true, and (R) is the correct explanation of (A). Cardiac muscles work tirelessly and rhythmically throughout an individual’s life. (The high number of mitochondria is a biological fact that correctly explains their tireless nature, though mitochondria are not explicitly detailed in this section’s cardiac text).
• 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.
  • Answer: (iv) (A) is false, but (R) is true. Tendons connect muscles to bones, not bone to bone (ligaments connect bone to bone). When a muscle contracts, the tendon successfully transmits this force to the bone for movement.
• D. Assertion: In a hinge joint, movement occurs primarily in one plane. Reason: The bone ends are shaped to allow sliding in all directions.
  • Answer: (iii) (A) is true, but (R) is false. A hinge joint, like the elbow or knee, bends and straightens in one direction only. Therefore, the bone ends are not shaped to allow sliding in all directions.

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 the Table 3.7.

(For plotting the graph, visualize Age on the X-axis mapping to both DBH and Annual Rings on the Y-axis. The line moves strictly upwards diagonally).

• (i) Analyse the graph in terms of the diameter of the stem over time and share the interpretation. As the age of the teak tree progresses, there is a steady and linear increase in its diameter at breast height (DBH).
• (ii) What is the relation between the diameter of the teak tree to the annual rings formed? There is a direct proportional relationship. Each year corresponds to the formation of exactly one annual growth ring, which contributes progressively to the increased diameter of the tree.
• (iii) Which specialised tissue is responsible for the girth of the stem and where is it located? The lateral meristem is responsible for increasing the girth (diameter) of the stem, and it is located along the circumference of the stem.

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? The bark consists of cork cells that provide a protective layer making the tree impermeable to water and gases. Removing it exposes the tree to mechanical injury, harmful microorganisms, and water loss.
• (ii) Which plant tissue would be affected by further damage to the tree trunk even after debarking? Further damage beneath the cork cells would directly affect the phloem, which sits just beneath the bark as part of the vascular tissue system.
• (iii) Which function of the tree would be hampered if the tissues beneath the bark were severely damaged? If the phloem is damaged, the tree will lose its ability to transport food prepared in the leaves down to the roots and other parts of the plant.
• (iv) What assumptions are you making to answer the questions above? How would the answer change if your assumptions are also changed? The assumption is that the elephant only stripped away the outer protective layers (dermal tissue/bark) and the phloem beneath it, but left the deeper xylem intact. If we assume the damage penetrated even deeper into the wood, then the xylem would also be destroyed, meaning the plant could no longer transport water and minerals from the roots up to the leaves.

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. Collenchyma tissue is responsible for providing this support and flexibility, allowing the stem to bend without breaking. If the collenchyma were replaced by sclerenchyma, the stem would become hard, woody, and inflexible due to the thick lignified walls of sclerenchyma cells. Consequently, the stem would snap and break under the heavy force of monsoon winds instead of bending.

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? Type ‘B’ cuttings were able to grow because they contained a node, which houses the intercalary meristem responsible for producing new branches and growth.
• (ii) What difference was present in type ‘B’ compared to type ‘A’? Type ‘B’ contained the node (the joint where meristematic tissue is present), whereas type ‘A’ was simply the internode section (the space between joints) completely lacking the intercalary meristem necessary for sprouting.
• (iii) What observation or measurement was made to determine whether this change had an effect? Sohan observed whether the cuttings successfully “sprouted and developed into sugarcane plants” over the span of a few weeks.
• (iv) What parameters should be kept the same for both types of cuttings to ensure a fair comparison?Environmental parameters such as the amount of sunlight, water, soil nutrient composition, and ambient temperature should be kept identical for both cuttings.

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. Rajiv’s counter-argument is scientifically accurate. While simple permanent tissues (like parenchyma, collenchyma, and sclerenchyma) are composed of only one single type of cell performing a uniform function , complex permanent tissues are made up of different types of cells working together. For example, the complex tissue xylem consists of tracheids, vessels, xylem parenchyma, and xylem fibres.

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. Sclerenchyma tissue provides this tough strength because it is composed of dead cells with thick walls heavily deposited with lignin, making them exceptionally hard and strong. Parenchyma couldn’t serve this purpose because it consists of living cells with thin, unlignified walls and loose intercellular spaces. While it is good for storing food and providing basic support, it lacks the rigid structural integrity needed to withstand the mechanical stress of a tough mat.

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? Vibha’s statement is incorrect. While apical meristems are indeed located at the root and shoot tips to increase length , plants also have lateral meristems along the circumference of stems to increase girth, and intercalary meristems at the base of nodes to help branches regenerate. Neha could ask Vibha: “If meristems are only at the tips of the roots and shoots to increase length, then what tissue is responsible for making a tree trunk wider, or helping grass grow back after it has been mowed?”

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

• (i) Which cell will have a larger vacuole? Give reasons.The mature plant cell will have a significantly larger vacuole. In permanent plant tissues, a large central vacuole is generally present to store nutrients and maintain the cell’s rigid structure. In contrast, animal cells either lack vacuoles entirely or possess only very small, temporary ones.
• (ii) What assumptions are you making to answer the question above? The main assumption is that the plant cell in question is a mature, differentiated cell (like a parenchyma cell). If it were an actively dividing meristematic plant cell, it would likely lack a vacuole entirely.

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?

To critically examine this statement, you could ask: “Are there any plant tissues that perform secondary functions in addition to their main role? Can a tissue’s function vary depending on its location in the plant?” To answer this, look at these examples:

• Parenchyma: Its primary function is to store food. However, it also performs photosynthesis in the green parts of the plant, and in aquatic plants, specialised parenchyma forms air spaces to help the plant float.
• Xylem: Its primary function is to transport water and minerals. However, because it contains thick-walled sclerenchymatous fibres, it also secondarily provides immense structural strength to the plant. Therefore, the textbook statement is an oversimplification; many plant tissues perform multiple, complementary functions.