Chapter 3: Tissues in Action

Cells are the basic units of life, and tissues are groups of cells working together to perform specific functions. Understanding how tissues are structured helps us know how organs function, how diseases arise (like cancer, where cell division goes uncontrolled), and how the body maintains itself. This knowledge has direct applications in medicine, such as developing treatments for tissue damage, growing replacement tissues (tissue engineering), and understanding how drugs affect the body. For human welfare, it enables advances in agriculture (crop improvement), medical diagnosis, and therapies like skin grafting.

Plant tissues differ from animal tissues mainly because plants are stationary and must synthesise their own food through photosynthesis, while animals move and obtain food from external sources. Plants have rigid cell walls and tissues like sclerenchyma for support, and meristematic tissues for continuous growth, which are absent in animals. Animal tissues are more diverse (epithelial, connective, muscular, nervous) to support locomotion, digestion, and coordination. The different lifestyles and modes of nutrition drive these structural and functional differences in tissues.

In multicellular organisms, different cells are specialised to perform different tasks — this is called division of labour. Cells of similar type group into tissues (e.g., muscle tissue for movement), tissues form organs (e.g., heart), and organs form organ systems (e.g., circulatory system). This hierarchy allows each unit to be highly efficient at its specific task, much like specialised workers in a factory. The structure of each tissue is directly suited to its function — for example, xylem cells are hollow and dead to allow smooth water transport, while nerve cells are long with branches to rapidly transmit signals.

Observation: Roots in Jar A (uncut) continue to grow steadily in length each day. Roots in Jar B (tips cut on Day 3) stop growing or grow much more slowly after the tips are removed.

Inference: Root growth occurs only from the tip region, which contains actively dividing meristematic cells (apical meristem). When the tip is removed, the source of new cells is lost and growth stops. This confirms that apical meristem at the root tip is responsible for the increase in root length.

Coconut husk fibres are composed mainly of sclerenchyma cells, which have thick walls impregnated with lignin. Lignin makes the walls extremely hard and rigid, and since these cells are dead at maturity, they provide tough, inflexible mechanical support. Coriander leaf stalks, on the other hand, are composed largely of collenchyma cells, which are living cells with unevenly thickened walls due to pectin deposition (not lignin). Pectin allows flexibility, so collenchyma provides support while still allowing the stalk to bend without breaking.

In desert plants, water is scarce and there is a constant risk of water loss through the leaf surface (transpiration). A thick cuticle, being a waxy, impermeable layer, reduces this water loss and helps the plant survive in dry conditions — making it highly advantageous. For aquatic plants, however, water and dissolved gases are absorbed directly from the surrounding water through the leaf surface. A thick cuticle would block this absorption and prevent gaseous exchange, making it disadvantageous and even harmful for the plant's survival.

Xylem vessels and tracheids are dead, hollow, and lignified, forming continuous tubes that offer low resistance to water flow. The movement of water is driven by transpiration pull — water evaporates from the stomata in leaves (a process in living leaf cells), creating a negative pressure (suction) at the top of the plant. This suction pulls water upward through the continuous xylem column, a phenomenon explained by the cohesion-tension theory. Thus, the living leaf cells create the driving force, while the dead xylem cells provide the structural pathway — both are essential for water transport against gravity.

Stomata are tiny pores in the epidermis that serve two critical functions: gaseous exchange (CO₂ in for photosynthesis, O₂ out) and transpiration (loss of water vapour). Without stomata, carbon dioxide could not enter the leaves, making photosynthesis impossible, and the plant would eventually die. Additionally, the transpiration pull that drives water movement upward through xylem would be eliminated, disrupting water and mineral transport to all parts of the plant. The plant would also be unable to regulate its internal temperature through evaporative cooling.

Question: Why does the infected area become red and swollen?

Question: Why do we breathe faster and why does the face turn red during exercise?

When you lift a heavy object, you feel the muscles pulling on bones — this force is transmitted through tendons (connective tissue connecting muscle to bone). When you bend your knee or elbow, the stability at the joint is maintained by ligaments (connecting bone to bone). The smooth movement at the joint is cushioned by cartilage at the ends of bones. Blood as a fluid connective tissue is involved whenever there is internal activity requiring oxygen and nutrient delivery to the working muscles.

Steps to calculate:

• Record your total body weight (e.g., 45 kg).
• Use standard percentages: bone mass ≈ 12–15%, muscle mass ≈ 30–50% (varies by age and gender).
• Estimate bone weight = 45 × 0.14 ≈ 6.3 kg; muscle weight = 45 × 0.40 ≈ 18 kg.

Bone and muscle mass differ between individuals due to factors like age, gender, physical activity, and nutrition. Together, bones and muscles account for more than half the body weight, highlighting the importance of the musculoskeletal system in supporting and moving the body.

Different body parts allow different types of movement depending on the type of joint present. The shoulder and hip allow movement in all directions (ball and socket joint), the elbow and knee bend in one direction (hinge joint), and the neck allows rotation (pivot joint). Fixed joints in the skull allow no movement at all. By moving these body parts, we observe that the shape and structure of the joint determines the range and direction of movement possible.

Bones themselves cannot move on their own — they are moved by skeletal muscles. Muscles are attached to bones via tendons (tough connective tissue). When a muscle contracts (shortens), it pulls the tendon, which in turn pulls the bone, causing movement at the joint. This action is controlled voluntarily by the nervous system, which sends signals to the muscle fibres, triggering contraction. Thus, it is the coordinated action of muscles, tendons, bones, and nerves that causes bone movement.

In classical dance poses like Bharatanatyam or Kathak, multiple joints are actively used simultaneously. The hip joint (ball and socket) allows wide sideways and rotational leg movements; the knee joint (hinge) allows bending and straightening; the ankle joint (hinge) allows pointing and flexing of the foot; and the shoulder and wrist joints allow graceful arm and hand gestures. The spine (pivot and flexible vertebral joints) allows bending, twisting, and arching. These dances demonstrate the full range of motion available through different joint types in the human body.

The experiment on carrot cell regeneration (Fig. 3.19) shows that individual carrot phloem cells are totipotent — they retain the ability to dedifferentiate (revert to a meristematic state) and then redifferentiate into all cell types needed to form a complete new plant. This means each phloem cell carries the full genetic information of the organism and can express any part of it under the right conditions. This property demonstrates that differentiation in plants is not necessarily permanent and is reversible under appropriate nutritional and hormonal stimulation.

The highest biomass would be obtained in the combination with both nutrients and hormones, because nutrients provide the raw materials (carbon, nitrogen, minerals) for cell growth while hormones (like auxins and cytokinins) stimulate cell division and differentiation. The lowest biomass would result from the medium with neither nutrients nor hormones, as cells would lack both the building blocks and the signals needed for division and growth. The medium with only one component (nutrients or hormones alone) would show intermediate results, confirming that both are necessary for optimal plant cell growth.

No, you will not get the same results with animal cells. Most animal cells are not totipotent — once differentiated, they generally cannot revert to an undifferentiated state and regenerate an entire organism under simple culture conditions. While embryonic stem cells have some pluripotency (ability to form many cell types), adult animal cells like muscle or nerve cells are highly specialised and cannot form a complete animal from a single cell. This is a fundamental difference between plant and animal cells in terms of developmental plasticity.

Two commercial applications:

1. Micropropagation (Tissue Culture Agriculture): Large numbers of identical, disease-free plants (e.g., orchids, bananas, potatoes) can be produced rapidly from small tissue samples, ensuring uniform quality and high yields for commercial farming.
2. Production of secondary metabolites: Plant cells in culture can be used to produce valuable compounds such as medicines (e.g., vincristine from periwinkle for cancer treatment), flavours, fragrances, and dyes on an industrial scale without needing to grow full plants.

✔ Correct Answer: (iii)

Why (iii) is correct: Meristematic cells have thin, flexible cell walls that allow rapid cell division without hindrance. Their dense cytoplasm is packed with organelles needed for active metabolism, and the large, prominent nucleus contains the genetic material required to control continuous and rapid cell division.

Why other options are wrong:

• (i) Thick walls are a feature of sclerenchyma (permanent tissue), not meristematic tissue; thick walls would actually hinder cell division.
• (ii) Meristematic cells typically lack large vacuoles — vacuoles would take up space and reduce cytoplasmic density, making division less efficient.
• (iv) Meristematic cells are undifferentiated, not functionally differentiated; differentiation is what converts them into permanent tissues.

✔ Correct Answer: (ii)

Why (ii) is correct: Phloem is the conducting tissue responsible for transporting food (sugars synthesised during photosynthesis) from the leaves (source) to other parts of the plant including roots (sink) — a process called translocation. Sieve tubes and companion cells in phloem facilitate this downward transport of food.

Why other options are wrong:

• (i) Xylem transports water and minerals upward from roots to leaves, not food.
• (iii) Epidermis is a protective tissue that forms the outer covering; it is not involved in food transport.
• (iv) Sclerenchyma is a supporting tissue with thick lignified walls; it provides structural strength, not transport.

✔ Correct Answer: (iii)

Why (iii) is correct: Epithelial tissues lining organs like the lungs (alveoli) and blood capillaries need to be thin so that gases, nutrients, and waste products can diffuse across them quickly and efficiently. A thinner tissue means a shorter diffusion distance, which speeds up the exchange of materials essential for the body's functioning.

Why other options are wrong:

• (i) Food storage is the function of fat tissue (adipose tissue), not epithelial tissue.
• (ii) Maximum strength requires multiple layers (stratified epithelium for protection in skin), which would be counterproductive for exchange surfaces.
• (iv) Reducing friction is the function of mucus secreted by epithelial cells, not the thickness of the tissue itself.

In a straight-leg jump, the ankles and knees remain stiff and do not bend, so all the impact force upon landing is absorbed directly by the bones and fixed joints, making the landing jarring and uncomfortable. In a normal jump, the ankles, knees, and hips all bend (flex) during take-off and landing, acting as natural shock absorbers. The hip moves backward and downward, the knee bends significantly, and the ankle flexes to cushion the landing. This demonstrates that hinge joints (knee and ankle) and the ball and socket joint (hip) work together to distribute impact force and protect the skeletal system.

✔ Correct Answer: (ii)

Why (ii) is correct: The knee and ankle are hinge joints, which allow movement in only one plane — bending (flexion) and straightening (extension) — just like the hinge of a door. This design allows powerful, controlled movement in one direction, which is why knees and ankles bend during jumping and walking.

Why other options are wrong:

• (i) Ball and socket joints (like shoulder and hip) allow free movement in multiple directions, which is not the case for knees and ankles.
• (iii) Pivot joints allow rotational movement only (like the neck rotating side to side), not the bending action seen at the knee and ankle.

✔ Correct Answer: (iii) — (A) is true, but (R) is false.

Epithelium in the lungs (simple squamous epithelium) is indeed well-suited for gas exchange because it is a single layer of very thin, flat cells that allow rapid diffusion of oxygen and carbon dioxide. However, the reason given is incorrect — the lung epithelium does NOT consist of multiple layers of tall cells. Multiple layers would slow diffusion, but the actual lung epithelium is a single, thin layer specifically designed to maximise diffusion speed.

✔ Correct Answer: (i) — Both (A) and (R) are true, and (R) is the correct explanation of (A).

Cardiac muscles must contract rhythmically and continuously throughout life without tiring. This is possible because cardiac muscle cells are packed with a very high number of mitochondria — the powerhouses of the cell — which generate enormous amounts of ATP energy continuously. The heart also has its own abundant blood supply (via coronary arteries) that ensures constant delivery of oxygen and nutrients, sustaining non-stop contraction. Thus, the reason directly and correctly explains the assertion.

✔ Correct Answer: (iv) — (A) is false, but (R) is true.

The assertion is incorrect because tendons connect muscles to bones (not bone to bone). It is ligaments that connect bone to bone and provide joint stability. The reason, however, is correct — tendons are indeed made of tough, fibrous connective tissue (rich in collagen fibres) and their function is to transmit the force generated by muscle contractions to the bones, thereby causing movement.

✔ Correct Answer: (iii) — (A) is true, but (R) is false.

The assertion is correct — hinge joints like the knee and elbow do allow movement in only one plane (flexion and extension). However, the reason is false — the bone ends in a hinge joint are NOT shaped to allow sliding in all directions. Instead, the complementary shapes of the bone ends at a hinge joint are designed to restrict movement to a single axis, like a door hinge, preventing unwanted movements in other directions.

(i) Analysis of stem diameter over time: As the age of the tree increases, the diameter of the stem also increases steadily. The graph shows a positive, roughly linear relationship between age and diameter — older trees have wider trunks. This increase in diameter (girth) corresponds to the addition of new layers of wood each year by the lateral meristem.

(ii) Relationship between diameter and annual rings: The number of annual rings equals the age of the tree, and the diameter increases proportionally with the number of rings. Each ring represents one year of growth — a wider ring indicates a year of good growth conditions (adequate water and nutrients), while a narrow ring indicates unfavourable conditions. Thus, diameter and the number of annual rings are directly proportional.

(iii) Tissue responsible for increase in girth: The lateral meristem (also called vascular cambium) is responsible for the increase in girth of the stem. It is located as a ring of actively dividing meristematic cells arranged along the circumference of the stem. These cells divide and produce new xylem cells towards the inside and new phloem cells towards the outside, leading to an annual increase in stem diameter visible as growth rings.

(i) Functions hampered by debarking: The bark contains phloem (which transports food from leaves to roots) and cork (which provides protection against water loss, mechanical injury, and pathogens). Debarking disrupts food transport downward and removes the protective layer, exposing the inner tissues to infection, desiccation, and physical damage.

(ii) Tissue affected by further damage to trunk: If damage extends deeper into the trunk beyond the bark, the xylem (wood) would be affected. Xylem is the water-conducting tissue that transports water and minerals from roots to leaves. Damage to xylem would disrupt this upward transport.

(iii) Function hampered if tissues beneath bark are damaged: If the xylem beneath the bark is severely damaged, the transport of water and dissolved minerals from the roots to the leaves would be severely hampered. Without water, photosynthesis would stop, and the tree would eventually die.

(iv) Assumptions and changes: We assume the debarking is complete around the circumference (ring-barking) and that the xylem is intact. If only partial debarking has occurred, the tree may survive if some phloem pathways remain. If the lateral meristem (cambium) just beneath the bark is intact, the tree may regenerate new phloem and cork over time. If cambium is destroyed along with the bark, regeneration becomes impossible.

The tissue responsible for flexibility in young stems is collenchyma. Collenchyma cells are living cells with unevenly thickened corners due to deposition of pectin (not lignin), which allows the cells to bend and flex without breaking, providing mechanical support while remaining pliable. If collenchyma were replaced by sclerenchyma, the stem would become extremely rigid and brittle due to the thick lignified walls of sclerenchyma cells. Under monsoon winds, such a rigid stem would not be able to bend and absorb the force, and the sapling would likely snap or break rather than flex and survive.

(i) Why type B grew but type A did not: Type B cuttings were able to grow because they contained at least one node — the point on the stem where intercalary meristem and axillary buds are present. These meristematic cells can divide and differentiate to produce new shoots and roots. Type A cuttings lacked nodes (they were internodal segments) and hence had no meristematic tissue to initiate new growth.

(ii) Difference in type B compared to type A: Type B cuttings had nodes (with buds and intercalary meristem), while type A cuttings were internodal segments without any meristematic tissue or buds. The presence of a node is essential for vegetative propagation through cuttings.

(iii) Observation made to determine change: The appearance of new shoots and roots (sprouting) from the cuttings after a few weeks was the key observation. Growth in type B and absence of growth in type A confirmed that nodes are necessary for regeneration.

(iv) Parameters to keep the same for a fair comparison: Both cuttings should be kept in the same: soil type and quality, amount of water and irrigation, temperature and light conditions, length and thickness of the cutting, and time period of observation. Only the variable being tested (presence/absence of node) should differ between the two types.

Rohan's statement is correct for simple permanent tissues like parenchyma, collenchyma, and sclerenchyma — each is made of only one type of cell, and all cells in the tissue look similar and perform the same function (storage, flexibility, or mechanical support respectively). However, Rajiv is also correct — complex permanent tissues like xylem and phloem are made up of more than one type of cell (e.g., xylem contains vessels, tracheids, xylem parenchyma, and xylem fibres), each with a different structure. Yet all these different cell types work together as a unit to perform a single overall function (water transport in xylem). So, complex tissues are groups of different cells performing a single combined function, refining Rohan's definition.

Coconut husk fibres are composed of sclerenchyma tissue. Sclerenchyma cells have extremely thick cell walls impregnated with lignin — a hard, waterproof polymer — making them very strong, rigid, and durable. By the time sclerenchyma cells are mature and functional, they are dead (their living contents disappear), leaving only the tough wall behind, which is ideal for making fibrous mats. Parenchyma cells, being living cells with only thin primary walls and large central vacuoles, are soft, flexible, and easily compressed or degraded. They lack the mechanical strength, rigidity, and resistance to wear that sclerenchyma provides, making them completely unsuitable for making durable fibrous mats.

Vibha's statement is incomplete and incorrect. While apical meristems are indeed located at root and shoot tips, meristematic tissue is also found at two other locations. Lateral meristem is located as a ring along the circumference of stems and is responsible for increasing stem girth (seen as annual rings). Intercalary meristem is located at the base of internodes or just above nodes and helps in regrowth after cutting (e.g., in grasses). Neha could ask Vibha: 'If meristematic tissue exists only at root and shoot tips, then how does a stem increase in thickness over years, and how does grass grow back after it is mowed?' This would prompt Vibha to think about lateral and intercalary meristems.

(i) Plant cell has a larger vacuole: A plant cell of the same size would have a larger (central) vacuole than an animal cell. In mature plant cells, a single large central vacuole can occupy up to 90% of the cell volume. This vacuole stores water, ions, nutrients, and waste products, and also helps maintain turgor pressure to keep the cell rigid and support the plant structure. Animal cells either lack vacuoles or have only small, temporary food/contractile vacuoles. The plant cell wall allows the cell to withstand the pressure exerted by a large, water-filled vacuole without bursting.

(ii) Assumptions made: We assume the plant cell is a mature, fully differentiated cell (not a meristematic cell, which lacks vacuoles). We also assume the animal cell is a typical somatic cell and not a specialised storage cell. We assume both cells are from multicellular organisms under normal physiological conditions.

This statement is an oversimplification and can be critically examined by asking: 'Does parenchyma perform only one function? Does epidermis only protect?' For example, parenchyma performs multiple functions — it stores food, stores water (in succulents), performs photosynthesis (chlorenchyma), and provides buoyancy (aerenchyma in aquatic plants). Epidermis not only protects the plant but also regulates water loss through the cuticle, enables gaseous exchange through stomata, and absorbs water through root hairs. Xylem transports water AND provides mechanical support through xylem fibres. These examples show that many plant tissues are multifunctional, making the textbook statement an oversimplification that needs revision.

Ligament rupture occurs when the fibrous connective tissue connecting bones at a joint is torn, often due to sudden twisting or impact (e.g., ankle sprain). It causes pain, swelling, and instability at the joint. Cartilage rupture (e.g., meniscus tear in the knee) damages the cushioning tissue at bone ends, causing pain and limiting smooth joint movement; cartilage has poor blood supply so heals slowly. Bone fracture is a crack or complete break in a bone due to excessive force, and requires immobilisation (cast) or surgery followed by weeks of healing.

To reduce risk: Regular weight-bearing exercise strengthens bones and muscles; yoga and stretching improve joint flexibility and ligament strength; a diet rich in calcium (milk, greens), Vitamin D (sunlight, fish), and protein supports bone density and tissue repair. Avoiding smoking and excessive alcohol, maintaining a healthy weight, and wearing proper footwear also significantly reduce risk of musculoskeletal injuries.

When you point your toes downward (plantarflexion) and upward (dorsiflexion), you can clearly feel the Achilles tendon (calcaneal tendon) moving beneath your fingers at the back of the ankle. This is the thickest and strongest tendon in the body, connecting the calf muscles (gastrocnemius and soleus) to the heel bone (calcaneus). Tendons are made of dense fibrous connective tissue (collagen fibres) that transmit the force of muscle contraction to the bone, enabling movement. Other easily felt tendons include those at the back of the knee, the wrist (flex your fingers to feel them), and the front of the elbow.

Physical practices like yoga involve stretching, balancing, and weight-bearing poses that stimulate bone remodelling and increase bone density, reducing the risk of osteoporosis. The isometric and isotonic muscle contractions in yoga build muscle strength, improve joint flexibility, and strengthen tendons and ligaments. Kabaddi involves intense bursts of running, wrestling, and dodging, which build skeletal muscle mass, improve cardiovascular fitness, and strengthen bones through repeated mechanical loading. Regular physical activity of any kind stimulates osteoblasts (bone-forming cells) and maintains the musculoskeletal system in an optimal functional state throughout life.

Pruning removes apical buds, causing intercalary and lateral meristems at nodes to become active, resulting in bushier growth with more branches — it redirects meristematic activity. Grafting joins the conducting tissues (xylem and phloem) of two plants so water, minerals, and food can flow between them, combining desirable traits of both. Irrigation ensures adequate water supply to maintain turgor in parenchyma cells, keeps xylem transport efficient, and supports photosynthesis in mesophyll tissue. Crop rotation prevents soil nutrient depletion, ensuring that conducting and supporting tissues of crops receive adequate mineral nutrition (like calcium for cell walls and iron for chloroplasts) for healthy growth.

Desert leaves (e.g., cacti, aloe) have thick cuticles, reduced surface area, and sunken stomata — adaptations of epidermal and mesophyll tissues to minimise water loss. Moist habitat leaves (e.g., ferns, banana) are broad with thin cuticles, maximising surface area for photosynthesis and gaseous exchange. Aquatic leaves (e.g., lotus, water hyacinth) have air-filled cavities (aerenchyma parenchyma) for buoyancy and lack cuticle to absorb water and gases directly. Traditional knowledge — such as using banana leaves as plates (waxy cuticle prevents staining), neem leaves as insect repellents (secondary metabolites in epidermal cells), or palash leaves stitched into plates — reflects an intuitive understanding of leaf tissue structure and chemistry.

Tribal dance forms like Chhau (Jharkhand/West Bengal), Ghoomar (Rajasthan), Bihu (Assam), or Koli dance (Maharashtra) involve a wide variety of joint movements. For example, spinning involves the pivot joint of the neck and ball and socket joints of the hips; stamping involves hinge joints of the knees and ankles; arm gestures involve the ball and socket joint of the shoulder and hinge joint of the elbow. A drama or dance performance on joint movements can show: pivot joint = head rotation scene, hinge joint = walking/jumping scene, ball and socket = throwing/catching scene, and fixed joint = head protection scene. This creative approach makes the concept of joints memorable and experiential for all students.

Unlike plant cells, most adult animal cells are not totipotent — they are highly specialised (differentiated) and cannot normally revert to generate a complete organism. However, embryonic stem cells are pluripotent and with advanced technology (like somatic cell nuclear transfer, used in cloning), it is theoretically possible to generate an animal from a single cell. Advantages would include: regenerating lost or damaged organs for transplantation, preserving endangered species, and advancing medicine through personalised cell therapies. Challenges include: complex ethical concerns (especially with human cells), technical difficulties in reprogramming adult cells, risk of genetic errors leading to tumours, high failure rates, and potential misuse in human cloning. While exciting scientifically, this development requires stringent ethical guidelines and regulatory oversight.