Tissues in Action — NCERT Solutions

The study of cells and tissues helps us understand how living organisms grow, develop and perform life processes. Since cells are the basic unit of life and tissues are groups of cells working together, this study explains the structure–function relationship in plants and animals. It is also significant for human welfare because knowledge of tissues helps researchers understand growth, repair, movement, transport and control in the body, and it supports advances such as plant tissue culture, crop improvement, and medical use of stem cells and transplants.

Plant and animal tissues are different because plants and animals have different body structures, life styles and needs. Plants are usually fixed in one place, so they need tissues for support, protection, transport of water and food, and growth at certain regions. Animals move from place to place, so they have tissues for movement, coordination, sensory functions, and internal support. Also, plant cells have a cell wall, while animal cells do not, so plant tissues are generally more rigid and animal tissues are more flexible. These differences arise because the structure and function of tissues are adapted to the way plants and animals live.

In multicellular organisms, there is a division of labour at different levels. Cells of similar type form a tissue, tissues form organs, organs form organ systems, and organ systems together form the organism. Each level is specialised for certain functions, so the body works more efficiently. For example, in animals muscle tissue helps movement and nervous tissue carries messages, while in plants xylem transports water and minerals and phloem transports food. Thus, the structure of each tissue or organ is closely related to its function, and specialisation increases efficiency and allows complex life processes.

The data show that roots in Jar A continue to grow in length, but roots in Jar B stop growing after the tips are cut. Yes, this is similar to the graph in Fig. 3.2. The inference is that roots grow only from their tips because the tips contain actively dividing cells. This means the apical meristem at the root tip is responsible for growth in length.

Coconut husk fibres are made of sclerenchyma. Sclerenchyma cells have thick, lignified walls, so they are hard and brittle. Coriander leaf stalks are soft and flexible because they have more collenchyma/parenchyma, which provide flexibility instead of hardness.

A thick cuticle is useful for a desert plant because it reduces water loss by transpiration, helping the plant survive in dry conditions. But underwater plants do not need to conserve water, so a thick cuticle is unnecessary and disadvantageous because it can hinder exchange with the surrounding water and is not helpful for water conservation.

The dead xylem cells form tubes that transport water upward from the roots. At the top, the living cells of the leaves lose water through transpiration. This creates a transpiration pull that helps draw more water up through the xylem against gravity. So, xylem conducts water, and transpiration in the leaves keeps the water moving continuously.

If there were no stomata in the epidermis of stems or leaves, the plant would have serious problems in gas exchange and transpiration. Carbon dioxide would not enter easily for photosynthesis, oxygen exchange would be affected, and transpiration pull would be reduced, so water transport would also be disturbed. The plant would also lose an important way of removing water vapour and wastes.

The poses of dances involve movements at different joints such as the shoulder, elbow, wrist, neck, hip, knee and ankle. The shoulder and hip are ball-and-socket joints, allowing movement in many directions. The elbow, knee, ankle and finger joints mainly act as hinge joints, allowing bending and straightening in one plane. The neck has a pivot joint, which allows the head to turn side to side. These joints make the various dance movements possible.

Platelets help in blood clotting at the site of injury.

Bone and muscle mass differ between individuals because of differences in age, gender, body composition, nutrition, and physical activity. For example, adult males usually have a higher muscle percentage than adult females, while bone mass is about 12–15% in adults on average. Together, bones and muscles contribute a large part of the body weight: bones give support and rigidity, while muscles provide movement and also add to body mass. Their proportions vary from person to person, so total body weight differs.

The bones move because of the action of muscles, especially the skeletal muscles attached to bones by tendons. Joints only allow movement; they do not move the bones by themselves. When a muscle contracts, it pulls on the bone through the tendon, producing movement at the joint. Different kinds of joints also limit the direction of movement, so some body parts move in many directions while others move in only one direction.

This happens because the body has different types of joints. Some joints, like the shoulder and hip, are ball-and-socket joints, so they allow movement in many directions. Others, like the elbow, knee and ankle, are hinge joints, so they allow movement mainly in one direction. Joints do not move the bones themselves; muscles cause the movement.

Meristematic cells divide repeatedly because they have thin cell walls, dense cytoplasm, and a large prominent nucleus. These features support rapid cell division and continuous growth.

Food is transported from leaves to roots by phloem. So if the plant cannot transport food, phloem is malfunctioning.

Epithelial tissues lining internal organs are usually only one or a few cells thick so that substances can pass across them quickly and efficiently. Thin layers are best for rapid exchange of gases, nutrients and wastes.

In the straight-leg jump, the knees and ankles remain stiff, so there is very little bending at those joints and the hips also move less naturally. In the normal jump, the ankles, knees and hips bend to help absorb the force and make the jump smoother. So the difference is that the normal jump uses flexion at the joints, while the straight-leg jump keeps them rigid.

When you bend your knees and ankles, the joints involved are hinge joints, because they allow movement mainly in one direction.

• A: Epithelium for gas exchange is true, but the reason is false because gas exchange needs a single thin layer, not multiple tall layers. So (iii).
• B: Cardiac muscle can contract continuously without fatigue is true, and the reason is also true and explains it. So (i).
• C: Tendons connect muscle to bone, not bone to bone, so the assertion is false. The reason is true. So (iv).
• D: In a hinge joint, movement is mainly in one plane is true. The reason is false because the bone ends are not shaped to allow sliding in all directions. So (iii) would be expected for D if using the standard list, but since the correct matching from the statement is that the assertion is true and the reason is false, the option is (iii).

Therefore the answers are: A-iii, B-i, C-iv, D-iii.

Plot a graph with age of the tree on the x-axis and diameter and number of annual rings on the y-axis using the data points:

• Age 5 years → diameter 4 cm, rings 5
• Age 10 years → diameter 8 cm, rings 10
• Age 20 years → diameter 24 cm, rings 20
• Age 25 years → diameter 28 cm, rings 25
• Age 30 years → diameter 32 cm, rings 30
• Age 40 years → diameter 40 cm, rings 40

Join the points smoothly or as separate line graphs for diameter and annual rings. The graph will show an increasing trend for both diameter and rings with age.

The graph shows that the diameter of the stem increases with age. In young trees the increase may be slower, and with time the trunk becomes thicker. This means the tree grows in girth as it gets older, and the increase in diameter reflects the activity of lateral meristem.

The relation is that as the diameter of the teak tree increases, the number of annual rings also increases. In the given data, the number of annual rings is equal to the age of the tree, and wider or narrower rings reflect growth conditions during different years. So, annual rings can be used to estimate the tree’s age, while the diameter shows its increase in girth.

The specialised tissue responsible for the girth of the stem is the lateral meristem. It is located in a ring along the circumference of the stem.

Debarking would hamper the tree’s protection and transport of food because the bark protects the inner tissues and the tissues beneath the bark include conducting tissues. If the damage goes deeper, the tree may not be able to transport water and food properly, and the stem may also lose support and become vulnerable to injury and disease.

Debarking hampers the tree’s protective function because the outer covering is removed, exposing inner tissues to injury and infection. It may also disturb transport if tissues like phloem are damaged, because bark region includes tissues involved in movement of food.