Exploration Class 9 Science Chapter 3: Tissues in Action — NCERT Solutions

Explanation: The kind of movement a joint allows depends on its structure: the ball and socket joint of the shoulder lets the bone rotate in almost all directions, the hinge joints of the elbow, knee, fingers and toes allow bending in only one plane, the pivot joint of the neck lets the head turn from side to side and nod, and the gliding joint of the wrist permits bending and limited turning. The joint, however, only permits a movement; it does not produce it. Movement is produced by muscles - skeletal muscle is joined to bone by tough tendons, and when the muscle contracts it shortens and pulls the bone along with it. Since a muscle can only pull and never push, muscles are arranged in antagonistic pairs such as the biceps and triceps at the elbow: one contracts to bend the joint while the other relaxes, and the reverse action straightens it.

Explanation: Growth in thickness is called secondary growth, and it is brought about by the lateral meristem, also called cambium, which runs lengthwise along the sides of the stem and root as a thin ring of small, thin-walled, actively dividing cells. These cells divide again and again and add new cells on both sides - secondary xylem (wood) towards the centre and secondary phloem towards the outside - so the diameter of the stem keeps increasing year after year. The cambium is more active in the favourable growing season, when it forms wide, large cells that appear as light-coloured wood, and less active in the unfavourable season, when it forms narrow, darker cells. This alternation produces the light and dark bands seen in the figure as annual growth rings, and counting these rings gives the approximate age of the tree.

Explanation: A sugarcane stem is made mostly of permanent tissue, whose cells have lost the power to divide, so a plain internode piece cannot by itself produce a new plant. At each node, however, there is a bud (the eye) that contains meristematic tissue - small, thin-walled cells with dense cytoplasm that divide continuously. When such a cutting is planted, the dividing cells of the bud give rise to a new shoot and roots arise at the node, so only the cuttings carrying a bud sprout. This is why sugarcane is propagated from stem pieces called setts, each of which must carry at least one node with a healthy bud. For the comparison to be fair, only the presence of the bud should differ between the two sets, because if any other condition also changed, the difference in sprouting could not be attributed to the bud.

Explanation: A tissue is a group of cells of similar structure that work together at a common job, and a stem contains several such tissues, so its cells cannot all be alike. The outermost single layer of flat, closely packed cells is the epidermis, covered by a waxy cuticle and bearing epidermal hairs; it protects the stem and checks water loss. Below it lies the ground tissue, made of collenchyma (living cells thickened at their corners, giving flexibility to the young stem), parenchyma (living, thin-walled cells with intercellular spaces that store food) and sclerenchyma (dead cells with thick lignified walls that make the stem hard and strong). Further in are the vascular bundles arranged in a ring, in which xylem (largely dead cells with thick lignified walls and wide lumens) carries water and minerals upward and phloem carries food, with a narrow band of lateral meristem (cambium) between them whose small, actively dividing cells add new xylem and phloem. The differences in cell shape, in wall thickness and in whether the cells are living or dead all correspond to the different functions each tissue performs.

Explanation: Blood is a fluid connective tissue whose matrix is the liquid plasma, in which red blood cells, white blood cells and platelets float. When a blood vessel is cut, red blood oozes out and the platelets (thrombocytes) gather at the injured spot and release chemicals called clotting factors. Together with calcium and other factors present in the plasma, these convert the soluble plasma protein fibrinogen into long, insoluble threads of fibrin. The fibrin threads form a net across the wound that traps blood cells, and this mass dries and hardens into a clot. The clot plugs the cut, stops further loss of blood and prevents germs from entering the body.

Explanation: Up to day 4 the two curves almost coincide, so the roots in both jars were growing at the same rate to begin with. The sudden vertical fall of exactly 1 cm in the Jar B line on day 4 marks the point where about 1 cm of the root tips was cut off, and after that the Jar B line stays perfectly flat at 1.6 cm while Jar A keeps climbing to about 5.8 cm. Since removing the tip was the only change made to Jar B, and growth stopped completely after it, the region that makes a root longer must be the tip itself. The tip carries the apical meristem, a group of small, thin-walled, actively dividing cells with dense cytoplasm that keep producing new cells; these new cells then enlarge and increase the length of the root. Once the tip is cut off the meristem is gone, so no new cells are formed and the root can no longer grow in length.

Explanation: The assertion is true; lung epithelium is specialized for gas exchange. The reason is false; it is made of a single layer of very thin, flat (squamous) cells to minimize diffusion distance, not multiple layers of tall cells.

Explanation: A thin epithelial layer, such as in the alveoli of the lungs or the lining of blood capillaries, provides a short diffusion path, allowing for the efficient and rapid exchange of gases, nutrients, and waste products.

Explanation: The assertion is true; cardiac muscle works continuously. The reason is also true and correctly explains the assertion. The high number of mitochondria provides constant ATP for energy, and the rich blood supply ensures a steady flow of oxygen and nutrients.

Explanation: Meristematic cells are metabolically active with dense cytoplasm, a large nucleus for controlling cell division, and thin walls to allow for growth. They lack large vacuoles which would otherwise hinder rapid division.

Explanation: The knee and ankle joints are primarily hinge joints, which allow for movement in one plane, such as flexion and extension (bending and straightening).

Explanation: The assertion is false; tendons connect muscle to bone, while ligaments connect bone to bone. The reason is a true statement describing the composition and function of tendons.

Explanation: The assertion is true; hinge joints like the elbow or knee allow movement primarily in one plane (flexion/extension). The reason is false; the specific shape of bones in a hinge joint is what restricts movement to one plane, unlike gliding joints which allow sliding in multiple directions.

Explanation: Phloem is the vascular tissue responsible for translocation, which is the transport of soluble organic compounds (food, primarily sucrose) from the leaves to other parts of the plant like the roots.