NCERT Solutions for Class 10 Science Chapter 6 Control and Coordination

A reflex action is an involuntary, automatic response to a specific stimulus, routed through a fixed pathway called the reflex arc in the spinal cord. The body acts before the brain consciously thinks.

Walking is a voluntary, learned action controlled by the thinking part of the brain (the fore-brain), which sends instructions to leg muscles based on a conscious decision to move.

The key biological distinction is the routing of the signal. A reflex bypasses the thinking brain to save time in dangerous situations. Walking requires the brain to plan, coordinate, and adjust muscle movements continuously.

A neuron’s electrical impulse cannot leap across the physical gap between two cells. At the synapse, the arriving impulse triggers the first neuron’s axon ending to release chemical neurotransmitters into the gap. These chemicals diffuse across, bind to receptors on the next neuron’s dendrite, and start a fresh electrical impulse there. This ensures one-way, controlled signal transmission (NCERT, p. 101).

The cerebellum, a part of the hind-brain, maintains the posture and equilibrium of the body. It is responsible for the precision of voluntary actions like walking in a straight line and riding a bicycle (NCERT, p. 104). It constantly adjusts muscle signals to keep the body balanced.

Smell detection works as a signal pathway. The agarbatti’s smoke carries chemical particles into the nose. Olfactory receptors in the nasal cavity detect these chemicals and convert them into electrical impulses. These impulses travel along the olfactory nerve to the fore-brain, where specific regions interpret the signals as the sensation of smell (NCERT, p. 103).

The reflex arc is formed in the spinal cord to produce a fast, automatic response without waiting for the brain’s thinking process. However, the brain still receives the sensory information afterwards.

Its role is to process the event consciously (you feel pain after pulling your hand away from a flame) and store the memory so the body avoids the danger next time (NCERT, p. 102).

Plant hormones are chemical substances produced naturally in one part of a plant that diffuse to other parts to control growth, development, and responses to the environment. The main examples are auxin, gibberellin, cytokinin, and abscisic acid (NCERT, p. 107).

The sensitive plant (chhui-mui) folds its leaves due to turgor pressure changes—water moves out of cells, causing them to change shape. This is a growth-independent, immediate movement. A shoot bending towards light is a growth-dependent movement caused by the hormone auxin diffusing to the shady side, making cells there elongate faster.

The first is a quick electrical-chemical response; the second is a slow, directional tropism.

Auxin is a plant hormone that promotes growth. It is synthesised at the shoot tip and helps cells grow longer. Gibberellin also promotes growth by helping the stem elongate (NCERT, p. 107).

When a tendril touches a support, auxin concentrates on the side away from the support. This high auxin concentration stimulates those cells to grow longer faster than the contact side. The unequal growth rate causes the tendril to curve inward, wrapping around the object.

Principle: Hydrotropism is the directional growth of a plant root toward water. To demonstrate it, we must show that roots bend toward a water source even against the pull of gravity.

1. Setup: Take a porous pot filled with water and bury it slightly off-centre in a larger container holding dry soil. Plant freshly germinated bean seeds in the dry soil, a few centimetres away from the porous pot.
2. Control: Plant identical seeds in a second container with uniformly moist soil and no localized water source.
3. Observation: After a few days, dig carefully. The roots in the experimental setup will bend toward the water-filled porous pot. In the control, roots grow normally downward.
4. Conclusion: Roots exhibit positive hydrotropism, growing toward the water source despite gravity.

Chemical coordination is managed by the endocrine system. Specialised glands secrete hormones directly into the bloodstream. The blood carries these hormones throughout the body, but they act only on specific target organs that have matching receptors. This provides a slower but body-wide, persistent method of control compared to the fast, targeted nervous system.

Iodine is essential for the thyroid gland to synthesize the thyroxin hormone, which regulates carbohydrate, protein, and fat metabolism. Without enough iodine, thyroxin cannot be produced, leading to goitre, a condition where the thyroid gland in the neck swells. Iodised salt prevents this dietary deficiency (NCERT, p. 109).

Adrenaline prepares the body for a fight-or-flight response with several simultaneous actions: the heart beats faster to supply more oxygen to muscles, blood is diverted away from the digestive system and skin to the skeletal muscles, and the breathing rate increases for faster oxygen intake. These integrated changes prepare the body for rapid physical action (NCERT, p. 109).

Insulin, produced by the pancreas, regulates blood sugar levels by helping cells absorb glucose. In diabetics, the pancreas does not produce enough insulin, causing blood sugar to rise dangerously. External insulin injections compensate for the shortfall, regulating sugar levels artificially (NCERT, p. 110).

Correct answer: (d) Cytokinin.

Cytokinin promotes cell division in plants. Insulin regulates blood sugar in animals, thyroxin regulates metabolism in animals, and oestrogen is an animal sex hormone.

Correct answer: (b) synapse.

The synapse is the junction between two neurons where chemical neurotransmitters transfer the signal. (a) is a part of the neuron, (c) is the long fibre, and (d) is the signal itself.

Correct answer: (d) all of the above.

The fore-brain handles thinking, the medulla in the hind-brain regulates heartbeat, and the cerebellum balances the body.

Receptors are specialised tips of nerve cells located in sense organs that detect stimuli (light, sound, taste, smell, touch) and convert them into electrical impulses. If receptors are damaged, the body fails to detect environmental changes.

For example, if olfactory receptors are damaged, you lose the sense of smell. If gustatory receptors fail, you cannot taste food, which can lead to eating spoiled food unknowingly. Damaged pain receptors mean you might not pull your hand away from a hot surface, causing burns.

A neuron has three main parts: Dendrites (branch-like structures that receive impulses), the Cell body (containing the nucleus, where the impulse passes through), and the Axon (a long fibre that carries the electrical impulse to the end). The axon ends form a synapse with the next cell.

Function: The neuron acquires information at the dendritic tip, transmits it as an electrical impulse along the axon, and passes it to the next neuron or a muscle cell by releasing chemicals at the synapse.

When light shines from one side, the plant hormone auxin diffuses toward the shady side of the shoot. This higher auxin concentration makes the cells on the shady side grow longer faster than the lit side. The unequal growth causes the shoot to bend toward the light.

A spinal cord injury disrupts both: (1) Motor signals from the brain to muscles below the injury, causing paralysis; and (2) Sensory signals from body parts below the injury to the brain, causing loss of sensation. Reflex arcs routed through that spinal segment may also be lost, as the relay pathway is broken.

Plants use hormones for chemical coordination. These are synthesised at one site (e.g., auxin at the shoot tip) and diffuse to target areas. Auxin and gibberellin promote growth; cytokinin promotes cell division; abscisic acid inhibits growth (NCERT, p. 107).

Organisms must constantly detect environmental changes and respond appropriately to survive. In multicellular bodies, specialised tissues and systems are needed to integrate these signals. The nervous system provides fast, targeted responses, while hormones provide slower, body-wide, persistent regulation. Together, they ensure controlled, coordinated actions.

Both happen without conscious thought, but they differ in mechanism and purpose. Involuntary actions (like heartbeat, breathing, digestion) are ongoing, automatic processes controlled by the mid-brain and hind-brain (medulla). Reflex actions are fast, specific responses to a sudden stimulus, routed through a fixed pathway (the reflex arc) in the spinal cord for immediate protection.

The sensitive plant moves its leaves by changing turgor pressure—water moves in or out of plant cells, causing them to swell or shrink. This is independent of growth. Leg movement relies on muscle cells with special proteins that change shape in response to nerve impulses, shortening the muscle.

The plant uses water movement; the animal uses protein shape change driven by nerves (NCERT, p. 106).