NCERT Solutions Class 10 Science Chapter 6 Control and Coordination

A reflex action is a sudden, automatic and quick response to a stimulus that needs no thinking. It is controlled by the spinal cord through a reflex arc. Walking is a voluntary action: it is done on purpose, under our control, and is managed by the fore-brain.

• A reflex happens on its own, like pulling the hand back the moment we touch a hot plate. We do not decide to do it.
• In a reflex the message is handled quickly in the spinal cord, so the response is very fast; the brain learns of it only afterwards.
• Walking is something we choose to do, including the direction and the speed.
• Walking is controlled by the brain and uses voluntary muscles, so it stays under conscious control.

Answer: A reflex action is fast, automatic and controlled by the spinal cord (no thinking needed), while walking is a slow, voluntary action that we decide to do and that the brain controls.

A synapse is the tiny gap between the axon end of one neuron and the dendrite of the next. A nerve message travels inside a neuron as an electrical impulse, but it cannot jump the gap as electricity, so at the synapse it is changed into a chemical signal.

• The electrical impulse travels along the axon and reaches its end at the synapse.
• The impulse triggers the release of chemicals (neurotransmitters) into the gap.
• These chemicals cross the gap by diffusion and reach the dendrite of the next neuron.
• On reaching it, the chemicals start a fresh electrical impulse, so the message is passed on.

Answer: At the synapse the electrical impulse of the first neuron releases chemicals; these cross the gap and start a new electrical impulse in the next neuron, so the message moves forward.

The brain has three main parts: the fore-brain, the mid-brain and the hind-brain. The hind-brain has a part called the cerebellum, which looks after the body's posture and balance.

• Posture means keeping the body steady and upright; equilibrium means keeping balance.
• These need the muscles to work together smoothly and with the right force.
• The cerebellum controls this fine coordination of voluntary muscle action.
• This is why walking straight, riding a bicycle or picking up a pencil is smooth and well balanced.

Answer: The cerebellum, a part of the hind-brain, maintains the posture and equilibrium (balance) of the body.

Smell is detected by special nerve cells called olfactory receptors in the lining of the nose. A receptor picks up a particular stimulus and starts a nerve impulse. The smell is finally recognised by the fore-brain.

• The burning agarbatti gives off scent particles into the air.
• These particles reach the nose, where the olfactory receptors detect them.
• The receptors set off an electrical impulse in the sensory nerves.
• This impulse travels to the smell area of the fore-brain, which reads the signal, so we sense the smell.

Answer: Scent particles from the agarbatti are detected by olfactory receptors in the nose, which send a nerve impulse to the smell area of the fore-brain, where the smell is recognised.

A reflex action is controlled by the reflex arc in the spinal cord, not the brain. The brain's role is limited: the message is also carried to the brain so we become aware of what happened, but the brain does not control the quick response.

• The receptor detects the stimulus and sends an impulse along a sensory neuron to the spinal cord.
• The spinal cord at once passes the signal to a motor neuron, which makes the muscle act. This loop is the reflex arc.
• Because the spinal cord handles the response, the action happens before the brain can think about it.
• The information still travels up to the brain, so the brain comes to know about the reflex just after it has happened.

Answer: The brain does not control a reflex action; the spinal cord does, through the reflex arc, for speed. The brain only receives the information afterwards so that we become aware of the action.

Plant hormones (phytohormones) are chemicals made by plants that control and coordinate their growth, development and responses to the environment. They are made in one part of the plant and move to another part where they act.

• Plants have no nervous system, so they use chemicals, not nerves, to coordinate their activities.
• These chemicals are the plant hormones, made in small amounts.
• Some promote growth, for example auxin, gibberellin and cytokinin.
• Some slow or stop growth, for example abscisic acid, which causes wilting of leaves.

Answer: Plant hormones are chemicals made by plants that control their growth, development and responses to stimuli; examples include auxin, gibberellin, cytokinin (promoters) and abscisic acid (inhibitor).

The folding of the sensitive plant (touch-me-not) leaves is a quick movement that does not involve growth and is not directional. The bending of a shoot towards light is a slow movement caused by growth and is directional (a tropic movement).

Answer: Sensitive-plant leaf movement is fast, non-directional and caused by water changes with no growth; shoot movement towards light is slow, directional and caused by growth due to auxin.

Plant hormones that increase growth are called growth promoters. The main examples named in the chapter are auxin, gibberellin and cytokinin.

• Auxin helps cells grow longer (cell elongation) and causes shoots to bend towards light.
• Gibberellin helps in the growth of the stem.
• Cytokinin promotes cell division and is found in higher amounts in fruits and seeds.
• Any one of these is a correct example of a growth-promoting plant hormone.

Answer: Auxin is an example of a plant hormone that promotes growth (gibberellin and cytokinin also promote growth).

A tendril is a thread-like part of some climbing plants that is sensitive to touch. Its coiling around a support is caused by uneven growth, controlled by the hormone auxin.

• When a tendril touches a support, the side in contact stops growing fast.
• Auxin moves away from the contact side and collects on the side away from the support.
• The extra auxin on the far side makes those cells grow longer than the contact side.
• Because one side grows more, the tendril bends and coils around the support, so the plant can climb.

Answer: When a tendril touches a support, auxin shifts to the side away from the support and makes that side grow faster; this uneven growth makes the tendril coil around the support.

Hydrotropism is the growth of a plant part in response to water: roots grow towards water (positive hydrotropism). To show it, we make water available on only one side and watch which way the roots grow.

• Take a wide trough, fill it with soil, and sow a few germinating seeds in the middle.
• Bury a small porous clay pot of water near one side, a little away from the seeds, so water seeps out only on that side.
• Keep the soil on the other side dry, and place the set-up where light and gravity act equally on both sides.
• Water gently near the buried pot for a few days, then carefully dig out the seedlings.
• The roots will have bent and grown towards the side with the water, even though the seeds were sown in the middle. This shows hydrotropism.

Answer: Sow germinating seeds in soil with a buried porous pot of water on one side and dry soil on the other; after a few days the roots bend towards the water, demonstrating hydrotropism.

In animals, chemical coordination is done by hormones made by endocrine glands. These glands have no ducts, so they pour their hormones directly into the blood, which carries them to the target organs where they act.

• Endocrine glands such as the thyroid, pituitary, pancreas and adrenal glands produce hormones.
• The hormones are released straight into the blood, in very small but precise amounts.
• Blood carries them all over the body, but each hormone acts only on its particular target organ.
• At the target organ the hormone brings about the required change; the amount is kept correct by feedback control.

Answer: Chemical coordination in animals is done by hormones; endocrine glands secrete them into the blood, which carries them to target organs where they produce the needed response.

The thyroid gland needs the element iodine to make the hormone thyroxin, which controls the metabolism of carbohydrates, proteins and fats. Iodised salt is common salt to which iodine has been added.

• Thyroxin is made by the thyroid gland, and iodine is needed to make it.
• If our diet lacks iodine, the thyroid cannot make enough thyroxin.
• A shortage of thyroxin disturbs metabolism and can cause goitre, in which the neck swells.
• Using iodised salt supplies the iodine the thyroid needs, so enough thyroxin is made and goitre is prevented.

Answer: Iodised salt provides iodine, which the thyroid needs to make thyroxin. Enough iodine keeps thyroxin levels normal and prevents the iodine-deficiency disease goitre.

Adrenaline is a hormone secreted by the adrenal glands in times of fear, anger or stress. It is released into the blood and prepares the body for quick action (the fight-or-flight response).

• Adrenaline makes the heart beat faster, so more oxygen-rich blood is pumped.
• Small arteries to the digestive system and skin narrow, so blood is diverted to the skeletal muscles.
• The breathing rate increases, so more oxygen is taken in.
• Together these changes give the muscles more oxygen and energy, readying the body to fight or run.

Answer: Adrenaline makes the heart beat faster, raises the breathing rate, and diverts blood from the digestive system and skin to the skeletal muscles, preparing the body for fight or flight.

Insulin is a hormone made by the pancreas. Its job is to control (lower) the level of sugar in the blood. Diabetes is a condition in which the blood sugar level is too high.

• In some diabetic patients, the pancreas does not make enough insulin.
• Without enough insulin, the blood sugar level rises and stays high, which can be harmful.
• To make up for the shortage, such patients are given insulin from outside, as injections.
• The injected insulin helps regulate the blood sugar and keeps it within the normal range.

Answer: In these patients the pancreas does not secrete enough insulin, so blood sugar rises. Insulin injections supply the missing hormone and bring the blood sugar back to normal.

A plant hormone is a chemical made by plants to control their growth. We pick the only option that is a plant hormone; the rest are animal hormones.

• Insulin is an animal hormone (from the pancreas) that controls blood sugar. Not a plant hormone.
• Thyroxin is an animal hormone (from the thyroid). Not a plant hormone.
• Oestrogen is an animal hormone (a female sex hormone). Not a plant hormone.
• Cytokinin is made by plants and promotes cell division. It is a plant hormone.

Answer: (d) Cytokinin.

The very small gap between the axon end of one neuron and the dendrite of the next is called a synapse. We match each option to its meaning.

• A dendrite is the branched part of a neuron that receives signals. Not a gap.
• A synapse is the gap between two neurons across which the signal is passed. This is the gap.
• An axon is the long fibre that carries the impulse. Not a gap.
• An impulse is the electrical signal itself, not a gap.

Answer: (b) Synapse.

Different parts of the brain do different jobs. The fore-brain handles thinking, the medulla regulates the heartbeat, and the cerebellum keeps the body balanced. So the brain as a whole is responsible for all three.

• Thinking is done by the fore-brain, so (a) is true.
• Heartbeat is an involuntary action controlled by the medulla, so (b) is true.
• Balance and posture are controlled by the cerebellum, so (c) is true.
• Since (a), (b) and (c) are all correct, the answer is (d).

Answer: (d) All of the above.

Receptors are specialised nerve cell tips in our sense organs that detect a particular stimulus and start a nerve impulse. Different receptors detect different stimuli: gustatory for taste, olfactory for smell, and so on.

• Function: receptors detect changes such as light, sound, taste, smell, heat and touch, and pass this information into the nervous system as impulses.
• They are located in the sense organs: the eye, ear, nose, tongue and skin.
• When receptors fail: the body cannot detect that stimulus, so it cannot respond in time.
• Problems: if heat or pain receptors fail, a person may get burnt or injured without knowing; if taste or smell receptors fail, spoilt food might be eaten; ear-receptor failure can affect hearing and balance.

Answer: Receptors detect stimuli and start nerve impulses. If they fail, the body cannot sense those changes, so a person may get burnt, eat spoilt food, or fail to react to danger in time.

A neuron (nerve cell) is the basic unit of the nervous system. It is specialised to receive information and carry it as an electrical impulse from one part of the body to another. Its main parts are the dendrites, the cell body, the axon and the nerve endings near the synapse.

• Dendrites: the short, branched fibres that receive information from receptors or other neurons.
• Cell body: contains the nucleus; the impulse passes from the dendrites through the cell body.
• Axon: the long fibre that carries the impulse away from the cell body towards its end.
• Nerve endings and synapse: at the axon end, the impulse triggers chemicals that cross the synapse to the next neuron or muscle.

Function. A neuron receives information at the dendrites, converts it into an electrical impulse, carries it along the axon, and releases chemicals at the synapse to pass the message on.

Answer: A neuron has dendrites (receive), a cell body (with nucleus), an axon (carries the impulse) and nerve endings at the synapse. Its function is to receive information and conduct it as an electrical impulse across the body, passing it on chemically at the synapse.

Phototropism is the directional growth of a plant part in response to light: shoots grow towards light (positive) and roots grow away from light (negative). It is controlled by the hormone auxin.

• Auxin is made at the tip of the shoot and helps cells grow longer.
• When light falls on one side, auxin moves to the shady side.
• The extra auxin on the shady side makes those cells grow longer than the cells on the lit side.
• Because one side grows more, the shoot bends and grows towards the light. This is phototropism.

Answer: In phototropism, auxin made at the shoot tip moves to the shady side when light comes from one side; the extra auxin makes the shady side grow faster, so the shoot bends towards the light.

The spinal cord is the main pathway that carries messages between the body and the brain, and it forms the reflex arcs. So an injury to the spinal cord breaks these pathways.

• All nerves from the body pass through the spinal cord on their way to and from the brain.
• Sensory signals going from the body up to the brain (touch, pain, temperature) get disrupted.
• Motor signals (instructions) going from the brain down to the muscles also get disrupted, so the affected muscles cannot move properly.
• The reflex arcs that pass through the injured part are broken too, so reflexes in that region are lost.

Answer: A spinal cord injury disrupts the signals passing between the body and the brain, the sensory signals going up and the motor signals coming down, as well as the reflexes handled by the injured part.

Plants have no nervous system, so they coordinate only by chemical means, using plant hormones (phytohormones). These are made in one part of the plant and move to another part to bring about the response.

• When a plant detects a stimulus (light, gravity or touch), its cells make a suitable plant hormone.
• The hormone is made at one place but acts at another, so it diffuses to where it is needed.
• There the hormone changes the growth of cells, for example making them grow longer, which coordinates the response.
• Examples: auxin bends shoots to light; gibberellin and cytokinin promote growth; abscisic acid stops growth and causes wilting.

Answer: Chemical coordination in plants is done by plant hormones (auxin, gibberellin, cytokinin, abscisic acid). They are made in one part of the plant, diffuse to where they are needed, and control growth and responses.

The body of a multicellular organism is made of many organs and tissues. For the body to work as one unit, these parts must work together in a controlled way and respond correctly to the environment. This is the job of the control and coordination systems.

• An organism must detect changes (stimuli) and respond suitably, for example moving away from danger or towards food.
• Different organs do different jobs, so their actions must be coordinated to work smoothly as one whole.
• The response to each stimulus must be controlled and appropriate.
• Control and coordination also keep internal conditions steady (such as blood sugar) and protect the body. The nervous system and hormones together do this.

Answer: A system of control and coordination lets an organism detect changes and respond suitably, make its organs work together as one unit, give correct controlled responses, and keep its internal conditions steady and protected.

Both involuntary actions and reflex actions happen without our conscious will, but they differ in which part of the nervous system controls them and how fast they are. Reflexes are controlled by the spinal cord; most other involuntary actions are controlled by the mid-brain and hind-brain.

Answer: Reflex actions are sudden, very fast responses controlled by the spinal cord (e.g. pulling the hand off a flame). Involuntary actions are ongoing internal activities like heartbeat and breathing, controlled by the mid-brain and hind-brain.

Animals coordinate their bodies in two ways: the nervous mechanism, which uses fast electrical impulses along nerves, and the hormonal mechanism, which uses chemical hormones carried by the blood. Both control body activities and work together.

Answer: Nervous control uses fast electrical impulses, gives quick but short-lived responses, and reaches only nerve-connected cells. Hormonal control uses chemical hormones in the blood, is slower but longer-lasting, and reaches all cells. Both together coordinate the body.

The folding of sensitive plant leaves and the movement of our legs are both responses, but the way they happen is very different. The plant has no nerves or muscles; our legs are moved by the nervous system acting on muscles.

Answer: In the sensitive plant, leaves move because cells change their water content with no nerves or muscles. Our legs move because nerves carry impulses to leg muscles, whose proteins change shape and contract. The plant's movement is a quick touch response; the leg movement is voluntary and muscle-driven.