Control and Coordination — Important Questions

Answer: (c) Synapse. At the synapse the electrical impulse causes the release of a chemical (neurotransmitter) that crosses the gap and starts a new impulse in the next neuron. This also makes sure the impulse can travel in only one direction.

Answer: (c) Auxin. Auxin is synthesised at the shoot tip and promotes cell elongation. When light falls on one side, auxin moves to the shaded side, making those cells grow longer so the shoot bends towards the light (phototropism).

Answer: (b) Cerebellum. The cerebellum (part of the hindbrain) is responsible for maintaining posture and balance and for the precision of voluntary movements. (The cerebrum handles thinking and voluntary decisions; the medulla controls involuntary actions like heartbeat and breathing.)

Answer: (c) Iodine. Iodine is required by the thyroid gland to make the hormone thyroxine, which regulates metabolism. A deficiency of iodine reduces thyroxine production and causes the thyroid to enlarge — the swelling of the neck called goitre. This is why iodised salt is recommended.

Answer: (a) Both A and R are true and R is the correct explanation of A. A reflex response is generated by the reflex arc through the spinal cord, so the motor command is issued before the information reaches the conscious, thinking part of the brain. Bypassing the brain makes the response fast and involuntary, which is exactly why it does not involve thinking.

(a) Phototropism: the directional growth movement of a plant part in response to light. A shoot shows positive phototropism (grows towards light), while a root shows negative phototropism (grows away from light).

(b) Geotropism (gravitropism): the directional growth movement of a plant part in response to gravity. A root shows positive geotropism (grows downwards, towards gravity), while a shoot grows upwards (negative geotropism).

Difference: The nervous system transmits information as fast electrical impulses along neurons, giving a rapid but short-lasting response; hormones travel slowly through the blood as chemical messengers and produce slower but longer-lasting effects on target organs.

Usefulness: Nerve impulses cannot reach every cell, and a cell cannot generate impulses continuously; hormonal signalling allows steady, widespread and sustained control of processes (like growth and metabolism) that need to be regulated over long periods.

(a) Reflex action: a sudden, automatic and involuntary response of the body to a stimulus, which takes place without conscious thought. Example: quickly withdrawing the hand on touching a hot object (or blinking when something approaches the eye).

(b) Reflex arc pathway:

The stimulus is detected by the receptor, carried by the sensory neuron to the spinal cord, which sends a command through the motor neuron to the effector muscle, producing an instant response.

Labelled diagram of a neuron:

Functions:
(a) Dendrites — receive information (stimulus) and carry the impulse towards the cell body.
(b) Axon — conducts the impulse away from the cell body to the nerve endings.
(c) Nerve endings (axon terminals) — pass the impulse across the synapse to the next neuron or to the muscle/gland by releasing chemicals.

(a) Auxin: synthesised at the shoot tip, it promotes cell elongation. When light comes from one side, auxin accumulates on the shaded side; those cells elongate more, so the shoot bends towards the light (phototropism).

(b) Cytokinin: promotes cell division. It is present in greater amounts in rapidly dividing regions such as fruits and seeds, and also helps delay the ageing of leaves.

(c) Abscisic acid (ABA): an inhibitory hormone that slows down growth. It causes wilting of leaves and triggers the closing of stomata during water shortage, helping the plant conserve water (a stress hormone).

The brain is the main coordinating centre of the body and has three major regions:

1. Forebrain (Cerebrum):

• The largest and most complex part.
• Controls thinking, reasoning, memory, intelligence and will (voluntary actions).
• Receives sensory impulses (sight, smell, hearing, touch, taste) and interprets them.
• Contains centres associated with hunger and other sensations.

2. Midbrain:

• Connects the forebrain and hindbrain.
• Controls certain reflex movements of the head, neck and eye muscles (e.g. changes in pupil size and reflexes of the eye).

3. Hindbrain — has three parts:

• Cerebellum: maintains posture and balance and provides precision to voluntary actions (e.g. walking, writing, picking up a pencil).
• Pons: helps regulate breathing.
• Medulla oblongata: controls involuntary actions such as heartbeat, breathing, blood pressure, salivation and vomiting.

Protection of the brain: The brain is enclosed in a bony box, the skull (cranium). Inside the skull it is surrounded by three membranes called meninges and is cushioned by a shock-absorbing fluid, the cerebrospinal fluid (CSF), which protects it from mechanical injury and jerks.

(a)

• Insulin — secreted by the pancreas; lowers blood glucose by helping cells absorb and store sugar.
• Adrenaline — secreted by the adrenal glands; prepares the body for emergency ('fight-or-flight') by raising the heartbeat, breathing rate and blood supply to the muscles.
• Growth hormone — secreted by the pituitary gland; controls the growth and development of the body (bones and tissues).

(b) In diabetes the hormone insulin is deficient (or not used properly), so blood sugar stays high. It is managed by injecting insulin and by controlling the diet (and lifestyle).

(c) Feedback regulation: The amount of a hormone secreted is controlled by feedback based on the level of the substance it regulates. For example, when the blood sugar level rises (e.g. after a meal), the pancreas is stimulated to secrete more insulin, which brings the sugar level down. When the blood sugar level falls back to normal, insulin secretion is automatically reduced. This self-regulating feedback keeps the internal conditions of the body stable (homeostasis).

(a) The withdrawal of the foot is a reflex action (an involuntary, automatic response).

(b) Correct order of the reflex arc:
Receptor (skin of foot) to Sensory neuron to Spinal cord to Motor neuron to Effector (leg muscle) to Response (foot withdrawn).

(c) The response is controlled by the spinal cord, not by the conscious brain. Since the man's spinal cord was undamaged, the reflex arc could still work: the sensory impulse reached the spinal cord, which directly sent a motor command to withdraw the foot — even though the injured brain meant he was unconscious and felt nothing.

(d) Such a reflex is very fast because it does not wait for the brain to think. This allows the body to respond instantly to harmful stimuli (like a pinprick, heat or sharp objects) and thus protects it from injury.