Control and Coordination

In a shoot receiving light from one side, auxin becomes distributed unevenly, with more auxin generally accumulating on the shaded side. In shoots, this promotes greater cell elongation on that side. The unequal elongation produces curvature, so the stem bends towards the light. Plants have no blood, bones, or muscles; therefore option A is correct.

Mimosa, commonly called touch-me-not, shows a rapid response when its leaves are touched: the leaflets fold and the leaf may droop. This is a plant movement caused by changes in cells near the leaf base, rather than by muscles or a nervous system. Swimming, running, and flying are animal movements, so option A is the correct example.

A tendril shows thigmotropism, which is a directional growth response to touch. After contacting a support, growth becomes faster on the side away from the contact and slower on the touched side. This unequal growth causes the tendril to curve and wrap around the support. Blood flow, heartbeat, and stopped respiration do not explain this plant movement, so option A is correct.

Hormones are chemical messengers released by endocrine glands. They enter the bloodstream and are carried throughout the body, but they produce a response mainly in specific target organs or target cells that have suitable receptors. Thus, the idea is not that hormones act only on teeth, hair, or nails. Option A correctly describes their destination.

Nervous control uses electrical impulses travelling along neurons, so it usually produces a rapid and short-lived response at a particular site. Hormonal control uses chemical messengers carried by blood; its response generally begins more slowly, lasts longer, and may affect several target tissues. Therefore, option A states the main school-level difference. The other statements reverse or deny this distinction.

In a coordinated response, a receptor detects a stimulus and the nervous system sends a message to an effector. Muscles act as effectors: after receiving the message, they contract or relax, producing movement or another physical response. They do not make food, form blood as their sole function, or only produce hormones. Thus, option A correctly describes their role.

The ear has two important sensory functions. The cochlea of the inner ear is associated mainly with hearing, while the vestibular apparatus, including the semicircular canals, detects head movement and helps maintain balance and posture. Digestion, blood formation, and sugar production are not functions of the ear. Therefore, option A is the correct response.

The nose is a chemosensory organ. Receptors in the nasal lining detect particular airborne chemical molecules, and the brain interprets the resulting nerve signals as different smells or odours. Light is detected by photoreceptors in the retina, sound by receptors in the inner ear, and pressure mainly by receptors in the skin. Hence, option A correctly identifies the stimulus detected by nasal receptors.

The tongue contains taste receptors, especially in taste buds, that respond to dissolved chemicals in food and help the nervous system identify taste sensations such as sweet, salty, sour, bitter, and umami. Light is detected by the retina, sound by receptors in the inner ear, and blood is pumped by the heart. Therefore, option A correctly states the function of tongue receptors.

Skin is a major sensory organ containing different receptors. Mechanoreceptors respond to touch and pressure, thermoreceptors detect warmth or cold, and other receptors can signal pain. Thus, skin does not detect only one sensation. Light is mainly detected by the retina, taste by the tongue, and smell by the nose. Option A is correct because touch and temperature are clear examples of skin-detected stimuli.

The governing idea of control and coordination is that organisms detect stimuli or changes in their surroundings and then produce suitable responses. Animals may use receptors, nerves, the brain, and hormones, while plants mainly use chemical signals and growth responses. Thus option A gives the complete chapter message. Options B, C, and D are false because organisms do respond and both plants and animals show coordination.

A neuron is the structural and functional unit of the nervous system. It is a specialized cell that receives, processes, and transmits nerve impulses through its dendrites, cell body, and axon. A hormone is a chemical messenger, a gland releases substances, and an effector carries out a response. Hence option A is correct because only neuron means a nerve cell.

Dendrites are the short, branched projections of a neuron that generally receive signals from receptors or nearby neurons and conduct them toward the cell body. The axon carries impulses away from the cell body, while the nucleus controls cell activities and is not the receiving projection. Therefore, option A is correct. The wording refers to the standard school-level function of dendrites.

The axon is the long fibre of a neuron that conducts nerve impulses away from the cell body toward another neuron, muscle, or gland. Dendrites generally receive incoming signals and carry them toward the cell body. The nucleus manages cellular functions, and a vacuole is not the conducting part of a neuron. Thus option A is the only correct answer under the standard neuron pathway.

The small junction or gap between two communicating neurons is called a synapse. When an impulse reaches the end of one neuron, chemicals called neurotransmitters are released and cross the synaptic gap to influence the next neuron. A gland secretes substances, an effector performs a response, and a receptor detects stimuli. Therefore, option A is correct.

The governing concept is nervous coordination. Voluntary actions are performed consciously according to our will, such as walking, writing, speaking, and picking up an object. The brain receives sensory information, processes it, and sends motor commands to appropriate muscles. Therefore, option A, the brain, is correct. The spinal cord mainly conducts impulses and coordinates many reflexes, while the pancreas and thyroid are endocrine glands, not the main centres for voluntary movement.

The governing concept is the organisation of the nervous system. The brain acts as a major control and processing centre, and the spinal cord connects it with nerves and also participates in reflex pathways. Together, these two structures constitute the central nervous system, so option A is correct. The peripheral nervous system consists mainly of nerves outside the brain and spinal cord; the endocrine and digestive systems are different body systems.

The governing concept is division of labour among brain regions. The cerebrum is the largest part of the brain and is associated with conscious awareness, thinking, learning, memory, and interpretation of sensory information. Hence option A, the cerebrum, is correct. The cerebellum chiefly coordinates posture and balance, the medulla controls important involuntary activities, and the spinal cord conducts impulses and mediates many reflexes.

The governing concept is coordination of movement. The cerebellum receives information about body position and helps coordinate the timing and force of muscle contractions. This allows stable posture, balance, and smooth, accurate movements, so option A is correct. The cerebrum is mainly linked with conscious thought and learning; an olfactory receptor detects smell; and the pancreas performs digestive and hormonal functions rather than maintaining balance.

The governing concept is autonomic or involuntary control by the brainstem. Breathing rhythm and heartbeat continue without conscious decision, and the medulla oblongata contains centres that regulate these vital automatic activities. Therefore, option A is correct. The cerebrum is mainly responsible for conscious thought and voluntary actions, the cerebellum coordinates balance and movement, and the eye is a sensory organ rather than a control centre.

The governing concept is the stimulus-response pathway. A receptor detects a change, or stimulus, in the external or internal environment and converts it into a nerve signal or other biological message. Thus option A, receptor, is correct. An effector produces the response, such as muscle contraction or gland secretion; a hormone is a chemical messenger; and the nucleus is the genetic-control centre of a cell, not a sensory receiving structure.

The governing concept is sensory specificity. Receptors are specialised cells or structures that respond best to particular stimuli. The retina contains photoreceptors, including rods and cones, which respond to light and help begin the process of vision. Therefore option A is correct. Smell receptors respond to odor molecules, taste receptors to dissolved chemicals, and pressure receptors to mechanical force, so none of those describes the light-sensitive receptors of the eye.

The governing concept is sensory reception and stimulus specificity. Receptors in the nasal lining respond to chemical molecules present in the air and generate signals that are interpreted by the brain as smell. Therefore option A is correct. Light is detected by photoreceptors in the eye, sound by receptors in the inner ear, and temperature by thermoreceptors in the skin and other tissues. Thus the nose is not the primary receptor organ for the other listed stimuli.

The governing concept is the response side of a stimulus-response pathway. An effector is a body part that carries out the response after receiving a command from the nervous or hormonal system. Muscles contract and glands release secretions, so both can act as effectors; therefore option A is correct. A receptor detects the stimulus, a synapse passes a signal between cells, and a dendrite receives nerve impulses in a neuron.

Phototropism is the directional growth of a plant part in response to light. In a stem, auxin becomes unevenly distributed and promotes greater elongation on the shaded side, so the stem bends toward the light. Therefore, option A is correct. Geotropism responds to gravity, hydrotropism to water, and chemotropism to chemicals, so those distractors describe different stimuli.