Control and Coordination

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.

Geotropism, also called gravitropism, is directional growth in response to gravity. Roots generally grow downward, in the same direction as gravitational pull, so their response is called positive geotropism. Hence option A is correct. Phototropism concerns light, hydrotropism concerns water, and thigmotropism concerns touch; none of these explains the stated stimulus.

A stem usually grows upward, whereas gravity acts downward. Growth in a direction opposite to the gravitational pull is therefore called negative geotropism, making option A correct. Positive geotropism means growth with gravity, as in roots. Hydrotropism is a response to water, thigmotropism to touch, and chemotropism to chemical substances, so those options do not fit.

Hydrotropism is the directional growth of a plant part in response to water or moisture. When roots grow toward a region containing more water, they display positive hydrotropism; therefore option A is correct. Phototropism is caused by light, geotropism by gravity, and thigmotropism by touch. Although roots also respond to gravity, the stimulus named here is water.

Thigmotropism is directional growth caused by contact or touch. When one side of a tendril touches a support, unequal growth makes it curve and coil around that support, so option A is correct. Phototropism is guided by light, hydrotropism by water, and geotropism by gravity. The essential stimulus in this example is physical contact with the support.

Chemotropism is directional growth in response to chemical substances. During plant reproduction, chemical signals associated with the ovule guide the pollen tube toward it, enabling the male gamete to reach the female reproductive structure. Thus option A is correct. Phototropism responds to light, thigmotropism to touch, and hydrotropism to water, so they are not the governing stimuli here.

A green stem generally shows positive phototropism, meaning it grows toward a source of light. Light causes auxin to redistribute toward the shaded side, where cells elongate more rapidly; this unequal growth bends the stem toward the illuminated side. Hence option A is correct. B describes the opposite response, while C and D refer to directions unrelated to the stated light stimulus.

Touch-me-not leaves fold rapidly after contact because water movement changes the turgor pressure in special motor cells at the leaf base. The response does not depend on the direction from which the touch comes and is not a growth movement, so it is a non-directional touch-induced movement, commonly called thigmonasty or seismonasty. Thus option A is correct; the other choices are directional tropisms.

Tropism is a directional growth response of a plant part to an external stimulus. The direction may be toward the stimulus, called positive tropism, or away from it, called negative tropism. Light, gravity, water, touch, and chemicals can act as stimuli. Therefore option A is correct; digestion, blood flow, and breathing rate are animal or physiological processes, not plant tropic growth.

The governing concept is hormonal or chemical coordination. Endocrine glands produce hormones, which are chemical messengers released into the blood and carried to specific target organs. Therefore, option A is correct. Nerve fibres transmit electrical impulses rather than hormones, bones provide support, and red blood cells mainly transport oxygen and carbon dioxide; none of these performs chemical coordination.

The governing concept is hormonal regulation of blood glucose. Insulin, secreted by the pancreas, promotes the uptake of glucose by body cells and supports its storage as glycogen, thereby lowering an elevated blood-glucose level. Hence option A is correct. Adrenaline can raise blood glucose during stress, while thyroxine regulates metabolism and growth hormone has broader growth-related effects; neither is the principal answer here.