Control and Coordination

The governing concept is chemotropism, a directional growth response to chemical substances. The pollen tube grows toward the ovule because chemical signals released by tissues around the ovule guide its direction. This movement is important for delivering the male gamete to the female gametophyte before fertilisation. Therefore, option A is correct. Phototropism responds to light, geotropism to gravity, and hydrotropism to water, so none of those describes the main stimulus here.

The governing concept is a rapid nastic movement in response to touch. Mimosa leaves fold when touched because a signal causes changes in ion and water movement in the pulvinus cells, changing their turgor pressure; this is not directional growth. Therefore, option A is correct. Light-induced, water-induced, and gravity-induced growth describe tropic responses that occur more slowly and depend on the direction of the stimulus.

Phototropism is a growth response controlled by unequal distribution of auxin. When light comes from one side, more auxin collects on the shaded side of a shoot. Cells there elongate more than cells on the illuminated side, so the stem curves toward the light. Thus option A is correct. The other choices describe neither the cause of bending nor the actual response of a shoot.

The governing concept is the role of plant hormones in growth and development. Cytokinins stimulate cell division, or cytokinesis, and therefore help form new cells in growing plant tissues. Hence option A is correct. Blood-pressure regulation and nerve-message transmission are animal functions, while stopping respiration is not a normal function of cytokinin. The distractors belong to unrelated body systems or describe an incorrect effect.

The brain is a delicate central nervous-system organ, so it is enclosed within a strong bony case called the skull or cranium. This structure surrounds the brain and reduces the risk of injury from external impact. Therefore option A is correct. The vertebral column protects the spinal cord, ribs protect organs in the chest, and the thigh bone supports the leg, so those options do not protect the brain.

The spinal cord is a delicate part of the central nervous system that carries messages between the brain and the rest of the body. It lies inside the vertebral canal, which is formed by the vertebral column or backbone. Thus option A is correct. The skull encloses the brain, ribs protect organs in the chest, and the shoulder bone supports the upper limb rather than surrounding the spinal cord.

Taste receptors are specialised sensory receptors that respond to dissolved chemicals in food. They are concentrated mainly in taste buds located on the tongue, although some taste buds may occur elsewhere in the mouth and throat. Therefore, option A is correct. The eye detects light, the ear detects sound, and the skin detects touch, pressure, temperature, and pain, so options B, C, and D do not match taste perception.

Sound is a mechanical stimulus produced by vibrations. Receptors in the inner ear, especially the sensory hair cells of the cochlea, convert these vibrations into nerve signals that the brain interprets as sound. Thus, option A, the ear, is correct. The eye contains photoreceptors for light, the nose has receptors for smell, and the tongue has taste receptors, so options B, C, and D detect different stimuli.

The nose contains olfactory receptors in the nasal lining. These receptors respond to odor molecules dissolved in the mucus and send nerve impulses to the brain, producing the sensation of smell. Therefore, option A is correct. Light is detected by photoreceptors in the eye, taste by receptors in the tongue, and pressure mainly by mechanoreceptors in the skin, so B, C, and D are unsuitable.

A neuron transmits information through changes in the electrical potential across its cell membrane. When sufficiently stimulated, an action potential travels along the axon as an electrical nerve impulse. At a synapse, chemical neurotransmitters may carry the message across the gap, but within the neuron the impulse is primarily electrical. Hence option A is correct; the other choices are unrelated materials, not message forms.

A target organ is a tissue or organ containing receptors that specifically recognise a particular hormone. After a hormone travels through the blood, it produces a response only in cells with the appropriate receptors. Therefore, option A is correct. Digestion, blood formation, and bone formation describe particular functions, not the defining relationship between a hormone and its responsive organ.

The nervous system and endocrine system are major coordinating systems. The nervous system uses rapid electrical impulses and neurotransmitters, while the endocrine system uses hormones carried in blood; together they regulate responses, growth, metabolism, and internal balance. Hence option A is correct. Food production, bone formation, and waste storage are particular biological functions, not the common purpose of both systems.

The governing concept is reflex action, a rapid, automatic protective response. Heat stimulates pain receptors in the skin; sensory neurons carry the impulse to the spinal cord, where an interneuron connects it to a motor neuron. The motor neuron activates arm muscles before the brain has fully analysed the sensation, so withdrawal is quick. The brain later receives information and becomes aware of pain. The other options deny normal nerve or muscle function.

The governing concept is directional transmission in a neuron. Dendrites are short, branched extensions that receive stimuli or nerve impulses from receptors or nearby neurons. The impulse then passes through the cell body and travels along the axon toward its terminal, where it can affect another cell. Therefore, option B correctly states both functions; the other options assign unrelated functions such as food-making, blood formation, breathing, or water storage.

A synapse is the tiny junction or gap between two neurons. When an electrical impulse reaches the end of the first neuron, it causes chemical messenger substances to be released. These chemicals cross the synaptic gap and stimulate the next neuron, which then carries the impulse onward. Thus option C is correct. Bones, blood cells, and digestive juice do not transmit messages across a synapse.

The governing concept is endocrine coordination by chemical messengers. Hormones are released by endocrine glands into the blood and may remain in circulation or continue affecting target cells after the initial release. Their action usually begins more slowly than a nerve impulse but can persist longer and influence several organs. Thus option A is correct; the remaining options describe false or unrelated processes.

The governing concept is the emergency or fight-or-flight response. Adrenaline released by the adrenal glands prepares the body for rapid action by increasing heart rate, breathing rate, and blood supply to active muscles. These changes help deliver oxygen and energy quickly. Therefore option A is correct. Sleep, absence of blood changes, and complete muscle stoppage are opposite to the normal action of adrenaline.

The governing concept is the role of iodine in thyroid hormone synthesis. The thyroid gland requires iodine to produce thyroxine, a hormone involved in regulating metabolism and development. When iodine intake is insufficient, thyroxine production may fall, and prolonged deficiency can contribute to enlargement of the thyroid gland called goitre. Therefore option A is correct; the other options describe no valid physiological relationship.

The governing concept is the division of functions among brain regions. The cerebellum coordinates the activity of muscles and helps maintain posture, balance and precise voluntary movement. Therefore, damage to it most directly causes difficulty in coordinated movement and balance. Blood-sugar regulation is mainly hormonal, thyroxine is produced by the thyroid gland, and skin colour is not the cerebellum’s principal function.

The governing concept is phototropism caused by unequal growth. When light comes from one side, auxin becomes distributed unevenly in the shoot. In a typical shoot, the shaded side receives more auxin and its cells elongate more than cells on the lighted side. This difference in elongation produces curvature, so the stem bends toward the light. Blood, roots pulling light and stopping photosynthesis do not explain it.