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 sensory reception. Receptors in the nose respond to chemical molecules responsible for smell, and receptors in the tongue respond to dissolved chemicals responsible for taste. Thus both detect chemical stimuli, making option A correct. Light is detected by photoreceptors in the eye, sound by auditory receptors in the ear, and gravity is not the specific stimulus for these nose and tongue receptors.

The inner ear contains balance-related structures that detect head position and movement. These structures generate nerve signals about orientation and motion, and the brain combines them with information from the eyes and muscles to maintain posture. Therefore, option A is correct. The other options describe digestion, blood-sugar regulation, and hormone secretion, which are not functions of the inner ear.

The cerebrum is the largest part of the forebrain and controls higher mental activities such as thinking, learning, memory, reasoning, and conscious actions. Damage to it can therefore affect thinking and memory, making option A correct. Heart-muscle formation, thyroxine production, and downward root growth are controlled by developmental, endocrine, or plant processes rather than the cerebrum.

A voluntary action is performed consciously and according to a choice, such as raising a hand, writing, or walking. The cerebrum evaluates information, makes a decision, and sends motor commands through nerves to the appropriate muscles. Therefore option A is correct. The other statements are biologically false or unrelated to conscious movement.

Plants do not generally have muscles or a nervous system that can produce rapid coordinated movement. Many plant movements result from unequal growth on two sides of an organ, as in a shoot bending toward light. Since cell division and elongation take time, such responses are usually slow. Therefore option A is correct; the other statements are factually incorrect.

Hydrotropism means directional growth in response to water, so a root growing toward moisture demonstrates hydrotropism. Phototropism means directional growth in response to light, so a stem bending or growing toward light demonstrates phototropism. Therefore option A correctly names both responses. The other options confuse growth responses with life processes or deny the observed responses.

Hormones are chemical substances released by endocrine glands into the blood. The circulation carries them to particular target cells or organs, where they bind to receptors and change the activity of those targets. Because they transmit regulatory information chemically rather than through nerve impulses, they are called chemical messengers. Thus option A is correct; the other options misrepresent their nature.

The governing concept is hormonal control of blood glucose. Insulin, secreted by the pancreas, helps body cells take up and use glucose and promotes its storage as glycogen. If insulin is deficient, glucose remains in the bloodstream, so blood sugar rises and diabetes may develop. Adrenaline can temporarily raise glucose during stress, while thyroxine and estrogen have different primary functions; therefore option A is correct.

The governing concept is the emergency or fight-or-flight response. In a frightening situation, the adrenal glands release adrenaline. This hormone increases heart rate, breathing rate and blood flow to muscles so that the body can respond rapidly and obtain more oxygen and energy. Thyroxine regulates general metabolism, insulin regulates blood glucose, and growth hormone supports growth; none gives this immediate fear response. Hence option C is correct.

The governing concept is tropism, a directional growth response to an external stimulus. Water provides hydrotropic stimulation and gravity provides geotropic stimulation. Roots grow toward water and generally downward in response to gravity, showing that plant growth can be coordinated without a nervous system. Photosynthesis, nerves and blood circulation do not explain this directional growth. Therefore option A correctly identifies the feature.

The governing concept is geotropism, also called gravitropism. Positive tropism means growth in the direction of the stimulus. Gravity acts downward, and roots usually grow downward in response to it; therefore downward root growth is positive geotropism. Upward stem growth is generally negative geotropism, tendril coiling is related to touch, and pollen-tube growth toward an ovule is chemotropism. Hence option B is correct.

The governing concept is negative geotropism, or negative gravitropism. Negative tropism means growth away from the direction of the stimulus. Since gravity acts downward, the stem or shoot growing upward is growing opposite to gravity and therefore shows negative geotropism. Root growth toward water is hydrotropism, tendril bending is a touch response, and pollen-tube growth toward a chemical signal is chemotropism. Thus option B is correct.

The usual direction of impulse flow within a neuron is from the receiving dendrites to the cell body and then along the axon to the axon terminal. Dendrites collect signals from receptors or neighbouring neurons, while the axon carries the processed impulse away from the cell body. Therefore option A gives the correct sequence. The other choices reverse or mix the normal structural pathway.