Control and Coordination

Iodine is required by the thyroid gland to produce thyroxine. If iodine intake is insufficient, thyroxine production may fall, and the thyroid gland can enlarge as it attempts to capture more iodine. This enlargement is called goitre, so option A is correct. Diabetes is associated with problems in insulin regulation, night blindness commonly relates to vitamin A deficiency, and malaria is caused by a parasite transmitted by mosquitoes.

The pituitary gland is called the master gland because it secretes hormones that regulate the activity of several other endocrine glands, including the thyroid, adrenal glands, and gonads. Its hormones also influence growth and reproduction. Therefore option A is correct. Digestion is mainly performed by the digestive system, blood filtration is associated with the kidneys, and bone formation is not the defining role of the pituitary.

Growth hormone is secreted by the pituitary gland and supports the growth of bones and other body tissues, especially during childhood and adolescence. A suitable amount is important for normal height and physical development, so option A is correct. Growth hormone does not determine the colour of blood, the taste of food, or dental cleanliness; those features are governed by different substances and processes.

The governing concept is tropic movement, a directional growth response to an external stimulus. When a plant organ, especially a shoot, grows towards a light source, the response is called positive phototropism. Geotropism responds to gravity, hydrotropism to water, and chemotropism to chemicals. Therefore, option A is correct.

Plants require light for photosynthesis, the process by which they manufacture food from carbon dioxide and water. A stem and its leaves therefore tend to grow towards available light, showing positive phototropism. Growing to lose water, escape soil, or become a root is not the biological purpose of this response. Thus, option A is correct.

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.