Chapter 2: Control and Coordination

Solution

The neuron (nerve cell) is the structural and functional unit of the nervous system. It is specialised to transmit electrical impulses (nerve signals) from one part of the body to another. A nephron is the functional unit of the kidney, a nucleotide is the monomer of DNA/RNA, and a neutrophil is a type of white blood cell.

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The part that insulates the axon and speeds up impulse is the Myelin sheath.

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The part that contains the nucleus is the Cell body (soma).

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The part that receives signals from other neurons is the Dendrite.

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The part that carries impulse away from cell body is the Axon.

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The part that is the junction between two neurons is the Synapse.

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The part that releases neurotransmitters is the Axon terminal (synaptic knob).

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At the synapse (the tiny gap between two neurons), the electrical impulse cannot jump across directly. Instead, the axon terminal of the first neuron releases chemical substances called neurotransmitters (such as acetylcholine) into the synaptic cleft. These chemicals diffuse across the gap and bind to receptors on the dendrite of the next neuron, generating a new electrical impulse. This ensures unidirectional transmission of the impulse.

Solution

The correct sequence of a reflex arc is: Receptor → Sensory neuron → Relay neuron (in spinal cord) → Motor neuron → Effector. The receptor detects the stimulus, the sensory neuron carries the impulse to the spinal cord, the relay neuron (interneuron) connects sensory to motor neurons within the spinal cord, the motor neuron carries the impulse to the effector (muscle or gland), which performs the response.

Solution

The medulla oblongata (part of the hindbrain) controls essential involuntary actions such as breathing, heart rate, blood pressure, vomiting, sneezing, and swallowing. Damage to the medulla can be fatal because these are life-sustaining functions. The cerebrum handles voluntary actions and thinking, the cerebellum controls balance and coordination, and the hypothalamus regulates hunger, thirst, and body temperature.

Solution

When a plant shoot is illuminated from one side, auxin redistributes and accumulates on the shaded side. Since auxin promotes cell elongation, cells on the shaded side grow faster and become longer than those on the illuminated side. This uneven growth causes the shoot to bend toward the light (positive phototropism). This was first demonstrated by experiments involving coleoptiles (Darwin, Went).

Solution

The folding of Mimosa pudica leaves is a nastic movement (specifically, thigmonasty). It is non-directional — the leaves fold the same way regardless of which direction the touch comes from. It is caused by a sudden loss of turgor pressure (water) in the cells at the base of the leaves (pulvinus), not by growth. Tropisms are directional growth responses, which this is not.

Solution

Ethylene is a gaseous plant hormone that promotes fruit ripening. This is why placing an unripe fruit alongside ripe fruits (which emit ethylene) helps it ripen faster. Ethylene is used commercially to ripen fruits like bananas and tomatoes during transport. Auxin promotes cell elongation, gibberellins promote stem growth, and cytokinins promote cell division.

Solution

The thyroid gland requires iodine to synthesise the hormone thyroxine. When iodine is deficient, the thyroid cannot produce enough thyroxine, and it enlarges in an attempt to compensate — this swelling is called goitre. This is why the government mandates the use of iodised salt to prevent iodine deficiency. Diabetes is caused by insulin deficiency, dwarfism by growth hormone deficiency, and gigantism by growth hormone excess.

Solution

Adrenaline (epinephrine), secreted by the adrenal glands (located on top of the kidneys), is called the 'fight or flight' hormone. During emergencies, stress, or danger, adrenaline is released into the blood. It increases heart rate, breathing rate, blood pressure, and blood supply to muscles — preparing the body to either fight the threat or run away from it.

Solution

The growth of a pollen tube toward the ovule is an example of chemotropism — growth in response to a chemical stimulus. The ovule secretes certain chemicals that attract the pollen tube, guiding it through the style to reach the ovule for fertilisation. This is a positive chemotropism and is essential for successful sexual reproduction in flowering plants.

Solution

The axon is a long fibre that carries nerve impulses away from the cell body toward other neurons, muscles, or glands. Dendrites carry impulses toward the cell body. The cell body (cyton) contains the nucleus. A synapse is the junction between two neurons, not a part of a single neuron.

Solution

Testosterone is the primary male sex hormone, secreted by the testes. It is responsible for the development of male secondary sexual characteristics during puberty — deepening of voice, growth of facial and body hair, increased muscle mass, and development of male reproductive organs. The ovaries in females secrete oestrogen and progesterone.

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 28 = 28+28 = total AT%

G + C = 100 - 2x28. So G = (100 - 2x28)/2 = 22%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 20 = 20+20 = total AT%

G + C = 100 - 2x20. So G = (100 - 2x20)/2 = 30%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 32 = 32+32 = total AT%

G + C = 100 - 2x32. So G = (100 - 2x32)/2 = 18%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 30 = 30+30 = total AT%

G + C = 100 - 2x30. So G = (100 - 2x30)/2 = 20%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 25 = 25+25 = total AT%

G + C = 100 - 2x25. So G = (100 - 2x25)/2 = 25%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 18 = 18+18 = total AT%

G + C = 100 - 2x18. So G = (100 - 2x18)/2 = 32%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 35 = 35+35 = total AT%

G + C = 100 - 2x35. So G = (100 - 2x35)/2 = 15%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 22 = 22+22 = total AT%

G + C = 100 - 2x22. So G = (100 - 2x22)/2 = 28%

Solution

By Chargaff's rule: A = T and G = C.

A + T = 2 x 15 = 15+15 = total AT%

G + C = 100 - 2x15. So G = (100 - 2x15)/2 = 35%

Solution

The hormone responsible for stem elongation and seed germination is: Gibberellin.

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The hormone responsible for cell elongation and phototropism is: Auxin.

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The hormone responsible for inhibiting growth and closing stomata during drought is: Abscisic acid (ABA).

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The hormone responsible for fruit ripening is: Ethylene.

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The hormone responsible for promoting cell division is: Cytokinin.

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The hormone responsible for apical dominance is: Auxin.

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Description: Preserved remains of organisms found in rocks of different ages

This is: Fossil evidence

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Description: Embryos of fish, reptile, bird, and mammal look similar in early stages

This is: Embryological evidence

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Description: Human arm, whale flipper, and bat wing have same bone structure but different functions

This is: Homologous organs

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Description: Wings of birds and wings of insects perform same function but have different origin

This is: Analogous organs

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Description: Non-functional remnants like appendix in humans

This is: Vestigial organs

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Description: Similarity in DNA sequences across different species

This is: Molecular (DNA) evidence

Solution

The correct order of a reflex arc is:

Receptor -> Sensory neuron -> Spinal cord -> Motor neuron -> Effector

Stimulus -> Receptor -> Sensory neuron -> CNS -> Motor neuron -> Effector -> Response

Solution

The correct order of a reflex arc is:

Hot object stimulus -> Nerve impulse to spinal cord -> Spinal cord processing -> Nerve impulse to muscle -> Hand withdrawal

Stimulus -> Receptor -> Sensory neuron -> CNS -> Motor neuron -> Effector -> Response

Solution

The correct order of a reflex arc is:

Skin receptor -> Sensory neuron -> Relay neuron -> Motor neuron -> Muscle

Stimulus -> Receptor -> Sensory neuron -> CNS -> Motor neuron -> Effector -> Response

Solution

Reflex actions are processed by the spinal cord because speed is critical. If you touch a hot pan, the impulse travels to the spinal cord and back to the hand muscles much faster than if it had to travel all the way to the brain for processing. The brain is informed of the reflex action afterward but is not involved in the immediate response. This speed advantage protects the body from serious injury.

Solution

The cerebellum is responsible for maintaining balance, posture, and coordination of voluntary movements. It ensures that movements like walking, writing, or riding a bicycle are smooth and precise. Damage to the cerebellum results in ataxia — shaky, uncoordinated movements and difficulty maintaining balance, even though the person can still decide to move (cerebrum is intact).

Solution

Abscisic acid (ABA) is the growth inhibitor among plant hormones. It promotes leaf fall (abscission), seed dormancy, and closing of stomata during water stress. It counteracts the growth-promoting effects of auxin, gibberellin, and cytokinin. ABA is sometimes called the 'stress hormone' of plants because its levels rise during drought, salinity, and temperature stress.

Solution

Insulin, secreted by the beta cells of the Islets of Langerhans in the pancreas, lowers blood sugar levels by promoting glucose uptake into cells. Deficiency of insulin (or cells becoming resistant to it) causes diabetes mellitus — a condition where blood sugar remains abnormally high. This can damage blood vessels, kidneys, eyes, and nerves over time. Patients may need insulin injections.

Solution

The adrenal gland secretes adrenaline (the fight-or-flight hormone) in response to stress or danger. Adrenaline causes increased heart rate, rapid breathing, dilated pupils, increased blood flow to muscles, and heightened alertness — all preparing the body to respond to the threat. The thyroid (metabolism), pituitary (growth), and pancreas (blood sugar) are not primarily involved in acute stress responses.

Solution

Gibberellins are commercially used to produce seedless fruits (parthenocarpy). When gibberellin is applied to grape flowers, fruit development occurs without fertilisation, resulting in seedless grapes. Gibberellins also promote stem elongation, break seed dormancy, and stimulate flowering. This is an important commercial application of plant hormones.

Solution

The pituitary gland is called the 'master gland' because it secretes hormones that control and regulate the activity of several other endocrine glands. For example, it produces thyroid-stimulating hormone (TSH) to control the thyroid, adrenocorticotropic hormone (ACTH) to control the adrenal cortex, and gonadotropins (FSH, LH) to control the gonads. However, the pituitary itself is controlled by the hypothalamus.

Solution

The brain is protected by three layers of defence: (1) The skull (cranium) — a hard bony case that provides physical protection. (2) Three membranes called meninges that cover the brain (dura mater, arachnoid mater, pia mater). (3) Cerebrospinal fluid (CSF) that fills the space between the meninges and acts as a shock absorber, cushioning the brain from jolts and impacts.

Solution

Tt x Tt Punnett square: TT, Tt, Tt, tt

Genotype ratio: 1 TT : 2 Tt : 1 tt

Fraction that are tall = 3/4

Solution

Tt x Tt Punnett square: TT, Tt, Tt, tt

Genotype ratio: 1 TT : 2 Tt : 1 tt

Fraction that are homozygous (TT or tt) = 1/2

Solution

Tt x Tt Punnett square: TT, Tt, Tt, tt

Genotype ratio: 1 TT : 2 Tt : 1 tt

Fraction that are homozygous tall (TT) = 1/4

Solution

Tt x Tt Punnett square: TT, Tt, Tt, tt

Genotype ratio: 1 TT : 2 Tt : 1 tt

Fraction that are short (dwarf) = 1/4

Solution

Tt x Tt Punnett square: TT, Tt, Tt, tt

Genotype ratio: 1 TT : 2 Tt : 1 tt

Fraction that are heterozygous (Tt) = 1/2

Solution

Both statements are true and the Reason correctly explains the Assertion. At a synapse, the nerve impulse is converted to a chemical signal (neurotransmitters released from vesicles in the axon terminal). These neurotransmitters can only bind to receptors on the dendrite of the next neuron — the dendrite does not release neurotransmitters, and the axon terminal does not have receptors. This structural arrangement ensures that impulses travel in one direction only (unidirectional transmission).

Solution

Both are true and the Reason correctly explains the Assertion. Roots show positive geotropism (grow toward gravity, i.e., downward) and negative phototropism (grow away from light). These tropisms are advantageous because growing downward helps roots anchor the plant firmly and reach water and minerals in the deeper layers of soil. Auxin plays a key role — in roots, higher auxin concentration on the lower side inhibits growth, causing the root to bend downward.

Solution

When blood sugar rises after a meal, the pancreas detects the high glucose level and the beta cells of the Islets of Langerhans release insulin. Insulin promotes the uptake of glucose by body cells (especially liver and muscle cells), where it is either used for energy or stored as glycogen. This brings the blood sugar level back down to normal. Glucagon does the opposite — it raises blood sugar when it drops too low. This is a classic example of a negative feedback mechanism.

Solution

The pancreas is a dual-function (mixed) gland. As an endocrine gland, it secretes hormones (insulin and glucagon) directly into the blood through the Islets of Langerhans. As an exocrine gland, it secretes pancreatic juice (containing digestive enzymes — trypsin, lipase, amylase) through a duct into the duodenum of the small intestine. No other option has this dual function — thyroid, adrenal, and pituitary are purely endocrine.

Solution

Excess growth hormone (GH) during childhood (while growth plates in bones are still open) leads to gigantism — a condition where the person grows abnormally tall. GH stimulates the liver to produce insulin-like growth factor (IGF-1), which promotes bone and cartilage growth. If excess GH occurs after growth plates close (in adults), it causes acromegaly (enlargement of hands, feet, and facial features) instead. Deficiency of GH causes dwarfism — the opposite condition.