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Cambridge IGCSE Biology · 0610

Chapter 14: Coordination and response (Part 4)

Temperature regulation

Optimum
Humans maintain a constant 37°C for optimum enzyme activity.
Control
Regulated by the hypothalamus in the brain.
Mechanisms
  • Too cold: Shivering (muscle contraction produces heat) and vasoconstriction (arterioles constrict to reduce blood flow to skin surface capillaries, reducing heat loss).
  • Too hot: Sweating (evaporation reduces surface temperature) and vasodilation (arterioles dilate to increase blood flow to surface capillaries, increasing heat loss).
Skin structure
Includes hairs, hair erector muscles, sweat glands, receptors, and fatty tissue for insulation.
Structure of the skin showing epidermis, dermis, hair, hair erector muscle, sweat gland, receptors, sensory neurones, blood vessels, and fatty tissue
Diagram 1: Structure of the skin. The epidermis, dermis, hair, hair erector muscle, sweat gland, receptors, sensory neurones, blood vessels, and fatty tissue are labelled.

Exam Traps

  • Do not say vasodilation occurs when cold — arterioles dilate when hot to increase surface blood flow.

Tropic responses

Gravitropism
A growth response to gravity (shoots are negative; roots are positive).
Phototropism
A growth response to light (shoots are positive; roots are negative).

Exam Traps

  • Do not say roots are positively phototropic — roots grow away from light.
  • Avoid saying shoots are positively gravitropic — shoots grow upwards against gravity.

Chemical control in plants

Auxins
Plant hormones that control growth by causing cell elongation.
Mechanism in shoots
  1. Auxin is made in the shoot tip and diffuses down the plant.
  2. Light or gravity causes unequal distribution (auxin moves to the shady side or lower side).
  3. Higher auxin concentration stimulates faster cell elongation on that side.
  4. The unequal growth causes the shoot to bend toward the light or away from gravity.
Phototropism in a shoot showing auxin moving to the shaded side and causing bending toward the light
Diagram 2: Phototropism in a shoot. A shoot tip receives light from one side. Auxin moves to the shaded side, causing those cells to elongate more and the shoot to bend towards the light source.

Exam Traps

  • Do not say auxin is destroyed on the lit side — it moves away from light to the shaded side.
  • Avoid claiming auxin inhibits growth on the shaded side — higher auxin speeds elongation there in shoots.

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