Response and coordination

Unit 9 Response and coordination

Key unit competence: Explain how plants and animals detect and respond to stimuli through nervous and hormonal coordination.

Key ideas

• Sensitivity is the ability to detect a stimulus (a change in the environment) and respond. Coordination links receptors and effectors so that the organism responds correctly.
• Plants respond slowly by growth movements. Tropisms are directional growth responses: phototropism (to light), gravitropism (geotropism) (to gravity), hydrotropism (to water), thigmotropism (to touch). Shoots are positively phototropic; roots are positively gravitropic.
• Auxin (IAA) is made in the shoot tip and moves down. In one-sided light, auxin moves to the shaded side, causing cells there to elongate more, so the shoot bends towards the light. In roots, high auxin inhibits elongation.
• Nastic movements are non-directional responses: e.g. the sensitive plant (Mimosa) folds its leaves when touched (by loss of turgor), and some flowers open in light and close in darkness.
• The human nervous system: the central nervous system (CNS) = brain and spinal cord; the peripheral nervous system (PNS) = nerves linking the CNS to the body, including the autonomic nervous system (controls involuntary actions such as heartbeat).
• Parts of the brain: cerebrum (thinking, memory, voluntary actions, senses), cerebellum (balance and coordination of movements), medulla oblongata (breathing, heartbeat, blood pressure), hypothalamus (temperature, water balance, links with the pituitary gland).
• Neurones: sensory neurones carry impulses from receptors to the CNS; relay (intermediate) neurones in the CNS connect them; motor neurones carry impulses from the CNS to effectors (muscles and glands). A neurone has a cell body, dendrites, an axon, often a myelin sheath (speeds impulses) and nerve endings.
• A synapse is a tiny gap between neurones. The impulse causes release of a neurotransmitter that diffuses across the gap and starts an impulse in the next neurone; synapses make impulses travel one way.
• A reflex action is a rapid, automatic response to a stimulus that protects the body, e.g. withdrawing a hand from a hot object, blinking, the knee jerk. Reflex arc: receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector. A voluntary action is controlled consciously by the brain.
• Hormonal (endocrine) control: endocrine glands secrete hormones directly into the blood; they act on target organs. Examples: insulin (pancreas: lowers blood glucose), adrenaline (adrenal glands: prepares for "fight or flight"), thyroxine (thyroid: metabolism, needs iodine), growth hormone (pituitary), oestrogen and testosterone (ovaries, testes). Hormonal responses are slower and longer-lasting than nervous ones.

Common misconceptions

• "Plants bend towards light because light pulls them." No: auxin moves to the shaded side and makes those cells grow longer.
• "Reflexes are controlled by the brain." No: most simple reflexes pass through the spinal cord; the brain is informed afterwards.
• "Hormones travel along nerves." No: hormones are carried in the blood.
• "Nervous and hormonal responses are equally fast." No: nerve impulses are fast and short-lived; hormones are slower and longer-lasting.

Worked examples

Practice questions

1. Define the terms stimulus, receptor and effector.

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   Stimulus: a change in the environment detected by an organism. Receptor: a cell or organ that detects a stimulus. Effector: a muscle or gland that responds.

   Common mistake: Calling the response a stimulus.

2. Explain how auxin causes a shoot to bend towards light.

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   Auxin is made at the shoot tip and moves to the shaded side in one-sided light; it makes cells there elongate more than on the lit side, so the shoot bends towards light.

   Common mistake: Saying auxin moves to the lit side.

3. Name three parts of the brain and give one function of each.

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   Cerebrum: thinking, memory, voluntary actions. Cerebellum: balance and coordination. Medulla oblongata: breathing, heartbeat. Hypothalamus: temperature and water control.

   Common mistake: Mixing up cerebrum and cerebellum.

4. Draw a reflex arc in words, naming all parts.

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   Stimulus → receptor → sensory neurone → relay neurone in spinal cord → motor neurone → effector → response.

   Common mistake: Leaving out the relay neurone.

5. Give three differences between nervous and hormonal coordination.

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   Nervous: electrical impulses along neurones, very fast, short-lived, localised. Hormonal: chemicals in blood, slower, longer-lasting, may affect many organs.

   Common mistake: Saying hormones travel in nerves.

6. Distinguish between a reflex action and a voluntary action, with examples.

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   Reflex: automatic, fast, not consciously controlled, through spinal cord (blinking, withdrawing hand). Voluntary: conscious, controlled by cerebrum, slower (writing, walking).

   Common mistake: Calling walking a reflex.

7. Name four endocrine glands and a hormone from each.

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   Pancreas: insulin. Adrenal: adrenaline. Thyroid: thyroxine. Pituitary: growth hormone/ADH. Ovaries: oestrogen. Testes: testosterone.

   Common mistake: Naming salivary glands as endocrine.

8. State two differences between tropisms and nastic movements.

   Show answer

   Tropisms are directional growth responses, slow and permanent; nastic movements are non-directional, often fast, usually by turgor changes and reversible.

   Common mistake: Saying Mimosa shows phototropism when touched.

Matching and drag-and-drop

1. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. The hormone that lowers blood glucose is:	a. medulla oblongata
   2. Hormones are carried around the body by the:	b. insulin
   3. The part of the brain that controls breathing and heartbeat is the:	c. shaded side of the shoot
   4. In one-sided light, auxin collects on the:	d. blood

   Show answer

   1 → b (insulin) · 2 → d (blood) · 3 → a (medulla oblongata) · 4 → c (shaded side of the shoot)

   • It is made by the pancreas.
   • Endocrine glands are ductless.
   • These are involuntary actions.
   • Shaded cells grow longer.

2. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   positive gravitropism  ·  nastic movement  ·  positive phototropism  ·  synapse
   1. The sensitive plant folding its leaves when touched is a .
   2. Roots growing downwards show .
   3. The gap between two neurones is a .
   Show answer

   1: nastic movement · 2: positive gravitropism · 3: synapse

   • It is not directional.
   • Gravity is the stimulus.
   • A chemical crosses it.

3. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. The plant hormone that controls cell elongation in shoots is:	a. motor neurones
   2. Tendrils coiling around a support show:	b. auxin
   3. The part of the brain that controls balance is the:	c. cerebellum
   4. Neurones carrying impulses to muscles are:	d. thigmotropism

   Show answer

   1 → b (auxin) · 2 → d (thigmotropism) · 3 → c (cerebellum) · 4 → a (motor neurones)

   • It is made at the shoot tip.
   • Touch is the stimulus.
   • It coordinates movements.
   • Muscles are effectors.

4. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   brain and spinal cord  ·  cerebrum  ·  medulla oblongata  ·  thigh muscle
   1. The central nervous system consists of the .
   2. In the knee jerk reflex, the effector is the .
   3. Thinking and memory are functions of the .
   Show answer

   1: brain and spinal cord · 2: thigh muscle · 3: cerebrum

   • The rest is the peripheral system.
   • The muscle contracts and the leg kicks.
   • It is the largest part of the brain.

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