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2.10Structures and Functions in Living Organisms

Co-ordination and Response — Edexcel Linear - OL Revision Notes & Topical Questions

15 min read 24 parts Lesson 25 of 25 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

Response to the Environment, Homeostasis and Plant Responses — EDEXCEL LINEAR - OL notes: Explain why organisms respond to stimuli, how homeostasis is controlled by negative feedback and how auxin produces phototropism and geotropism.Organisms must detect changes (stimuli) in the…

  • positive phototropism
  • negative geotropism
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Part 1 of 5Responding to change

Organisms must detect changes (stimuli) in their environment and respond to them to survive. In animals a stimulus is detected by a receptor, a coordinator (brain or spinal cord) decides the response, and an effector (muscle or gland) carries it out. Plants have no nervous system; they respond to stimuli by growth, controlled by hormones.

Part 2 of 5Homeostasis

Homeostasis is the maintenance of a constant internal environment.

It keeps conditions such as body temperature, blood glucose, water content and carbon dioxide level within narrow limits, where enzymes work best.
Negative feedback
Homeostasis is under involuntary control, using the nervous system and hormones,
  • and works by negative feedback: if a level rises above normal, the response lowers it,
  • and if it falls, the response raises it.
Each system has a receptor to detect change, a coordination centre and an effector.

A coordinated response, such as that required when there is not enough water in the blood, needs a stimulus (low blood water potential detected by the brain), a coordinator (the brain and pituitary gland), a hormone (ADH) and an effector (the kidney), producing a response (more water reabsorbed).

Part 3 of 5Plant responses: tropisms and auxin

Unilateral light, rotating clinostat and dark controls in a phototropism investigation
Unilateral light, rotating clinostat and dark controls in a phototropism investigation

Plants grow in response to stimuli. Plant shoots display positive phototropism (growing towards a light source) and negative geotropism; roots display positive geotropism (growing down) and negative phototropism. The growth responses are controlled by the hormone auxin, produced in the shoot tip.

1
Light from one side causes auxin to move to the shaded side of the shoot.
2
Auxin accumulating on the shaded side stimulates cells there to elongate (grow longer) faster than the cells on the lit side.
3
The shoot therefore bends towards the light.
4
In roots, higher auxin concentrations on the lower side inhibit elongation, so the root bends downward.

The benefit

  • shoots growing towards light can photosynthesise more;
    • roots growing downwards anchor the plant and reach water and minerals.
  • Experiments with a shoot tip covered by foil, removed or with a mica barrier show that the tip detects light and makes auxin.

Part 4 of 5Exam practice: sequences and experiments

For a homeostasis question, write the sequence as a chain: change in the variable, detection by receptors, signal to the coordination centre, signal to the effector, response, and return to normal.

1
For tropisms, remember classic experiments.
2
Charles Darwin and his son found that a shoot tip covered by an opaque cap did not bend towards light,
  • whereas a shoot with the base covered did;
    • later experiments with agar blocks showed that a chemical (auxin) produced in the tip moves down to the shaded side.
3
When asked to design an experiment, cover the tip of one seedling with foil, remove the tip of another,
  • and leave a third as a control, with light from one side; measure the angle of bend after a set time.
4
Control the species, age, light intensity and temperature, and use several seedlings to obtain a mean.

Remember that plant hormones act more slowly than animal nervous responses and that the response is a growth movement, not a movement by muscles. A further term is that auxin distribution is affected by gravity in roots and shoots: auxin collects on the lower side, which stimulates shoot cells to elongate (shoots grow up) but inhibits root cells (roots grow down).

Part 5 of 5Worked examples

  1. Define homeostasis. [1]1 mark

  2. Explain how phototropism happens in a shoot. [4]4 marks

  3. Why is negative feedback important? [2]2 marks

  4. What is the effect of auxin on roots? [1]1 mark

  5. State the order of components in a response in animals. [3]3 marks

Key terms

TermMeaning
StimulusA change in the environment detected by an organism.
HomeostasisMaintenance of a constant internal environment.
Negative feedbackA response that reverses a change.
AuxinA plant hormone controlling growth responses.
PhototropismGrowth in response to light.
GeotropismGrowth in response to gravity.

Avoid these mistakes

1
Saying phototropism is a movement of the whole plant.
2
Saying auxin moves to the lit side.
3
Confusing positive and negative feedback.
4
Saying plants respond using nerves.

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