Form Three Biology Notes
Chapter One: Coordination in Plants · Ordinary Level (Form Three) · Study Notes. Read this chapter free, then continue every chapter in the Click Online Academy app.
All Form Three Biology Chapters
- Chapter 1Coordination in PlantsRead below · Free
- Chapter 2Coordination in AnimalsContinue in the app
- Chapter 3Endocrine SystemContinue in the app
- Chapter 4Excretion in AnimalsContinue in the app
- Chapter 5RegulationContinue in the app
Learning Outcomes
By the end of this chapter, you should be able to:
- explain the concept of coordination in plants;
- describe the main plant hormones (phytohormones) and their roles;
- distinguish between tropic and nastic movements;
- identify the common types of tropisms and give examples;
- explain how auxins control phototropism and geotropism;
- state the importance of coordination in plants and its applications in agriculture.
Key Terms and Concepts
| Term | Meaning |
|---|---|
| Coordination | The working together of different parts of an organism in an orderly way so that it responds correctly to stimuli. |
| Stimulus | A change in the internal or external environment that can be detected by an organism (e.g. light, gravity, touch, water). |
| Response | The reaction of an organism or its part to a stimulus. |
| Phytohormone | A chemical substance produced in plants that regulates growth, development and responses (also called plant hormone or plant growth regulator). |
| Tropism | A directional growth movement of a plant part towards or away from a unilateral stimulus. |
| Nastic movement | A non-directional movement of a plant part in response to a diffuse (non-directional) stimulus; often due to changes in turgor. |
| Auxin | A plant hormone (mainly indoleacetic acid, IAA) produced at shoot tips that promotes cell elongation and controls many tropisms. |
| Phototropism | Growth movement in response to the direction of light. |
| Geotropism | Growth movement in response to gravity (also called gravitropism). |
| Hydrotropism | Growth movement in response to water or moisture. |
| Thigmotropism | Growth movement in response to touch or contact (also called haptotropism). |
| Apical dominance | The inhibition of growth of lateral buds by the terminal (apical) bud, caused mainly by auxins. |
1. Concept of Coordination in Plants
Plants are made of different parts (roots, stems, leaves, flowers) that work together in a coordinated way to perform life functions. The interaction of plant structures in response to external and internal stimuli is called coordination.
Unlike animals, plants do not have a nervous system or muscles. They cannot move from place to place (no locomotion). However, they can detect changes in their environment (stimuli) and respond by growth movements or changes in turgor. These responses help the plant obtain light, water, nutrients and support, and protect itself.
Plant responses are usually slow and continuous. They are controlled mainly by chemical messengers called plant hormones (phytohormones).
Main ideas
- Plants detect stimuli such as light, gravity, water, chemicals and touch.
- Responses often take the form of movement of curvature (bending due to unequal growth on two sides of an organ).
- Coordination allows the plant to adapt and survive in its habitat.
TEST YOUR UNDERSTANDING
Q1. What is coordination in plants?
Q2. Why do plants need coordination even though they do not move from place to place?
2. Plant Hormones (Phytohormones)
Plant hormones are chemical substances produced in very small amounts in one part of the plant and transported to other parts where they regulate growth, development and responses. The main groups are:
| Hormone | Main production site | Key effects |
|---|---|---|
| Auxins (e.g. IAA) | Shoot tips, young leaves, developing seeds | Promote cell elongation; control phototropism and geotropism; apical dominance; root formation; fruit development; can act as selective herbicides at high concentration. |
| Gibberellins | Young leaves, roots, seeds | Stem elongation (especially internodes); break seed dormancy; promote germination; induce parthenocarpy in some fruits. |
| Cytokinins | Root tips, young fruits | Promote cell division; delay leaf senescence; work with auxins in organ formation. |
| Ethylene (ethene) | Ripening fruits, senescing leaves | Promotes fruit ripening; stimulates leaf and fruit abscission; involved in some stress responses. |
| Abscisic acid (ABA) | Leaves, roots, seeds | Closes stomata during water stress; maintains seed and bud dormancy; promotes abscission in some cases. |
Auxins and growth
Auxins loosen cell walls so that cells can elongate under turgor pressure. Concentration matters:
- In shoots, moderate to higher auxin levels promote elongation.
- In roots, the same higher levels usually inhibit elongation; roots respond best to very low concentrations.
This difference explains why the same hormone produces opposite curvature in shoots and roots under gravity or unilateral light.
TEST YOUR UNDERSTANDING
Q3. Name five main plant hormones and state one function of each.
Q4. What is apical dominance and how is it used in horticulture?
3. Tropic Movements (Tropisms)
Tropisms are directional growth movements of a plant organ towards (+) or away from (−) a unilateral stimulus. They are slow, irreversible and controlled largely by unequal distribution of auxins.
| Tropism | Stimulus | Typical response | Importance |
|---|---|---|---|
| Phototropism | Light | Shoots positive (grow towards light); many roots negative or insensitive | Maximises light capture for photosynthesis |
| Geotropism (gravitropism) | Gravity | Roots positive (grow downwards); shoots negative (grow upwards) | Anchorage and access to water/minerals; shoots reach light |
| Hydrotropism | Water / moisture | Roots positive | Enables roots to find water |
| Thigmotropism (haptotropism) | Touch / contact | Tendrils and climbing stems coil around supports | Support for weak-stemmed plants |
| Chemotropism | Chemicals | Pollen tubes grow towards ovules (positive) | Enables fertilisation |
How auxin causes phototropism
- Light from one side causes auxin to accumulate on the shaded side of the shoot tip.
- Higher auxin on the shaded side makes those cells elongate faster.
- The shoot bends towards the light (positive phototropism).
How auxin causes geotropism
- When a seedling is placed horizontally, gravity causes more auxin to settle on the lower side of both root and shoot.
- In the shoot, higher auxin promotes growth on the lower side → shoot curves upwards (negative geotropism).
- In the root, higher auxin inhibits growth on the lower side → upper side grows faster → root curves downwards (positive geotropism).
TEST YOUR UNDERSTANDING
Q5. Differentiate between positive and negative tropism. Give one example of each.
Q6. Explain why a horizontally placed root grows downwards while the shoot grows upwards.
Q7. Name the tropism shown by: (a) a tendril coiling round a stick, (b) a root growing towards moist soil, (c) a pollen tube growing towards an ovule.
4. Nastic Movements
Nastic movements are non-directional responses. The direction of the movement does not depend on the direction of the stimulus. They are often rapid and caused by changes in turgor pressure in specialised cells rather than by permanent growth.
| Type | Stimulus | Example |
|---|---|---|
| Seismonasty (thigmonasty) | Touch, shock or vibration | Leaves of Mimosa pudica (sensitive plant / touch-me-not) fold quickly when touched |
| Photonasty | Change in light intensity | Flowers of some plants open in the morning and close at night |
| Nyctinasty | Day–night cycle (light and temperature) | “Sleep movements” of leaves or flowers that fold at night |
| Thermonasty | Temperature change | Opening or closing of some flowers with temperature |
Tropism versus nastic movement
| Feature | Tropism | Nastic movement |
|---|---|---|
| Direction | Related to the direction of the stimulus | Independent of the direction of the stimulus |
| Speed | Usually slow | Often rapid |
| Mechanism | Unequal growth (mainly auxin) | Often change in turgor pressure |
| Reversibility | Generally irreversible | Often reversible |
| Example | Shoot bending towards light | Mimosa leaves folding on touch |
TEST YOUR UNDERSTANDING
Q8. What is the main difference between a tropic movement and a nastic movement?
Q9. Give one example of a nastic movement and name the type of stimulus involved.
5. Importance and Applications of Coordination in Plants
| Area | How knowledge of plant coordination helps |
|---|---|
| Survival of the plant | Roots find water and provide anchorage; shoots capture light; climbing plants gain support; flowers open at the right time for pollinators. |
| Agriculture & horticulture | Pruning to overcome apical dominance and produce bushier crops; use of rooting powders (synthetic auxins) on cuttings; control of fruit ripening with ethylene; induction of seedless fruits (parthenocarpy); selective herbicides based on auxins. |
| Crop management | Understanding photoperiodism and dormancy helps farmers time planting, storage of tubers and use of growth regulators. |
| Research & biotechnology | Tissue culture and micropropagation rely on the balance of auxins and cytokinins to form roots or shoots. |
TEST YOUR UNDERSTANDING
Q10. State three practical applications of knowledge of plant hormones.
Chapter Summary
- Coordination in plants is the orderly working of plant parts so that the plant detects and responds to stimuli. It is chemical, not nervous.
- Plant hormones (phytohormones) — auxins, gibberellins, cytokinins, ethylene and abscisic acid — regulate growth, development and responses.
- Tropisms are directional growth movements (phototropism, geotropism, hydrotropism, thigmotropism, chemotropism) controlled mainly by unequal distribution of auxins.
- Nastic movements are non-directional and often rapid (e.g. seismonasty in Mimosa), usually caused by turgor changes.
- Knowledge of plant coordination is applied in pruning, rooting cuttings, fruit ripening, weed control and tissue culture.
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Continue in the appEnd-of-Topic Questions
Attempt all questions in your exercise book before checking the suggested answers. (New Syllabus 2027)
- Section A: Multiple choice. Choose the correct answer. (i) The chemical substances that coordinate growth and responses in plants are called: A. Enzymes B. Phytohormones C. Neurotransmitters D. Antibodies. (ii) Growth of a shoot towards light is an example of: A. Negative phototropism B. Positive phototropism C. Nastic movement D. Geotropism. (iii) Which hormone is mainly responsible for apical dominance? A. Ethylene B. Abscisic acid C. Auxin D. Cytokinin. (iv) The folding of Mimosa leaves when touched is a: A. Tropism B. Nastic movement C. Tactic movement D. Tactic response. (v) Roots growing downwards in response to gravity show: A. Positive geotropism B. Negative geotropism C. Positive phototropism D. Hydrotropism. (5 marks)
- Match the items in Column A with those in Column B: (i) Phototropism (ii) Geotropism (iii) Hydrotropism (iv) Thigmotropism (v) Chemotropism. Column B: A. Response to gravity B. Response to water C. Response to light D. Response to chemicals E. Response to touch. (5 marks)
- (a) Define coordination in plants. (b) List the five main groups of plant hormones. (7 marks)
- Distinguish between tropic and nastic movements. Give one example of each. (6 marks)
- Explain how auxin causes a shoot to bend towards unilateral light. (6 marks)
- State the meaning of the following terms: (a) stimulus, (b) apical dominance, (c) phytohormone, (d) positive tropism. (4 marks)
- Briefly explain five ways in which knowledge of plant coordination is useful to farmers or gardeners. (10 marks)
- A learner places a potted bean seedling on its side in a dark room. After three days the shoot has curved upwards and the root downwards. Explain these observations. (8 marks)
Suggested Answers
- (i) B. Phytohormones. (ii) B. Positive phototropism. (iii) C. Auxin. (iv) B. Nastic movement. (v) A. Positive geotropism.
- (i) C. (ii) A. (iii) B. (iv) E. (v) D.
- (a) The working together of different plant parts so that the plant detects and responds appropriately to stimuli. (b) Auxins, gibberellins, cytokinins, ethylene, abscisic acid.
- Tropisms are directional growth movements related to the direction of the stimulus (e.g. shoot towards light); nastic movements are non-directional and often due to turgor changes (e.g. Mimosa leaf folding).
- Light from one side causes auxin to accumulate on the shaded side; cells on the shaded side elongate faster than those on the lit side, so the shoot bends towards the light.
- (a) A change in the environment that can be detected. (b) Inhibition of lateral buds by the terminal bud. (c) A chemical produced in plants that regulates growth and responses. (d) Growth towards the stimulus.
- Pruning to overcome apical dominance; using rooting hormones on cuttings; controlling fruit ripening; selective weed killers; inducing seedless fruits; managing dormancy of seeds/tubers; tissue culture (any five, each explained).
- Gravity causes more auxin to collect on the lower side. In the shoot the higher concentration promotes growth so the shoot curves upwards (negative geotropism). In the root the higher concentration inhibits growth so the upper side grows faster and the root curves downwards (positive geotropism).
These are suggested answers. Credit is also given to correct points expressed in your own words.
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