✅ Thermoregulation in Plants – Notes
Thermoregulation is a type of homeostasis in which plants maintain their internal temperature and protect their cellular functions from damage due to high or low environmental temperature.
Unlike animals, plants cannot move to escape temperature extremes. Instead, they use physiological and biochemical adaptations to tolerate heat and cold stress.
🌡️ A. High Temperature Stress in Plants
Effects of High Temperature
- Causes enzyme denaturation.
- Disturbs metabolism and photosynthesis.
- Increases transpiration and water loss.
- When water is limited, stomata close, reducing cooling and causing heat stress.
Adaptations to High Temperature
Examples of HSPs:
- HSP70
- HSP90
- Small HSPs (sHSPs)
Functions of Heat Shock Proteins
- Bind and stabilize proteins.
- Prevent denaturation.
- Help refold damaged proteins under heat stress.
❄️ B. Low Temperature Stress in Plants
Effects of Low Temperature
- Reduces membrane fluidity.
- Lipids of the cell membrane become solidified (crystalline).
- Affects transport of solutes across membranes.
- Membrane proteins lose function.
- Freezing temperature causes ice crystal formation, damaging cells.
✅ Substances That Increase Membrane Fluidity During Cold Stress
Plants maintain membrane fluidity in low temperature by changing the composition of their cell membrane. The following substances help prevent the membrane from becoming rigid:
🔹 1. Unsaturated Fatty Acids
- Examples: Oleic acid, Linoleic acid, Linolenic acid
- Role: These fatty acids have double bonds that prevent tight packing of lipid molecules.
- Function: They stop crystallization of membrane lipids and keep the membrane flexible in cold conditions.
🔹 2. Membrane Phospholipids
- Examples: Phosphatidylcholine and Phosphatidylethanolamine
- Function: These phospholipids help in maintaining flexibility and proper structure of the phospholipid bilayer under cold stress.
🔹 3. Plant Sterols
- Examples: Sitosterol and Stigmasterol
- Function: These sterols act like cholesterol in animal cells. They stabilize the cell membrane and prevent it from becoming too rigid in low temperature.
🔹 4. Fatty Acid Desaturase Enzymes
- Enzyme group: FAD enzymes (Fatty acid desaturases)
- Function: These enzymes introduce double bonds into fatty acid chains, increasing unsaturation, which helps in maintaining membrane fluidity during cold stress.
✅ Supercooling Compounds (Cold Tolerance) – Paragraph Form
To protect themselves from freezing damage, plants accumulate special cryoprotectant compounds inside their cells. These substances prevent ice from forming within the cytoplasm and allow the cell contents to supercool below 0°C without freezing. Among these compounds, sugars such as sucrose, raffinose, and trehalose play an important role by lowering the freezing point of the cell sap. In addition, sugar alcohols like mannitol, sorbitol, and glycerol prevent the growth of ice crystals and help maintain cell structure.
Plants also accumulate the amino acid proline, which protects proteins and maintains osmotic balance during low temperature stress. Another important adaptation involves antifreeze proteins (AFPs) that bind to small ice crystals and inhibit their growth inside cells, thus preventing serious cellular damage. Furthermore, organic acids such as malate and citrate contribute to osmotic adjustment and help stabilize cellular functions during cold stress.
These cryoprotective substances are especially important because ice formation outside the cell wall (extracellular freezing) is usually not harmful and plants can survive it. However, ice formation inside the cytoplasm (intracellular freezing) is deadly because it ruptures membranes and organelles, causing cell death. Therefore, supercooling is a vital survival strategy for plants exposed to freezing temperatures.
✅ Examples of Cold-Resistant Plants
- Maple
- Oak
- Pine
- Roses
- Winter wheat
- Spruce
🌿 Types of Movements in Plants
1. Tropic Movements (Tropisms)
- Definition: These are directional growth movements in plants in response to an external stimulus.
- Nature: The plant part moves towards (positive tropism) or away from (negative tropism) the stimulus.
- Mechanism: Caused by differential growth (unequal growth on opposite sides of an organ such as stem or root).
- Examples of external stimuli: Light, gravity, touch, chemicals, and water.
✅ Major Types of Tropic Movements in Plants
🔹 1. Phototropism (Response to Light)
- Stimulus: Light
- Explanation: Growth movement of plant parts in response to light.
- Example: Shoots show positive phototropism as they bend towards light.
- Roots show negative phototropism as they grow away from light.
🔹 2. Geotropism / Gravitropism (Response to Gravity)
- Stimulus: Gravity
- Explanation: Growth movement due to the pull of gravity.
- Example: Roots show positive geotropism (grow downward).
- Shoots show negative geotropism (grow upward against gravity).
🔹 3. Thigmotropism (Response to Touch)
- Stimulus: Touch or contact
- Explanation: Growth movement in response to contact with a solid object.
- Example: Tendrils of climbing plants (peas, grapevine) coil around a support.
🔹 4. Chemotropism (Response to Chemicals)
- Stimulus: Chemicals
- Explanation: Growth directed by a chemical substance.
- Example: Growth of pollen tube toward ovule during fertilization.
🔹 5. Hydrotropism (Response to Water)
- Stimulus: Water
- Explanation: Growth movement toward moisture.
- Example: Roots show positive hydrotropism as they grow toward water.
🔬 Short Descriptions
1. Phototropism
- Growth response to light.
- Auxin (plant growth hormone) plays a major role.
- Example: Shoot tips bend towards light (positive phototropism).
2. Geotropism
- Growth response to gravity.
- Roots show positive geotropism (grow downward).
- Shoots show negative geotropism (grow upward).
3. Thigmotropism
- Response to touch or contact.
- Seen in climbing plants like peas and grapevines.
- Tendrils coil around a support after touching it.
4. Chemotropism
- Response to chemicals.
- Example: Pollen tube grows toward chemicals released by ovule during fertilization.
✅ Short Questions and Answers (20)
1. What is thermoregulation in plants?
Answer: Thermoregulation is the ability of plants to maintain normal internal temperature under high or low environmental temperatures.
2. Why is high temperature harmful for plants?
Answer: High temperature denatures enzymes and disturbs metabolism in plants.
3. How do plants cool themselves during high temperature?
Answer: Plants use evaporative cooling through transpiration to reduce temperature.
4. What are heat shock proteins?
Answer: Heat Shock Proteins (HSPs) are special protective proteins produced during heat stress that prevent enzyme denaturation.
5. Give two examples of heat shock proteins.
Answer: HSP70 and HSP90.
6. Why is low temperature harmful for plant membranes?
Answer: Low temperature reduces membrane fluidity by converting membrane lipids into crystalline (solid) form.
7. Name one substance that increases membrane fluidity during cold stress.
Answer: Linolenic acid (an unsaturated fatty acid).
8. What enzyme increases unsaturated fatty acids in cold conditions?
Answer: Fatty Acid Desaturase (FAD) enzyme.
9. What is supercooling in plants?
Answer: Supercooling is the ability of plants to prevent freezing inside cells by lowering the freezing point of cell sap.
10. What are cryoprotectants?
Answer: Cryoprotectants are substances like sugars and proteins that prevent ice crystal formation in plant cells.
11. Give two examples of cryoprotectants.
Answer: Sucrose and Proline.
12. Why is extracellular ice formation less harmful than intracellular freezing?
Answer: Extracellular ice forms outside the cell wall and does not damage organelles, while intracellular ice ruptures membranes and kills the cell.
🌿 Movements in Plants
13. What are plant movements?
Answer: Plant movements are responses of plants to external or internal stimuli, usually by changing growth patterns.
14. What are tropic movements?
Answer: Tropic movements are directional growth movements in plants in response to external stimuli.
15. What is phototropism?
Answer: Phototropism is the growth movement in response to light, such as shoots bending toward light.
16. What is geotropism?
Answer: Geotropism is the growth movement in response to gravity. Roots show positive geotropism.
17. What is thigmotropism?
Answer: Thigmotropism is movement in response to touch, seen in tendrils of climbing plants.
18. What is chemotropism?
Answer: Chemotropism is growth in response to chemicals, like the pollen tube growing toward the ovule.
19. Give one example of hydrotropism.
Answer: Roots growing toward moisture in the soil show hydrotropism.
20. Why do shoots show positive phototropism?
Answer: Shoots show positive phototropism to receive more light for photosynthesis.
✅ MCQs on Tropic Movements (With Answers & Explanations)
1. Tropic movements in plants are always
A. Temporary
B. Directional
C. Non-directional
D. Random
✅ Answer: B – Tropic movements occur in response to a directional stimulus.
2. Bending of shoots toward light is called
A. Geotropism
B. Hydrotropism
C. Phototropism
D. Thigmotropism
✅ Answer: C – Phototropism is growth towards light.
3. Positive geotropism is shown by
A. Shoot
B. Leaf
C. Fruit
D. Root
✅ Answer: D – Roots grow toward gravity.
4. Which hormone is mainly involved in phototropism?
A. Cytokinin
B. Auxin
C. ABA
D. Ethylene
✅ Answer: B – Auxins control curvature in phototropism.
5. The coiling of climber tendrils around a support is
A. Hydrotropism
B. Thigmotropism
C. Chemotropism
D. Phototropism
✅ Answer: B – Thigmotropism is response to touch.
6. Growth of pollen tube toward ovule is
A. Geotropism
B. Chemotropism
C. Hydrotropism
D. Phototropism
✅ Answer: B – Guided by chemicals from ovule.
7. Positive hydrotropism occurs in
A. Stem
B. Leaves
C. Roots
D. Flowers
✅ Answer: C – Roots grow toward water.
8. Tropism shown by climbing plants is
A. Phototropism
B. Hydrotropism
C. Thigmotropism
D. Geotropism
✅ Answer: C – Tendrils respond to touch.
9. Negative tropism means
A. Fast response
B. Movement toward stimulus
C. Movement away from stimulus
D. Random growth
✅ Answer: C – Away from stimulus (e.g., shoot in geotropism).
10. Which of the following is a growth movement?
A. Tropic movement
B. Nastic movement
C. Sleep movement
D. Turgor movement
✅ Answer: A – Tropism involves growth.
11. Roots show negative phototropism because
A. They dislike light
B. They grow toward water
C. They avoid light to absorb water and minerals
D. They lack chlorophyll
✅ Answer: C – Roots avoid light to stay underground.
12. The site of light perception in phototropism is
A. Roots
B. Flower
C. Shoot tip
D. Leaf
✅ Answer: C – Tip of shoot detects light.
13. Which hormone moves to the shaded side in phototropism?
A. Gibberellin
B. Auxin
C. Cytokinin
D. ABA
✅ Answer: B – Auxin migrates to shaded side, causing bending.
14. Hydrotropism in roots overrides
A. Phototropism
B. Geotropism
C. Thigmotropism
D. Thermotropism
✅ Answer: B – Roots may ignore gravity to grow toward water.
15. Tropic movements are mostly seen in
A. Animals
B. Lower bacteria
C. Plants
D. Fungi
✅ Answer: C – Plants show growth-based responses.
16. The growth response to chemical stimuli is
A. Hydrotropism
B. Chemotropism
C. Phototropism
D. Heliotropism
✅ Answer: B – Response to chemicals.
17. Growth of stem toward light is
A. Positive phototropism
B. Negative hydrotropism
C. Positive geotropism
D. Negative chemotropism
✅ Answer: A – Shoot moves toward light.
18. What type of movement helps root to find water?
A. Phototropism
B. Hydrotropism
C. Heliotropism
D. Thigmotropism
✅ Answer: B – Hydrotropism.
19. Opening of a flower bud toward light is
A. Tropic movement
B. Nastic movement
C. Taxic movement
D. Geotropism
✅ Answer: B – Not growth-based and non-directional stimulus.
20. Auxin distribution is unequal in
A. Geotropism only
B. Hydrotropism
C. Phototropism and geotropism
D. Thigmotropism
✅ Answer: C – Both phototropism and geotropism involve auxin redistribution.
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Interactive MCQs — Tropic Movements (20)
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