Form 4 - Complete Study Guide
The reaction of an organism to a stimulus detected in its environment.
The ability to detect and react to changes. Plants have limited sensitivity primarily related to their nutritional needs.
Growth responses to the direction of a stimulus. Controlled by plant hormones called auxins.
Growth towards light
Growth in response to gravity
Growth towards water
Growth towards chemicals
Response to touch
Gradual turning towards light
Seedlings bend towards the light source due to phototropism
Seedlings grow straight up - no bending due to uniform light exposure
The clinostat ensures uniform light exposure, preventing directional growth and proving that phototropism is a response to directional light stimulus.
Non-directional movement responses not influenced by the direction of the stimulus.
Quick response to light intensity changes
Example: Morning glory flowers opening
Response to touch or vibration
Examples:
• Mimosa pudica leaves folding
• Venus flytrap trapping insects
Response to chemical/nutrient concentration
Example: Sundew tentacles
Response to water/humidity
Example: Flowers opening/closing
Insect touches trigger hairs
Trap snaps shut
Enzymes digest insect
Shedding of leaves, flowers, or fruits in response to adverse conditions
Mechanism: Cells near leaf base dry out, leading to leaf fall
Period of drastically reduced metabolic activity
Occurs in seeds and buds, halting growth completely
Locomotory response in unicellular organisms
Phototaxis: Movement toward/away from light
Chemotaxis: Movement toward/away from chemicals
Chloroplasts in palisade mesophyll cells move to upper leaf surface to capture more light, increasing photosynthesis rate
Motile gametes move toward specific chemicals during fertilization process
Produced at the tips of shoots or roots, controlling various plant responses and growth patterns.
Promotes rapid growth in region below shoot tip; decreases down the stem
Retains leaves on plant; inhibits early abscission
Stimulates root development at low concentrations; inhibits at high concentrations
Promotes fruit development
Promotes growth
Inhibits side branch growth
Stimulates adventitious and lateral roots
Facilitates elongation in growth zone
Synthetic auxins used as weed killers
Promote rapid root growth in stem cuttings
Help manage apical dominance
Prevent premature fruit drop
| Aspect | Tropisms | Nasties |
|---|---|---|
| Direction | Directional (toward/away from stimulus) | Non-directional |
| Type of Response | Growth response | Movement response |
| Speed | Gradual/slow | Quick/rapid |
| Control | Auxin hormones | Turgor pressure changes |
| Reversibility | Generally permanent | Often reversible |
Why is a clinostat used in phototropism experiments?
Answer: To ensure uniform light exposure from all directions, preventing directional growth and serving as a control to prove phototropism occurs due to directional light stimulus.
Is the Venus flytrap's response to insects a tropism or nastic movement? Explain.
Answer: Nastic movement (thigmonasty). It's a quick, non-directional response to touch that doesn't depend on the direction of the stimulus.
Explain how auxins control phototropism in plant stems.
Answer: Auxins accumulate on the shaded side of the stem, promoting faster cell growth on that side. This unequal growth causes the stem to bend toward the light source.