Response to Stimuli (AQA A-Level Biology): Revision Notes
Plant Growth Factors
Plants lack a nervous system yet successfully respond to environmental changes to survive. This response system relies on chemical messengers called plant growth factors that coordinate growth and development throughout the plant.
How plants respond to environmental stimuli
Plants demonstrate remarkable sensitivity to three key environmental factors:
Light - Shoots exhibit positive phototropism by growing towards light sources. This response ensures optimal positioning for photosynthesis, as light energy is essential for this vital process.
Gravity - Plant roots show positive gravitropism by growing downwards in the direction of gravitational pull. This anchoring response helps roots access water and nutrients from the soil.
Water - Most plant roots display positive hydrotropism by growing towards water sources. This behaviour supports both photosynthesis and other metabolic processes, while providing structural support.
These three tropistic responses work together to ensure plants can efficiently access the resources they need for survival: light for energy, water for metabolic processes, and stable positioning through gravity orientation.
Understanding plant growth factors
Plant growth factors are chemical messengers that influence plant development. Unlike animal hormones, these substances have distinctive characteristics:
Plant growth factors differ significantly from animal hormones in three key ways:
- They affect growth by influencing the tissues that produce them, rather than acting on distant target organs
- They are manufactured by cells distributed throughout the plant, not concentrated in specific organs
- They are produced in relatively small quantities but have significant effects
The most important plant growth factor is indoleacetic acid (IAA), which belongs to a group of compounds called auxins. IAA plays a central role in controlling plant cell elongation and directional growth responses.
Control of tropisms by IAA
A tropism describes the directional growth response of a plant to a directional environmental stimulus. IAA orchestrates these responses through its uneven distribution within plant tissues.
Phototropism in flowering plants
When light strikes a plant shoot from one direction, a complex sequence of cellular events occurs:
Worked Example: Phototropic Response Mechanism
Step 1: IAA Production Cells at the shoot tip produce IAA, which begins transport down the stem
Step 2: Initial Distribution IAA initially spreads evenly throughout all regions as it moves downward
Step 3: Light-Induced Migration Light exposure causes IAA to migrate from the illuminated side to the shaded side
Step 4: Concentration Gradient Higher IAA concentrations build up on the shaded side compared to the lit side
Step 5: Differential Growth The increased IAA concentration stimulates greater cell elongation on the shaded side
Step 6: Bending Response This differential growth causes the shoot tip to bend towards the light source
The result is positive phototropism - growth towards light that maximises the plant's photosynthetic potential.
Gravitropism in flowering plants
Gravity influences plant growth through a similar IAA-mediated mechanism:
Worked Example: Gravitropic Response in Roots
Step 1: IAA Production Root tip cells produce IAA, which travels along the root
Step 2: Initial Distribution IAA initially distributes evenly to all sides of the root
Step 3: Gravitational Influence Gravitational force influences IAA movement towards the lower side of the root
Step 4: Concentration Accumulation Greater IAA concentrations accumulate on the lower side than the upper side
Step 5: Growth Inhibition In roots, high IAA concentrations inhibit rather than stimulate cell elongation
Step 6: Directional Bending Reduced elongation on the lower side causes the root to bend downwards towards gravity
This creates positive gravitropism in roots. Interestingly, shoots display the opposite response - they grow away from gravity's pull, showing negative gravitropism.
Role of IAA in elongation growth
IAA transport occurs in one direction only - away from the tips of shoots and roots where it is produced. The hormone affects plant cells by increasing the flexibility of their cell walls, but this response only occurs in young, actively growing regions.
The unidirectional transport of IAA is crucial for maintaining proper plant architecture and ensuring that growth responses occur in the correct locations within the plant.
The mechanism behind IAA's action is explained by the acid growth hypothesis. This theory proposes that IAA stimulates the active transport of hydrogen ions from the cytoplasm into spaces within the cell wall. This acidification makes the cell wall more plastic and allows the cell to elongate through expansion.
As cells mature, they develop greater rigidity, which explains why older parts of shoots and roots cannot respond to IAA stimulation.
Scientific discoveries behind IAA understanding
The understanding of plant growth factors developed through a series of ingenious experiments conducted by pioneering scientists over several decades.
Darwin's groundbreaking experiments
Charles Darwin was among the first scientists to investigate plant responses to light. He observed that young grass shoots grew towards windows and proposed that the shoot tip detected the light stimulus.
Darwin's Three-Part Investigation
Experiment 1: Control Observation Unilateral light caused normal bending towards the light source
Experiment 2: Tip Removal Removing the shoot tip prevented any response to light
Experiment 3: Tip Coverage Covering the tip with an opaque cover also prevented the response
Conclusion: The tip must detect the stimulus and produce some factor that causes the response.
Boysen-Jensen's chemical messenger experiments
Building on Darwin's work, scientist Peter Boysen-Jensen designed experiments to prove the "messenger" was chemical rather than electrical. Using mica (which blocks chemicals but not electrical signals) and gelatin (which blocks electrical signals but allows chemicals to pass), he conducted crucial tests.
Boysen-Jensen's Material Barrier Tests
Test 1: Mica on the illuminated side allowed normal bending
Test 2: Mica on the shaded side prevented bending
Test 3: Gelatin blocks allowed normal bending
Conclusion: These results confirmed the messenger was chemical in nature.
Paal's displacement experiments
Arpad Paal investigated how the chemical messenger worked by removing shoot tips and placing them on one side of the cut surface.
Paal's Asymmetric Tip Placement
Method: Removed shoot tips and placed them on one side of the cut surface in complete darkness
Result: Shoots bent towards the side where no tip was present
Significance: Proved that the chemical messenger could work without light stimulus
Briggs's distribution experiments
Winslow Briggs and colleagues determined how light affected IAA distribution through carefully designed collection experiments.
Briggs's IAA Distribution Study
Condition 1 - Darkness: IAA collected equally from both sides of shoots
Condition 2 - Light with blocked lateral transfer: IAA still collected equally from both sides
Condition 3 - Light with possible lateral transfer: 70% of IAA collected on the shaded side and only 30% on the illuminated side
Key Finding: Light causes uneven IAA distribution, not uneven IAA production.
Key Points to Remember:
- Plant growth factors are chemical messengers that control plant responses to environmental stimuli without requiring a nervous system
- IAA (indoleacetic acid) is the primary auxin that controls cell elongation and tropistic responses
- Phototropism occurs because light causes IAA to accumulate on the shaded side, promoting greater cell elongation and bending towards light
- Gravitropism results from gravity-induced IAA accumulation on the lower side, with opposite effects in shoots (growth away from gravity) and roots (growth towards gravity)
- The acid growth hypothesis explains how IAA increases cell wall plasticity by promoting hydrogen ion transport into cell wall spaces