Plant Responses - Tropisms and IAA — AQA A-level Biology

SPEC 3.6.1.1 - RESPONDING TO THE ENVIRONMENT

Plant Responses & Simple Reflex Arc

IAA, phototropism, gravitropism, taxes, kineses, and the three-neurone reflex

Survival Through Response

Organisms increase their chance of survival by responding to changes in their environment. Without the ability to detect and respond to stimuli, organisms cannot find food, avoid predators, or maintain suitable conditions for metabolism.

Stimulus

A detectable change in the internal or external environment that produces a response in an organism.

Response

A change in behaviour or physiology of an organism as a result of detecting a stimulus.

Spec Statement

The AQA spec states: "Organisms increase their chance of survival by responding to changes in their environment." This underpins all responses in this topic — tropisms, taxes, kineses, and the reflex arc.

IAA — The Plant Growth Substance

IAA (Indoleacetic Acid)

The main auxin in plants; a growth substance produced in meristematic tissue at shoot and root tips that promotes cell elongation. Roots are far more sensitive to IAA than shoots — the same IAA concentration that maximally promotes shoot elongation inhibits root elongation.

IAA is produced at the tip of shoots and roots. It moves to the zone of elongation where it controls cell elongation — cells get longer, not more numerous.

Differential Sensitivity

Roots are far more sensitive to IAA than shoots. This single principle drives both phototropism and gravitropism.

IAA ConcentrationEffect on ShootEffect on Root

Very lowLittle effectPromotes elongation

Moderate (optimal shoot)Maximum promotionInhibits elongation

HighInhibits elongationStrongly inhibits

Phototropism

The growth response of a plant to a directional light stimulus; shoots are positively phototropic (grow toward light).

The mechanism involves differential IAA distribution driven by unilateral (one-sided) light:

1

Light falls on one side of the shoot tip — unilateral illumination.

2

IAA produced at the tip moves laterally, accumulating on the shaded side.

3

Shaded side: higher [IAA] → greater cell elongation in the zone of elongation.

4

Illuminated side: lower [IAA] → less cell elongation.

5

Unequal elongation causes the shoot to bend toward the light — positive phototropism.

Examiner Alert

IAA does NOT cause cells to divide — it causes cell elongation. The shoot bends because the shaded side elongates more, not because the illuminated side is directly stimulated.

Phototropism — Diagram

Gravitropism

The growth response of a plant to gravity; roots are positively gravitropic (grow toward gravity); shoots are negatively gravitropic (grow away from gravity).

Consider a seed germinating with the root tip placed horizontally:

1

IAA accumulates on the lower side of the root tip — gravity causes lateral redistribution downward.

2

Lower side: high [IAA] — roots are highly sensitive → elongation is inhibited on the lower side.

3

Upper side: low [IAA] → elongation is promoted on the upper side.

4

Upper side grows faster → root curves downward — positive gravitropism.

Shoot vs Root

In the shoot, the same high [IAA] on the lower side would promote elongation there — making the shoot curve upward (negative gravitropism). Same redistribution, opposite outcome — explained entirely by differential IAA sensitivity.

Gravitropism — Root Diagram

Taxes — Directional Responses

Taxis (pl. Taxes)

A directional movement of a whole organism toward (positive taxis) or away from (negative taxis) a stimulus. The direction of movement is determined by the direction of the stimulus source.

Positive Taxis

Positive phototaxis in Euglena : moves toward light to maximise photosynthesis.

Positive chemotaxis in bacteria: move toward nutrient gradients.

Negative Taxis

Negative phototaxis in Euglena : moves away from very bright light (which could damage photosynthetic pigments).

Negative chemotaxis: organisms move away from harmful chemicals.

Note: Woodlice in choice chambers demonstrate kinesis, not taxis.

Naming Taxes

Name format: stimulus type + taxis. Light → phototaxis. Chemicals → chemotaxis. Always state positive or negative.

Kineses — Non-Directional Responses

Kinesis (pl. Kineses)

A non-directional change in the rate of movement or frequency of turning of a whole organism in response to stimulus intensity. The organism does not move toward or away from the stimulus.

The organism does not know where the stimulus is — it only responds to how intense it is. By moving faster or turning more in unfavourable conditions, organisms accumulate by chance in favourable areas.

Classic Example — Woodlice in Humidity

In dry conditions: woodlice move faster and turn more frequently.

In humid conditions: woodlice slow down and turn less frequently.

Result: woodlice spend more time in humid areas — not by navigating there, but because their random movement slows when they happen to be there.

FeatureTaxisKinesis

Directional?YES — toward/away from stimulusNO — random but rate changes

Stimulus property usedDirection of sourceIntensity of stimulus

OutcomeDirect navigation to/from sourceAccumulation in favourable areas by chance

The Simple Reflex Arc

Reflex

A rapid, automatic, involuntary response to a stimulus that does not involve conscious thought. Protective reflexes allow organisms to respond before pain is consciously registered.

The three-neurone reflex arc is the basic circuit. AQA does not require details of spinal cord anatomy (dorsal/ventral roots).

1

Receptor — detects the stimulus (e.g. heat receptor in skin).

2

Sensory neurone — carries impulse from receptor to spinal cord.

3

Relay neurone — in the spinal cord; connects sensory to motor neurone without going to the brain first.

4

Motor neurone — carries impulse from spinal cord to effector.

5

Effector — muscle contracts or gland secretes; the response occurs.

Why Reflexes Are Fast

The pathway bypasses the brain — the relay neurone connects directly to the motor neurone in the spinal cord. The brain receives a signal in parallel, after the reflex has already occurred.

Reflex Arc — Diagram

Required Practical 10

Investigation into the effect of an environmental variable on the movement of an animal using either a choice chamber or a maze.

VariableDetail

Independent variableHumidity (dry vs humid) or light level

Dependent variableNumber of animals in each section after set time (e.g. 5 minutes)

Control variable 1Temperature — kept constant; temperature independently affects movement rate

Control variable 2Number of animals — same each repeat; ensures fair comparison

Control variable 3Starting position — all animals placed in centre; prevents positional bias

Expected resultMore animals found in the humid section after the set time — consistent with kinesis (woodlice slow down and turn less frequently in humid conditions, spending more time there by chance)

Interpreting Results

Animals accumulate in humid chamber = woodlice show kinesis. This is not taxis — there is no directed movement toward humidity.

Common Exam Errors

Examiner Alert 1 — Taxis vs Kinesis

Students describe woodlouse accumulation as positive taxis. It is kinesis — the movement is non-directional; the organism does not navigate toward humidity, it simply slows down when it happens to be there.

Examiner Alert 2 — IAA causes elongation, NOT division

IAA promotes cell elongation of existing cells in the zone of elongation. Never say IAA causes cells to multiply.

Examiner Alert 3 — Root vs Shoot IAA sensitivity

Roots are far more sensitive to IAA than shoots. A concentration that maximally promotes shoot growth inhibits root growth. Reversing this is the single most common error on tropism questions.

Examiner Alert 4 — Reflex pathway

The reflex arc goes through the spinal cord via the relay neurone. The brain becomes aware of the stimulus after the reflex has already occurred.

DEEP Framework — Extended Answer

Model answer for: "Explain how unilateral light causes a shoot to bend toward the light source." [6 marks]

DEEP Chain

D

Define: IAA (indoleacetic acid) is an auxin — a plant growth substance that promotes cell elongation in shoots. Phototropism is the growth response of a shoot to a directional light stimulus.

E

Explain mechanism: Unilateral light causes IAA to move laterally from the illuminated side to the shaded side of the shoot tip. This creates a higher concentration of IAA on the shaded side than the illuminated side.

E

Evidence/Example: In the zone of elongation below the tip, cells on the shaded side are exposed to higher [IAA] and elongate more than cells on the illuminated side. Went's agar block experiments demonstrated that the tip must be present for bending to occur, confirming IAA is produced at the tip.

P

Practical/Quantitative: The differential in cell length between the two sides results in curvature toward the light. This is adaptive — it maximises light absorption for photosynthesis.

Level 3 Tip

Every step must name the molecule (IAA) and explain what it does (promotes cell elongation). The examiner wants: molecule → mechanism → consequence.

Practice Question

A student placed a seedling horizontally on a clinostat (a device that rotates slowly, negating the effect of gravity). She observed that the root did not curve downward as it would normally.

(a) Explain, in terms of IAA, why a root placed horizontally in normal conditions grows downward. [4 marks]

(b) Predict what would happen to the shoot of the same horizontally-placed seedling if unilateral light were applied from above. [3 marks]

D:

IAA accumulates on the lower side of the root tip due to gravity causing lateral redistribution of IAA downward.

E:

Roots are very sensitive to IAA — the high concentration of IAA on the lower side inhibits cell elongation on the lower side. The upper side has a lower IAA concentration, which promotes elongation on the upper side.

E:

The upper side elongates more than the lower side, causing the root tip to curve downward — positive gravitropism. The clinostat prevents this by continuously rotating the seedling, so IAA cannot accumulate on one side.

P:

Part (b): When the shoot is horizontal, IAA accumulates on the lower side due to gravity. In the shoot, higher [IAA] promotes elongation, so the lower side elongates more than the upper side, causing the shoot to curve upward (negative gravitropism). Light from above would also cause IAA to redistribute away from the illuminated upper surface, further increasing [IAA] on the lower side — both stimuli reinforce upward shoot curvature.

Synoptic Links

Synoptic Link — Paper 3

IAA and cell elongation → Spec 3.2 (Cells): Cell elongation requires uptake of water by osmosis down a water potential gradient, vacuole enlargement, and expansion of the cell wall. A synoptic question could ask you to link the IAA mechanism to osmosis or water potential.

Synoptic Link — Paper 3

Reflex arc → Spec 3.6.2/3.6.3 (Neurones, Synapses): The reflex arc uses neurones connected by synapses. The next lessons cover action potentials and synaptic transmission. A synoptic question could ask you to trace a signal through a modified reflex arc incorporating a novel scenario.

Synoptic Link — Essay

AQA 2023 Examiner Report: Tropisms, taxes, and kineses all fall under "survival and response" (3.6.1.1). An essay on "the importance of interactions between organisms and their environment" could include all three. They are not separate topics — they are all mechanisms of survival response.

Summary

Key Concepts — Lesson 6.01

ConceptKey Point

IAAPlant growth substance; promotes elongation in shoots; same concentration inhibits root elongation (roots more sensitive)

PhototropismIAA moves to shaded side → more elongation there → shoot bends toward light

GravitropismIAA to lower side; inhibits root elongation there → upper grows more → root curves down

TaxisDirectional movement toward/away from stimulus source; e.g. positive phototaxis in Euglena

KinesisNon-directional; rate of movement/turning changes with stimulus intensity; e.g. woodlice in humidity

Simple reflex arcReceptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector; rapid, automatic, involuntary

Top Exam Tip

In any tropism question: state (1) where IAA accumulates, (2) the effect on cell elongation (promotion or inhibition), (3) the consequence for growth direction. Name IAA explicitly. For taxis vs kinesis: the single deciding factor is whether the movement is directional toward/away from the source.