Survival and Plant Growth Responses Revision Notes | A Level AQA Biology

1. Why Responding to the Environment Matters

A seedling pushing through soil towards light, or a root curving downward into the earth — these are not accidents. They are precisely coordinated survival strategies. Plants that cannot detect and respond to directional stimuli are less likely to photosynthesise efficiently, anchor themselves, or absorb water. Response to the environment is, at its core, a matter of survival.

The AQA specification makes this explicit: organisms increase their chance of survival by responding to changes in their environment. For plants, this means detecting stimuli such as light direction and gravity, then adjusting growth accordingly. The mechanism that drives this is elegant — a single class of chemical signal called a growth factor.

2. Growth Factors and How They Move

In flowering plants, growth is not uniform — it is directed. Specific growth factors are produced in actively dividing and elongating regions, particularly the tips of shoots and roots. From these growing regions, the growth factors move to other tissues, where they regulate how much cells elongate.

Picture the shoot tip as a factory producing IAA. Under uniform conditions, IAA spreads evenly downward and cells on all sides elongate at the same rate — the shoot grows straight. The interesting biology begins when a directional stimulus, such as unilateral light or gravity, causes IAA to redistribute unevenly across the tissue.

This redistribution is the key to understanding both phototropism and gravitropism. The side of the shoot or root that receives more IAA behaves differently from the side that receives less — and because roots and shoots respond to the same concentration of IAA in opposite ways, the outcomes are strikingly different in each organ.

3. IAA and Cell Elongation: The Concentration Effect

Here is where the biology becomes genuinely fascinating — and where many students lose marks by oversimplifying. IAA does not have a single universal effect. Its effect depends on both the concentration present and the tissue type being considered.

IAA promotes cell elongation by binding to receptors in the cell membrane, triggering the secretion of hydrogen ions into the cell wall. This acidifies the wall, activating enzymes that break bonds between cellulose microfibrils. The loosened wall allows the cell to take up water by osmosis and expand. Higher IAA concentrations drive more of this wall-loosening activity — up to a point.

The critical point is that roots are far more sensitive to IAA than shoots. A concentration of IAA that is optimal for shoot elongation is actually inhibitory for root elongation. This differential sensitivity is the entire basis for how gravitropism produces opposite curvature in roots and shoots when IAA accumulates on one side.

4. Phototropism: Growing Towards Light

Phototropism is the directional growth response of a plant to a light stimulus. Shoots show positive phototropism — they grow towards the light source. This is directly explained by the unequal distribution of IAA caused by unilateral (one-sided) illumination.

Worked example: Explaining Shoot Phototropism Using IAA

Explain, using IAA distribution, why a shoot bends towards a unilateral light source.

1. Unilateral light strikes the shoot tip from one side. IAA is produced at the shoot tip (apical meristem). — The tip is the site of IAA production — this is where redistribution begins.
2. IAA moves laterally away from the illuminated side towards the shaded side of the shoot tip. — Light causes IAA to migrate to the shaded side, creating an unequal distribution across the shoot.
3. The shaded side now has a higher concentration of IAA than the illuminated side. — This asymmetry in IAA concentration is the direct cause of differential growth.
4. Higher IAA on the shaded side promotes greater cell elongation on that side. Lower IAA on the illuminated side means less elongation. — In shoots, higher IAA concentration promotes elongation — the shaded side grows faster.
5. Cells on the shaded side elongate more than cells on the illuminated side. The shoot bends towards the light source. — Differential elongation on opposite sides of the shoot produces the observed curvature — positive phototropism.

Answer: IAA redistributes to the shaded side of the shoot tip. Higher IAA concentration on the shaded side promotes greater cell elongation there. The illuminated side elongates less. The shoot therefore bends towards the light source — positive phototropism.

Notice the logic chain: stimulus detected at tip → IAA redistributes laterally → concentration gradient established → differential elongation → curvature. Every step must be present in an exam answer for full marks. Missing the word 'differential' or failing to link IAA concentration to elongation rate are the two most common reasons students drop marks here.

5. Gravitropism: Responding to Gravity

Gravitropism follows the same underlying mechanism as phototropism — IAA redistribution causing differential elongation — but the stimulus is gravity rather than light, and the responses in roots and shoots are opposite to each other. This is where the differential sensitivity of roots and shoots to IAA becomes essential to your explanation.

When a plant is placed horizontally, IAA accumulates on the lower side of both the shoot and the root due to gravity. In the shoot, the lower side has more IAA, which promotes more elongation there — the shoot curves upward (negative gravitropism). In the root, the same higher IAA concentration on the lower side actually inhibits elongation there, because roots are far more sensitive to IAA. The upper side of the root, with less IAA, elongates more — so the root curves downward (positive gravitropism).

Worked example: Explaining Root and Shoot Gravitropism Using IAA

A seedling is placed horizontally. Explain, using IAA, why the shoot grows upward and the root grows downward.

1. Gravity causes IAA to accumulate on the lower side of both the horizontal shoot and the horizontal root. — IAA moves to the lower side in both organs — the same redistribution occurs in both, but the outcomes differ.
2. Lower side of shoot: higher IAA concentration → promotes cell elongation. Upper side of shoot: lower IAA concentration → less elongation. — Shoots respond positively to higher IAA concentrations within the physiological range — more IAA means more elongation.
3. The lower side of the shoot elongates more than the upper side. The shoot curves upward — negative gravitropism. — The shoot grows away from gravity because the lower side grows faster.
4. Lower side of root: higher IAA concentration → inhibits cell elongation (supraoptimal concentration for roots). Upper side of root: lower IAA concentration → promotes elongation. — Roots are far more sensitive to IAA. The concentration that promotes shoot elongation is inhibitory for roots.
5. The upper side of the root elongates more than the lower side. The root curves downward — positive gravitropism. — The root grows towards gravity because the upper side (with less IAA) grows faster than the inhibited lower side.

Answer: IAA accumulates on the lower side of both organs. In the shoot, this promotes more elongation on the lower side, so the shoot curves upward (negative gravitropism). In the root, this concentration inhibits elongation on the lower side, so the upper side elongates more and the root curves downward (positive gravitropism).

The elegance here is that the same IAA redistribution produces opposite curvatures in roots and shoots — entirely because of the difference in sensitivity between the two tissue types. This is a classic example of how one signal can produce different responses depending on the target tissue.

6. Bringing It Together: Survival Value of Tropisms

Returning to the first specification requirement — organisms increase their chance of survival by responding to changes in their environment — tropisms are a direct illustration of this principle in plants. Positive phototropism ensures shoots are oriented towards the light source, maximising the surface area of leaves exposed to light for photosynthesis. Positive gravitropism in roots ensures they grow into the soil, improving anchorage and access to water and mineral ions.

These are not passive processes. They require the detection of a directional stimulus, the active redistribution of a chemical signal, and a precisely calibrated cellular response. The fact that roots and shoots have evolved different sensitivities to IAA means a single molecular signal can coordinate two complementary growth responses simultaneously — an efficient solution to the challenge of establishing a seedling in a competitive environment.

7. Exam Technique and Common Mistakes

Questions on IAA and tropisms appear regularly at A Level and tend to reward students who use precise language and complete logical chains. The most common reason for losing marks is writing a partial explanation — for example, stating that IAA accumulates on one side without then linking this to differential elongation and the resulting direction of curvature.

When a question asks you to 'explain' phototropism or gravitropism, the examiner expects a chain of reasoning: stimulus → IAA redistribution → concentration difference → differential elongation → direction of curvature. Each arrow in that chain represents a mark. Stopping at 'IAA moves to the shaded side' without explaining what that does to cell elongation will cost you marks every time.

For gravitropism questions, the most common error is applying the same logic to both roots and shoots without acknowledging the difference in sensitivity. Always state explicitly that roots are more sensitive to IAA than shoots, and that a concentration which promotes elongation in shoots inhibits elongation in roots. This distinction is what separates a mid-level answer from a top-level one.