Control and coordination in plants

· Plants coordinate responses using chemical signals and, in some cases, rapid electrical changes.
· Key examples for CIE: Venus fly trap closure, auxin in elongation growth, and gibberellin in barley germination.
· Exam answers should link the stimulus → signal → response → advantage.
· Avoid vague phrases like “plants have nerves”; plants do not have a nervous system, but some responses involve electrical impulses/action potentials.

Venus fly trap: rapid response to stimulation

· The Venus fly trap has modified leaves with two lobes forming a trap.
· Each lobe has sensitive trigger hairs on its inner surface.
· Touching a trigger hair stimulates an electrical impulse/action potential across the leaf.
· Closure usually requires two stimulations close together, helping avoid wasting energy on rain or debris.
· The action potential causes rapid changes in ion movement and water movement between cells.
· Cells on different sides of the leaf change shape due to changes in turgor pressure.
· This causes the lobes to snap shut around prey.
· Continued movement of prey causes further stimulation, leading to tighter closure and later digestion.
· High-mark explanation: trigger hair stimulation → action potentials → ion movement → water movement by osmosis → turgor changes → lobes close.

The trigger hairs are the sensory structures that detect prey movement. Stimulation of these hairs starts the electrical signalling that leads to trap closure. This image is useful for identifying where the stimulus is detected. Source

This diagram shows the sequence from rapid trap closure to digestion and reopening. It helps connect the rapid response with the plant’s nutritional advantage. Use it to visualise the overall response after trigger hairs are stimulated. Source

Auxin and elongation growth

· Auxin is a plant growth regulator involved in elongation growth.
· Auxin stimulates proton pumps in the cell surface membrane.
· Proton pumps move H⁺ ions from the cytoplasm into the cell wall.
· This acidifies the cell wall, lowering its pH.
· The lower pH activates wall-loosening proteins/enzymes, especially expansins.
· Expansins weaken bonds between cellulose microfibrils and other wall components.
· The cell wall becomes more flexible and can stretch.
· Water enters the cell by osmosis, increasing turgor pressure.
· The softened wall expands under pressure, so the cell elongates.
· High-mark sequence: auxin → proton pumps activated → H⁺ pumped into wall → wall pH falls → wall loosens → water enters → turgor-driven elongation.

This diagram shows the acid growth mechanism required for CIE. Auxin activates proton pumping, which acidifies and loosens the cell wall. The loosened wall allows elongation when water enters the cell. Source

This diagram supports the auxin section by showing the wall structure that must be loosened during elongation. Cellulose microfibrils give the wall strength, so weakening interactions between wall components allows expansion. It is useful for linking wall structure to growth. Source

Gibberellin and germination of barley

· Gibberellin is a plant growth regulator involved in seed germination.
· In barley seeds, water uptake by the seed is called imbibition.
· Imbibition stimulates the embryo to produce gibberellin.
· Gibberellin diffuses to the aleurone layer surrounding the endosperm.
· Gibberellin stimulates aleurone cells to produce and secrete amylase.
· Amylase hydrolyses starch stored in the endosperm into soluble sugars, such as maltose.
· Sugars are transported to the embryo and used in respiration.
· Respiration releases ATP for growth of the embryo.
· This allows growth of the radicle and early seedling development.
· High-mark sequence: water uptake → embryo releases gibberellin → gibberellin acts on aleurone layer → amylase produced → starch hydrolysed → sugars respired → ATP for embryo growth.

This page supports the role of gibberellin in seed germination. Gibberellin promotes amylase production, which releases sugars from stored starch. These sugars provide energy for early growth before photosynthesis is established. Source

Common exam traps

· Do not say auxin “provides energy”; auxin stimulates proton pumping, while ATP is needed for active transport.
· Do not say gibberellin directly digests starch; gibberellin stimulates production of amylase, which digests starch.
· Do not say Venus fly traps use muscles; closure is caused by turgor changes in plant cells.
· Do not omit osmosis when explaining turgor-related movement.
· Always name the key structures: trigger hairs, proton pumps, cell wall, aleurone layer, endosperm, amylase.