15.2 Control and Coordination in Plants

The Venus flytrap converts touch to a rapid electrical response that closes its leaf trap. Sensory hairs detect movement, and the resulting signal causes hinge cells to swell so the two leaf lobes fold together.

1. An insect touches the sensory hairs on the leaf.
2. Calcium ion channels open in hinge cells at the base of a sensory hair, so calcium ions enter and generate a receptor potential.
3. Repeated stimulation is required: two hairs stimulated together, or one hair stimulated twice within a short interval, can reach the condition for an action potential. The action potential is propagated across the trap cells; without the required repeat stimulation, the trap resets.
4. Water moves into the hinge cells, making them swell. This change in cell shape causes the two lobes to fold together and close the trap.

Continued movement by prey can maintain the closed state. Further stimulation can cause calcium ions to enter gland cells and stimulate release of digestive enzymes, but this is a downstream response after the electrical closure pathway.

This is plant electrical coordination, not an animal nervous pathway: the card does not require neurones, synapses, or neurotransmitters. The electrical signal is the trigger, while water-driven hinge-cell swelling provides the immediate mechanical change that closes the trap.

Auxin is a plant growth regulator that promotes elongation growth. Its effect is local and cell-specific: auxin binds to a receptor on the cell-surface membrane and activates changes that allow the cell to extend.

1. Auxin binds to a receptor protein on the cell-surface membrane.
2. Proton pumps move H+ into the cell wall, lowering its pH. This activates expansins, which loosen bonds between cellulose microfibrils.
3. Auxin also opens potassium ion channels. More K+ in the cytoplasm lowers its water potential, so water enters by osmosis through aquaporins.
4. Water entry increases internal pressure; with the loosened wall able to stretch, the cell elongates.

The result is auxin-controlled elongation growth in roots or shoots, contributing to plant growth and form. The reliable mechanism here is receptor activation → wall loosening plus water uptake → turgor-driven cell elongation; this card does not assert a particular shaded-side or phototropic redistribution unless separately supported by the matched objective source.

Auxin does not lengthen a cell by supplying water directly, and water entry alone is insufficient if the cellulose wall cannot stretch. The card stays with the general elongation mechanism and does not replace it with gibberellin signalling or an unsupported phototropism-specific explanation.

In a dormant barley seed, water uptake starts germination and enables the embryo to produce gibberellin. Gibberellin coordinates the mobilisation of stored starch so the embryo receives soluble sugars for growth.

1. The dry, dormant seed absorbs water. The embryo then produces and releases gibberellin.
2. Gibberellin diffuses to the protein-rich aleurone layer surrounding the starch-containing endosperm.
3. In aleurone cells, gibberellin regulates gene expression, increasing transcription of mRNA coding for the hydrolytic enzyme amylase.
4. Amylase is released into the endosperm and hydrolyses stored starch to soluble maltose. Maltose is converted to glucose, which is transported to the embryo.

The embryo respires the glucose to obtain energy for growth, linking hormone signalling to seedling development. The causal chain is water uptake → embryo gibberellin → aleurone gene expression → amylase → starch hydrolysis → soluble sugar → embryo respiration and growth.

Gibberellin is a signal, not the digestive enzyme: amylase performs the starch hydrolysis. The aleurone layer makes the enzyme, while the endosperm stores the starch and the embryo uses the resulting sugars. This card does not describe auxin elongation or Venus flytrap electrical signalling.