Stomata, auxin and gibberellin

Last time

Before we start: last time, in Homeostasis and the control of blood glucose, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Plants have no nerves

A plant cannot run from drought or reach for light with a muscle. Yet it closes its pores when water is short, bends towards a window, and wakes up a seed at the right moment. It does all this with chemical messengers, plant hormones, and this lesson covers three of them: abscisic acid, auxin and gibberellin.

Why stomata open and close

Stomata must be open for carbon dioxide to enter for photosynthesis. But open stomata also let water vapour out by transpiration. So stomata open in the light when photosynthesis is possible and water is plentiful, and close when the plant is short of water or in the dark. This balance reduces water loss without starving the plant of carbon dioxide.

Opening a stoma

Guard cells have thicker inner walls and thinner outer walls, with cellulose microfibrils that stop them from stretching along their length. In the light, proton pumps in the guard cell membrane use A T P to pump hydrogen ions out. The inside becomes more negative, and potassium ion channels open, so potassium ions enter. The water potential in the guard cells falls, so water enters by osmosis. The cells become turgid, and because of their wall structure, they bow outwards and the stoma opens.

A past-paper example

May June 2024, Paper 43, question 6. Describe the mechanism in guard cells that leads to the opening of a stoma. Six marks. Proton pumps pump hydrogen ions out, potassium ions enter, the water potential falls, and water enters by osmosis. The guard cells become turgid. The thick inner wall and the microfibrils make the cells curve, opening the stoma.

Abscisic acid

When a plant is short of water, the roots and leaves make abscisic acid, A B A. It travels to the guard cells. A B A binds to receptors on the guard cell membrane. It causes calcium ions to enter the cytoplasm, which opens channels so that potassium and chloride ions leave. The water potential of the guard cells rises, so water leaves by osmosis. The cells become flaccid and the stoma closes. This saves water.