International A Level Biology / Coordination in animals and plants
Plant responses: auxin and gibberellins revision notes
Edexcel International A Level Biology (YBI11), chapter 24: Coordination in animals and plants, specification point 8.11. Free lesson.
Watch the video lesson · International A Level Biology past papers
Auxin (IAA) and gibberellin are plant hormones that control growth by changing how cells behave and which genes are transcribed. IAA helps shoots grow towards light, while gibberellin helps stems elongate and seeds germinate.
How IAA makes a shoot grow towards light
A shoot growing towards light shows positive phototropism. A tropism is a growth response to a stimulus from one direction. IAA is made in the shoot tip. In even light it spreads down both sides, but light from one side makes it move across to the shaded side. Cells there receive more IAA and elongate more than cells on the lit side. The unequal growth makes the shoot curve towards the light, helping its leaves receive light for photosynthesis. Use the exact conclusion: the shoot grows towards the light.
How IAA makes a cell elongate
Cells in the elongation zone, just behind the shoot tip, get longer as they grow. Their cellulose walls must loosen first. IAA binds to a receptor in the cell surface membrane, activating proton pumps. These move H⁺ ions into the wall, lowering its pH. The more acidic conditions activate expansins, which break hydrogen bonds between cellulose microfibrils. The wall loosens, allowing the microfibrils to slide apart. Meanwhile, K⁺ ions enter the cell and lower its water potential. Water enters by osmosis, pushes against the loosened wall and stretches it, so the cell elongates. Name the…
How gibberellin helps a seed germinate
When a dry cereal grain such as barley takes up water, its embryo becomes active. The embryo makes gibberellin (GA), which diffuses to the aleurone layer, a ring of living cells around the starchy endosperm. Gibberellin binds to a receptor there, causing DELLA repressor proteins to break down. A transcription factor is freed and switches on the amylase gene. RNA polymerase transcribes the gene, and the aleurone cells make and secrete amylase into the endosperm. Amylase hydrolyses starch to maltose, which is then hydrolysed to glucose. These sugars move to the embryo for respiration and…
How the two hormones affect genes
Both hormones change transcription, the process of making RNA from a gene. Neither hormone is itself a transcription factor. Each binds to a receptor, which leads to a transcription factor being freed so it can switch genes on. IAA is made in shoot tips. It promotes cell elongation and helps shoots grow towards light. It also inhibits side buds from growing, called apical dominance. Gibberellin is made in young leaves and seeds, including by the embryo in a cereal grain. It promotes stem elongation and seed germination. Some dwarf varieties grow tall when given gibberellin.