IB Biology HLCoordinating Body SystemsPaper 1 & 2~11 min read
Plant Hormones
A plant cell is a box inside a rigid wall. To grow longer it cannot simply inflate — it has to loosen the box first, then push. Auxin does both jobs at once, and the way it manages that is one of the neatest mechanisms in the syllabus.
📚 What you need to know
Phytohormones regulate growth, development, reproduction, longevity and death. They are also called plant growth regulators.
The common auxin is IAA (indole-3-acetic acid), produced in cells at the growing tip of a shoot.
In shoots auxin causes cells to elongate. In roots it inhibits growth, and at very high concentrations it inhibits shoot growth too.
Auxin enters cells by simple diffusion but needs auxin efflux carriers (PIN3 proteins) to leave.
Cells place efflux carriers on one side only, giving one-way movement. It uses ATP, so it is active transport.
Auxin causes elongation by acidifying the cell wall, activating expansins, and drawing water in by osmosis.
The phytohormones
Plant hormones are chemicals that regulate growth, development, reproductive processes, longevity and even death. Many different chemicals act as phytohormones, and you should be able to match each one to its main effect.
Phytohormone
Main effect
Auxins
Cell elongation
Abscisic acid
Suppresses the growth of plants
Cytokinins
Increase the rate of cell division
Ethylene
Promotes fruit ripening
Gibberellin
Controls cell elongation, seed germination, flowering and dormancy
Brassinosteroids
Regulate growth, development and responses to stresses
Two of these do cell elongation. Auxin and gibberellin both cause cells to lengthen. If a question is about a shoot bending, it is auxin. If it is about germination, dormancy or flowering, it is gibberellin.
Auxin and where it goes
Auxins are a group of hormones influencing many aspects of plant growth. The one you need to name is IAA, indole-3-acetic acid.
In shoots, auxin is produced in cells at the growing tip, then moves away into the surrounding tissues. Its main role there is regulating shoot growth: it causes cells to elongate, which lengthens the stem. But the effect depends entirely on where the auxin is and how much of it there is:
In shoots, auxin causes cells to elongate.
In roots, auxin inhibits cell growth — the opposite effect.
At very high concentrations, auxin can inhibit shoot growth as well.
Auxin efflux carriers
Auxin enters a cell by simple diffusion. Getting out again is the hard part: to exit a cell, and therefore to move on to the next one, auxin needs membrane proteins called auxin efflux carriers. Efflux just means an outward flow — auxin is pumped out of one cell and into another.
These carriers are proteins called PIN3 proteins, and here is the crucial detail: a plant cell can distribute its efflux carriers on one side of the cell only. That makes the movement of auxin one way. The process requires ATP, so it is a form of active transport.
Those pumps are what establish an auxin gradient across a stem or a root in response to a stimulus such as light or gravity. Light is thought to affect the expression of the genes coding for PIN3 proteins, so light shining on one side of a stem more than the other leads to an uneven distribution of efflux pumps — and an uneven distribution of pumps is what creates the gradient.
In, easy. Out, expensive. Auxin diffusing in is passive and undirected. Auxin being pumped out is active and directed. Because only the exit is controlled, controlling the position of the pumps controls the direction of the whole flow.
How auxin actually makes a cell longer
This is the acid growth mechanism. A plant cell is surrounded by a rigid wall of cellulose microfibrils, so the cell cannot get longer until that wall is loosened. Auxin arranges both the loosening and the pushing.
Written out in full, the sequence is:
Auxin molecules bind to a receptor protein on the cell surface membrane.
Auxin stimulates ATPase proton pumps to pump hydrogen ions from the cytoplasm into the cell wall.
This acidifies the cell wall, lowering its pH.
The low pH activates proteins called expansins, which loosen the bonds between cellulose microfibrils.
At the same time, potassium ion channels are stimulated to open.
Potassium concentration in the cytoplasm increases, which decreases the water potential of the cytoplasm.
Water is therefore absorbed by osmosis, entering through aquaporins.
The internal pressure of the cell increases, causing the loosened cell wall to stretch.
The cell elongates.
🧠
Expansins expand
The name is the definition. Expansins let the wall expand by loosening the bonds holding the cellulose microfibrils in place. And "acid growth" tells you the trigger — it is the drop in pH inside the wall that switches them on. Two words, two thirds of the mechanism.
Worked examples
WE 1
Explain the transport of auxin between cells
Explain why the movement of auxin through a plant tissue is described as active transport, and why it occurs in only one direction. (3 marks)
Step 1: how auxin gets in and out
Auxin enters a cell by simple diffusion, but it cannot leave without auxin efflux carriers (PIN3 proteins) in the membrane.
Step 2: why it is active
Pumping auxin out through these carriers requires ATP, which is the defining feature of active transport.
Step 3: why it is directional
A cell distributes its efflux carriers on one side only, so auxin can only be pumped out on that side and therefore only ever passes to the neighbouring cell in that direction.
ATP makes it active; the position of the carriers makes it one wayboth halves are needed. Saying "it uses ATP" explains active but not directional, and vice versa
WE 2
Describe how auxin causes cell elongation
Describe the mechanism by which auxin causes a plant cell to elongate. (5 marks)
Step 1: binding
Auxin binds to a receptor protein on the cell surface membrane.
Step 2: acidify the wall
This stimulates ATPase proton pumps to move hydrogen ions from the cytoplasm into the cell wall, lowering its pH.
Step 3: loosen the wall
The low pH activates expansins, which loosen the bonds between cellulose microfibrils, so the wall is no longer rigid.
Step 4: draw water inPotassium ion channels open, potassium enters the cytoplasm and decreases its water potential, so water enters by osmosis through aquaporins.
Step 5: stretch
The internal pressure rises and pushes against the loosened wall, which stretches, so the cell elongates.
Acidify, loosen, draw water in, stretchthe wall must be loosened and the pressure must rise. Answers that mention only one of the two rarely score above three
WE 3
Explain a contradictory-looking result
A researcher applies the same auxin solution to a shoot and to a root. The shoot grows longer; the root grows less than an untreated control. Explain this difference. (3 marks)
Step 1: the effect on the shoot
In shoots, auxin causes cells to elongate, so the shoot lengthens.
Step 2: the effect on the root
In roots, auxin has the opposite effect and inhibits cell growth, so the treated root grows less than the control.
Step 3: the general principle
The response to a phytohormone depends on the tissue it reaches and on the concentration. Auxin also inhibits shoot growth at very high concentrations.
Same hormone, opposite responses, because the tissues respond differentlythe phrase "the same hormone can have different effects in different tissues" is a mark-earning generalisation. Add it after you have dealt with both cases
💡 Exam tips
Learn IAA by its full name once — indole-3-acetic acid — then use IAA.
The words worth memorising in the acid growth mechanism are ATPase, expansins, aquaporins and water potential.
When you write about osmosis, say water potential decreases, not "gets more concentrated".
If asked why transport is active, mention ATP and the fact that carriers are needed to exit the cell.
Efflux means out. If you can define the term in the answer, do.
⚠ Common mistakes
Saying auxin causes cell division. That is cytokinin. Auxin causes elongation.
Saying auxin always promotes growth. Not in roots, and not at very high concentrations in shoots.
Writing that auxin is pumped into cells. It diffuses in; the pumps move it out.
Saying the cell wall dissolves. Expansins loosen bonds between microfibrils; the wall stays intact.
Saying water enters because the cell needs it. It enters down a water potential gradient, by osmosis.
Forgetting the potassium step. Without it there is no reason for water to move in.
Calling ethylene a growth inhibitor generally. Its named role here is promoting fruit ripening.
Up next: Plant Growth & Fruit Ripening — how auxin and cytokinin balance root and shoot growth from opposite ends of the plant, and why a single ripening banana can take the whole bunch with it.
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