IB Biology HLCoordinating Body SystemsPaper 1 & 2~8 min read
Phototropism
A plant on a windowsill leans towards the glass. It looks like the lit side is being pulled across. It is the opposite: the dark side is doing the growing, and it pushes the shoot over. Once you see that, the whole mechanism falls into place.
📚 What you need to know
Plant shoots are positively phototropic — they grow towards light. Phototropism affects shoots and the tops of stems.
This maximises the light absorbed for photosynthesis, and it matters because plants cannot move.
Light causes auxin to be transported from the illuminated side to the shaded side by PIN3 proteins.
This sets up an auxin gradient: more auxin on the shaded side, less on the illuminated side.
Higher auxin means a faster rate of cell elongation, so the shaded side grows faster and the shoot bends towards the light.
What phototropism is
Plant shoots grow towards light. That is a positive tropism, because the growth is towards the stimulus; a negative tropism would be growth away from it.
The point of it is straightforward. A shoot that leans into the light intercepts more of it, so the leaves photosynthesise faster. And because a plant is fixed in place, growing in a new direction is the only kind of moving it can do.
🧠
Reading the word
Photo means light and tropos means a turning. Phototropism is a light-turning. Every tropism name is built the same way: gravi-tropism is a gravity-turning, hydro-tropism a water-turning. Learn the ending once and the rest is vocabulary you already have.
The mechanism
Light affects the growth of plant shoots because it affects where the auxin ends up. The chain runs like this:
The concentration of auxin determines the rate of cell elongation in the stem. Higher auxin means faster elongation.
If the auxin concentration is not uniform across the stem, then growth is not uniform either — you get uneven cell growth.
When light shines on a stem from one side, auxin is transported by PIN3 proteins from the illuminated side to the shaded side.
An auxin gradient is established: more auxin on the shaded side, less on the illuminated side.
The higher auxin concentration on the shaded side causes a faster rate of cell elongation there, so that side lengthens more and the shoot bends towards the light.
The bending side is the dark side. Cells on the shaded side get longer, and since one side of a stem is now longer than the other, the stem curves — towards the shorter, illuminated side. Nothing is pulling the plant towards the window.
Why light changes where auxin goes
Auxin does not simply drift towards the shade. It is pumped. Auxin leaves a cell through membrane proteins called auxin efflux carriers, which are PIN3 proteins, and a cell can place those carriers on one particular side of itself. That makes the movement one-way.
Light is thought to affect the expression of the genes coding for PIN3 proteins. Light hitting one side of a stem more than the other therefore produces an uneven distribution of efflux pumps, and an uneven distribution of pumps produces the auxin gradient. You will meet this in full detail on the next page.
The classic tip experiments
Long before anyone knew what auxin was, a simple set of experiments showed that the tip is what detects light, even though the bending happens further down. You may be asked to interpret them.
Compare the last two carefully. Cutting the tip off stops growth altogether, because the tip is the source of the auxin. Covering the tip leaves growth intact but removes the bending, because the tip can no longer detect which side the light is on. Two different results, two different conclusions.
Worked examples
WE 1
Explain positive phototropism
A seedling is illuminated from one side only. Explain how it comes to bend towards the light. (4 marks)
Step 1: the redistribution
Light causes auxin to be transported by PIN3 proteins from the illuminated side of the shoot to the shaded side.
Step 2: the gradient
An auxin gradient is established, with a higher concentration on the shaded side.
Step 3: the effect on cells
A higher auxin concentration increases the rate of cell elongation, so cells on the shaded side become longer than those on the illuminated side.
Step 4: the consequence for the shoot
Because one side of the stem is now longer than the other, the shoot curves towards the light, increasing light absorbed for photosynthesis.
Auxin to the shaded side, faster elongation there, the shoot curves overthe phrase cell elongation is essential. Writing "the cells divide more" is a different process and will not score
WE 2
Interpret an experimental result
In one setup the shoot tip is cut off and the seedling neither grows nor bends. In another the tip is covered with an opaque cap; the seedling grows normally but does not bend. Explain what each result shows. (3 marks)
Step 1: the removed tip
No growth at all means the tip is the source of the auxin that drives cell elongation. Without it there is no auxin to produce growth.
Step 2: the covered tip
Growth continues, so auxin is still being produced and the tip is still present. But there is no bending, so the tip can no longer detect the direction of the light.
Step 3: combine the two
Together they show the tip performs two roles — producing auxin and detecting light — while the response, elongation, happens further down the shoot.
The tip senses and supplies; the region below respondsanswer each setup separately before combining them. Questions like this award a mark for the comparison, not just for the two observations
WE 3
Process auxin distribution data
In an illuminated shoot, 65% of the auxin was found on the shaded side and 35% on the illuminated side. Calculate the ratio of auxin on the two sides and explain what effect this has. (3 marks)
Step 1: the ratio
65 ÷ 35 = 1.86, so roughly 1.9 : 1 shaded to illuminated
Step 2: the difference
65 − 35 = 30 percentage points more auxin on the shaded side
Step 3: the biological effect
The shaded side has almost twice the auxin, so its cells elongate substantially faster, that side of the stem lengthens more, and the shoot curves towards the light.
About 1.9 : 1 in favour of the shaded side, causing uneven elongationa ratio and a difference are not the same thing. If a question asks for a ratio, divide; if it asks for a difference, subtract
💡 Exam tips
Always say auxin moves from the illuminated side to the shaded side. Naming both sides is safer than saying "to one side".
Use cell elongation, not growth, cell division or swelling.
Name PIN3 if the question asks how auxin is transported.
Finish the chain: uneven elongation causes the curvature. Do not stop at the gradient.
Link the response to a benefit — more light absorbed, so a higher rate of photosynthesis.
⚠ Common mistakes
Saying the illuminated side grows faster. It is the shaded side.
Saying the plant is attracted to the light. Plants do not move; differential growth changes their direction.
Writing that auxin diffuses to the shaded side. It is actively pumped by efflux carriers.
Saying light destroys auxin on the lit side. The auxin is redistributed, not destroyed.
Confusing removing the tip with covering it. One stops growth; the other stops only the bending.
Applying shoot behaviour to roots. Auxin has the opposite effect in roots.
Up next: Plant Hormones — the full cast of phytohormones, how PIN3 efflux carriers create a one-way flow of auxin, and the step-by-step mechanism by which auxin actually makes a cell longer.
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