Photosynthesis · Grade 7
Why are leaves green?
Because chlorophyll absorbs red and blue light and reflects green. The colour we see is precisely the colour the plant does not use — it is bounced back to our eyes instead of being taken in. That is also a real limitation: some of the light energy reaching the leaf is simply never collected.
Learning objectives
- Locate chlorophyll inside the chloroplast and say why those cells sit nearest the light
- Read an absorption graph and state which colours of light chlorophyll uses
- Explain why a green leaf reflects the one colour it has no use for
- Predict how a plant grown under green light alone will grow
The colour you see is the colour rejected
An object looks a certain colour because it reflects light of that colour and absorbs the rest. A red shirt absorbs blue and green and sends back red.
So a green leaf is a leaf that absorbs red and blue and reflects green. What we see is exactly the part of the light the plant found no use for.
That sounds wasteful, and to a degree it is. If chlorophyll absorbed green too, leaves would look black and the plant would collect more energy.
Where the chlorophyll sits
Chlorophyll is not spread over the leaf. It sits inside an organelle called a chloroplast, in cells near the leaf's upper surface — where the light is strongest.
That is why the top of most leaves is a darker green than the underside. It is not accidental but arranged: the cells that collect light are placed where the light is.
Reading an absorption graph
An absorption graph shows how much light is taken in at each wavelength. Chlorophyll has two peaks — one in the blue, one in the red — and a trough between them, in the green.
From that you can predict: a plant lit only with green light receives very little usable energy and barely grows, even though the lamp is on and the leaf looks well lit.
Worked examples
From the absorption graph, will a plant grow faster in red light or green light?
- The graph has an absorption peak in the red
- In the green there is a trough — low absorption
- More absorption means more available energy
Answer: In red light
Why do leaves change colour in autumn?
- Chlorophyll breaks down first, when the tree stops investing in the leaf
- Yellow and orange pigments were there all along
- Without the dominant green, they are revealed
Answer: The other colours were always there; the green was covering them
A plant is kept under foil for a month except for one strip. What happens?
- Only the strip received light
- Only there did photosynthesis happen and sugar form
- The rest of the leaf pales, since chlorophyll is not maintained without light
Answer: A green strip on a pale leaf — and the only part that responds to an iodine test
Common mistakes
- Thinking chlorophyll absorbs green
- It reflects green, which is why we see green. Red and blue are what it absorbs, and that is where the energy comes from.
- Assuming all light is equally good
- Absorption depends on wavelength. Strong green light can illuminate a leaf beautifully and contribute almost nothing to growth.
- Treating chlorophyll as merely a pigment
- It is a molecule that captures light energy and passes it on. The colour is a consequence of its job, not its purpose.
What to remember
- The colour seen is the colour reflected.
- Chlorophyll absorbs red and blue.
- It sits in the chloroplast, near the light.
- Green light alone is nearly useless.
More in Photosynthesis
- Where does a plant's mass actually come from?
- How is a leaf built and what do the stomata do?
- What is the equation of photosynthesis and where does the oxygen come from?
- How do you test a leaf for starch and what makes an experiment fair?
- What is the difference between photosynthesis and cellular respiration?