Coaching Centre
Photosynthesis and respiration
The word and symbol equations for photosynthesis and respiration, how a leaf is built to photosynthesise, how a plant gets carbon dioxide, light and water, limiting factors, and what happens to the glucose a plant makes.
A gum tree that weighs several tonnes was built almost entirely out of thin air. Roughly half of the dry mass of wood is carbon, and every bit of it came from carbon dioxide that drifted in through the leaves. The soil supplied water and a few minerals. Sunlight supplied the energy to put it all together.
Photosynthesis: making food from light
Photosynthesis is the process plants use to turn light energy into chemical energy stored in sugar. It happens inside chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs red and blue light strongly and reflects most of the green, which is why leaves look green.
carbon dioxide + water → glucose + oxygen (light energy absorbed by chlorophyll)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Although it's written as one equation, photosynthesis is really dozens of reactions, each run by its own enzyme. First chlorophyll captures light energy and uses it to split water, releasing oxygen. Then that energy is used to join carbon dioxide molecules into glucose.
A leaf is a food factory
Every part of a leaf is shaped for photosynthesis. Leaves are broad and thin, so they catch lots of light and gases don't have far to travel.
| Part | How it helps |
|---|---|
| Waxy cuticle | A clear, waterproof layer that cuts water loss but lets light through. |
| Upper epidermis | A single layer of transparent cells that protects the leaf. |
| Palisade layer | Tall cells packed with chloroplasts, lined up near the top surface where the light is strongest. Most photosynthesis happens here. |
| Spongy layer | Rounded cells with air spaces between them, so carbon dioxide can reach every cell. |
| Xylem and phloem | The leaf's veins. Xylem brings water in; phloem carries sugar out. |
| Stomata and guard cells | Pores, mostly on the underside, that let carbon dioxide in and oxygen and water vapour out. |
So a plant gets its three requirements from three different places: light from the sun, water from the soil via the roots and xylem, and carbon dioxide from the air via the stomata. That's the root and shoot systems working together.
What limits photosynthesis?
Photosynthesis can only go as fast as its scarcest ingredient allows. Whatever is in shortest supply is called the limiting factor. It could be light, carbon dioxide or temperature. Water pondweed is handy for testing this, because the oxygen it makes comes out as bubbles you can count.
- Count bubbles with the lamp at 10, 20, 30, 40, 50 and 60 cm. Describe the shape of your graph.
- With the lamp at 10 cm, switch between "Some" and "Lots" of baking soda. Which factor was limiting at 10 cm?
- Try the green filter. Why does green light work so badly?
- Warm the water to 50 °C. Why do the bubbles stop, even in bright light?
Commercial greenhouse growers use this idea. Tomatoes and cucumbers grown in glasshouses are often given extra light, warmth and extra carbon dioxide, which can raise yields by around a fifth or more. Adding just one of these won't help much if another is still limiting.
Respiration: using the food
Making glucose is only half the story. To actually use the energy in glucose, every living cell (plant and animal) carries out cellular respiration in its mitochondria:
glucose + oxygen → carbon dioxide + water + energy
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Respiration is basically photosynthesis run backwards. The products of one are exactly the reactants of the other.
| Photosynthesis | Respiration | |
|---|---|---|
| Where | Chloroplasts (green cells only) | Mitochondria (every living cell) |
| When | Only in the light | All the time, day and night |
| Takes in | Carbon dioxide and water | Glucose and oxygen |
| Gives out | Glucose and oxygen | Carbon dioxide and water |
| Energy | Stored (light → chemical) | Released for the cell to use |
In daylight a plant photosynthesises much faster than it respires, so overall it takes in carbon dioxide and gives out oxygen. At night photosynthesis stops but respiration carries on, so the plant takes in oxygen and gives out carbon dioxide. Around dawn and dusk the two rates are about equal and there's no overall gas exchange at all.
Where does the glucose go?
A plant makes far more glucose in daylight than it needs at that moment. The phloem carries it, dissolved as sugar, to wherever it's needed:
- Respiration, to power every process in every cell, including the roots.
- Cellulose, joined into long chains to build new cell walls.
- Starch, a compact store of glucose packed into roots, tubers and seeds. Potatoes, carrots, rice and wheat grains are full of it, which is why they're such good food for us.
- Other substances: proteins (using nitrogen from the soil), oils in seeds, and sugars in fruit and nectar to attract animals.
Test yourself
Which are the products of photosynthesis?
Carbon dioxide + water → glucose + oxygen.
How does carbon dioxide get into a leaf?
CO₂ diffuses in through the stomata and spreads through the air spaces.
Why are palisade cells near the top of the leaf?
They are packed with chloroplasts and do most of the photosynthesis.
At midnight, a plant is
No light means no photosynthesis, but respiration continues.
A pondweed with lots of baking soda makes more bubbles when the lamp moves closer. What is the limiting factor?
Increasing light increases the rate, so light was in shortest supply.
Potatoes store spare glucose as
Starch is a compact store of glucose.
Label the parts of the leaf.
Photosynthesis, respiration or both?
In photosynthesis, absorbs light energy, which is used to turn carbon dioxide and water into and . This happens in the . Carbon dioxide enters through the . Spare glucose can be stored as or used to make for cell walls.
A student says, "Plants photosynthesise and animals respire, so plants are good for the air and animals are bad." Evaluate this statement using your knowledge of both processes.
The statement is only partly correct. Plants do photosynthesise, but they also respire, in every living cell, all the time, just like animals. In daylight a plant photosynthesises much faster than it respires, so overall it takes in carbon dioxide and gives out oxygen. At night, photosynthesis stops but respiration continues, so the plant takes in oxygen and gives out carbon dioxide.
Over a whole day a healthy, growing plant still releases more oxygen than it uses, so it is fair to say plants add oxygen to the air. However, calling animals "bad" ignores how the two processes are linked: the carbon dioxide released by animal respiration is a raw material for photosynthesis, and the oxygen from photosynthesis is used in respiration. Each depends on the other, so the statement is too simple.