Parts of a plant and their jobs
A flowering plant has three main parts. Roots hold the plant in the soil and take in water and minerals. The stem holds the leaves up to the light and has two kinds of pipes. Xylem pipes carry water up. Phloem pipes carry sugar to every part. The leaf is the place where food is made.
A leaf is thin and flat so that light can reach all its cells. Its upper cells hold many green chloroplasts. The green colour comes from chlorophyll, which catches light energy. On the leaf skin are tiny holes called stomata (one hole is a stoma). Each stoma has two bean-shaped guard cells that open and close it like a door.
Photosynthesis in leaves
Photosynthesis means making food with light. The leaf takes carbon dioxide from the air through the stomata, and water from the soil through the xylem. Chlorophyll uses light energy to join them into sugar (glucose). Oxygen is the leftover and goes out through the stomata.
Carbon dioxide + water, with light and chlorophyll, gives glucose + oxygen.
6CO2 + 6H2O → C6H12O6 + 6O2
The sugar is used at once, or turned into starch and stored. Phloem carries sugar to roots, fruits and growing tips. Photosynthesis needs light, so it happens only by day. More light gives more food, up to a limit.
Respiration in plants
Sugar is stored energy. To use it, every living cell does respiration: it uses oxygen to break sugar down and release energy. Carbon dioxide and water are the waste.
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Plants respire all day and all night, in leaves, stems and roots. Roots also need oxygen from air gaps in the soil, which is why waterlogged soil can kill a plant. At night only respiration is going on, so the plant gives out carbon dioxide. In the day photosynthesis is faster, so the plant gives out more oxygen than it uses.
Photosynthesis stores energy; respiration releases it. They are opposite jobs, not the same job.
Transpiration and water transport
Water enters the roots through thin root hairs. It moves up the xylem, a long chain of hollow tubes, to the leaf. From the leaf cells, water turns into vapour and leaves through the open stomata. This loss is transpiration.
Transpiration is not only a loss. As water leaves the leaf, it pulls more water up the xylem behind it, like sucking on a straw. This pull also brings up minerals and cools the leaf on a hot day.
Guard cells control the gate. When the plant has plenty of water, they swell and open the stoma. When the air is very dry or the plant is short of water, they go slack and the stoma closes. This saves water but also stops carbon dioxide coming in. Transpiration is faster in hot, dry, windy and bright conditions, and slower in cool, humid and still air.
One gate, three jobs: putting it together
The stoma is the meeting point. Carbon dioxide comes in and oxygen goes out for photosynthesis. Oxygen comes in and carbon dioxide goes out for respiration. Water vapour goes out for transpiration. When a stoma closes, all three slow down. The plant chooses between getting food and saving water.
| Process | Where | Needs | Gives |
|---|---|---|---|
| Photosynthesis | Green cells (leaf) | Light, CO2, water | Sugar, O2 |
| Respiration | All living cells | Sugar, O2 | Energy, CO2, water |
| Transpiration | Stomata of leaves | Water, open stomata | Vapour, upward pull |
Try it
In the 3D: before you move the sliders, guess what will happen to the food made if the air gets very dry. Then slide Dry air to the top and check.
At home (bag test): in the morning, tie a clear plastic bag round a leafy branch of a potted plant, and leave it in the sun for two hours. Small water drops appear inside the bag. That is the water vapour that came out through the stomata. Try the same with a bag on a bare stick: no drops.
Key formulas and definitions
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (photosynthesis, needs light and chlorophyll)
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (respiration)
- Rate of water loss = water lost ÷ time
- Stoma = tiny gate; guard cells = the two cells that open and close it
- Xylem = water up; phloem = sugar to all parts
- Transpiration = loss of water vapour from leaves through stomata
Worked examples
1. A leaf makes 1 molecule of glucose by photosynthesis. How many molecules of carbon dioxide and oxygen are involved?
From 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂: 6 molecules of CO₂ go in and 6 molecules of O₂ come out for every 1 glucose.
2. A potted plant loses 18 mL of water in 6 hours. What is the rate of transpiration per hour?
Rate = water lost ÷ time = 18 ÷ 6 = 3 mL per hour.
3. At noon on a hot dry day, a plant wilts and its stomata close. Explain what happens to its food making and why.
Closed stomata stop carbon dioxide entering, so there is less raw material and photosynthesis slows even though there is light. The plant saved water but made less food.
Common mistakes
- Saying plants only respire at night. They respire all the time; at night respiration is just the only one you can notice.
- Mixing up xylem and phloem. Remember "X" for up (xy-lem lifts water), "P" for plenty of sugar sent everywhere.
- Thinking respiration is the same as breathing in and out. Respiration is the energy-releasing work inside cells; gas exchange is only the delivery.
- Thinking transpiration is only a waste. It also pulls water and minerals up and cools the leaf.