Plants get carbon from carbon dioxide in the air, through photosynthesis, and get mineral ions from the soil, through the roots. The two supplies do different jobs, and questions often test whether you can keep them apart.
This lesson completes the route through plant nutrition. The way roots absorb ions links to movement across membranes.
What does each supply do for the plant?
Carbon and energy. Carbon dioxide and water are turned into glucose using light energy. The glucose supplies energy through respiration and is the starting material for starch, sucrose and cellulose.
Mineral ions. These are absorbed from the soil solution, often by active transport in root hair cells, because the ions can be at a lower concentration in the soil than inside the cell. They are not energy sources. They are building blocks:
| Ion | Used to make | If it is short |
|---|---|---|
| Nitrate | Amino acids, then proteins | Stunted growth, yellow leaves |
| Magnesium | Chlorophyll | Yellow leaves (chlorosis) |
Much of the dry mass of a plant is built from carbon that came from the air, not from the soil.
Worked example
The data are invented for practice. Identical seedlings were grown for three weeks in water with different mineral supplies. All other conditions were the same.
| Solution | Mean height after 3 weeks (cm) | Leaf colour |
|---|---|---|
| Complete mineral solution | 18 | Green |
| No nitrate | 7 | Pale yellow |
| No magnesium | 14 | Yellow, veins green |
Question: What do the results show about nitrate and magnesium, and why does the no-magnesium seedling still grow?
Step 1, compare with the control. The complete solution gives 18 cm. The no-nitrate seedling is 18 − 7 = 11 cm shorter, which is 11 ÷ 18 × 100 ≈ 61% shorter.
Step 2, magnesium. The no-magnesium seedling is 18 − 14 = 4 cm shorter, which is 4 ÷ 18 × 100 ≈ 22% shorter.
Step 3, link to function. Nitrate supplies nitrogen for proteins, so growth of new cells is badly restricted. Magnesium makes chlorophyll, so the leaves are yellow and absorb less light, but some photosynthesis continues.
Step 4, explain growth without magnesium. The seedling still has carbon dioxide and water, and some chlorophyll, so it can still make some glucose. Carbon acquisition is reduced, not stopped.
Answer: Nitrate deficiency restricts growth much more than magnesium deficiency in this data (about 61% versus 22% shorter). Magnesium affects chlorophyll, so photosynthesis falls but continues.
The mistake to watch for
A common slip is to call minerals “plant food” that gives energy.
Mistaken answer: “The plant is yellow because it did not get enough food from the soil.”
Soil minerals contain no usable energy. The plant’s food is glucose, made in the leaves.
The correction is to name the ion and its job: “not enough magnesium, so less chlorophyll, so less light absorbed and less photosynthesis”. Use the word “raw material” for carbon dioxide and water, and “mineral ion” for nitrate and magnesium.
Check yourself
Try these, then open each answer.
1. Name the substance plants use as their source of carbon, and state where it enters.
Show answer
Carbon dioxide, entering through the stomata in the leaves.
2. Why can nitrate not be used to give a plant energy?
Show answer
Nitrate is a mineral ion used to make amino acids and proteins. Energy comes from glucose, released in respiration.
3. A plant has pale yellow leaves and very short stems. Suggest one mineral shortage and explain.
Show answer
Nitrate shortage, because too little protein is made for growth. Magnesium shortage would also cause yellow leaves, but stunted growth points strongly to nitrate.
Where this leads next
Put the whole module together in the plant nutrition practice set. If you want to revisit why roots can take up ions against a gradient, see movement across membranes.
Many students lose marks when one answer needs photosynthesis, transport and uptake together. A teacher in online one-to-one Biology tuition can help you link them clearly.