Photosynthesis
Senior 2 · Biology
Photosynthesis
Lesson 4 of 4
22 min
Let's begin
Walk past a maize field where the lower leaves have turned yellow, and you are probably looking at a hungry plant, one short of minerals rather than water or light. Rwandan farmers buy fertilisers such as NPK and urea to prevent this. Used well, fertilisers feed the nation; used carelessly, they poison lakes and rivers.
Photosynthesis makes carbohydrates, which contain only carbon, hydrogen and oxygen. To make chlorophyll, proteins and DNA, a plant also needs other elements. It absorbs them from the soil water as ions (charged particles) through its root hairs. Elements needed in large amounts are macronutrients; those needed in tiny amounts are micronutrients. Too little of one causes a deficiency, which shows as poor growth or as chlorosis (yellowing of leaves).
| Ion | Why the plant needs it | Deficiency symptoms |
|---|---|---|
| Magnesium (Mg²⁺) | Magnesium is at the centre of every chlorophyll molecule. | Yellowing between the veins, starting in the older leaves (the plant moves its magnesium to young leaves); less photosynthesis. |
| Nitrate (NO₃⁻) | Provides the nitrogen for amino acids, and so for proteins (including enzymes), and for chlorophyll and DNA. | Stunted growth, weak stems, and pale yellow older leaves. |
| Phosphate (PO₄³⁻) | Needed for DNA, cell membranes and energy transfer. | Poor root growth; purplish leaves. |
| Potassium (K⁺) | Helps enzymes work and controls water movement in cells. | Yellow or scorched leaf edges. |
Worked example
Three identical bean seedlings are grown for three weeks with their roots in different solutions (this is called water culture):
Interpretation: A is the control, showing normal growth. B shows that magnesium is needed to make chlorophyll. C shows that nitrogen is needed for growth, because without it the plant cannot make proteins for new cells. Only one mineral was missing in each case, so each difference can be blamed on that mineral.
Each harvest removes minerals from the soil. Fertilisers put them back.
The label NPK 17-17-17 means the fertiliser supplies 17% nitrogen, 17% phosphorus (counted as P₂O₅) and 17% potassium (counted as K₂O) by mass.
Worked example
How much nitrogen is in a 50 kg bag of urea, which is 46% nitrogen? 50 × 0.46 = 23 kg. The same bag of NPK 17-17-17 holds only 50 × 0.17 = 8.5 kg of nitrogen.
Eutrophication is the build-up of nutrients in a lake or river, which leads to so much plant and algal growth that the water runs out of oxygen. It happens like this:
Other dangers:
Good practice: apply the recommended amount at the right time (not just before heavy rain), mix inorganic fertiliser with manure or compost, keep strips of grass or trees between fields and water, and use terraces on slopes to reduce run-off.
Tip
Mg makes the green (chlorophyll); N makes the growth (proteins).
Eutrophication chain: Nutrients → Algae bloom → Darkness below → Death of plants → Decomposers → Deoxygenation → fish die. Examiners give a mark for each link, so write every step.
Why does a plant lacking magnesium turn yellow even when it has plenty of light and water?
Solution: Magnesium is part of the chlorophyll molecule. Without it the plant cannot make chlorophyll, so its leaves lose their green colour (chlorosis) and photosynthesise less.
Why are the symptoms of magnesium deficiency seen first in the older leaves?
Solution: The plant moves magnesium out of older leaves to the young, growing leaves, so the older leaves run short first.
A farmer applies a 25 kg bag of NPK 17-17-17. What mass of nitrogen does this supply?
Solution: 25 × 0.17 = 4.25 kg of nitrogen.
Explain why fish die in a lake after fertiliser runs into it.
Solution: The nutrients cause an algal bloom. Plants below are shaded and die. Bacteria decomposing them respire and use up the dissolved oxygen, so the fish suffocate.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
