Mineral Requirements

Plants are autotrophic — they make their own food. But food (glucose) is only carbon, hydrogen and oxygen. Plants also need nitrogen, magnesium, phosphorus, potassium, sulfur, and traces of other elements to build everything else they use.

Where do they get them?

• C, H, O — from CO₂ + water (photosynthesis).
• N, Mg, P, K, etc. — from MINERAL IONS dissolved in soil water, taken up through roots.

Cambridge focus. The syllabus highlights TWO key minerals: nitrates and magnesium ions. Memorise the role + the deficiency symptom for each.

Why plants need nitrogen.

• Amino acids (and so proteins) all contain nitrogen — none of which can come from glucose alone.
• Plants make amino acids by combining a glucose-derived skeleton with nitrate.
• Amino acids → proteins (enzymes, structural, etc.).
• DNA, chlorophyll, ATP — all also require nitrogen.

Why NITRATE, not N₂?

• 78% of the air is nitrogen GAS (N₂), but plants CANNOT use N₂ directly.
• The triple bond in N₂ is too strong to break.
• Plants take in NITRATE IONS (NO₃⁻) dissolved in soil water.
• Nitrogen-fixing bacteria (in soil and root nodules) convert N₂ to nitrates that plants can use.

Deficiency symptoms.

• Stunted growth (less protein → less new tissue).
• Older leaves yellow (chlorosis) — the plant moves nitrogen out of older leaves to keep growing tips alive.
• Overall plant looks pale, small, weak.

Worked qualitative. Why do farmers add NITROGEN-CONTAINING FERTILISERS to fields?

• Plants use up soil nitrate as they grow.
• Without replacement, soil becomes nitrogen-depleted → poor crops.
• Adding ammonium nitrate / urea → more nitrate available → faster growth, bigger yield.
• Side effect: excess fertiliser washes into waterways → eutrophication.

Cambridge tip. Always say "amino acids and proteins". Saying just "for protein" misses a mark — the chain of reasoning is glucose + nitrate → amino acid → protein.

Why plants need magnesium.

• The chlorophyll molecule has a magnesium ATOM at its centre.
• Without magnesium, chlorophyll can't be made.
• Less chlorophyll → less light absorbed → less photosynthesis → less glucose → less growth.

Deficiency symptoms.

• Yellow leaves (chlorosis) — usually starting BETWEEN the veins. The veins themselves often stay green (they get magnesium first).
• Affects older leaves first (magnesium is moved from older to newer leaves).
• Stunted growth (because of reduced photosynthesis).

How plants take up magnesium.

• Mg²⁺ ions in soil water.
• Absorbed by ROOT HAIR cells, mostly by ACTIVE TRANSPORT (because soil concentration is low).
• Carried up the xylem to leaves.

Worked qualitative. A houseplant has yellow leaves between green veins. Likely cause?

• Classic magnesium deficiency.
• Solution: add magnesium-rich fertiliser (or Epsom salts = MgSO₄).

Cambridge tip. "Yellow leaves" is partial — say "yellow leaves due to lack of chlorophyll".

The challenge. Soil concentration of mineral ions is usually LOWER than concentration inside root cells. Diffusion would push ions OUT, not in.

The solution: active transport.

1. Mineral ions in the soil water reach the ROOT HAIR cells.
2. PROTEIN PUMPS in the root hair cell membrane bind to the ion.
3. Pumps use ENERGY (ATP) from respiration to push the ion INTO the cell — against the gradient.
4. Ions move from cell to cell into the xylem.
5. Xylem carries them UP to leaves.

Why root hair cells are good at this.

• Long, thin EXTENSION → huge SURFACE AREA for absorption.
• Many MITOCHONDRIA → lots of ATP for active transport.
• Thin cell wall → close contact with soil water.

Worked qualitative. Why does waterlogged soil kill plants?

• Air can't reach the roots.
• Roots can't respire aerobically.
• No ATP → active transport stops.
• No mineral uptake → plant deficient → wilts and dies.

Cambridge tip. When asked HOW minerals enter, say "active transport" — and pair it with "requires energy from respiration". Avoid saying "diffusion": that only works when soil > root, which is rare.