Plants & Mineral Ions

Plants make glucose by photosynthesis, but glucose contains only carbon, hydrogen and oxygen. To build everything else a plant needs — chlorophyll, proteins, DNA — it must take in extra mineral ions from the soil.

• Mineral ions are simple dissolved substances in the soil water (for example nitrate ions and magnesium ions).
• They are absorbed from the soil through the roots, dissolved in water.
• They are needed for healthy growth — without them, plants grow poorly and show clear deficiency symptoms.

Key idea. Mineral ions are not food. They do not provide energy. They are raw materials the plant uses to make important molecules.

For Double Award you only need two mineral ions: magnesium ions and nitrate ions.

Magnesium ions are needed to make chlorophyll.

• Chlorophyll is the green pigment in chloroplasts that absorbs light for photosynthesis.
• If a plant is short of magnesium ions, it cannot make enough chlorophyll.

Deficiency symptom — yellow leaves (chlorosis):

• Without chlorophyll the leaves cannot stay green, so they turn yellow. This yellowing is called chlorosis.
• Less chlorophyll → less photosynthesis → less glucose → the plant grows poorly too.

Exam wording. The mark scheme wants the chain: magnesium ions → make chlorophyll → (deficiency) yellow leaves / chlorosis. Naming chlorophyll is the key marking point — do not just say "to stay green".

Nitrate ions are needed to make amino acids (and therefore proteins).

• The plant uses nitrate ions (which contain nitrogen) together with glucose to make amino acids.
• Amino acids join together to build proteins.
• Proteins are needed for growth (building new cells) and to make enzymes.

Deficiency symptom — stunted / poor growth:

• Without enough nitrate, the plant cannot make enough proteins.
• This means it cannot build new cells properly, so it shows stunted growth (small, weak plants with poor growth).

Exam wording. Examiners want: nitrate ions → make amino acids → make proteins → (deficiency) stunted growth. Writing "nitrate for proteins" is fine, but mentioning amino acids secures the spec point in full.

Mineral ions are absorbed from the soil by the roots — mainly through the root hair cells, which have a large surface area for absorption.

• The concentration of mineral ions is usually higher inside the root cells than in the soil water.
• So the ions must be moved against a concentration gradient (from low concentration in the soil to high concentration in the root).
• Moving substances against a concentration gradient is called active transport, and it requires energy.
• This energy comes from respiration in the root cells.

This is why active transport is different from diffusion: diffusion moves substances down a gradient and needs no energy, but active transport moves them up the gradient and does need energy.

Exam tip. If a question asks how the ions are absorbed, the answer is active transport, and you should add "against the concentration gradient" and "requires energy (from respiration)" to get full marks.

Paper 1 (4WSD0/1B) often describes an unhealthy plant and asks which ion is missing and why the symptom appears. Work backwards from the symptom:

Symptom seen	Missing ion	Why the symptom happens
Yellow leaves (chlorosis)	Magnesium ions	Cannot make chlorophyll, so leaves are not green
Stunted / poor growth	Nitrate ions	Cannot make amino acids / proteins, so cannot build new cells

How to write a full-mark answer:

1. Name the ion that is missing.
2. State what that ion is used to make (chlorophyll, or amino acids/proteins).
3. Link this to the symptom described.

For example: "The plant lacks magnesium ions (1). Magnesium is needed to make chlorophyll (1), so the leaves cannot stay green and turn yellow / show chlorosis (1)."

Watch out. Do not muddle the two: magnesium is for chlorophyll (a colour problem → yellow leaves); nitrate is for proteins (a building problem → stunted growth).