Inorganic ionsEdexcel A-Level Biology B: Revision notes
Section 1
Why plants need mineral ions
Plants make carbohydrates by photosynthesis from carbon dioxide and water, but those molecules contain only carbon, hydrogen and oxygen. To build proteins, nucleic acids, chlorophyll, cell walls and ATP, plants need other elements. These are absorbed from the soil solution as inorganic ions by the root hair cells, mostly by active transport, which needs ATP.
Four ions are required: nitrate (NO₃⁻), calcium (Ca²⁺), magnesium (Mg²⁺) and phosphate (PO₄³⁻). Each has a specific role, and one ion cannot be replaced by another. A deficiency of any ion limits growth and often produces visible symptoms.
Section 2
Nitrate ions: amino acids and DNA
Nitrate ions supply nitrogen. Plants use it to make:
- Amino acids, which are joined by peptide bonds into proteins, including enzymes, channel and carrier proteins
- DNA (and RNA), because the nitrogenous bases (adenine, guanine, cytosine and thymine) contain nitrogen
A plant short of nitrate cannot make enough protein or DNA, so cell division and growth slow and the plant is stunted. Older leaves often turn pale yellow as nitrogen is moved to young growth.
Always name both products of nitrate: amino acids (so proteins) and DNA. One alone usually gains only one mark.
Section 3
Calcium ions: calcium pectate
Plant cell walls are stuck to each other by a layer called the middle lamella. It is made mainly of pectin, and calcium ions link the pectin molecules together to form calcium pectate, which makes the layer firm and holds neighbouring cells together.
Without enough calcium, less calcium pectate forms, so the middle lamellae are weak and cells separate easily. Growing points such as shoot and root tips are the most affected, and new tissue is soft and fragile.
The middle lamella is not made of cellulose. Cellulose is in the cell wall itself; calcium pectate is in the middle lamella.
Section 4
Magnesium ions: chlorophyll
Magnesium ions (Mg²⁺) are the central ion of every chlorophyll molecule. Chlorophyll is the main photosynthetic pigment: it absorbs light energy, mainly red and blue wavelengths, and passes it into the light-dependent reactions of photosynthesis.
A plant short of magnesium makes less chlorophyll, so its leaves turn yellow, often between the veins (chlorosis). Less light is absorbed, so the rate of photosynthesis falls and growth is poor.
Section 5
Phosphate ions: ADP and ATP
Phosphate ions (PO₄³⁻), written Pᵢ when free, join to ADP to make ATP:
ADP + Pᵢ → ATP + H₂O (needs energy, for example from respiration or photosynthesis)
ATP is the immediate source of energy for the cell. When it is hydrolysed back to ADP and Pᵢ, the energy released is used for processes such as active transport, synthesis of proteins and cell division.
A plant short of phosphate makes less ATP, so root growth and ion uptake by active transport are poor, and growth is stunted. Leaves may look dull or dark.
A shortage of phosphate affects active transport, because active transport uses ATP. This links two topics in one answer.
Must know
- Nitrate: nitrogen for amino acids (proteins) and DNA
- Calcium: calcium pectate in the middle lamellae, holding cells together
- Magnesium: central ion of chlorophyll
- Phosphate: makes ADP and ATP
- Ions are taken up by roots, mainly by active transport
- A deficiency of any one ion limits growth
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Inorganic ions
- A grower raises young tomato plants in hydroponic culture, where the roots sit in a nutrient solution. Plants given a complete solution grow normally. Plants given a solution lacking nitrate ions grow slowly and their older leaves turn pale yellow. Plants given a solution lacking magnesium ions have leaves that turn yellow between the veins.Explain why plants lacking nitrate ions grow slowly.2 marks
- A strawberry grower compares three nutrient solutions. In solution 1 all essential ions are present and the plants grow well. In solution 2 phosphate ions are missing: the roots grow poorly, the plants take up fewer ions by active transport and the leaves are dull and dark. In solution 3 calcium ions are missing: new tissue at the shoot and root tips is weak and soft, and its cells separate easily when handled.Explain why plants in solution 2 take up fewer ions by active transport.2 marks
- Young wheat plants in a sandy field are stunted and have pale yellow leaves. A farmer adds ammonium nitrate to one half of the field and magnesium sulfate to the other half. Three weeks later, plants in the nitrate-treated half have green leaves and have grown well. Plants in the magnesium-treated half show no improvement. Chlorophyll content was low in the plants before treatment and is still low in the magnesium-treated plants, but is high in the nitrate-treated plants.Deduce which ion was lacking in the soil, and explain why the stunted growth is consistent with this.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).