Recommended additional practical: plant mineral deficiencies

The biology you need (spec 4.8)

• Nitrate ions: a source of nitrogen for amino acids (so proteins and enzymes), nucleotides (DNA, RNA, ATP) and chlorophyll. Deficiency: stunted growth and yellowing, starting in the older leaves.
• Magnesium ions: part of the chlorophyll molecule. Deficiency: yellowing (chlorosis), often between the veins and starting in the older leaves, so less photosynthesis and slower growth.
• Calcium ions: form calcium pectate in the middle lamella that holds neighbouring plant cell walls together. Deficiency: poor growth of new tissue, so young leaves and growing tips are stunted or die back.
• Water: a solvent and transport medium, a reactant in photosynthesis, and keeps cells turgid for support.

Method

Grow identical plants in culture solutions that each lack one ion. Duckweed (Lemna) is ideal because it is small, grows quickly and reproduces by budding off new fronds; seedlings such as wheat or radish also work.

1. Make a complete nutrient solution and solutions each missing one ion (no nitrate, no magnesium, no calcium), plus distilled water.

2. Put the same volume of each solution in identical containers; add the same number of healthy plants of the same species and similar size (for example 4 duckweed fronds).

3. Cover the sides of the containers (foil or black paper) so algae do not grow in the solution.

4. Keep all containers in the same light intensity, day length and temperature (a growth cabinet or the same windowsill, moving the positions each day).

5. After a set time (for example 14 days), record the number of fronds, or seedling height, mass or leaf number, and the leaf colour against a colour chart.

6. At least three containers for each solution; calculate means and standard deviations.

The complete solution is the control: it shows how plants grow when no ion is missing.

Processing and evaluating

• Use percentage change or the increase in frond number, so containers that started slightly differently can be compared.
• Leaf colour by eye is subjective: use a colour chart, or extract chlorophyll in ethanol and measure absorbance with a colorimeter.
• Dry mass is the most valid measure of growth, but drying kills the plants, so it can only be done at the end.
• Plants from the same parent (duckweed buds) reduce genetic variation.
• Containers must be washed and rinsed in distilled water: traces of ions on glassware or in tap water could supply the missing ion.
• A bar chart of mean frond number with standard deviation bars suits the categoric independent variable.

Duckweed example data: every container started with 4 fronds

Worked example

Using the example data, calculate the percentage increase in frond number in the complete solution and in the solution with no nitrate, and explain the difference.

1. Complete: (52 - 4) / 4 x 100 = 1200%
2. No nitrate: (14 - 4) / 4 x 100 = 250%
3. Without nitrate the plants cannot make enough amino acids, proteins, nucleotides or chlorophyll, so new fronds form much more slowly.

Answer: 1200% and 250%: growth is far lower without nitrate because nitrogen is needed for proteins, nucleic acids and chlorophyll.

Check yourself

1. Duckweed in a solution with no magnesium went from 4 to 22 fronds in 14 days. Calculate the percentage increase.
   Show answer

   450 %. (22 - 4) / 4 x 100 = 450%.

2. Three containers of duckweed in complete solution had 50, 55, 51 fronds. Calculate the mean.
   Show answer

   52. (50 + 55 + 51) / 3 = 52.

3. Calculate the standard deviation of 50, 55, 51 fronds, using s = √[Σ(x - x̄)² / (n - 1)], to 3 significant figures.
   Show answer

   2.65. Mean 52; squared differences 4, 9, 1 = 14; ÷ 2 = 7; s = 2.65.

4. In complete solution the mean number of fronds rose from 4 to 52 in 14 days. Calculate the mean rate of increase in fronds per day, to 2 significant figures.
   Show answer

   3.4 fronds per day. (52 - 4) / 14 = 3.43 fronds per day.

5. Plants grown without magnesium ions have yellow leaves. Why?
   • Magnesium is needed to make calcium pectate
   • Magnesium is a source of nitrogen for amino acids
   • Magnesium makes the leaves absorb yellow light
   • Magnesium is part of the chlorophyll molecule, so less chlorophyll is made
   Show answer

   Magnesium is part of the chlorophyll molecule, so less chlorophyll is made. Without magnesium, chlorophyll cannot be made, so leaves are pale (chlorosis).

6. Why do plants lacking calcium ions show poor growth of young tissue?
   • Calcium is part of chlorophyll
   • Calcium pectate is needed in the middle lamella between new cell walls
   • Calcium is needed to make amino acids
   • Calcium is the main solute for osmosis
   Show answer

   Calcium pectate is needed in the middle lamella between new cell walls. New cells need calcium pectate to hold their walls together.

7. What is the control in a mineral deficiency investigation?
   • Plants grown in distilled water only
   • Keeping all the plants at the same temperature
   • Using the same species of plant
   • Plants grown in a complete culture solution containing all the ions
   Show answer

   Plants grown in a complete culture solution containing all the ions. The complete solution shows normal growth for comparison; distilled water lacks every ion, so it does not isolate one ion.

8. Why are the sides of the culture containers covered with foil?
   • To keep the plants warm
   • To stop the plants photosynthesising
   • To stop algae growing in the solution and using the ions
   • To make the leaves greener
   Show answer

   To stop algae growing in the solution and using the ions. Algae would compete for ions and change the solutions.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills

Last updated 8 November 2026. Spotted a mistake? Tell us.