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5.4.2Energy Transfers In and Between Organisms (A Level only)

Investigating Minerals and Plant Growth — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 24 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

5.4.2 Investigating Minerals and Plant Growth — AQA A Level Biology 7402 notes: Plan and evaluate an investigation into how mineral availability affects plant growth, including the use of controls and statistics.Mineral ions are absorbed from the soil by root hair cells, mostly by active transport.

  • culture solution
  • Student's t-test
  • correlation
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Part 1 of 6Why minerals matter to plants

Mineral ions are absorbed from the soil by root hair cells, mostly by active transport. They are needed in small quantities but are essential for building specific molecules. Deficiency produces characteristic symptoms:

IonUsed forDeficiency symptom
Nitrate (N)Amino acids, proteins, nucleotides, chlorophyllStunted growth, yellowing of older leaves (chlorosis)
Phosphate (P)ATP, nucleotides, phospholipidsPoor root growth, purple-tinged leaves
Magnesium (Mg2+)Centre of chlorophyllChlorosis between veins, especially in older leaves
Potassium (K+)Enzyme activation, stomatal opening, osmotic balanceYellow edges to leaves, premature leaf death

Yellowing is caused by a failure to make chlorophyll, which reduces light absorption and therefore the light-dependent reaction and rate of photosynthesis.

Part 2 of 6Planning the investigation

A standard method uses seedlings grown in culture solution (hydroponics) so the concentration of each ion can be controlled exactly.

Prepare a set of solutions: a complete culture solution as the control,
  • and others each lacking one mineral made up from separate salts.
    • (for example no nitrate, no magnesium)
  • A further set can use a range of concentrations of one mineral.
Use the same species, age and mass of seedlings, with the same number in each container.
  • Many replicates reduce the effect of individual variation.
    • (at least 5 per treatment)
Keep the volume of solution, light intensity, temperature, and duration constant.
  • Use opaque containers or foil to prevent algae growing,
    • and aerate the solution with an air pump so roots can respire aerobically;
      • otherwise active transport of ions would be affected.
Grow for a set period (e.g. 3–4 weeks), changing the solution regularly.
Dry mass
Measure the response variable: height, number of leaves, leaf area,
  • or best of all the dry mass of the plants, plus observation of leaf colour.
    • (dried to constant mass)

Part 3 of 6Handling results and statistics

Calculate the mean growth for each treatment, with standard deviation to show variation.

Present as a table and bar chart, with error bars of ±1 standard deviation (or 95% confidence intervals).
When error bars do not overlap, the difference between means is likely to be significant.
Student's t-test
A Student's t-test can compare the means of two treatments (for example complete solution versus nitrate-free); the null hypothesis is that there is no significant difference.
If the calculated t exceeds the critical value at p = 0.05, the null hypothesis is rejected.
Correlation
Where there is a range of concentrations, a correlation test may show whether growth increases with concentration,
  • but correlation does not prove cause.

Where measuring dry mass is used, remember that it is destructive, so each replicate is a different plant; fresh mass is not a good measure because of variation in water content.

Part 4 of 6Evaluation and sources of error

1
The plants may differ genetically; reduce this with seeds from one pure-bred variety.
2
Reagent salts can carry trace contaminants, so a "lacking" solution may contain a little of the mineral.
3
Seeds contain their own reserves of minerals, so deficiency symptoms take time; use seedlings of the same age and a long enough period.
4
Light intensity varies across a bench; rotate or randomise the positions.
5
Small sample sizes make results unreliable. Increase replicates.

Hydroponics differs from soil, where mineral availability depends on pH, soil structure and microorganisms, so results cannot be applied directly to a field without further trials.

Part 5 of 6Worked data example

Suppose five replicate groups of ten seedlings are grown for four weeks and the mean dry masses per plant are: complete solution 0.84 g, no nitrate 0.31 g, no magnesium 0.58 g, no phosphate 0.42 g.

Worked data example
The percentage reduction in the nitrate-free group is (0.84 − 0.31) / 0.84 × 100 = 63%.
This indicates that nitrate has a larger effect on growth than magnesium, whose deficiency reduces chlorophyll but still leaves some photosynthesis (31% reduction).
You should state the comparison with the control, give the calculation,
  • and offer the biological explanation: nitrate is needed for amino acids and enzymes,
  • so protein synthesis and cell division are affected at once.

When the question asks how to make the investigation more valid, mention that only one variable is changed at a time, the volume of solution per plant is equal, and the potential sources of error such as seed reserves are reduced. For ethics, plants raise no significant issues, which makes them suitable for large replicates.

Part 6 of 6Exam-style questions

  1. A student grows seedlings in culture solution lacking magnesium. Predict and explain the appearance of the leaves. [3]3 marks

  2. Suggest why the culture solution is aerated. [2]2 marks

  3. Give two ways of reducing the effect of variation between plants in this experiment. [2]2 marks

Key terms

TermMeaning
Culture solutionWater containing a known mix of mineral ions for growing plants without soil.
ChlorosisYellowing of leaves due to lack of chlorophyll.
ControlTreatment that does not receive the independent variable change, used for comparison.
ReplicateRepeat of a treatment to allow a mean to be calculated.
Null hypothesisStatement that there is no significant difference or correlation.
Dry massMass of tissue after removing all water.

Avoid these mistakes

1
Measuring fresh mass instead of dry mass.
2
Not including a complete culture solution as a control.
3
Forgetting to aerate the solution.
4
Concluding that a correlation proves a mineral causes growth without controlling other variables.

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