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Structures and Functions in Living Organisms

Nutrition — Edexcel Linear - OL Revision Notes & Topical Questions

15 min read 26 parts Lesson 12 of 25 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Lesson summary

Plant Nutrition: Minerals and Photosynthesis Practicals — EDEXCEL LINEAR - OL notes: Explain why plants need mineral ions and carry out and evaluate the practicals that show oxygen release and the need for light and carbon dioxide.Photosynthesis provides carbohydrates, but plants…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Worked example 5
  • Iodine solution

Part 1 of 5Why plants need more than carbohydrates

Healthy plant compared with nitrate and magnesium deficiency
Healthy plant compared with nitrate and magnesium deficiency

Photosynthesis provides carbohydrates, but plants also contain and need many other biological molecules: proteins, lipids and nucleic acids (DNA).

As plants do not eat, they must make these themselves.
Nitrogen
Carbohydrates contain carbon, hydrogen and oxygen, but proteins also contain nitrogen (and certain amino acids other elements).
Nitrate
Two fundamental mineral ions are nitrate and magnesium; without them plants cannot photosynthesise or grow properly.
Mineral ionUsed to makeDeficiency symptom
NitrateAmino acids and proteinsStunted growth, yellow older leaves
MagnesiumChlorophyllYellow leaves (chlorosis), especially between the veins

Part 2 of 5Practical: oxygen released by a water plant

Investigating light intensity and photosynthesis using pondweed and oxygen collection
Investigating light intensity and photosynthesis using pondweed and oxygen collection

Oxygen produced by photosynthesis can be shown using a water plant such as Elodea (pondweed).

Practical: oxygen released by a water plant
Place the pondweed in a beaker of water (with sodium hydrogencarbonate as a source of carbon dioxide) under an inverted funnel and test tube.
Shine a bright light on it.
Oxygen is released as bubbles from the cut end of the pondweed; collect the gas in the test tube.

As the plant is in water, the oxygen released can be seen as bubbles; the rate of bubbles shows the rate of photosynthesis.

Part 3 of 5Practical: testing a leaf for starch to show light is needed

Testing a leaf for starch with hot water, ethanol in a water bath and iodine
Testing a leaf for starch with hot water, ethanol in a water bath and iodine

The glucose made is quickly used up, converted into other substances and transported or stored as starch, so a leaf cannot be reliably tested for glucose. Starch is stored in chloroplasts where photosynthesis occurs, so testing a leaf for starch is a reliable indicator of where photosynthesis has happened.

De-starch
De-starch the plant by placing it in a dark cupboard for 24 hours so any starch is used up.
Cover part of a leaf with aluminium foil (or a stencil) and place the plant in bright light.
Remove the leaf and dip it in boiling water for about a minute to kill it and stop reactions.
Boil it in ethanol in a water bath (never heat ethanol directly,
  • because it is extremely flammable;
    • use an electric water bath or water from a just-boiled kettle) to remove the chlorophyll.
Rinse in water to soften the leaf, spread it on a white tile and cover with iodine solution.

The exposed area turns blue-black (starch present); the covered area stays yellow-brown or orange-brown, because without light it could not photosynthesise and the cells broke down starch to use the glucose in respiration.

Practical: testing a leaf for starch to show light is needed
This proves that light is needed for photosynthesis.
For a partly covered leaf,
  • such as a strip or triangle of foil, most of the leaf turns blue-black with an orange-brown strip containing a blue-black triangle in the centre.

Part 4 of 5Practical: carbon dioxide and the CORMS evaluation

To show carbon dioxide is needed, de-starch the plant, then enclose one leaf in a conical flask containing potassium hydroxide (which absorbs carbon dioxide) and another leaf in a control flask without potassium hydroxide.

Practical: carbon dioxide and the CORMS evaluation
After light exposure test both for starch.
The leaf from the potassium hydroxide flask stays orange-brown (no CO2, no photosynthesis); the control leaf turns blue-black.

Evaluate with CORMS: Change one thing (light or no light), Observe/measure, Repeat to make results reliable, Measure the outcome, keep the leaves from the same plant or species and of the same age and size, Same conditions otherwise (controlled variables).

Part 5 of 5Worked examples

  1. Why does a plant need nitrate ions? [2]2 marks

  2. Describe how to remove chlorophyll from a leaf before the starch test and why this is done. [3]3 marks

  3. A leaf partly covered with foil is tested with iodine. State and explain the result. [3]3 marks

  4. Why is the plant put in the dark for 24 hours before the experiment? [2]2 marks

  5. What is the purpose of potassium hydroxide in the carbon dioxide experiment? [1]1 mark

Key terms

TermMeaning
NitrateA mineral ion needed to make amino acids and proteins.
MagnesiumA mineral ion needed to make chlorophyll.
De-starchRemove starch from a plant by keeping it in the dark.
Iodine solutionReagent that turns blue-black with starch.
ControlA set-up where the factor being tested is kept unchanged for comparison.

Avoid these mistakes

1
Heating ethanol directly with a flame — use a water bath.
2
Testing the leaf for glucose rather than starch.
3
Forgetting to de-starch the plant first.
4
Saying magnesium is needed to make proteins — it is needed for chlorophyll.

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