8.3 How Plants Use Glucose

A free GCSE Biology lesson for AQA (8461) from OBA School: respiration, storage as starch, fats and oils, cellulose, and making proteins with nitrate ions. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• List the ways a plant uses the glucose it makes in photosynthesis
• Explain why plants store glucose as insoluble starch
• Explain why plants need nitrate ions from the soil to make proteins
• Interpret data about the stores in seeds and other plant organs

Key words

• Respiration: The reaction in every living cell that transfers energy from glucose.
• Starch: An insoluble carbohydrate made from many glucose molecules joined together. The main plant store.
• Cellulose: A carbohydrate made from glucose that forms strong plant cell walls.
• Amino acids: The small units that join to make proteins. Plants make them from glucose and nitrate ions.
• Nitrate ions: Mineral ions absorbed from the soil by active transport. They supply the nitrogen in amino acids.
• Insoluble: Does not dissolve in water, so it does not affect the water concentration in a cell.

Five main uses of glucose

Glucose made in the leaves is used in the leaf, or turned into sucrose and carried in the phloem to other parts of the plant. AQA expects you to know these uses:

• Respiration; Why the plant needs it Releases energy for active transport, building molecules, growth and cell division. Plants respire day and night.
• Starch for storage; Why the plant needs it Kept in leaves, roots, tubers (e.g. potatoes) and seeds, ready to turn back into glucose.
• Fats and oils for storage; Why the plant needs it Common in seeds such as sunflower and oilseed rape. They store a lot of energy in a small mass.
• Cellulose; Why the plant needs it Strengthens cell walls so cells and the whole plant keep their shape.
• Amino acids → proteins; Why the plant needs it Glucose is combined with nitrate ions to make amino acids, which join to make proteins such as enzymes.

Why store starch, not glucose?

Glucose is soluble. Lots of dissolved glucose would lower the water concentration inside a cell, so water would move in by osmosis and upset the cell. Starch is insoluble, so it can be stored in large amounts without affecting the water balance. It is also compact.

Exam tip: for “why starch?” give two points: it is insoluble, so it has no osmotic effect on the cell. Then you can add that it is compact.

Making proteins needs nitrate

Glucose contains only carbon, hydrogen and oxygen. Amino acids also contain nitrogen. Plants get it as nitrate ions from the soil. Root hair cells absorb nitrate by active transport, because the concentration in the soil is usually lower than in the root cells.

A plant short of nitrate cannot make enough protein, so it grows slowly and its older leaves turn yellow. Farmers add nitrate fertilisers to replace the nitrate taken away when crops are harvested.

Worked example: explaining stunted growth

Two sets of bean plants are grown in sand. One set gets a full mineral solution, the other gets the same solution without nitrate. After 4 weeks the second set is much smaller. Explain why.
Without nitrate the plants cannot make amino acids, so they cannot make proteins.
Proteins are needed to build new cells and make enzymes, so growth is limited.

Answer: No nitrate → no amino acids → no proteins → less growth

Working with data on plant stores

Different plant organs store different products. The values below are typical approximate percentages by mass of dry seed.

• Wheat grain; Starch (%) 65; Oil (%) 2; Protein (%) 13
• Dried pea; Starch (%) 45; Oil (%) 2; Protein (%) 23
• Sunflower seed; Starch (%) 2; Oil (%) 50; Protein (%) 21

Worked example: a mass calculation

A farmer harvests 2.5 tonnes of dry sunflower seed. Estimate the mass of oil in it, using the table.
Oil = 50% of 2.5 tonnes = 0.50 × 2.5 = 1.25 tonnes.

Answer: About 1.25 tonnes of oil

Common mistakes

• Saying plants use glucose “for energy”. Write “for respiration, which releases energy”.
• Thinking plants only respire at night. They respire all the time; in daylight photosynthesis is usually faster.
• Saying starch is stored because it is soluble. It is stored because it is insoluble.
• Forgetting nitrate when asked how plants make proteins. Glucose alone has no nitrogen.

Revision questions

Try these, then check your answers below each one.

Question 1. Why do plants store glucose as starch? (Choose from: Starch is soluble and moves easily, Starch is insoluble so it does not affect the water balance of cells, Starch releases more energy than glucose, Starch is needed to make cell walls.)

Answer: Starch is insoluble so it does not affect the water balance of cells

Working: Starch is insoluble, so stored starch does not cause water to move into cells by osmosis.

Question 2. Name the carbohydrate made from glucose that strengthens plant cell walls.

Answer: cellulose

Working: Cellulose is made from glucose and forms strong cell walls.

Question 3. Plants make amino acids from glucose and which ions absorbed from the soil? (Choose from: Magnesium ions, Nitrate ions, Sodium ions, Chloride ions.)

Answer: Nitrate ions

Working: Nitrate ions supply the nitrogen needed to make amino acids, which join to make proteins.

Related lessons

• Previous lesson: 8.2 The Rate of Photosynthesis
• Next lesson: 8.4 Making the Most of Photosynthesis
• In this chapter: 8.1 Photosynthesis
• In this chapter: 8.5 Required Practicals: Photosynthesis

Continue in the full course

This lesson is section 8.3 (AGCBH0803) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.