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B8.3 How Plants Use Glucose

GCSE Combined Science · AQA · Chapter B8: Photosynthesis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: respiration, starch, fats and oils, cellulose, and proteins made 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 five ways a plant uses the glucose made in photosynthesis
  • Explain why plants store carbohydrate as insoluble starch
  • Explain why plants need nitrate ions from the soil to make proteins
  • Use data about the stores in seeds and other plant organs in calculations

Key words

  • Respiration: The reaction in every living cell that transfers energy from glucose.
  • Starch: An insoluble carbohydrate made of many glucose molecules joined together.
  • Cellulose: A carbohydrate made from glucose that forms strong plant cell walls.
  • Amino acids: Small molecules that join to make proteins. Plants make them from glucose and nitrate ions.
  • Nitrate ions: Mineral ions absorbed from the soil, usually by active transport. They supply nitrogen.
  • Insoluble: Does not dissolve in water, so it has no effect on osmosis.

Five uses of glucose

Glucose made in the leaves is used there, or changed to sugars that are carried in the phloem to growing tips, roots, fruits and seeds. You need to know five uses:

  • 1 Respiration; Why the plant needs it Releases energy for active transport, building large molecules and cell division. Plants respire day and night.
  • 2 Insoluble starch; Why the plant needs it Stored in leaves, roots, tubers (such as potatoes) and seeds, and turned back into glucose when needed.
  • 3 Fats and oils; Why the plant needs it Stored mainly in seeds, e.g. sunflower and oilseed rape. They store a lot of energy in a small mass.
  • 4 Cellulose; Why the plant needs it Strengthens cell walls so cells, and the plant, keep their shape.
  • 5 Amino acids → proteins; Why the plant needs it Glucose combines with nitrate ions to make amino acids, which join to make proteins such as enzymes.

Why store starch, not glucose?

Glucose is soluble. A lot of dissolved glucose would make the cell contents more concentrated, so water would move in by osmosis and the cell could swell. 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?” make two linked points: it is insoluble, so it has no osmotic effect / does not draw water into the cell.

Making proteins needs nitrate

Glucose contains only carbon, hydrogen and oxygen, but amino acids also contain nitrogen. Plants absorb it as nitrate ions from the soil. Root hair cells use active transport, because nitrate is usually more concentrated inside the root cells than in the soil water. The energy comes from respiration.

Without enough nitrate a plant cannot make enough protein, so it grows slowly and its older leaves turn yellow. Farmers add nitrate fertilisers to replace the nitrate removed when crops are harvested.

Worked example: explaining a result

Two trays of radish seedlings are grown in sand. Tray 1 gets a full mineral solution; tray 2 gets the same solution without nitrate. After 3 weeks tray 2 plants are much smaller. Explain why.
Without nitrate the plants cannot make amino acids, so they cannot make proteins.
Proteins are needed for new cells and enzymes, so growth is reduced.

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

Using data on plant stores

Different seeds store different products. The chart shows approximate typical values.

Worked example: a mass calculation

A farmer harvests 3.5 tonnes of dry oilseed rape. Estimate the mass of oil, using the chart (42%).
Oil = 0.42 × 3.5 = 1.47 tonnes.

Answer: About 1.47 tonnes of oil

Worked example: a ratio

In broad bean seeds, what is the ratio of starch to protein in the form n : 1?
40 : 26 = (40 ÷ 26) : 1 = 1.54 : 1 (to 3 significant figures).

Answer: 1.54 : 1

Common mistakes

  • Writing “glucose is used for energy”. Say “used in respiration, which releases energy”.
  • Thinking plants only respire at night. They respire all the time.
  • Saying starch is stored because it is soluble. It is insoluble.
  • Forgetting nitrate when explaining how plants make proteins.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows five ways a plant uses glucose. The box marked ? is a carbohydrate used to strengthen plant cell walls. What is it? (Choose from: Starch, Cellulose, Glycogen, Sucrose.)

Answer: Cellulose

Working: Glucose is used to make cellulose, which strengthens plant cell walls.

Question 2. Why do plants store glucose as starch rather than as glucose? (Choose from: Starch is soluble, so it moves easily around the plant, Starch is insoluble, so it does not affect the movement of water into and out of cells, Starch releases more energy in respiration than glucose, Starch can be used directly in photosynthesis.)

Answer: Starch is insoluble, so it does not affect the movement of water into and out of cells

Working: Starch is insoluble, so storing it has no osmotic effect: water does not rush in. It is also compact.

Question 3. Plants combine glucose with ions from the soil to make amino acids. Name these ions.

Answer: nitrate

Working: Nitrate ions, absorbed by the roots, supply the nitrogen needed to make amino acids and then proteins.

Related lessons

Continue in the full course

This lesson is section B8.3 (AGCSC0803) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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