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5.2.6Topic 5 · On the Wild Side

Products of Photosynthesis — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 18 of 34 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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5.2.6 Products of Photosynthesis — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how the products of photosynthesis are used by plants and how the products of the Calvin cycle are converted into other organic molecules.Oxygen, produced by photolysis, is the immediate gaseo…

  • triose phosphate (TP)
  • amino acids
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Amyloplast

Section 1 of 8Immediate and intermediate products

Oxygen, produced by photolysis, is the immediate gaseous product.

Some is used by the plant's mitochondria in respiration, and the rest diffuses out through the stomata.
Triose phosphate (TP)
The main organic product of the Calvin cycle is triose phosphate (TP), which is converted to other substances.
Because TP is a three-carbon sugar, plants can combine two molecules to make glucose, or reorganise carbon skeletons into a wide range of molecules.

Section 2 of 8Carbohydrates

A large proportion of the carbon fixed is used to make carbohydrates:

Glucose
Glucose is used in respiration to release energy as ATP, and as a building block.
Sucrose
(glucose + fructose) is the main transport sugar.
  • It is soluble, not very reactive, and is carried in the phloem to sinks such as roots, fruits and growing tips.
Starch
Starch is the storage polysaccharide, made from alpha glucose and stored in chloroplasts temporarily and in amyloplasts in seeds, roots and tubers.
  • It is insoluble and compact, so does not affect water potential.
Cellulose
Cellulose from beta glucose is used for the cell walls of new cells, and is needed for growth.

Starch accumulates in leaves during the day, and is broken down at night to sucrose, which is exported so that the plant can continue to grow when it is dark.

Section 3 of 8Lipids, proteins and nucleic acids

Some carbon from TP and GP is converted to lipids: glycerol is derived from triose phosphate and fatty acids are built from two-carbon units.

Lipids are used for membranes, and as energy stores in oil-rich seeds such as sunflower and rape.
Amino acids
To make amino acids, plants need nitrogen taken up as nitrate; carbon skeletons from the Calvin cycle and from respiration receive an amino group.
The amino acids are assembled into proteins including enzymes, such as rubisco, which is probably the most abundant protein on Earth.
Similarly, nucleotides need nitrogen and phosphate ions; thus plants need mineral nutrients, not only the products of photosynthesis.
This is why a nitrate deficiency stunts growth even where light is plentiful (see 4.2.3).

Section 4 of 8Measuring and applying productivity

ProductUse in the plant
GlucoseRespiration, synthesis
SucroseTransport in phloem
StarchStorage
CelluloseCell walls
LipidsMembranes, storage
Amino acids and proteinsEnzymes, structure

The rate of photosynthesis can be measured in a number of ways: counting the bubbles or measuring the volume of oxygen released by water plants such as Elodea under different light intensities; using a photosynthometer; measuring uptake of CO2 using a CO2 sensor or hydrogencarbonate indicator; or measuring increase in dry mass.

Measuring and applying productivity
Dry mass over time is the basis of net primary productivity (5.1.6).
Distance of a lamp from the plant can be used to vary light intensity, with light intensity proportional to 1 / distance2.
A heat shield, such as a water bath, avoids temperature changes.

Products end up in the human diet as food, fibre, fuel and many medicines, which is the connection with the resources from plants studied in 4.2.

Section 5 of 8Seasonal and agricultural applications

Plants adjust the use of their products over time.

1
In temperate deciduous trees, sugars made in summer are stored as starch in roots and stems and used in spring to produce new leaves before photosynthesis restarts.
2
Seeds and fruits are important sinks for lipid, starch and protein, and humans exploit this by growing cereals, potatoes, oil seed rape and beans.
3
Farmers can increase the proportion of photosynthetic product that ends in the harvested part: dwarf wheat varieties put more into grain and less into stem, an example of how selective breeding changes the allocation of resources.
4
Understanding the products also helps to explain nutrient requirements: a crop making protein-rich seeds requires more nitrate,
  • whereas one making starch might need less.
5
Greenhouse growers measure yield as dry mass per unit area,
  • and economic analysis compares the extra yield with the cost of fertiliser, lighting and heating, a common context for evaluation questions.

Section 6 of 8Exam-style questions

  1. Explain why plants transport sucrose rather than glucose in the phloem. [2]2 marks

  2. Describe how the products of the Calvin cycle are used to make a protein. [3]3 marks

  3. In an experiment using a pondweed, 12 bubbles per minute were recorded with the lamp at 10 cm and 3 bubbles per minute at 20 cm. Calculate the relative light intensity (1/d2) at each distance, and the ratio, and comment. [3]3 marks

Section 7 of 8Key terms

TermMeaning
Triose phosphateThe three-carbon product of the Calvin cycle used to make other molecules.
SucroseDisaccharide transported in the phloem.
StarchStorage polysaccharide of plants made from alpha glucose.
CellulosePolysaccharide of beta glucose in cell walls.
AmyloplastPlastid specialised for starch storage.
PhotosynthometerApparatus for measuring the volume of oxygen produced by a water plant.
SinkPart of a plant that uses or stores sugars.
Dry massMass of a sample after water removal, used to estimate biomass.

Section 8 of 8Exam technique

1
Believing that the only product of photosynthesis is glucose.
2
Forgetting that nitrogen from the soil is needed to make amino acids.
3
Saying starch is transported in the phloem: the transport sugar is sucrose.
4
Suggesting that photosynthesis takes place only in the day and respiration only at night.
5
Using distance rather than 1/d2 as light intensity in the calculation.

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