5.3.2Energy Transfers In and Between Organisms (A Level only)

Photosynthetic Products — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 14 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision

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5.3.2 Photosynthetic Products — AQA A Level Biology 7402 notes: Describe how the products of photosynthesis are used and stored by plants and passed on to consumers.The immediate product of the Calvin cycle that leaves the cycle is triose phosphate (TP).For every six turns of the…

Carbohydrates
Amino acids and proteins
Nucleotides
Respiration
Defence and signalling
compensation point

Section 1 of 8From triose phosphate to complex molecules

The immediate product of the Calvin cycle that leaves the cycle is triose phosphate (TP).

From triose phosphate to complex molecules

For every six turns of the cycle, twelve molecules of TP are formed, but only two leave to make a hexose; ten are needed to regenerate RuBP.

From TP a plant can build almost every organic molecule it needs, using minerals absorbed from the soil.

Carbohydrates

TP is converted to glucose, then to sucrose for transport, starch for storage and cellulose for cell walls.

Lipids

Lipids glycerol and fatty acids made from TP and from acetyl groups form triglycerides and phospholipids for membranes and energy-rich seed stores.

Amino acids and proteins

Amino acids and proteins carbon skeletons from the cycle combine with nitrate-derived nitrogen to make amino acids, then enzymes and structural proteins.

Nucleotides

Nucleotides need phosphate and nitrogen for DNA, RNA and ATP.

Because a plant needs nitrogen, phosphorus, magnesium and other elements to complete these syntheses, mineral shortage limits how much of the fixed carbon can be turned into useful structures, even if light and carbon dioxide are plentiful.

Section 2 of 8Storage and transport

Plants store carbohydrate as starch, an insoluble polymer of α-glucose made of amylose and amylopectin. Being insoluble, starch does not change the water potential of cells and is compact. It is stored in chloroplasts temporarily and in amyloplasts of tubers, seeds and roots for longer periods. Starch is hydrolysed back to glucose when needed for respiration.

Sugars are moved from sources (leaves) to sinks (roots, growing shoots, fruits, storage organs) as sucrose in the phloem.

Storage and transport

Sucrose is soluble, non-reducing and relatively unreactive, which suits transport.

This movement is called translocation and is explained by the mass flow hypothesis: sucrose is actively loaded into the phloem at the source, lowering water potential so water enters by osmosis, and the resulting hydrostatic pressure pushes sap towards the sink where sucrose is removed.

Section 3 of 8Uses of the products inside the plant

The organic products are used in several ways:

Respiration

Respiration glucose is oxidised to release ATP for active transport of minerals, protein synthesis, cell division and growth. A plant respires all the time, including in the light.

Growth

Growth cellulose for new cell walls, proteins and lipids for new cytoplasm and membranes.

Storage

Storage starch, oils and, in some species, fructans for use in winter or germination.

Defence and signalling

Defence and signalling some products are made into secondary metabolites such as tannins and alkaloids that deter herbivores.

The balance between photosynthesis and respiration changes with the light intensity. At the compensation point the rate of photosynthesis exactly equals the rate of respiration, so there is no net gas exchange; below it the plant uses stored reserves.

Section 4 of 8Passing products to consumers

Herbivores eat plant tissue and digest the large organic molecules into small soluble ones that are absorbed.

The absorbed carbon, nitrogen and phosphorus are rebuilt into the herbivore's own molecules.

A carnivore then eats the herbivore.

Cellulose is very hard for many animals to digest, so a large amount of the plant's chemical energy is not absorbed and leaves as faeces, which becomes available to decomposers.

This link between the chemistry of products and what animals can digest affects how efficiently energy passes through a food chain and is developed in the topics on biomass and efficiency.

Product	Main use	Why suited
Sucrose	Transport	Soluble, unreactive
Starch	Storage	Insoluble, compact, osmotically inactive
Cellulose	Cell wall	Strong microfibrils held by hydrogen bonds
Lipid	Membranes, seed oil	Hydrophobic; high energy density

Section 5 of 8Linking to other topics

The fate of photosynthetic products connects several areas of the course.

Respiration of the sugars made links to the ATP cycle and to the mitochondrion.

Transport of sucrose links to mass flow in the phloem and to the structure of sieve tube elements and companion cells.

The use of nitrate and phosphate links to nutrient cycles and to fertilisers.

When examination questions ask where a particular atom in a molecule came from, trace it: the carbon in a plant's cellulose came from atmospheric carbon dioxide fixed by rubisco, the hydrogen from water split in photolysis, and the nitrogen from nitrate taken up through root hair cells by active transport.

Section 6 of 8Exam-style questions

Explain why plants store carbohydrate as starch rather than glucose. [3]3 marks
Name two substances made from the products of photosynthesis that contain nitrogen and give one use of each. [2]2 marks

Section 7 of 8Key terms

Term	Meaning
Triose phosphate	Three-carbon sugar phosphate formed from GP, the starting point for synthesising organic molecules.
Starch	Insoluble storage polysaccharide of alpha-glucose.
Sucrose	Soluble disaccharide used to transport carbohydrate in phloem.
Translocation	Movement of organic solutes in phloem from source to sink.
Compensation point	Light intensity at which photosynthesis rate equals respiration rate.
Sink	Region of a plant where sugars are used or stored.

Section 8 of 8Exam technique

Saying plants transport starch in the phloem. Sucrose is the transport sugar.

Forgetting that plants also respire; not all glucose made is stored.

Stating that glucose is made directly in the light-dependent reaction. It is made from TP in the light-independent stage.

Describing cellulose as a storage molecule. It is structural.

Last updated: 13 October 2026