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

10 min read 17 parts Lesson 12 of 34 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

Photosynthesis Overview, ATP and Chloroplasts — EDEXCEL A LEVEL BIOLOGY A (SNAB) 9BI0 — REVISION NOTES notes: Summarise photosynthesis, explain the role of ATP and relate chloroplast structure to function.Photosynthesis is a series of chemical reactions in producers such as plants and algae.

  • chloroplast envelope
  • Thylakoids
  • light-dependent reactions
  • light-independent reactions
  • Worked example 1
  • Worked example 2

Part 1 of 5Overview of photosynthesis

Photosynthesis is a series of chemical reactions in producers such as plants and algae.

1
It converts light energy into chemical energy stored in the biomass of producers.
2
Light energy is used to split strong bonds in water molecules (H2O), releasing hydrogen and oxygen.
3
The hydrogen combines with carbon dioxide and is stored in glucose, which fuels respiration; oxygen is released as a waste product.
4
The overall equation is 6CO2 + 6H2O + light energy → C6H12O6 + 6O2.

Part 2 of 5ATP as an energy supply

All organisms need a constant supply of energy to maintain their cells and stay alive: for building new molecules from the products of digestion (anabolic reactions), for moving substances across membranes by active transport or moving them within cells, and for muscle contraction.

ATP
The immediate energy source is ATP (adenosine triphosphate), which contains adenine, a ribose sugar and three phosphate groups.
Removal of one phosphate creates ADP (adenosine diphosphate) and removal of two creates AMP.
ATP is hydrolysed to ADP and inorganic phosphate (Pi) by the enzyme ATP hydrolase, releasing a small, manageable amount of energy.
ATP is regenerated by adding Pi to ADP, in a condensation reaction catalysed by ATP synthase, using energy from respiration (or from light in photosynthesis).
ATP is a good energy currency because it releases energy in small amounts, quickly and in one step, and is soluble, but it cannot be stored, so it is constantly recycled.

Part 3 of 5Chloroplast structure and function

Chloroplasts are the organelles in plant cells where photosynthesis occurs. Each is surrounded by a double membrane called the chloroplast envelope, each membrane being a phospholipid bilayer. The double membrane encloses the chloroplast, keeping together all the components needed for photosynthesis, and transport proteins in the inner membrane control the flow of molecules between the stroma and the cytoplasm.

Chloroplast structure and function
Thylakoids
Thylakoids flattened sacs of membrane containing chlorophyll and other pigments, forming the site of the light-dependent reactions; stacked into grana, which give a large surface area for light absorption and ATP production.
Stroma
Stroma the fluid matrix that contains enzymes (including rubisco) for the light-independent reactions, plus starch grains, lipid droplets, ribosomes and a loop of DNA.
Lamellae
Lamellae membranes linking grana.

Part 4 of 5Two stages of photosynthesis

Photosynthesis takes place in two stagesthe light-dependent reactions, which rely directly on light, and the light-independent reactions, which do not use light directly but depend on the products of the light-dependent reactions (ATP and reduced NADP).

Part 5 of 5Worked examples

  1. Name the products of the light-dependent stage that are used in the light-independent stage. [2]2 marks

  2. Explain how the structure of the chloroplast is suited to photosynthesis. [4]4 marks

  3. Why is ATP a suitable immediate energy source? [2]2 marks

Key terms

TermMeaning
ATPMolecule that is the immediate energy source in cells.
ADPAdenosine diphosphate.
ThylakoidMembrane sac in the chloroplast containing chlorophyll.
GranumStack of thylakoids.
StromaFluid matrix of the chloroplast.

Avoid these mistakes

1
Saying oxygen comes from carbon dioxide — it comes from water.
2
Saying ATP is stored in large amounts.
3
Saying chlorophyll is in the stroma — it is on the thylakoid membranes.
4
Saying ATP is used up permanently rather than being recycled.

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