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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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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

Section 1 of 17Overview 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.

Section 2 of 17ATP 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.

Section 3 of 17Chloroplast 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.

Section 4 of 17Two 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).

Section 5 of 17Worked 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

Section 6 of 17The light-dependent reactions

The light-dependent reactions occur on the thylakoid membranes.

Their products are ATP, reduced NADP (NADPH) and oxygen.
Oxygen is a waste product (released by photolysis of water), while ATP and NADPH pass to the light-independent reactions.
The ADP and NADP produced in the light-independent reactions pass back to the thylakoids so that more ATP and NADPH can be made.
Photophosphorylation
ATP and NADPH are made by photophosphorylation: phosphorylation is the addition of phosphate, here to ADP to form ATP,
  • and the energy released as electrons pass down the electron transport chain is used to make ATP.

Section 7 of 17Non-cyclic photophosphorylation

This involves photosystems II and I and produces both ATP and NADPH. (Photosystem II comes first only because of the order in which the photosystems were discovered.)

Light energy hits photosystem II in the thylakoid membrane; two electrons in chlorophyll gain energy and are excited to a higher energy level and leave the photosystem.
The excited electrons pass along the electron transport chain, releasing energy as they do so.
Chemiosmosis
This energy drives chemiosmosis: H+ ions are pumped from a low concentration in the stroma into the thylakoid space, building a concentration gradient across the thylakoid membrane.
H+ ions flow back down the gradient through ATP synthase, which makes ATP from ADP and Pi.
Photolysis of water replaces electrons lost from photosystem II: H2O → 2H+ + 2e- + ½O2.
Light also excites electrons in photosystem I; these are picked up with H+ by NADP to form reduced NADP.

In cyclic photophosphorylation, only photosystem I is used and electrons return to it, making ATP only.

Section 8 of 17The light-independent reactions (Calvin cycle)

These reactions take place in the stroma.

1
They do not need light directly, but need ATP and reduced NADP from the light-dependent stage.
2
Rubisco
In the Calvin cycle:
  • Carbon dioxide combines with the 5-carbon ribulose bisphosphate (RuBP), catalysed by the enzyme rubisco, to form an unstable 6-carbon compound that splits into two molecules of the 3-carbon glycerate 3-phosphate (GP).
  • GP is reduced to glyceraldehyde 3-phosphate (GALP) (also called triose phosphate) using ATP and reduced NADP.
  • Five sixths of the GALP is used to regenerate RuBP (using ATP).
  • The rest is used to make glucose and other organic molecules.
3
Glucose is a 6-carbon molecule, so six turns of the cycle are required to produce one molecule of glucose.

Section 9 of 17Products of photosynthesis

Intermediates such as GP and GALP are used to make other biological molecules the plant needs: carbohydrates (glucose, then starch and cellulose), amino acids for protein synthesis (GP provides carbon skeletons, with nitrate supplying nitrogen), fatty acids and glycerol that form lipids such as triglycerides and phospholipids (fatty acids form the tails), and nucleotides for nucleic acids. The products therefore include amino acids, polysaccharides, lipids and nucleic acids.

Section 10 of 17Worked examples

  1. Explain how ATP is produced on the thylakoid membrane. [4]4 marks

  2. How many turns of the Calvin cycle are needed to make one glucose molecule? Explain. [2]2 marks

  3. State what happens to five sixths of the GALP. [1]1 mark

Section 11 of 17Principle of the Hill reaction

The light-dependent reactions take place on the thylakoid membrane.

High-energy electrons are released from chlorophyll in the photosystems and picked up by the coenzyme NADP along with hydrogen ions from photolysis,
  • forming reduced NADP (NADPH).
NADP is the electron acceptor.
Hill reaction
In the Hill reaction, an artificial electron acceptor replaces NADP.
DCPIP (dichlorophenolindophenol) and methylene blue are indicators that accept electrons and change from blue to colourless when reduced.
The rate at which the colour change happens shows the rate of the light-dependent reaction.

Section 12 of 17Method

1
Prepare an isolation solution of sucrose, potassium chloride and a buffer at pH 7.
  • It is kept cold and has the same water potential as the chloroplasts,
  • so that the chloroplasts do not burst or shrink by osmosis and enzymes are not damaged by extreme pH.
2
Grind leaves (e.g. spinach) with about 20 cm3 of ice-cold isolation solution in a pestle and mortar or blend for 10 seconds; this breaks apart the tissue and releases the chloroplasts.
3
Filter through muslin to remove debris, and centrifuge to concentrate the chloroplasts, keeping the extract on ice.
4
Set up tubes containing chloroplast extract and DCPIP, with a lamp at a fixed distance; use a colorimeter with a red filter to measure absorbance.
5
Take a sample at the start and then every minute for 10 minutes, recording absorbance.
6
Repeat at least twice more, keeping the distance from the lamp and the timings the same.

Controlsa tube kept in the dark (wrapped in foil) or a tube with boiled chloroplasts or without chloroplasts, to show that the colour change needs light and active chloroplasts.

Section 13 of 17Interpreting results

The absorbance reading should decrease over the 10 minute period because the DCPIP is reduced by electrons from the chloroplasts: the solution changes from blue to colourless, so less light is absorbed.

Interpreting results
The steeper the fall, the faster the rate of the light-dependent reaction.
Results can be improved by using different light intensities or wavelengths, keeping the temperature constant,
  • and calculating a mean rate from the repeats.
A tube in the dark should show no colour change, because no electrons are released without light energy.

Section 14 of 17Evaluating the Hill reaction practical

Limitations

  • DCPIP is reduced slowly, so the change may be difficult to see by eye, which is why a colorimeter is used;
    • chloroplasts become less active over time after isolation, so work quickly and keep extracts on ice;
    • stray light can affect the result, so keep the experiment shielded.
  • Plotting absorbance against time for each light intensity or distance from the lamp lets you compare initial rates from the gradient, and the rate of the light-dependent reaction should increase with light intensity until another factor becomes limiting.

Section 15 of 17Worked examples

  1. Explain why the leaves are ground in cold isolation solution containing sucrose. [3]3 marks

  2. Why does absorbance fall during the experiment? [2]2 marks

  3. State the purpose of a tube in the dark. [1]1 mark

Section 16 of 17Key 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.
TermMeaning
PhotophosphorylationMaking ATP using light energy.
ChemiosmosisMovement of ions down a gradient through ATP synthase to make ATP.
PhotolysisLight-driven splitting of water.
RubiscoEnzyme that fixes CO2 onto RuBP.
GALP3-carbon sugar phosphate made in the Calvin cycle.
RuBP5-carbon CO2 acceptor, regenerated in the Calvin cycle.
TermMeaning
Hill reactionReduction of an artificial electron acceptor by isolated chloroplasts in light.
DCPIPBlue redox dye that becomes colourless when reduced.
ColorimeterInstrument that measures absorbance of light by a solution.
Isolation mediumCold buffered solution with a water potential matching the chloroplasts.

Section 17 of 17Exam technique

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.
1
Saying the light-independent reactions can occur in the dark without any dependence on light — they need ATP and NADPH.
2
Saying oxygen comes from carbon dioxide.
3
Saying photosystem I is first in the sequence.
4
Saying GP is the 5-carbon compound.
Avoid these mistakes
Saying DCPIP turns blue when reduced.
Using a high temperature that denatures enzymes.
Forgetting that DCPIP replaces NADP as electron acceptor.

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