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

Section 1 of 8Photosynthesis

Essential knowledge

Photosynthesis is the route by which energy enters almost all ecosystems: plants are autotrophs that build organic molecules from CO₂ and water, and these molecules supply energy and carbon to consumers.

ATP is a phosphorylated nucleotide; energy is released when the terminal phosphate is removed by hydrolysis (ATPase). ATP is made in photosynthesis by photophosphorylation and in respiration by oxidative and substrate-level phosphorylation. It is an immediate, small-packet energy source, not an energy store.

Chlorophyll molecules and accessory pigments are grouped in photosystems in the thylakoid membranes. Light energy raises electrons to a higher energy level; they are passed to an electron acceptor and then along electron carriers (a series of redox reactions).

As electrons pass along the chain, energy released pumps protons (H⁺) into the thylakoid space. Protons flow back into the stroma through ATP synthase channels, providing energy to join ADP and P_i (chemiosmosis).

Water is split by light (photolysis). Electrons replace those lost by chlorophyll, protons join with electrons to reduce NADP (NADP + 2H⁺ + 2e⁻ → reduced NADP), and oxygen diffuses out of the leaf or is used in respiration.

In non-cyclic photophosphorylation both ATP and reduced NADP are made; in cyclic photophosphorylation electrons return to the same photosystem, so only ATP is made.

In the stroma, rubisco combines CO₂ with RuBP to form an unstable 6C compound that splits into two molecules of GP. GP is reduced to GALP using hydrogen from reduced NADP and energy from ATP.

Five-sixths of GALP is used to regenerate RuBP (using more ATP), so the cycle can continue; the rest makes glucose, sucrose, starch, cellulose, amino acids (with nitrogen from nitrate) and lipids (glycerol and fatty acids).

The light-independent reactions are not directly light-dependent but stop soon after darkness because they need a supply of ATP and reduced NADP; in the dark GP accumulates and RuBP falls.

Chloroplasts are adapted

Chloroplasts are adapted grana give a large surface area of thylakoid membrane for light absorption; the stroma contains enzymes; they contain DNA and ribosomes to make some of their own proteins.

At low light intensity, light is limiting; as intensity rises, rate rises until another factor (CO₂ or temperature) becomes limiting and the graph plateaus. High temperatures denature enzymes such as rubisco, reducing rate.

Rate can be measured by collecting O₂ from pondweed (e.g. Cabomba/Elodea) in a photosynthometer, counting bubbles, or timing floating leaf discs; controls include temperature (water bath/heat shield), CO₂ (sodium hydrogencarbonate solution) and same plant.

Hill reaction

Isolated chloroplasts in light can reduce an artificial electron acceptor in place of NADP. DCPIP or methylene blue turns from blue to colourless when reduced, so the rate of decolourisation measures the rate of the light-dependent reactions.

Required practical

DCPIP

Mix chloroplast extract with DCPIP at a fixed distance from a lamp behind a heat shield; take colorimeter absorbance readings every minute. Absorbance falls as DCPIP is reduced. Controls: extract kept in dark, and DCPIP with buffer only. Vary light intensity, wavelength or temperature.

Chloroplast isolation

Blend leaves in ice-cold isolation medium (sucrose for isotonic conditions, buffer for pH, KCl), filter out debris, then centrifuge so chloroplasts form a pellet. Resuspend the pellet in fresh medium and keep it on ice to slow enzyme activity and damage.

Photosystems I and II

Photosystems contain chlorophyll a, chlorophyll b and carotenoids. PSII absorbs best at 680 nm and PSI at 700 nm. In non-cyclic flow electrons go PSII → electron carriers → PSI → NADP; cyclic flow uses PSI only.

Uses of ATP

ATP supplies energy for active transport, muscle contraction, nerve impulse transmission and anabolic reactions such as protein synthesis. Removing one phosphate gives ADP, removing two gives AMP; ADP and P_i are recycled to make more ATP.

Redox and coenzymes

Oxidation is loss of electrons or hydrogen (or gain of oxygen); reduction is gain of electrons or hydrogen. NADP is a coenzyme that carries hydrogen: reduced NADP donates it to reduce GP in the Calvin cycle.

GP and GALP products

GP is used to make amino acids and fatty acids. GALP is used to make hexoses (glucose, then sucrose, starch and cellulose), glycerol for lipids, and nucleic acids. These molecules form plant biomass passed to consumers.

Grana and lamellae

Thylakoid stacks (grana) are linked by membrane channels called lamellae. The thylakoid space has a very small volume, so pumping protons into it builds a steep gradient quickly for ATP synthesis.

Section 2 of 8Process & sequence

1

Light is absorbed by chlorophyll in a photosystem; energy excites a pair of electrons to a higher energy level and they leave the chlorophyll (photoionisation).

2

Excited electrons pass along a chain of electron carriers in the thylakoid membrane, losing energy at each step.

3

The energy released is used to pump H⁺ into the thylakoid space; H⁺ diffuse back through ATP synthase,

making ATP from ADP + P_i .
- (photophosphorylation)

4

Photolysis of water replaces the lost electrons, releasing H⁺ and O₂; electrons and H⁺ reduce NADP to reduced NADP.

5

In the stroma, rubisco fixes CO₂ with RuBP (5C) to form two GP (3C).

6

GP is reduced to GALP using reduced NADP and ATP.

7

Most GALP regenerates RuBP (using ATP); the remainder is converted to glucose and other organic molecules.

Measuring rate of photosynthesis with pondweedwrite these steps in this order; each step is a marking point.

1

Place a freshly cut piece of pondweed, cut end up, in a boiling tube of 1% sodium hydrogencarbonate solution (excess CO₂) in a water bath at constant temperature.

2

Position a lamp at a measured distance; place a clear heat shield/water tank between lamp and tube; allow 5 minutes to acclimatise.

3

Collect gas via a capillary tube (photosynthometer) for a fixed time, e.g. 5 minutes,

and measure the length of the bubble; convert to volume using πr²l.

4

Repeat three times at each distance and calculate a mean; repeat at different distances in a darkened room.

5

Calculate rate = volume ÷ time and plot rate against light intensity (1/d²).

Section 3 of 8Compare

Light-dependent vs light-independent reactions
Feature	Light-dependent	Light-independent
Site	Thylakoid membranes (grana)	Stroma
Needs light directly?	Yes	No (but needs ATP and reduced NADP from the light-dependent stage)
Inputs	Water, ADP + P_i, NADP, light energy	CO₂, RuBP, ATP, reduced NADP
Outputs	ATP, reduced NADP, O₂	GALP (→ glucose etc.), ADP + P_i, NADP, regenerated RuBP
Key events	Excitation of electrons, electron transport, photolysis, chemiosmosis	Carbon fixation by rubisco, reduction of GP, regeneration of RuBP

Both stages stop quickly in the dark because the Calvin cycle depends on ATP and reduced NADP.

Chloroplast structure related to function
Structure	Function
Double membrane (envelope)	Controls entry and exit of substances
Thylakoid membranes stacked into grana	Large surface area for chlorophyll, electron carriers and ATP synthase
Stroma	Fluid containing Calvin cycle enzymes (e.g. rubisco), starch grains, lipid droplets
Circular DNA and 70S ribosomes	Code for and make some chloroplast proteins
Thylakoid space	Accumulates H⁺ to create a proton gradient for ATP synthesis

Section 4 of 8Worked examples

One quarter

A pondweed placed 20 cm from a lamp produced 0.54 cm³ of O₂ in 6 minutes. Calculate the rate of photosynthesis and predict the relative light intensity at 40 cm compared with 20 cm.

Rate = volume ÷ time = 0.54 ÷ 6 = 0.090 cm³ min⁻¹.
- Light intensity ∝ 1/d²: at 20 cm, 1/20² = 1/400 = 0.0025; at 40 cm, 1/1600 = 0.000625.
  - Ratio = 0.000625 ÷ 0.0025 = 0.25, so intensity at 40 cm is one quarter of that at 20 cm.

6 turns

How many turns of the Calvin cycle, and how many molecules of ATP and reduced NADP, are needed to make one glucose molecule? (Each turn uses 3 ATP and 2 reduced NADP.)

One CO₂ is fixed per turn and glucose has 6 C, so 6 turns.
- ATP = 6 × 3 = 18 ATP;
  - reduced NADP = 6 × 2 = 12 reduced NADP.

100%

Leaf discs took 250 s to float at one light intensity and 125 s at a higher intensity. Calculate the rates and the percentage increase.

Rate = 1/t.
- Rate 1 = 1/250 = 0.0040 s⁻¹;
  - rate 2 = 1/125 = 0.0080 s⁻¹. % increase = (0.0080 − 0.0040) ÷ 0.0040 × 100 = 100%.

Section 5 of 8Keep going

Photosynthesis Edexcel A Level Biology SNAB: what to do next

These Photosynthesis Edexcel A Level Biology SNAB notes covered the key ideas, worked examples and exam technique for this topic. Check your Photosynthesis Edexcel A Level Biology SNAB understanding with the practice questions below, then reread any Photosynthesis Edexcel A Level Biology SNAB point you were unsure about before moving on.

Photosynthesis Edexcel A Level Biology SNAB: revision checklist

1

Revise the main Photosynthesis Edexcel A Level Biology SNAB definitions until you can write them from memory.

2

Practise Photosynthesis Edexcel A Level Biology SNAB exam questions under timed conditions.

3

Link each Photosynthesis Edexcel A Level Biology SNAB idea to a diagram or a worked example.

4

List the common Photosynthesis Edexcel A Level Biology SNAB mistakes and how to avoid them.

5

Explain one Photosynthesis Edexcel A Level Biology SNAB idea out loud as if teaching a friend.

6

Summarise Photosynthesis Edexcel A Level Biology SNAB in five key points on a revision card.

7

Check which command words appear most often in Photosynthesis Edexcel A Level Biology SNAB questions.

8

Come back to Photosynthesis Edexcel A Level Biology SNAB after a week for spaced practice.

9

Mark your answers to Photosynthesis Edexcel A Level Biology SNAB questions against the key ideas above.

10

Highlight the Photosynthesis Edexcel A Level Biology SNAB keywords that examiners expect to see.

1

Practice: Photosynthesis Edexcel A Level Biology SNAB practice questions

2

Previous lesson: 4.2 Resources from Plants — 9BI0 Notes

3

Next lesson: 5.2 Ecosystems & Energy Transfer — 9BI0 Notes

4

Course: Edexcel A Level Biology A (snab) 9BI₀ — all units

5

Official syllabus: Pearson Edexcel A Levels

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Photosynthesis · Autotroph · ATP · Photophosphorylation · Photolysis · Light-dependent reactions · Light-independent reactions (Calvin cycle) · RuBP · Rubisco · GP · GALP · Chlorophyll · Limiting factor

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1

Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).

2Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

3Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

4

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

5Avoid vague words

“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.

6

Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

1

plants make energy by photosynthesislight energy is transferred to chemical energy in glucose

2

the light-independent reactions happen at nightthe Calvin cycle does not directly need light but requires ATP and reduced NADP from the light-dependent reactions

3

CO₂ is split to release oxygenoxygen comes from the photolysis of water

4

ATP stores energy long termATP is an immediate energy source releasing energy in small amounts when hydrolysed

5

NADP is used to reduce GPreduced NADP provides hydrogen (and ATP provides energy) to reduce GP to GALP

6

temperature affects the light-dependent reactions mosttemperature mainly limits the enzyme-controlled Calvin cycle (e.g. rubisco)