Pearson Edexcel International A Level Biology: Photosynthesis (YBI11)

This guide teaches photosynthesis from scratch for the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. It covers statements 5.1 to 5.8 of Topic 5, including Core Practical 10. Topics 1 to 4 are International AS (IAS) and Topics 5 to 8 are International A2 (IA2), so this material is A2. There are no tiers. Pair it with the revision notes and the practice questions. The course hub and the printable checklist show the rest of the course, and the free diagnostics show where to start.

What this guide covers

Statement	What you must be able to do
5.1	Explain the overall reaction: light energy splits water, hydrogen is stored in glucose
5.2	Link photophosphorylation of ADP to ATP hydrolysis as an immediate energy supply
5.3	Describe the light-dependent reactions, cyclic and non-cyclic, and photolysis
5.4 (i)	Describe the Calvin cycle: GP, GALP, RuBP and RUBISCO
5.4 (ii)	State how simple sugars are used in respiration and to make new biological molecules
5.5	Relate chloroplast structure to function
5.6	Define absorption spectrum and action spectrum
5.7	Separate pigments by chromatography and use Rf values
5.8	Core Practical 10: factors affecting the rate of photosynthesis in an aquatic plant

5.1 The overall reaction

The word equation is carbon dioxide + water → glucose + oxygen, driven by light energy. The balanced symbol equation is:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Twelve water molecules go in and six come out, so the net use is six. The specification wants you to see the reaction as a transfer of hydrogen. Light energy splits the strong bonds in water. The hydrogen is combined with carbon dioxide and stored in a fuel, glucose. The oxygen from the water is released to the atmosphere. Glucose is energy-rich because it carries that stored hydrogen.

5.2 ATP in photosynthesis

ADP + Pi → ATP needs energy. In photosynthesis the energy comes from light, so the process is photophosphorylation. ATP made this way is used within the chloroplast, mainly in the Calvin cycle.

ATP is not stored in bulk. When it is hydrolysed, ATP + H₂O → ADP + Pi, a small, useful amount of energy is released at once, for roughly 30 kJ per mole of ATP. That makes ATP an immediate supply for processes such as active transport and synthesis, and it is quickly rebuilt from ADP. The cycle ADP → ATP → ADP is how energy is passed between processes without losing most of it as heat.

5.3 The light-dependent reactions

These happen on the thylakoid membranes. Chlorophyll and other pigments sit in photosystems. Photosystem II (PSII) and photosystem I (PSI) each contain a primary pigment at the reaction centre.

When a pigment absorbs a photon, an electron in chlorophyll is raised to a higher energy level. It is said to be excited, and it leaves the chlorophyll. Excited electrons are passed from carrier to carrier along an electron transport chain in the thylakoid membrane. They lose energy at each step.

That energy is used to pump H⁺ ions into the thylakoid space. A high concentration of H⁺ builds up inside, giving a gradient across the membrane. H⁺ flows back out through ATP synthase, which makes ATP from ADP and Pi. This is chemiosmosis.

Non-cyclic photophosphorylation

1. Light excites electrons in PSII, and they leave the chlorophyll.
2. The electrons move along the electron transport chain; ATP is made as they go.
3. Light excites electrons in PSI. Electrons from the chain replace those lost.
4. The electrons from PSI reduce NADP, giving reduced NADP, with H⁺ from the stroma.
5. PSII needs replacement electrons. They come from photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂.

Products: ATP, reduced NADP and oxygen. Photolysis is the only source of oxygen and the source of the electrons and H⁺ for reducing NADP.

Cyclic photophosphorylation

Only PSI is used. Excited electrons leave PSI, pass along a chain and return to PSI. They are not given to NADP.

Products: ATP only. There is no reduced NADP, no photolysis and no oxygen. The cycle gives extra ATP when the Calvin cycle needs more ATP than reduced NADP.

Feature	Non-cyclic	Cyclic
Photosystems	PSII and PSI	PSI only
Electron source	Water (photolysis)	PSI itself
Final electron acceptor	NADP	Returns to PSI
ATP made	Yes	Yes
Reduced NADP made	Yes	No
Oxygen released	Yes	No

5.4 The light-independent reactions

(i) These reactions occur in the stroma and need ATP and reduced NADP from the light-dependent stage. They are called light-independent because no light is absorbed in them, but they stop in the dark because the supply of those two products stops.

The Calvin cycle is carbon fixation followed by reduction:

1. CO₂ combines with ribulose bisphosphate (RuBP, a 5-carbon compound), catalysed by RUBISCO. The unstable 6-carbon product splits at once into two molecules of glycerate 3-phosphate (GP, 3 carbons).
2. GP is reduced to glyceraldehyde 3-phosphate (GALP, 3 carbons). ATP gives energy and reduced NADP gives hydrogen.
3. Most GALP is used to regenerate RuBP, which also uses ATP. A fraction leaves the cycle to make carbohydrate.

Worked example: building one glucose

Count carbons. Six turns fix 6 CO₂, giving 12 GP and then 12 GALP (36 carbons). Two GALP (6 carbons) leave to make one glucose. The remaining 10 GALP (30 carbons) are rearranged into 6 RuBP (30 carbons).

ATP: 12 to reduce GP, plus 6 to regenerate RuBP, gives 18. Reduced NADP: 12. So each CO₂ fixed costs 3 ATP and 2 reduced NADP. A suspension that fixes 4.8 µmol of CO₂ needs 14.4 µmol of ATP and 9.6 µmol of reduced NADP, and could yield 4.8 ÷ 6 = 0.80 µmol of glucose.

Effect of stopping the light

In the dark, ATP and reduced NADP fall. GP cannot be reduced, so GP accumulates. RuBP cannot be regenerated, but it is still being used by RUBISCO, so RuBP falls. If CO₂ is removed instead, RuBP rises and GP falls. Graph questions on GP and RuBP levels rely on this reasoning.

(ii) The products are simple sugars. Plants, animals and other organisms use them in respiration, and to synthesise new molecules: polysaccharides (starch for storage, cellulose for cell walls), amino acids and so proteins (with nitrogen from nitrate), lipids (from glycerol and fatty acids made from GALP-derived compounds) and nucleic acids (sugars and the phosphate-containing nucleotides). Animals depend on this because they eat plants or other organisms that did.

5.5 Chloroplast structure and function

Feature	Role
Double membrane (envelope)	Controls what enters and leaves; keeps the stroma separate from the cytoplasm
Thylakoid membranes	Large surface area for pigments, photosystems, electron carriers and ATP synthase
Grana (stacks of thylakoids)	Pack many photosystems close together for light capture
Intergranal lamellae	Link grana so that products can pass between them
Thylakoid space	Small volume, so a H⁺ gradient builds up quickly
Stroma	Contains RUBISCO and the other Calvin cycle enzymes, plus starch grains and lipid droplets
Chloroplast DNA and ribosomes	Make some of the chloroplast’s own proteins

Chloroplast ribosomes are smaller than those of the cytoplasm. The pigments are arranged in photosystems so that energy is funnelled to the reaction centre. Having the light-dependent stage on membranes and the light-independent stage in the surrounding stroma means the products of one are right next to the enzymes that need them.

5.6 Absorption and action spectra

An absorption spectrum plots the absorbance of a pigment against wavelength. An action spectrum is a graph of the rate of photosynthesis (for example oxygen release) at each wavelength.

Chlorophyll absorbs mostly in the blue (about 430 to 450 nm) and red (about 650 to 680 nm) parts of the spectrum and reflects most green light. Accessory pigments such as carotenoids absorb other blue-green wavelengths and pass the energy on.

The action spectrum is similar in shape to the combined absorption spectra, which is evidence that the pigments are the ones driving photosynthesis. It is not identical, because only light absorbed by pigments linked to photosystems can drive the reactions.

Worked interpretation

At 450 nm the rate is 78 units and at 550 nm it is 20 units. At 550 nm the rate is 20 ÷ 78 × 100 = 26% of the rate at 450 nm. Green light is not useless: some is absorbed by accessory pigments, so the rate is low but not zero.

5.7 Separating pigments by chromatography

A concentrated pigment extract is made by grinding leaf in a solvent such as propanone. A small spot is placed on a pencil line (the origin) near the bottom of a chromatography strip. The strip stands in solvent with the origin above the liquid. As the solvent rises, each pigment moves at a rate that depends on how soluble it is in the solvent and how strongly it sticks to the paper. Mark the solvent front before it dries.

Rf = (distance from origin to pigment spot) ÷ (distance from origin to solvent front)

Measure each pigment to the middle of its spot. Rf is a ratio of two lengths, so it has no unit and lies between 0 and 1. Compare Rf values with reference values obtained with the same solvent and paper to identify the pigments.

Worked example. Solvent front 8.0 cm. Spots at 7.6, 6.0, 4.0 and 2.4 cm.

Rf = 7.6 ÷ 8.0 = 0.95; 6.0 ÷ 8.0 = 0.75; 4.0 ÷ 8.0 = 0.50; 2.4 ÷ 8.0 = 0.30.

The pigment with the highest Rf is the most soluble in the solvent. Pencil is used for the origin because ink would dissolve and run.

5.8 Core Practical 10: rate of photosynthesis in an aquatic plant

Aim: investigate how light intensity, light wavelength, temperature and availability of carbon dioxide affect the rate of photosynthesis, using a suitable aquatic plant such as Canadian pondweed.

Method outline: a cut shoot is placed in water in a beaker or tube. The rate is measured as bubbles of oxygen released per minute, or as the volume of gas collected in a capillary tube. Change one factor only and keep the others constant.

Factor	How to vary it	What to control
Light intensity	Move a lamp to different distances	Temperature, CO₂
Wavelength	Coloured filters	Intensity, temperature
Temperature	Water baths at set temperatures	Light, CO₂
CO₂	Different concentrations of sodium hydrogencarbonate solution	Light, temperature

Light intensity is proportional to 1 ÷ distance². Moving the lamp from 30 cm to 15 cm gives (30 ÷ 15)² = 4 times the intensity.

Worked example. Counts in 5 minutes: 41, 44, 38. Mean = 41. Rate = 41 ÷ 5 = 8.2 bubbles per minute.

Gas volume: a bubble 12.0 mm long in a tube of radius 0.5 mm has volume πr²l = π × 0.5² × 12.0 = 9.4 mm³.

Improving reliability: allow the plant to adjust to each new condition for a few minutes, repeat readings and take a mean, and use a water bath or heat shield so that the lamp does not heat the water. Limitations: bubble counting assumes bubbles are the same size, and some oxygen dissolves or is used in respiration.

Making a 0.05 mol dm⁻³ solution from a 0.10 mol dm⁻³ stock: 25 cm³ stock + 25 cm³ water gives 50 cm³.

Common errors

• Saying oxygen comes from carbon dioxide. It comes from water.
• Saying cyclic photophosphorylation makes reduced NADP.
• Writing that the Calvin cycle “does not need light” and ignoring its dependence on ATP and reduced NADP.
• Mixing up absorption and action spectra. One measures absorbed light, the other measures rate.
• Measuring Rf to the edge of a spot, or using ink for the origin.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology (YBI11) specification, Issue 2, published by Pearson Education Limited.