Photosynthesis: Study Guide (Biology 3.4)

Capturing light energy

In Topic 3.3 you saw that every living system needs an energy input. For almost all life on Earth, that energy first enters through photosynthesis. Plants, algae and cyanobacteria capture energy from sunlight and store it in the chemical bonds of carbohydrates.

The overall equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy absorbed by chlorophyll)

• Reactants: carbon dioxide, water and light energy.
• Products: carbohydrate (written here as glucose) and oxygen.

The sugars can be used straight away, broken down in respiration to make ATP, or built into larger molecules. Plants store them as starch and use them to make cellulose, proteins and lipids. Every animal that eats a plant, and every animal that eats that animal, depends on this stored energy.

The equation is a summary. It hides two linked sets of reactions:

1. The light reactions capture light energy and use it to make ATP and NADPH. They split water and release O₂.
2. The Calvin cycle uses that ATP and NADPH to build carbohydrate from CO₂.

Where photosynthesis came from

Photosynthesis first evolved in prokaryotes, long before plants existed.

• Rock layers show that free oxygen in Earth’s atmosphere rose sharply about 2.4 billion years ago. This is known as the Great Oxidation Event.
• The evidence points to cyanobacteria, prokaryotes that release O₂ in photosynthesis, as the cause. The oldest fossils that most scientists accept as eukaryotes are much younger than this rise in oxygen, so eukaryotes are very unlikely to have produced it.
• Eukaryotic photosynthesis was built on the prokaryotic pathway. A chloroplast is descended from a cyanobacterium that was taken in by an early eukaryotic cell (endosymbiosis, Unit 2). Chloroplasts still have a double membrane, their own circular DNA and bacteria-like ribosomes, and they use two photosystems, just as cyanobacteria do.

So the chemistry happening in a leaf today is a version of the chemistry that changed the planet’s atmosphere.

Inside the chloroplast

The structure of the chloroplast matches its two jobs.

Structure	What it is	Its job
Stroma	fluid inside the inner membrane, outside the thylakoids	site of the Calvin cycle (carbon fixation)
Thylakoid	flattened sac of membrane	its membrane holds chlorophyll in two photosystems, plus electron transport proteins and ATP synthase
Thylakoid space	the inside of a thylakoid	collects H⁺ ions to build a proton gradient
Granum (plural grana)	a stack of thylakoids	site of the light reactions; stacking packs a large membrane area into a small space

The light reactions

The light reactions are a chain of coordinated steps across the thylakoid membrane. Follow Figure 2 as you read.

1. Light boosts electrons in photosystem II. Chlorophyll molecules absorb light energy. The energy raises electrons to a higher energy level, and they leave the photosystem.
2. Water is split. To replace the lost electrons, photosystem II takes electrons from water. Water splits into electrons, H⁺ ions and oxygen. The H⁺ ions stay inside the thylakoid space. The oxygen is released as O₂, a by-product.
3. Electrons pass along the electron transport chain (ETC). The ETC is a series of proteins in the thylakoid membrane that connects photosystem II to photosystem I. Electrons move from carrier to carrier in oxidation–reduction (redox) reactions: each carrier is reduced when it gains electrons and oxidised when it passes them on. At each step the electrons lose a little energy. Some of that energy is used to pump H⁺ from the stroma into the thylakoid space. (You do not need the names of the carriers.)
4. Light boosts the electrons again in photosystem I. The electrons arrive with less energy than they started with. Light absorbed by photosystem I raises them to a high energy level once more.
5. NADP⁺ is reduced to NADPH. The electrons from photosystem I are passed to NADP⁺, which also picks up H⁺, forming NADPH on the stroma side. NADPH carries high-energy electrons to the Calvin cycle.
6. The proton gradient makes ATP. Water splitting and pumping leave a high H⁺ concentration inside the thylakoid and a low concentration outside, in the stroma. This electrochemical gradient stores energy. H⁺ ions can flow back out only through ATP synthase, a protein channel in the membrane. As they flow, ATP synthase joins ADP and Pᵢ into ATP. Making ATP by this flow of protons is chemiosmosis; when the energy came from light, the whole process is called photophosphorylation.

Why two photosystems? One light-driven boost does not give an electron enough energy to travel all the way from water to NADP⁺. Photosystem II takes electrons from water and powers the proton pumping; photosystem I gives the electrons the extra energy needed to reduce NADP⁺. Without both, the plant could not make both ATP and NADPH.

ETCs are not only in chloroplasts. Electron transport chains also run in the inner membrane of mitochondria (Topic 3.5) and across the plasma membranes of prokaryotes. In every case, electron flow builds a proton gradient that ATP synthase uses.

The Calvin cycle

The Calvin cycle takes place in the stroma. It uses the products of the light reactions to fix CO₂ into carbohydrate.

In	Out
CO₂ (from the air, through stomata)	carbohydrate (a three-carbon sugar, used to build glucose, sucrose and starch)
ATP (from the light reactions)	ADP + Pᵢ (back to the thylakoids)
NADPH (from the light reactions)	NADP⁺ (back to the thylakoids)

ATP supplies energy and NADPH supplies high-energy electrons. Together they convert low-energy CO₂ into energy-rich carbohydrate. The cycle regenerates its starting molecule, which is why it is a cycle. You do not need to learn its steps, intermediates or enzymes.

The two stages depend on each other. Without light, ATP and NADPH run out within a short time and the Calvin cycle stops. Without the Calvin cycle, ADP and NADP⁺ are not returned, and the light reactions slow down.

Worked example 1: using the equation

Question. In a fictional experiment, a potted plant takes up 2.64 g of CO₂ in one day. Assume all of it is turned into glucose. Calculate (a) the mass of glucose made, (b) the mass of O₂ released and (c) the mass of water used, and check that mass is conserved. Use the molar masses in the calculator note.

1. Moles of CO₂ = mass ÷ molar mass = 2.64 g ÷ 44.01 g mol⁻¹ = 0.0600 mol.
2. (a) From the equation, 6 mol CO₂ → 1 mol glucose. Moles of glucose = 0.0600 ÷ 6 = 0.0100 mol. Mass = 0.0100 mol × 180.16 g mol⁻¹ = 1.80 g.
3. (b) 6 mol CO₂ → 6 mol O₂, so 0.0600 mol O₂. Mass = 0.0600 × 32.00 = 1.92 g.
4. (c) 6 mol CO₂ uses 6 mol H₂O, so 0.0600 mol H₂O. Mass = 0.0600 × 18.02 = 1.08 g.
5. Check. Mass in = 2.64 + 1.08 = 3.72 g. Mass out = 1.80 + 1.92 = 3.72 g. Mass is conserved.

Interpretation. A real plant would gain less than 1.80 g of glucose, because it also breaks down sugar in respiration, day and night. The equation gives the maximum. Remember too that the O₂ comes from the water, not from the CO₂, even though the masses balance either way.

Worked example 2: what a herbicide reveals

Question. A fictional herbicide H blocks the transfer of electrons from photosystem II to the electron transport chain. Leaf discs in bright light are measured with and without H.

Measurement (fictional data)	Control	With H
O₂ release / µmol h⁻¹	12.0	0.6
pH in thylakoid space	5.2	7.6
pH in stroma	7.9	7.9
ATP made (% of control)	100	9

(a) Calculate the percentage fall in O₂ release. (b) Calculate how many times more concentrated H⁺ is in the thylakoid space than in the stroma, with and without H. (c) Use the data to support the claim that H stops the plant making carbohydrate.

(a) (12.0 − 0.6) ÷ 12.0 × 100 = 95% fall.

(b) Each pH unit is a tenfold change in H⁺ concentration.

1. Control: 7.9 − 5.2 = 2.7 units, so 10^2.7 ≈ 500 times more H⁺ inside.
2. With H: 7.9 − 7.6 = 0.3 units, so 10^0.3 ≈ 2 times. The gradient has almost gone.

(c)

1. Claim. H stops carbohydrate production.
2. Evidence and reasoning, step by step. Electrons cannot leave photosystem II, so it cannot accept more electrons from water. Water splitting stops, which is why O₂ release falls by 95%. With no electron flow along the ETC, no H⁺ is pumped and none is released from water, so the H⁺ gradient collapses (500 times → 2 times). Without the gradient, ATP synthase makes little ATP (9% of control). No electrons reach photosystem I, so no NADPH is made either.
3. Link to the Calvin cycle. The Calvin cycle needs both ATP and NADPH to turn CO₂ into carbohydrate. With both missing, carbohydrate production stops, and the plant cannot store energy.

Common misconceptions

• “The O₂ released comes from CO₂.” It comes from water split at photosystem II.
• “The Calvin cycle is the ‘dark reactions’, so it happens at night.” It happens in the light, because it needs a constant supply of ATP and NADPH from the light reactions.
• “Plants photosynthesise instead of respiring.” Plant cells respire all the time, in light and dark. Photosynthesis is extra, not a replacement.
• “ATP synthase pumps protons.” ATP synthase lets protons flow down their gradient and uses that flow to make ATP. The ETC does the pumping.
• “The H⁺ concentration is highest in the stroma.” It is highest inside the thylakoid space.
• “Photosystem I comes first.” The names reflect the order in which they were discovered. Electrons go from photosystem II to photosystem I.
• “Photosynthesis started in plants.” It began in prokaryotes; chloroplasts descend from cyanobacteria.
• “Chlorophyll makes energy.” Chlorophyll absorbs light energy and passes it to electrons; energy is converted, not created.

Where this leads

Photosynthesis stores light energy in carbohydrate. Next, in Topic 3.5, Cellular Respiration, you will see how cells release that energy as ATP, using another electron transport chain and another ATP synthase, this time in mitochondria. Look back at Topic 3.3 for why energy must keep flowing. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.