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C. Metabolic Processes · Metabolic Processes

The Light-Dependent Reactions: Photolysis, Photosystems and Photophosphorylation

C3.2 C2.1 C3.3 C1.1About 75 minutes
Learning goals
  • I can describe how photosystem II and photosystem I use light energy to move electrons from water to NADP⁺.
  • I can explain photolysis and the origin of the O₂ released by photosynthesis.
  • I can explain how chemiosmosis in the thylakoid makes ATP (photophosphorylation).
  • I can compare noncyclic and cyclic photophosphorylation, and compare chemiosmosis in chloroplasts and mitochondria.

Words to know

photolysis
The splitting of water using energy from light (via photosystem II): 2H₂O → 4H⁺ + 4e⁻ + O₂.
photosystem II (PSII)
The photosystem whose reaction centre (P680) passes excited electrons to the ETC and replaces them with electrons from water.
photosystem I (PSI)
The photosystem whose reaction centre (P700) re-energizes electrons and passes them, via ferredoxin, to NADP⁺.
plastoquinone (PQ)
A mobile carrier in the thylakoid membrane that carries electrons from PSII to the cytochrome b₆f complex and moves H⁺ into the lumen.
cytochrome b₆f complex
A proton pump in the thylakoid membrane that passes electrons from PQ to plastocyanin.
plastocyanin (PC)
A small copper-containing protein in the lumen that carries electrons to PSI.
ferredoxin (Fd)
An iron-sulfur protein on the stroma side that carries electrons from PSI to NADP⁺ reductase.
photophosphorylation
Making ATP using light energy, through electron transport, an H⁺ gradient and ATP synthase.
noncyclic photophosphorylation
Linear electron flow from water through PSII and PSI to NADP⁺; produces O₂, NADPH and ATP.
cyclic photophosphorylation
Electron flow from PSI back to the cytochrome complex and to PSI again; produces ATP only (no NADPH, no O₂).

Turning sunlight into electricity, the biological way

A solar panel on a farm roof in southern Ontario uses light to push electrons through a wire. A thylakoid membrane does something similar: it uses light to push electrons "uphill" from water, where they are held tightly, to NADPH, where they have high energy. Along the way, it uses some of the energy to build an H⁺ gradient and make ATP, just as mitochondria do. Then it hands the ATP and NADPH to the Calvin cycle.

Remember from Lessons 3, 6 and 11

From Lesson 11: light reactions happen in the thylakoid membranes; photosystems have an antenna complex and a reaction centre (P680 in PSII, P700 in PSI). From Lesson 6: an ETC pumps H⁺, and H⁺ flowing back through ATP synthase makes ATP (chemiosmosis). From Lesson 3: water is a poor electron donor, because oxygen holds electrons tightly; moving them "uphill" needs energy.

Step by step: noncyclic electron flow

  1. PSII absorbs light. Energy passed through the antenna pigments excites an electron in P680. The excited electron is captured by a primary electron acceptor. P680 is now missing an electron and becomes one of the strongest biological oxidizing agents known.
  2. Photolysis. An enzyme cluster on the lumen side of PSII (the oxygen-evolving complex) splits water to replace the lost electrons: 2H₂O → 4H⁺ + 4e⁻ + O₂. The O₂ diffuses out of the leaf as a by-product; the H⁺ stays in the lumen.
  3. Electron transport. From the acceptor, electrons pass to plastoquinone (PQ), the cytochrome b₆f complex and plastocyanin (PC). As electrons pass through the cytochrome complex, H⁺ is pumped from the stroma into the lumen.
  4. PSI absorbs light. P700 is excited and passes an electron to its own acceptor; the gap is filled by the electron arriving from PC.
  5. NADPH is made. Electrons pass via ferredoxin (Fd) to the enzyme NADP⁺ reductase, which reduces NADP⁺ + 2e⁻ + H⁺ → NADPH in the stroma.

Because the electrons get two energy boosts and go down an electron transport chain in between, the energy diagram looks like a Z: it's called the Z-scheme.

energy of electronsP680 (PSII)2H₂O → 4H⁺ + O₂4e⁻ replacePSII's electronslightacceptorPQcytochromeb₆fPCH⁺ pumped into lumen→ ATP (chemiosmosis)P700 (PSI)lightacceptorFdNADP⁺ + H⁺→ NADPHNADP⁺ reductaseZ-scheme: light giveselectrons two "boosts"
The Z-scheme of noncyclic electron flow. Light boosts electrons at PSII and again at PSI. Between them, electrons fall through PQ, the cytochrome b₆f complex and PC, which pumps H⁺ for ATP synthesis. Water supplies the electrons; NADP⁺ is the final acceptor.

Making ATP: photophosphorylation

Three things build up the H⁺ concentration in the thylakoid lumen:

  • H⁺ released by photolysis of water (in the lumen);
  • H⁺ pumped by the cytochrome b₆f complex as electrons pass from PQ;
  • H⁺ removed from the stroma when NADP⁺ is reduced.

The lumen becomes much more acidic than the stroma (textbooks often quote about 3 pH units in bright light, a thousandfold difference in H⁺ concentration; measurements in living leaves suggest it is usually smaller, closer to 2 units, because part of the stored energy is a charge difference). H⁺ flows back into the stroma through ATP synthase, which makes ATP in the stroma, right where the Calvin cycle needs it. This is chemiosmosis again, powered by light: photophosphorylation.

Products of noncyclic flow: O₂ (released), NADPH and ATP (both in the stroma).

stroma (low [H⁺])thylakoid lumen (high [H⁺])PSIIPQcytb₆fPCPSIFdNADP⁺reductaseNADP⁺ + H⁺ → NADPHATPsynthaseADP + Pᵢ → ATPH⁺lightlight2H₂O → 4H⁺ + O₂H⁺H⁺ into lumene⁻ path (dashed)
The light reactions in the thylakoid membrane. Water is split on the lumen side; electrons (dashed) flow PSII → PQ → cytochrome b₆f → PC → PSI → Fd → NADP⁺. H⁺ builds up in the lumen and flows back through ATP synthase, making ATP in the stroma.

Cyclic photophosphorylation

The Calvin cycle needs 3 ATP for every 2 NADPH (you'll see why in the next lesson), but noncyclic flow makes a little less ATP than that. Chloroplasts make up the difference with cyclic electron flow:

  • Excited electrons from PSI go to ferredoxin, but instead of reducing NADP⁺, they are passed back to the cytochrome b₆f complex and then to PC and PSI again.
  • Each loop pumps H⁺ into the lumen, so ATP is made.
  • No NADPH is made, no water is split and no O₂ is released. PSII isn't involved.

When NADPH builds up (and NADP⁺ is scarce), more electrons take the cyclic route, which helps balance ATP and NADPH supply.

FeatureMitochondrion (respiration)Chloroplast (photosynthesis)
Energy source for electron flowOxidation of food (NADH, FADH₂)Light absorbed by photosystems
Electron donor → final acceptorNADH/FADH₂ → O₂H₂O → NADP⁺
H⁺ pumped intoIntermembrane spaceThylakoid lumen
ATP synthase faces / ATP made inMatrixStroma
GasUses O₂Releases O₂
Name of ATP productionOxidative phosphorylationPhotophosphorylation
Chemiosmosis in mitochondria and chloroplasts: the same mechanism, different energy sources and directions.
Worked example

Let's trace the electrons and count the products

For every O₂ molecule released by a chloroplast in noncyclic flow, how many electrons pass through the chain, how many NADPH are made, and how many photons are absorbed (assume one photon per electron at each photosystem)?

  1. Photolysis: 2H₂O → 4H⁺ + 4e⁻ + 1 O₂. So 4 electrons per O₂.
  2. Each NADPH needs 2 electrons: 4 ÷ 2 = 2 NADPH.
  3. Each electron is boosted twice (once at PSII, once at PSI): 4 × 2 = 8 photons.
  4. ATP: the H⁺ gradient from these 4 electrons makes roughly 3 ATP (an estimate).

Per O₂: 4 electrons, 2 NADPH, about 8 photons absorbed (and roughly 3 ATP).

Worked example

Let's predict the effect of a herbicide

The herbicide atrazine, widely used in corn fields, blocks the transfer of electrons from PSII to plastoquinone. Predict the effects on (a) O₂ release, (b) NADPH production, (c) ATP production, and (d) the Calvin cycle.

  1. With PQ unable to accept electrons, PSII can't pass on its excited electrons, so it can't take new ones from water: (a) O₂ release stops.
  2. No electrons reach PSI from water, so (b) noncyclic NADPH production stops.
  3. The cytochrome complex no longer pumps H⁺ from noncyclic flow, so (c) ATP production falls sharply (some cyclic flow around PSI may continue briefly).
  4. Without NADPH and enough ATP, (d) the Calvin cycle stops, so no sugar is made and the plant starves. (Excess light energy trapped in PSII also causes damage.)

Blocking PSII → PQ stops O₂ release, NADPH and most ATP production, which shuts down the Calvin cycle and kills the plant.

Just for fun

Plants split water using sunlight at room temperature, something human engineers are still working hard to copy cheaply in "artificial leaf" devices. In a contest of chemistry at room temperature, the maple tree in your yard is still ahead.

Hands-on activity

See the light reactions with a floating leaf disk test

You need: fresh spinach leaves, a drinking straw (or a single-hole punch), baking soda, dish soap, a 10 mL plastic syringe without a needle (from a pharmacy), 2 clear cups, a desk lamp or bright window, a timer.

Safety note: use only a needle-free syringe. Keep the lamp at a safe distance so nothing overheats.

  1. Make bicarbonate solution: 1/8 teaspoon baking soda in 300 mL of water, plus one drop of dish soap. (Bicarbonate supplies CO₂; the soap helps the solution soak into the leaf.)
  2. Punch 10 leaf disks with the straw, avoiding big veins.
  3. Pull the plunger out, drop the disks into the syringe barrel, push the plunger back in, and draw up about 5 mL of solution. Push out any air.
  4. Hold your finger over the tip and pull the plunger back to make a vacuum for 10 seconds, then release. Repeat 3 or 4 times until the disks sink. (The vacuum pulls air out of the leaf's spaces.)
  5. Pour the disks and solution into a cup under the lamp. Set up a second cup the same way but cover it with foil or put it in a dark cupboard.
  6. Every minute, count how many disks are floating. Record for 20 minutes.

Questions:

  • What gas makes the disks float? Which reaction produces it?
  • What does the cup in the dark show?
  • Graph floating disks vs time; find the time when 5 disks (half) are floating.

What to look for: disks in the light rise one by one as O₂ from photolysis builds up inside the leaf spaces; disks in the dark stay down. The half-float time (about 5 to 15 minutes is typical) is a measure of the rate of photosynthesis. You'll use this method again in the photosynthesis investigation lesson.

Your turn

1

Write the equation for photolysis. Where does it occur?

2

What is the final electron acceptor of noncyclic electron flow?

3

List the carriers in order from PSII to NADP⁺.

4

Give three reasons the H⁺ concentration is high in the thylakoid lumen.

5

Where in the chloroplast is ATP made? Why is that location useful?

6

What are the products of cyclic photophosphorylation?

7

Why is the energy diagram of noncyclic flow called the Z-scheme?

More practice: warm-up

1

Which photosystem splits water?

2

What is P700?

3

What does ferredoxin carry?

4

Is O₂ released in cyclic photophosphorylation?

5

What is ATP production using light energy called?

More practice: on your own

1

Explain why noncyclic electron flow alone does not supply enough ATP for the Calvin cycle, and how the chloroplast solves this.

2

A chloroplast suspension is given DCMU, which blocks electron flow out of PSII. A chemical that donates electrons directly to PSI is then added. Predict whether NADPH and O₂ are produced. Explain.

3

Compare where H⁺ accumulates and where ATP is made in mitochondria and chloroplasts.

4

In 1966 André Jagendorf soaked isolated thylakoids in an acid solution (pH 4) in the dark, then moved them to a pH 8 solution with ADP and Pᵢ. They made ATP in the dark. What does this show?

5

Explain why P680⁺ (after losing its electron) can pull electrons from water, which almost nothing else in a cell can do.

Challenge

1

Calculate the minimum number of photons needed to make one molecule of glucose in noncyclic flow, given that the Calvin cycle needs 12 NADPH per glucose and each electron requires one photon at each photosystem.

2

Some herbicides (such as paraquat) accept electrons from PSI and pass them to O₂, forming highly reactive oxygen species. Predict the effect on NADPH production and explain why these herbicides kill plant cells quickly in bright light.

3

Use the second law of thermodynamics to explain why the light reactions need two photosystems rather than one, given the energy needed to move electrons from water to NADP⁺.

Check your understanding

Quiz

1.

Photolysis, the splitting of water, is associated with:

2.

The final electron acceptor in noncyclic electron flow is:

3.

In the chloroplast, H⁺ accumulates in the:

4.

Cyclic photophosphorylation produces:

5.

Which statement correctly compares chemiosmosis in chloroplasts and mitochondria?

6.

A herbicide blocks electron transfer from PSII to plastoquinone. Which process stops first?

For parents and teachers

Big idea: The light reactions (C3.2) use two photosystems to move electrons from water to NADP⁺, releasing O₂ and building an H⁺ gradient that drives ATP synthase (photophosphorylation). Students should compare this with oxidative phosphorylation (C3.4) and know both cyclic and noncyclic flow (C2.1).

How to help: The floating leaf disk activity needs a needle-free syringe (sold at pharmacies) and works best with fresh spinach. Ask your teen to trace an electron from water to NADPH aloud, naming each carrier, and then compare the thylakoid with the mitochondrion using the table.

Common difficulties:

  • Thinking O₂ comes from CO₂, or that PSI splits water.
  • Getting the direction of H⁺ pumping backwards (into the lumen, then back out to the stroma through ATP synthase).
  • Thinking PSI comes first because it is numbered I (it was discovered first; electrons pass through PSII first).
  • Forgetting that cyclic flow makes no NADPH and no O₂.