Light Reactions and Pigments

The light reactions of photosynthesis trap light energy in pigments and turn it into the chemical energy of ATP and NADPH, splitting water and releasing oxygen. This page covers leaf pigments, absorption and action spectra, the light harvesting complex, photosystems I and II, the Z scheme, splitting of water, cyclic and non-cyclic photophosphorylation and the chemiosmotic hypothesis. NEET regularly asks P680 and P700, the Z scheme sequence, cyclic versus non-cyclic photophosphorylation and the parts of ATP synthase in the light reactions of photosynthesis.

1. Photosynthetic Pigments

1.1 Four pigments in the leaf

• Leaves, even on the same plant, show many shades of green.
• The leaf pigments can be separated by paper chromatography.
• ★ Exam imp The colour of a leaf is due not to one pigment but to four pigments.

• Pigments: substances that can absorb light at specific wavelengths.
• Chlorophyll a is the most abundant plant pigment in the world.

1.2 Absorption spectrum and action spectrum

• Absorption spectrum: a graph of how much light a pigment absorbs at different wavelengths.
• Action spectrum: a graph of the rate of photosynthesis at different wavelengths. Here the rate is measured by ${\mathrm{O}}_2$ release.
• Visible light runs from violet to red (VIBGYOR), about 400 to 700 nm.

• ★ Exam imp Chlorophyll a shows its maximum absorption in the blue region and a second peak in the red region (graph a).
• The rate of photosynthesis is also higher in the blue and red regions (graph b).
• Hence chlorophyll a is the chief pigment associated with photosynthesis.
• ★ Exam imp Graph (c) shows that there is no complete one-to-one overlap between the absorption spectrum of chlorophyll a and the action spectrum.
• Most photosynthesis takes place in the blue and red regions; some takes place at the other visible wavelengths too.

1.3 Accessory pigments

• Chlorophyll is the major pigment that traps light.
• Other thylakoid pigments, chlorophyll b, xanthophylls and carotenoids, are called accessory pigments.
• They also absorb light and transfer the energy to chlorophyll a.

2. The Light Reaction

• Light reactions, or the photochemical phase, include four events:

1. Light absorption.
2. Water splitting.
3. Oxygen release.
4. Formation of the high-energy chemical intermediates ATP and NADPH.

2.1 Light harvesting complexes and photosystems

• Several protein complexes take part in the light reaction.
• The pigments are organised into two discrete photochemical light harvesting complexes (LHC), within Photosystem I (PS I) and Photosystem II (PS II).
• ★ Exam imp The photosystems are named in the sequence of their discovery, not in the sequence in which they function.
• Each LHC is made up of hundreds of pigment molecules bound to proteins.
• ★ Exam imp In each photosystem, all the pigments except one molecule of chlorophyll a form a light harvesting system called the antennae.
• By absorbing different wavelengths of light, the antenna pigments make photosynthesis more efficient.
• The single chlorophyll a molecule forms the reaction centre. It is different in the two photosystems.

3. Electron Transport and the Z Scheme

The steps below follow one electron from PS II to $\mathrm{NAD}{\mathrm{P}}^{+}$.

1. In PS II, the reaction centre chlorophyll a absorbs 680 nm red light. Its electrons become excited and jump into an orbit farther from the nucleus.
2. An electron acceptor picks up these electrons.
3. The acceptor passes them to an electron transport system made of cytochromes.
4. This movement is downhill on the oxidation-reduction (redox) potential scale. The electrons are not used up; they pass on to the pigments of PS I.
5. At the same time, electrons in the reaction centre of PS I are excited by 700 nm red light. They pass to another acceptor of greater redox potential.
6. These electrons move downhill again to a molecule of energy-rich $\mathrm{NAD}{\mathrm{P}}^{+}$, reducing it to NADPH + ${\mathrm{H}}^{+}$.

3.1 Splitting of water

• PS II keeps losing electrons, so they must be replaced continuously.
• The replacement electrons come from the splitting of water.
• ★ Exam imp Water splitting is associated with PS II. Water splits into 2${\mathrm{H}}^{+}$, [O] and electrons.
• This creates oxygen, one of the net products of photosynthesis.
• The electrons needed to replace those removed from PS I are provided by PS II.

• The water splitting complex is associated with PS II, which lies on the inner side of the thylakoid membrane.
• So the protons and ${\mathrm{O}}_2$ formed are released into the lumen of the thylakoid.

4. Cyclic and Non-cyclic Photophosphorylation

• Living organisms extract energy from oxidisable substances and store it as bond energy.
• Special substances such as ATP carry this energy in their chemical bonds.
• Phosphorylation: the process by which cells synthesise ATP, in mitochondria and chloroplasts.
• Photophosphorylation: the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

4.1 Non-cyclic photophosphorylation

• The two photosystems work in series, first PS II and then PS I.
• They are connected through the electron transport chain, as in the Z scheme.
• ★ Exam imp Both ATP and NADPH + ${\mathrm{H}}^{+}$ are synthesised by this electron flow.

4.2 Cyclic photophosphorylation

• Here only PS I is functional.
• The excited electron is circulated within the photosystem, and phosphorylation occurs due to this cyclic flow.
• A possible site is the stroma lamellae.
• ★ Exam imp The grana lamellae have both PS I and PS II. The stroma lamellae lack PS II and the NADP reductase enzyme.
• So the excited electron does not pass to $\mathrm{NAD}{\mathrm{P}}^{+}$. It returns to the PS I complex through the electron transport chain.
• ★ Exam imp Cyclic flow therefore makes only ATP, not NADPH + ${\mathrm{H}}^{+}$.
• ★ Exam imp Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm is available for excitation.

5. The Chemiosmotic Hypothesis

• The chemiosmotic hypothesis explains how ATP is actually synthesised in the chloroplast.
• As in respiration, ATP synthesis is linked to a proton gradient across a membrane, here the thylakoid membrane.
• ★ Exam imp One difference: in chloroplasts protons accumulate inside, in the lumen. In respiration they accumulate in the intermembrane space of mitochondria as electrons move through the ETS.

5.1 How the proton gradient forms

1. Water splitting happens on the inner side of the membrane, so its protons accumulate within the lumen.
2. The primary acceptor of electrons, located towards the outer side, passes its electron to an H carrier, not an electron carrier. This carrier takes a proton from the stroma with each electron, and releases the proton into the lumen when it hands the electron to a carrier on the inner side.
3. The NADP reductase enzyme is on the stroma side. It removes protons from the stroma to reduce $\mathrm{NAD}{\mathrm{P}}^{+}$ to NADPH + ${\mathrm{H}}^{+}$.

• Hence protons decrease in the stroma and accumulate in the lumen.
• This creates a proton gradient across the thylakoid membrane and a measurable decrease in pH in the lumen.

5.2 ATP synthase and ATP formation

• The breakdown of the gradient is what leads to ATP synthesis.
• Protons move back across the membrane to the stroma through the transmembrane channel of CF₀ of the ATP synthase.

• The breakdown of the gradient gives enough energy for the conformational change in CF₁.
• Energy is first used to pump protons into the lumen, creating a high proton concentration there.
• Diffusion of protons back through ATP synthase releases enough energy to activate the enzyme, which catalyses ATP formation.
• This ATP and the NADPH are used immediately in the stroma, in the reactions that fix $\mathrm{C}{\mathrm{O}}_2$ and synthesise sugars.

5.3 Reading the chemiosmosis figure

• PS II (P680) oxidises water on the lumen side: each ${\mathrm{H}}_2\mathrm{O}$ gives ½${\mathrm{O}}_2$, 2${\mathrm{H}}^{+}$ and 2 electrons.
• Plastoquinone (PQ, reduced form $\mathrm{PQ}{\mathrm{H}}_2$) carries electrons and picks up ${\mathrm{H}}^{+}$ from the stroma.
• Cytochrome ${\mathrm{b}}_6\mathrm{f}$ passes electrons on and releases ${\mathrm{H}}^{+}$ into the lumen.
• Plastocyanin (PC) carries electrons to PS I (P700).
• On the stroma side, Fd (ferredoxin) and FNR (ferredoxin-NADP⁺ reductase, the NADP reductase) reduce $\mathrm{NAD}{\mathrm{P}}^{+}$ + ${\mathrm{H}}^{+}$ to NADPH.
• The lumen has high ${\mathrm{H}}^{+}$ and the stroma low ${\mathrm{H}}^{+}$: a high electrochemical potential gradient across the membrane.

6. Exam Essentials

Pairs to Match

Numbers to Remember

• Four leaf pigments; one chlorophyll a molecule forms each reaction centre.
• LHC: hundreds of pigment molecules bound to proteins.
• PS I: absorption peak 700 nm (P700). PS II: absorption maximum 680 nm (P680).
• Cyclic photophosphorylation also occurs with light beyond 680 nm only.
• $2{\mathrm{H}}_2\mathrm{O}$ → $4{\mathrm{H}}^{+}$ + ${\mathrm{O}}_2$ + $4{\mathrm{e}}^{-}$; per water molecule: 2${\mathrm{H}}^{+}$, [O] and 2 electrons.
• ATP synthase has two parts, CF₀ and CF₁; chemiosmosis needs four things.

7. Quick Revision

• Chromatography shows four pigments: chlorophyll a (blue green), b (yellow green), xanthophylls (yellow), carotenoids (yellow-orange).
• Chlorophyll a absorbs most in blue and red; photosynthesis is also highest there.
• Absorption and action spectra overlap roughly, not exactly.
• Accessory pigments pass energy to chlorophyll a, widen the spectrum and prevent photo-oxidation.
• Light reactions: light absorption, water splitting, ${\mathrm{O}}_2$ release, ATP and NADPH formation.
• LHC = hundreds of pigments on proteins; antenna = all except one chlorophyll a.
• Reaction centres: P700 (PS I) and P680 (PS II); named by order of discovery.
• Z scheme: PS II → acceptor → cytochromes → PS I → acceptor → $\mathrm{NAD}{\mathrm{P}}^{+}$, drawn on a redox scale.
• Water splitting at PS II, lumen side: $2{\mathrm{H}}_2\mathrm{O}$ → $4{\mathrm{H}}^{+}$ + ${\mathrm{O}}_2$ + $4{\mathrm{e}}^{-}$.
• Non-cyclic (PS II + PS I): ATP + NADPH. Cyclic (PS I only, stroma lamellae): ATP only.
• Cyclic flow also occurs when only light beyond 680 nm is available.
• Chemiosmosis: water splitting and the H carrier add protons to the lumen (lower pH); NADP reductase removes protons from the stroma.
• Protons return through CF₀; CF₁ changes shape and makes ATP.
• Chemiosmosis needs a membrane, a proton pump, a proton gradient and ATP synthase.

8. Solved Examples

9. Practice Questions

Common Mistakes to Avoid