C1.3: HL (C1.3.9-C1.3.19)

What is a photosystem?

A molecular array of chlorophyll and other accessory pigments, with a special chlorophyll acting as a reaction centre from which an excited electron is emitted.

• Always located in membranes

• Found in the thylakoids of chloroplasts (photosynthetic eukaryotes) and in the membranes of cyanobacteria

What are the advantages of a structured array of different pigment molecules in a photosystem?

• Maximizes the range of wavelengths of light absorbed → sufficient energy transfer

• Excited electrons are controlled

• Electrons can be directed along the electron transport chain

• A single pigment alone would not be enough — photosynthesis wouldn't happen

What is the role of Photosystem II (PSII)?

Accepts de-energized electrons from water and donates excited electrons to an electron transport chain (to make ATP).

• Photolysis of water in PSII generates protons and electrons used in the light-dependent reactions

• O₂ is released as waste

Why is photosynthesis significant for Earth's atmospheric oxygen?

Photosynthesis is the only significant source of O₂ known on Earth.

• Photosynthetic organisms saturated the environment with O₂, transforming oceans and geological processes

• Earth's oceans initially contained high iron → O₂ reacted with it to form iron oxide (precipitate) → Banded Iron Formations (BIFs)

• Once iron was fully consumed, O₂ began accumulating in the atmosphere

• Free O₂ was toxic to obligate anaerobes (many died) but necessary for aerobes

• Eventually led to the formation of the ozone layer

What are Banded Iron Formations (BIFs)?

Geological deposits that formed when O₂ produced by early photosynthesis reacted with iron dissolved in Earth's oceans, precipitating iron oxide in alternating layers.

Describe ATP production by chemiosmosis in the light-dependent reactions.

• Electrons from the photosystems pass along the electron transport chain (composed of multiple electron-shuttling carrier proteins)

• Electrons lose energy — this energy is used to pump H⁺ from the stroma into the thylakoid lumen

• H⁺ build up creates a concentration gradient across the thylakoid membrane

• H⁺ return to the stroma down their gradient via ATP synthase (= chemiosmosis)

• ATP synthase uses the kinetic energy of proton passage to catalyse synthesis of ATP from ADP + Pi

• This process is called photophosphorylation (light provided the initial energy)

What is photophosphorylation?

The production of ATP in the light-dependent reactions of photosynthesis, using energy that originally came from light.

• Light excites electrons in the photosystems

• Energy lost by electrons in the ETC is used to build a H⁺ gradient

• Proton flow through ATP synthase drives ATP synthesis

How is NADP reduced in Photosystem I?

• NADP is the electron carrier in photosynthesis — it must be reduced to form NADPH

• The reaction is catalysed by the enzyme NADP reductase

• NADP reductase uses electrons (from PSI) and H⁺ to reduce NADP

• De-energized electrons coming from PSII (via the ETC) replace those emitted from PSI

How should NADP terminology be paired?

Pair either:

• "NADP" and "reduced NADP", OR

• "NADP⁺" and "NADPH"

Never mix the two notations in the same answer.

What are thylakoids?

The site of the light-dependent reactions of photosynthesis.

The light-dependent reactions include:

• Photolysis of H₂O

• Synthesis of ATP by chemiosmosis

• Reduction of NADP

In cyanobacteria, thylakoids are embedded in the cell membrane.

What is the Calvin cycle?

The light-independent reactions of photosynthesis, whose end goal is to produce C₆H₁₂O₆ (glucose).

Three stages:

1. Carbon fixation

2. Reduction

3. RuBP regeneration

What is carbon fixation in the Calvin cycle?

The attachment of a CO₂ molecule to RuBP, catalysed by rubisco.

• CO₂ + RuBP (5C) → unstable 6-carbon intermediate

• The intermediate almost immediately breaks down into 2 molecules of glycerate-3-phosphate (GP, 3C)

• For 1 glucose: 6 CO₂ are fixed to 6 RuBP → 12 GP

What is rubisco?

Ribulose Bisphosphate Carboxylase — the enzyme responsible for carbon fixation in the Calvin cycle.

• The most abundant enzyme on Earth

• Needed in high concentrations in the stroma because it works slowly

What is RuBP?

Ribulose bisphosphate — a 5-carbon compound that CO₂ attaches to during carbon fixation.

Describe the reduction stage of the Calvin cycle.

Causes the synthesis of triose phosphate (TP, also called G3P).

• GP is reduced to TP (the number of carbons does not change — both are 3C)

• Requires the NADPH and ATP made in the light-dependent reactions

• ATP provides phosphate and energy

• NADPH provides electrons (gets oxidized back to NADP⁺)

• For 1 glucose: 12 GP + 12 ATP + 12 NADPH → 12 TP

Describe RuBP regeneration in the Calvin cycle.

• A cycle is more efficient than a linear pathway, so RuBP must be regenerated

• 5/6 of the TP produced is used to regenerate RuBP

• For 1 glucose: 10 of the 12 TP are used to regenerate 6 RuBP, while the remaining 2 TP leave the cycle to form glucose

• Additional ATP is required for this regeneration

Give the overall stoichiometry of the Calvin cycle per glucose molecule.

To produce 1 glucose (C₆H₁₂O₆):

• 6 CO₂ are fixed

• 12 NADPH are used (in the reduction stage)

• ATP is used in both the reduction and regeneration stages

• 12 TP are produced → 2 leave the cycle to form glucose, 10 regenerate RuBP

What can the products of the Calvin cycle be used for?

Glucose can be used as:

• A respiration substrate (in mitochondria)

• Cellulose (for the cell wall)

• Stored as starch

TP can also be converted to amino acids and lipids via other metabolic pathways.

• These conversions require extra minerals from the soil (e.g. nitrogen, sulfur, phosphorus)

How are the light-dependent (LD) and light-independent (LID) reactions interdependent?

• LD reactions produce ATP and reduced NADP

• LID reactions (Calvin cycle) require ATP and reduced NADP to continue

• Photosystem II produces ATP (from ADP)

• Photosystem I produces reduced NADP (from NADP)

• A lack of CO₂ stops the Calvin cycle → NADP⁺ and ADP are not regenerated → the light-dependent reactions are also inhibited

How does light intensity affect the rate of photosynthesis?

• At low light intensity: limited production of ATP and reduced NADP → carbon fixation is slow

• As light intensity increases: rate of photosynthesis increases proportionally

• At high light intensity: carbon fixation reaches a maximum / plateau — further increases in light don't raise the rate (another factor becomes limiting, e.g. CO₂)

Why is the Calvin cycle cyclical rather than linear?

A cycle is more efficient than a linear pathway because the starting material (RuBP) is regenerated and reused.

• 5/6 of the TP produced is used to regenerate RuBP

• This allows the cycle to continue indefinitely, as long as CO₂, ATP and NADPH are availab