1B: Photosynthesis (Light Dependent Reactions)

What is Photosynthesis?

Photosynthesis is the biochemical process by which phototrophic organisms—such as plants, algae, and certain bacteria—convert light energy into chemical energy.

What are the main substances involved in photosynthesis?

Photosynthesis involves carbon dioxide (CO₂) and water (H₂O) as key reactants and produces glucose (C₆H₁₂O₆) and oxygen (O₂) as products.

Photosynthesis involves carbon dioxide (CO₂) and water (H₂O) as key reactants and produces glucose (C₆H₁₂O₆) and oxygen (O₂) as products.

The key energy products of light reactions are ATP and NADPH

What is the role of ATP in photosynthesis?

ATP serves as the energy currency of the cell, providing the necessary energy for various cellular processes.

What is the role of NADPH in photosynthesis?

NADPH functions as a reducing power, carrying electrons and hydrogen ions to be used in the Calvin cycle for CO₂ fixation.

Where do light reactions occur in the chloroplast?

Thylakoid membranes

What are the components of thylakoids?

Thylakoids are membrane-bound compartments containing chlorophyll and other pigments, essential for absorbing light energy.

What are the inputs to light reactions?

The inputs to light reactions are light, water (H₂O), NADP⁺, and ADP.

What are the outputs of light reactions?

The outputs of light reactions include oxygen (O₂), ATP, and NADPH.

What is the role of chlorophyll in photosynthesis?

Chlorophyll, the primary photosynthetic pigment, absorbs specific wavelengths of light and acts as the main electron donor during light reactions.

What are accessory pigments in photosynthesis?

Accessory pigments, like chlorophyll b, help capture light energy and increase the range of wavelengths usable for photosynthesis.

What are the two types of photosystems?

The two types of photosystems are Photosystem II (PSII, with P680) and Photosystem I (PSI, with P700).

What is the Z-Scheme?

The Z-Scheme is a diagram that illustrates the energy levels of electrons as they move from water (H₂O) to NADP⁺ during light reactions.

What is Chemiosmosis in photosynthesis?

Chemiosmosis is the process that couples the electron transport chain with ATP synthesis using a transmembrane proton gradient.

What is Cyclic Electron Flow?

Cyclic Electron Flow (CEF) is a process that occurs when light reactions produce insufficient ATP relative to NADPH, redirecting electrons to produce extra ATP.

What is Photosystem II (PSII)?

Photosystem II (PSII) is one of the two photosystems involved in photosynthesis. It has a reaction center chlorophyll known as P680, which absorbs light energy and initiates the process of photolysis (splitting of water). During this process, water (H₂O) is split into protons (H⁺), electrons (e⁻), and oxygen (O₂). The electrons released from water are energized by light and subsequently enter the electron transport chain.

How does PSII function in light reactions?

• Light is absorbed by chlorophyll molecules, exciting electrons in the P680 reaction center.

• The energized electrons are transferred to a primary acceptor and then passed through a series of proteins in the electron transport chain, including plastoquinone.

• During this transfer, energy is released, which is used to pump protons into the thylakoid lumen, creating an electrochemical gradient.

• The splitting of water replenishes the electrons in PSII, releasing O₂ as a byproduct.

What is Photosystem I (PSI)?

Photosystem I (PSI) is the second photosystem in the light reactions of photosynthesis. Its reaction center contains chlorophyll P700, which absorbs light energy at a different wavelength compared to PSII. PSI's main function is to facilitate the reduction of NADP⁺ to NADPH.

How does PSI operate in light reactions?

• Light is absorbed by the P700 reaction center, energizing its electrons.

• These excited electrons are transferred to ferredoxin, a protein that carries electrons.

• Ferredoxin then reduces NADP⁺ to NADPH, which acts as a reducing agent in the Calvin cycle.

• Unlike PSII, PSI does not contribute to splitting water, but it receives electrons from the electron transport chain, which come from PSII.

What is the role of the cytochrome b6f complex?

The cytochrome b6f complex plays a critical role in photosynthesis as part of the electron transport chain between PSII and PSI. Its functions include:

• Accepting electrons from plastoquinone after they have passed through PSII.

• Transferring these electrons to plastocyanin, which then delivers them to PSI.

• Using the energy from the electron transfer to pump protons into the thylakoid lumen, contributing to the proton gradient needed for ATP synthesis through chemiosmosis.

How does the electron transport chain function between PSII and PSI?

Electrons from PSII are transferred to plastoquinone, which carries them to the cytochrome b6f complex.

• The cytochrome b6f complex facilitates the transfer of electrons to plastocyanin.

• This process also involves the pumping of protons into the thylakoid lumen, enhancing the proton gradient.

• The electrons then travel to PSI, where they energize the P700 reaction center.

What is the significance of splitting water in photosynthesis?

Splitting water in photosynthesis is vital for several reasons:

• It replenishes the electrons lost by PSII, allowing the continuation of the electron transport process.

• It produces oxygen (O₂) as a byproduct, which is essential for aerobic respiration in many living organisms.

• The protons released contribute to the proton gradient used to generate ATP during photophosphorylation.

How do C3, C4, and CAM photosynthesis differ in how they supply CO₂ to the Calvin cycle (and thus use ATP/NADPH from light reactions)?

• C3: CO₂ directly enters Calvin cycle → uses ATP + NADPH immediately

• C4: CO₂ concentrated in bundle sheath before Calvin cycle → requires extra ATP

• CAM: CO₂ stored at night, released in day → uses ATP + NADPH during day when light reactions are active

Why do C4 and CAM pathways require more ATP from light reactions than C3?

• Both use additional steps to concentrate/store CO₂

• C4: ATP used to transport/regenerate PEP

• CAM: ATP used in storage + release processes

• Result: higher ATP demand → relies on photophosphorylation (and cyclic flow if needed)

How do environmental conditions affect whether C3, C4, or CAM pathways are advantageous in relation to light reactions?

• C3: Optimal in moderate light, low temperature → less ATP demand

• C4: High light → supports extra ATP production needed

• CAM: Hot/dry → stomata closed in day, but light reactions still supply ATP/NADPH for daytime carbon fixation

What is photophosphorylation and how is ATP produced?

Photophosphorylation = light-driven ATP synthesis

• Light excites electrons in PSII → PSI (Z-scheme)

• Electron transport pumps H⁺ into thylakoid lumen

• Creates proton motive force (ΔμH)

• H⁺ flows through ATP synthase (CF₀–CF₁) → ATP formed

Differentiate cyclic and non-cyclic photophosphorylation

• Non-cyclic:

  • PSII → PSI → NADPH

  • Produces ATP + NADPH + O₂

• Cyclic:

  • PSI only (electrons cycle back)

  • Produces ATP only (no NADPH, no O₂)
    Function: increases ATP supply to match Calvin cycle demand

Why is cyclic photophosphorylation required in photosynthesis?

• Calvin cycle needs 3 ATP : 2 NADPH per CO₂

• Non-cyclic flow does not produce enough ATP

• Cyclic flow increases proton gradient → more ATP without making NADPH

What is chemiosmosis and how does it generate ATP in the light-dependent reactions?

• Chemiosmosis = ATP production using a proton gradient (ΔμH) (proton motive force) across the thylakoid membrane

• Electron transport (PSII → PSI) pumps H⁺ into the thylakoid lumen

• Photolysis of water adds additional H⁺ to lumen

• Creates electrochemical gradient (ΔpH + ΔΨ)

• H⁺ flows back into stroma via ATP synthase (CF₀–CF₁)
  → Drives photophosphorylation (ADP + Pi → ATP)

Key idea: Converts energy from electron transport into ATP via proton flow

What are CF₀ and CF₁ in chloroplast ATP synthase and how do they function?

Couples the Proton Gradient to ATP Synthesis.

• CF₀: Membrane-embedded subunit

  • Forms H⁺ channel across thylakoid membrane

  • Allows proton flow from lumen → stroma

• CF₁: Peripheral (stroma-facing) subunit

  • Contains catalytic sites

  • Synthesizes ATP from ADP + Pi

Mechanism:
H⁺ flow through CF₀ drives conformational changes in CF₁ → ATP production

What is the overall outcome and significance of the light-dependent reactions?

• Convert light energy → chemical energy (ATP + NADPH)

• Use H₂O as electron source → release O₂ (photolysis)

• Generate proton motive force (ΔμH) via electron transport → drives ATP synthesis

• Supply ATP + NADPH to the Calvin cycle for carbon fixation

Overall: Energy capture + conversion stage of photosynthesis enabling CO₂ reduction