I HATE THIS CLASS

What does oxidation mean in photosynthesis?

Loss of electrons (often hydrogen) from a molecule

What does reduction mean in photosynthesis?

Gain of electrons (often hydrogen) by a molecule

Which molecule is oxidized in the light reactions?

Water (H₂O)

Which molecule is reduced in the light reactions?

NADP⁺

Where do the light reactions occur?

In the thylakoid membrane

Where does the Calvin cycle occur?

In the chloroplast stroma

What is the function of Photosystem II (PSII)?

Uses light to oxidize water and send electrons through the electron transport chain

What is the reaction center of PSII?

P680

What is the first electron acceptor after PSII excitation?

Pheophytin

What is the role of the Oxygen-Evolving Complex (OEC)?

Splits water molecules to release O₂

What ions make up the OEC cluster?

Manganese (Mn) and calcium (Ca)

Where do the electrons from water go first?

To P680⁺ in PSII

What happens when P680 absorbs a photon?

It becomes excited (P680*) and transfers an electron to pheophytin

What is the primary quinone electron acceptor in PSII?

QA

What is the secondary quinone electron acceptor in PSII?

QB

What is plastoquinone (PQ)?

A mobile electron and proton carrier in the thylakoid membrane

What does PQ do after it accepts two electrons?

It picks up two protons from the stroma and carries them to the cytochrome b₆f complex

What is the Z-scheme?

A diagram showing energy changes of electrons as they move from water to NADP⁺

What are the two excitation peaks in the Z-scheme?

P680 (PSII) and P700 (PSI)

What is the function of Photosystem I (PSI)?

Uses light energy to reduce NADP⁺ to NADPH

What is the reaction center chlorophyll of PSI?

P700

What is the final electron acceptor in the light reactions?

NADP⁺

What enzyme catalyzes NADPH formation?

Ferredoxin-NADP⁺ reductase (FNR)

What is plastocyanin (PC)?

A small copper-containing protein that transfers electrons from cytochrome b₆f to PSI

What does cytochrome b₆f do?

Mediates electron transfer from PQH₂ to PC and pumps protons into the lumen

What is the Q-cycle?

A process in cytochrome b₆f that pumps protons while recycling PQ molecules

How many protons are pumped per two electrons through the Q-cycle?

Four protons

Which side of the thylakoid membrane becomes acidic?

The lumen

What drives ATP synthesis in chloroplasts?

The proton gradient across the thylakoid membrane

What is the structure of ATP synthase?

CF₀ (proton channel) + CF₁ (ATP-forming headpiece)

What direction do protons flow to make ATP?

From the lumen into the stroma

What is photophosphorylation?

The synthesis of ATP using light energy–driven proton gradients

What is linear electron flow?

Electron transport from water to NADP⁺ that produces ATP

What is cyclic electron flow?

Electrons cycle around PSI to make ATP but no NADPH or O₂

When does cyclic flow occur?

When ATP demand exceeds NADPH demand

Which photosystems are involved in linear electron flow?

Both PSII and PSI

Which photosystem participates in cyclic electron flow?

PSI only

Which flow pathway produces oxygen?

Linear electron flow

Which flow pathway produces only ATP?

Cyclic electron flow

What is the overall energy output of the light reactions?

ATP and NADPH

How many photons are needed to move one electron from water to NADP⁺?

Eight photons total (four for PSII

What enzyme fixes CO₂ in the Calvin cycle?

Rubisco (Ribulose-1

What molecule does Rubisco act on?

RuBP (Ribulose-1

What is the first product of CO₂ fixation?

3-Phosphoglycerate (3-PGA)

Where does the Calvin cycle take place?

In the stroma of the chloroplast

What are the three phases of the Calvin cycle?

Carboxylation

How many ATP and NADPH are required to fix three CO₂?

9 ATP and 6 NADPH

What enzyme converts 3-PGA to 1 3-bisphosphoglycerate?

What enzyme converts 1 3-bisphosphoglycerate to G3P?

Glyceraldehyde 3-phosphate dehydrogenase (GAPDH)

How many G3P molecules exit the cycle per three CO₂ fixed?

One G3P

How many are recycled to regenerate RuBP?

Five G3P

What activates Rubisco?

Carbamylation of a lysine residue by CO₂ and binding of Mg²⁺

Why is Rubisco considered inefficient?

It can also bind O₂ instead of CO₂

What is photorespiration (PCO cycle)?

A process where Rubisco uses O₂

What are the products of Rubisco’s oxygenase activity?

One 3-PGA and one 2-phosphoglycolate

Where does glycolate processing occur?

In the peroxisome

Why is photorespiration wasteful?

It consumes energy and releases fixed carbon

What environmental conditions favor photorespiration?

High temperature and low CO₂

How do C₄ plants reduce photorespiration?

By concentrating CO₂ around Rubisco in bundle sheath cells

What is the first enzyme in the C₄ pathway?

PEP carboxylase

What is the first stable product in the C₄ pathway?

Oxaloacetate

What happens to malate in C₄ plants?

It is decarboxylated in bundle sheath cells to release CO₂ for the Calvin cycle

How many extra ATP are used in C₄ fixation compared to C₃?

About two extra ATP per CO₂

How do CAM plants minimize water loss?

They open stomata at night and close them during the day

When do CAM plants fix CO₂?

At night using PEP carboxylase

What do CAM plants store CO₂ as?

Malate (in vacuoles at night)

When do CAM plants release stored CO₂?

During the day for the Calvin cycle

Which type of photosynthesis is most water-efficient?

CAM

What connects the light reactions to the Calvin cycle?

ATP and NADPH

What role does the cytochrome b₆f complex play in energy transduction?

It links PSII and PSI while generating the proton gradient

Which molecule provides electrons to the photosynthetic electron transport chain?

Water

What is the main difference between PSII and PSI?

PSII oxidizes water

PSI reduces NADP⁺

What is the energy ratio needed for the Calvin cycle?

3 ATP : 2 NADPH

What would happen if Cyt b₆f stopped working?

Proton gradient would fail and ATP synthesis would stop

What metal is essential for electron transfer in plastocyanin?

Copper (Cu)

What metal is essential for electron transfer in the OEC?

Manganese (Mn)

What does ferredoxin do?

Transfers electrons from PSI to FNR

What are the products of photosynthesis?

Glucose (from G3P)

What is the overall chemical equation for photosynthesis?

6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O