6.2 Light Dependent Reactions

Light dependent reaction products

ATP, NADPH, O₂

Light independent reactions/Calvin cycle

Convert CO₂ → glucose

Light Dependent Reactions

Include cyclic and noncyclic photophosphorylation using photsystems

Photosystems (PS)

Large chlorophyll containing proteins found in the thylakoid membrane

Action spectrum

Plots wavelength of light that are most effective for photosynthesis

Red/blue light

Highest rate of photosynthesis

Green light

Lowest rate of photosynthesis (chlorophyll reflects green wavelength of light instead of absorbing it; this is why plants are green)

Noncyclic photophosphorylation

Occurs in thylakoid membrane using PSII and PSI

Step 1 of noncyclic photophosphorylation

Photons from sunlight excite electrons in PSII to a higher energy level

Step 2 of noncyclic photophosphorylation

High-energy electrons are passed to a primary electron acceptor

Step 3 of noncyclic photophosphorylation

Electrons are passed through the electron transport chain providing energy to pump protons from the stroma to the thylakoid lumen (creates an electrochemical gradient)

Step 4 of noncyclic photophosphorylation

Proton flow down their concentration gradient through ATP synthase converts to ADP to ATP

Purpose of ATP being produced in photosynthesis

Used to power photosynthesis; cellular respiration still occurs to produce energy in plants

Step 5 of noncyclic photophosphorylation

Electrons (now low energy) arrive at PS I; photons from sunlight re-excite electrons

Step 6 of noncyclic photophosphorylation

Electrons are passed though another ETC

Step 7 of noncyclic photophosphorylation

High energy electrons combine with NADP⁺ to form NADPH

NADPH

Transports high energy electrons to the Calvin cycle for glucose production

Photolysis

Splitting of water by light that occurs in PSII

What water is split into during photolysis

• H⁺ (used in concentration gradients)

• Electrons (stored in phosystems to be excited by sunlight)

• Oxygen (released into the atmosphere)

Cyclic photophosphorylation

Occurs in the stroma lamellae; only involves PS I

Steps of cyclic photophosphorylation

1. Electrons in PS I get excited by sunlight to a high energy state

2. Electrons get recycled passed back to the first ETC, allowing more pumping of H+ to make more ATP instead of NADPH

Purpose of cyclic photophosphorylation

Light indepndent reactions uses more ATP than NADPH