6.2- Light Dependent Reaction

Photosynthesis includes:

1. Light Dependent Reactions (Light Reactions).

a. Products: ATP, NADPH, O2

2. Light Independent Reactions (Dark Reactions/ Calvin Cycle).

a. Convert CO2 → glucose

Light-Dependent Reactions include…

cyclic and noncyclic photophosphorylation using photosystems.

Photosystems (PS)

large chlorophyll containing proteins found in the thylakoid membrane.

Action Spectrum

plots wavelengths 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 the ________ and uses__________.

• Thylakoid

• PS II and PS I

Noncyclic Photophosphorylation steps

1. Photons from sunlight excite electrons in PS II to a higher energy level.

2. High-energy electrons are passed to a primary electron acceptor.

3. Electrons are passed through the electron transport chain (ETC), providing energy to pump protons (H+) from stroma  thylakoid lumen (creates an electrochemical gradient).

4. H+ flow down their concentration gradient

   through ATP synthase, which converts ADP to

   ATP.

   a. This ATP is used to power photosynthesis.

   Cellular respiration still occurs to produce

   energy in plants.

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

6. Electrons are passed through another ETC.

7. High-energy electrons combine with NADP+ to form NADPH.

   a. NADPH transports high energy electrons to

   the Calvin cycle for glucose production.

Photolysis

• splitting of water by light that occurs in PS II. Water is split into:

  1. H+ (used in concentration gradients).

  2. Electrons (stored in photosystems to be excited by sunlight).

  3. Oxygen (released into atmosphere – source of oxygen we breathe).

Cyclic Photophosphorylation

• occurs in the stroma lamellae; only involves PS I

  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+ and making more ATP instead of NADPH (replenishes the ATP used in Calvin Cycle).