Photosynthesis

Stroma

• Fluid in which light independent reactions take place, second part of photosynthesis

Thylakoid

- contains pigments for light dependent photosynthesis, contains enzymes for this (first part of photosynthesis). Thylakoids made of membrane

Stages of photosynthesis

Photosynthesis is split into two stages

• The light dependent reaction, which occurs in the thylakoid membranes

• The light independent reaction, which occurs in the stroma

• The light independent stage can occur in the absence of light however it requires producings produced in the light dependent stage

• The light dependent reaction has two main parts

• Production of ATP

• Photolysis of water

Production of ATP

The thylakoid membranes contain the proteins and enzymes needed for the

light dependent reaction

The membranes contain two trans-membrane complexes called

photosystems I and ll. They are made of proteins and photosynthetic

pigments.

PSII mostly absorbs light of 680 nm

PSI mostly absorbs light of 700 nm

When light is absorbed by chlorophyll in PSII, a pair of electrons are excited

to a higher energy level — they leave the chlorophyll molecule.

The electrons are passed through a series of electron carrier molecules (also

found in thylakoid membrane) in a series of oxidation-reduction reactions

These carrier molecules form an electron transport chain. Energy is lost as

the electrons are passed from one carrier to the next.

This energy is used to produce ATP from ADP and Pi through chemiosmosis

Photosystem I also forms part of the electron transport chain

Production of ATP - Chemiosmosis

Uses proton pumps (proteins) in the thylakoid membrane

H + ions (protons) are pumped from the stroma into the thylakoid lumen against

their concentration gradient - requires energy released from the electron

transport chain

This maintains a high concentration of H + ions in the thylakoid lumen

The H + ions then move by diffusion across the thylakoid membrane back into the

stroma. They pass through the ATP synthase channel protein.

H + causes structural changes to the ATP synthase enzyme, activating it to catalyse

the formation of ATP: ADP + Pi ATP

ATP is needed for the light independent reaction (second stage of photosynthesis).

Photolysis of water

Photolysis = splitting of water using light energy

Water is split into oxygen, hydrogen ions and electrons:

The electrons replace those lost from photosystem Il — Why is this

important?

• When PSII absorbs light, it excites electrons to a higher energy level.

• These excited electrons leave PSII and move down the electron transport chain to eventually help make energy molecules (ATP and NADPH).

• Once those electrons leave, PSII is left missing electrons—kind of like a battery that has lost its charge.

• To continue absorbing light and exciting more electrons, PSII must replace the lost electrons. Otherwise, it can't pass on energy anymore because it has no electrons left to excite.

H+ contributes to the high concentration inside the thylakoid lumen — it can

then diffuse through the ATP synthase channel protein

What happens to the oxygen?

• The oxygen atoms from water combine to form O₂ gas (molecular oxygen).

• This oxygen is released as a byproduct of photosynthesis.

• It diffuses out of the thylakoid and eventually leaves the plant cell into the atmosphere.

• This is the oxygen that we and other aerobic organisms breathe!

Production of reduced NADP

NADP is an electron carrier molecule and the final molecule in the electron

transport chain

NADP is reduced by H+ and electrons to form reduced NADP (NADPH)

NADP++ + 2e-» NADPH +

Reduced NADP, along with ATP, is needed for the next stage of

photosynthesis: the light independent reaction