light dependant

Light dependant stage of photosynthesis

• Takes part in the thylakoid membranes of chloroplasts
• Photosystem 1 is mainly (and photosystem 2 is almost exclusively) on the intergranal lamellae
• The pigments trap light energy (usually from sunlight) to convert it to chemical energy as ATP

Water's role

• Water is one of the raw materials used in photosynthesis and is the source of the by product O2
• Photosystem 2 contains an enzyme that splits water in the presence of light.
• The water is split into:
• .H+ ions (protons) which are used in chemiosmosis
• .electrons to replace those lost by oxidised chlorophyll
• .oxygen
• This process is called photolysis

Products of photolysis

• The protons (H+ ions) are used in chemiosmosis to produce ATP
• They're also accepted by coenzyme NADP to reduce it ready for the light independent stage to reduce CO2 and produce organic molecules
• The electrons replace those lost by oxidised chlorophyll
• Some of the oxygen produced from photolysis is used in aerobic respiration
• The rest of it diffuses out the leaf via the stomata to keep the concentration of O2 in the leaf low

Photophosphorylation

• When photons hit a chlorophyll molecule two electrons become excited
• These electrons are captured by electron acceptors
• They're then passed along a chain of electron carriers
• Energy is released as the electrons are passed along the carriers which pumps protons across the thylakoid membrane into the thylakoid space
• A gradient is formed and the protons flow down the gradient through channels associated with ATP synthase enzymes
• This flow is called chemiosmosis
• Chemiosmosis produces kinetic energy which is converted to chemical enrgy in the ATP molecules

Cyclic photophosphorylation

• Chemiosmosis produces a force that joins ADP and Pi (inorganic phosphate) to make ATP
• The making of ATP using light energy is called photophosphorylation
• Cyclic photophosphorylation uses photosystem 1
• They pass to the electron acceptor and back to the chloropyll molecule they were lost from
• Small amounts of ATP are produced but no photolysis of water is needed and no NADP is reduced
• This is used in the light independent reaction or to swell guard cells to open the stomata.

non-cyclic photophosphorylation

• Involves both photosystems
• light strikes photosystem 2 and excites and releases a pair of electrons from the primary pigment reaction centre of the chlorophyll molecule
• The electrons pass along a chain of electron carriers and the energy produced is used to synthesise ATP
• light has also struck photosystem 1 and two electrons has been lost
• These electrons and protons produced from photolysis join NADP which reduces it
• The electrons from photosystem 2 replace the electrons lost from PS1
• electrons lost from PS2 are replaced by electrons from photolysis
• Protons from photolysis are used in chemiosmosis and captured by NADP