A2 Biology: Photosynthesis

• Takes place in green plants
• Light energy from te sun is trapped and concerted into a cheicl energy which can be storeed in molecules of carbohydrate.
• 6CO2 + 6H2O --light energy--> C6H12O6 + 6O2
• The leaf is the main photosynthetic organ.
• The chloroplast is the organelle where photosynthesis is carried out.

Chloroplast features

• Chloroplast envelope - double membrane permeable to glucose, CO2, O2, and some ions
• Ribosome
• Circular DNA
• Thylakoid membranes (stack = graum) - contains chlorophyll for photosynthesis
• Starch grain - insoluble carbohydrate storage product of photosynthesis
• Stroma - matrix of chloroplast
• Lipid droplet - energy store made from sugars during this process

Chloroplast structure

• Outermembrane is permeable to small ions, the inner is less permeable.
• Transport proteins allow movement across.
• Inside the chloroplast is a system of membranes running through a colourless, structureless substance - matrix aka stroma.

Stroma

• gel-like substance which contains enzymes for photosynthesis.
• Site of the light independent stage of photosynthesis (Calvin cycle)
• Other structure found in the stroma of chloroplasts are starch grains, DNA, lipid droplets and ribosomes.

Thylakoids

• membrane system is the site of the light dependant stage of photosynthesis.
• membrane system consists of any flattened fluid filled sacs called thylakoids.
• forms stacks called grana at intervals
• membrane provides a large surface area covered with photosynthetic pigments, enzymes and electron carriers.
• The grana are interconnected with intergranal thylakoids/lamellae

Photosynthetic pigments

• photosynthetic pigments absorb light of certain wavelengths and reflects others.
• They are found in funnel shaped structure called photosystems which harvest lights and are situated in the thylakoid membranes.
• These include chlorophylls and the carotenoids, carotene and xanthophyll. 

Chlorophyll

• Two types: chlorophyll a (P680) and chlorophyll a (P700)
• Both absorb red light but at slightly different wavelengths and appear yellow/green.
• Found at the centre of the photosystems and are known as primary pigment reaction centre.

Chlorophyll and photosystems

• Chlorophyll a (p700) is found in photosystem I and has a peak absorption of 700 nm
• Chlorophyll a (P680) is found in photosystem II and has a peak absorption of 680 nm.

Breakdown of photosynthesis.

Two main reactions

1. Light Dependant Reaction - Produces energy from the photons (solar power) in form of ATP and NADPH - takes place in thylakoid membrane

2. Light Independent Reaction/Calvin Cycle/Carbon Fixation - Uses energy ATP and NADPH from light reaction to make glucose. - takes place in stroma]

LIGHT DEPENDENT STAGE

3 major events:

• Photolysis of water
• Light harvesting
• photophosphorylation

Photolysis of Water

• In photosystem II there is an enzyme in presents of light that can split water into protons, electrons and oxygen. This is called photolysis.
• H2O   2e- + 2H+  + 1/2O2
• Oxygen is lost through the stomata into the air
• Electrons replaces those lost by the chlorophyll during energy tansduction
• Protons are used in chemiosmosis to produce ATP and reduce the coenzyme NADP (both needed for light independent stage)

Light harvesting

• 2 distinct light harvesting systems: PS.I and PS.II
• This is where all the light energy is harvested by the chlorophylls is transferred to the few chlorophyll molecules t the centre of the two reaction centres.
• These are known as primary pigment reaction centre molecule and absorb wavelength of 700 nm in PSI and 68 in PS.II

PHOTOSYSTEM I

Cyclic phosphorylation

• Excited electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost.
• No photolysis of water and no generation of reduced NADP.
• Small amounts of ATP made by chemiosmosis.
• This may be used in the light dependant reaction or y the guard cells to open the stomata.

PHOTOSYSTEM I & II

Non cyclic-phosphorylation

• PS.II contains chlorophyll a molecules called p680
• after excitation these molecules  by light, a pair of e- are ejected from primary pigment centre
• these pass to electron carriers and the energy released is used to synthesis ATP by chemiosmosis.

• PS.I contains chlorophyll a p700
• after excitation of these molecules by light, e- are rejected from primary pigment reaction centre
• These e- along with protons (H+) join NADP (coenzyme) and it is reduced
• The e- from PS.II will now replace e- lost by PS.I
• e- from photolysis of water replace the electrons lost by PS.II
• Protons from photolysis of water takes part in chemiosmosis to make ATP and the join with NADP

Photophosphorylation 

• Energy is released a electrons pass along the chain of electron carer in the thylakoid membrane.
• this pumps proton across the thylakoid membranes into the thylakoid space where they accumulate
• a proton gradient is formed across the membrane and protons flow down their gradient through channels associated with ATP synthase enzymes.
• this flow is called chemiosmosis. it produces a force that join ADP and Pi to make ATP. 
• kinetic energy from the proton flow is converted to chemical energy in the ATP
• this formation of ATP and occurs through the processes of cyclic and non-cyclic phosphorylation

LIGHT INDEPENDENT STAGE

• Takes place in the stroma of chloroplast
• aka Calvin Cycle
• products of light dependent stage are used
• CO2 is the source of carbon for the production of organic molecules.

Calvin Cycle

• CO2 (enters via stomata) combines with ribulose bisphosphate (RuBP) (5C) to make 2 molecules of glycerate 3-phosphate (GP) (2 x 3C)
• The enzyme RUBISCO catalyses this reaction
• CO2 has now been fixed
• GP is phosphorylated and reduced to form triose-phosphate (TP) using AT and reduced NADP from the light dependent stage
• 5/6 f TP molecules are recycled back to 3 molecules of RuBP which fixes more CO2 in a cycle. The rest of the TP is used to make other compounds such as lipids, amino acids or carbohydrates.