3.5.1 Photosynthesis

light-dependent reaction

-requires light

-occurs in the thylakoids of chloroplasts

-chloroplasts contain photosynthetic pigments

coenzyme

a molecule that links 2 processes together by carrying substances

e.g. NADP transfers H to other molecules - NADP reduced to form NADPH

photophosphorylation - light dependent reaction

some of the energy absorbed is used to add Pi to ADP and form ATP and some is used to form NADPH

NON-CYCLIC photophosphorylation 1) light energy excites electrons

-light energy is absorbed by PSII which excites electrons to a higher energy level

-these high energy electrons are released from chlorophyll and pass through down the electron transfer chain = photoionsation

-as electrons will lose energy and move down energy levels to reach PSI

NON-CYCLIC photophosphorylation 2) photolysis of water

-light energy splits water into protons, electrons + oxygen

-water = source of electrons

H2O ⇌ ½ O₂ + 2H⁺ + 2e^-

-protons = remain in thylakoid lumen

-electrons = replace electrons lost from chlorophyll

-oxygen = used for respiration or diffuses out

NON-CYCLIC photophosphorylation 3) formation of ATP

-energy lost from excited electrons is used to transport protons into thylakoid lumen so thylakoid has higher conc of protons than stroma

-forms electrochemical H⁺ gradient across thylakoid membrane

-protons diffuse through ATP synthase embedded in membrane which changes shape, unlocking ADP + Pi binding sites to form ATP = substrate level phosphorylation

-some energy produced is also used in active transport of H⁺ from stroma to thylakoid lumen

NON-CYCLIC photophosphorylation 4) formation of NADPH

-light energy is absorbed by PSI which excites electrons to an even higher energy level

-electrons are transferred to NADP along with a proton from stroma to form NADPH

CYCLIC photophosphorylation

-produces ATP and only uses PSI

-electrons from chlorophyll are not passed onto NADPH but passed back to PSI by electron carriers

-so electrons are recycled and can repeatedly flow through PSI

-only occurs sometimes when conc of NADPH is too high or demands for ATP are very high

light-independent reaction

-occurs in the stroma of chloroplasts

-uses the products NADPH and ATP from light-dependent reaction to catalyse reactions

STAGES of light-independent 1) carbon fixation

-CO2 diffuses into stroma and is then trapped by the enzyme rubisco

-CO2 combines with ribulose biphosphate (RuBP) which is catalysed by rubisco

-this forms an unstable 6-carbon intermediate which then breaks down to produce two glycerate 3-phosphate (GP)

STAGES of light-independent 2) reduction 

-2 GP is reduced to triose phosphate (TP)

-uses energy from hydrolysis of 2 ATP and uses H+ ions from NADPH

-some of the TP is converted into glucose (1/6 carbons)

STAGES of light-independent 3) regeneration

-after the 1 carbon leaves, the 5/6 carbons left form ribulose monophosphate (RuP)

-this then regenerates and produces RuBP using ATP for the phosphate group

uses of ATP + NADPH

ATP = provides energy and provides Pi to convert RuP to RuBP

NADPH = GP reduced to TP