3.5.1 photosynthesis

• outer membrane • intermembrane space • inner membrane • stroma (aqueous fluid) • thylakoid • lumen (inside of thylakoid) • grana (stacks of thylakoids) • lamellae (thylakoid membranes)

photosynthetic pigments - there are different kinds, and each has evolved to absorb specific wavelengths (colours) of visible light

• light-dependent reaction (LDR) • light-independent reaction (LIR)/Calvin cycle

• the photosynthetic pigments eg. chlorophyll which are attached to proteins within the thylakoid membrane • they are involved in the light-dependent reaction

they absorb different wavelengths of light

• photosystem I (PSI/700) • photosystem II (PSII/680)

• chlorophyll absorbs light, leading to the photoionisation of chlorophyll • some of the energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP • the production of ATP involves electron transfer associated with the transfer of electrons down the electron transfer chain and passage of protons across chloroplast membranes and is catalysed by ATP synthase embedded in these membranes (chemiosmotic theory) • photolysis of water produces protons, electrons and oxygen

• chlorophyll in PSII absorbs light leading to photoionisation - the Mg in chlorophyll is ionised to Mg^2+ and releases two electrons • the photolysis of water produces two electrons to replace those lost from Mg (so more light can be absorbed), as well as oxygen and protons • the electrons released from Mg start moving down the electron transport chain - the energy they release is used to transport protons from the stroma to the lumen of the thylakoid • the electrons reach PSI where more light is absorbed to excite the electrons again • the electrons move down another, shorter electron transport chain, and NADP^+ reacts with a proton to form reduced NADP (NADPH) which is the final electron carrier • protons move through ATP synthase from the lumen back to the stroma, and this causes a conformational change to ATP synthase leading to the production of ATP from ADP and Pi

PSII absorbs light with a wavelength of 680nm which is higher frequency and therefore energy

the process which occurs in relation to PSI where after electrons have moved down the second electron transport chain, they are picked up by NADP, and protons diffuse through ATP synthase leading to the phosphorylation of ADP to produce ATP

• cyclic phosphorylation • this happens when there is no NADP to pick up the electrons - instead they go back into PSI • ATP is still produced by ATP synthase

• irom atoms within the proteins are reduced • this alter the permeability of the thylakoid membrane, so protons can diffuse into the thylakoid

stroma

• use reduced NADP from the light-dependent reaction to form a simple sugar • the hydrolysis of ATP from the light-dependent reaction provides additional energy for this

• carbon dioxide reacts with ribulose biphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP), which is catalysed by the enzyme rubisco • ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate (GP accepts hydrogen from reduced NADP) • some of the triose phosphate is used to regenerate RuBP in the Calvin cycle • some of the triose phosphate is converted to useful organic substances

• carboxylation - RuBP reacts with carbon dioxide (reaction catalysed by rubisco) forming an unstable 6C intermediate compound which quickly becomes 2 molecules of GP (some of them leave cycle here to form amino acids/fatty acids) • reduction - each GP molecule is reduced to a molecule of triose phosphate using NADPH, and the hydrolysis of one ATP molecule per GP molecule provides energy for this (one triose phosphate molecule every 3 cycles used to make useful organic substances) • regeneration - RuBP is regenerated from triose phosphate and the hydrolysis of one ATP molecule provides energy

• 2 cycles occur and then it is regenerated from 2 molecules of triose phosphate leaving 1 C atom • 3 cycles occur and it is regenerated from 6 molecules of triose phosphate leaving 1 C atom from each cycle which form triose phosphate

• hexose sugars (from triose phosphate) • glycerol (from triose phosphate) and fatty acids (from glycerate 3-phosphate) which form lipids • amino acids (from glycerate 3-phosphate)

• light intensity • temperature • CO2 concentration

• as light intensity/CO2 concentration increases, rate of photosynthesis increases • the maximum rate of photosynthesis is achieved • light intensity/CO2 concentration continues to increase but rate of photosynthesis plateaus because a new factor has become the limiting factor

• as temperature increases, rate of photosynthesis increases • the maximum rate of photosynthesis is achieved • temperature continues to increase but rate of photosynthesis decreases because the enzymes involved in the reaction denature