LECTURE - PHOTOSYNTHESIS DR AKM SHAFIQUL ISLAM

ELECTRON TRANSPORT PATHWAY Occurs within the inner mitochondrial membrane Electrons are removed from NADH and shuttled through a series of electron acceptors –Energy is removed from the electrons with each transfer This energy is used to make ATP –NADH  3 ATP –FADH 2  2 ATP –O 2 is the terminal electron acceptor ½O 2 + 2H + + 2e -  H 2 O –Anaerobic respiration utilizes a molecule other than O 2 as the terminal electron acceptor e.g., NO 3 -, SO 4 2-, CO 2, etc.

Respiratory chain reaction NADH + H + +1/2 O ADP + 3P i NAD H 2 O + 3 ATP and FADH 2 + ½ O ADP + 2P i FAD + 3 H 2 O + 2 ATP Overall reaction Glucose + 6 O 2 6 CO H 2 O  G o ’= -696 kcal/mol

Review of Cellular Respiration –one 6-C glucose oxidized to 6 CO 2 molecules –2 ATP from substrate level phosphorylation in glycolysis –2 ATP from substrate level phosphorylation in citric acid cycle –Each NADH generates a maximum of 3 ATP 10 NADH = maximum of 30 ATP –Each FADH 2 generates a 2 ATP 2 FADH 2 = 4 ATP –TOTAL possible ATP = 38

Photophosphorylation Only occurs in photosynthetic cells which contain light trapping pigments such as chlorophyll Light causes chlorophyll to give up electrons Energy released from the transfer of electrons (oxidation) of chlorophyll through a system of carrier molecules is used to generate ATP

Photosynthesis Photosynthesis is largely reverse of respiration Energy in the form of light is captured and used for conversion of carbon dioxide to glucose and its polymers Photosynthesis is the prime supplier of energy for biosphere 6 CO H 2 O + light  C 6 H 12 O 6 + O 2

PHOTOSYNTHESIS Who does photosynthesis? Though plants can photosynthesize, microorganisms are responsible for the majority of the photosynthesis occurring on the planet

CHLOROPLASTS Chloroplasts are the site of photosynthesis –photosynthetic bacteria don’t have chloroplasts, they essentially are chloroplasts Chloroplasts contain the pigment chlorophyll –Pigments absorb light –Multiple similar forms of chlorophyll exist –The light absorbed is ultimately used to reduce CO 2 to glucose

In procaryctes, (cyanobacteria, green sulfur bacteria, purple sulfur bacteria) photosynthesis occurs in stacked membranes While organelle called the chloroplast conducts photosynthesis for eucaryotes (algae, plants) Both systems contain chlorophylls which strongly absorb visible light

Two different light-harvesting and reaction system. 1.Photosystem I – activated about 430 nm. 2.While phosphosystem II activated by light with wavelength below 680 nm.

In the case of phosphosystem I, the electron used to reduce NADP +. Excited electron from phosphsystem II are passed to phosphosystem I with ADP phosphorylation accomplished once for every excited electron pair transported. Thus the overall stoichiometry for the photosynthetic eucaryotes is H 2 O + 4hv + NADP+ + ADP + P i  NADPH + H+ +1/2 O 2 + (ATP + H 2 O) Additional ATPs may be generated from absorbed light energy via cyclic photophosphorylation

Additional ATPs may be generated from absorbed light energy via cyclic photophosphorylation. Electron excited from P700 to P430 flow to cyt b 563 and back to P700, causing ADP phosphorylation in the process. 3hv + ADP + P i  ATP + H 2 O

Biosynthesis The phenomenon that characterize the microbial process are –Substrate or nutrient utilization –Cell growth –Product release Biosynthesis influences all the three

Nutrient requirements are directed by cells need for precursor molecule, stored chemical energy and reducing power The rate of cell growth is determined by the rate of biosysnthesis, the rate at which new cell materials are formed Cell growth rate – varies widely. E. coli bacteria can double in 20 min.

Synthesis of Small Molecules Monomeric building blocks are constructed. Approximately 70 different compounds are required in this purpose –4 ribonucleotides –4 hydroxyribonucleotides –20 amino acid –about 15 monosaccharides –about 20 fatty acids and lipids ATP, NAD, other carrier and coenzymes involved in the synthesis

Living cell assimilate nitrogen by incorporating it into the amino acids glutamic acid and glutamine First, glutamic acid is formed by reaction between ammonia and  -ketoglutamic acid, one of the TCA cycle intermediate HOOC(CH 2 ) 2 COCOOH + NH NADH HOOC(CH 2 ) 2 CHNH 2 COOH + NAD + + H 2 O glutamate dehydrogenase

Additional ammonia can be accepted by adding it to glutamic acid to give glutamine HOOC(CH 2 ) 2 CHNH 2 COOH + NH ATP HOOCNH 2 CH(CH 2 ) 2 CONH 2 + ADP + P i + H + The second reaction require metabolic energy and ammonia. In some bacteria direct amination of pyruvate to alanine occurs with consumption of NADH glutamine synthase

Other amino acids are formed by conversion of glutamate or by transfer of its amino group to other carbon skeletons –Example, glutamate is converted to poline in a sequence of two enzyme-catalyzed reaction plus a spontaneous hydrolysis step C 5 NH 8 O ATP + 2(NADP + H + ) C 5 NH 8 O ADP + P i + 2NADP + + H 2 O

Transpotation from glutamate to alanine and aspartate Glutamate + oxaloacetate   -ketoglutarate + aspartate Glutamate + pyruvate   -ketoglutarate + alanine