Question

compare and contrast photosystems 1 and 2

how many molecules of ATP does a cell ultimately gain when 4 molecules of glucose are broken down via glycolysis. explain.

Answer 1

Answer 1: Green plants have two types of grouping system of the light harvesting pigments, their comparison is discussed below

	Photosystem I	Photosystem II
Reaction centre	P700	P680
Wavelength	Longer wavelength 700nm is absorbed	Shorter wavelength 680nm is absorbed
location	On the outer surface of the thylakoid membrane	On the inner surface of the thylakoid membrane
LHC	Light harvesting complex is firmly attached to it	LHC is detachable from PSII
Chlorophyll a/b ratio	high	low
Chlorophyll/carotenoids ratio	25:1	5:1
Cyclic and non cyclic photophosphorylation	Takes part in both	Only in non-cyclic photophophorylation
Source of electrons	From PSII	From photolysis of water
Passing electrons to	To NADP+	To PSI

Answer 2: Glycolysis is breaking down of glucose, it is the first step in aerobic as well as anaerobic respiration. It takes place in cytoplasm and does not require presence of oxygen.

ATP generation in Glycolysis takes place by substrate level Phosphorylation i.e when one compound is converted to another, the energy generated is able to phosphorylate ADP and form ATP.

In the preparatory phase of Glycolysis 2 molecules of ATP are used up hence in final calculation of ATP generation, these numbers of molecules will be subtracted from overall gain of ATP.

When one molecule of glucose is broken down, two molecules of 3-carbon compound (glyceraldehyde-3-phophate and dihydroxy acetone phosphate) are formed and

In the substrate level Phosphorylation

when 1,3-bisphosphoglycerate(2 molecules) is converted to 3-phosphoglycerate (2 molecules)

and phosphoenol Pyruvic acid (2 molecules) is converted to Pyruvic acid (2 molecules)

there is gain of 4 molecules of ATP, 2 in each of above reactions.

Hence number of ATP molecules generated will be 4 when one glucose is broken down but as mentioned earlier two molecules of ATP are used in the preparatory phase hence subtracting it, the net gain of ATP is 2 molecules.

If we have to consider it for 4 molecules of glucose then multiplying 2 by 4 we get 8 molecules as the gain of ATP in Glycolysis.

Additional information: In the process of Glycolysis when one molecule of glucose is converted to two molecules of Pyruvate, two molecules of NADH will be formed which will gain 5 molecules of ATP (one molecule of NADH is equivalent to 2.5 molecules of ATP) hence if aerobic respiration is considered in which final step is electron transport chain and oxidative Phosphorylation. To calculate the gain of ATP, we will have to consider gain of ATP by NADH also. Therefore if it is aerobic respiration, one NADH gives 2.5 molecules of ATP, hence 2 molecules will give 5ATP. And if 4 glucose molecules are utilised then 5 multiplied by 4 will give us 20 molecules of ATP

Hence net gain of ATP will be 8 +20 = 28 in aerobic respiration.

In the above question ATP gain from Glycolysis is asked, hence if it is anaerobic respiration where electron transport chain does not take place, net gain of ATP by utilisation of 4 glucose molecules will be 8.

note: if any doubt,please ask in comments section.