The Biochemistry of Photosynthesis

Question 1

what are the 3 stages of photosynthesis?

Answer 1

-light harvesting
-the light dependent stage
-the light independent stage

Question 2

what are accessory pigments?

Answer 2

-the chlorophyll and other pigments which are located in the thylakoid membranes

Question 3

what is the antenna complex?

Answer 3

-the accessor pigments which are arranged in clusters with each cluster containing hundreds of these pigments

Question 4

how is the one molecule of chlorophyll a situated in the antenna complex?

Answer 4

-a molecule of chlorophyll a, the primary pigment, is found at the base of the antenna complex in a region called the reaction center

Question 5

how does the antenna complex maximize light absorption?

Answer 5

-the whole antenna harvests light over a range of different wavelengths

Question 6

what is the process of resonance energy transfer?

Answer 6

-light energy which is passed from adjacent molecules by special proteins
-this energy is funneled towards the reaction center at the bottom of the antenna complex

Question 7

what does each antenna complex form?

Answer 7

-a photosystem

Question 8

what are the two stages of photosynthesis?

Answer 8

-light dependent and independent stage

Question 9

where does the light dependent stage occur

Answer 9

-in the thylakoid membranes of the chlorophyll

Question 10

describe the process of the light dependent stage in photosystem 2?

Answer 10

-if enough light energy is hitting electrons in chlorophyll a (the primary pigment) the electrons become excited and are emitted through the process of photoactivation
-the emitted electrons are taken up by an electron acceptor
-the electrons then are passed down an electron carrier chain which is made up primarily cytochromes
-a molecule of ATP is formed by photophosphorylation

Question 11

describe the process of the light dependent stage during photosystem 1 including photolysis

Answer 11

-these electrons after giving out a molecule of ATP are at lower energy, when they enter PS1 more light energy hits them making them excited again
-again they are emitted through photoactivation to another electron acceptor they then are passed to NADPH
-photolysis occurs which is the splitting of water into hydrogen ions and electrons and oxygen
-the hydrogen ions combine with the electrons in the acceptor to form hydrogen which reduces the NADP into NADPH which is passed to the light independent stage
-the electrons from photolysis are used to replace the electrons lost in photoactivation
-the oxygen is given off as a waste product

Question 12

what are the end products of the light dependent stage?

Answer 12

-ATP and NADPH

Question 13

what us the absorption peak of chlorophyll a in PS11 and PS1?

Answer 13

-in PS11 it is 680nm
-in PS1 it is 700nm

Question 14

what is another name for the light independent stage?

Answer 14

-the calvin cycle

Question 15

where does the light independent stage occur?

Answer 15

-in the stroma

Question 16

what does the light independent stage rely on?

Answer 16

-the products from the light dependent reaction
-ATP and NADPH

Question 17

explain the process of the light independent stage

Answer 17

-CO2 is fixed with the 5 carbon compound ribulose bisphosphate (RuBP)
-this is catalysed by the enzyme rubisco
-this forms two molecules of glycerate phosphate (GP)
-the NADPH is used to reduce the GP into 2 molecules of triose phosphate by releasing its hydrogen molecule
-during this a molecule of ATP is used which results in the formation of ADP + Pi
-5/6 molecules of triose phosphate are recycled to continue the cycle to produce Ribulose phosphate
-the ribulose phosphate is then converted into ribulose bisphosphate and another molecule of ATP is used to provide energy for this to happen
-1/6 of the triose phosphate is used to produce useful organic molecules e.g. glucose, starch and lipids

Question 18

how many times must the light independent reaction occur to produce a molecule of glucose and why?

Answer 18

-6 times
-because 1/6 of the triose phosphate is used to produce useful products like glucose and starch

Question 19

how many molecules of CO2 is needed to produce a molecule of glucose?

Answer 19

-6

Question 20

what is the effect of light being removed on the concentration of glycerate phosphate and ribulose bisphosphate?

Answer 20

-when light is removed the light dependent reaction stops
-ATP and NADPH not made
-they cannot be used in the light independent reaction
-glycerate phosphate cannot be converted to triose phosphate
-buildup of glycerate phosphate and a reduction of ribulose bisphosphate
-cycle stops

Question 21

what is the effect of CO2 being removed on the concentration of glycerate phosphate and ribulose bisphosphate?

Answer 21

-CO2 can no longer be fixed to combine with ribulose bisphosphate therefore glycerate phosphate cannot be formed
-the RB builds up and the GP that was already made is used up so concentration decreases
-cycle stops