Module 5: Photosynthesis continued

Question 1

Within light dependent photosynthesis , name the 3 processes that can occur

Answer 1

non-cyclic photophosphorylation
cyclic photophosphorylation
photolysis

Question 2

Non cyclic-

1)What photosystems are involved?
2)Light is absorbed by each photosystem , what effect does this have?

Answer 2

1)PS2 and PS1
2)this excites electrons at the reaction centres

Question 3

Describe non cyclic photophosphorylation , there are 5 steps

(start with after electrons have been excited)

Answer 3

1)Excited electrons are released from PS2 and are passed to an electron transport chain
2)Meanwhile ATP is produced by chemiosmosis
3)Electrons lost from PS2 are replaced by the break down of water using light (photolysis)
4)Electrons are excited again by light absorbed into PS1 and are then passed to another electron transport chain(ATP again produced by chemiosmosis)
5)These electrons , alongside hydrogen ions are accepted by the coenzyme NADP forming NADPH

Question 4

Briefly describe cyclic photophosphorylation

make 3 points

Answer 4

-electrons that leave the electron transport chain after PS1 can be replaced to PS2 , instead of being used to reduce NADP
-reduced NADP is not produced
-ATP is still produced , but in a smaller quantity

Question 5

What is photolysis?
What is the purpose of photolysis in the light dependent stage of photosynthesis?

Answer 5

the break down of water molecules using light energy
to produce electrons that are used to replace ones lost from PS2

Question 6

Give the equation for photolysis

Answer 6

H2O — 1/2 O2 + 2H+ + 2e-

Question 7

What happens to the hydrogen ions ( protons ) produced during photolysis?

make 3 points about this process

Answer 7

-they are released into the lumen of the thylakoids , which increases the proton concentration across the membrane
-they move back through the membrane down their electrochemical gradient , which drives the formation of ATP
-once they are returned to the stroma , they combine with NADP and an electron from PS1 to form NADPH

Question 8

What does the removal of hydrogen ions from the stroma maintain?

Answer 8

a proton gradient across the thylakoid membranes

Question 9

Name the cycle that is involved in the light independent stage of photosynthesis

Answer 9

The Calvin Cycle

Question 10

Give the first 2 steps of the Calvin cycle

(start with entrance of CO2)

How many carbons are in a molecule of RuBP?

Answer 10

1)carbon dioxide enters intracellular space within the spongy mesophyll of leaved by diffusion through open stomata
2)It diffuses into cells and into stroma of chloroplasts where it combines with a molecule called RuBP

5 carbons

Question 11

Give the next 2 steps of the Calvin cycle

Answer 11

3)The enzyme RuBisCO catalyses the fixation of carbon dioxide and RuBP , which produces an unstable six carbon molecule
4)This unstable molecule is immediately broken down into 2 three carbon molecules , known as GP molecules

Question 12

Give the final 2 steps of the Calvin cycle

Answer 12

5)Each GP molecule is converted into another 3 carbon molecule known as TP , using a H atom from reduced NADP and energy supplied by ATP
6)TP is either used to regenerate RuBP so the Calvin cycle can continue , or its used to begin the synthesis of complex organic molecules

Question 13

Give 4 examples of complex organic molecules that TP can be used to synthesise

Answer 13

carbohydrates such as glucose
lipids
proteins
nucleic acids

Question 14

How many carbon dioxide molecules have to enter the Calvin cycle in order to produce 1 molecule of glucose?

Therefore how many TP molecules are also produced?
How many out of this number are used to make glucose?
The remaining are used for what?

Answer 14

6

12

2 are used to make the glucose molecule , so the remaining 10 are recycled to regenerate 6 RuBP molecules