Component 1.2 - Photosynthesis LDS

Question 1

What is an absorption spectrum?

Answer 1

A graph that shows how much light is absorbed at different wavelengths

Question 2

What is an action spectrum?

Answer 2

A graph showing the rate of photosynthesis at different wavelengths

Question 3

What are the two main classes of pigments that act as transducers?

Answer 3

Chlorophylls and carotenoids

Question 4

Action spectrum and absorption spectrum have a close correlation, what does this mean?

Answer 4

It suggests that the pigments responsible for absorbing light are used in photosynthesis

Question 5

What is a photosystem made up of?

Answer 5

1) An antenna complex - harvests light energy and passes the excitation to the reaction centre from one pigment molecule to another (made up of 400 photosynthetic pigments)
2) A reaction centre (within the antenna complex) contains two molecules of chlorophyll a which emits an electron when it is excited

Question 6

What are the similarities and the difference between photosystem 1 and photosystem 2?

Answer 6

Similarities:
Both have an antenna complex of all pigment types and a reaction centre of two chlorophyll a molecules

Differences:
They absorb light at different wavelengths and therefore their absorption peaks are different

Question 7

What are the products of the light dependent stage ?

Answer 7

ATP - to synthesise energy rich molecules such as glucose

Reduced NADP

Question 8

What is a by-product/waste product of the light dependent stage?

Answer 8

Oxygen, derived from water.

Question 9

What happens in cyclic photophosphorylation?

Answer 9

1) PS1 absorbs photons and electrons in chlorophyll a are excited
2) These are emitted and picked up by an electron acceptor which passes them down a chain of carriers
3) Energy released as electron passes down transport chain phosphorylates ADP to ATP
4) Electrons can pass back to PSI via proton pump

Question 10

Describe the steps of non-cyclic photophosphorylation

Answer 10

1) Electrons in chlorophyll a molecules (in reaction centre)of PS2 are excited and emitted
2) They are picked up by an electron acceptor which passes them onto an electron transport chain to PS1
3) Electron passage down transport chain makes energy available to phosphorylase ADP.
4) Electrons from chlorophyll a molecules in PS1 are excited, emitted and picked up by an electron acceptor

• Chlorophyll molecules in PSII are left with positive charge - oxidised

Question 11

Describe the photolysis of water

Answer 11

Water molecules in thylakoid space is split into 2H+, 2e- + 1/2O2 by absorbing photons

1) Electrons produced replace those lost from PS2 in non-cyclic photophosphorylation
2) 2 protons from water and the 2 electrons lost from PS1 reduce NADP

Question 12

What is the passage of protons in the light dependent stage?

Answer 12

1) As electrons pass through a proton pump in the thylakoid membrane they provide energy to pump protons from the stroma into the thylakoid space
2) Electrochemical gradient - more inside thylakoid space than in stroma (lower PH)
3) Chemiosmosis - H+ ions diffuse down an electrochemical gradient through ATP synthetase releasing energy so that ADP is phosphorylated to ATP

Question 13

What 3 factors maintain the proton gradient ?

Answer 13

1) photolysis of water in the thylakoid space
2) removal of protons from the stroma to reduce NADP
3) proton pumps pushing protons into thylakoid space

Question 14

What is the function of the pigments in chloroplasts?

Answer 14

• Absorb light energy and begin its conversion to storable chemical energy
• Different pigments absorb photons at different wavelengths

Question 15

Where does the light dependent stage occur?

Answer 15

Thylakoid membranes

Question 16

Why is it bad if non-cyclic photophosphoryation cannot take place?

Answer 16

• It stops electrons from PSII being moved to PSI
• This blocks the reduction of NAD to reduced NAD
• Calvin cycle cannot work without reduced NAD
• Cannot form glucose for respiration