Light-dependent Reaction

Question 1

Describe and explain where photosynthetic pigments are found within chloroplasts

Answer 1

Chloroplasts contain photosynthetic pigments (e.g. chlorophyll a, chlorophyll b, carotene).
These are coloured substances that absorb light energy needed for photosynthesis. The pigments are found in the thylakoids membranes - they’re attached to proteins. The protein and the pigment is called a photosystem.

Question 2

Describe and explain photo systems

Answer 2

• contains two types of photosynthetic pigments - primary pigments and accessory pigments
• primary pigments are reaction centres where electrons are excited during the light dependent reaction.
• in most chloroplasts the primary pigment is chlorophyll
• accessory pigments make up light-harvesting systems
• these surround reactions centres and transfer light energy to them to boost the energy available for electron excitement to take place
• e.g. of accessory pigments are carotenoids, chlorophyll b

There are two types of photosystems used by the plant to capture energy:
- photosystem I (PSI) absorbs light best at a wavelength of 700nm
- photosystem II (PSII) absorbs light best at a wavelength of 680 nm

Question 3

Describe the two stages of photosynthesis

Answer 3

1) The light-dependent reaction
- needs light
- takes place in the thylakoid membranes of the chloroplasts
- here light is absorbed by photosynthetic pigments in the photosystems and converted to chemical energy
- the light energy is used to add a phosphate group to ADP to form ATP, and to reduce NADP to form reduced NADP (an energy rich molecule that can transfer hydrogen, and so electrons, to other molecules)
- ATP transfers energy and reduced NADP transfers hydrogen to the light independent reactions
- during the process water is oxidised to form oxygen

Light independent reaction:
- relies on ATP and reduced NADP made in light dependent reactions
- takes place in the stroma of the chloroplast
- ATP and the reduced NADP from the light dependent reaction supply the energy and hydrogen to make glucose from CO2

Question 4

What are the three things light energy absorbed by the photosystems is used to do?

Answer 4

1) making ATP from ADP and inorganic phosphate (phosphorylation)
2) making reduced NADP from NADP
3) splitting water into H+ ions, electrons and oxygen (this is called photolysis)

Question 5

Describe and explain non-cyclic photophosphorylation

Answer 5

Non-cyclic photophosphorylation produces ATP, reduced NADP, and oxygen
Photosystems in the thylakoids membranes are linked by electron carriers (proteins that transfer electrons). The photosystems and electron carriers form an electron transport chain (a chain of proteins through which excited electrons flow)

Question 6

Describe and explain the stages of non-cyclic photophosphorylation

Answer 6

1) light energy excites electrons in chlorophyll
- light energy is absorbed by PSII (P680)
- light energy excites electrons in chlorophyll
- the high energy electrons move along the electron transport chain to PSI (P700)

2) photolysis of water produced protons, electrons and oxygen
- as the excited electrons from chlorophyll leave PSII to move along the electron transport chain, they must be replaced
- light energy splits water into protons (H+ ions), electrons and oxygen (so the oxygen in photosynthesis comes from water)
Reaction: H2O —> 2H+ + 1/2O2

3) energy from the excited electron makes ATP
- the excited electrons lose energy as they move along the electron transport chain
- this energy is used to transport protons (H+) into the thylakoids, via membrane proteins called protein pumps, so that the thylakoid has a high conc of protons than the stroma
- this forms a proton gradient across the membrane
- protons move down their conc gradient and into the stroma via an enzyme called ATP synthase
- the energy from this movement combines ADP and inorganic phosphate to form ATP

4) Energy from the excited electrons generates reduced NADP
- light energy is absorbed by PSI, which excites the electrons again to an even higher energy level
- finally the electron are transferred to NADP, along with a proton from the stroma to form reduced NADP

Question 7

Describe and explain cyclic photophosphorylation

Answer 7

• only produces ATP and only uses PSI
• its called ‘cyclic’ because the electrons from the chlorophyll molecule are passed onto NADP, but are passed back to PSI via electron carriers.
• this means the electrons are recycled and can repeatedly flow through PSI
• no NADP and oxygen are produced