5.2.1 Photosynthesis

Question 1

How are photosynthesis and respiration linked ?

Answer 1

•CO2 and H2O are the reactants of photosynthesis and produce C6H12O6 + O2
•C6H12O6 and O2 are the reactants of respiration and produce CO2 + H2O.

Question 2

Describe the structure of a chloroplast

Answer 2

*Disc shaped
*Double membrane
*Thylakoids ; flattened discs that form grana
*Stroma : fluid filled matrix with high enzyme conc and it’s own loop of DNA

Question 3

Where do the light dependant and light independent reactions occur in the plant ?

Answer 3

Light dependant : in the thylakoids of the chloroplasts
Light independent : The stroma of the chloroplasts.

Question 4

What is the role of the photosynthetic pigments ?

Answer 4

Molecules which are embedded within the thylakoid membrane, they absorb different wavelengths of light in order to maximise the rate of photosynthesis

Question 5

Name two main groups of photosynthetic pigments

Answer 5

•Primary pigments: chlorophyll (made from chlorophyll a + b)
•Accessory pigments : carotenoids (carotene + xanthophylls).

Question 6

Name the processes in the light dependant reaction

Answer 6

Cyclic
•Photoionisation (excitation of chlorophyll e- for chain)
•Electron transfer chain
•Chemiosmosis

Non cyclic only
•Reduction of NADP
•Photolysis of water

Question 7

Explain the role of light in photoionisation

Answer 7

•Chlorophyll molecules absorb energy from photons of light
•This excites 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll.

Question 8

What happens in the electron transfer chain ?

Answer 8

•Electrons released from chlorophyll move down a series of carrier proteins in the thylakoid membrane
•Redox reactions occur which release energy

Question 9

How does chemiosmosis produce ATP in the light dependant stage ?

Answer 9

•Some energy released from the ETC is used for the active transport of H+ from the stroma to the thylakoid space
•H+ ions move down the concentration gradient from thylakoid space into stroma via transmembrane channel protein ATP synthase
•ATP synthase catalysts ADP + Pi -> ATP.

Question 10

Describe non-cyclic phosphorylation

Answer 10

•Uses photosystems I & II
•Excited electrons enter ETC to produce ATP
•NADP acts as final electron acceptor and is reduced, water is photolysed to release e- and replace those lost in PSI & PSII
•The purpose is to produce ATP and rNADP for the Calvin cycle, which produces biological compounds.

Question 11

Describe cyclic phosphorylation

Answer 11

•Uses only PS I
•Excited e- enter ETC and produce ATP then return directly to PS, therefore there is no photolysis or reducing of NADP
•Purpose is to produce ATP and to meet surplus energy needs of cell.

Question 12

What happens in the photolysis of water ?

Answer 12

•Light energy is used to split water molecules into O2 and H+
•2H2O -> O2 + 4H+ + 4e-

Question 13

What happens to the products of the photolysis of water ?

Answer 13

•H+ ions move out of the thylakoid space via ATP synthase
•e- replace the lost electrons from chlorophyll
•O2 is used in respiration or diffuses out as a waste gas.

Question 14

How and where is reduced NADP produced in the light dependant reaction ?

Answer 14

•NADP + 2H+ (lysis of water) + 2e- -> reduced NADP
•Catalysed via dehydrogenase enzymes
•Occurs in the stroma of chloroplasts.

Question 15

Name the 3 main stages in the light independant reaction

Answer 15

•Carbon fixation
•Reduction
•Regeneration

Question 16

What happens during carbon fixation ?

Answer 16

•Reactions between CO2 and ribulose bisphosphate (RuBP), catalysed between ribulose bisphosphate carboxylase (RuBisCo)
•This forms an unstable 6 carbon intermediate which breaks down into 2x glycerate phosphate.

Question 17

What happens during reduction in the light independent reaction ?

Answer 17

•2 glycerate phosphate are reduced into 3 triose phosphate
•Requires 2rNADP + 2ATP, forms 2NADP and 2ADP.

Question 18

Outline the roles of triose phosphate from the light independent reaction

Answer 18

•Raw material : 1C leaves the cycle per rotation to make monosaccharides, amino acids and other bio molecules
•Involved in the regeneration of RuBP, after 1C leaves cycle 5C compound forms: RuP
•1ATP is used to turn it into RuBP.

Question 19

How many carbons are in the following compounds : RuBP, GP, GALP ?

Answer 19

•RuBP : 5C
•GP : 3C
•GALP : 3C

Question 20

Define limiting factor

Answer 20

A factor that determines the maximum rate of reaction even if other factors change to become more favourable

Question 21

Name 4 environmental factors that can limit the rate of photosynthesis

Answer 21

•Light intensity (light dependant stage)
•CO2 levels (light independent stage)
•Temperature (enzyme controller)
•Magnesium levels (chloroplasts).

Question 22

How does light intensity affect the of reaction of photosynthesis?

Answer 22

•Low light intensity : slower light dependant reaction, less ATP and rNADP produced, less GP able to be converted to TP for light independent