8.3 Photosynthesis

Question 1

Location of Light-Dependent Reactions

Answer 1

in thylakoid space and across thylakoid membranes

Question 2

Products of Light-Dependent Reactions

Answer 2

light energy converted to chemical energy in the form of ATP and NADPH (reduced NADP) - both serve as energy sources for light-independent reactions

Question 3

Location of Light-Independent Reactions / Calvin Cycle

Answer 3

stroma

Question 4

Photosystems

Answer 4

large light harvesting arrays made up of chlorophyll and the accessory pigments
located in thylakoids
two types: Photosystem I and Photosystem II
both have reaction centers

Question 5

Photoactivation

Answer 5

many chlorophyll molecules in the photosystems absorb light energy and pass it to two special chlorophyll molecules in the reaction center of the photosystem
when the special chlorophyll molecules absorb the energy from a photon of light an electron within the molecule becomes excited (chlorophyll is then photoactivated)

Question 6

Plastoquinone

Answer 6

the electron acceptor for Photosystem II
the chlorophylls at the reaction center have the special property of being able to donate excited electrons to an electron acceptor
plastoquinone collects two excited electrons from Photosystem II then moves away to another position in the membrane it’s hydrophobic so stays within the membrane even though it’s not in a fixed position
absorption of 2 photons of light causes the production of 1 reduced plastoquinone
plastoquinone carries the electrons to the start of the chain of electron carriers

Question 7

Photolysis

Answer 7

the splitting of H2O - it’s how oxygen is generated in photosynthesis (oxygen is a waste product and diffuses away)
once the plastoquinone becomes reduced the chlorophyll in the reaction center is then a powerful oxidizing agent and causes the H2O molecules around it to split and give up electrons to replace those it has lost

Question 8

Photophosphorylation

Answer 8

the production of ATP using energy derived from light

carried out by thylakoids

Question 9

Thylakoid membranes contain:

Answer 9

Photosystem II
ATP synthase
a chain of electron carriers
Photosystem I

Question 10

Electron Transport Chain

Answer 10

plastoquinone transfer its electrons
the electrons are passed from carrier to carrier
as electrons pass energy is released which is used to pump protons across the thylakoid membrane into the space inside the thylakoids
a concentration gradient of protons develops which is a store of potential energy
photolysis (which takes place in the fluid in the thylakoids) also contributes to the proton gradient

Question 11

Chemiosmosis

Answer 11

protons travel down the concentration by passing through the enzyme ATP synthase
energy released by the passage of protons is used to make ATP from ADP and an inorganic phosphate

Question 12

Plastocyanin

Answer 12

when the electrons reach the end of the chain of carriers they are passed to plastocyanin a water-soluble electron acceptor in the fluid inside the thylakoids
reduced plastocyanin is needed for the next stage of photosynthesis

Question 13

Photosystem I

Answer 13

chlorophyll molecules absorb light energy and pass it to the 2 special chlorophyll molecules in the reaction center
raises an electron in one of the molecules to a high energy level (photoactivation)
the excited electron passes along a chain of carriers in Photosystem I at the end of which it is passed to ferredoxin (a protein in the fluid outside the thylakoid)
2 molecules of reduced ferredoxin are used to reduce NADP (NADPH)
the electron that Photosystem I donated to the chain of electron carriers is replaced by an electron carried by plastocyanin

Question 14

Cyclic phosphorylation

Answer 14

when the supply of NADP runs out electrons return to the electron transport chain rather than being passed to NADP
as the electrons flow back along the transport chain to Photosystem I they cause pumping of protons which allows ATP production

Question 15

Carbon Fixation

Answer 15

occurs in stroma
CO2 reacts with ribulose bisphosphate (RuBP) to produce 2 molecules of glycerate 3-phosphate
enzyme: ribulose bisphosphate carboxylase (RUBISCO)

Question 16

Role of NADPH and ATP in the Calvin Cycle

Answer 16

Glycerate 3-phosphate is reduced to triose phosphate using NADPH and ATP
NADPH provides the hydrogen atoms
ATP provides the energy
needs to happen to turn Glycerate 3-phosphate into a carbohydrate because carbohydrates need to have a 2:1 hydrogen to oxygen ratio and when RuBP was converted to Glycerate 3-phosphate carbon and oxygen were added meaning that the amount of hydrogen in relation to oxygen is reduced so Glycerate 3-phosphate needs to gain hydrogen

Question 17

The Fate of Triose Phosphate

Answer 17

1st carbohydrate produced by the light-independent reactions: triose phosphate
2 triose phosphate molecules can be combined to form hexose phosphate and that can be combined using condensation reactions to form starch
BUT if all triose phosphate was converted to hexose or starch the supplies of RuBP in the chloroplast would soon be used up
some triose phosphate has to be used to regenerate RuBP

Question 18

RuBP Regeneration

Answer 18

series of enzyme-catalyzed reactions convert triose phosphate into RuBP
uses ATP
IF 3 RuBP molecules used, 6 triose phosphates produced, 1 is converted to hexose or starch, 5 are used to regenerate 3 RuBPs using 3 ATPs
(6 turns of Calvin cycle to produce 1 glucose)
(as much RuBP must be produced as consumed)

Question 19

Chloroplast

Answer 19

chloroplast envelope: a double membrane
thylakoids: an extensive system of internal membranes which are an intense green color
small fluid-filled spaces inside thylakoids
stroma: a colorless fluid around the thylakoids that contains many different enzymes
grana: stacks of thylakoids
If a chloroplast has been photosynthesizing rapidly then there may be starch grains or lipid droplets in the stroma