8.3 Photosynthesis

Question 1

Where do the light-dependent reactions take place?

Answer 1

The light dependent reactions take place in the inter-membrane space of the thylakoids.

Question 2

What are the thylakoids?

Answer 2

The chloroplast has an outer membrane and an inner membrane. The inner membrane encloses a third system of interconnected membranes called the thylakoid membranes. Within the thylakoid is a compartment called the thylakoid space. The light-dependent reactions take place in the thylakoid space and across the thylakoid membranes.

Question 3

What are the products of the light-dependent reactions?

Answer 3

Reduced NADP and ATP so NADPH + H+ are produced in the light dependent reactions.

Light energy is converted into chemical energy in the form of ATP and reduced NADP in the light reactions. The ATP and reduced NADP serve as energy sources for the light independent reactions.

Question 4

Where do the light independent reactions take place?

Answer 4

Light independent reactions take place in the stroma. The inner membrane of the chloroplast encloses a compartment called the stroma. This is a thick protein rich medium containing enzymes for use in the light independent reactions, also known as the calvin cycle.

Question 5

What is the Calvin cycle?

Answer 5

The Calvin cycle is the light independent reactions of photosynthesis. It is an anabolic (building up) pathway that requires endergonic (absorbing energy) reactions to be coupled with the hydrolysis of ATP and the oxidation of reduced NADP.

Question 6

What is the stroma?

Answer 6

The stroma is a compartment enclosed by the inner membrane of the chloroplast. This is a thick protein rich medium containing enzymes for use in the light independent reactions.

Question 7

Outline the light dependent reactions of photosynthesis?

Answer 7

The light dependent reactions use photosynthetic pigments (organised into photostems) to convert light energy into chemical energy in the form of reduced NADP and ATP.

1) Light is absorbed by photosystem 2. A photosystem is a group of accessory photosynthetic pigments with a reaction centre made of chlorophyll. When a photosystem absorbs light energy delocalised electrons within the pigments become energised or ‘excited’. The chlorophyll is then photo-activated, the chlorophylls at the reaction centre have the special property of being able to donate excited electrons to an electron acceptor. (Energy is passed down the pigments to chlorophyll, which becomes excited and is able to pass delocalised electrons on which have been raised in energy levels). Photosystem 2 absorbs light first.
2) These excited electrons are transferred to carrier molecules or electron acceptor molecules within the thylakoid membrane. Photosystem 2 is first. The electron acceptor for photosystem 2 is plastoquinone. Plastoquinone collects two excited electrons from photosystem 2 and then moves away to another position in the membrane. Plastoquinone is hydophobic so although it is not at a fixed position in the membrane it does remain in the membrane.
3) Having accepted these electrons plastoquinone is then reduced, because it has gained 2 electrons. It then repeats this, and another molecule of plastoquinone accepts another 2 electrons from chlorophyll in photosystem 2, so it has lost 4 electrons and 2 molecules of plastoquinone have been reduced.

4) Once the plastoquinone becomes reduced, the chlorophyll in the reaction centre is then a powerful oxidising agent and causes the water molecules nearest it to split and give up electrons, to replace those that it has lost.
2H2O = O2 + 4H+ + 4e-
The splitting of water is called photolysis and it is how oxygen is produced in photosynthesis. Oxygen is a waste product and diffuses away. The useful product of photosystem 2 is the reduced plastoquinone which not only carries a pair of electrons but lots of the energy absorbed from light.

5) The reduced plastoquinone carries the pair of excited electrons from the reaction centre of photosystem 2 to the start of the chain of electron carriers.
6) Excited electrons from Photosystem 2 are used to generate a proton gradient. Once plastoquinone transfers its electrons, the electrons are then passed from carrier to carrier in the chain. As the 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 across the thylakoid membrane, which is a store of potential energy. Photolysis, which takes place in the fluid inside the thylakoids, also contributes to the proton gradient. This is because water is broken down creating more H+.
7) The protons can travel back across the membrane, down a concentration gradient, by passing through the enzyme ATP synthase. The energy released by the passage of protons down their concentration gradient is used to make ATP from ADP and inorganic phosphate.. This method of producing ATP is strikingly similar to the process that occurs inside the mitochondrion and is given the same name of chemiosmosis.
8) When 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 in the next stage of photosynthesis.
9) Chlorophyll molecules within photosystem 1 absorb light energy and pass it to the special two chlorophyll molecules in the reaction centre. This raises an electron in one of the chlorophylls to a high energy level. As with Photosystem 2, this is called photoactivation.
10) The excited electron passes along a chain of carriers in Photosystem 1, at the end of which it is passed to ferredoxin, a protein in the fluid outside the thylakoid.
11) Two molecules of reduced ferredoxin are then used to reduce NADP to NADPH + H+

The electron that Photosystem 1 donated to the chain of electron carriers is replaced by an electron in plastocyanin. Photosystem 1 and 2 are therefore linked; electrons excited in Photosystem 2 are passed along the chain of carriers to plastocyanin, which transfers them to Photosystem 1. The electrons are re-excited with light energy and are eventually used to reduce NADP.

The supply of NADP sometimes runs out. When this happens the electrons return to the electron transport chain that links the two photosystems, rather than being passed to NADP. As the electrons flow back along the electron transport chain to Photosystem 1, they cause pumping of protons, which allows ATP production. This process is cyclic photophosphorylation.

Question 8

What are photosystems?

Answer 8

Photostems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane. There are 2 types, photosystem 1 and photosystem 2.
They are almost like a funnel, with photosynthetic pigments as a large light-harvesting array with a reaction centre made of chlorophyll further down.

Question 9

What wavelength does photosystem 1 absorb?

Answer 9

700nm (bigger so discovered first)

Question 10

What wavelength does photosystem 2 absorb?

Answer 10

680nm

Question 11

Where does the light dependent part of photosynthesis start?

Answer 11

Light is absorbed by photosystem 2.

Question 12

What is the electron acceptor for photosystem 2?

Answer 12

Plastoquinone.

Question 13

What is photolysis?

Answer 13

The splitting of water and it is how oxygen is generated in photosynthesis. It occurs after photosystem 2 has given 2 electrons each to 2 molecules of its electron acceptor plastoquinone.

Question 14

What is the useful product of photosystem 2?

Answer 14

Reduced plastoquinone, which not only carries a pair of electrons but also much of the energy absorbed from light. This energy drives all the subsequent reactions of photosynthesis.

Oxygen is also produced by photolysis, when the oxidised chlorophyll breaks the water molecules apart.

Question 15

What structures do the thylakoid membranes contain?

Answer 15

• Photosytem 2
• ATP synthase
• a chain of electron carriers
• Photosystem 1

Question 16

What is the production of ATP with light called?

Answer 16

Photophosphorylation, it is carried out by thylakoids.

Question 17

What is the electron acceptor for Photosystem 1?

Answer 17

Plastocyanin

Question 18

What is the electron acceptor at the end of the electron chain after photosystem 1 called?

Answer 18

Ferredoxin, a protein in the fluid outside the thylakoid.

Question 19

How do electrons not run out in Photosystem 2 and Photosystem 1?

Answer 19

The electrons lost in photosystem 1 are replaced by de-energised electrons from photosystem 2.

Electrons lost from photosystem 2 are replaced by electrons from the splitting of water.

Question 20

What is cyclic photophosphorylation?

Answer 20

This is the process in which when NADP runs out, the electrons return to the transport chain that links the two photosystems, rather than being passed to NADP. As the electrons flow back along the electron transport chain to photosystem 1 they cause pumping of protons, which allows ATP production.

It involves the use of only one photosystem, and is used when there is no more NADP so that although no NADPH is being made, ATP can still be made. Light is absorbed by photosystem 1 and the excited electrons may enter the electron transport chain to pump H+ ions through the membrane so they follow a concentration gradient and go through ATP synthase making ATP. What is different to the normal phosphorylation is that this time the de-energised electron returns to the photosystem restoring its energy supply. In this type, water is NOT needed to replenish the electron supply.

Question 21

What are the products of the light dependent reactions?

Answer 21

ATP and NADPH

Question 22

Outline the light independent stage of reactions?

Answer 22

Light independent reactions use the products from the light dependent reactions to form organic molecules. They occur in the stoma.

CARBON FIXATION
1) It begins with a 5C compound called ribulose bisphosphate (RuBP), an enzyme RuBP carboxylase or rubisco catalyses the attachment of a CO2 molecule to the ribulose bisphosphate. The stroma contains a lot of rubisco.

2) The resulting 6C compound is therefore unstable and breaks down into 2 3C compounds called glycerate - 3 - phosphate.
3) This happens 3 times, and 3 molecules of ribulose bisphosphate have a CO2 attached and then break down into 6 molecules of glycerate-3-phosphate.

REDUCTION
4) 6 molecules of ATP reduce the 6 molecules of glycerate-3-phosphate.

5) 6 molecules of NADPH then reduce the 6 molecules of glycerate-3-phosphate again. This makes it triose phosphate. Each glycerate-3-phosphate required 1 ATP and 1 NADPH.
- These two steps need to happen because when carbon is added in step 1 to ribulose bisphosphate the hydrogen to oxygen ratio decreases. In sugars the ratio needs to be 2:1, so hydrogen has to be added. This needs to be done through reduction as reduction is the addition of hydrogen. This is done with the ATP and the NADPH. ATP provides the energy for the reduction and NADPH provides the hydrogen.
6) Of the 6 molecules of triose phosphate, one may be used to make half a glucose molecule. Hence two cycles are necessary to produce glucose and more to produce polysaccharides like starch. The remaining 5 molecules of triose phosphate are used to make more ribulose bisphosphate (5 x 3C = 3 x 5C). Remaking RuBP requires energy from the hydrolysis of ATP.
7) The 5 triose phosphates to be made back into RuBP are provided with 3 ATP, and as the ATP is dephosphorylated the triose phosphates are given the energy to turn back into 3 ribulose bisphosphates.

Question 23

Compare cyclic and non cyclic phosphorylation?

Answer 23

Cyclic phosphorylation produces ATP, and does not produce NADPH and does not need water to replenish electrons. It only uses one photosystem instead of 2. It does not produce oxygen.
This sounds better, however ATP is a highly reactive molecule and cannot be stored in the cell, and therefore it is often better to make NADPH in addition to ATP. NADPH is essential for the long term storage of energy because it is required to make organic molecules such as glucose. Therefore non-cyclic is the only one that can produce these organic molecules and allows for longer term storage.

Cyclic is therefore used to produce additional ATP in order to produce all the cells energy demands whereas non cyclic makes products used for the light independent reactions.

Question 24

Where do light independent reactions occur?

Answer 24

In the stroma

Question 25

Why are ATP and NADPH required in the light independent stage of photosynthesis?

Answer 25

When carbon dioxide is added to ribulose bisphosphate to make a 6C compound. More oxygen is in the compound. This decreases the hydrogen to oxygen ratio in the compound and sugars have the ratio 2:1 so it is necessary to increase the amount of hydrogen to make sugar. This is done through ATP and NADPH. ATP provides the energy for the reaction, and NADPH provides the hydrogen to reduce the 6C compound.

Question 26

What is triose phosphate used for?

Answer 26

Triose phosphate is used for 2 things, to regenerate RuBP and also to produce carbohydrates for example glucose or starch.
You need to use some of the triose phosphate to regenerate RuBP because if all the triose phosphate was used to form glucose or starch then supplies in the chloroplast would soon be used up. Some therefore have to be used to regenerate RuBP. This process is a conversion of 3-carbon sugars into 5-carbon sugars and it cannot be done in a single step. Instead a series of reactions take place.

1 of the molecules is used to make glucose or starch. 5 of the molecules are used for regeneration. Because ribulose bisphosphate is both consumed and produced in this set of reactions they form a cycle. It is called the Calvin cycle to honour Melvin Calvin who was given the Nobel prize for Chemistry in 1961 for his work on the process.

Question 27

How is ribulose bisphosphate regenerated?

Answer 27

Ribulose bisphosphate is reformed using ATP. In the last phase of the Calvin cycle, a series of enzyme-catalysed reactions convert triose phosphate molecules into RuBP. 3 ATP are used to provide the energy for this process and ATP is changed into ADP + Pi

Question 28

What was a huge scientific discovery that helped Melvin discover the Calvin cycle?

Answer 28

The discover of 14C really helped in working out the process of the Calvin cycle. When 2 scientists discovered 14 C in 1945 it was found to have the ideal half life to trace the pathway of photosynthesis. Melvin was able to set up an apparatus that modelled photosynthesis. He placed algae in a bottle with access to light, water and CO2. Then at the start of his experiment he replaced the CO2 store from 12CO2 to 14CO2 and then took samples of the algae at regular intervals to see what compounds in the algae contained 14C.

Question 29

Label a chloroplast?

Answer 29

• It has a double membrane forming the outer chloroplast envelope.
• An extensive system of internal membranes called thylakoids, which are an intense green colour.
• Small fluid-filled spaces inside the thylakoids.
• A colourless fluid around the thylakoids called stroma that contains many different enzymes.
• In most chloroplasts there are stacks of thylakoids, called grana.
• If a chloroplast has been photosynthesising rapidly then there may be starch grains or liquid drops in the stroma.

Question 30

What is a grana?

Answer 30

A stack of thylakoids.

Question 31

How are chloroplasts adapted?

Answer 31

• The large surface area of the thylakoid membranes ensures that the chloroplast has a large-light absorbing capacity.
• The thylakoids are often arranged in stacks called grana. Leaves that are brightly illuminated (in the sun) typically have chloroplasts with deep grana, which allow more light to be absorbed.
• The volume of fluid inside the thylakoids is very small so when protons are pumped in, a proton gradient develops after relatively few photons of light have been absorbed, allowing ATP synthesis to begin quickly.
• The stroma contains the enzymes to assist with the Calvin cycle, but having membranes and sections within the chloroplast it allows the conditions to be optimised for the reactions there. The stroma also has lots of NADPH and ATP produced by the thylakoids.