Module 5.5 - Photosynthesis

Question 1

What is photophosporylation?

Answer 1

Adding phosphate to a molecule using light.

Question 2

What is photolysis?

Answer 2

The splitting of a molecule using light energy.

Question 3

What is decarboxylation?

Answer 3

The removal of carbon dioxide from a molecule.

Question 4

What is dehydrogenation?

Answer 4

The removal of hydrogen from a molecule.

Question 5

What different photosynthetic pigments are in chloroplasts and what are their function?

Answer 5

> Chlorophyll a, chlorophyll b and carotene.

>These are coloured substances that absorb the light energy needed for photosynthesis.

Question 6

Where are the photosynthetic pigments found?

Answer 6

The pigments are found in the thylakoid membranes - they’re attached to proteins. The protein and pigment is called a photosystem.

Question 7

How many types of photosynthetic pigments does a photosystem contain and what are they called?

Answer 7

> Two types - primary and accessory pigments.

Question 8

What is the function of the primary pigments?

Answer 8

Reaction centres where electrons are excited during the light-dependent reaction.

Question 9

What is the function of the accessory pigments?

Answer 9

They make up the light-harvesting systems. These surround reaction centres and transfer light energy to them to boost the energy available for electron excitement to take place.

Question 10

Where is the stroma found?

Answer 10

Contained within the inner membrane of the chloroplast and surrounding the thylakoids is a gel-like substance called the stroma.

Question 11

What does the stroma contain?

Answer 11

> Enzymes, sugars and organic acids.
They have their own circular DNA, there can be multiple copies in each chloroplast.
Carbohydrates produced by photosynthesis and not used straight away are stored as starch grains in the stroma.

Question 12

What is the difference between the light photosystem I and II absorb?

Answer 12

> Photosystem I absorbs light best at a wavelength of 700nm.

>Photosystem I absorbs light best at a wavelength of 680nm.

Question 13

Where does the light-dependent reaction take place?

Answer 13

In the thylakoid membranes of the chloroplasts. Light energy is absorbed by photosynthetic pigments in the photosystems and converted to chemical energy.

Question 14

What is the purpose of the light energy and what does it do?

Answer 14

> Used to add a phosphate group to ADP to form ATP
Reduce NADP to form reduced NADP
ATP transfers energy and reduced NADP transfers hydrogen to the light-independent reaction.
During the process H2O is oxidised to O2

Question 15

What is the other name for the light-independent reaction and where does it take place?

Answer 15

> The Calvin Cycle

>In the stroma of the chloroplasts.

Question 16

Describe an experiment that can be used to seperate photosynthetic pigments?

Answer 16

Thin Layer Chromatography (TLC) :

1) Grind up several leaves with some anhydrous sodium sulfate and some propanone.
2) Transfer the liquid to a test tube, add some petroleum ether and gently shake the tube. Two distinct layers will form - the top layer is the pigments mixed in with the petroleum ether.
3) Transfer some of the liquid from the top layer into a 2nd test tube with some anhydrous sodium sulfate.
4) Draw a horizontal pencil line near the bottom of the chromatography paper and place drops of the liquid on the line - this is the point of origin
5) Once the point of origin is dry, put the plate into a glass beaker with some prepared solvent and as the solvent spreads up the plate, the different pigments will separate.
6) Mark the solvent front.
7) Can calculate the Rf values.

Question 17

What are the 3 things the light energy in the light-dependent reaction are used for?

Answer 17

> Making ATP from ADP and inorganic phosphate. This reaction is called photophosphorylation.
Making reduced NADP from NADP.
Splitting water into protons (H+ ions), electrons and oxygen. This is called photolysis.

Question 18

What are electron carriers?

Answer 18

Proteins that transfer electrons.

Question 19

What do the photosystems and electron carriers form together?

Answer 19

Form an electron transport chain - a chain of proteins through which excited electrons flow.

Question 20

What happens when light energy excites electrons in chlorophyll?

Answer 20

> Light energy is absorbed by PSII.
The light energy excites electrons in chlorophyll.
The electrons move to a higher energy level.
These high-energy electrons move along the electron transport chain to PSI.

Question 21

What happens during the photolysis of water?

Answer 21

> As the excited electrons from chlorophyll leave PSII to move along the electron transport chain, they must be replaced.
Light energy splits water into H+ ions (protons), electrons and oxygen.
The reaction is: H2O -> 2H+ + 1/2O2

Question 22

How does the energy from the excited electrons make ATP?

Answer 22

> The excited electrons lose energy as they move along the electron transport chain.
This energy is used to transport protons into the thylakoid, via membrane proteins called proton pumps, so that the thylakoid has a higher conc. of protons than the stroma. This forms a proton gradient across the membrane.
Protons move down their conc. gradient into the stroma, via an enzyme called ATP synthase. The energy from this movement combines ADP and inorganic phosphate (Pi) to form ATP.

Question 23

How does non-cyclic photophosphorylation generate reduced NADP?

Answer 23

> Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level.
Finally the electrons are transferred to NADP, along with a H+ ion from the stroma, to form reduced NADP.

Question 24

Why is cyclic photophosphorylation ‘cyclic’?

Answer 24

> The electrons from the chlorophyll molecule aren’t passed onto NADP, but are passed back to PSI via electron carriers. This means the electrons are recycled and can repeatedly flow through PSI. This process doesn’t produce any reduced NADP or O2 - it only produces small amounts of ATP.

Question 25

What is chemiosmosis?

Answer 25

The process of electrons flowing down the electron transport chain and creating a proton gradient across the membrane to drive ATP synthesis.

Question 26

Describe the stage of the calvin cycle where the carbon dioxide is combined with ribulose biphosphate?

Answer 26

> CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast.
Here CO2 is combined with RuBP(ribulose biphosphate), a 5-carbon compound and this reaction is catalysed by Ribulose biphosphate carboxylase (RuBisCO).
This gives an unstable 6-carbon compound which quickly breaks down into 2 molecules of a 3-carbon compound called glycerate 3-phosphate (GP).

Question 27

Describe how from 2 molecules of GP you get TP and many useful organic compounds?

Answer 27

> Now ATP from the light-dependent reaction, provides energy to turn GP into TP. The reaction aslo requires H+ ions, coming from reduced NADP (also from the light-dependent reaction)
Reduced NADP is recycled to NADP (for use in the light-dependent reaction again).
TP is then converted into many useful organic compounds, e.g. glucose.

Question 28

Describe how RuBP is regenerated in the final step of the calvin cycle?

Answer 28

> 5 out of every 6 molecules of TP produced in the cycle aren’t used to make hexose sugars, but to regenerate RuBP.
Regenerating RuBP uses the rest of the ATP produced by the light-dependent reaction.

Question 29

Describe how TP is used to make carbohydrates?

Answer 29

> Hexose sugars are made by joining 2 triose phosphate molecules together.
Larger carbohydrates are made by joining hexose sugars together in different ways.

Question 30

Describe how TP and GP are used to make lipids?

Answer 30

Made from glycerol, which is synthesised from Tp and fatty acids, which are synthesised from GP.

Question 31

Describe how TP or GP are used to make amino acids?

Answer 31

Some amino acids are made from GP.

Question 32

How many times must the cycle turn to produce two molecules of TP that can be used to make one hexose sugar and explain your reasoning?

Answer 32

> 3 turns of the cycle produces 6 molecules of TP for 2 molecules of TP are made for every 1 CO2 molecule is used.
5 out of 6 of these TP molecules are used to regenerate RuBP.
This means that for 3 turns of the cycle only 1 TP is produced that’s used to make a hexose sugar.
A hexose sugar had 6 carbons, so 2 TP molecules are needed to form 1 hexose sugar.
This means the cycle must turn 6 times to produce 2 molecules of TP that can be used to make 1 hexose sugar.

Question 33

How many ATP and NADP molecules do 6 turns of the cycle need and where do these come from?

Answer 33

> 18 ATP
12 reduced NADP
From the light-dependent reaction.

Question 34

What are the ideal conditions for photosynthesis for most plants in temperate climates?

Answer 34

> High light intensity
Temperature around 25 degrees
Carbon dioxide at 0.4%

Question 35

Describe the rate of photosynthesis to light intensity graph?

Answer 35

Light is needed to provide energy for the light-dependent reaction so the higher the intensity of light, the more energy it provides. The rate of photosynthesis increases until it reaches a saturation point where increasing the light intensity after this point makes no difference for something else has become the limiting factor. The graph now levels off.

Question 36

How would high temperatures above 25 degrees affect the plant?

Answer 36

> Stomata close to avoid losing too much water causing photosynthesis to slow down for less CO2 enters the leaf when the stomata are closed.
The thylakoid membranes may be damaged, reducing the rate of the light-dependent stage reactions by reducing the number of sites available for electron transfer.
The membranes around the chloroplasts could be damaged which could cause enzymes important in the Calvin cycle to be released into the cell. This would reduce the rate of the light-dependent stage reactions.
Chlorophyll could be damaged. This would reduce the amount of pigment that can absorb light energy, which would reduce the rate of the light-dependent stage reactions.

Question 37

What is water stress and what is its impact on photosynthesis?

Answer 37

When plants don’t have enough water, their stomata will close to preserve what little water they do have, leading to less CO2 entering the leaf for the Calvin cycle and slowing photosynthesis down.

Question 38

How does light intensity affect the levels of GP, RuBP and TP in the Calvin cycle?

Answer 38

> In low light intensities, the products of the light-dependent stage (reduced NADP and ATP) will be in short supply.
This means that conversion of GP to TP and RuBP is slow.
So the level of GP will rise (as it is still being made) and levels of TP and RuBP will fall (as they’re used to make GP).

Question 39

How does low temperature affect the levels of GP, RuBP and TP in the Calvin cycle?

Answer 39

> All the reactions in the Calvin cycle are catalysed by enzymes.
At low temperatures, all of the reactions will be slower as the enzymes will work more slowly.
This means the levels of RuBP, GP and TP will fall.

Question 40

How do high temperatures affect the levels of GP, RuBP and TP in the Calvin cycle?

Answer 40

GP, TP and RuBP are affect in the same way at very high temperatures because the enzymes will start to denature.

Question 41

How does low carbon dioxide concentrations affect the levels of GP, RuBP and TP in the Calvin cycle?

Answer 41

> Conversions of RuBP to GP is also slow (less CO2 to combine with RuBP to make GP).
The level of RuBP will rise (as it’s still being made) and levels of GP and TP will fall (being used up to make RuBP).

Question 42

Describe an experiment you could perform to measure the effect of light intensity on photosynthesis?

Answer 42

1) A test tube containing the pondweed and water is connected to a capillary tube full of water. The tube of water is connected to a syringe.
2) A source of white light is placed at a specific distance from the pondweed.
3) The pondweed is left to photosynthesise for a set amount of time. As it photosynthesises, the oxygen released will collect in the capillary tube.
4) At the end of the experiment, the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the gas bubble is measured. This is proportional to the volume of O2 produced.
5) The experiment should be repeated and the average length of gas bubble is calculated, to make the results more precise.
6) The whole experiment is then repeated with the light source placed at different distances from the pondweed.