3.5.1 Photosynthesis (A-level only)

Question 1

Why do plants need energy?

Answer 1

Plants need energy for:

• Photosynthesis.
• Active transport (mineral uptake to root hair cells).
• DNA replication.
• Cell division.
• Protein synthesis.

Question 2

Why do animals need energy?

Answer 2

Animals need energy for:

• Muscle contraction.
• Maintenance of body temperature
• Active transport.
• DNA replication.
• Cell division.
• Protein synthesis.

Question 3

Define Photosynthesis.

Answer 3

= The process in which energy from light is used to make glucose from H2O and CO2 (the light energy is converted to chemical energy in the form of glucose.

Question 4

Comment on ATP’s role as the immediate source of energy in a cell.

Answer 4

• A cell can’t get its energy directly from glucose.
• ATP’s phosphates contain chemical energy.
• ATP diffuses to the part of the cell that needs energy.
• There hydrolysed back to ADP + Pi (catalysed by ATP hydrolase).
  => Chemical energy is released from the phosphate bond —> used by cell.
  => ADP and Pi recycled and the process starts again.

Question 5

Comment on ATP’s role as the immediate source of energy in a cell.

Answer 5

• A cell can’t get its energy directly from glucose.
• ATP’s phosphates contain chemical energy.
• ATP diffuses to the part of the cell that needs energy.
• There hydrolysed back to ADP + Pi (catalysed by ATP hydrolase).
  => Chemical energy is released from the phosphate bond —> used by cell.
  => ADP and Pi recycled and the process starts again.

Question 6

What are ATP’s specific properties that make it a good energy source?

Answer 6

1. Releases a small, manageable amount of energy at a time —> no energy wasted as heat.
2. Small, soluble molecule so can be easily transported around the cell.
3. Easily broken down, so energy can be released instantaneously.
4. Can be quickly re-made.
5. Can phosphorylate other molecules (transferring one of its phosphate groups to them) —> making them more reactive.
6. ATP can’t pass out of the cell, so the cell always has an immediate supply of energy.

Question 7

Define metabolic pathway.

Answer 7

= A series of small reactions controlled by enzymes —> respiration and photosynthesis.

Question 8

Define phosphorylation.

Answer 8

= Adding phosphate groups to a molecule —> ADP is phosphorylated to ATP.

Question 9

Define photophosphorylation.

Answer 9

= Adding phosphate groups to a molecule using light.

Question 10

Define photolysis.

Answer 10

= The splitting of a molecule using light energy.

Question 11

Define photoionisation.

Answer 11

= Light energy excites electrons in an atom / molecule, giving them more energy so e- released. This loss of e- oxidises the species —> becomes a cation.

Question 12

Define hydrolysis.

Answer 12

= The splitting of a molecule using water.

Question 13

Define decarboxylation.

Answer 13

= The removal of CO2 from a molecule.

Question 14

Define dehydrogenation.

Answer 14

= The removal of hydrogen from a molecule.

Question 15

Define redox reactions.

Answer 15

= Reactions involving both oxidation and reduction.

Question 16

How can something be oxidised?

Answer 16

1. Lost electrons.
2. Lost hydrogen
3. Gained oxygen.

Question 17

How can something be reduced?

Answer 17

1. Gained electrons.
2. Gained hydrogen.
3. Lost oxygen.

Question 18

What is a coenzyme?
How do they work?
Examples?

Answer 18

= A molecule that aids the function of an enzyme.

• They work by transferring a chemical group from one molecule to another.
  1. NADP used in photosynthesis - transfers H from one molecule to another (reduces one molecule and oxidises another).
  2. NAD and FAD used in respiration - transfer H from molecule to another (reduces one molecule and oxidises another).
  3. Coenzyme A (CoA) transfers acetate between molecules.

Question 19

Describe chloroplasts (site of photosynthesis).

Answer 19

• Flattened organelles surrounded by double membrane.
• Thylakoids (fluid-filled sacs) stacked up in the chloroplast intro structures called grana.
• Contained within the inner membrane and surrounding the thylakoids is a gel-like substance called the stroma - contains enzymes, sugars and organic acids.

Question 20

Comment on photosynthetic pigments and photosystems.

Answer 20

Chloroplasts contain photosynthetic pigments (chlorophylls A and B / Carotene) = coloured substances that absorb the light energy needed for photosynthesis.

• Photosynthetic pigments found in thylakoid membranes attached to proteins = photosystems.
• 2 photosystems used by plants to capture light energy - PSII (680nm optimum light wavelength for absorption) and PSI (700nm optimum light wavelength for absorption).

Question 21

Briefly summarise the Light-Dependent Reaction.

Answer 21

1. Reaction requiring light in the thylakoid membranes of the chloroplasts.
2. Light energy absorbed by chlorophyll and other photosynthetic pigments in the photosystems.
3. Light energy excites the electrons in the chlorophyll, leading to their eventual release from the molecule, photoionising the the chlorophyll.
4. Some energy from released e- used to phosphorylate ADP —> ATP, and some used to reduce NADP.
   => ATP transfers energy and NADP transfers H to the LIR.
5. During the process H2O is oxidised to O2.

Question 22

Briefly summarise the Light-Independent Reaction.

Answer 22

1. Calvin Cycle - doesn’t directly use light energy, but does rely on the products of light-dependent reaction.
2. Takes place in the stroma of the chloroplast.
3. Here, the ATP and NADPH from the LDR supply energy and H necessary to make simple sugars from CO2.

Question 23

In the LDR, what is the energy from the photoionisation of chlorophyll used for?

Answer 23

1. Making ATP from ADP and Pi - photophosphorylation.
2. Reducing NADP —> NADPH.
3. Photolysis of water - splitting of water to protons, electrons and oxygen.

Question 24

Define electron carriers.

Answer 24

= Proteins that transfer electrons.

Question 25

Define electron transport chain.

Answer 25

= A chain of proteins through which excited electrons flow.

Question 26

Non-cyclic photophosphorylation produces?

Answer 26

ATP, NADH, O2.

Question 27

Outline stage 1 of the LDR - Light energy excites e- in chlorophyll.

Answer 27

1. Light energy absorbed by chlorophyll in PSII.
2. Light energy excited electrons in chlorophyll.
3. These excited electrons move to a higher energy level.
4. High-energy electrons are released from the chlorophyll and move down the ETC to PSI.

Question 28

Outline stage 2 of the LDR - Photolysis of water produces protons, electrons and O2.

Answer 28

5. Excited electrons from chlorophyll leave PSII to move down ETC —> must be replaced.
6. Light energy splits water into protons, electrons and oxygen (photolysis).
7. Reaction is:
   - H2O —> 2H+ (+) 1/2 O2.

Question 29

Outline stage 3 of the LDR - Energy from the excited electrons makes ATP (chemiosmosis).

Answer 29

8. Excited electrons lose energy as they move down ETC.
9. This energy used to transport protons into thylakoid —> thylakoid has a higher [H+] than stroma, forming a proton gradient across the membrane.
10. Protons move down their concentration gradient into the stroma, via the ATP synthase enzyme, embedded in the thylakoid membrane.
11. The energy from this movement combines ADP and Pi to form ATP.

Question 30

Outline stage 4 of the LDR - Energy from the excited electrons generated NADPH.

Answer 30

12. Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level.
13. Finally, electrons transferred to NADP, along with a proton from the stroma, to form NADPH.

Question 31

Define chemiosmosis.

Answer 31

= The process of electrons flowing down an electron transport chain and creating a proton gradient across the membrane to drive ATP synthesis. Described by chemiosmotic theory.
Cyclic photophosphorylation only produces?
- ATP.

Question 32

Outline the process of cyclic photophosphorylation.

Answer 32

1. Only uses PSI - called cyclic as electrons from chlorophyll aren’t passed passed onto NADP, but are instead passed back to PSI via electron carriers.
2. Electrons are therefore recycled and can repeatedly flow through PSI.
3. Does NOT produce any NADPH / O2. Only produces small amounts of ATP.

Question 33

Where does the Calvin Cycle take place?

Answer 33

The Calvin Cycle takes place in the stroma of the chloroplasts.

Question 34

Draw the Calvin Cycle.

Answer 34

Check if correct based on next few cards.

Question 35

Outline stage 1 of the Calvin Cycle - CO2 combined with RuBP.

Answer 35

1. CO2 enters the leaf via stomata and diffuses into the stroma of the chloroplast.
2. Here, CO2 is combines with RuBP (5C), catalysed by the RUBISCO enzyme.
3. This gives an unstable 6C carbon, which quickly breaks down into two molecules of a 3C carbon called Glycerate-3-phosphate (GP).

Question 36

Outline stage 2 of the Calvin Cycle - ATP and NADPH reduce GP to TP —> organic substances.

Answer 36

4. Hydrolysis of ATP (from LDR) provides energy to turn GP (3C) into Triose Phosphate (3C).
5. This reaction also requires H+ ions, which come from NADPH (also from LDR). NADPH is recycled to NADP.
6. Some TP is then converted into useful organic substances (e.g. glucose) and some continues in the Calvin Cycle to regenerate RuBP.

Question 37

Outline how RuBP is regenerated.

Answer 37

7. 5/6 molecules of TP produced in the cycle are used to regenerate RuBP.
8. Regenerating RuBP uses the rest of the ATP produced by the LDR.

Question 38

GP and TP can be converted to?

Answer 38

Can be converted to useful organic substances such as glucose.

1. Carbohydrates - hexose sugars are made by joining two triose phosphate molecules together and larger carbohydrates (e.g. sucrose / starch / cellulose) are made by joining hexose sugars together in different ways.
2. Lipids - made using glycerol, synthesised from TP, and fatty acids, which are synthesised from GP.
3. Amino Acids - some amino acids can be made from GP.

Question 39

Justify why the Calvin Cycle needs to turn six times to make one hexose sugar.

Answer 39

• 3 turns of the cycle produces 6 molecules of TP, as 2 TP molecules are made for every CO2 molecule used.
• 5/6 of these TP molecules are used to regenerate RuBP.
• 3 turns = one TP molecule produced for hexose sugar.
• Hexose sugar has 6C, so 2 x TP molecules needed.
• Therefore cycle needs to turn 6 times.
  NB => One turn of the cycle requires 3ATP (2 on right) and 2 NADH (2 on right).
  NB => Six turns of the cycle require 18 ATP and 12 NADH from the LDR.

Question 40

What are the optimum conditions for photosynthesis - light?

Answer 40

1. High light intensity of a certain wavelength:
   - Light needed to provide energy for the LDR => higher intensity, more energy it provides.
   - Only certain wavelengths of light used for photosynthesis.
   - Photosynthetic pigments chlorophyll A and B an carotene absorb the red and blue light in sunlight (green light is reflected, which is why plants look green).

Question 41

What are the optimum conditions for photosynthesis - temperature?

Answer 41

2. Temperature around 25C:
   - Photosynthesis involves enzymes (ATP synthase, RUBISCO etc). If the temperature falls below 10C, the enzymes become inactive, but if the temperature is more than 45C they may start to denature.
   - At high temperatures, stomata close to avoid losing too much water —> causes photosynthesis to slow down as less CO2 is entering the leaf when the stomata are closed.

Question 42

What are the optimum conditions for photosynthesis - [CO2]?

Answer 42

3. [CO2] at 0.4%
   - CO2 makes up 0.04% of the gases in the atmosphere.
   - Increasing this to 0.4% gives a higher rate of photosynthesis, but any higher and the stomata start to close.

Question 43

What are the optimum conditions for photosynthesis - water?

Answer 43

4. Constant supply of water:
   - Too little water and photosynthesis has to stop.
   - Too much water and soils become waterlogged (reducing uptake of minerals such as Mg, necessary to make chlorophyll A).

Question 44

List three of photosynthesis’ limiting factors.

Answer 44

1. Light.
2. Temperature.
3. CO2.

Question 45

Outline how limiting factors limit the rate of photosynthesis.

Answer 45

• If any of these factors is too low / too high, it will limit the rate of photosynthesis.
• Even if the other two factors are at the perfect level, it won’t make any difference to the rate as long as that factor is at the wrong level.

Question 46

Draw a graph of rate of photosynthesis (y) against light intensity (x).

Answer 46

• A to B, rate increases with light intensity - therefore light intensity is limiting the rate of photosynthesis.
• At point B, the graph levels off as saturation point is reached —> new limiting factor.

Question 47

Draw a graph of rate of photosynthesis (y) against temperature (x).

Answer 47

• Increasing with temperature, until a point where graph levels off as something else becomes the limiting factor.
• Same general curve for each temperature, but higher temperatures —> higher curve with slightly steeper gradient.

Question 48

Draw a graph of rate of photosynthesis (y) against [CO2] (x).

Answer 48

• Increasing with [CO2], until a point where graph levels off as something else becomes the limiting factor.
• Same general curve for each [CO2], but higher [CO2] —> higher curve with slightly steeper gradient.

Question 49

What do growers do with information about limiting factors?

Answer 49

Growers use information about limiting factors to increase plant growth.

• They try to create an environment with optimum conditions for photosynthesis, increasing growth, yield and profit.
• Either in glasshouse, or employ similar techniques in polytunnels - tunnels made of polythene, in which plants can be grown.

Question 50

How can we manage [CO2] in a glasshouse?

Answer 50

• CO2 added to the air, by burning a small amount of propane in a CO2 generator.

Question 51

How can we manage light intensity in a glasshouse?

Answer 51

• Light can get in through the glass.

- Lamps provide light at night-time.

Question 52

How can we manage temperature in a glasshouse?

Answer 52

• Glasshouses trap heat energy from sunlight, which warms the air.
• Heaters and cooling systems can also be used to keep a constant optimum temperatures.
• Air circulation systems make sure the temperature is even throughout the glasshouse.