Topic 5: photosynthesis

Question 1

Where do the light-dependent and light-independent reactions occur in plants?

Answer 1

light-dependent: in the thylakoids of chloroplasts
light-independent: in the stroma

Question 2

Explain the role of light in photoionisation

Answer 2

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

Question 3

Name the two main stages involved in the production of ATP in the light-dependent reaction.

Answer 3

1) electron transfer train
2) chemiosmosis

Question 4

What happens in the electron transfer chain?

Answer 4

Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions, which releases energy.

Question 5

How is a proton gradient established during chemiosmosis?

Answer 5

Some energy released from the ETC is coupled to the active transport of H+ ions from the stroma into the thylakoid space

Question 6

How does chemiosmosis produce ATP in the light-dependent stage?

Answer 6

H+ ions move down their conc gradient from the thylakoid space into the stroma via channel protein ATP synthase.
ATP synthase catalyses ADP + Pi –> ATP

Question 7

Explain the role of light in photolysis

Answer 7

Light energy splits molecules of water
2H2O –> 4H+ + O2

Question 8

What happens to the products of photolysis of water?

Answer 8

• H+ ions: move out of thylakoid space via ATP synthase and are used to reduce the coenzyme NADP
  -e- : replace electrons lost from chlorophyll
  O2: used for respiration or diffuses out of leaf as waste gas

Question 9

How and where is reduced NADP produced in the light-dependent reactions?

Answer 9

• NADP +2H+ +2e- –> reduced NADP
• Catalysed by dehyrdogenase enzymes
• Stroma of chloroplasts

Question 10

Where do H+ ions and electrons used to reduce NADP come from?

Answer 10

H+ ions: photolysis of water
Electrons: NADP acts as final electron acceptor of ECT

Question 11

How does the light-independent reaction result in the production of useful organic substances?

Answer 11

1C /6 leaves the cycle and converted into useful organic molecules

Question 12

State the roles of ATP and reduced NADP in the light-independent reaction

Answer 12

ATP: reduction of GP to TP and provides phosphate group to convert RuP into RuBP
reduced NADP: coenzyme transports electrons needed for reduction of GP to TP

Question 13

State the no. of carbon atoms in RuBP, GP and TP

Answer 13

RuBP: 5
GP: 3
TP: 3

Question 14

Describe the structure of a chloroplast

Answer 14

• usually disc-shaped
• double membrane
• thylakoids
• stroma

Question 15

How does the structure of the chloroplast maximise the rate of the light-dependent reaction?

Answer 15

• ATP synthase channels with granal membrane
• large surface area of thylakoid membrane for ETC
• photosystems position of chlorophyll to enable max absorption of light

Question 16

How does the structure of the chloroplast maximise the rate of the light-independent reaction?

Answer 16

• own DNA and ribosomes for synthesis of enzymes e.g. rubisco
• concentration of enzymes and substrates in stroma is high

Question 17

Define limiting factor

Answer 17

factor that determines the max rate of a reaction, even if other factors change to become more favourable

Question 18

Name 4 environmental factors that can limit the rate of photosynthesis

Answer 18

• light intensity
• CO2 levels
• temperature
• mineral/ magnesium levels

Question 19

Write the symbol equation for photolysis.

Answer 19

H2O–> 1⁄2O2 + 2e- + 2H+

Question 20

Describe photoionisation of chlorophyll.

Answer 20

Light energy is absorbed by the chlorophyll and the energy results in the electrons becoming excited and raising up an energy level to leave the chlorophyll. Therefore the chlorophyll has been ionised by light. Some of the energy from the released electrons is used to make ATP and reduced NADP in chemiosmosis.

Question 21

Where does the light-dependent reaction (LDR) occur?

Answer 21

Thylakoid membrane

Question 22

What are the three key reactions in the LDR?

Answer 22

• Photoionisation of chlorophyll -
• Photolysis
• Chemiosmosis

Question 23

Describe chemiosmosis in 4 key stages

Answer 23

Step 1: The electrons that gained energy and left the chlorophyll move along a series of proteins embedded within the thylakoid membrane.

Step 2: As they move along, they release energy and some of the energy from electrons is used to pump the protons across chloroplast membranes.

Step 3: An electrochemical gradient is created. The protons pass through the enzyme ATP synthase, which results in the production of ATP.

Step 4: The protons combine with the co-enzyme NADP to become reduced NADP. As the protons move from a high to low concentration gradient this is known as chemiosmosis.

Question 24

Why is the LIR temperature sensitive?

Answer 24

It involves the enzyme Rubisco

Question 25

What is the role of ATP in the Calvin cycle?

Answer 25

• To provide the energy to reduce GP to TP
• To provide the energy to regenerate RuBP from TP

Question 26

What is the role of NADPH in the Calvin cycle?

Answer 26

To provide an H to reduce GP to TP

Question 27

What is the triose phosphate used for?

Answer 27

1- To donate one carbon each turn of the cycle to go towards making a hexose sugar

2- To regenerate RuBP so the cycle can continue

Question 28

Which molecules from the LDR are used in the LIR?

Answer 28

1 ATP
2 Reduced NADP

Question 29

Where does the LIR occur?

Answer 29

The stroma of the chloroplast

Question 30

What can the hexose sugars made be used for?

Answer 30

Whilst glucose is the product, this monosaccharide can join to form disaccharides such as sucrose, and
polysaccharides such as cellulose and starch. It can also be converted into glycerol and therefore combine with fatty acids to make lipids for the plant.

Question 31

Draw the Calvin cycle.

Answer 31

check work sheets