Photosynthesis

Question 1

How are photosynthesis and respiration related

Answer 1

CO2 and H2O are the raw materials for photosynthesis and the products of respiration
O2 and Glucose are the raw materials for respiration and the products of photosynthesis

Question 2

Describe the structure of a chloroplast

Answer 2

Usually disk shaped
Double membrane (envelope)
Thylakoids: flattened discs stack to form grana
Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
Stroma: fluid filled matrix with enzyme and substrate concentration and own loop of DNA

Question 3

Where do light dependent and light independent reactions occur in plants

Answer 3

Light-dependent: In the thylakoids of chloroplasts
Light-independent: stroma of chloroplasts

Question 4

What is the role of photosynthetic pigments?

Answer 4

Embedded within thylakoid membrane. Absord different wavelengths of light to maximise rate of photosynthesis

Question 5

Name the two main groups of photosynthetic pigment

Answer 5

Primary pigment: Chlorophyll (made of chlorophyll a & chlorophyll b) found in photosystems
Accessory pigments: carotenoids (carotene & xanthophylls) found in light harvesting systems

Question 6

Name the processes in the light-dependent reaction

Answer 6

• Photoionisation
• Electron transfer chain
• Chemiosmosis

Non-cyclic only:
- Reduction of NADP
- Photolysis of water

Question 7

Explain the role of light in photoionisation

Answer 7

Chlorophyll molecules absorb energy from photons of light
This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be release from the chlorophyll

Question 8

What happens in the electron transfer chain (ETC)?

Answer 8

Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & undergo a series of REDOX reactions which releases energy

Question 9

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

Answer 9

Some energy released from the ETC is coupled to active transport of H+ ions from stroma into thylakoid space
H+ ions move down concentration gradient from thylakoid space into stroma via transmembrane channel protein ATP synthase
ATP synthase catalyses ADP + Pi —> ATP

Question 10

Describe non-cyclic photophosphorylation

Answer 10

Uses photosystems I and II. Excited electrons enter ETC to produce ATP. NADP acts as final electron acceptor and is reduced. Water is photolysed to released electrons to replace those lost from PSII
Purpose is to produce ATP and reduced NADP for Calvin cycle to produce biological compounds

Question 11

Describe cyclic photophosphorylation

Answer 11

Uses only photosystem I. Excited electrons enter ETC to produce ATP then return directly to photosystem (so no reduction of NADP and no water needed to replace lost electrons)
Purpose is to produce additional ATP to meet surplus energy demands of cell

Question 12

What happens in photolysis of water?

Answer 12

Light energy splits molecules of water
2H2O —> 4H+ + 4e- + O2

Question 13

What happens to the products of the photolysis of water?

Answer 13

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 14

How and where is reduced NADP produced in the light dependant reaction

Answer 14

NADP + 2H+ (from photolysis of water) + 2e- (from acting as electron acceptor in ETC) —> Reduced NADP
Catalysed by dehydrogenase enzymes
Stroma of chloroplasts

Question 15

Name the three main stages in the light-dependant reaction

Answer 15

1. Carbon fixation
2. Reduction
3. Regeneration

Question 16

What happens during carbon fixation

Answer 16

Reaction between CO2 and ribulose bisphosphate (RuBP) catalysed by ribulose bisphosphate carboxylase (RuBisCo)
Forms unstable 6C intermediate that breaks down into 2 x glycerate 3-phosphate

Question 17

What happens during reduction in the light-dependant reaction

Answer 17

2 x GP are reduced to 2 x Triose Phosphate (TP)
Requires 2 x reduced NADP and 2 x ATP
Forms 2 x NADP and 2 x ADP

Question 18

Outline the roles of TP in the light dependent reaction

Answer 18

Raw material - 1C leaves the cycle to produce monosaccharides, amino acids and other biological molecules
Involved in regeneration of RuBP: After 1C leaves cycle, the 5C compound RuP forms. RuP is then converted into RuBP using 1 x ATP. Forms 1 x ADP

Question 19

State the number of carbon atoms in RuBP, GP and GALP

Answer 19

RuBP: 5
GP: 3
GALP: 3

Question 20

Define limiting factor

Answer 20

Factor that determines maximum rate of a reaction even if other factors change to become more favourable

Question 21

Name 4 environmental factors that can limit the rate of photosynthesis

Answer 21

• Light intensity (light-dependent stage)
• CO2 levels (light-independent stage)
• Temperature (enzyme-controlled steps)
• Mineral / Magnesium levels (maintain normal functioning of chlorophyll)

Question 22

How does light intensity affect the rate of photosynthesis

Answer 22

Low light intensity = slow light-dependent reaction = less ATP and NADPH produced to convert GP to TP in light-dependent reaction
- GP level rises
- TP level falls = RuBP level falls

Question 23

Describe the implications of water stress