5.2.1: Photosynthesis

Question 1

How are photosynthesis and respiration related?

Answer 1

CO2 & H2O are the raw materials for
photosynthesis and the products of respiration.
O2 & glucose are the raw materials for
photosynthesis and the products of respiration

Question 2

Describe the structure of a chloroplast.

Answer 2

● Usually disc-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 high enzyme &
substrate concentration & own loop of DNA.

Question 3

Where do the light-dependent & 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? Name

the 2 main groups

Answer 4

Embedded within thylakoid membrane. Absorb different
wavelengths of light to maximise rate of photosynthesis.
● Primary pigment: chlorophyll (made of chlorophyll a &
chlorophyll b) found in photosystems.
● Accessory pigments: carotenoids (carotene & xanthophylls) found in light-harvesting systems.

Question 5

Name the processes in the light-dependent reaction.

Answer 5

● photoionisation
● electron transfer chain
● chemiosmosis
non-cyclic only:
● reduction of NADP
● photolysis of water

Question 6

Explain the role of light in photoionisation.

Answer 6

Chlorophyll molecules absorb energy from photons of light.

This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll.

Question 7

What happens in the electron transfer chain (ETC)?

Answer 7

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 8

How does chemiosmosis produce ATP in the

light-dependent stage?

Answer 8

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 9

Describe non-cyclic photophosphorylation.

Answer 9

Uses Photosystems Ⅰ & Ⅱ. Excited electrons enter
ETC to produce ATP. NADP acts as final electron
acceptor & is reduced. Water is photolysed to
release electrons to replace those lost from PS Ⅱ.
Purpose is to produce ATP & reduced NADP for
Calvin cycle to produce biological compounds.

Question 10

Describe cyclic photophosphorylation.

Answer 10

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

Question 11

What happens in photolysis of water?

Answer 11

Light energy splits molecules of water

2H2O → 4H+ + 4e- + O2

Question 12

What happens to the products of the photolysis of

water?

Answer 12

H+ ions: move out of thylakoid space via ATP
synthase & 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 13

How and where is reduced NADP produced in the

light-dependent reaction?

Answer 13

NADP + 2H+ (from photolysis of water) + 2e- (from
acting as final electron acceptor in ETC) → reduced NADP.
Catalysed by dehydrogenase enzymes.
Stroma of chloroplasts.

Question 14

Name the 3 main stages in the light-independent

reaction

Answer 14

1. Carbon fixation
2. Reduction
3. Regeneration

Question 15

What happens during carbon fixation?

Answer 15

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

Question 16

What happens during reduction in the light-independent reaction?

Answer 16

2 x GP are reduced to 2 x triose phosphate (TP)
Requires 2 x reduced NADP & 2 x ATP
Forms 2 x NADP & 2 x ADP

Question 17

Outline the roles of TP from the light-independent

reaction.

Answer 17

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

Question 18

State the number of carbon atoms in RuBP, GP &

GALP.

Answer 18

RuBP: 5
GP: 3
GALP: 3

Question 19

Define ‘limiting factor’.

Answer 19

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

Question 20

Name 4 environmental factors that can limit the rate

of photosynthesis.

Answer 20

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

Question 21

How does light intensity affect the rate of

photosynthesis?

Answer 21

Low light intensity = slower light- dependent reaction = less ATP & NADPH produced to convert GP to TP in light-independent reaction.
● GP level rises
● TP level falls = RuBP level falls

Question 22

Describe the implications of water stress.

Answer 22

1. Abscisic acid binds to complementary receptors on guard cell membrane, causing Ca2+ ion channels on tonoplast to open. Ca2+ ions diffuse from vacuole into cytosol.
2. Positive feedback triggers other ion channels to open.
   Other ions e.g. K+ diffuse out of guard cell.
3. Water potential of guard cell becomes more positive.
   Water diffuses out via osmosis.
4. Guard cells become flaccid so stomata close.

Question 23

State the purpose and principle of paper chromatography

Answer 23

Molecules in a mixture are separated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (TLC plate, usually coated in a silicate)

Question 24

Outline a method for extracting photosynthetic

pigments.

Answer 24

Use a pestle and mortar to grind a leaf
with an extraction solvent e.g.
propanone.

Question 25

Outline how TLC can be used to separate

photosynthetic pigments.

Answer 25

1. Use a capillary tube to spot pigment extract onto
   pencil ‘start line’ (origin) 1 cm above bottom of plate.
2. Place chromatography paper in solvent (origin should
   be above solvent level).
3. Allow solvent to run until it almost touches the other
   end of the paper. Pigments move different distances.