5.1 - Photosynthesis

Question 1

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

Answer 1

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

Question 2

Explain the role of light in

photoionisation.

Answer 2

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 3

Name the 2 main stages involved in ATP
production in the light-dependent
reaction.

Answer 3

1. electron transfer chain

2. chemiosmosis

Question 4

What happens in the electron transfer

chain (ETC)?

Answer 4

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 5

How is a proton concentration gradient

established during chemiosmosis?

Answer 5

Some energy released from the ETC is
coupled to the active transport of H
\+
ions (protons) from the stroma into the
thylakoid space.

Question 6

How does chemiosmosis produce ATP in

the light-dependent stage?

Answer 6

H+ ions (protons) move down their
concentration gradient from the thylakoid
space into the stroma via the 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+ + 4e- + O2

Question 8

What happens to the products of the

photolysis of water?

Answer 8

● 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 9

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

Answer 9

● NADP + 2H+ + 2e- → reduced NADP.
● Catalysed by dehydrogenase
enzymes.
● Stroma of chloroplasts.

Question 10

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

Answer 10

● H+ ions: photolysis of water
● Electrons: NADP acts as the final
electron acceptor of the electron
transfer chain

Question 11

Name the 3 main stages in the Calvin

cycle.

Answer 11

1. Carbon fixation
2. Reduction
3. Regeneration

Question 12

What happens during carbon fixation?

Answer 12

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

Question 13

What happens during reduction (in the

Calvin cycle)?

Answer 13

● 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 14

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

Answer 14

1C leaves the cycle (i.e. some of the TP
is converted into useful organic
molecules).

Question 15

What happens during regeneration (in

the Calvin cycle)?

Answer 15

● After 1C leaves the cycle, the 5C
compound RuP forms
● RuBP is regenerated from RuP using 1x
ATP
● Forms 1x ADP

Question 16

State the roles of ATP & (reduced) NADP

in the light-independent reaction.

Answer 16

● ATP: reduction of GP to TP & provides
phosphate group to convert RuP into
RuBP.
● (reduced) NADP: coenzyme transports
electrons needed for reduction of GP to TP

Question 17

State the number of carbon atoms in

RuBP, GP & TP.

Answer 17

RuBP: 5
GP: 3
TP: 3

Question 18

Describe the structure of a chloroplast.

Answer 18

● 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.

Question 19

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

Answer 19

● ATP synthase channels within granal
membrane.
● large surface area of thylakoid membrane for
ETC.
● photosystems position chlorophyll to enable
maximum absorption of light.

Question 20

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

Answer 20

● Own DNA & ribosomes for synthesis of
enzymes e.g. rubisco.
● Concentration of enzymes &
substrates in stroma is high.

Question 21

Define ‘limiting factor’.

Answer 21

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

Question 22

Name 4 environmental factors that can

limit the rate of photosynthesis.

Answer 22

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

Question 23

Outline some common agricultural
practices used to overcome the effect of
limiting factors in photosynthesis.

Answer 23

● Artificial light, especially at night.
● Artificial heating.
● Addition of CO2
 to greenhouse
atmosphere.

Question 24

Why do farmers try to overcome the

effect of limiting factors?

Answer 24

● To increase yield.
● Additional cost must be balanced with
yield to ensure maximum profit.

Question 25

Suggest how a student could investigate
the effect of a named variable on the rate
of photosynthesis.

Answer 25

dependent variable: rate of O2
 production/ CO2
consumption
1. Use a potometer
2. Place balls of calcium alginate containing green
algae in hydrogencarbonate indicator (colour change
orange → magenta as CO2
 is consumed & pH
increases).
www.pmt.ed

Question 26

State the purpose and principle of paper

chromatography

Answer 26

Molecules in a mixture are separated based
on their relative attraction to the mobile
phase (running solvent) vs the stationary
phase (chromatography paper).

Question 27

Outline a method for extracting

photosynthetic pigments.

Answer 27

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

Question 28

Outline how paper chromatography can
be used to separate photosynthetic
pigments.

Answer 28

1. Use a capillary tube to spot pigment extract onto
   pencil ‘start line’ (origin) 1 cm above bottom of paper.
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.

Question 29

What are Rf values? How can they be

calculated?

Answer 29

● Ratios that allow comparison of how far
molecules have moved in chromatograms.
● Rf value = distance between origin and
centre of pigment spot / distance between
origin and solvent front.