chapter 13

Question 1

Where in the chloroplast does the light-dependent stage of photosynthesis occur?

Answer 1

In the thylakoids.

Question 1

In which cell organelle does
photosynthesis occur?

Answer 1

In the chloroplasts of plant cells.

Question 2

Where in the chloroplast does the light-independent stage of photosynthesis occur?

Answer 2

In the stroma.

Question 3

Why are there many different photosynthetic pigments?

Answer 3

Each pigment absorbs light most efficiently at
specific wavelengths. Having multiple pigments
allows the organism to capture more energy from
the Sun, compared to having just one pigment.

Question 4

Which photosynthetic pigment is found in all photosynthetic plants?

Answer 4

Chlorophyll a.

Question 5

Which colours of light do chlorophyll pigments absorb, and which do they reflect?

Answer 5

Chlorophyll a and b absorb red light
(wavelength 650-700 nm) and blue light
(wavelength 400-450 nm).

The chlorophyll pigments reflect green
light (wavelength 500-550 nm). This is
why most plants are green.

Question 6

Plants containing carotenoid pigments (xanthophyll and carotene) tend to be which colours?

Answer 6

Red, orange and yellow. The carotenoids absorb
blue light (wavelength 400-450 nm) and reflect red,
orange and yellow (wavelength 550-700 nm). The
presence of these pigments gives carrots, pumpkins
and tomatoes their colour.

Question 7

Explain the difference between an absorption spectrum and an action spectrum.

Answer 7

An absorption spectrum measures
which wavelengths of light are
absorbed by the chlorophyll pigments.

An action spectrum measures the rate
of photosynthesis occurring at particular
wavelengths.

Question 8

Name the technique used to separate chloroplast pigments.

Answer 8

Chromatography.

Question 9

How is the Rf value calculated in chromatography?

Answer 9

Retention value = Distance travelled by component/Distance travelled by solvent

Question 10

Define photoactivation.

Answer 10

The process by which two electrons in the
chlorophyll molecule become excited (due to
absorption of light energy) which causes them
to leave the molecule. This results in the
ionisation of the chlorophyll molecule.

Question 11

What happens to the electrons that leave the chlorophyll molecule during photoactivation?

Answer 11

The two electrons are taken up by an electron
acceptor. From here, they move down electron
carriers within the thylakoid membrane in the
electron transfer chain (ETC). The electrons
release energy as they move down the ETC.

Question 12

How is the energy from the ETC used in the light-dependent stage?

Answer 12

The energy lost from the electrons is used to pump
two H+ ions from the stroma into the thylakoid
space. This creates a H+ gradient across the
thylakoid membrane.

Question 13

How is the H+ gradient used to make ATP?

Answer 13

The H+ ions in the thylakoid space exit via an ATP synthase enzyme in the thylakoid membrane. This enzyme catalyses the formation of ATP, using the energy in the H+ ion gradient.

Once in the stroma, NADP accepts the H+ ions becoming reduced NADP.

Question 14

What happens to water during the light-dependent stage of photosynthesis?

Answer 14

Water is split using light energy (photolysis).

2H2O → 4H+ + 4e- + O2

Question 15

State what happens to the products of the photolysis of water.

Answer 15

● H+ ions taken up by a hydrogen acceptor, NADP

● Electrons replace those lost from the chlorophyll
molecules during photoactivation

● O2 given off as a waste product

Question 16

Outline cyclic phosphorylation.

Answer 16

● Excited electrons from photosystem I enter the ETC
● Energy from electrons used to produce ATP. The electrons return to photosystem I (hence the process is cyclic)
● There is no reduction of NADP, and no need to photolysis of water to
replace the electrons
● Cyclic photophosphorylation is performed to produce extra ATP for
immediate energy

Question 17

Outline non-cyclic photophosphorylation.

Answer 17

● Electrons from photosystem II are excited and enter the ETC. This produces ATP and reduced NADP via generation of the proton gradient and subsequent
movement through the thylakoid membrane.
● The electrons then move to photosystem I, so photolysis of water is required to replace the lost electrons from photosystem II (hence the term non-cyclic).
● Non-cyclic photophosphorylation produces ATP and reduced NADP, which are important for the light-independent stage.

Question 18

What are the two important products of the light-dependent stage that are used in the light-independent stage?

Answer 18

ATP and reduced NADP - these
compounds transfer energy from the
light-dependent stage which is used later.

Question 19

Name the three stages of the Calvin cycle in the light-independent stage.

Answer 19

1. Fixation of carbon dioxide
2. Reduction
3. Regeneration

Question 20

During fixation, which molecule does CO2 combine with?

Answer 20

Ribulose bisphosphate (RuBP), a 5-carbon sugar.

Question 21

Name the enzyme that catalyses the fixation of CO2.

Answer 21

Rubisco (in the stroma)

Question 22

What is the product of the fixation of CO2

Answer 22

2 molecules of glycerate 3-phosphate (GP), a 3-carbon compound.

Question 23

Describe the reduction step of the light-independent stage.

Answer 23

● Two molecules of GP are reduced to two molecules of triose phosphate (TP). ATP and reduced NADP are required for this step.

● ATP is hydrolysed to provide energy for the conversion. Reduced NADP is used to reduce GP.

● This results in two ADP molecules and two NADP molecules. NADP returns to the light-dependent stage to be reduced again.

Question 24

What happens during the regeneration step of the light-independent stage?

Answer 24

TP is converted into RuBP, to be used again for fixation of CO2 . This process requires ATP.

Question 25

What other molecules (aside from RuBP) can TP be converted into?

Answer 25

● Sugars e.g. glucose, starch, cellulose
● Amino acids and proteins
● Lipids

Question 26

Explain the term ‘limiting factor’.

Answer 26

A limiting factor is one which limits the rate
of a reaction (e.g. photosynthesis), often
because it is in short supply, regardless of
the levels of other factors.

Question 27

Name the potential limiting factors of photosynthesis.

Answer 27

● Light intensity
● CO2 concentration
● Temperature

Question 28

Explain how changing light intensity affects the rate of photosynthesis.

Answer 28

● At low light intensities, the rate of the light-dependent stage is low. Little ATP and reduced
NADP is produced, so the rate of the light-independent stage is low.

● As light intensity increases, the rate increases. Eventually the volume of O2 produced and CO2 absorbed will equal the CO2 released and O2 absorbed, i.e. photosynthesis and respiration rates are equal. This is the compensation point.

● Past this point, increases in light intensity cause a proportional increase in photosynthesis.
This continues until factor becomes limiting.

Question 29

Explain how changing CO2 concentration affects the rate of photosynthesis.

Answer 29

CO2 is required to combine with RuBP in
the light-independent stage of
photosynthesis. Increasing the CO2
concentration will increase the rate of
photosynthesis until another factor
becomes limiting.

Question 30

How can a knowledge of limiting factors be used to optimise crop yields in greenhouses?

Answer 30

● Light intensity is increased by installing lamps and
using wavelengths of light chloroplasts can absorb

● The temperature is controlled

● CO2 concentration may be increased

Question 31

What is DCPIP?

Answer 31

DCPIP is a redox indicator. When DCPIP
is reduced (by the H+ ions from the photolysis of water), it turns from blue to colourless.

Question 32

Describe how the structure of a chloroplast is adapted to its function.

Answer 32

● The thylakoid membranes, arranged in a stack, provide a large surface area for the absorption of light by photopigments
● The double membrane is permeable to gases and products of photosynthesis
● The thylakoid space accommodates the H+ ions; the thylakoid membrane maintains the gradient
● The enzymes for the light-independent stage are all found in fluid stroma