Photosynthesis

Question 1

What is a chloroplast?

Answer 1

An organelle found in plants and algae that is the site of photosynthesis.

Question 2

Describe the structure of a chloroplast.

Answer 2

Double membrane (chloroplast envelope)
· Grana - stacks of flattened disks (thylakoids) that contain photosystems, electron transport chain, ATP synthase
Grana connected by intergranal lamellae
* Stroma - fluid-filled matrix containing enzymes

Question 3

Where are chloroplasts located in an angiosperm leaf?

Answer 3

Mainly found in the mesophyll layers.

Question 4

How are chloroplasts adapted for
photosynthesis?

Answer 4

-Thylakoids give a large surface area for light-independent reactions
-Photosynthetic pigments arranged into photosystems to maximise light absorption
-Stroma directly surrounds grana - products of photosynthesis diffuse directly into the stroma.
-Contain their own DNA and ribosomes
-Inner chloroplast membrane less permeable than outer enabling control over the movement of substances

Question 5

How are angiosperm leaves adapted for photosynthesis?

Answer 5

Large surface area to maximise light absorption
Thin to reduce the diffusion distance for CO2
Upper epidermis is transparent, allowing light to strike mesophyll layers

Palisade cells are densely packed and contain many chloroplasts

Air spaces reduce the diffusion distance for CO2
The vein network transports water and minerals to the leaf and takes sugars away
Stomata allow CO2 to diffuse into the leaf

Question 6

What is a transducer? Why are chloroplasts described as transducers?

Answer 6

Something that converts one type of energy into another. Chloroplasts transduce light energy into the chemical energy of ATP

Question 7

What is a photosynthetic pigment?

Answer 7

A molecule present in chloroplasts that absorbs certain wavelengths of light.

Question 8

Give some examples of photosynthetic
pigments.

Answer 8

Chlorophylls a and b
Beta carotene
Xanthophylls

Question 9

What is the purpose of chromatography?

Answer 9

To separate different products from a mixture.

Question 10

What is photosynthesis?

Answer 10

A complex metabolic pathway that synthesises organic molecules in the presence of light. Overall:
6CO2+6H2O→ C6H12O6+602

Question 11

Name the two stages of photosynthesis.

Answer 11

Light-dependent stage
Light-independent stage

Question 12

What is a Photosystem?

Answer 12

Protein complex consisting of an antenna complex and reaction centre
Involved in the absorption of light and the transfer of electrons in photosynthesis. Two types; Photosystems I and II.

Question 13

How do Photosystems I and II differ?

Answer 13

They absorb different wavelengths of
light.

Question 14

Describe the process of light harvesting

Answer 14

The antenna complex absorbs light energy of varying wavelengths and transfers it to the reaction centre
Energy absorbed by two chlorophyll a molecules which emit ‘excited’ electrons

Question 15

What is an absorption spectra?

Answer 15

A graph showing the wavelengths of light absorbed by a pigment.

Question 16

What is an action spectra?

Answer 16

A graph of the rate of photosynthesis against each wavelength of light absorbed by a pigment.

Question 17

Describe the light-dependent stage of
photosynthesis.

Answer 17

• First stage of photosynthesis
• Takes place in the thylakoids of the chloroplast
• Uses light energy to produce ATP, reduced NADP and

Question 18

State the sources of electrons for the electron transport chain.

Answer 18

Cyclic and non-cyclic photophosphorylation.

Question 19

Define cyclic photophosphorylation.

Answer 19

The formation of ATP involving Photosystem I only.

Question 20

Outline cyclic photophosphorylation.

Answer 20

Involves Photosystem I only
* Excited electrons enter the electron transport chain to produce ATP and then return to Photosystem I
* No reduction of NADP and no water required to replace lost electrons

Question 21

What is the purpose of cyclic photophosphorylation?

Answer 21

Produces additional ATP to meet surplus energy demands of the cell.

Question 22

Define non-cyclic photophosphorylation.

Answer 22

The formation of ATP and reduced NADP involving both Photosystems I and II

Question 23

Outline non-cyclic photophosphorylation.

Answer 23

Involves Photosystem I and II
Excited electrons enter the electron transport chain to produce ATP
NADP acts as a final electron acceptor and is reduced
* Water is photolysed to compensate for electrons lost from Photosystem II

Question 24

What is the purpose of non-cyclic photophosphorylation?

Answer 24

Produces ATP and reduced NADP for
the Calvin cycle.

Question 25

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

Answer 25

Protons flow down their concentration gradient from the thylakoid space into the stroma via ATP synthase
ATP synthase phosphorylates ADP to form ATP as
protons flow through it

Question 26

Describe photolysis.

Answer 26

The splitting of a molecule of water in the presence of light that occurs during the light-dependent stage of photosynthesis. This produces protons, electrons and
oxygen:
H2O→ 2H+ + 2e+1⁄20,

Question 27

What happens to the products of photolysis?

Answer 27

H+ - used in proton pumping and to reduce NADP
e-replaces electrons lost from chlorophyll a in PSII
O2 - by-product, used for respiration or diffuses out of the leaf as waste gas

Question 28

Explain how the electron transfer chain results in the production of reduced
NADP.

Answer 28

NADP acts as a final electron acceptor, and is subsequently reduced.

Question 29

Describe the light-independent stage of photosynthesis.

Answer 29

• Second stage of photosynthesis
  Does not require light energy and takes place in the stroma
• Uses carbon dioxide and the products of the light-dependent stage to build organic molecules

Question 30

What is the light-independent stage also known as?

Answer 30

The Calvin Cycle

Question 31

Name the three main stages of the
Calvin cycle.

Answer 31

1. Carbon fixation
2. Reduction
3. Regeneration

Question 32

What happens during carbon fixation of
the Calvin cycle?

Answer 32

Reaction between CO, and ribulose bisphosphate (RuBP) catalysed by enzyme RuBisCo
Forms unstable 6C intermediate that breaks down into two molecules of glycerate 3-phosphate (GP)

Question 33

What happens during reduction of the Calvin cycle?

Answer 33

2x GP are reduced to 2x triose phosphate (TP)
Requires 2x reduced NADP and 2× ATP formed during the light-dependent reaction
Forms 2x NADP and 2x ADP that enter the
light-dependent reaction

Question 34

What happens during regeneration of the Calvin cycle?

Answer 34

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

Question 35

How are nutrients produced as a result of photosynthesis?

Answer 35

• Formation of amino acids from GP (requires nitrates and sulfates)
• TP molecules used to produce sugars e.g. glucose, fructose, sucrose

Question 36

What is a limiting factor?

Answer 36

A variable that limits the rate of a
particular reaction.

Question 37

Name the factors that limit the rate of photosynthesis. What stage do they limit?

Answer 37

• Light intensity - light-dependent stage
  Light wavelength - absorption by chlorophyll
• CO2 levels - light-independent stage
• Temperature - enzyme-controlled reactions pH - enzyme-controlled reactions

Question 38

Describe the role of nitrogen in plant
metabolism.

Answer 38

Synthesis of amino acids, nucleotides and chlorophyll.

Question 39

What does nitrogen deficiency in plants cause?

Answer 39

• Stunted growth
• Chlorosis (yellowing of the leaves)

Question 40

Describe the role of magnesium in plant
metabolism.

Answer 40

Synthesis of chlorophyll.

Question 41

What does magnesium deficiency in plants cause?

Answer 41

• Chlorosis (yellowing of the leaves)