3.5.1 - Photosynthesis

Question 1

Organisms that undergo photosynthesis are known as

Answer 1

Photoautotrophs

Question 2

What are the stages
of photosynthesis?

Answer 2

1. Light dependent reaction
2. Light independent reaction

Question 3

Where does the light dependent reaction occur?

Answer 3

Thylakoid membrane of chloroplast

Question 4

Where does the light independent reaction occur?

Answer 4

Stroma of chloroplast

Question 5

Structure of a chloroplast

Answer 5

• Double membrane (chloroplast membrane)
  > outer membrane and inner membrane
• Stroma - which contains:
  > Thylakoid membrane
  > Small/70s ribosomes
  > Circular DNA
  > Starch grains/lipid droplets
  > Lamella
  > Granum

Question 6

Inter-granal lamellae

Answer 6

thylakoid linking grana

Question 7

Grana

Answer 7

Stacks of thylakoid

Question 8

Thylakoids membranes contain…

Answer 8

• The pigment “Chlorophyll”
• enzymes and electron carriers required for photosynthesis

Question 9

Stroma

Answer 9

Fluid filled matrix where the light - independent stage of photosynthesis takes place

Question 10

Adaptations of membrane system in chloroplasts for light dependent reaction

Answer 10

• membranes of the grana provide a large surface area to increase the number of light dependent reactions that can occur
• membrane system provides a large number of pigment molecules in an arrangement that ensures as much light as necessary is absorbed

Question 11

Describe photoionisation in the light-dependent reaction (LDR)

Answer 11

● Chlorophyll absorbs light energy which excites its electrons to a higher energy level
● So electrons are released from chlorophyll (chlorophyll becomes positively charged)

Question 12

Describe what happens after photoionisation in the Light dependent reaction

Answer 12

Some energy from electrons released in photoionisation is conserved in the production of ATP / reduced NADP
(chemiosmotic theory):

1. Electrons move along electron transfer chain (electron carriers), releasing energy
2. This energy is used to actively pump protons from stroma into thylakoid
3. Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase
4. Energy used to join ADP and Pi to form ATP (photophosphorylation)
5. NADP accepts a proton and an electron to become reduced NADP

Question 13

How does chlorophyll a harness energy from sunlight?

Answer 13

• thylakoid membranes contain pigments, enzymes and electron carriers required for light dependent reactions

> membranes of thylakoid (grana) create a large surface area to increase number of light dependent reactions that occur

> membrane system provides large amount of pigment molecules
arrangement of these pigments is important and ensure as much light is absorbed as possible.

> pigments are arranged in photosystems

Question 14

Two types of photosystem

Answer 14

• Photosystem 1
  > PSI
  > P700
  > pigments absorb longer wavelengths of light
• Photosystem 2
  > PSII
  >P680
  > pigments absorb shorter wavelengths of light

Question 15

How do photosystem I and II differ slightly?

Answer 15

• The chlorophyll a pigments in the reaction centres of the photosystems are associated with different proteins
• This gives them different absorption peaks (wavelengths at which they absorb most light)

Question 16

Oxidation

Answer 16

gains oxygen/loses hydrogen/loses electrons

Question 17

Reduction

Answer 17

loses oxygen/gains hydrogen/gains electrons

Question 18

Oxidation and reduction always…

Answer 18

Take place together
(redox)

Question 19

Describe photolysis of water in the Light Dependent Reaction

Answer 19

● Water splits to produce protons, electrons and oxygen
(H2O → 1⁄2 O2 + 2e- + 2H+)
○ Electrons replace those lost from chlorophyll

Question 20

Why is the photolysis of water necessary?

Answer 20

• loss of electrons when light strikes a chlorophyll molecule leaves it dhort of electrons
• if chlorphyll molecule is to keep absorbing light energy, electrons must be replaced.
• replacement of electrons are provided by photolysis of water

Question 21

The photolysis of water also yields protons…

Answer 21

• These protons pass out of the thylakoid space through ATP synthase channels and are taken up by an electron carrier called NAD.
• NADP becomes reduced (NADPH)
  > NADPH required in light independent reaction

Question 22

What happens to the oxygen produced in the photolysis of water?

Answer 22

The oxygen by-product from the photolysis of water is used in respiration or diffuses out of the leaf as a waste product of photosynthesis.

Question 23

How are chloroplasts structurly adapted to capture sunlight and carry out the light-dependent reaction of photosynthesis?

Answer 23

• Thylakpid membranes provide a large surface area for attachment of chlorophyll, electron carriers and enzymes that carry out light-independent reaction.
• Network of proteins in grana hold chlorophyll in precise manner - allowing maximum absorption of light.
• Granal membranes have ATP synthase channels within them - which catalyse production of ATP
  > also selectively permeable which allows establishment of proton gradient.
• Chloroplasts contain DNA + ribosomes so they can quickly manufacture some proteins involved in light-dependent reaction.

Question 24

Products of the light dependent reaction which are required for the light independent reaction of photosynthesis:

Answer 24

• ATP
• Reduced NADPH

Question 25

Why does the light independent reaction of photosynthesis eventually stop in the absence of light?

Answer 25

Whilst it doesn't need light directly, the products ATP and reduced NADPH are required to reduce glycerate 3-phosphate in the light independent reaction and so it relies on the light dependent reaction

Question 26

Describe the light-independent reaction of photosynthesis (Calvin cycle)

Answer 26

1. CO2 reacts with ribulose bisphosphate (RuBP) ○ Catalysed by the enzyme rubisco 2. Forming 2 glycerate 3-phosphate (GP) molecules 3. GP reduced to triose phosphate ○ Using products from light-dependent reaction - reduced NADP and energy from ATP ○ reduced NADP is oxidised to NADP > which returns to the light dependent reaction to be reduced again by accepting more protons ○ ATP is hydrolysed by ATP hydrolase into ADP+Pi 4. Some TP converted to useful organic substances (eg. glucose/starch/cellulose/lipids etc... 5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from hydrolysis of ATP to ADP + Pi)

Question 27

How does CO2 enter the calvin cycle?

Answer 27

- CO2 from the atmosphere diffuses into leaf through stomata and dissolves in water around the walls of mesophyll cells. - It then diffuses through the cell surface membranes, cytoplasm and chloroplast membranes to stroma of chloroplasts > where CO2 reacts with RuBP and the calvin cycle occurs.

Question 28

How are chloroplasts adapted to their function of the light independent reaction of photosynthesis?

Answer 28

- Fluid of stroma contains enzymes required for light independent reaction > Stromal fluid is membrane-bound in chloroplast, so it has a chemical environment with a high concentration of enzymes and substrates that can be maintained. - Stroma fluid surrounds grana so products of light-dependent reaction in thylakoid (of grana) can readily diffuse into stroma. - Contains DNA + ribosomes so can easily and quickly manufacture proteins involved.

Question 29

Describe and explain how temperature affects rate of photosynthesis

Answer 29

1. As temperature increases, rate increases ○ Enzymes eg. rubisco gain kinetic energy ○ So more enzyme-substrate complexes form 2. Above an optimum temperature, rate decreases ○ Enzymes denature as H bonds in tertiary structure break ○ So fewer enzyme-substrate complexes form

Question 30

Describe and explain how light intensity affects rate of photosynthesis

Answer 30

1. As light intensity increases, rate increases ○ Light-dependent reaction increases (eg. more photoionisation of chlorophyll) so more ATP and reduced NADP produced ○ So light-independent reaction increases as more GP reduced to Triose phosphate P and more Triose Phosphate regenerates RuBP 2. Above a certain light intensity, rate stops increasing ○ Another factor is limiting eg. temperature / CO2 concentration

Question 31

Describe and explain how CO2 concentration affects rate of photosynthesis

Answer 31

1. As CO2 concentration increases, rate increases ○ Light-independent reaction increases ○ As more CO2 combines with RuBP to form GP ○ So more GP reduced to Triose Phosphate ○ So more Triose Phosphate converted to organic substances and more RuBP regenerated 2. Above a certain CO2 concentration, rate stops increasing ○ Another factor is limiting eg. temperature / light intensity

Question 32

Explain the important consideration when evaluating data relating to agricultural practices used to overcome the effect of limiting factors

Answer 32

● Agricultural practice should increase rate of photosynthesis, leading to increased yield ○ As more glucose produced for faster respiration ○ So more ATP to release energy for growth eg. cell division, protein synthesis ● But profit from extra yield should be greater than costs (money & environmental costs)