17.3 Photosynthesis

Question 1

What are pigment molecules?

Answer 1

Molecules which absorb specific wavelengths of light

& direct energy to the reaction centre

Question 2

After absorbing light energy, to which site does a pigment molecule direct this energy to?

Answer 2

reaction centre

(where reactions of photosynthesis occur)

Question 3

What is the primary pigment molecule in plants?

Answer 3

chlorophyll a

Question 4

Why may a plant contain accessory pigments as well as chlorphyll a?

Answer 4

accessory pigments absorb wavelengths of light which chlorophyll a do not

therefore maximise efficiency of energy absorption from sun

Question 5

What accessory pigments may a plant contain?

Answer 5

chlorphyll b

xanthophylls

carotenoids

Question 6

Where in the ultrastructure of a chloroplast are pigment molecules present?

Answer 6

thylakoid membrane

Question 7

What is the reaction centre & what molecule is present there?

Answer 7

Site of reactions involved in photosynthesis

Chlorophyll a is present

Question 8

What 2 parts is a photosystem made from?

Answer 8

1. antennae complex
2. reaction centre

Question 9

What is photophosphorylation?

Answer 9

phosphorylation of ADP ⟶ ATP

using energy from sunlight

Question 10

Describe & explain the steps of non-cyclic photophosphorylation:

Answer 10

• PSII absorbs light energy/photon, which excites electron at reaction centre
• Excited electron passes through electron transport chain
  
  • Releases energy when transferred between subsequent electron carriers
  • Replaced by photolysis of water
• Energy released used to actively transport H⁺ ions across thylakoid membrane
  
  • Creates proton gradient
• H⁺ diffuses back by facilitated diffusion using ATP synthase membrane protein
  
  • Diffusion of H⁺ down concentration gradient releases energy
  • Energy used to form bond between ADP + P_i ⟶ ATP
• Electron + H⁺ from photolysis of H₂O + NADP ⟶ Reduced NADPH
  
  • H⁺ + e^- + NADP ⟶ NADPH

Question 11

Describe & explain the steps of cyclic photophosphorylation:

Answer 11

• PSI absorbs light energy/photon, which excites electron at reaction centre.
• Excited electron passes through electron transport chain
  
  • Releases energy when transferred between subsequent electron carriers
• Energy released used to actively transport H⁺ ions across thylakoid membrane
  
  • Creates proton gradient
• H⁺ diffuses back by facilitated diffusion using ATP synthase membrane protein
  
  • Diffusion of H⁺ down concentration gradient releases energy
  • Energy used to form bond between ADP + P_i ⟶ ATP

After chemiosmosis, electron in electron transport chain returns to PSI, resets process

Question 12

After joining the electron transport chain, from where s the electron from PSII replaced?

Answer 12

photolysis of water

(H₂O ⟶ 2H⁺ + 2e^- + ½O₂)

Question 13

During photosynthesis, H⁺ + e^- + NADP ⟶ NADPH

What provides the proton & electron in this reaction?

Answer 13

electron from the electron transport chain

hydrogen from the photolysis of water

Question 14

What is the electron transport chain?

Answer 14

series of redox reactions & electron carriers

in which an excited electron releases energy at each stage

(this energy used to create a proton gradient, which drives the formation of ATP)

Question 15

What is chemiosmosis?

Answer 15

synthesis of ATP

driven by flow of protons across a membrane

Question 16

What is the equation for the photolysis of water?

Answer 16

H₂O → 2H⁺ + ½O₂ + 2e^-

Question 17

What are the products of non-cyclic photophosphorylation?

Answer 17

1. ATP
2. Reduced NADP

Question 18

What are the products of cyclic photophosphorylation?

Answer 18

ATP ONLY

Question 19

How do the products of cyclic & non-cyclic photophosphorylation differ?

Answer 19

CYCLIC: produces ATP ONLY

NON-CYCLIC: produces ATP & NADPH

Question 20

Which photosystem(s) are involved in cyclic photophosphorylation?

Answer 20

PSI ONLY

(electron leaves PSI to electron transport chain, then returns)

Question 21

Which photosystem(s) are involved in non-cyclic photophosphorylation?

Answer 21

PSII AND PSI

Question 22

Which phosphorylation produces reduced NADP as a product?

Answer 22

non-cyclic (since electron is used up along with H⁺ to form reduced NADP, stopping the cycle)

Question 23

What occurs in cyclic phosphorylation?

Answer 23

1. pigments in PSI absorbs photons of light
2. excites electron in the reaction centre
3. transported by electron transport chain, releasing energy
4. energy released used to actively transport H⁺ across thylakoid membrane
5. form proton gradient, as H⁺ moves back by facilitated diffusion through ATP synthase protein, drives ATP formation
6. this electron can undergo process again, once sufficient energy is absorbed to excite it

(reduced NADPH is not a product in cyclic phosphorylation)

Question 24

What is photophosphorylation?

Answer 24

phosphorylation of ADP ⟶ ATP

using energy from sunlight

Question 25

Where does the reactions of the calvin cycle occur?

Answer 25

spongy mesophyll layer (because contains air pockets containing CO₂, which is reactant)

Question 26

Outline the process of the calvin cycle:

Answer 26

* CO₂ + RuBP ⟶ **unstable 6-carbon intermediate** * **Fixation** of CO₂ catalysed by **RuBisCO enzyme**, Ribulose Biphosphate carboxylase * 6-carbon intermediate **breaks down** ⟶ 2**GP**, glycerate phosphate * GP **reduced** ⟶ **TP**, triose phosphate * H supplied by NADPH, energy supplied by ATP, both from light-dependent stage * For every 6 TP, 5TP used to **regenerate** 3 molecules of **RuBP** * Remaining TP used to form carbohydrates, proteins, lipids etc

Question 27

During the calvin cycle, why does the conversion of GP ⟶ TP require ATP & NADPH?

Answer 27

NADPH supplies hydrogen atom for reduction ATP supplies energy for reaction

Question 28

During the calvin cycle, where does the ATP & NADPH come from?

Answer 28

both are products of the **light-dependent stage** (therefore light-independent stage is technically not completely light-independent)

Question 29

How many of each product is made in the calvin cycle per 1 CO₂ molecule?

Answer 29

1 6-carbon unstable intermediate 2 GP 2 TP

Question 30

If 6 TP are made during photosynthesis, how many of these molecules will go towards forming carbohydrates/lipids/proteins?

Answer 30

1 TP (rest 5 are used to regenerate RuBP)

Question 31

What is photorespiration?

Answer 31

O₂ is **competitive inhibitor** of **RuBisCO enzyme** RuBisCO enzyme catalyses reaction with O₂ rather than CO₂ forms **phosphoglycolate**, **toxic** 2 carbon compound **requires ATP** to break down. photorespiration wastes approx 25% of products from calvin cycle occurs at low concentrations of CO₂

Question 32

If 6 TP are made during photosynthesis, how many of these molecules will go towards forming carbohydrates/lipids/proteins?

Answer 32

1 TP (rest 5 are used to regenerate RuBP)