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 centres where

Question 3

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

Answer 3

accessory pigments absorb wavelengths of light which chlorophyll a do not
therefore maximising efficiency of energy absorption from sun

Question 4

What accessory pigments may a plant contain?

Answer 4

chlorphyll b
xanthophylls
carotenoids

Question 5

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

Answer 5

thylakoid membrane

Question 6

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

Answer 6

Site of reactions involved in photosynthesis
Chlorophyll a is present

Question 7

what two parts is a photosystem made from

Answer 7

an antennae complex and reaction centre

Question 8

what is photophosphorylation

Answer 8

phosphorylation of ADP + Pi ⟶ ATP
using energy from sunlight

Question 9

describe the steps of non-cyclic photophosphorylation

Answer 9

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

Question 10

describe the steps of cyclic photophosphorylation

Answer 10

1.PSI absorbs light energy/photon, which excites electron at reaction centre.
2.Excited electron passes through electron transport chain
3.Releases energy when transferred between subsequent electron carriers
4.Energy released used to actively transport H+ ions across thylakoid membrane
5.Creates proton gradient
H+ diffuses back by facilitated diffusion using ATP synthase membrane protein
6.Diffusion of H+ down concentration gradient releases energy
7.Energy used to form bond between ADP + Pi ⟶ ATP
8.After chemiosmosis, electron in electron transport chain returns to PSI, resets process

Question 11

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

Answer 11

photolysis of water

(H2O ⟶ 2H+ + 2e- + ½O2)

Question 12

During photosynthesis, H+ + e- + NADP ⟶ NADPH
What provides the proton & electron in this reaction?

Answer 12

electron from the electron transport chain
hydrogen from the photolysis of water

Question 13

what is the electrons transport chain

Answer 13

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 14

what is chemiosmosis

Answer 14

the synthesis of ATP
driven by flow of protons across a membrane

Question 15

what are the products of non cyclic photophosphorylation

Answer 15

reduced NADP (NADPH)
ATP & O2

Question 16

what are the products of cyclic photophosphorylation

Answer 16

ATP

Question 17

what is NADP

Answer 17

A coenzyme involved in photosynthesis. It accepts electrons to be reduced to NADPH.

Question 18

describe photolysis

Answer 18

1) Light hits a water mol. An enzyme helps split H2O into 2H* and O.
2) H* Ions reduce NADP into NADPH.
3) The electrons replace those lost from PSII in non-cyclic photophosphorylation.
4) O2 is released as a waste gas.

Question 19

Whats the light independent reaction also called?

Answer 19

the calvin cycle

Question 20

describe the calvin cycle

Answer 20

1)CO2 + RuBP ⟶ unstable 6-carbon intermediate
Fixation of CO2 catalysed by RuBisCO enzyme, Ribulose Biphosphate carboxylase
6-carbon intermediate breaks down ⟶ 2GP, 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 21

whats chemiosmosis

Answer 21

The diffusion of protons from a high to a low conc releasing energy.

Question 22

What reaction is catalysed by rubisco?

Answer 22

the carboxylation of RuBp

Question 23

what does GP stand for

Answer 23

Glycerate - 3 - phosphate

Question 24

what does TP stand for

Answer 24

Triose phospate

Question 25

what does RuBP stand for

Answer 25

ribulose biphosphate

Question 26

what is used to regenerate RuBP

Answer 26

Five out of every six molecules of TP produced in the cycle aren’t used to make hexose sugars, but to regenerate RuBP
- Regenerating RuBP uses the rest of the ATP produced by the light-dependent reaction

Question 27

How is triose phosphate (TP) turned into carbohydrates?

Answer 27

Hexose sugars (eg. glucose) are made by joining 2 triose phosphate molecules together and larger carbohydrates (eg. sucrose, starch, cellulose) are made by joining hexose sugars together in different ways

Question 28

Which photosystem(s) are involved in cyclic photophosphorylation?

Answer 28

PS1 only (electron leaves PSI to electron transport chain, then returns)

Question 29

what photosystems are involved in non cyclic photophosphorylation

Answer 29

PS1 and PS11
PS11 (p680) first then to PS1 (p700)

Question 30

what occurs in cyclic photophosphorylation

Answer 30

pigments in PSI absorbs photons of light
excites electron in the reaction centre
transported by electron transport chain, releasing energy
energy released used to actively transport H+ across thylakoid membrane
form proton gradient, as H+ moves back by facilitated diffusion through ATP synthase protein, drives ATP formation
this electron can undergo process again, once sufficient energy is absorbed to excite it
(reduced NADPH is not a product in cyclic phosphorylation)

Question 31

where does the reaction of the calvin cycle occur

Answer 31

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

Question 32

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

Answer 32

NADPH supplies hydrogen atom for reduction

ATP supplies energy for reaction

Question 33

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

Answer 33

both are products of the light-dependent stage

(therefore light-independent stage is technically not completely light-independent)

Question 34

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

Answer 34

1 x 6-carbon unstable intermediate
2 x GP
2 x TP

Question 35

what is photorespiration

Answer 35

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