17.3 Photosynthesis Flashcards
What are pigment molecules?
Molecules which absorb specific wavelengths of light
& direct energy to the reaction centre
After absorbing light energy, to which site does a pigment molecule direct this energy to?
reaction centre
(where reactions of photosynthesis occur)
What is the primary pigment molecule in plants?
chlorophyll a
Why may a plant contain accessory pigments as well as chlorphyll a?
accessory pigments absorb wavelengths of light which chlorophyll a do not
therefore maximise efficiency of energy absorption from sun
What accessory pigments may a plant contain?
chlorphyll b
xanthophylls
carotenoids
Where in the ultrastructure of a chloroplast are pigment molecules present?
thylakoid membrane
What is the reaction centre & what molecule is present there?
Site of reactions involved in photosynthesis
Chlorophyll a is present
What 2 parts is a photosystem made from?
- antennae complex
- reaction centre
What is photophosphorylation?
phosphorylation of ADP ⟶ ATP
using energy from sunlight
Describe & explain the steps of non-cyclic photophosphorylation:
- 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 + Pi ⟶ ATP
- Electron + H+ from photolysis of H2O + NADP ⟶ Reduced NADPH
- H+ + e- + NADP ⟶ NADPH
Describe & explain the steps of cyclic photophosphorylation:
- 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 + Pi ⟶ ATP
After chemiosmosis, electron in electron transport chain returns to PSI, resets process
After joining the electron transport chain, from where s the electron from PSII replaced?
photolysis of water
(H2O ⟶ 2H+ + 2e- + ½O2)
During photosynthesis, H+ + e- + NADP ⟶ NADPH
What provides the proton & electron in this reaction?
electron from the electron transport chain
hydrogen from the photolysis of water
What is the electron transport chain?
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)
What is chemiosmosis?
synthesis of ATP
driven by flow of protons across a membrane
What is the equation for the photolysis of water?
H2O → 2H+ + ½O2 + 2e-
What are the products of non-cyclic photophosphorylation?
- ATP
- Reduced NADP
What are the products of cyclic photophosphorylation?
ATP ONLY
How do the products of cyclic & non-cyclic photophosphorylation differ?
CYCLIC: produces ATP ONLY
NON-CYCLIC: produces ATP & NADPH
Which photosystem(s) are involved in cyclic photophosphorylation?
PSI ONLY
(electron leaves PSI to electron transport chain, then returns)
Which photosystem(s) are involved in non-cyclic photophosphorylation?
PSII AND PSI
Which phosphorylation produces reduced NADP as a product?
non-cyclic (since electron is used up along with H+ to form reduced NADP, stopping the cycle)
What occurs in cyclic phosphorylation?
- 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)
What is photophosphorylation?
phosphorylation of ADP ⟶ ATP
using energy from sunlight
Where does the reactions of the calvin cycle occur?
spongy mesophyll layer
(because contains air pockets containing CO2, which is reactant)
Outline the process of the calvin cycle:
- 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
During the calvin cycle, why does the conversion of GP ⟶ TP require ATP & NADPH?
NADPH supplies hydrogen atom for reduction
ATP supplies energy for reaction
During the calvin cycle, where does the ATP & NADPH come from?
both are products of the light-dependent stage
(therefore light-independent stage is technically not completely light-independent)
How many of each product is made in the calvin cycle per 1 CO2 molecule?
1 6-carbon unstable intermediate
2 GP
2 TP
If 6 TP are made during photosynthesis, how many of these molecules will go towards forming carbohydrates/lipids/proteins?
1 TP
(rest 5 are used to regenerate RuBP)
What is photorespiration?
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
If 6 TP are made during photosynthesis, how many of these molecules will go towards forming carbohydrates/lipids/proteins?
1 TP
(rest 5 are used to regenerate RuBP)
