5.2.1: Photosynthesis Flashcards
How are photosynthesis and respiration related?
CO2 & H2O are the raw materials for
photosynthesis and the products of respiration.
O2 & glucose are the raw materials for
photosynthesis and the products of respiration
Describe the structure of a chloroplast.
● Usually disc-shaped.
● Double membrane (envelope).
● Thylakoids: flattened discs stack to form grana.
● Intergranal lamellae: tubular extensions attach
thylakoids in adjacent grana.
● Stroma: fluid-filled matrix with high enzyme &
substrate concentration & own loop of DNA.
Where do the light-dependent & light-independent
reactions occur in plants?
Light-dependent: in the thylakoids of chloroplasts.
Light-independent: stroma of chloroplasts.
What is the role of photosynthetic pigments? Name
the 2 main groups
Embedded within thylakoid membrane. Absorb different
wavelengths of light to maximise rate of photosynthesis.
● Primary pigment: chlorophyll (made of chlorophyll a &
chlorophyll b) found in photosystems.
● Accessory pigments: carotenoids (carotene & xanthophylls) found in light-harvesting systems.
Name the processes in the light-dependent reaction.
● photoionisation ● electron transfer chain ● chemiosmosis non-cyclic only: ● reduction of NADP ● photolysis of water
Explain the role of light in photoionisation.
Chlorophyll molecules absorb energy from photons of light.
This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll.
What happens in the electron transfer chain (ETC)?
Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & undergo a series of redox reactions, which releases energy.
How does chemiosmosis produce ATP in the
light-dependent stage?
Some energy released from the ETC is coupled to active
transport of H+ ions from stroma into thylakoid space.
H+ ions move down concentration gradient from thylakoid space into stroma via transmembrane channel protein ATP synthase.
ATP synthase catalyses ADP + Pi → ATP
Describe non-cyclic photophosphorylation.
Uses Photosystems Ⅰ & Ⅱ. Excited electrons enter
ETC to produce ATP. NADP acts as final electron
acceptor & is reduced. Water is photolysed to
release electrons to replace those lost from PS Ⅱ.
Purpose is to produce ATP & reduced NADP for
Calvin cycle to produce biological compounds.
Describe cyclic photophosphorylation.
Uses only Photosystem Ⅰ. Excited electrons enter
ETC to produce ATP then return directly to
photosystem (so no reduction of NADP & no water
needed to replace lost electrons).
Purpose is to produce additional ATP to meet surplus
energy demands of cell.
What happens in photolysis of water?
Light energy splits molecules of water
2H2O → 4H+ + 4e- + O2
What happens to the products of the photolysis of
water?
H+ ions: move out of thylakoid space via ATP
synthase & are used to reduce the coenzyme NADP.
e-: replace electrons lost from chlorophyll.
O2: used for respiration or diffuses out of leaf as
waste gas
How and where is reduced NADP produced in the
light-dependent reaction?
NADP + 2H+ (from photolysis of water) + 2e- (from
acting as final electron acceptor in ETC) → reduced NADP.
Catalysed by dehydrogenase enzymes.
Stroma of chloroplasts.
Name the 3 main stages in the light-independent
reaction
- Carbon fixation
- Reduction
- Regeneration
What happens during carbon fixation?
Reaction between CO2 & ribulose
bisphosphate (RuBP) catalysed by ribulose bisphosphate carboxylase (RuBisCo).
Forms unstable 6C intermediate that breaks down into 2 x glycerate 3-phosphate (GP).
What happens during reduction in the light-independent reaction?
2 x GP are reduced to 2 x triose phosphate (TP)
Requires 2 x reduced NADP & 2 x ATP
Forms 2 x NADP & 2 x ADP
Outline the roles of TP from the light-independent
reaction.
Raw material: 1C leaves the cycle to produce
monosaccharides, amino acids & other biological
molecules.
Involved in regeneration of RuBP: After 1C leaves
cycle, the 5C compound RuP forms. RuP is converted
into RuBP using 1x ATP. Forms 1x ADP.
State the number of carbon atoms in RuBP, GP &
GALP.
RuBP: 5
GP: 3
GALP: 3
Define ‘limiting factor’.
Factor that determines maximum rate of a reaction, even if other factors change to become more favourable.
Name 4 environmental factors that can limit the rate
of photosynthesis.
● Light intensity (light-dependent stage).
● CO2 levels (light-independent stage).
● Temperature (enzyme-controlled steps).
● Mineral/magnesium levels (maintain normal functioning of chlorophyll)
How does light intensity affect the rate of
photosynthesis?
Low light intensity = slower light- dependent reaction = less ATP & NADPH produced to convert GP to TP in light-independent reaction.
● GP level rises
● TP level falls = RuBP level falls
Describe the implications of water stress.
- Abscisic acid binds to complementary receptors on guard cell membrane, causing Ca2+ ion channels on tonoplast to open. Ca2+ ions diffuse from vacuole into cytosol.
- Positive feedback triggers other ion channels to open.
Other ions e.g. K+ diffuse out of guard cell. - Water potential of guard cell becomes more positive.
Water diffuses out via osmosis. - Guard cells become flaccid so stomata close.
State the purpose and principle of paper chromatography
Molecules in a mixture are separated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (TLC plate, usually coated in a silicate)
Outline a method for extracting photosynthetic
pigments.
Use a pestle and mortar to grind a leaf
with an extraction solvent e.g.
propanone.
Outline how TLC can be used to separate
photosynthetic pigments.
- Use a capillary tube to spot pigment extract onto
pencil ‘start line’ (origin) 1 cm above bottom of plate. - Place chromatography paper in solvent (origin should
be above solvent level). - Allow solvent to run until it almost touches the other
end of the paper. Pigments move different distances.
