Clear, concise, effective summary notes covering all points in Topic 13: Photosynthesis of the new 2022-24 CIE A Level Biology syllabus. A must-have revision tool that helped me achieve an A* in A-Level Biology.
Topic 13: Photosynthesis
Chloroplast structure & function
● how chloroplast structure is suitable for its functions
Stroma contains 70S ribosomes & loops of Enable prod. of chloroplast proteins & enzymes for
chloroplast DNA light-independent reactions
Stroma contains large starch grains Stores sugars formed by photosynthesis as starch
→ insoluble
Thylakoids stacked into grana & connected by Create ↑ SA for ↑ photosynthetic pigments → ↑ no.
lamellae of light-dependent reactions occurring at same
time → as much light absorbed as possible
Pigment molecules in photosystem arranged in Pass E. down to reaction centre
funnel-like structures
Photosynthesis
● overview of stages
○ light-dependent reactions – light drives transfer of e-s and H+s to carrier NADP+ & reduce
it to NADPH + prod. ATP by photophosphorylation
■ occurs in thylakoid membrane
○ light-independent reactions – CO2 incorporated into organic molecules by carbon fixation
& then reduced to carbs. by NADPH using E. in ATP
■ occurs in stroma
● photosynthetic pigments
○ substance that absorbs visible light
○ we see the colour most transmitted/reflected by pigment
■ chlorophyll absorbs violet-blue & red light, reflects/transmits green light ∴ why
leaves are green
Pigment group Name Colour reflected Functions
Chlorophylls chlorophyll a yellow-green ● participates directly in light-dependent
reactions
xanthophyll yellow ● widen action spectrum of wavelengths
that can drive photosynthesis
● photoprotection – absorbs & dissipates
excessive & harmful light E.
, ● wavelength spectra
○ absorption spectrum
■ shows absorbance of different wavelengths of light by a particular photosynthetic
pigment
■ eg. chlorophyll a peaks at ~470nm & ~680nm → absorbs lights from blue-violet &
red regions
○ action spectrum
■ shows rate of photosynthesis at different wavelengths of light
■ ie. cumulation of absorption spectra of all pigments
■ ∴ peaks in blue-violet, red regions, trough in green-yellow region
● light-dependent reactions
○ makes ATP & NADPH for light-independent reaction
■ can be cyclic/non-cyclic depending on e- flow (non-linear/linear)
○ photosystems
■ reaction-centre complex surrounded by several light-harvesting complexes
● reaction-centre complex – an organised association of proteins holding a
special pair of chlorophyll a molecules and a primary electron acceptor
● light-harvesting complex – various pigment molecules bound to proteins
○ wide variety & large number of pigment molecules → broader
spectrum of wavelengths of light can be harvested at high SA
○ act as antennae molecules for reaction-centre complex
■ found in thylakoid membrane of chloroplasts
■ photosystem II (PS II) & photosystem I (PS I) cooperate in light reactions
● each has particular kind of primary pigment = pair of chlorophyll a
molecules
● PS II pair = P680 ∵ best absorption at 680 nm
● PS I pair = P700 ∵ best absorption at 700 nm
○ photoactivation
■ ie. excitation of chlorophyll by light
■ upon absorption of light photons, pigment molecule changes from ground state →
excited state
● ∵ an e- elevated to higher energy orbital
■ e- instantaneously drops back to ground state orbital → releasing E. doing so as
heat → excites e- of nearby pigment molecule
■ photons are also released → release of light = fluorescence
● difficult to see unless w/ UV light
○ photolysis
■ enzyme = oxygen-evolving complex/water-splitting complex splits H2O → 2e- +
2H+ + ½O2
■ e-s fill gap in P680 left by photoexcited e-s
■ O2 exits as by-product
■ H+ forms NADPH from NADP+
○ photophosphorylation
■ photoexcited e-s pass down ETC → generates E. w/ each step → pumps H+ from
stroma into thylakoid space by chemiosmosis→ proton grad. established → H+
diffuse back into stroma via ATP synthase → ATP created
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