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Summary AQA A level Biology Paper 2 topic 5 notes
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A detailed summary of the AQA A level Biology paper 2 topic 5 which has been edited to learn help the content and the mark scheme too.
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🌿T5: energy transfers in and
between organisms
3.5.1 photosynthesis
what are the adaptations of a leaf?
-arranged so they dont overlap to prevent shadows forming on the
leaves below
-large SA to maximise light absorption
-transparent cuticle and epidermis to let light through to chloroplasts in
mesophyll cells
-network of xylem and phloem for efficient transport of sugars and
water
-upper mesophyll cells contain chloroplasts so light can be absorbed as
they are more exposed at the top
-air spaces within mesophyll to allow rapid diffusion of gases though
the leaf
-thin so there is a short diffusion pathway
-many stomata which open and close to balance exchange with water
loss
what is the structure of the chloroplast?
T5: energy transfers in and between organisms 1
, what are the adaptations of the chloroplast?
-contain DNA and ribosomes to manufacture proteins for
photosynthesis
-stroma fluid has enzymes and substrates enclosed by membrane so
they are kept distinct from cytoplasm
-stroma fluid surrounds grana so there is short diffusion pathway for
products of light dependent reaction to diffuse from grana to stroma
-thylakoid membranes have a large SA so there is more space for
attachment of chloroplasts, electron carriers, enzymes
-proteins in grana to hold chlorophyll in correct position so it absorbs as
much as light as possible
-granal membranes contain selectively permeable ATP synthase
channels to synthesise ATP and establish protein conc. gradient
where does the light dependent reaction take place?
-in the thylakoid membranes
what is photoionisation?
-when chlorophyll absorbs sunlight, electrons become excited
(photoexcitation) so they leave the chlorophyll molecule
-the chlorophyll becomes ionised and this is photoionisation
what happens to the electrons that leave the chlorophyll?
-they are taken up by the electron carriers by series of redox reactions
-they form an electron transfer chain
-each new carrier loses energy
T5: energy transfers in and between organisms 2
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