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Summary OCR BIOLOGY A 2.5 BIOLOGICAL MEMBRANES
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BIOLOGY 2.5EMILY KINZLER
, CELL MEMBRANES ARE PARTIALLY PERMEABLE BARRIERS
separate cell content from exterior enviro
!
permeability = ability to let substances through
very small mols diffuse in between structural mols of memb
some subs dissolve in lipid layer + pass through NOT ALL MEMBRANES HAVE SAME COMPOSITION
some pass through protein channels/carrier proteins
partially permeable = dont let all types of mol enter neurones - protein channels/carriers in memb covering long axon allow entry/exit of ions to bring abt
conduction of electrical impulses along the length
myelin sheath - 20% protein + 76%lipid
wbc - membs have protein receptors that enable them to recognise antigens on foreign cells
root hair cells - many carrier proteins to actively transport nitrate ions from soil
inner memb of mitochondria are 76% protein + 24% lipid, as memb has many electron carriers made of protein
+ hydrogen ion channels associated w atp synthase enz
ROLES OF MEMBRANES
AT SURFACE OF CELLS STRUCTURE OF CELL MEMBRANES
plasma memb:
sep cells components from ext enviro (external surroundings/tissue fluid)
reg transport of materials in/out
has antigens so immune system can recognise cell as being “self” + not attack
may release chemicals that signal to other cells
have receptors for chemical signals, hormones/drugs bind to memb bound receptors
site of chemical reactions
FLUID MOSAIC MODEL
membrane consists of phospholipid bilayer w proteins floating in it making mosaic pattern
lipid mols can change places + some proteins can move, so fluid
WITHIN CELLS hydrophilic heads in contact w watery int/exterior
hydrophobic tail at centre of memb away from water
memb around organelles seperate contents from cytoplasm
some proteins have pores as channels to allow ions (electric charge + surrounded by
cristae on mitochondria give large sa for aerobic resp + localise some enz
water+ to pass through
needed for resp
some proteins are carriers - change shape to carry specific mols across memb
thylakoid membranes house chlorophyll, reactions for photosynth occur
some proteins attached to carrier proteins + function as enz/antigens/receptor sites for
(chloroplast)
complementary shaped signalling chemicals (hormones)
digestive enz on plasma memb of epithelial cells that line small intestine,
eukaryote membranes have cholesterol, helps reg fluidity, maintain mechanical stability,
catalyse final stage in breakdown of suagrs
resist temp change effect
thickness - 5-10 nm
glycocalyx - outside memb, formed from carb chains attached to glyco lipid/proteins in
memb
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