MEMBRANES
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1.
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Describe, explain “fluid mosaic model” of
membrane structure
2. Include outline of roles of phospholipids,
cholesterol, glycolipids, proteins,
glycoprotein
3. Outline roles/f(x)s of membranes
within/at surface of cells
4. D&E diffusion, osmosis, active transport,
endocytosis, exocytosis, soln of diff ψ
Membrane Structure
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Phospholipid
2 non-polar hydrophobic fatty acid
tails attached to 2C of 3C glycerol
-ve charge phosphate grp attached to
3rd carbon of glycerol (interact with
water, hydrophilic)
“amphipathic”- Hphilic,phobic regions
Cholesterol
4 ring structure, largely hydrophobic
hydrophilic hydroxyl grp, hydrophobic
ring structure (1/2 cell membrane)
Proteins
polypeptide chain(s) in 3D conformatn
hydrophilic/hydrophobic regions
(polar/acid/basic) – hydrophilic
trans-membrane proteins, peripheral
membrane protein, integral
membrane protein
Carbohydrates
covalently assoc. with lipids/proteins,
forming glycolipids/ glycoproteins
Fluid Mosaic Model
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membrane is fluid bilayer of phospholipid
moledules in which proteins are
dispersed/embedded/ assoc superficially
proteins constantly changing/moving
hydrophilic portions of
proteins/phospholipids maximally exposed
= results stable membrane structure
hydrophobic portionis non-aq bilayer core
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phospholipids in membrane held by weak
hydrophobic interactions of VdW’s forces
weak interactions allow membrane
lipids/proteins to drift laterally within
membrane plane
molecules rarely flip transversely across
membrane/switch phospholipids layer
- large size, large drag force, anchored to
cytoskeleton/extracellular matrix
phospholipids move quickly along
membrane’s plane, protein < lipid
Temp ↓ membrane ↓ fluid until
phospholipids settle in closely-packed
arrangem  membrane solidifies
Temp for solidifyg (depends cholesterol,
phospholipids composition)
Freeze at ↓ temperature if ↑ proportion of
phospholipids with unsaturated HC tails
cold tolerant plants (winter wheat) ↑
unsaturated phospholipids conc. in
autumn, prevent plasma membranes’
solidifying in winter.
OH grp of cholesterol aligns with
phosphate heads of phospholipids while
remaining portion is tucked into fatty acid
portion of membrane.
Steroid ring (4 HC rings) closely attracted
to fatty acid chains nearest phospholipids
heads = restrains motion, less fluid even
at warmer temp.
++firmness/integrity, maintains fluidity
1. Integral proteins
- penetrate hydrophobic core of lipidbilayer (hydrophobic regions surrounded
by hydrocarbon portions of phospholipids)
- hydrophobic regions consist of non-polar
a.a. coiled in alpha helices
- hydrophilic parts exposed to aq. Soln
a) Unilateral proteins – ½ across membrane
b) Transmembrane proteins
2. Peripheral/extrinsic proteins
- not embedded in lipid bilayer
- held by fibres of extracellular matrix/
cytoskeleton (maintain cell shape, fix
location for membrane proteins)
o
membrane carbohydrates- cell-cell recog
- sort out embryo’s cell into tissue/organs
- immune sys. reject foreign cells
Membrane carbohydrates:
- branched oligo(<15)saccharides
- covalent bond to lipid/protein
- A, B, AB, O blood groups
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Selectively permeable plasma membrane
regulates (sugars, a.a., nutrients,
metabolic waste pdts) traffic
 lipids permit non-polar molecules
 specific integral transport proteins
 Non-polar (hydrophobic) molecules
- HC, CO2 , O2 (dissolve, cross membrane)
- smaller (same lipid solubility) faster
 Polar, charged (hydrophilic) molecules
- small enough to pass hydrophobic core
- charged ions (Na+, H+) canNot.
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Retain cytoplasm
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Maintain homeostatic condition
appropriate to biochemical reactions
necessary to sustain life
Compartmentalization
Cell level, inter organelle, intra organelle
Chemical Rxns
- membrane has specialized enzymes for
biosynthetic f(x)s and energy generation
- During respiration, org cmpds that contain
high energy e- are broken down, releasing eto do work.
These e- reach membrane (cristae of
mitochondrion and thylakoid membrane of
chloroplast), pass down series of e- carriers.
- Protons pumped into a specialized
compartment creating proton gradient
forming ATP
Communications
TO - - - obtain nutrients for energy/raw materials
- excrete waste/ secrete useful subst.
- generate essential ionic gradients (nervous,
muscular activity)
- maintain suitable pH, ionic conc. for enzyme
1.
2.
3.
4.
Diffusion
Osmosis
Active Transport
Bulk Transport (endo/exo-cytosis)
A) Active Process
- active transport
- movement of subs. Across membranes,
against conc/e-chemical gradient, use ATP
- Sodium-potassium pump
B) Passive Process
- Diffusion & Osmosis
Diffusion (nerves)
“ Movement of molecules/ions frm region of
high conc. to region of low conc, down
diffusion gradient” [spontaneous, passive]
(O2 from lungs to blood, CO2 Opp.)
1. molecular size ( smaller, faster)
2. solubility in lipid bilayer (non polar
faster)
3. Charge of molecule (charged specie
attract more water, form hydration
shells, increase effective size of ion –
ions difficult to pass thru l.b.)
4. Conc gradient (steeper, faster)
5. Kinetic energy of molecules (high
temp, high energy, high diffusion rate)
too high temp affects integrity of
membrane, loss of selectivity
6. Surface Area ( larger, faster)
(microvilli increase absorption)
7. Distance (longer, slower)
Facilitated Diffusion (w transport protein)
 facilitate substs insoluble in phospholipids
 provide hydrophilic channels
(transmembrane channel proteins) OR
carrier proteins (conformation change)
Channel Proteins
- fixed shape, trans membrane
- selective hydrophilic channel
water channel protein in cells lining
collecting ducts in kidney
Carrier Proteins
- rapid changes in shape, exists in 2 alternate
conformations (entry of glucose molecules
into RBCs, movement of chloride and
hydrogencarbonate ions into/out of RBC)
- direction cannot work against conc. gradient
- chance collision (trans protein, substrate)
- more binding sites, faster facilitated diffusn
Active transport
“Energy (atp)-consuming transport of
molecules/ions across membrane, against conc.
gradient”
- ATP manufactured by respiration
- Carrier proteins in cell membrane needs ATP
to keep changing shape
- Usually one way, maintain integral conc. of
small molecules that differ from conc. in envir.
Endocytosis
“infolding or extension of cell surface
membrane to form vesicle or vacuole, thus
allowing cell to acquire macromolecules and
particulate matter”
Exocytosis
“secretion of macromolecules(waste
material/undigested remains/useful products)
by fusion of vesicles with plasma membrane”
- lipid molecules of 2 bilayers rearrange (fuse)
- pancreas (insulin exocytosis to blood)
- neurons (release acetylcholine, stimulate
other neurons/muscle cells)
- stomach/intestine cells release enzymes,
mucus (release mucin from goblet cells lining
intestinal wall for lubrication)
- amoeba pumps out excess water
Active process, but not active transport