Membranes and Transport
Three Major Constituents of Membranes
(1) Lipids
– 25-75% by weight – unusual structure (found only in membrane)
phospholipid – glycerol – 1
st
two carbons attached to long chain fatty acids, phosphate ester to alcohol attached to third
alcohol is usually choline (a quarternary amine permanent charge); also ethanolamine, serine, and rarely inisitol
phospholipid without alcohol is called
phosphitidyl
, e.g. phosphitidyl choline = phospholipid with choline
sometimes long chain fatty acids will have double bonds –
always cis
unlike triglycerides, membrane phospholipids are
amphipathic
(hydrophobic on one end, hydrophilic on other)
hence, they form
bilayers
(like two-dimensional liquid – free to float around) – only 70 Å
permeable to water and gasses, impermeable to most polar molecules
Function:
provide barrier to diffusion and electrical insulation
also found in form of cholesterol and sphingomyelin
(2) Proteins
– 25-75% by weight – two types:
(1)
integral proteins
– associate with hydrophobic core of lipid bilayer (e.g. transmembrane proteins)
need 20 amino acids in
a
-helix to traverse membrane – may have more than one transmembrane region
(2)
peripheral proteins
– anchored by binding to lipid heads or to other proteins – interact with membrane surface
bilayer allows translation and rotation about axis perpendicular to membrane, but no trans-bilayer (flip-flop) movement
bilateral asymmetry of membrane – two types:
(1) absolute asymmetry (every protein has same orientation)
(2) compositional asymmetry (lipids found on both sides but in different concentrations)
(3) Carbohydrates
– 0-5% by weight, mostly added to proteins (i.e., glycoproteins)
Function of Membrane Proteins
(1) Transport
– carried out by transport proteins – like enzymes (highly specific, can be saturated)
e.g. Na
+
/K
+
pump – uses 20% of all ATP produced by cell – all face same direction
types of transport:
(a)
Facilitated Diffusion
– solutes only go down concentration gradient – e.g. glucose channels
can be cotransporter – symport (same direction) or antiport (opposite direction)
indirect active transport
– cotransport with another molecule going down its gradient
(b)
Active Transport
– move solute
against
concentration gradient – three families
P-type
– Na
+
/K
+
ATPase, Ca
++
ATPase (10 times less inside cell), H
+
/K
+
ATPase (stomach)
F-type
– F
0
F
1
ATPase – runs in reverse in mitochondria to make ATP from H
+
gradient
V-type
– proton pump to create lysosome acidity
(c)
Pores
– less selective, higher flux, always down concentration gradient – dependent only on size (e.g., gap jct)
(d)
Channels
– more selective than pores, usually carry ions, frequently regulated (ligand-gated or voltage-gated)
(2) Communication Across Membranes
– e.g., insulin receptor – surface protein tells cell how much insulin is around
(3) Adhesion and Recognition
– e.g., among progenitor cells during development, or white blood cell-antigen
capable of force transmission – cytoplasmic elements get shorter in muscle, allowing it to hold up to mechanical stress
(4) Electrical Activity
– propagate electrical impulses (action potentials) through nerve or muscle cells
(5) Fission and Fusion of Cells
– fission during cell division
fusion is normally very thermodynamically unfavorable – trapped water/etc. – does not happen spontaneously
e.g., transitional element
Þ
Golgi apparatus
Þ
vesicle
Þ
Golgi apparatus