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BC368 Biochemistry of the Cell II. Biological Membranes Chapter 11: Part 1 February 11, 2014. Plasma Membrane.

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Bc368 biochemistry of the cell ii

BC368Biochemistry of the Cell II

Biological Membranes

Chapter 11: Part 1

February 11, 2014


Plasma Membrane

“Possibly the decisive step [in the origin of life] was the formation of the first cell, in which chain molecules were enclosed by a semi-permeable membrane which kept them together but let their food in.”

J. B. S. Haldane, 1954



Plasma Membrane

Membrane is composed of:

  • Lipids

  • Phospholipids

  • Sterols

  • B. Proteins

  • Integral

  • Peripheral

  • C. Carbohydrates

  • Glycolipids

  • Glycoproteins


Plasma Membrane

  • Variable components in different membrane types


Membrane lipids
Membrane Lipids

  • Amphiphilic lipids

  • Major types: phospholipids, glycolipids, sterols

sphingosine

Glycolipid

glycerophospholipid

sphingolipid


Phospholipids
Phospholipids

  • Two classes: glycerophospholipids (aka phosphoglycerides) and sphingolipids

Fig 10-7


Membrane lipids 1a glycerophospholipids
Membrane Lipids: 1A. Glycerophospholipids

  • Two fatty acids and a polar “head group” on glycerol.

  • Vary in the FA’s and head group.


Membrane lipids 1b sphingolipids
Membrane Lipids: 1B. Sphingolipids

  • Named for the enigmatic Sphinx

  • Common in nerve and brain cell membranes


Membrane lipids 1b sphingolipids1

note amide

linkage

Membrane Lipids: 1B. Sphingolipids

  • Named for the enigmatic Sphinx

  • Sphingosine replaces glycerol, so only 1 FA tail


Membrane lipids 1b sphingolipids2
Membrane Lipids: 1B. Sphingolipids

  • Example: sphingomyelin


Glycolipids
Glycolipids

  • Two classes: glycosphingolipids and galactolipids

Fig 10-7


Membrane lipids 2a glycosphingolipids
Membrane Lipids: 2A. Glycosphingolipids

  • Sphingolipids with carbohydrate head group; common on cell surfaces

  • Examples: cerebrosides and gangliosides


Membrane lipids 2b galactolipids
Membrane Lipids: 2B. Galactolipids

  • Diglycerides with galatose groups

  • Common in plant (thylakoid) membranes


Membrane lipids 3 sterols
Membrane Lipids: 3. Sterols

  • Cholesterol and cholesterol-like compounds


Lipid Components of Membranes

  • Lipid composition varies across different membranes.

Fig 11-2



Defects in membrane turnover
Defects in Membrane Turnover

Deposits of gangliosides in Tay Sachs brain


Lipid Aggregates

  • Lipids spontaneously aggregate in water as a result of the Hydrophobic Effect.


Lipid Aggregates

  • Amphiphilic lipids form structures that solvate their head groups and keep their hydrophobic tails away from water.

  • Above the critical micelle concentration, single-tailed lipids form micelles.

Fig 11-4


Lipid Aggregates

Fig 11-4

  • Double-tailed lipids form bilayers, the basis of cell membranes.

  • Bilayers can form vesicles enclosing an aqueous cavity (liposomes).

Fig 11-4


Lipid Components of Membranes

  • Different types of membranes have characteristic lipid compositions.


Lipid Components of Membranes

  • Lipid composition varies across the two leaflets of the same membrane.


Membrane Proteins

  • Integral proteins (includes lipid-linked): need detergents to remove

  • Peripheralproteins: removed by salt, pH changes

  • Amphitropicproteins: sometimes attached, sometimes not


Single Transmembrane Segment Proteins

  • Usually alpha-helical, ~20-25 residues, mostly nonpolar.

  • Example: glycophorin of the erythrocyte.

Fig 11-8


Multiple Transmembrane Segment Proteins

  • 7 alpha-helix motif is very common.

  • Example: bacteriorhodopsin

Fig 11-10


Beta Barrel Transmembrane Proteins

  • Multiple transmembrane segments form β sheets that line a cylinder.

  • Example: porins.


Lipid-Linked Membrane Proteins

  • Attached lipid provides a hydrophobic anchor.

Fig. 11-14

  • An important lipid anchor is GPI (glycosylated phosphatidylinositol.


Membrane Carbohydrates

  • On exoplasmic face only


Membrane Carbohydrates

  • On exoplasmic face only

  • An example is the blood group antigens


Membrane Dynamics

  • At its transition temperature (TM), the bilayer goes from an ordered crystalline state to an a disordered fluid one.

Fig 11-16


Membrane Dynamics

  • Phospholipids in a bilayer have free lateral diffusion.

Fig 11-17


Membrane Dynamics

  • Phospholipids in a bilayer have restricted movement between the two faces.

Fig 11-17


Membrane Dynamics

  • Flippases, floppases, and scramblases catalyze movement between the two faces.



Fluorescent Recovery After Photobleaching

  • Fluorescent tag is attached to a membrane component (lipid, protein, or carbohydrate).

  • Fluorescence is bleached with a laser.

  • Recovery is monitored over time.



Protein Mobility in the Membrane

  • Some membrane proteins have restricted movement.

  • May be anchored to internal structures (e.g., glycophorin is tethered to spectrin).

Fig. 11-20


Protein Mobility in the Membrane

  • Lipid rafts are membrane microdomains enriched in sphingolipids, cholesterol, and certain lipid-linked proteins.

  • Thicker and less fluid than neighboring domains.

Fig. 11-21


Protein Mobility in the Membrane

  • Lipid rafts are membrane microdomains enriched in sphingolipids, cholesterol, and certain lipid-linked proteins.

  • Thicker and less fluid than neighboring domains.

Lipid Rafts


Nature reviews molecular cell biology 4 414 418 may 2003

Nature Reviews Molecular Cell Biology 4, 414-418 (May 2003)

Domains of gel/fluid lipid segregation in a model membrane vesicle, which is a mixture of fluid dilaurylphosphatidylcholine phospholipids with short, disordered chains and gel dipalmitoylphosphatidylcholine phospholipids with long, ordered chains. A red fluorescent lipid analogue (DiIC18) partitions into the more ordered lipids, whereas a green fluorescent lipid analogue (BODIY PC) partitions into domains of more fluid lipids. These domains in a model membrane are much larger than the domains of cell membranes.


Membrane Permeability

  • Membranes are selectively permeable.

  • Permeable to nonpolars and small polar molecules.

  • Impermeable to ions and large polar molecules.


Membrane Permeability

  • What actually gets across a membrane depends on several factors:

  • Solubility in the nonpolar lipid environment

  • The concentration gradient

  • Whether a protein transporter exists


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