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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

slide2

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

slide4

Plasma Membrane

Membrane is composed of:

  • Lipids
  • Phospholipids
  • Sterols
  • B. Proteins
  • Integral
  • Peripheral
  • C. Carbohydrates
  • Glycolipids
  • Glycoproteins
slide5

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
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
slide16

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

slide19

Lipid Aggregates

  • Lipids spontaneously aggregate in water as a result of the Hydrophobic Effect.
slide20

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

slide21

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

slide22

Lipid Components of Membranes

  • Different types of membranes have characteristic lipid compositions.
slide23

Lipid Components of Membranes

  • Lipid composition varies across the two leaflets of the same membrane.
slide24

Membrane Proteins

  • Integral proteins (includes lipid-linked): need detergents to remove
  • Peripheralproteins: removed by salt, pH changes
  • Amphitropicproteins: sometimes attached, sometimes not
slide25

Single Transmembrane Segment Proteins

  • Usually alpha-helical, ~20-25 residues, mostly nonpolar.
  • Example: glycophorin of the erythrocyte.

Fig 11-8

slide26

Multiple Transmembrane Segment Proteins

  • 7 alpha-helix motif is very common.
  • Example: bacteriorhodopsin

Fig 11-10

slide27

Beta Barrel Transmembrane Proteins

  • Multiple transmembrane segments form β sheets that line a cylinder.
  • Example: porins.
slide28

Lipid-Linked Membrane Proteins

  • Attached lipid provides a hydrophobic anchor.

Fig. 11-14

  • An important lipid anchor is GPI (glycosylated phosphatidylinositol.
slide29

Membrane Carbohydrates

  • On exoplasmic face only
slide30

Membrane Carbohydrates

  • On exoplasmic face only
  • An example is the blood group antigens
slide31

Membrane Dynamics

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

Fig 11-16

slide32

Membrane Dynamics

  • Phospholipids in a bilayer have free lateral diffusion.

Fig 11-17

slide33

Membrane Dynamics

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

Fig 11-17

slide34

Membrane Dynamics

  • Flippases, floppases, and scramblases catalyze movement between the two faces.
slide36

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.
slide38

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

slide39

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

slide40

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.

slide42

Membrane Permeability

  • Membranes are selectively permeable.
  • Permeable to nonpolars and small polar molecules.
  • Impermeable to ions and large polar molecules.
slide43

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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