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Membranes. Structure and function. Phospholipids. Lipid micelles Micelles: vesicles in which polar head groups of lipids face the external aqueous solution and hydrophobic tails collect together inside. (Fig. 4.5a,b). Figure 4.5. Lipid micelles. Lipid bilayers. Water. No water.

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membranes
Membranes

Structure and function

phospholipids
Phospholipids
  • Lipid micelles
    • Micelles: vesicles in which polar head groups of lipids face the external aqueous solution and hydrophobic tails collect together inside.

(Fig. 4.5a,b)

slide4

Figure 4.5

Lipid micelles

Lipid bilayers

Water

No water

Serine

Phosphate

Hydrophilic heads interact with water

Hydrophobic tails interact with each other

Hydrophilic heads interact with water

lipid bilayer
Lipid Bilayer
    • Amphipathic lipids can form a bilayer structure in an aqueous solution.
      • Liposomes: vesicles composed of a bilayer enclosing an internal aqueous solution. (Fig. 4.1a,b; 4.6a)
    • Micelle, bilayer, and liposome formation is spontaneous
  • Raises questions about property of membranes: permeability? fluidity? Is this selective?
the importance of permeability
The importance of permeability
  • If certain molecules or ions pass though a lipid bilayer more readily than others, it means that the internal environment of a vesicle can become different from the outside!
  • This difference in internal and external conditions is vital and a key characteristic of living cells.
slide8

Figure 4.6a

Liposomes: Artificial membrane-bound vesicles

Water

Water

62.5 nm

62.5 nm

artificial membranes as an experimental system
Artificial membranes as an experimental system
  • Planar bilayers: are formed in a hole in a divider between two aqueous solutions. (Fig. 4.6b)
  • Artificial membranes can be used to study permeability of lipid bilayers.
slide10

Figure 4.6b

Planar bilayers: Artificial membranes

Water

Water

Lipid

bilayer

membranes are composed of lipids
Membranes are Composed of Lipids
  • Artificial lipid structures are used as experimental systems to determine membrane properties.
    • Selective permeability of the membrane to molecules and ions: polar, charged, and/or large compounds are least likely to cross a bilayer. (Fig. 4.6c, 4.7a,b)
slide12

Figure 4.6c

Artificial membrane experiments

How rapidly can different

solutes cross the membrane

(if at all) when….

1. Different types of

phospholipids are used to make the membrane?

Solute

(ion or

molecule)

?

2. Proteins or other

molecules are added to the membrane?

slide13

Figure 4.7b

Summary of relative permeabilities

Phospholipid bilayer

Hydrophobic molecules

O2, CO2, N2

Small, uncharged polar molecules

H2O, glycerol

Large, uncharged polar molecules

Glucose, sucrose

H+,Na+,NCO3–,

Ca2+,CL-,Mg2+,K+

Ions

slide14
Unsaturated lipids: double bonds create kinks, prevents close packing of lipid tails, increases fluidity.
  • Saturated lipids pack more tightly; membrane is less fluid. (Fig. 4.8a,b)
slide15

Figure 4.8a

The angles of carbon bonds

C

C

120º

109.5º

C

C

Single bonds

Double bonds

slide16

Figure 4.8b

Double bonds cause

kinks in hydrocarbons.

H2C

CH2

H2C

CH2

H2C

CH

Kink

CH

H2C

CH2

H2C

Unsaturated

fatty acid

Saturated

fatty acid

CH2

H2C

slide17

Figure 4.8c

Kinks change the permeability of membranes.

Lipid bilayer

with no

unsaturated

fatty acids

Low permeability

Lipid bilayer

with many

unsaturated

fatty acids

High permeability

slide18

Figure 4.9a

Cholesterol: fills spaces between lipid tails, allows closer packing, decreases fluidity.

Polar

Nonpolar

  • Temperature effects: higher temperatures increase fluidity and permeability.
temperature and fluidity
Temperature and Fluidity
  • At 25ºC phospholipids are liquid.
  • Bilayers have consistency of olive oil.
  • As temps drop, molecules move slower, fluidity decreases… solidify.
slide20
At room temp, phospholipids have been clocked at 2um/sec.
    • Can travel the length of a bacteria cell every second.
  • Phospholipid molecules whiz around each layer while water and small molecules shoot in and out of the membrane.
movement of substances across membranes
Movement of Substances Across Membranes
  • Diffusion
    • Net directed movement of molecules or ions, driven by thermal energy. (Fig. 4.10)
    • Diffusion is a spontaneous, passive process.
      • Molecules and ions move downhill along electrochemical gradients.
    • Facilitated diffusion: assisted by a type of membrane protein called an ionophore.
    • http://www.biosci.ohiou.edu/introbioslab/Bios170/diffusion/Diffusion.html
    • http://northonline.sccd.ctc.edu/judylearn/NTR%20150/GUIDE_WEEK_1.htm
slide22

Figure 4.10

Lipid

bilayer

DIFFUSION ACROSS A SEMI-PERMEABLE MEMBRANE

1. Start with two

different molecules

on opposite sides

of a semipermeable

membrane

(a phospholipid bilayer).

2. Molecules

diffuse across the

membrane - each

along its own

concentration

gradient.

3. Equilibrium is

established.

Molecules

continue to move

back and forth

across the membrane

but at equal rates.

movement of substances across membranes1
Movement of Substances Across Membranes
  • Osmosis
      • Diffusion of water across a membrane towards regions of higher solute concentration. (Fig. 4.11)
slide24

OSMOSIS

1. Start with more solute on one side of the lipid bilayer than the other using molecules that cannot cross the semipermeable

membrane.

Lipid

bilayer

2. Water moves from

the region of low

concentration of solutes (high concentration of water)to the region of high

concentration of solutes (low concentration of water).

Osmosis

movement of substances across membranes2
Movement of Substances Across Membranes
  • Osmosis
    • Osmosis can cause shrinking or swelling of cells or vesicles:
      • Water enters vesicle if internal solution is hypertonic to the external solution. (Fig. 4.12)
      • Water leaves vesicle if internal solution is hypotonic to the external solution.