1. Cell Membranes and Transport
Chapter 4 Part 1

2. Cell Membrane What does it do for cell? Controls what goes in and out
Regulates molecules moving from one liquid side of the cell to the other liquid side of the cell
Protects
Supports

3. Cell Membrane Lipid bilayer A cell membrane is made of two
What are lipids?
What does bi- mean?
What’s a layer?
A cell membrane is made of two
layers of lipid molecules

4. Micelles
Stable, ball-like structure made when phospholipids are placed in water
Hydrophilic heads face outward (toward water) and hydrophobic tails face inward

5. Bilayers Two layered structures Made of phospholipids
Can form in sheets
Found in the cell membrane as a phospholipid bilayer

6. Liposomes Artificially prepared membrane-bound compartments (vesicles)
Made by breaking up biological membranes and allowing them to reseal themselves
Used to deliver drugs, cosmetic uses (lotions), food supplements

7. Cell membrane Phospholipids bilayer
Made of a negatively charged phosphate “head”
PO43-
Attracts water because the phosphate is charged (-)
Water is a polar , slightly positive ends and slightly negative ends
Attached to the phosphate group are 2 fatty acid chains
Hydrophobic= don’t like water
So the inside of the cell membrane doesn’t let water in but the outside allows cells to be dissolved in aqueous environments
Amphipathic molecule – has hydrophilic & hydrophobic regions

9. Other things in the membrane…
Proteins embedded in lipid bilayer
Carbohydrates attached to proteins
Cholesterol scattered inside
FLUID MOSAIC MODEL
Singer and Nicolson, 1972
Fluid b/c molecules within the membrane can move around in their own layers
Mosaic b/c protein and cholesterol are scattered throughout (as well as carbohydrates)
Model no one has seen the cell membrane in detail….model explain behavior of molecules we have observed in experiments

11. Membrane Structure and Function
Collagen
proteoglycan
Fibronectin

12. Membrane Structure and Function (con’t)
Plasma Membrane
boundary that separates living cell from
its non-living surroundings
+ 7 nm thick
+ selectively permeable
+ unique structure relates to function

16. Membrane Structure and Function (con’t)
Fluid Mosaic Model (Singer and Nicolson, 1972)
The Fluid Quality of Membranes
+ held together by hydrophobic interactions
- lipids/proteins drift about laterally
+ unsaturated hydrocarbon tails
- maintain fluidity at low temperatures
+ cholesterol
- stabilizes the membrane
restrains movement at high temp.
hinders close packing at low temp.

18. Proteins Proteins help things get across membrane Different Types
Channels
Pumps
Carriers

19. Intrinsic proteins Extrinsic proteins Integral proteins
Found in inner layer, outer layer or spanning the entire membrane
“transmembrane” protein
Hydrophobic portion that crosses membrane is made of one or more alpha-helical chains
Hydrophilic portion made from hydrophilic amino acids
Mobile “icebergs” in the phospholipid bilayer
Extrinsic proteins
Peripheral proteins
Found on inner or outer surface of cell membrane
Many bound to intrinsic proteins
Others bound to other molecules or phospholipids

20. Different types of membrane proteins

22. Transport Proteins Carrier proteins Channel Proteins
Can be 1 single protein or many working together
Transport Proteins
Carrier proteins
Channel Proteins
Constantly flip between two shapes
Binding site opening alternates
Water filled pores
Fixed shape
Allow charged substances to diffuse across
Usually “gated”
Part of membrane can move to open or close
Two examples
Na ion pump
K+ ion pump
BOTH are HIGHLY specific
BOTH provide HYDROHILIC CHANNELS for ions and polar molecules to pass through (this means that the amino acids of these proteins lining the inside of them must be hydrophilic R-groups)
Many are GATED part of protein molecule on the inside surface of the membrane can move to open or close the pore

24. Carbohydrates… Chemical identification cards “ID” card of cell
Helps individual cells id each other
Glycoproteins and glycolipids
Receptor molecules
Consist of short carbohydrate chains

25. Carbohydrates Glycoproteins Glycolipids
Short carbohydrate chain attached to intrinsic protein
Important in cell to cell recognition
Form sugary coating of cell called “glycocalyx”
In animal cellsmade of glycoproteins
Short carbohydrate chains attached to phospholipids
Good for adhesion to other cells...form tissues
Form sugary coating of cell called “glycocalyx”
In plant cellsmade of glycolipids
Both Glycoproteins and Glycolipids can act as cell markers or ANTIGENS, allowing cell-to-cell recognition

27. Carbohydrate receptor molecules:
Cell markers
Antigens
Cell-cell recognition
Signaling receptors
Help coordinate activities of animal cells
Recognize hormones and neurotransmitters
Endocytosis receptors
Ligand activated endocytosis
Binding receptors
Cell adhesion
Binds cells to other cells in tissues and organs

29. Plasma membrane synthesis

30. Functions of Membrane Proteins
Transport
Enzymatic Activity
Signal Transduction
Intercellular joining
Cell-cell recognition
Attachment to the cytoskeleton and ECM

31. Cell Walls are RIGID but PERMEABLE!
Not selective!!!
Outside cell membrane (does NOT replace membrane!!!)
Plants, algae, fungi, prokaryotes
Have pores to allow things in
Function: SUPPORT and PROTECT

33. Cell Walls are RIGID but PERMEABLE!
Cell walls made of…
TOUGH CARBOHYDRATES
RIGID for support but very permeable
Plant Cell Walls
CELLULOSE
Fungi Cell Walls
CHITIN
Bacterial Cell Walls
PEPTIOGLYCAN
Cell Walls are RIGID but PERMEABLE!
Not selective!!!

34. Types of Membrane Proteins

37. Glycoprotein

38. Glycolipid

39. Cholesterol
Reduces membrane fluidity by reducing phospholipid movement
Hinders solidification at low temperatures

40. Cholesterol is a steroid