1. MEMBRANE STRUCTURE &
FUNCTIONS

2. MEMBRANE STRUCTURE AND FUNCTION
Recall: Membranes are made of phospholipids, which have
a phosphate group attached to fatty acid (lipid) chains.
Phospholipids have a polar and non-polar end, making
them amphipathic

3. MEMBRANE STRUCTURE AND FUNCTION
The amphipathic nature of phospholipids means that it
forms a bilayer or a single-layer micelle spontaneously in
water.
Vesicle (bilayer)
Micelle (unilayer)
Phospholipid bilayer

4. MEMBRANE STRUCTURE AND FUNCTION
A bilayer arrangement allows the membrane to have
water on either side.
Water
Water
Hence the cell membrane is called the phospholipid bilayer.

5. MEMBRANE STRUCTURE AND FUNCTION
This bilayer is not static; it is dynamic. There is fluidity in
the membrane.

6. MEMBRANE STRUCTURE AND FUNCTION
Fluidity is assisted by sterols (i.e. Cholesterol in animal cells)
Sterols stabilize the membrane.
Hydrophilic
end
Hydrophobic
end
Hydrophobic
tail

7. MEMBRANE STRUCTURE AND FUNCTION
At high temperatures, sterols restrain movement of
phospholipids
At low temperatures, sterols occupy spaces between
phospholipids to prevent the lipids from sticking together
(enhances fluidity)

8. MEMBRANE STRUCTURE AND FUNCTION
Fluid mosaic model: Membranes are fluid (not rigid);
proteins embedded in the lipids float freely.

9. MEMBRANE STRUCTURE AND FUNCTION
Two types of membrane proteins:
1) Integral membrane protein – protein embedded in the
lipid bilayer
2) Peripheral membrane protein – protein on the surface
of the membrane

10. MEMBRANE STRUCTURE AND FUNCTION
Two types of membrane proteins:

11. MEMBRANE STRUCTURE AND FUNCTION
Some integral proteins are transmembrane proteins
because they cross the thickness of the lipid bilayer.
Protein
crosses the
membrane

12. MEMBRANE STRUCTURE AND FUNCTION
How are proteins embedded in the membrane?
The amino acids of the
protein that cross the cell
membrane have
hydrophobic R-groups.
The parts that are
exposed to the
environment have
hydrophilic R-groups.
Gly
Ala
Val
Leu
Ile
Phe
Trp
Pro
Ala
Leu
Ile
Phe
Trp
Pro Gly
Ala
Leu
Ile
Phe
Trp
Pro

13. MEMBRANE STRUCTURE AND FUNCTION
How are proteins embedded in the membrane?
These nonpolar (hydrophobic) amino acids are often
found in the transmembrane sections of proteins.

14. MEMBRANE STRUCTURE AND FUNCTION
How are proteins embedded in the membrane?
hydrophobic
hydrophilic
hydrophilic

15. MEMBRANE STRUCTURE AND FUNCTION
How are proteins embedded in the membrane?
Hydrophobic section with
nonpolar R-groups

16. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
1) Transport
2) Receptor
3) Enzyme
4) Anchor (attachment)/recognition
Some membrane proteins have multiple functions

17. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
1) Transport proteins: Move molecules and ions across a
membrane

18. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
1) Transport proteins: can be channel and carrier proteins
Channel protein Carrier protein

19. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
1) Transport proteins: Some can transport two substances in
opposite directions simultaneously (i.e. sodium potassium
pump)

20. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
2) Receptor proteins: relays signals between cells internal
and external environments

21. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
3) Enzyme proteins: catalyzes reactions at the membrane

22. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
4) Anchor (attachment)/recognition proteins: Attachment
points for cytoskeleton and ECM components; also for cell-
cell recognition (i.e. by immune system)

23. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
4) Anchor (attachment)/recognition proteins: Immune cells
can identify which cells are foreign, and which cells are
infected

24. MEMBRANE STRUCTURE AND FUNCTION
Functions of membrane proteins:
4) Anchor (attachment)/recognition proteins: Often have
carbohydrate groups on the extracellular side; these are
called glycoproteins

25. MEMBRANE STRUCTURE AND FUNCTION
Carbohydrates are often used as “markers” in recognition:
Glycoproteins: Protein with a carbohydrate attached
Glycolipid: Lipid with a carbohydrate attached