1. Membrane structure and function

2. Key questions
What is the structure and function of cellular membranes?
How do different entities cross the plasma membrane?

3. The big picture Cell membrane function:
Physically separate a cell from its environment
Control what goes in and out of the cell

4. Revisiting the lipid bilayer
Cell membrane structure:
When placed in water, phospholipids spontaneously form into two layers because of the amphipathic nature of the molecule
Hydrophilic heads face the outside of the membrane
Hydrophobic tails are in the interior
This is the framework of all membranes

5. Fluid mosaic model
Fluid because most of the components of the cell membrane move laterally within the membrane

6. Fluid mosaic model
Fluid because most of the components of the cell membrane move laterally within the membrane
Mosaic because the membrane contains a mixture of substances including:
proteins
cholesterol

7. Membrane proteins and their functions
There are two types of membrane proteins
Integral-penetrate the hydrophobic core of the lipid bi-layer
Some span the entire membrane
Peripheral
Not embedded in the bilayer
Peripheral appendages 
Hydrophobic region
Adhesion Protein
Enzyme
Receptor Protein
Recognition Protein
Passive Transporter
Active Transporter
Hydrophilic region
Stepped Art
Fig. 5-5, pp

8. Membrane proteins and their functions
Transporters
Can act as enzymes (peripheral)
Signal transduction
Recognition proteins
Fasten cells to adjacent cells
Attachment
Signaling molecule 
Enzymes
Receptor
ATP
(a) Transport
(b) Enzymatic activity
(c) Signal transduction
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)

9. Membrane permeability
The plasma membrane is selectively permeable

10. Membrane permeability
The plasma membrane is selectively permeable
What controls the flow of substances that can pass freely across the membrane?

11. What controls the flow of substances that can pass freely across the membrane?
Concentration gradient occurs when there is a different in concentration between two adjacent regions
Region 2
Region 1

12. What controls the flow of substances that can pass freely across the membrane?
Concentration gradient occurs when there is a different in concentration between two adjacent regions
Substances will move down their concentration gradients (from an area of high to low concentration)
Region 2
Region 1
This movement is called diffusion

13. What controls the flow of substances that can pass freely across the membrane?
Diffusion-the movement of molecules or ions down a concentration gradient

14. What controls the flow of substances that can pass freely across the membrane?
Diffusion-the movement of molecules or ions down a concentration gradient
Why does diffusion occur?

15. What controls the flow of substances that can pass freely across the membrane?
Diffusion occurs because:
Molecules are always in motion
Therefore they collide
They will collide more if they are
crowded
These collisions tend to spread
them out
Diffusion of a substance across a membrane does not require energy….. It is considered passive transport.

16. What controls the flow of substances that can pass freely across the membrane?
How quickly a substances diffuses depends on several factors:
Size
Temperature
Steepness of the concentration gradient
Charge
Pressure

17. What controls the flow of substances that can pass freely across the membrane?

18. Some things can pass freely across the membrane while others cannot

19. Some things can pass freely across the membrane while others cannot
How do molecules and ions that cannot freely cross the bilayer on their own enter or exit a cell?

20. Some things can pass freely across the membrane while others cannot
How do molecules and ions that cannot freely cross the bilayer on their own enter or exit a cell?
With the help of transport proteins
By exocytosis and endocytosis

21. How do molecules and ions that cannot freely cross the bilayer on their own enter and exit a cell?
Transport protein
Selective permeability depends on membrane structure AND presence of transport proteins
Each type of transport protein moves a certain molecule across a membrane
There are several types of transport proteins
Channel proteins (hydrophilic channel)
Ion channels allow ions to cross the membrane (many are gated)
Carrier proteins bind with passenger molecules, change shape and then let the passengers out on the other side

22. How do molecules and ions that cannot freely cross the bilayer on their own enter and exit a cell?
There are two kinds of transport
1.) Facilitated diffusion
A concentration gradient drives the diffusion of a substance across a membrane with the assistance of a membrane protein.
No energy is required.
Ie. glucose

23. How do molecules and ions that cannot freely cross the bilayer on their own enter and exit a cell?
There are two kinds of transport
1.) Facilitated diffusion
A concentration gradient drives the diffusion of a substance across a membrane with the assistance of a membrane protein.
No energy is required.
Ie. glucose
2.) Active transport
A membrane protein assists in the transportation of a solute against its concentration gradient
Requires energy
Allows cells to maintain different internal conditions

24. How do molecules and ions that cannot freely cross the bilayer on their own enter and exit a cell?
More on active transport:
Electrogenic pumps create a membrane potential (electrical potential energy due to a separation of opposite charges).
Two forces drive the passage of ions across a membrane: concentration gradient and charge (electrochemical gradient)
EXTRACELLULAR
FLUID –
ATP – + H+ H+
Proton pump H+ – + H+ H+ – +
CYTOPLASM H+ – +

25. How do molecules and ions that cannot freely cross the bilayer on their own enter and exit a cell?
More on active transport:
Cotransport
An ATP-powered pump that transports a specific solute can indirectly drive the active transport of other solute – +
ATP H+ H+ – +
Proton pump H+ H+ – + H+ H+ – + H+
Sucrose-H+
cotransporter
Diffusion
of H+ H+ –
Sucrose + – +
Sucrose

26. Review of passive and active transport
Passive transport
Diffusion
Facilitated diffusion
Active transport
ATP

27. Bulk transport Exocytosis A vesicle moves to the cell’s surface
Its membrane fuses with the plasma membrane
Contents of the vesicle are released outside of the cell

28. Bulk transport Endocytosis
Substances near the cell’s surface are taken into the cell
A small section of plasma membrane balloons inward and then pinches off
Contents are now vesicles which are delivered to their destination within the cell

29. Both endo and exocytosis involve motor proteins and require energy
Bulk transport
Endocytosis
Substances near the cell’s surface are taken into the cell
A small section of plasma membrane balloons inward and then pinches off
Contents are now vesicles which are delivered to their destination within the cell
Both endo and exocytosis involve motor proteins and require energy

30. Take-home messages You should understand:
Membrane structure and function
The types and roles of membrane proteins
Membrane permeability and how substances that can and cannot flow freely across the membrane enter and exit cells