1. The Cell Membrane

2. Overview
The cell membrane separates a living cell from its nonliving surroundings
thin barrier = 8nm thick (n=nano=10-9)
Controls traffic in & out of the cell
selectively permeable
allows some substances to cross more easily than others- “choosy”
Made of phospholipids, proteins , cholesterol, and carbohydrates.

3. Why do the phospholipids arrange themselves like this?
Phosphate
Tails are made of fatty acids
Hydrophobic
“Water fearing”
Heads are made of phosphate
Hydrophilic
“Water loving”
Arranged as a bilayer
Fatty acid
Inside cell
Why do the phospholipids
arrange themselves like this?
Outside cell

4. Cholesterol also makes up the cell membrane structure
Cholesterol also makes up the cell membrane structure. It is between the tails of the phospholipids.
Fluid outside the cell
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
Phospholipids
Cholesterol
Cytoplasm

5. Membrane Proteins peripheral proteins
There are 2 types of membrane proteins:
peripheral proteins
loosely bound to the surface of the membrane
integral proteins
Pass through the lipid bilayer

6. Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter
Enzyme activity
Cell surface receptor
Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.
Cell surface identity marker
Cell adhesion
Attachment to the cytoskeleton

7. Membrane carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish one cell from another
basis for rejection of foreign cells by immune system
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

8. Fluid Mosaic Model
In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer
The composition of the cell membrane is called the fluid mosaic model because the phospholipid bilayer and the embedded proteins can move laterally in the membrane like a “fluid” to let compounds into and out of the cell.

9. What is the natural movement of molecules into and out of the cell?
Movement from high concentration of that substance to low concentration of that substance.

10. Simple Diffusion Movement from HIGH to LOW concentration diffusion
“passive transport”
no energy needed (ATP)
diffusion

11. Simple diffusion through phospholipid bilayer
What molecules can get through directly?
fats & other lipids (hydrophobic)
tiny molecules like O2 and CO2
Sometimes H2O can sneak through
lipid
inside cell
outside cell
What molecules can NOT get through directly?
Sometimes H2O
Large molecules
Salts
Sugars
salt
NH3
sugar aa
H2O

12. How do the non-fat compounds get in/out?
Membrane becomes selectively-permeable with protein channels formed by integral proteins.
This is called facilitated diffusion
facilitate = to help
high
low
Donuts!
Each transport protein is specific as to the substances that it will translocate (move).
For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.
Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.
These channel proteins allow fast transport.
For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion.
“The Bodyguard”

13. Facilitated Diffusion
no energy needed (ATP)
still allowing molecules to move from high to low concentration
Integral proteins are specific to what they transport because of their shape
inside cell
H2O aa
sugar
salt
outside cell
NH3

14. Osmosis Water is very important to cell function
Diffusion of water from high concentration of water to low concentration of water is osmosis
across a selectively-permeable membrane

15. The direction of osmosis is determined by comparing solute concentrations on each side of the membrane. What is a solute?
Hypertonic - more solute, less water than another solution
Hypotonic - less solute, more water than another solution
Isotonic - equal solute, equal water than another solution
hypotonic
hypertonic
water
net movement of water

16. For each cell, label the solutions as hypertonic, hypotonic or isotonic and draw an arrow to show the direction of water movement.

17. Managing water balance
Cell survival depends on balancing water uptake & loss
freshwater
balanced
saltwater

18. Managing water balance
A cell in fresh water
example: Paramecium
What type of environment?
problem: gains water, swells & can burst
solution: contractile vacuole
pumps water out of cell which
requires ATP (energy)
ATP
freshwater

19. Water regulation
Contractile vacuole in Paramecium
ATP

20. Managing water balance
Another example:
Plant cells
When in a hypotonic (freshwater) environment they are constantly taking up water.
Do they burst?
No!
Plant cells have a cell wall that prevents bursting.
They build up turgor pressure which makes the plant stand tall.

21. Managing water balance
Another example:
Plant cells
How do they deal with a hypertonic environment?
They lose water causing the cell membrane to pull away from the cell wall. This is called plasmolysis.

23. Getting through cell membrane
Passive Transport
diffusion with the concentration gradient
high  low
Simple diffusion
diffusion of hydrophobic molecules, small molecules and sometimes water
Facilitated diffusion
diffusion of hydrophilic and large molecules
through a protein channel
high  low concentration gradient
Active transport
diffusion against the concentration gradient
low  high
uses a protein pump
requires ATP
ATP

24. Transport summary simple diffusion facilitated diffusion
ATP
active transport

25. conformational change
Active Transport
Why is active transport necessary?
The Na+/K+ pump is the way that our nervous system works. By pumping ions up their concentration gradients, electricity is generated.
conformational change
Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport.
ATP

26. Na+/K+ Pump The steps of the pump are as follows:
3 Na+ ions inside the cell bind to the carrier protein (pump).
A phosphate from ATP attaches to the pump and the pump changes shape.
The 3 Na+ ions are dumped outside the cell.
2 K+ ions outside the cell bind to the pump.
The phosphate detaches from the pump and the pump goes back to its original shape.
The 2 K+ ions are dumped inside the cell.

28. How about large molecules and fluids?
Endocytosis- Active Transport
The taking in of large molecules (too big to pass through the membrane) or a large amount of a fluid by “engulfing”. To engulf means that the cell membrane pinches in and surrounds what is being ingested. The molecules are then digested in a pouch called a vesicle.
phagocytosis = Ingesting large molecules
pinocytosis = Ingesting large amounts of a fluid
Draw endocytosis:

29. Exocytosis- Active Transport
The opposite of endocytosis. The vesicle fuses with the cell membrane and it then opens up to release the contents.