1. Membrane Structure and Function
Chapter 5
BIOL1000
Dr. Mohamad Termos

2. Objectives Following this lecture, you should be able to describe:
Membrane Structure
Diffusion
- Simple diffusion
- Facilitated diffusion
- Channels
- Carrier Proteins
Osmosis
Active transport
- Transport by Pumps
- Transport by vesicles

3. Phospholipids Phospholipids: 1- hydrophilic phosphate head
2- two hydrophobic fatty acid chain tails
Phospholipids line up in a bilayer to form the plasma membrane.
Hydrophilic heads on the inside and outside
Hydrophobic tails in the inter-membranous space

4. Other Membrane Components
Proteins, cholesterol, and carbohydrates.
Proteins: embedded in the membrane, act as receptor or transporters
Cholesterol: embedded in the bilayer, improve fluidity of the membrane
Carbohydrate chains: attached to proteins or phospholipids, ac as recognition sites.

5. Membrane Proteins Protein functions:
Act as enzymes: facilitate reactions on the membrane
Act as receptors: receptors of extra-cellular messengers (e.g. hormones), and transfer the signal onside the cell
Act as channels: or pores that mediate passage of certain molecules

6. Diffusion Diffusion: molecule spread out.
Equilibrium: number of molecules is equal on sides of a membrane
Concentration gradient: molecules are not at equilibrium. They will move along the gradient until they reach their equilibrium point.

7. Osmosis
- Osmosis: spread out of water down their concentration gradient.
Hypotonic solution: solution with fewer solute particles.
Hypertonic solution: solution with many solute particles.
Isotonic solution: water moves from hypotonic towards hypertonic till it reaches equilibrium state called isotonic.
Semi-permeable membrane: permeable to water but not solute.

8. Semi-permeable Cell Membrane
Cell seeks to live in an isotonic environment
- If the surrounding solution is hypotonic (less solute), water would flow into the cell, causing it to swell and later burst.
- If the cell is in a hypertonic solution (more solute), water would leave the cell, and shrivel it up

9. Simple Vs. Facilitated Diffusion
Simple Diffusion: Small, non-polar molecules (e.g. O2, CO2)are able to cross the hydrophobic barrier.
Facilitated Diffusion: charged or polar molecules need help by special proteins (e.g. channel proteins) to pass through.
NOTE: Molecules travel with their natural concentration gradient.

10. Facilitated Diffusion
Carrier protein: Another kind of protein that can be used for facilitated diffusion.
Carrier proteins allow hydrophilic molecules to pass through the hydrophobic membrane, like channel proteins. But instead of acting like a pore they act like a revolving door.

11. Carrier proteins
They trap a molecule, then change conformation to let it out on the other side.
Again, since the molecules are traveling with their concentration gradient, this process is till a type of diffusion

12. Active Transport
Active Transport: Movement of molecules against their concentration gradient. This needs energy (ATP) unlike the other types of movements.
In this diagram, ATP is being used to shuttle sodium ions out of the cell, and potassium ions into the cell. This is called sodium-potassium ATPase pump, and is very common on most cell membranes, especially nerve cells.

13. Transport by Vesicles
Endocytosis: Is the process of the membrane creating a pocket around molecules, and pinching off into the cell. Often, this process will bring in several molecules at once. Inside the cell, the membranous pocket can be broken down and the contents can be let out, into the cytoplasm.
Exocytosis: The opposite to endo. Molecules inside of the cell need to leave, they can be packaged inside membranous vesicles and pushed towards the cell membrane. When the vesicle arrives, its wall fuses with the cell’s membrane, and lets the contents of the vesicle spill out into the extra-cellular fluid.