1. Membrane Proteins: Transport

2. Membrane Structure
All cells, as well as the organelles, are surrounded by membrane.
Membranes are made primarily of phospholipids arranged in a “bilayer”, that is the membrane is two molecules thick (8000 membranes stacked would be about the thickness of a sheet of paper)
Proteins “float” around in the phospholipid bilayer, this arrangement is called the “fluid mosaic” model

3. RBC Protein Arrangement
“Cell and Molecular Biology” is an attempt to elucidate the way in which cells work.

4. Some Proteins are Complex Structures
The photosynthetic reaction center of Rhodopseudomonas vividis contains 4 different subunits: two are multipass integral membrane peptides, one is an integral protein facing the cytosol, and one is a peripheral peptide facing the exterior of the cell

5. Membrane Proteins

6. Membranes are Selectively Permeable
Some molecules can diffuse through membranes.
O2, CO2, and H2O can enter and leave a cell by diffusing through the membrane.
Salts (ions) and many other molecules such as sugars and amino acids cannot diffuse through membranes
Diffusion is the movement of solute particles from a region of higher concentration to a region of lower concentration. (Actually from higher free energy to lower free energy, charge also has an effect here.)

7. Osmosis
If a membrane separates two solutions of different concentrations, and the solute is not able to diffuse through the membrane, the H2O will move through the membrane to equalize the concentrations on each side.
This process is called osmosis
The higher concentration solution is said to be hypertonic
The lower concentration solution is hypotonic
Two solutions of equal concentration are isotonic

8. Cells are Affected by the Osmotic Conditions of their Surroundings
Hypotonic conditions cause a cell to swell as water moves in.
Hypertonic conditions cause a cell to shrivel, as water moves out. Plant cells undergo a change called plasmolysis as the cell membrane pulls away from the cell wall
Animal cells will become “leaky”, and may lyse, if exposed to hypotonic solutions.
Plant cells are protected from excessive swelling by the cell wall.

9. Transport Across the Membrane
Some molecules enter and leave the cell by passive transport: they diffuse through the membrane
Many molecules are actively moved across the membrane by transport proteins in the membrane
Some molecules move through protein pores or channels in the membrane, moving from high concentration to low concentration. This process is called facilitated diffusion
Active transport can move molecules from low concentration to high concentration and requires energy (often ATP)

10. Movement into Cells vs. Lipid Solubility and Size
Different molecules diffuse into cells at different rates
Lipid molecules tend to diffuse more readily
Small molecules tend to enter more easily
Polar and charged molecules do not diffuse across membranes easily

11. Transport Kinetics How do you distinguish transport from diffusion?
If a protein is involved in transporting a molecule across a membrane, saturation kinetics will be seen

12. Transport or Diffusion?
Generally several orders of magnitude faster than diffusion
Saturation kinetics observed (like Michaelis-Menton); diffusion rate is directly proportional to concentration of solute
Simple diffusion: similar molecules enter at similar rates; transport, specificity for particular molecules
Inhibition is possible for transport

13. Types of Transporters
Different types of transport proteins can move : one solute (uniports), two solutes in the same direction (symports), or two solutes in opposite directions (antiports).

14. Identifying Transport Proteins
Transport proteins are tentatively identified by isolating proteins bound to transport inhibitors
Definitive identification requires reconstituting membranes with the protein and demonstrating transport activity

15. Ionophores: Permit Ions to Cross Membranes
Ionophores are common growth enhancers in livestock feed, and are used in veterinary medicine as a coccidiostat in poultry

16. Determining the Type of Ionophore
Movement of a carrier will slow in a gel compared to a liquid
A channel will not be effected as much by te membrane phase change

17. Transport Proteins show Specificity
Many properties of transport proteins are similar to properties of enzymes

18. Models of Transport
Movement of solute across the membrane is accomplished by changes in the protein’s conformation.

19. Band 3 Anion Antiport
Anion (negative ion) transporters in the RBC are responsible for the ability of the body to rid itself of CO2

20. Types of Movement Across Membranes
Properties of solute transport can be used to distinguish between simple diffusion, facilitated diffusion, and active transport.

21. Ion Channels Facilitated diffusion of small ions
Some protein channels are large non-specific channels
But, separate channels exist for Na+, K+, Ca2+, Cl-
These are called ion channels
They are not “just openings” in the membrane

22. Controlling Ion Channels
Controlled ion channels are referred to as “gates”
These can be controlled by:
signal molecules; Ligand gated Channels
Electrical differences; voltage gated channels
Mechanical effects; receptors

23. Importance of Ion Channels
Changes in Na+ and K+ allow nerve impulses to be transmitted down a nerve cell
Many toxins and anesthetics interfere with ion channels
Japanese puffer fish toxin (Tetrodotoxin) blocks Na+ channels: a few micrograms can cause paralysis and death if eaten
Cystic fibrosis is caused by a defect in Cl- channels; decreased fluid secretion causes thick mucus which clogs airways and other passages