Membrane Proteins: Transport

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

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

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

Membrane Proteins

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.)

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

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.

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)

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

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

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

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).

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

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

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

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

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

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

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

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

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

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