Biological Membranes

Biological membranes consist of phospholipid bilayers

• Hydrophilic heads (consisting of phosphate groups) face outwards.
• Hydrophobic tails of fatty acids face towards interior of membrane.

Fluid mosaic model explains current understanding of membrane structure.

• Proteins can occupy one side of the membrane (peripheral) or can cross from one end to the other through the membrane (transmembrane.)
• In transmembrane proteins, hydrophilic regions will face outwards, while hydrophobic regions will be found in interior of membrane.
• Biological membranes are two-dimensional fluids, where phospholipid molecules of each layer are mobile.
• Various molecules, such as glycoproteins, glycolipids, histocompatibility proteins will protrude from surface of plasma membrane.
• Vesicles form and fuse easily from phospholipid bilayer, as evidenced by the formation of vesicles from the Golgi apparatus. In so doing, cells can "package" and release cellular products.
• Receptor molecules are proteins found on the exterior surface of the plasma membrane. When the appropriate molecule binds to it, the receptor molecule will change shape. This change of shape may transmit information to the interior of the cell and induce signal transduction, in which an extracellular signal is converted to an intracellular signal that affects some function of the cell, e.g. insulin binding to receptor, and that induces the production of the intracellular signal molecule, cyclic AMP.

Cell membranes are selectively permeable

• Movement of molecules depends on size, molecular weight and charge.
• Small molecules move across membrane easily.
• Nonpolar substances move through the lipid bilayer rapidly.
• Polar molecules move through more slowly.

Diffusion is a physical process based on random motion

Video clip of diffusion:

http://www.youtube.com/watch?v=H7QsDs8ZRMI&feature=related

• Solvent molecules will push solute molecules from an area of high concentration to an area of low concentration until dynamic equilibrium is reached. This movement is along a concentration gradient. The movement of solute molecules across a membrane is called dialysis.
• Water can also diffuse across a membrane from an area of high concentration to an area of low concentration. This is called osmosis.
• In some cases, the diffusion of a solute is faster than expected, based on the size of the solute molecules. Such molecules diffuse through a membrane via specific channels. This movement is still along a concentration gradient and is called facilitated diffusion. An example is glucose.

Active transport pushes substances (usually ions) against a concentration gradient

Video clip of active transport:
http://www.youtube.com/watch?v=STzOiRqzzL4

We have the technology to analyze the concentration of ions in the cytoplasm of neurons and in the extracellular fluid bathing neurons. Here are the results:

	Concentration outside the neuron	Concentration inside the neuron
K+ (Potassium)	5 mM	100 mM
Na+ (Sodium)	150 mM	15 mM
Cl- (Chloride)	150 mM	13 mM
Ca+2 (Calcium)	2 mM	0.0002 mM

If diffusion were acting alone, what should happen to the concentration of potassium and sodium ions inside and outside the cell? The fact that a concentration gradient is maintained indicates that there is carrier-mediated active transport, in which sodium ions are actively removed from and potassium ions are actively pulled into neurons. This requires the constant expenditure of energy, and we can measure the consumption of ATP as an indicator of energy consumption. This carrier-mediated active transport of neurons is commonly called the sodium-potassium pump.

There are several forms of gross movement of the plasma membrane, all of which require energy.

• In exocytosis, vesicles fuse with the plasma membrane, thereby ejecting the contents into the outside environment.
• In endocytosis, observable sections of the plasma membrane pinch inwards so that materials are taken into the cells.

A video clip showing distinction between endocytosis and exocytosis:
http://www.youtube.com/watch?v=4gLtk8Yc1Zc

• In phagocytosis, the larger of two cells surrounds the other, and the plasma membrane pinches off to form a food vacuole within the larger of the two cells.

A video clip showing phagocytosis by a neutrophil:
http://www.youtube.com/watch?v=fpOxgAU5fFQ&feature=related