Membranes

Membranes (e.g., plasma membrane or cell membrane)

• Define the boundaries of cells and give them form.

• Provide a barrier that limits the movement of substances into and out of the cell, thus protecting the cell from fluctuating or adverse conditions. Origin of the impermeability lies in the double layer of phospholipids which, being hydrophobic, doesn't allow the passage of  charged molecules.  Water, although not lipid-soluble, appears to be able to dissolve in membranes or pass through water channels.

• Intracellular organelles are surrounded by cell membranes which define compartments within the cell and which can maintain a different environment from the rest of the cell so as to support specialized functions.

I. Membrane Structure and Organization:

• Most membranes are composed primarily of lipid and protein. As a general rule the more metabolically active membranes have a higher proportion of protein.

• Membranes also contain carbohydrate.  Carbohydrates are attached primarily on the outer surface of the membrane as glycoproteins and glycolipids.

• Organized in a lipid bilayer.

• Lipids are amphipathic (have a hydrophilic and a hydrophobic end). Amphipathic refers to molecules that have different properties.

• Have self-repair capability.

• Most abundant lipids are phospholipids.

• Fatty acids of the phospholipids influence membrane fluidity.

• Contain cholesterol which makes bilayers less fluid (because it binds weakly to adjacent phospholipids), but stronger.

• Phospholipids exchange freely along monolayer, but rarely cross to the opposite monolayer.

Proteins:

• Integral (penetrate the lipid bilayer) vs. peripheral (float on or drift among the hydrophilic heads or are only partially embedded in one face of the membrane).

• Proteins can diffuse laterally in the monolayer, but rarely cross to the opposite monolayer.

• Proteins serve a variety of functions within the membrane. They can be ion channels, carriers, pumps, receptors, enzymes, antigens, or provide structural support.

Fluid Mosaic Model:

• Accepted model of the structure of the cell membrane in which globular proteins are integrated with the phospholipid bilayer.

• The membrane is fluid and depends on lipid composition and amount of cholesterol.

• Carbohydrates are attached to the outer surface.

Bulk Transport (Endocytosis vs Exocytosis)

• Cellular uptake (endocytosis) of materials into a cell and secretion (exocytosis)  of materials out of the cell into the extracellular environment.

• Types of endocytosis: Pinocytosis (ingestion of extracellular fluid); phagocytosis (ingestion of solid particles)

• Process of endocytosis

• Membrane invaginates, fuses, and pinches off to form a vesicle.

• Mechanism of exocytosis

• Molecules (e.g., neurotransmitters) are sequestered within secretory vesicles which, under the proper stimulation, move to the membrane and release their contents to the outside.

Cilia - Tiny hairlike structures that project from the surface of a cell into the extracellular fluid.

Flagella - Whiplike structures that propel sperm through their environment.

Membranes are highly selective in terms of which molecules can pass (they are selectively permeable).  This is the result of the nature of the lipid bilayer.

Diffusion - Movement of substances from a region of high concentration to a region of low concentration due to random molecular motion as a result of thermal (heat) energy.

• Rate of diffusion (flux) through a membrane is highly dependent upon such factors as:

1. the magnitude of the concentration difference, or concentration gradient across the membrane

2. the permeability of the membrane to the diffusing substances

3. the temperature of the solution

4. the surface area of membrane through which the substances are diffusing

5. molecular structure/shape

Osmosis - Net movement (diffusion) of water from a region of high concentration to a region of low concentration through a selectively permeable membrane.

Requirements for osmosis:

 1)  Concentration difference of a solute on the two sides of a selectively permeable membrane

 2) Membrane must be relatively impermeable to the solute.  The solute is thus said to be osmotically active.

In some membranes, water passes through special water channels called aquaporins that allow water to pass through more rapidly.

• Osmotic pressure (O.P.) = A measure of the tendency for a solution to gain water by osmosis.  O.P. is proportional to the solute concentration and is the pressure required to just prevent osmotic flow between 2 solutions.

• Pure water has an O.P. = 0.

• A solution which is 2X as concentrated as another solution will have 2X the O.P.

Osmolality (Osm) - Measure of the total concentration of solutes in a solution.

• Solutions can be isosmotic or hypoosmotic or hyperosmotic relative to one another.

Tonicity - Measure of the effect of a solution on the osmotic movement of water. It can often be defined by the response of cells immersed in a solution.

• Isotonic vs. hypotonic vs. hypertonic solutions

• Water always moves from a hypotonic to a hypertonic solution.

• In what direction do solutes move, if the membrane is permeable to them? _______________________________________

Passive (Transmembrane) Transport

• Does not directly require metabolic energy.

• 3 types:

• 1) Molecule simply diffuses through membrane.

• Moves from aqueous to lipid to aqueous phases.

• 2) Solute molecule diffuses through aqueous channels.

• Many proteins have pores or channels which enable specific solutes to pass.

• 3) Solute combines with a protein carrier molecule  in the membrane which facilitates the movement of the molecule across the membrane = carrier-mediated transport. If only a single type of molecule is moved by a carrier, the carrier is called a uniporter.

• Example - Facilitated diffusion of glucose across the membrane.

• Carrier proteins exhibit a transport maximum (T_m) due to their ability to saturate at a particular concentration.

Primary Active Transport (Occurs via membrane pumps which are integral proteins)

• - Transport occurs against a concentration gradient (from a region of lower to a region of higher concentration).

• - Requires energy obtained from ATP.

• - Transports ions selectively.

• - Can be inhibited by specific inhibitors or poisons which cause active transport to stop, but not passive transport.

• - Na⁺/K⁺ pump is the classic example of a primary active transport process. It transports 3 Na⁺ out of a cell and 2 K⁺ into a cell.

Secondary Active Transport (Coupled transport)

• Energy required for the "uphill" transport a molecule or ion is obtained from the "downhill" transport of another molecule into the cell.

• Symport (cotransport) vs antiport (countertransport)

Junctions between cells

• Gap junctions - Provide communication between cells and couple cells electrically and metabolically.

• Tight junctions - Seal cells together so molecules can't pass between cells.

Cell Signaling

• Compare paracrine vs synaptic vs endocrine.