Cell Membranes

Mark RobertsonProfessor of Biology

Delta College

Membrane PropertiesMembrane made of phospholipid bilayer, proteins, cholesterol, and carbohydrates and described by Fluid Mosaic Model✦ Proteins aid in cellular recognition, movement of substances through the membrane (channels and pumps), cellular connections, and controlling reactions (enzymes)

✦ Cholesterol molecules stabilize the membrane’s movement and yet maintain the membrane fluidity

✦ Glycoproteins (glycogen jutting out of imbedded protein) act as markers and binding sites on the membrane surface

✦ Phospholipids form matrix of themembrane (physical barrier)

Transmembrane ProteinsTransmembrane proteins (through the membrane) either act as “doors” to facilitate entry/exit or act as pumps to move items in/out of the cell✦ Ion Channels

✦ Active (requires ATP to open or close; sometimes used to concentrate products; ex: lipids in adipocytes or glycogenin liver cells to regulate blood sugar)

✦ Passive (always open; ex: toxins)✦ Ion Pumps

✦ Only Active (requires ATP)✦ Best example is Na+/K+ pump (move

3 Na+ ions out for 2 K+ ions in usingone ATP per cycle)

✦ Useful in cardiac and nervous tissues

Na+/K+ Pumps

Common Definitions✦Diffusion

✦Net movement of molecules from high to low concentration

✦All molecules diffuse eventually (reversed w/energy)

✦Dynamic Equilibrium✦Molecular movement occurs, but concentration of

substances is equal, so no net change over time✦Occurs with blood, tears, saliva (isotonic solutions)

✦Concentration Gradient (source to sink)✦Gradual concentration change over a distance (from areas of high concentration to low)

✦Useful for moving solutes through organism

Hypo/Hyper/IsotonicTonicity is a measure of osmotic pressure of two solutions separated by a semipermeable membrane. It is commonly used when describing the response of cells immersed in anexternal solution.

✦ Hypertonic (higher pressure)✦ Solution with more solutes

✦ Hypotonic( lower pressure)✦ Solution with less solutes

✦ Isotonic (same pressure)✦ Total solutes are equal

✦ NOTE: water always moves from hypo to hyper!

Cellular PermeabilitySelective permeability is the ability of a living cell to alter what may pass through at any given time. The rate depends on a variety of factors:✦ Solute size

✦ Smaller molecules diffuse faster than larger ones (ex: kidney dialysis machines)✦ Solute concentration

✦ More concentrated molecules diffuse faster than less concentrated molecules

✦ Polarity✦ Polar molecules move faster in water,

non-polars diffuse faster in lipids✦ Temperature

✦ All molecules move faster at higher temperatures

Active/Passive Processes✦Facilitated (Passive) Diffusion

✦ Channel proteins have charged interior that is used to speed up ionic diffusion✦Active Transport

✦ Using membrane proteins to force molecules into area of higher concentration (up a concentration gradient) by burning ATP

✦Vesicular (Bulk) Transport✦ Endocytosis (transport inside cell)

✦ Phagocytosis (envelop solids)✦ Pinocytosis (envelop fluids)

✦ Exocytosis (expulsion of items)

Possible Journal Critique Topics

Sodium/Potassium Imbalances?

Ion Channels and Disease (Epilepsy, Myotonia, Cardiac Arrhythmia, etc.)?

Hypoglycemia and/or Hyperglycemia?

Hypocalcemia and/or Hypercalcemia?

Kidney Dialysis as a Potential Treatment?