Study set #4

Ion channels

Ions normally diffuse across the plasma membrane through____ _______ which are selective, gated, or ungated

Electrochemical gradient

It is a combination of concentration and electrical gradient. -movement of charged particles in or out of cell depend on this gradient.

Pumps

Most common primary active transport protein -use energy from ATP directly to transport ions "uphill" against electrochemical gradient. •NaK pump transports Na out and K into cell. - NaK pump activity is regulated by thyroid hormone

Fick's law of diffusion

Rate = P A (Cout - Cin) / (x) Rate is proportional to permeability (P), surface area (A), concentration gradient (Cout - Cin); inversely proportional to diffusion distance or membrane thickness (x)

Osmotic pressure

The driving force for osmosis is ____ ____

Hypotonic solution

Water moves in, cell expands

Hypertonic solution

Water moves out, cell shrinks

concentration gradient

What is required for osmosis to occur across a membrane?

~290 mOsm (plasma osmolarity)

What is the normal osmolarity of body fluids (ECF/ICF)?

Insulin

With respect to the membrane transport, the specific action of ______ is to promote insertion of GLUT4 into the membrane therefore glucose receives uptake via facilitated diffusion

Osmosis definition

a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane.

Active transport

requires energy; transport against gradient

Osmolarity

total concentration of all solutes in a solution; 1 Osm = 1 mole of solutes per liter

Sodium potassium pump

(Na+/K+-ATPase) transports Na+ out, K+ in - maintains ionic composition of ICF and ECF - K+ and Na+ gradients are the basis of electrical properties of cells (membrane potentials) - Na+ gradient provides potential energy for transport of other molecules - Na+/K+ pump activity is stimulated by thyroid hormones Other active transport pumps: Ca2+-ATPase in muscle, H+-ATPase in stomach, etc.

Transepithelial transport of NaCl

- apical membrane: Na+ enters via diffusion through channels (apical Na+ channels) - basolateral membrane: primary active transport (Na+/K+ pump) moves Na+ out to ECF - Cl- follows Na+ passively via diffusion through channels

Transepithelial transport of Glucose

- apical membrane: secondary active transport - cotransport with Na+ (SGLT) - basolateral membrane: facilitated diffusion of glucose out of cell (GLUT)

Passive transport

- does not require energy; substances move down gradient

Cell membrane permability

- the ability of a substance to pass through a membrane - permability is determined by 1) the phospholipid layer 2) membrane transport proteins

Simple diffusion

-Passive transport - results from random molecular motion - net movement is from high concentration to low concentration - Fick's Law of Diffusion gives the rate of diffusion

Membrane transport proteins

-helps ions and polar molecules to pass through •channels- water filled pores, for ions and water. •carriers- protein "shuttles" for small polar molecules •pumps-active support, mostly ions

Facilitated diffusion

-passive transport -protein-mediated transport -carrier proteins: mediate diffusion of certain polar molecules across the membrane - down concentration gradient, no energy required - specificity - each carrier is specific to particular molecule(s) - saturation - rate limited by number of carrier proteins in the membrane → transport maximum GLUT proteins - family of glucose transporters, present in many cell membranes GLUT4 transporters in skeletal muscle and adipose tissue are insulin dependent insulin promotes insertion of GLUT4 into the membrane → glucose uptake via facilitated diffusion

Osmosis

-passive transport -passive movement of water across a membrane* due to solute concentration difference * selectively-permeable membrane: permeable to H2O but not solutes - osmosis is the primary mechanism of H2O transport across biological membranes - H2O follows solutes, H2O moves from dilute to concentrated solution ("solutes suck water")

Diffusion through channels

-passive transport -protein-mediated transport -ion channels are protein passageways for ions through the membrane - most channels are selective for certain ions (K+, Na+, etc.) - ions diffuse down electrochemical gradients (combined concentration and electrical gradient) - channels may be gated or ungated - aquaporins are water channels, found in most cell membranes

Primary active transport

-protein-mediated transport -pumps are transport proteins that use energy from ATP directly (ATPases) - transport ions "uphill", against electrochemical gradients -ex: Na+/K+ pump

Secondary active transport

-protein-mediated transport - uses potential energy stored in ionic gradients to move other molecules - transport protein couples downhill flow of an ion to uphill movement of another molecule cotransport - movement of both molecules in the same direction (e.g., Na+-glucose transport) countertransport - movement in opposite directions (e.g., Na+/H+ exchanger) SGLT is a Na+-glucose cotransporter in the small intestine epithelium - moves glucose against its gradient from the lumen into the epithelial cell - uses energy in the Na+ gradient formed by the Na+/K+-ATPase glucose transport in small intestine and kidneys goes uphill so it requires secondary AT

Lipid bilayer

1. Molecular size: smaller means more permeable 2. Lipid solubility: -nonpolar molecules which are lipid soluble are more permeable -polar molecules and ions are less permeable or impermeable

Mechanisms of membrane transport

1. Simple diffusion 2. Osmosis 3. Diffusion through channels 4. Facilitated diffusion 5. Primary active support 6. Secondary active support

Carrier protein

A Transport protein that functions in facilitated diffusion - mediates diffusion of certain polar molecules across the membrane - if down concentration, no energy required.