3.3 Passive Membrane Transport

What happens when solutions of different osmolarity are separated by a membrane permeable to all molecules, both solutes and water cross membrane until equilibrium is reached?

Equilibrium: Same concentration of solutes and water molecules on both sides, with equal volume on both sides

What happens when solutions of different osmolarity are separated by a membrane permeable only to water, not solutes, osmosis will occur until equilibrium is reached?

Same concentration of solutes and water molecules on both sides with unequal volumes on both sides.

Concentration Gradient

The difference in concentration between a particular substance between two different areas.

Tonicity

ability of a solution to change the shape or tone of cells by altering the cells' internal water volume

Isotonic Soltion

has same osmolarity as inside the cell, so volume remains unchanged

Hydrostatic Pressures

pressure of water inside cell pushing on membrane

Osmotic Pressures

pressure of water outside cell pushing cell pushing to move into cell by osmosis - the more solutes inside a cell, the higher the osmotic pressure

Carrier-mediated facilitated diffusion

-Carriers are transmembrane integral proteins -Carriers transport specific polar molecules, such as sugars and amino acids, that are too large for membrane channels -Example of specificity: glucose carriers will carry only glucose molecules, nothing else -Binding of molecule causes carrier to change shape, moving molecule in process -Binding is limited by number of carriers present -Carriers are saturated when all are bound to molecules and are busy transporting

Facilitated Diffusion

-Certain hydrophobic molecules (e.g., glucose, amino acids, and ions) are transported passively down their concentration gradient by: -Carrier-mediated facilitated diffusion Substances bind to protein carriers -Channel-mediated facilitated diffusion Substances move through water-filled channels

Channel-mediated facilitated diffusion

-Channels with aqueous-filled cores are formed by transmembrane proteins -Channels transport molecules such as ions or water (osmosis) down their concentration gradient -Specificity based on pore size and/or charge -Water channels are called aquaporins -Two types: 1. Leakage channels -Always open 2. Gated channels -Controlled by chemical or electrical signals

Diffusion

-Collisions between molecules in areas of high concentration cause them to be scattered into areas with less concentration -Diffusion is movement of molecules down their concentration gradients (from high to low) -energy is not required -speed of diffusion is influenced by size of molecule and temperature

Passive Transport

-Membrane transport processes that move substances down their concentration gradient. Driven by kinetic energy, so does not energy (ATP) -2 types of passive transport: 1. Diffusion -simple diffusion -carrier-and channel-mediated facilitated diffusion -Osmosis 2. Filtration -type of transport that usually occurs across capillary walls

Osmosis

-Movement of solvent, such as water, across a selectively permeable membrane -Water diffuses through plasma membranes -Through lipid bilayer (even though water is polar, it is so small that some molecules can sneak past nonpolar phospholipid tails) -Through specific water channels called aquaporins (AQPs) -Flow occurs when water (or other solvent) concentration is different on the two sides of a membrane

Simple Diffusion

-Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through phospholipid bilayer -Examples: oxygen, carbon dioxide, fat-soluble vitamins

Intravenous Solution

-different tonicities can be given to patients suffering different ailments -Isotonic solutions are most commonly given when blood volume needs to be increased quickly -Hypertonic solutions are given to edematous (swollen) patients to pull water back into blood -Hypotonic solutions should not be given because they can result in dangerous lysing of red and white blood cells

Hypertonic Solution

-has higher osmolarity than inside cell, so water flows out of cell, resulting in cell shrinking -Shrinking is referred to as crenation

Hypotonic Solution

-has lower osmolarity than inside cell, so water flows into -cell, resulting in cell swelling -Can lead to cell bursting, referred to as lysing

Osmolarity

-measure of total concentration of solute particles -water concentration varies with number of solute particles because solute particles displace water molecules -when solute concentration goes up, water concentration goes down, and vice versa -water moves by osmosis from areas of low solute (high water) concentration to high areas of solute (low water) concentration