Solutes are substances that are dissolved in a liquid to form a solution. The liquid that the solutes are dissolved in is referred to as a solvent. In biological systems, molecules that must enter or leave cells are the solutes and water is the solvent.
All particles are constantly and randomly moving. As a result over time, particles tend to spread themselves out evenly. Diffusion is the tendency of particles to move from an area where they are more concentrated, and therefore more random collisions, to an area where they are less concentrated and have fewer collisions. Where there is an equal concentration of particles in all areas equilibrium is achieved.
Moving along a concentration gradient is known as the movement from an area of higher concentration to an area of lower concentration. This type of movement is referred to as passive transport because no energy is expended by the cell to accomplish the movement.
Diffusion is the driving force behind the movement of many molecules through the cell membrane.
Factors Affecting Diffusion:
1. Lipid solubility of the diffusing particle:
The greater the lipid solubility of the diffusing particle, the more permeable the membrane will be.
2. Size of the diffusing particle:
Small particles move through the membrane much faster as large particles can not easily squeeze through the tightly packed phospholipids
Note: O2, H2O and CO2 diffuse through the cell membrane rapidly
-Osmosis = the diffusion of water across a semi-permeable membrane
-Plasma membrane permeable to water but not to solute
-Water moves from a solution with a lower concentration of solutes (i.e. the concentration of water is high) to a solution with a higher concentration of solutes (i.e. the concentration of water is low)
-Water moves from dilute solution to concentrated solution
Solution Types Relative to Cell:
Hypertonic Solution: Solute concentration of solution higher than cell
More dissolved particles outside of cell than inside of cell
Hyper = more (think hyperactive); Tonic = dissolved particles
Water moves out of cell into solution; net water movement out of cell
Hypotonic Solution: Solute concentration of solution lower than cell
Less dissolved particles outside of cell than inside of cell
Hypo = less, under (think hypodermic, hypothermia); Tonic = dissolved particles
Water moves into cell from solution; net water movement into cell
Cell expands (and may burst)
Isotonic Solution: Solute concentration of solution equal to that of cell
No net water movement