Definition: Diffusion is when things move down a concentration gradient; in nature, substances tend to move from an area of greater concentration to an area of lower concentration.
Practical example: a dry sponge placed in a bowl of water absorbs water until saturated.
Practical uses:
A wet cell phone turned off, dried, and buried in an open bowl of dry, uncooked rice.
Uncooked rice in a salt shaker in humid weather helps keep the salt from clumping (moisture moves into the rice/salt system).
Diffusion
Passive movement of molecules from an area of relatively high concentration to an area of relatively low concentration until equilibrium occurs.
Described as moving "down a concentration gradient".
Examples mentioned:
Across a membrane; gases through capillary walls.
Sugar dissolving (dispersing) in a liquid.
Smoke from a cigarette/fire/chimney dispersing/diffusing in air.
Osmosis (diffusion of water)
Passive movement of water across a selectively (semi-)permeable membrane toward a solution containing a relatively high solute concentration.
Key point: solid particles (solute) cannot move across the membrane; only water moves.
Outcome: the solution on both sides of the membrane becomes equally diluted/concentrated.
Key terms
SOLUTE: The particles in a solution.
SOLVENT: The liquid in a solution.
SOLUTION: A liquid (solvent) with particles (solute) dissolved in it.
ACTIVE vs. PASSIVE
Active: requires energy or physical exertion; ATP provides energy.
Passive: requires no energy, or minimal exertion (e.g., diffusion).
SURFACE AREA
Greater surface area increases the ability to absorb and secrete.
Example: the surface area of millions of tiny air sacs in the lungs would be the size of a tennis court if spread out.
SEMI-SELECTIVELY PERMEABLE MEMBRANE
A membrane that selectively allows things to pass through.
NATURE vs. NURTURE
Almost all conditions have a percentage of both genetics (nature) and environment (nurture).
FORM FOLLOWS FUNCTION
The shape/form of things, cells, or tissues is often determined by how they need to function.
HOMEOSTASIS
From the Greek: homeo = same; stasis = standing still.
Moving toward balance or a normal internal body state.
Achieved by a system of control mechanisms activated by NEGATIVE FEEDBACK.
NEGATIVE FEEDBACK
A correcting process in which the body's response reverses the effect of a stimulus and restores homeostasis.
Additional note on moisture and air
Warmer air can hold moisture better than cooler air.
When warmer, moist air interacts with cooler air, the moisture in the cooler air tends to condense or precipitate.