Concentration Gradient

  • 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.