osmosis and diffusion

diffusion

- moves from high concentration to low concentration
- moving with the concentration gradient
- once molecules are evenly spread out, equilibrium is achieved
- each particle moves randomly, however, the population of particles has directional movement. over time, particles tend to spread themselves out evenly
- molecules that enter/leave a cell by diffusion are either small and/or lipid soluble such as: o2, co2, alcohol, and small lipids

osmosis

- diffusion of water (and only water) across a selectively permeable membrane

note: a solution contains a solute (a solid dissolved in the the water) and a solvent (the liquid)

solutions can be divided into 3 categories

a) isotonic solutions
b) hypotonic solutions
c) hypertonic solutions

isotonic solutions

iso = equal

- solute concentration equal inside and outside the cell
- result: no net movement of water in or out

ie 0.9% NaCl solution is isotonic to blood. intravenous solutions usually have this tonicity

hypotonic solutions

hypo = low or less

- the solute concentration of the fluid surrounding the cell is less than the cell's cytoplasm (ie more "watery" outside the cell)
- result: water enters the cell to balance concentrations
- cytolysis: cells burst. occurs in animal but not plant cells (due to rigid cell wall)
- plant cells simply have an increase in turgor presence

hypertonic solutions

hyper - high or greater

- the fluid surrounding the cell has a higher solute concentration than the cell's cytoplasm
- result: water leaves due to osmosis
- animal cell shrivels; known as crenation
- plant cells go through plasmolysis

HINT: salt sucks! (or any other solute for that matter) - water is sucked or drawn towards the solutes

summary of terms

hypotonic in animal cell: cytolysis
hypertonic in animal cell: crenation
hypotonic in plant cell: turgor pressure
hypertonic in plant cell: plasmolysis

the tonicity is a relative concept. it is based on which two solutions you are comparing. for example:

distilled water (hypotonic) vs tap water (hypertonic)
and
tap water (hypotonic) vs sea water (hypertonic)

osmoregulation

- animals living in water have an osmotic difference between their cells and the surrounding water
- in fresh water, salt in the body is more than salt in water. water wants to enter cells

to manage this influx of water they:
a) excrete large amounts of water in urine
b) contractile vacuoles in single cell organisms expel water

- in saltwater, salt in body is less than salt in water. water wants to leave cells
animals deal with this problem in several ways:

a) make their body surfaces less permeable to salts and water
b) pump salts out of the blood back into the sea water
c) produce a very small amount of urine to get rid of metabolic wastes (a source of water loss)
d) replace lost water by drinking
e) get rid of excess ions by active transport from the body fluids back into the water