Page 22 - Cell biology PDC 2024

three sodium ions are extruded from and two potassium ions are
imported into the cell

2) Passive transport
Passive transport involves the movement of material down
concentration gradient (high concentration ⇒ low concentration).
It does not require the expenditure of energy (ATP hydrolysis)
There are three main types of passive transport:

 Simple diffusion – movement of small or lipophilic molecules (e.g.
O2, CO2, etc.)

 Osmosis – movement of water molecules (dependent on solute
concentrations)

 Facilitated diffusion – movement of large or charged molecules via
membrane proteins (e.g. ions, sucrose, etc.)

Osmosis and tonicity
Osmolarity: is the measure of solute concentration per unit
VOLUME of solvent, It's not the same as tonicity! Osmolarity
concerns with all of the solute concentrations, not just the ones
that can't cross the semipermeable membrane
Tonicity: is the measure of the osmotic pressure gradient between
two solutions
Unlike osmolarity, tonicity is only influenced by solutes that
cannot cross this semipermeable membrane.
Thus, you can have iso-osmolar solutions which are not isotonic

Isotonic Solution: A cell in an isotonic solution is in equilibrium
with its surroundings i.e. the solute concentrations inside and
outside the cell are the same. In this state there is no
concentration gradient between inside and outside the cell and no
large movement of water in or out so Water molecules do freely
move in and out of the cell, however, and the rate of movement is
the same in both directions.

Hypertonic solutions: Hypertonic solutions have a higher solute
concentration than inside the cell. This causes water to rush out
making the cell wrinkle or shrivel ex: crenation of RBCs

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