Salt (NaCl) is completely dissociated into Na+ and Cl- when in an aqueous solution. Lipid bilayers, like the ones that make up the cell membranes are not permeable to charged ions.
All cells acquire the molecules and ions they need from their surrounding extracellular fluid (ECF). There is an unceasing traffic of molecules and ions
-in and out of the cell through its plasma membrane
o Examples: glucose, Na+, Ca2+
-In eukaryotic cells, there is also transport in and out of membrane-bounded intracellular compartments such as the nucleus, endoplasmic reticulum, and mitochondria.
o Examples: proteins, mRNA, Ca2+, ATP
Two problems to be considered:
1. Relative concentrations
Molecules and ions move spontaneously down their concentration gradient (i.e., from a region of higher to a region of lower concentration) by diffusion.
Molecules and ions can be moved against their concentration gradient, but this process, called active transport, requires the expenditure of energy (usually from ATP).
2. Lipid bilayers are impermeable to most essential molecules and ions.
The lipid bilayer is permeable to water molecules and a few other small, uncharged, molecules like
oxygen (O2) and carbon dioxide (CO2). These diffuse freely in and out of the cell. The diffusion of water through the plasma membrane is of such importance to the cell that it is given a special name: osmosis.
Lipid bilayers are not permeable to:
-ions such as
K+, Na+, Ca2+ (called cations because when subjected to an electric field they migrate toward the cathode [the negatively-charged electrode])
Cl-, HCO3- (called anions because they migrate toward the anode [the positively-charged electrode])
- small hydrophilic molecules like glucose
- macromolecules like proteins and RNA
Mechanisms by which cells solve the problem of transporting ions and small molecules across their membranes:
1. Transmembrane proteins create a water-filled pore through which ions and some small hydrophilic molecules can pass by diffusion. The channels can be opened (or closed) according to the needs of the cell.
2. Active transport. Transmembrane proteins, called transporters, use the energy of ATP to force ions or small molecules through the membrane against their concentration gradient.