Tonicity, Osmosis, and Diffusion

Cell Membranes

• Composed of membrane molecules containing a hydrophilic phosphate head and hydrophobic fatty acid tails.
• Fatty acid tails are positioned inside the lipid bilayer away from water, while phosphate heads face toward the aqueous environment.
• Small nonpolar molecules such as $O_2$, $CO_2$, and $N_2$ easily pass through the membrane via diffusion.

Diffusion and Osmosis

• Selective permeability is the ability of a cell membrane to control which substances enter and exit, and it is synonymous with semi-permeability.
• Oxygen, carbon dioxide, nitrogen, and water pass through the cell membrane easily.
• Moving down a concentration gradient means moving from a region of high concentration to low concentration through passive transport without spending energy.
• Diffusion is the spontaneous movement of particles down a concentration gradient from higher concentration to lower concentration.
• Osmosis is the movement of solvent across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration to equalize concentrations on both sides.