The cell membrane is selectively permeable. Molecules that need/want into the cell have to use either an active or passive process. Usually molecules will follow the [gradient], meaning that they go from a higher concentration to a lower concentration. Sometimes, a cell needs to get something out (Na+) and will push it against the gradient. These processes help maintain homeostasis.
Passive Processes - we discussed four of them. These processes do not require ATP (aka energy) to do.
Simple Diffusion occurs when a small noncharged lipid soluble molecule wants to cross the membrane. Movement always occurs down the [gradient] until equilibrium is met.
Facilitated diffusion requires a protein to help it get across the plasma membrane. This process allows larger molecules to get to the other side. Amino acids, glucose, K+, Cl- are some of the molecules that use facilitated diffusion. This process also occurs down the [gradient].
Osmosis is simple diffusion for water. Water moves from a high concentration to a low concentration. Tonicity is the ability of a solution to exert an osmotic pressure upon a membrane. When a cell gains water (high concentration of solute), the cell is in a hypotonic state. When a cell loses water (high conctration of water), it is because it is in a hypertonic state and there is a hypotonic state outside of the cell. Osmotic pressure is the force exerted on a membrane due to the osmotic movement of water.
Filtration only occurs in single layers of cells. Examples include blood pressure (aka hydrostatic pressure) and molecules leaving the blood.
Active Processes require the use of ATP. They often move substances against their [gradient]. Sometimes they are called pumps or revolving doors. 40% of ATP is used on active transport.
Endocytosis is when stuff leaves the cell
Exocytosis is when stuff enters the cell.