movement in and out of the cell

• All cells have a plasma membrane, which is selectively permeable, allowing some substances through but not all
• Cell membranes are made of lipids called phospholipids, with a head that is polar and attracts water (hydrophilic) and 2 tails that are nonpolar and do not attract water (hydrophobic)
• Functions of the plasma membrane include active & selective transport, cell identity (through glycoproteins), and receiving signals (transmembrane proteins act as receptors)
• Movement across membranes is influenced by molecular size, net charge, lipid solubility, and concentration gradient
• Diffusion is the passive movement of a substance from an area of high concentration to low concentration, down the concentration gradient
• Simple Diffusion involves no energy input and allows small, lipophilic molecules to move freely across the membrane from areas of high concentration to low
• Facilitated Diffusion requires no energy input and involves proteins that help the movement of charged, polar molecules through channel proteins or carrier proteins
• Osmosis is the net movement of water across a plasma membrane from an area of low solute concentration to high solute concentration
• Active transport is the movement of molecules from low concentration to high concentration, requiring energy (ATP)
• Endocytosis is the bulk movement of solids INTO the cell, requiring energy
• Exocytosis is the same as endocytosis, but particles move OUT of the cell, also requiring energy
• The size of a cell is limited by the surface-area-to-volume ratio (SA:V), influencing the rate of entry and exit of substances into and out of cells