10T2K© Membranes

Thing 1: Membranes are mostly phospholipids The basis of every membrane of every cell is a bilayer of phospholipids.

Thing 2: Membranes also contain proteins Proteins have different jobs in membranes. Transport proteins allow many molecules to move from one side of a membrane to the other.

Thing 3: Membranes also contain carbohydrates Carbohydrates may be attached to proteins or lipids. Carbohydrates get attached to proteins and lipids in the ER and Golgi body. Different carbohydrates on red blood cells make different blood types.

Thing 4: Membranes in animal cells contain cholesterol Cholesterol is a steroid that fits between phospholipids in membranes.

Thing 5: The fluid mosaic model The lipid bilayer is fluid. That means that the phospholipids, proteins, and cholesterol can move around in the membrane.

Thing 6: Some molecules diffuse directly across the lipid bilayer Only small nonpolar (water-repelling) molecules do this. Examples are O2, CO2, and some steroids.

Thing 7: Most things cross membranes through transport proteins Most polar (water-attracting) and ionic (also water-attracting) substances get across membranes through transport proteins. Examples include water, glucose, and Na+.

Thing 8: Endocytosis and exocytosis Endocytosis is another way to get material into a cell. A piece of cell membrane pinches off with something inside it. Examples include how amoebas eat other cells, and how cholesterol gets into our cells.

Thing 8, continued Exocytosis is the opposite process. Vesicles (small vacuoles) mix with the cell membrane to release materials. Examples of material released by exocytosis include neurotransmitters and insulin.

Thing 9: Passive transport Passive transport is when movement across a membrane doesn’t require energy. Diffusion directly across a lipid bilayer is passive transport. Diffusion through a transport proteins is passive transport.

Thing 10: Active transport Active transport is when movement across a membrane requires energy. Movement through transport proteins in the direction opposite of diffusion is active transport.