Membranes

“Membranes” - BiologyPod Episode 2 2009 brought to you by Simon Cohen – email me your feedback or questions [email protected] To download this podcast just follow the link to http://biologypod.podomatic.com/ or http://biologypod.podbean.com/ In this Podcast I look at the areas of cell membranes and membrane permeability. Membrane Construction • Cell membranes consist of a double layer (bilayer) of phospholipid molecules (fatty molecules) and also contain protein molecules of various sizes,carbohydrates and cholesterol. • In eukaryote cells, membranes include the plasma membrane, which encloses the cytoplasm, and an interconnected network of non-plasma membranes. • Membranes have many functions in cells, including controlling the passage of molecules into and out of the cell, establishing compartments within cells (organelles), & carrying receptors for recognition and communication. Membrane Permeability Permeability describes the degree to which substances can pass through a cell’s membrane. Cell membranes are semipermeable, ( only some molecules can pass through.) Because cell membranes are semipermeable, a concentration gradient exists such that some substances are present on one side of the membrane in higher concentrations than on the other side. Passive transport is movement down the concentration gradient. No energy is expended in this activity. Substances can be passively transported via diffusion, facilitated diffusion, or osmosis. Diffusion: Particles move spontaneously from an area of higher concentration to an area of lower concentration. Diffusion usually refers to small, uncharged particles, such as oxygen, carbon dioxide, and some ions but some larger lipid soluble molecules such as other lipids (eg steroids) or alcohol can also diffuse through a membrane. Facilitated diffusion: Particles move spontaneously across the cell membrane from an area of higher concentration to an area of lower concentration with the help of special transport proteins. Facilitated diffusion permits the passage of both large and charged molecules (eg glucose) through the cell membrane. Osmosis: Osmosis is the diffusion of water from an area of higher water concentration (low solute concentration - Hypotonic) to an area of lower water concentration (high solute concentration - Hypertonic.) The tonicity, or relative solute concentration, of a cell determines the direction in which osmosis occurs: Hypertonic: higher solute concentration Isotonic: equal solute concentration Hypotonic: lower solute concentration Active transport is movement up the concentration gradient. This activity is powered by the cell’s energy. The cell expends energy (from ATP) and is enabled through the use of carrier proteins, which bind to a substance and shuttle it through pores in the cell membrane. Carrier proteins can perform two methods of active transport: Endocytosis occurs when proteins bring substances into the cell through the vacuoles or vesicles. Consumption of solid matter, such as cholesterol, through endocytosis is called phagocytosis. Consumption of extracellular liquid matter is called pinocytosis. Exocytosis occurs when carrier proteins remove substances from the cell through the vacuoles or vesicles. Insulin is delivered from the cell to the bloodstream by exocytosis.