MODULE 15

• The cell membrane is one of the great multi-taskers of biology.
• The cell membrane provides structure for the cell, protects cellular contents from the environment, and allows cells to act as specialized units.
• A membrane is the cell's interface with the rest of the world - it is the gatekeeper, if you will.
• The cell membrane is a phospholipir bilayer that determines what molecules can move into or out of the cell, and so is in large part responsible for maintaining the delicate homeostasis of each cell.
• In cellular biology, membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes, which are lipid bilayers that contain proteins embedded in them.
• Transport means moving something from one area to another by passing through a gate or a membrane.
• Cells are made of many crucial parts that work together and perform specific functions.
• One of the functions of cell membranes is to control what passes into and out of the cell.
• There are several different types of membrane transport, depending on the characteristics of the substance being transported and the direction of transport.
• Types of membrane transport can be grouped into passive and active transport.
• Passive transport can be simple diffusion or facilitated diffusion.
• Active transport can be ion pumps, exocytosis, and endocytosis.
• In simple diffusion, small non-charged molecules or lipid soluble molecules pass between the phospholipids to enter or leave the cell, moving from areas of high concentration to areas of low concentration (they move down their concentration gradient)
• Oxygen and carbon dioxide and most lipids enter and leave cells by simple diffusion.
• Diffusion occurs when particles spread.
• Diffusion occurs when particles move from a region where they are in high concentrations to a region where they are in low concentration, it happens when the particles are free to move.
• Uniform distribution = Equilibrium
• Diffusion happens in an area where there is no barrier to move anywhere.
• Osmosis is a type of diffusion where molecules of water are moving from a region of high-water concentration to a region of less water concentration by passing through a semi-permeable membrane, a type of membrane that selective allow which substance will enter through it.
• Facilitated diffusion uses transport proteins to move ions and other small molecules across the plasma membrane.
• Channel proteins are water-filled transport proteins that open and close to allow substance to pass through.
• Carrier proteins are transport proteins that change shape to open and close as substances diffuse across the membrane.
• Facilitated diffusion is a process whereby a substance passes through a membrane with an aid of an intermediary or a facilitator.
• The facilitator is an integral membrane protein that spans the width of the membrane.
• The force that drives the molecule from one side of the membrabe to the other side is the force of diffusion.
• In the cell, examples of molecules that must use facilitated diffusiin to move in and out of the cell membrane are glucose, sodium ions, and potassium ions.
• In facilitated diffusion, glucose, sodium ionsz and potassium ions pass using carrier proteins through the cell membrane without energy along the concentration gradient.
• In facilitated diffusion, protein carriers are needed in order for large molecules to get inside the cell like Na and Cl.