cell membrane and transport

• All cells and organelles are surrounded by a partially permeable membrane composed of a sea of phospholipids with protein molecules between the phospholipid molecules.
• The main function of the membrane is controlling the movement of substances in and out of the cell/organelle.
• The membrane also contains receptors for other molecules to allow signalling between cells.
• The fluidity of the membrane and the mosaic arrangement of the protein give the structure of the membrane its name – fluid mosaic model.
• Glycoproteins are recognition sites and act as antigens.
• Phospholipids form a bilayer, make membrane fluid, have non-polar tails and hydrophilic heads, thus forming a barrier to most water soluble substances.
• Cholesterol waterproofs the membrane, controls stability of membrane, has hydrophilic heads and hydrophobic tails.
• Intrinsic proteins pass through membranes, some form channels or carriers for water soluble molecules.
• Channel proteins are a hydrophobic channel where diffusion of polar molecules and ions happens.
• Carrier proteins allow active and passive transport, change shape when the molecule enters the protein.
• Extrinsic proteins are found on the surface only, some act as enzymes.
• Glycolipids are short carbohydrate chains that help make membranes stable by forming hydrogen bonds with H2O, help cells attach to one another.
• Specific ligands are released from the cell which are transported to the target cell where they bind to specific receptors on the cell surface membrane, producing a response which may cause a cascade of more reactions.
• The permeability of a cell membrane can be affected by heat, ethanol, and pH.
• The movement of molecules through cell membrane depends on the properties of the molecule as well as the requirements of the cell.
• Diffusion is the passive movement of small, non-polar lipid soluble molecules such as carbon dioxide and oxygen from an area of high concentration to an area of low concentration, the molecules move directly through the phospholipid bilayer.
• Facilitated diffusion requires a channel protein in the cell membrane to transport polar molecules, charged and water soluble molecules across the membrane.
• Osmosis is the net diffusion of water molecules from an area of low solute concentration to an area to high solute concentration through a partially permeable membrane.
• Active transport can transport all types of molecules through carrier proteins from an area of low concentration to an area of high concentration, this process requires energy in the form of ATP.
• Cytosis is a form of active transport where parts of the plasma membrane form infoldings or outfoldings.
• Exocytosis and endocytosis are types of cytosis that transport large particles by enclosing them in vesicles made from the cell surface membrane, the vesicles are transported into the cell in endocytosis.
• Water potential is the pressure exerted by water molecules that are free to move in a system, measured in kPa.
• A solution’s water potential falls as solutes are added as water molecules cluster around the solute, the contribution of solute to the water potential is called the solute potential.