Biology Unit 1.2 Membrane Transport

• The phospholipid bilayer carries out the main function of the plasma membrane, providing a barrier to the movement of some substances into and out of the cell
• Additional functions are carried out by proteins in the membrane
• Transport proteins allow ions and polar molecules to travel across the membrane
• There are two types of transport proteins, channel and carrier proteins
• Channel proteins form holes, or pores, through which molecules can travel
• Carrier proteins change shape to transport a substance across the membrane
• Transport proteins allow the cell to control which substances enter or leave
• Receptors are for the binding of peptide hormones
• Immobilized enzymes are integral proteins with the active site exposed on the surface of the membrane
• Cell adhesion allows cells to attach to neighbouring cells within a tissue
• Glycoproteins act as cell markers, or antigens, for cell-to-cell recognition
• Simple diffusion is the net movement, as a result of the random motion of molecules or ions, of a substance from a region of higher concentration to a region of lower concentration
• In simple diffusion, the random movement is caused by the kinetic energy of the molecules or ions
• If diffusion takes place for a long enough time period, molecules eventually reach equilibrium
• The higher the temperature, the higher the rate of diffusion and molecules have more kinetic energy at high temperatures, so random movement of molecules is faster
• The greater the surface area the higher the rate of diffusion
• Large molecules diffuse more slowly as they require more energy to move
• Non-polar molecules diffuse more quickly as they are soluble in the non-polar phospholipid bilayer
• Osmosis is the diffusion of water molecules, from a dilute solution to a solution with a higher solute concentration, across a partially permeable membrane
• While water can move directly in between the phospholipids, channel proteins called aquaporins allow water to pass through membranes more freely
• Osmosis can also be described as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane
• Water potential describes the tendency of water to move
• Some substances cannot diffuse through the phospholipid bilayer of cell membranes such as large molecules, polar molecules, and ions
• The process by which these substances cross the plasma membrane is known as facilitated transport or active transport
• During facilitated diffusion the net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient
• Facilitated diffusion is a passive form of transport; it does not require energy
• Some channel proteins are gated, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
• By channel proteins being gated, it allows the them to control the exchange of ions
• carrier proteins can switch between two shapes
• Active transport is the movement of molecules and ions across a cell membrane, from a region of lower concentration to a region of higher concentration, using energy from respiration
• Active transport occurs against, or up, a concentration gradient
• Active transport requires carrier proteins
• Carrier proteins in active transport are sometimes known as pumps
• Although facilitated diffusion also uses carrier proteins, active transport is different as it requires energy
• Facilitated diffusion and active transport are mechanisms that allow cell membranes to be selectively permeable
• Selective permeability is the ability of the membrane to differentiate between different types of molecules, only allowing some molecules through while blocking others
• Simple diffusion provides less control for cell membranes, as it is dependent only on the size and hydrophobic or hydrophilic nature of the molecules diffusing
• Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles
• Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport
• Membranes play a role in cell signalling by acting as an interface for communication between cells