The Cell Membrane & Transport The Cell Membrane • The cell membrane is a skin-like structure surrounding the cytoplasm serving as a barrier to the cell’s environment. • The membrane serves two main purposes – It transports materials into the cell that the cell needs for survival (“Gatekeeper”) – Cell to Cell Communication What does a cell need to transport? Semi-Permeable • The membrane only allows certain molecules to pass through according to their size • Because only certain things can move across the membrane it is said to be semi permeable • Small molecules such as water can go in and out freely • Large molecules like Proteins and Carbohydrates cannot. Composition of the Cell Membrane • The cell membrane consists of: 1.phospholipid bilayer 2.proteins. • The phospholipid bilayer looks like: Phospholipid Bilayer • Just like oils avoid mixing with water, phospholipid molecules form a phospholipid bilayer to avoid their heads touching their tails. • The outer surfaces consist of hydrophilic heads (interacting with the watery environment inside and outside the cell) • The center of the bilayer consists of hydrophobic fatty acids Phospholipid Bilayer Fluid Mosaic Model • Together the phospholipids, proteins and other membrane components form a dynamic structure referred to as a Fluid Mosaic. • Membranes are fluid like rather than sheets of molecules locked rigidly into place • Most of the protiens float freely like “icebergs” in a “sea” of phospholipids Membrane Transport • There two main ways to move things across the membrane – Passive Transport – Active Transport Concentration Gradient • The movement of molecules across the membrane is affected by their concentration gradient -the difference in concentration on both sides of the membrane. Membrane Transport • Passive Transport • Molecules move on their own from an area of high concentration to low concentration • Requires No Energy Membrane Transport • Active Transport • Molecules are moved from an area low concentration to high concentration • Requires Energy Active Transport -Low to High w/ Energy Used Passive Transport -High to Low/ No Energy Needed Passive Transportation • Two examples of passive transport: –Simple Diffusion –Facilitated Diffusion Simple Diffusion • During Simple Diffusion molecules move randomly from an area of high concentration to an area of lower concentration until they are evenly spread out (Dynamic Equilibrium) Everyday Example of Diffusion Facilitated Diffusion • Some molecules need “help” to get across the membrane • Through Facilitated Diffusion, transport proteins in the membrane provide a pathway for certain molecules to pass from high to low concentration. (Facilitate means to help) Facilitated Diffusion Active Transport • Some substances need to be moved from low to high concentrations, this is accomplished by active transport • Two examples of Active Transport: –Ion Pumps –Bulk Transport Pumps • Pumps involve carrier proteins and energy • Carrier proteins transport molecules against a gradient with the use of energy. Bulk Transport • Cells also use energy to move things into and out of the cell in bulk by folding the membrane around what needs to be moved • Cells bring in bulk items by endocytosis . – Phagocytosis-take in large particles • Cells remove bulk items from the cell by exocytosis Membrane Transport Summary • There are three primary means that water and other small molecules cross into or out of cells. – Diffusion – Facilitated Diffusion – Active Transport Passive Transport • All play an important role in the regulation of body fluids and many other physiological functions.