The Cell Membrane & Transport The Cell Membrane The cell membrane is a skin-like structure surrounding the cytoplasm serving as a barrier to the cells environment. The Cell membrane functions as the Gatekeeper of the cell, transporting essential materials inside and out of the cell. What does a cell need to transport? 1.Food in 2.Water in/out 3.Oxygen in 4.Waste like CO 2 out Semi-Permeable Because only certain things can move across the membrane it is said to be semi permeable The membrane only allows small, uncharged molecules to cross. Can cross: Water, O 2, CO 2, Cannot cross: Large molecules like proteins or carbohydrates. Charged ions like Cl -, Na + Composition of the Cell Membrane The cell membrane consists of: 1.phospholipid bilayer 2.proteins. The phospholipid bilayer looks like: Phospholipid Bilayer Fluid Mosaic Model The analogy of the membrane: proteins are like icebergs floating in the phospholipid ocean. Concentration Gradient Concentration gradient = the difference in concentration (amount of dissolved stuff) on both sides of the membrane. Membrane Transport There two main ways to move things across the membrane Passive Transport Active Transport Membrane Transport Passive Transport Molecules move from an area of high concentration to low concentration Requires No Energy Active Transport Molecules are moved from an area low concentration to high concentration Requires Energy Passive Transport - High to Low No Energy Needed Active Transport - Low to High, Needs Energy Passive Transportation Two examples of passive transport: Simple Diffusion Facilitated Diffusion Simple Diffusion Simple Diffusion is when molecules move randomly from an area of high concentration to an area of lower concentration until they are evenly spread out (Dynamic Equilibrium) Facilitated Diffusion Some molecules need help to get across the membrane (Facilitate means to help) Through Facilitated Diffusion, proteins in the membrane provide a pathway for certain molecules to pass from high to low concentration. Active Transport Some substances need to be moved from low to high concentrations, this is accomplished by active transport Two examples of Active Transport: Pumps Bulk Transport Pumps Pumps involve carrier proteins and energy Carrier proteins Pump molecules against a gradient from Low to high concentration with the use of energy. Bulk Transport Cells also use energy to move large amount of 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. 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 All play an important role in the regulation of body fluids and many other physiological functions. Passive Transport