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The Plasma Membrane, Cellular Transport, and Homeostasis The Plasma Membrane All cells have a cell membrane Functions: • Controls what enters and exits the cell to maintain an internal balance called HOMEOSTASIS • Provides protection and support for the cell Structure of the Cell Membrane Lipid Bilayer- 2 layers of phospholipids phosphate head is polar (water loving) fatty acid tails are non-polar (water fearing) proteins embedded in the membrane Fluid Mosaic Model According to the model, the lipid bilayer behaves more like a fluid than a solid. The membranes lipids and proteins can move laterally within the lipid bilayer Watch animation Cell Membrane Structure continued... Cell membranes have pores (holes) in them Selectively permeable: Allows some molecules in and keeps other molecules out The structure helps it to be selective 2 Types of Cellular Transport Weee!! Passive Transport Cell doesn’t use energy Goes WITH the concentration gradient high low Active Transport Cell does use energy Goes AGAINST the concentration gradient high This is gonna be hard work!! low Diffusion Diffusion is the movement of molecules from an area of high concentration to an area of low concentration Movement WITH the concentration gradient HIGH LOW Diffusion occurs until equilibrium is reached Equilibrium- when the concentration of molecules is the same throughout a space they occupy Osmosis Water is one of the small, polar substances that can easily pass through the selectively permeable plasma membrane The diffusion of water across a membrane is called osmosis Water moves from high to low concentration Isotonic (Iso = Same) The concentration of water molecules is the same inside and outside the cell. Water molecules will move across the membrane at the same rate in both directions NO NET MOVEMENT Hypertonic (Hyper = Above) Hypertonic solution = high solutue/ lower water on outside Water molecules will move out of the cell and the cell shrivels Plant cells will lose Turgor Pressure ( the pressure water molecules exert on the cell wall) Called plasmolysis Hypotonic (Hypo = Below) Lower concentration of solute on outside of the cell when compared to the inside of the cell Water molecules will move IN the cell and the cell will swell or possibly burst (cytolysis) Plant cells will gain Turgor Pressure ( the pressure water molecules exert on the cell wall) Called deplasmolysis What type of solution are these cells in? A Hypertonic B Isotonic C Hypotonic Facilitated Diffusion Helps transport molecules WITH their concentration gradient that cannot diffuse by themselves (from HIGH to LOW concentration) Carrier proteins assist these molecules with facilitated diffusion Facilitated Diffusion continued… Different carrier proteins will transport different molecules Glucose is transported this way Molecules can be moved in OR out of the cell Passive Transport Summary No energy required Molecules move from HIGH --> LOW concentration Types of Passive Transport Diffusion Osmosis isotonic vs hypertonic vs hypotonic Facilitated diffusion Diffusion through ion channels Cellular Transport…a reminder Weee!! Passive Transport Cell doesn’t use energy Goes WITH the concentration gradient high low Active Transport Cell does use energy Goes AGAINST the concentration gradient high This is gonna be hard work!! low Cell Membrane Pumps Ion channels and carrier proteins sometimes assist in active transport Requires energy Against the concentration gradient LOW to HIGH Movement in Vesicles Some substances are too large to transport nto the cell through carrier proteins So, endocytosis and exocytosis are used, which incorporate the use of vesicles. Movement in Vesicles Endocytosis is the process by which cells ingest external fluid, macromolecules, and large particles, including other cells External materials are enclosed by a portion of the cell membrane, which folds in on itself until it pinches off to form a vesicle. Pinocytosis: transport of solutes or fluids; “cell drinking” Phagocytosis: transport of large particles or whole cells; “cell eating” Movement in Vesicles Exocytosis is the process by which substances are released form the cell through a vesicle that fuses with the plasma membrane to let the substances out. Used to release large molecules like proteins, waste products, and toxins Proteins packaged in the Golgi exit the cell in this way
