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Plasma membrane Cell membrane Phospholipid bilayer Outside of cell Proteins Carbohydrate chains Cell membrane Inside of cell (cytoplasm) Protein channel Lipid bilayer Cell membranes are mosaics of different molecules combined. Selective permeability • The plasma membrane enables only certain molecules to enter or exit the cell. • Why is this important? Selective transport 1. Diffusion 2. Facilitated diffusion 3. Active transport Chemicals usually move across the cell membrane from areas of higher to lower concentration. 1. Diffusion: simple movement from HL 2. Osmosis: water diffusion 3. Facilitated or passive diffusion: movement of chemicals through proteins from H-L Glucose molecules High Concentration Cell Membrane Low Concentration Protein channel Diffusion video Click on image to play video. Diffusion across a membrane Osmosis • Water diffusion from higher to lower concentration • When solutions of varying water concentrations are found across a membrane, the solutions are given names. – Hypertonic – Hypotonic – Isotonic Osmosis across a membrane Achieving equilibrium Osmosis video Click on image to play video. Solutions • Hypertonic solutions have less water than the solution across the membrane. • Hypotonic solutions have more water than the solution across the membrane. • Isotonic solutions have the same water concentration as the solution across the membrane. Cells in various solutions Hypotonic LYSE Isotonic NORMAL Hypertonic animal PLASMOLYZE Plant, fungus, algae, bacteria TURGID FLACCID Channel vs. Carrier Proteins Molecule to be carried Active transport • Moving chemicals in and out of a cell, past the plasma membrane, using energy and a carrier protein • Chemicals move from areas of lower concentration to areas of higher concentration. Energy Molecule being carried Active transport video Click on image to play video. Example of active transport Exporting and importing materials Click on image to play video. Surface to volume ratio • Because materials must be transported across cell membranes, maximizing the amount of membrane surface area increases cellular activity. • This is why larger organisms are multicellular. Comparing surface area to volume • Surface area: 6 mm x 6 mm x 6 sides = 216 mm2 • Volume: 216 mm3 • S/T = 1 • Surface area: 3 mm x 3 mm x 6 sides x 8 cubes = 432 mm2 • Volume = 216 mm3 • S/T = 2
