Chapter 5 Cell Membrane Structure and Function 5 1 How Is the Structure of a Membrane Related to Its Function Functions of the plasma membrane o Isolates cell contents from external environment o Regulates the exchange of essential substances o Allows communication between cells o Creates attachments within and between cells o Regulates biochemical reactions Membranes are fluid mosaics in which proteins move within layers of lipids Fluid Mosaic Model Singer and Nicholson o mosaics or a patchwork of different proteins o selectively permeable o allows materials to move in out of cell o Phospholipids and proteins move from place to place within the bilayer Phospholipid bilayer fluid part of plasma membrane o Face both exterior and interior watery environments o Polar hydrophilic heads face outside and inside cell o Nonpolar hydrophobic tails face the inside of the membrane o Flexible allows for cellular shape changes o Individual phospholipid molecules are NOT bonded to each other o Some phospholipids have unsaturated fatty acids that causes kinks into their tails o Double triple bonds in tails of phospholipid bilayer more fluid o Single bonds in tails of phospholipid bilayer less fluid o Kinks make the membrane fluid o Membranes become more fluid at high temperature more movement o Membranes become less fluid at low temperatures less movement o Cholesterol stabilizes membranes affecting fluidity and reducing permeability A variety of proteins form a mosaic within the membrane Proteins are embedded within or attached to the phospholipid bilayer 1 Receptor proteins trigger cellular responses upon binding of specific molecules hormones sent by other cells 2 Recognition proteins name tag are glycoproteins that serve as identification tags on 3 Enzymatic proteins are proteins that promote chemical reactions that synthesize or 4 Attachment proteins anchor the cell membrane to the inner cytoskeleton to proteins 5 Transport proteins regulate the movement of hydrophilic molecules through the the surface of a cell break apart biological molecules outside the cell and to other cells plasma membrane through the membrane o Channel proteins form channels to allow specific ions or water molecules to pass o Carrier proteins have binding sites that can temporarily attach to specific molecules on one side of the membrane and then move them through the membrane to the other side 5 2 How Do Substances Move Across Membranes Molecules in fluids move in response to gradients o A fluid is a substance whose molecules can flow past one another have no defined shape o A solute is a substance that can be dissolved atoms ions or molecules in a solvent o A solvent is a fluid capable of dissolving a solute o The concentration of a substance defines the amount of solute in a given amount of solvent o A gradient is a physical difference in temperature pressure charge or concentration between two adjoining regions of space o Gradients cause molecules to move from one place to another o Molecules and ions in solution are in constant random motion o An increase in temperature increases the rate of this random motion Diffusion net movement from regions of high concentration to regions of low concentration 2 types of movement thru membranes o Passive transport is movement of substances across cell membranes down concentration gradients no energy required o Energy requiring transport requires the use of cellular ENERGY 3 types of Passive transport 1 Simple diffusion substances move down their concentration gradients across a membrane evenly distributed 2 Facilitated diffusion substances move down their concentration gradients with the help of channel and carrier transport proteins o A charged ion Ex Chloride or molecule cannot diffuse down its concentration gradient without a channel to pass it thru the hydrophobic portion of the membrane 3 Osmosis diffusion of water across selectively permeable membranes from high concentration to low concentration o Isotonic solutions have equal concentrations of water and dissolved substances No net water movement occurs across the membrane o Hypertonic solution is one with a greater solute concentration Water moves across a membrane toward the hypertonic solution o Hypotonic solution has a lower solute concentration Water moves across a membrane away from the hypotonic solution o When cells are placed into a hypertonic solution they shrink water loss o When cells are placed into a hypotonic solution they swell water entry o Cells in isotonic solutions remain unaffected Osmosis explains the flow of water from the cytoplasm of plants into their central vacuole Water pressure within the central vacuole is called turgor pressure This pressure provides supports support Pushes cytoplasm against cell well and helps maintain cell s shape 3 types of Facilitated AKA Energy requiring transport 1 Active transport proteins use energy to move substances across plasma membranes against up their concentration gradients protein pumps 2 Endocytosis cells engulf particles and transport them using vesicles Also against gradient o Pinocytosis cell drinking moves liquids into the cell o Phagocytosis cell eating moves large particles into the cell 3 Exocytosis moves material out of the cell uses energy and vesicles to dispose of waste products As a spherical cell enlarges its innermost parts get farther away from the plasma membrane o Once a cell gets too large diffusion which is relatively slow can take too long to supply important processes deep within the cell 5 3 How Do Specialized Junctions Allow Cells to Connect and Communicate Desmosomes attach cells together o Found where cells need to adhere tightly together under the stresses of movement i e small intestine o Allows for flexible attachments among cells Tight junctions make cell attachments leak proof o Found where tubes and sacs must hold contents without leaking i e urinary bladder Gap junctions animals and plasmodesmata plants allow direct communication between cells Small charged ions must pass thru membranes via pores formed by the membrane proteins
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