Cell Membrane Structure and Chapter 5 Function 5 1 How Is the Structure of a Membrane Related to Its Function Functions of the plasma membrane 1 Isolates cell contents from external environment 2 Regulates the exchange of essential substances 3 Allows communication between cells 4 Creates attachments within and between cells 5 Regulates biochemical reactions Membranes are fluid mosaics in which proteins move within layers of lipids The fluid mosaic model of a membrane was proposed in 1972 by singer and Nicolson mosaics or a patchwork of different proteins Selectively permeable Allows materials to move in out of cell The Plasma Membrane extracellular fluid outside extra cellular matrix carbohydrate protein glycoprotein attachment protein binding site receptor protein phospholipid phospholipid bilayer pore transport protein cholesterol recognition protein enzyme cytoplasm inside Fig 5 1 cytoskeleton The phospholipid bilayer is the fluid portion of the membrane Plasma membranes face both exterior and interior watery environments Polar hydrophilic heads face outside inside cell Nonpolar hydrophobic tails face the inside of the membrane extracellular fluid watery environment Fig 5 3 cytoplasm watery environment Author Animation Plasma Membrane Structure The phospholipid bilayer is the fluid portion of the membrane continued The phospholipid bilayer s flexible fluid membrane allows for cellular shape changes Individual phospholipid molecules are NOT bonded to each other Some phospholipids have Unsaturated fatty acids that cause kinks into their tails The kinks make the membrane fluid Kinks Increase Fluidity DOUBLE TRIPLE BONDS MOSTLY SINGLE BONDS more fluid less fluid Fig 5 4 The phospholipid bilayer is the fluid portion of the membrane continued O2 CO2 small nonpolar molecules some water molecules Glucose large polar water soluable molecules ions most water molecules Author Animation Cell Membranes The phospholipid bilayer is the fluid portion of the membrane continued Membranes become more fluid at high temperatures more movement and less fluid at low temperatures less movement 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 Recognition proteins name tag 2 Enzymatic proteins 3 Attachment proteins 4 Transport proteins 5 A variety of proteins form a mosaic within the membrane continued 1 Receptor proteins trigger cellular responses upon binding of specific molecules hormones sent by others 2 Recognition proteins are glycoproteins that seve as indentification tags on the surface of a cell Receptor Protein Activation extracellular fluid hormone receptor 1 A hormone binds to the receptor 2 Hormone binding activates the receptor changing its shape 3 The activated receptor stimulates a response in the cell cytoplasm Fig 5 5 A variety of proteins form a mosaic within the membrane continued 3 Enzymatic proteins are proteins that promote chemical reactions that synthesize or break apart molecules 4 Attachment proteins anchor the cell membrane to the inner cytoskeleton to proteins outside of the cell and to other cells A variety of proteins form a mosaic within the membrane continued 5 Transport proteins regulate the movement of hydrophilic molecules through the plasma membrane Channel proteins form channels to allow specific ions or water molecules to pass through the membrane 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 A fluid is a substance whose molecules can flow past one another have no defined shape A solute is a substance that can be dissolved atoms ions or molecules in a solvent A solvent is a fluid capable of dissolving a solute The concentration of a substance defines the amount of solute in a given amount of solvent A gradient is a physical difference in temp pressure charge or concentration between two adjoining regions of space Molecules in fluids move in response to gradients continued Gradients cause molecules to move from one place to another Molecules and ions in solution are in constant random motion An increase in temperature increases the rate of this random motion Results in net movement from regions of high concentration to regions of low concentration by a process called diffusion Diffusion of a Dye in Water 1 A drop of dye is placed in water Dye molecules 2 diffuse into the water water molecules diffuse into the dye 3 Both dye molecules and water molecules are evenly dispersed drop of dye water molecule Fig 5 6 Movement through membranes occurs by A Passive transport is movement of substances across cell membranes down concentration gradients no energy required B Energy requiring transport requires the use of cellular ENERGY Table 5 1 Passive transport includes 1 Simple diffusion 2 3 Facilitated diffusion Osmosis extracellular fluid O2 phospho lipid bilayer Cl water glucose cytoplasm channel protein a Simple diffusion through the phospholipid bilayer b Facilitated diffusion through channel proteins aquaporin c Osmosis through aquaporins or the phospholipid bilayer carrier protein d Facilitated diffusion through carrier proteins Fig 5 7 Simple diffusion Substances move down their concentration gradients across a membrane TYPO This animation should say simple diffusion Facilitated diffusion substances move down their concentration gradients with the help of channel and carrier transport proteins Osmosis diffusion of water across selectively permeable membranes from high concentration to low concentration It is all about the concentration of water molecules relative to other molecules Isotonic solutions have equal concentrations of water and dissolved substances No net water movement occurs across the membrane Hypertonic solution is one with a greater solute concentration Water moves across a membrane toward the hypertonic solution Hypotonic solution has a lower solute concentration Water moves across a membrane away from the hypotonic solution The Effect of Solute Concentration on Osmosis No net flow of water Water flows out the balloon shrinks Water flows in the balloon swells a A balloon in an isotonic solution b A balloon in a hypertonic solution c A balloon in a hypotonic
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