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Biology Chapter 7 Notes

by: Alina Hasan

Biology Chapter 7 Notes BSC 1010C

Marketplace > Valencia College > Science > BSC 1010C > Biology Chapter 7 Notes
Alina Hasan

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General Biology - BSC 1010C Membrane Structure and Function
General Biology
Professor Bouyahyaoui
Class Notes
MCAT, Biology, Biological, sciences, basic, Plasma, Lipids, phospholipids, structure, function
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This 7 page Class Notes was uploaded by Alina Hasan on Monday October 10, 2016. The Class Notes belongs to BSC 1010C at Valencia College taught by Professor Bouyahyaoui in Fall 2016. Since its upload, it has received 5 views. For similar materials see General Biology in Science at Valencia College.


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Date Created: 10/10/16
Chapter 7: Membrane Structure and Function Plasma Membrane  The plasma membrane is the boundary that separates the living cell from its surroundings. The  plasma membrane exhibits selective permeability, allowing some substances to cross it more  easily than others. Cellular membranes are fluid mosaics of lipids and proteins  Phospholipids are the most abundant lipid in the plasma membrane.  Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions.  A phospholipid bilayer can exist as a stable boundary between two aqueous compartments. The Fluidity of Membranes  Phospholipids in the plasma membrane can move within the bilayer.  Most of the lipids, and some proteins, drift laterally.  Rarely, a lipid may flip­flop transversely across the membrane.  As temperatures cool, membranes switch from a fluid state to a solid state.  The temperature at which a membrane solidifies depends on the types of lipids.  Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids.  Membranes must be fluid to work properly; they are usually about as fluid as salad oil.  The steroid cholesterol has different effects on membrane fluidity at different temperatures.  At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids.  At cool temperatures, it maintains fluidity by preventing tight packing. Evolution of Differences in Membrane Lipid Composition  Variations in lipid composition of cell membranes of many species appear to be adaptations to  specific environmental conditions  Ability to change the lipid compositions in response to temperature changes has evolved in  organisms that live where temperatures vary Membrane Proteins and Their Functions  A membrane is a collage of different proteins, often grouped together, embedded in the fluid  matrix of the lipid bilayer.  Proteins determine most of the membrane’s specific functions  Peripheral proteins are bound to the surface of the membrane Chapter 7: Membrane Structure and Function  Integral proteins penetrate the hydrophobic core   Integral proteins that span the membrane are called transmembrane proteins  The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices.  Six major functions of membrane proteins  Transport  Enzymatic activity  Signal transduction  Cell­cell recognition  Intercellular joining  Attachment to the cytoskeleton and extracellular matrix (ECM) The Role of Membrane Carbohydrates in Cell­Cell Recognition  Cells recognize each other by binding to molecules, often containing carbohydrates, on the  extracellular surface of the plasma membrane.  Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more  commonly to proteins (forming glycoproteins).  Carbohydrates on the external side of the plasma membrane vary among species, individuals,  and even cell types in an individual. Synthesis and Sidedness of Membranes  Membranes have distinct inside and outside faces.  The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma  membrane is determined when the membrane is built by the ER and Golgi apparatus. Membrane structure results in selective permeability  A cell must exchange materials with its surroundings, a process controlled by the plasma membrane.  Plasma membranes are selectively permeable, regulating the cell’s molecular traffic. The Permeability of the Lipid Bilayer  Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and  pass through the membrane rapidly.  Hydrophilic molecules including ions and polar molecules do not cross the membrane easily. Chapter 7: Membrane Structure and Function Transport Proteins  Transport proteins allow passage of hydrophilic substances across the membrane.  Some transport proteins, called channel proteins, have a hydrophilic channel that certain  molecules or ions can use as a tunnel.  Channel proteins called aquaporins facilitate the passage of water.  Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle  them across the membrane.  A transport protein is specific for the substance it moves.  Passive transport is diffusion of a substance across a membrane with no energy investment  Diffusion is the tendency for molecules to spread out evenly into the available space.  Although each molecule moves randomly, diffusion of a population of molecules may be  directional.  At dynamic equilibrium, as many molecules cross the membrane in one direction as in  the other.  Substances diffuse down their concentration gradient, the region along which the density of a  chemical substance increases or decreases.  No work must be done to move substances down the concentration gradient.  The diffusion of a substance across a biological membrane is passive transport because no  energy is expended by the cell to make it happen. Effects of Osmosis on Water Balance  Osmosis is the diffusion of water across a selectively permeable membrane.  Water diffuses across a membrane from the region of lower solute concentration to the region of  higher solute concentration until the solute concentration is equal on both sides. Water Balance of Cells Without Cell Walls  Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.  Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane.  Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water.  Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water.  Hypertonic or hypotonic environments create osmotic problems for organisms. Chapter 7: Membrane Structure and Function  Osmoregulation, the control of solute concentrations and water balance, is a necessary  adaptation for life in such environments.  The protist Paramecium, which is hypertonic to its pond water environment, has a contractile  vacuole that acts as a pump. Water Balance of Cells with Cell Walls  Cell walls help maintain water balance.  A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid  (firm).  If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp).  In a hypertonic environment, plant cells lose water   The membrane pulls away from the cell wall causing the plant to wilt, a usually lethal effect  called plasmolysis Facilitated Diffusion: Passive Transport Aided by Proteins  In facilitated diffusion, transport proteins speed the passive movement of molecules across the  plasma membrane.  Transport proteins include channel proteins and carrier proteins.  Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane.  Aquaporins facilitate the diffusion of water.  Ion channels facilitate the diffusion of ions.  Some ion channels, called gated channels, open or close in response to a stimulus.  Carrier proteins undergo a subtle change in shape that translocates the solute­binding site across  the membrane. Active transport uses energy to move solutes against their gradients  Facilitated diffusion is still passive because the solute moves down its concentration gradient,  and the transport requires no energy  Some transport proteins, however, can move solutes against their concentration gradients The Need for Energy in Active Transport  Active transport moves substances against their concentration gradients  requires energy, usually in the form of ATP Chapter 7: Membrane Structure and Function  performed by specific proteins embedded in the membranes  allows cells to maintain concentration gradients that differ from their surroundings  The sodium­potassium pump is one type of active transport system How Ion Pumps Maintain Membrane Potential  Membrane potential is the voltage difference across a membrane  Voltage is created by differences in the distribution of positive and negative ions across a  membrane  Two combined forces, collectively called the electrochemical gradient, drive the diffusion of  ions across a membrane  A chemical force (the ion’s concentration gradient)  An electrical force (the effect of the membrane potential on the ion’s movement)  An electrogenic pump is a transport protein that generates voltage across a membrane  The sodium­potassium pump is the major electrogenic pump of animal cells  The main electrogenic pump of plants, fungi, and bacteria is a proton pump  Electrogenic pumps help store energy that can be used for cellular work Cotransport: Coupled Transport by a Membrane Protein  Cotransport occurs when active transport of a solute indirectly drives transport of other  substances   Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active  transport of nutrients into the cell Bulk transport across the plasma membrane occurs by exocytosis and endocytosis  Small molecules and water enter or leave the cell through the lipid bilayer or via transport  proteins  Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles  Bulk transport requires energy Exocytosis Chapter 7: Membrane Structure and Function  In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents  outside the cell  Many secretory cells use exocytosis to export their products Endocytosis  In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane  Endocytosis is a reversal of exocytosis, involving different proteins  There are three types of endocytosis  Phagocytosis (“cellular eating”)  Pinocytosis (“cellular drinking”)  Receptor­mediated endocytosis  In phagocytosis a cell engulfs a particle in a vacuole  The vacuole fuses with a lysosome to digest the particle  In pinocytosis, molecules dissolved in droplets are taken up when extracellular fluid is “gulped”  into tiny vesicles  In pinocytosis, molecules dissolved in droplets are taken up when extracellular fluid is “gulped”  into tiny vesicles  In receptor­mediated endocytosis, binding of ligands to receptors triggers vesicle formation  A ligand is any molecule that binds specifically to a receptor site of another molecule Chapter 7: Membrane Structure and Function


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