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Chapter 5


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BIOL 1000
Julie Clark

Tanya Sivamanoharan Fri, Nov, 4/11 Biology- chap#5 notes MEMBRANES AND TRANSPORT 5.1- An Overview of the Structure of Membranes -key to evolution of life was plasma membrane -plasma membrane selectively permeable barrier for cell, that uptakes import. minerals and gets rid of cell waste and keeps protected environment for metabolism to take place -development of internal membranes allowed for splitting processes and increased complexity (ex: nuclear envelope to protect nucleus of cell) 5.1a- The Fluid Mosaic Model of Membranes -fluid mosaic model membranes are not rigid with molecules embedded with proteins float around rather than being locked in place -lipid molecules of membrane exist in a double layer -lipid molecules move in various ways on a certain side of the double layered membrane and exchange places within a layer million times a second -therefore the lipid molecules in the membrane are known to be dynamic -maintaining fluidity in membrane is highly import. to overall membrane function -mosaic means membranes contain wide assortment of diff. types of proteins, with specific functions -membrane asymmetry proteins and other components on one side of the membrane bilayer is diff. from the components of the other side of the lipid bilayer - each side of the bilayer perform diff. functions of the cell -receptor proteins found on external surface of plasma membrane where hormones and growth factors bind to 5.1b- Experimental Evidence in Support of the Fluid Mosaic Model -fluid mosaic model of membrane structure is supported by two factors: -membranes are fluid: -human proteins were linked to red dyed molecules and mice proteins were linked to green dyed molecules. When the two membranes were fused the green and red dye (molecules) had exchanged and moved around between the fused membranes -membrane asymmetry -after splitting bilayer into two components (the outer and inner layers) and when studying the components attached to each side of the membrane they differ in size, number, and shape, providing evidence that the 2 sides are distinctly diff. 5.2- The Lipid Fabric of a Membrane -foundation or underlying fabric of all biological membranes is the lipid molecules -keeping membranes in fluid state is import, to membrane function 5.2a-Phospholipids Are the Dominant Lipids in Membranes -phospholipids dominant lipids found in membranes, consist of 2 fatty acid tails linked by phospholipids to an alcohol or amino acid -a property of phospholipids is that they are amphipathic -amphipathic each phospholipid molecule contains region that is hydrophobic (nonpolar) and hydrophilic (polar) -the tail of the phospholipid is hydrophobic (nonpolar) and the head is hydrophilic (polar) -phospholipids react in an aqueous solution and form a bilayer 5.2b- Membrane Fluidity -the fluidity of the lipid bilayer depends on 2 factors: -composition of lipid molecule that makes up membrane -saturated fatty acid tails allows lipids to pack more tightly -unsaturated fatty acids cause kinks or bends in tail and causes lipid molecules to pack together less closely -the temperature -membranes remain fluid over a wide range of temperature -if temp. goes too low the phospholipids become closely packed and membrane forms a highly viscous semisolid gel -unsaturated more fluid -saturated less fluid -the normal fluid state of membrane is achieved by saturated and unsaturated fatty acids 5.2c- Organisms Can Adjust Fatty Acid Composition -maintaining membranes in a fluid state is essential to cell function -low temp. cause higher viscosity where the membrane may solidify -if membrane solidify it will prevent the ETC from operating b/c the ETC requires fast movement of components -high temperatures cause membranes to become too fluid and cause increased motion of molecules causing leakage -ions will start to diffuse across the membrane freely, disrupting ion balance causing cell death -animals that can live in extremely low environments have the ability to increase the proportion of unsaturated fatty acids in their membranes so it does not solidify -desaturasesgroup of enzymes that produce unsaturated fatty acids by fatty acid synthesis -all fatty acids are originally saturated that are later altered by desaturases enzymes remove hydrogen atoms making double bonded carbons -each double bond in a fatty acid is altered by a specific. desaturases enzyme -besides lipids, a group of enzymes called sterols influence membrane fluidity -sterols include cholesterol, which are found in animals and not plants or prokaryotes -sterols act as membrane buffers: -at high temp. they help they restrain movement of lipid molecules therefore reducing fluidity of membrane -at low temp. sterols disrupt fatty acids and take up space between lipid molecules causing a slower transition to the solid state of the membrane 5.3- Membrane Proteins -lipids are backbone of a membrane but proteins of the membrane determine its function and makes each membrane unique 5.3a- The Key Functions of Membrane Proteins -membrane proteins can is divided into 4 main functional categories: -all 4 functions may exist in a single membrane and a single protein or protein complex may carry out more than 1 of the 4 functions: -transport many membranes cannot move freely through the membrane, so a protein can act as a hydrophilic channel to allow movement of specific compounds. Transport proteins can change shape and move molecules from one side of membrane to the other -enzymatic activity # of enzymes are membrane proteins, like enzymes associated with the respiratory and ETC -signal transduction membranes have receptor proteins on the outer layer to bind specific chemicals (hormones). These receptor proteins trigger changes in interior of cell -attachment/recognition proteins exposed to both interior and exterior of cell where cytoskeleton attaches to, and other components in cell-cell recognition -all membranes are either peripheral or integral proteins 5.3b- Integral Membrane Proteins -integral membrane proteins proteins embedded to phospholipid bilayer -most integral proteins are transmembrane proteins, that surround entire cell, therefore have regions exposed to both sides of the membrane -integral proteins have regions that are non-polar amino acids in order to interact with hydrophobic core 5.3c- Peripheral Membrane Proteins -2 major group of proteins are called peripheral proteins -peripheral proteinspositioned on surface of membrane, and does not interact with hydrophobic core of membrane -peripheral proteins are held to membrane surface by noncovalent bonds (hydrogen and ionic bonds) -most peripheral proteins are on cytosolic side of membrane -some peripheral proteins apart of cytoskeleton (microtubules, microfilaments, intermediate filaments, or proteins that hold cytoskeleton together) 5.4- Passive Membrane Transport -membrane considered cell’s gatekeeper, controlling what comes in and out -hydrophobic nature prevents unnecessary movement of molecules and substances -molecules such as oxygen can diffuse through membrane quickly -ions/charged molecules and macromolecules like sugar and protein don’t diffuse as quickly and easily 5.4a- Passive Transport is based on Diffusion -passive transport movement of a substance across a membrane without need to expend chem. energy such as ATP -passive transport takes place by diffusion, from high to low concentration -diffusion above absolute temp. (-273’c), molecules are in constant motion until uniformly distributed (equilibrium) -diffusion is primary method of solute movement within cell and cellular compartments separated by membranes -increase in entropy results as a result of diffusion -when one side has higher concentration than the other, there is low entropy but when diffusion takes place, entropy increases until molecules are distributed evenly -the rate of diffusion depends concentration difference/gradient -the bigger gradient ,the faster rate of diffusion -even after equilibrium is reached, molecules/io
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