Class Notes (809,199)
Canada (493,574)
Biology (2,348)
BIOLOGY 1A03 (716)

Biology Chapter 6.docx

13 Pages
Unlock Document

McMaster University
Lovaye Kajiura

Biology Chapter 6: Lipids, Membranes and the First Cells Bernard Ho September 21, 2010 Lipids − General term for carbon-containing compounds that are found in organisms and are largely non-polar and hydrophobic − Do dissolve in liquids consisting of non-polar organic compounds − Do not dissolve in water because they have a significant hydrocarbon component and electrons are shared equally in carbon-hydrogen bonds, making them nonpolar − Are defined by their physical property (solubility) instead of their chemical structure − As a result, structure of lipids varies widely − Types of lipids o Fats  Composed of three fatty acids that are linked to a three carbon molecule called glycerol  Also called triglycerides  Form when a dehydration reaction occurs between a hydroxyl group of glycerol and the carboxyl group of a fatty acid  Glycerol and fatty acid molecules become joined by an ester linkage  Ex. Cholesterol • Distinguished by hydrocarbon tail formed of isoprene subunits • An important component of plasma membranes in many organisms • Estrogen, progesterone and testosterone are examples of hormones derived from cholesterol o Phospholipids  Consist of glycerol linked to a phosphate group and to either two chains of isopropene or two fatty acids  In some cases phosphate group is bonded to another small organic molecule, such as choline  Phospholipids with isopropene tails are found in domain Archaea  Phospholipids composed of fatty acids are found in domains Bacteria and Eukarya o Steroids o RWE  Trans fats – why they are bad for us • Partially hydrogenated oils • Triglycerides, naturally unsaturated fatty acid tails have been chemically altered to be more saturated by adding H atoms (hydrogenation) • Used to maintain semi-solid texture and flavour longer • Can cause o Cardiovascular disease (increased LDL cholesterol levels, which solidifies arterial plaques) o Membrane integrity (will incorporate into membranes, which decreases fluidity and changes structure, thus changing function) - o Free radical damage (damaging molecules missing e , trans fats increases free radicals) − Structure of membrane lipids o Just two types of lipids are usually found in plasma membranes o Membrane-forming lipids have a polar, hydrophilic region in addition to the non-polar, hydrophobic region found in all lipids o Charges and polar bonds in the head region interact with water molecules when a phospholipid is placed in solution o In contrast, long isopropene or fatty-acid tails of a phospholipid are nonpolar o Water molecules cannot form hydrogen bonds with hydrocarbon tail, so they do not interact with this part of the molecule o Compounds that contain both hydrophilic and hydrophobic elements are amphipathic o Phospholipids and cholesterol are amphipathic − Why membranes are important o Keep reaction inside cell o Greater efficiency of reaction and greater concentration of reactants o Highly selective o Separates living and non-living o Compartmentalization  Compartments may contain different enzymes, produce different products and may increase metabolic capacity Phospholipid Bilayers − Phospholipids do not dissolve in water, instead they form two types of structures, micelles or lipid bilayers − Micelles o Tiny droplets created when the hydrophilic heads of phospholipids face the water and hydrophobic tails are forced together, away from the water o Lipids with compact tails tend to form micelles o Because their double chain tails are often too bulky to fit into the interior of a micelle, most phospholipids tend to form bilayers − Phospholipid bilayers o Created when two sheets of phospholipid molecules align o Hydrophilic heads in each layer face a surrounding solution while the hydrophobic tails face one another inside the bilayers o In this way hydrophilic heads interact with water, while hydrophobic tails interact with each other − Micelles tend to form from phospholipids with relatively short tails, bilayers tend to form from phospholipids with longer tails − Both structures form spontaneously as they are much more stable energetically than are independent phospholipids in solution − Independent phospholipids are unstable because their hydrophobic tails disrupt hydrogen bonds that otherwise would form between water molecules − Loss of energy outweighs decrease in entropy − Overall, free energy of system decreases − Lipid bilayers formation is exergonic and spontaneous − Artificial membranes o When lipid bilayers are agitated by shaking, layers break and reform as small, spherical structures o Resulting vesicles had water on inside as well as outside because hydrophilic heads of lipids faced outward on each side of bilayers o Artificially created membrane-bound vesicles are called liposomes − Selective permeability of lipid bilayers o Lipid bilayers are highly selective o Selective permeability means that some substances cross a membrane more easily than other substances o Small, nonpolar molecules move across bilayers quickly o Large molecules and charged substances cross the membrane slowly, if not at all o Best explanation so far is that charged compounds and large, polar molecules can’t pass through the nonpolar, hydrophobic tails of a lipid bilayers o Because of electrical charge, ions are more stable in solution where they form hydrogen bonds with water than they are in interior of membranes, which is electrically neutral − Type of lipid in a membrane affects permeability o Two aspects of a hydrocarbon chain  Number of double bonds • When carbon atoms form double bonds, attached atoms are found in a plane instead of a tetrahedron • Carbon atoms involved are locked into place and cannot rotate freely • As a result, a kink is produced • Chain is said to be unsaturated • Kinks created by double bonds produce spaces among the tightly packed tails • These spaces reduce the strength of hydrophobic interactions among tails • Because interior of membrane is held together less tightly, structure should become more fluid and more permeable  Length • Membranes dominated by phospholipids with long, saturated hydrocarbon chains should be stiffer and less permeable because interactions among tails are stronger  Phospholipids with long, saturated tails form membranes that are much less permeable than membranes consisting of phospholipids with shorter, unsaturated tails  Lipids that have extremely long hydrocarbon tails form solids at room temperature due to extensive hydrophobic interactions that occur  Highly unsaturated fats are liquid at room temperature − Temperature affects fluidity and permeability of membranes o Fluidity as well as membrane’s permeability decreases as temperature decreases o As temperature drops, individual molecules move more slowly o As a result, hydrophobic tails in interior of membranes pack more tightly o At very low temperatures, lipid bilayers begin to solidify − Absence or presence of cholesterol affects permeability of membranes o At moderate temperatures, cholesterol restrains prokaryote movement, which reduces membrane fluidity o At low temperatures, cholesterol prevents the packing of phospholipids, which enhances membrane fluidity and prevents solidification − These experiments on lipid and ion movement demonstrate that membranes are dynamic − Phospholipid molecules whiz around each layer, while water and small nonpolar molecules shoot in and out of membranes − How quickly molecules move within and across membranes is a function of temperature and structure of hydrocarbon tails in the bilayers Why Molecules Move across Lipid Bilayers − Movement of molecules and ions that results from their kinetic energy is known as diffusion − Diffusion o Because solutes change position randomly due to diffusion, they tend to move from a region of high concentration to a region of low concentration (down a concentration gradient) o A difference in solute concentrations creates a concentration gradient o Molecules and ions still move randomly in all directions when a concentration gradient exists, but there is a net movement from regions of high to low concentration o Diffusion across a concentration gradient is a spontaneous process because it results in an increase in entropy o Once molecules or ions are randomly distributed throughout a solution, equilibrium is established o At equilibrium, molecules continue to move back and forth across membrane, but at equal rates o Passive process − Osmosis o Spontaneous passive process and is the diffusion of water across a selectively permeable membrane o Like other substances that diffuse, water moves along its concentration gradient, from low solute concentration to high solute concentration o Can also say water diffuses from a region of high [free 2 O molecules] to low [free 2 O molecules] o Movement of water is a special case of diffusion that is called osmosis o Osmosis occurs only when solutions are separated by a membrane that is permeable to some molecules, but not others (selectively permeable membrane) o Movement is spontaneous, driven by entropy achieved when solute concentrations are equal on both sides of membrane o Movement of water by osmosis is important because it can swell or shrink a membrane-bound vesicle o If solution outside a membrane has a higher concentration of solutes than the interior has and the solutes cannot pass through the bilayer, water will move out of the vesicle into the solution outside o As a result, the vesicle will shrink and membrane shrivels o Such a solution is said to be hypertonic o Conversely, if solution outside membrane
More Less

Related notes for BIOLOGY 1A03

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.