Biological molecules 生物分子复习笔记

Chapter 2: Biological moleculesThe building blocks of life4 most common elements in living organisms: hydrogen, carbon, oxygen and nitrogen (in order of abundance) Organic molecules always contain carbon and hydrogenMonomers, polymers and macromoleculesMacromolecules: giant biological molecules-polysaccharides,proteins and nucleic acids (are all polymers)Polymers: giant molecules made from many similar repeatingsubunits (monomers) joined together in a chainMonomers: relatively simple molecules which are used as basicbuilding blocks for the synthesis of polymers-monosaccharides,amino acids and nucleotidesCarbohydratesAll carbohydrates contain the elements carbon, hydrogen and oxygenHydrate refers to waterGeneral formula: C x(H2O)yCarbohydrates are divided into 3 main groups: monosaccharides, disaccharides and polysaccharides Saccharide refers to a sugar or sweet substanceMonosaccharidesMonosaccharide: a molecule consisting of a single sugar unit with the general formula (CH2O)nGeneral formula: (CH2O)nMonosaccharides are sugarsClassified according to the number of carbon atoms in each molecule: trioses (3C) pentoses (5C) and hexoses (6C) Common hexoses: glucose, fructose and galactoseCommon pentoses: ribose and deoxyribose➢Molecular and structural formulaeMolecular formula for a hexose: C6H12O6Structural formula: a diagram that shows the arrangements of the atoms➢Ring structuresFor pentoses and hexoses, thechain of carbon atoms is longenough to close up on itself andform a more stable ring structureɑ-glucose: hydroxyl group, -OH,on carbon atom 1 is below theringβ-glocose: -OH is above the ringIsomers: two forms of the samechemical➢Roles of monosaccharides in living organismsTwo major functions:1.A source of energy in respiration: the carbon-hydrogen bonds can be broken to release a lot of energy, which is transferred to help make ATPGlucose-the most important monosaccharide in energy metabolism2.Building blocks for larger moleculesDisaccharides and the glycosidic bondDisaccharide: sugar molecule consisting of two monosaccharides joined together by a glycosidic bond Disaccharides are sugarsCommon disaccharides: maltose, sucrose and lactoseMaltose=glucose+glucoseSucrose (in plants) =glucose+fructoseLactose (in milk) =glucose+galactoseLactose is an important constituent of the diet of young mammalsCondensation: the process of two monosaccharides joining together with a glycosidic bond by losing waterTwo hydroxyl (-OH) groups line up alongside each otherOne combines with a hydrogen atom form the other to form a H2O moleculeGlycosidic bond: oxygen bridge between the two moleculesHydrolysis: the reverse of condensation-disaccharides or polysaccharides are broken down to monosaccharides by adding waterPolysaccharidesPolysaccharides: polymers whose subunits are monosaccharides joined together by glycosidic bondsCommon polysaccharides: starch, glycogen and cellulosePolysaccharides are NOT sugarsIf glucose accumulated in cells-dissolve and make the contents of the sells concentrated→affect osmotic properties-interfere with normal cell chemistry because glucose is reactiveSo glucose is converted into starch (in plants) and glycogen (in animals)➢Starch and glycogenStarch is a mixture of amylose and amylopectinAmylose is made by condensations between ɑ-glucose moleculesA long, unbranching chain of 1,4 linked glucose molecules--curved and coil up, a helix shape--makes the final molecule more compactAmylopectin is also made of many 1,4 linked ɑ-glucose molecules--shorter chains than in amylose--branch out to the sides, the branches are formed by 1,6 linkagesGlycogen is made of chains of 1,4 linked ɑ-glucose with 1,6 linkages forming branches--more branched than amylopectin moleculesGlycogen molecules clump together to form granules (visible in liver cells and muscle cells)CelluloseA polymer of β-glocose--as a structural role--one glucose molecule must be upside down relative to the other→this arrangement results in a strong moleculeCellulose make up about 20-40% of the average cell wallCellulose fibers:--have a very high tensile strength→very difficult to tostretch or break→makes it possible for a cell towithstand the large pressures that develop within it as aresult of osmosis--freely permeable, allowing water and solutes to reachor leave the cell membraneCell wall’s function:--provide support for the plant by making tissues rigid--responsible for cell expansion during growthWithout cell wall: burst when in a dilute solutionDipoles and hydrogen bondsWhen atoms in molecules are held together by covalent bonds, they share electrons with each otherEach shared pair of electrons forms one covalent bondHowever, the electrons are not shared absolutely equallyIn water, the oxygen atom gets slightly more than its fair share, and so has a small negative charge, written δ−(delta minus)The hydrogen atoms get slightly less than their fair share, and so have a small positive charge, written δ+ (delta plus)This unequal distribution of charge is called a dipoleHydrogen bond: the negatively charged oxygen of one molecule is attracted to a positively charged hydrogen of anotherDipoles occur in many different molecules, particularly wherever there is an –OH, –CO or –NH group. Hydrogen bonds can form between these groups,。