AS Biology 9700 Crash Course: Biological Molecules PDF

# AS Biology 9700 Crash Course: Biological Molecules ## Biological Molecules - The four most common elements in living organisms are: carbon, hydrogen, oxygen, and nitrogen - **Classification of biological molecules:** - Carbohydrates (C, H, O) - Proteins (C, H, O, N) - Lipids (C, H, O...

# AS Biology 9700 Crash Course: Biological Molecules ## Biological Molecules - The four most common elements in living organisms are: carbon, hydrogen, oxygen, and nitrogen - **Classification of biological molecules:** - Carbohydrates (C, H, O) - Proteins (C, H, O, N) - Lipids (C, H, O) - Nucleic acids (C, H, O, N, P) (RNA and DNA) - Water ($H_2O$) - **Macromolecule:** A very large molecule made up of simpler molecules - **Polymer:** A large molecule made up of small repeating units called *monomers* - **Monomer:** A small molecule that is the repeating unit of a polymer - **All polymers are macromolecules, but not all macromolecules are polymers.** ## Carbohydrates - All carbohydrates contain carbon, hydrogen, and oxygen. - The “hydrate” part of the name refers to water as hydrogen and oxygen are present in a 2:1 ratio, similar to the water molecule. - General formula of carbohydrates: $C_x(H_2O)_y$ - Carbohydrates are divided into 3 main groups: - **Monosaccharides** - **Hexoses (6 carbon atoms):** Glucose, Galactose - **Pentoses (5 carbon atoms):** Ribose, Deoxyribose - **Trioses (3 carbon atoms)** - **Disaccharides** - Sucrose - Maltose - Lactose - Cellubiose - **Polysaccharides** - Glycogen - Starch - Cellulose ### Monosaccharides #### Glucose - **α-Glucose** - **β-Glucose** - α → below - β → above ### Disaccharides (condensation reactions ) - Formed by the joining of 2 monosaccharides - **Glycosidic bond:** C-O-C link between 2 sugar molecules; covalent bond - Maltose (α glucose + α glucose) - Sucrose (α glucose + fructose) - Lactose ### Ring form of Fructose - Cottage shaped ### Sucrose - α glucose and fructose ### Cellulose - Disaccharide (β glucose + β glucose) - β-glucose would rotate/flip by 180º ### Reducing Sugars - So called because they can carry out a type of chemical reaction known as “reduction.” - In the process, they themselves are “oxidized.” - The reducing sugars include all monosaccharides and some disaccharides. - Sucrose is *NOT* a reducing sugar. #### Benedict’s Test - The test uses Benedict’s reagent which is a copper (II) sulfate in an alkaline solution. It is blue in colour. - Reducing sugars reduce the soluble blue copper (II) sulfate to insoluble brick-red copper oxide, containing copper (I). - Reducing Sugar + $Cu^{2+}$ (Benedict's solution) → Oxidized Sugar + Cu (red-brown ppt.) ## Polysaccharides - Made by joining many monosaccharide molecules by condensation - **Non-reducing sugars:** - Starch - Glycogen - Cellulose - Glucose is also a reactive molecule and would interfere with normal cell chemistry. - These problems are avoided when glucose is converted by condensation reactions to a storage polysaccharide. - The storage polysaccharide is a convenient, compact, inert (unreactive) and insoluble molecule. - The storage polysaccharide in plants is starch; in animals, it is glycogen. - When needed, glucose is quickly made available again by enzyme-controlled hydrolysis reactions. - Glucose → Osmotically active - Glucose (solute / solid) - ↑ solute conc. WPY → Osmosis H2O ### Starch (α - glucose) - Amylose - α-glucose monomers - Helical structures - α-1, 4- glycosidic bonds - Unbranched - More resistant to digestion - Relatively lower Mr (less no. of monomers) - Stains blue-black with iodine solution - Amylopectin - α-glucose monomers - Non-helical structure - α-1, 4- glycosidic bonds & α-1, 6- glycosidic bonds - Branched - Less resistant to digestion because it has more free ends because of branching. - Relatively greater Mr - More water soluble - Amylopectin does not cause color change of the iodine solution. ### Glycogen - α-glucose monomers - Storage polysaccharide - Present in animal and fungal cells - Similar to the structure of amylopectin **Similarities with amylopectin:** - Branched - α-1, 4- glycosidic bond - α-1, 6- glycosidic bond **Differences with amylopectin:** - More branching in glycogen - Mr is greater than amylopectin - Glycogen is less resistant to digestion than amylopectin ### Cellulose - Structural support - Present in plant cell wall. - β-glucose monomers - Cellulose molecule is a linear, unbranched chain of β glucose monomers where every alternate β glucose monomer is rotated by 180º. - **Cellulose microfibril:** 60-70 cellulose molecules combine through hydrogen bonds to form cellulose microfibrils. - **Cellulose fibers:** Numerous cellulose microfibrils combine through hydrogen bonds to form cellulose fibers. - High tensile strength - **Cellulose (β-glucose monomers)** - Most abundant organic molecule - Structural role → Mechanically strong molecule - General formula: $(C_6H_{10}O_5)_n$ - $n$ → number of monosaccharides joining - $nC_6H_{12}O_6$ → $(C_6H_{10}O_5)_n$ + (n-1) $H_2O$ ## Lipids - **Macromolecule:** C, H, O - Not a polymer because they are not composed of repeating units of the same molecules or atoms. - They are organic compounds which are insoluble in water but are soluble in organic solvents. - Most lipids are formed by fatty acids combining with an alcohol. ### Triglycerides - Made up of 3 molecules of fatty acids and 1 molecule of glycerol - **Glyceride:** An ester formed by a fatty acid combining with glycerol (alcohol) **Fatty acids** - **Saturated:** (C-C) - **Unsaturated:** (C=C) - **Monounsaturated:** If one double bond is present - **Polyunsaturated:** If more than one double bond is present - **Double bonds make fatty acids or lipids melt more easily.** ### Phospholipids - Amphipathic nature — consist of polar and non-polar parts. - **Hydrophilic phosphate head:** soluble (water loving) - **Hydrophobic fatty acid tails:** insoluble (water hating) - Membranes ## Proteins - Made up of amino acids ### Structure of an Amino Acid - **R group:** This can vary for every amino acid - **Amine (amino) group:** - **Carboxylic acid group:** - **Zwitter ion:** Electrically neutral ### Peptide Bonds - **Carboxylic acid + amino** - **Peptide bond:** ## Levels of Protein Folding - **Primary structure:** The sequence of amino acids in a polypeptide chain (peptide bonds) - **Secondary structure:** Regular folding of a polypeptide chain in an α-helix or β-pleated sheet which results due to hydrogen bonding between the amino acids in a chain. - **α-helix:** Helical - **β-sheet:** Ribbon / arrow shaped - **Tertiary structure:** Regular 3D folding of a polypeptide chain due to hydrogen bonds (b/w polar R groups), hydrophobic interactions (b/w non-polar R groups), ionic bonding (b/w charged R groups) and disulfide linkages (b/w 2 cysteine amino acids & groups). - **Quaternary structure:** Forms when 2 or more polypeptide chains (of a protein) associate via hydrogen bonds, hydrophobic interactions, ionic bonds, and for disulfide linkages. ## Types of Proteins - **Globular (soluble)** - Hydrophilic R groups pointing outwards making the protein water soluble. - Spherical shape - Sensitive to pH and temperature changes - **Examples:** Enzymes, Haemoglobin - **Fibrous (insoluble)** - Hydrophobic R groups pointing outwards making the protein water insoluble. - Long and narrow strands - Less sensitive to pH and temperature changes - **Examples:** Keratin, Collagen ## Haemoglobin - 1 Hb molecule has 4 polypeptide chains: - 2 identical α-chains (α-globins) - 2 identical β-chains (β-globins) - **Quaternary structure:** - Each polypeptide chain is associated with a heme group. - Each heme group has iron ($Fe^{2+}$) attached to its centre. - This iron can reversibly bind with one oxygen molecule ($O_2$). - So one haemoglobin molecule can hold 4 oxygen molecules: - Hb + $4O_2$ → HbO$_8$ (oxyhaemoglobin) ## Collagen - **Fibrous protein** - Found in skin, tendons, cartilages, bones, teeth, and walls of blood vessels. - Most common protein in animals. - **Structural protein:** Makes up to 25% of the total protein content of a mammalian body. - **High tensile strength:** - **Flexible:** Every 3rd amino acid as glycine enables the polypeptide chains to lie close to each other, and form a light coil of tropocollagen. - **Associate via H-bonds:** ## Water - 70%-95% cell mass - One body is about 60% water - **Polar molecule:** - **Non-linear, bent shape:** - **Excellent solvent for polar and ionic molecules:** - **High specific heat capacity:** - The amount of heat energy required to raise the temperature of one unit mass of water by 1°C/1K. - **Advantage:** Ensures stable temperature/environment of water - Ensures that human body temperature remains within narrow ranges thus ensuring that optimum enzyme activity is not hindered. - Large water bodies maintain their temperature within narrow limits hence the environment and enzyme activity of aquatic life is not disturbed. - **High latent heat of vaporization:** - Measure of the heat energy required to vaporize 1g of liquid at constant temperature. - **Advantage:** Enables minimum loss of water, in the form of sweat, to remove large amount of heat from the body. (Cooling effect) - **Cohesion and surface tension:** - **Cohesion:** The attraction between molecules of the same kind. - **Surface tension:** The tendency of liquid surfaces at rest to shrink into the minimum surface area possible - **Advantage:** Serves as a habitat for light-weighted animals (e.g., mosquito) - **Adhesion:** - Attraction of molecules for other molecules of a different kind. - **Advantage:** Gives water molecules the tendency to stick to the sides of the vessel. - **Cohesion & Adhesion are both essential for the movement of water up the xylem vessel in plants.** - **Maximum density of water at 4°C** - **Anomalous behavior:** (Not because of hydrogen bonding) - When liquid is heated → it expands → volume ↑, density ↓ - When liquid is cooled → It comes closer → volume ↓, density ↑ - **More space occupied by ice molecules → expansion to form this structure:**