AQA A level Biology 100% correct questions and answers

Monomer A small, basic unit that can form a polymer. E.g. monosaccharides, amino acids and nucleotides. Polymers Long complex molecules consisting of chain of identical monomers. Condensation Reactions Forming a chemical bond between monomers and releasing a molecule of water. Hydrolysis Breaking down polymers into monomers with the addition of water Sugars All carbohydrates contain Carbon, Hydrogen and oxygen. The monomers for carbohydrates are monosaccharides e.g. glucose, fructose and galactose (reducing sugars). Glucose Is a hexose sugar with six carbons in each molecule. There are two types: alpha and beta glucose. These are isomers which means the same chemical formula, but a different molecular arrangement. Disaccharides A disaccharide forms when two monosaccharides join together through a condensation reaction. This forms a glycosidic bond. Glucose + Glucose = maltose Glucose + Fructose = sucrose Glucose + Galactose = lactose. Benedict's Test for Reducing Sugars 1.Add Benedict's reagent 2.Bring to Boil 3.Forms a brick red precipitate if positive. To find how much sugar there is, filter the solution and weigh the precipitate Non Reducing Sugars Benedict's Test 1. Initial Benedict's Test - negative. 2. With a new sample, add dilute HCl to the sample and boil. 3. Neutralise with sodium hydrogen carbonate. 4. Repeat Benedict's Polysaccharides Carbohydrates that are made up of more than two monosaccharides. Polysaccharides are joined by 1-4 glycosidic bonds can be broken down to monosaccharides by hydrolysis. Starch Insoluble in water and therefore doesn't affect the water potential gradient. Has no impact on Osmosis. Good for storage. Not Sweet Large molecule so cannot leave the cell and is always available. Branched and unbranched. Starch is the long term energy store in plant cells, found as starch grains in the cytoplasm. Amylose (25%) - Long unbranched chain of alpha glucose, linked by a 1-4 glycosidic bond. - Hydrogen bonds give it a coiled, spiral structure so it is therefore very compact. -Hydroxyl groups project into the middle. They form hydrogen bonds with each other which stabilises the spiral. Iodine detects this as it gets stuck inside the spiral and forms an iodine starch complex. Amylopectin (75%) - Long, branched chain of alpha glucose -Side branches allow the enzymes to break down the molecule to get at the glycosidic bonds easily. This means glucose can be released quickly. -The molecule is branches at 1-4 glycosidic bonds and 1-6 glycosidic bonds. -Highly branched means it can be easily digested by enzymes to produce glucose for respiration for ATP production due to the large surface area. Glycogen -Iodine does not detect this as it only detects amylose and glycogen only has a branched structure. -It is a monomer of alpha glucose with 1,4 and 1,6 glycosidic bonds. -Animal cells store their excess glucose from carbohydrate as glycogen in muscles and the liver. This is large and insoluble and therefore has no effect on osmosis. -The branched points occur more often. Therefore they are less dense and more soluble. Enzymes can hydrolyse glycosidic bonds of glycogen into glucose for respiration and the consequent ATP production. Glucose can be produced more quickly. -Indicates the higher metabolic requirements for animals compared with plants as animals need muscle contractions, homeostasis, temperature regulation). -Compact so it is good for storage. Cellulose -Long, unbranched chains of beta glucose. When beta molecules bond, every other molecule is inverted 180 degrees, forming straight cellulose chains, linked by 1,2 glycosidic bonds. -Chains become linked together by many hydrogen bonds to form microfibrils (many hydrogen bonds provides strength and stability). These stack to form fibrils. -Used in the cell wall of plants. -Provides strength and rigidity to cell wall as it can resist turgid pressure from water, stopping the cell from bursting. Starch Test Add iodine dissolved in potassium iodide solution to the sample. If the starch is present, the sample will turn from orange to blue/black. Lipids These are not polymers as they are not made of identical repeated units and contain ester bonds not glycosidic. They are made form hydrocarbons, glycerol, phosphates and fatty acids. Triglyceride 1 glycerol and 3 fatty acids. The fatty acids have long 'tails' made of hydrocarbons which are hydrophobic so they repel water molecules. They are insoluble in water. Fatty Acids R is the variable region attached to the carboxyl group. Hydrocarbon chains varies in length and saturation. Saturated fatty acids do not have any carbon - carbon double bonds. The chain is "saturated" with carbon. This means the fat is solid at room temperature and has a higher melting and boiling point. e.g. hard fats like lard and butter. Unsaturated fatty acids do have double bonds between carbon atoms, which cause their chains to 'kink'. This means the chain has a lower melting and boiling point and is liquid at room temperature. e.g. fluid oils like sunflower or olive oil. Triglyceride Formation Removal of water between acidic carboxyl group of fatty acid and hydroxyl group of glycerol. This is a condensation reaction and so, for every ester bond formed, a molecule of water is released. Phosopholipids Found in cell membranes. 1 Phosphate group, 1 glycerol and 2 fatty acids. -They are energy storage molecules. -Contain 2x more energy than the same mass of carbohydrates which is useful for animals that move a lot and therefore require a lot of energy. -Long hydrocarbon tails store lots of chemical energy which is released when broken down. -They are insoluble in water and so do not affect the water potential of the cell and cause water to enter the cell by osmosis. -Form insoluble droplets in cells. -Used as hormones e.g. testosterone and respiratory substrates. Phospholipid Bilayer A double layer of phospholipids that makes up plasma and organelle membranes. It can form a ball known as a micelle or a larger ball called a liposome. The tails of the phospholipid are hydrophobic and so face inwards, whereas the glycerol heads are hydrophilic and so face outwards, forming a bilayer. The hydrophilic heads form hydrogen bonds with water on the outside. This controls what enters and leaves the cell. A double layer is formed with the hydrophilic heads facing out, hiding the hydrophobic tails. Water soluble substances can not pass through the barrier. Funcitons Form adipose tissue in mammals. Energy store. Insulation against cold. Protection for body organs. Buoyancy as it is less dense than water Waxes and oils are used in waterproofing. Phospholipids form cell membranes. Emulsion Test Add ethanol then shake and pour top liquid into test tube half filled with distilled water and shake again. A positive result for lipids is a cloudy, milky white emulsion. Proteins The monomers for proteins are amino acids. A dipeptide forms when 2 amino acids join together with peptide bonds. Proteins are made from 1 or more polypeptide. Amino Acid Only different and specific in the R group. The R group represents a side chain from the central 'alpha' carbon and can be anything from a simple hydrogen atom to a more complex ring structure. The R group defines the amino acid. Glycine is the only amino acid that does not contain carbon. 20 amino acids make up all living things. Dipeptide and Peptide Formation Amino acids are linked by condensation reactions to form dipeptides and polypeptides. -1 molecule of water is released during the reaction. -Peptide bonds are formed. -Hydrolysis is breaking down a polypeptide into an amino acid through the addition of water. Protein Structure When more amino acids are added to a dipeptide, a polypeptide is formed. A protein consists of one or more polypeptide chains folded into a highly specific 3D shape. 4 phases: 1. Primary 2. Secondary 3. Tertiary 4. Quaternary Primary Structure The sequence of amino acids in the polypeptide chain. They are held together by peptide bonds. Secondary Structure Hydrogen bonds form between amino acids in the chain, making it coil into an alpha helix (coiling) or a beta pleated sheet (folding). The hydrogen bonds form between the carboxyl group of 1 amino acid and the amino groups in the peptide backbone. Tertiary Structure The coiled or folded chain of amino acids is often coiled or folded further, forming a specific 3D shape for each protein. More bonds form between different parts of the chain including hydrogen bonds and ionic bonds and disulphide bridges (wherever 2 molecules of the amino acid cysteine come close together, the sulfur atom of one cysteine bonds to the sulfur atom in another). Disulphide bridges are very strong, ionic are slightly weaker and rarer and hydrogen bonds are weak. For proteins made from a single polypeptide, this is the final 3D structure. Proteins can also be bonded by hydrophilic interactions in which the molecule folds away from water, towards the inside of the proteins. This can be due to mutation e.g. sickle cell anaemia. Bonds depend on the primary structure of the protein. Bonds Shape is maintained by: Hydrogen bonds are involved in all levels of the structure and are weak. Hydrophobic interaction are between the non polar sections of the protein and are strong. Disulphide bonds are the strongest and most important types of bond in proteins. They occur between two cysteine amino acids. Quaternary Structure Many different polypeptide chains are held together by bonds. This structure is the way these polypeptide chains are assembled together. They are held together and associated with prosthetic groups (non protein groups e.g. iron ions in haemoglobin). This is the proteins final specific 3D structure e.g. haemoglobin (4 polypeptides), insulin and collagen. Biuret Test Crush and add distilled water to make the sample a solution. Add sodium hydroxide to make the solution alkaline. Add copper (II) sulphate solution. If there is protein the colour will change from blue to violet. Function Shape determines function e.g. haemoglobin is compact and soluble. Enzymes - spherical due to tight folding. -Soluble and often have roles in metabolism e.g. digestion. -Other enzymes help to synthesise large molecules. Antibodies - immune response, made of 2 short polypeptide chains and 2 long and heavy chains bonded together. -They have a specific active site determined by the primary structure. Transport proteins - e.g. channel proteins which contain hydrophilic and hydrophobic amino acids which forms the channel. These transport molecules and ions across membranes. Structural Proteins - physically strong and consist of long polypeptide chains lying parallel to each other with cross links between them. Collagen has 3 polypeptides wound together which makes it strong. This is a great supportive tissue in animals.