Topic 1 - Biological Molecules

• Covalent bonding : when atoms share a pair of electrons in their outer shells, which fills the outer shells of both atoms and forms a stable molecule.
• Ionic bonding : when ions of opposite charges attract each other to form an electrostatic attraction known as an ionic bond. Ionic bonds are weaker than covalent bonds.
• Hydrogen bonding : when the negative region of one polar molecule attracts the positive region of another polar molecule. Polar molecules occur due to uneven distribution of electrons in a molecule. Hydrogen bonds are weak electrostatic bonds.
• Monomers can be linked together to form long chains called polymers, and the process by which these chains are formed is polymerisation. Examples of synthetic polymers include polythene and polyesters, and examples of natural polymers include polysaccharides and polynucleotides.
• Condensation reactions occur when monomers are joined together with chemical bonds to form polymers in a reaction, involving the elimination of a water molecule in the process.
• Some examples of polymers include polysaccharides, polynucleotides and polypeptides. Furthermore, some examples of monomers include monosaccharides, mononucleotides and peptides.
• Hydrolysis is the chemical breakdown between monomers in a polymer, involving the addition of water.
• Metabolism is the chemical processes that occur within a living organism in order to maintain life.
• A mole is a unit for measuring the amount of a substance present. One mole contains the same number of particles as there are in 12g of carbon atoms. Avogadro's constant (6.02 x 10^23) represents the number of particles in one mole of any substance.
• A molar solution is a solution with one mole of solute per litre of solution.
• Carbohydrates are carbon molecules combined with water (made up of carbon, hydrogen and oxygen). Carbon atoms form the basis for life on Earth.
• Monosaccharides are individual sugar monomers from which larger carbohydrates are made. They have the general formula (CH2O)n (n is a number from 3 to 7). Examples of monosaccharides include : glucose, fructose and galactose.
• Glucose is a hexose (6-carbon) sugar with the formula C6H12O6, and has two isomers : alpha glucose and beta glucose.
• A reducing sugar can donate electrons to another chemical in order to reduce it (acts as a reducing agent). Testing for reducing sugars is done using the Benedict's Test method. If a reducing sugar is heated with Benedict's reagent, it forms an insoluble red precipitate of copper (I) oxide (positive result).
• When using the Benedict's test for reducing sugars, firstly you add 2cm3 of the sample to a test tube. If the sample is solid, grind it up with water first. Next, add 2cm3 Benedict's reagent to the test tube and heat the test tube in a gently boiling water bath for five minutes. If a reducing sugar is present, the solution should turn orangey-red.
• Reduction is the process of gaining electrons or hydrogen in a reaction.
• Benedict's reagent is an alkaline solution of copper (II) sulfate.
• Disaccharides are molecules formed from the condensation of two monosaccharides.
• Examples of disaccharides include maltose, sucrose and lactose. Maltose is formed from the condensation of two glucose monosaccharides. Sucrose is formed from the condensation of glucose and fructose monosaccharides. Lactose is formed from the condensation of glucose and galactose monosaccharides.
• A glycosidic bond forms when a condensation reaction occurs between two monosaccharides to form a disaccharide. This glycosidic bond can also be destroyed through hydrolysis of the disaccharide, by adding water to break it down.
• The Benedict's test can also be used to test for non-reducing sugars (e.g. sucrose) in a sample. First, the Benedict's test for reducing sugars should be carried out (solution should stay blue). Next, another 2cm3 sample should be added to 2cm3 HCl and this should be boiled to hydrolyse remaining disaccharides in the sample. Then sodium hydrogencarbonate solution should be added to neutralise the HCl. The pH should be tested to verify an alkaline solution, and then the original Benedict's test should be carried out again. If non-reducing sugars are present, the solution will turn orangey-red.
• Polysaccharides are large, insoluble molecules, making them suitable for storage. When they are hydrolysed, they break down into disaccharides and monosaccharides.
• Starch can be tested for using an iodine test. The test is carried out at room temperature, and first a 2cm3 sample is added into a test tube/spotting tile. Next, 2 drops of potassium iodide solution should be added and the mixture should be shaken/stirred. If the solution turns from yellow to blue/black, starch is present in the sample.
• Starch is a polysaccharide found in many parts of a plant in the form of small grains. It is made up of chains of alpha glucose monosaccharides linked by glycosidic bonds formed through condensation reactions. Starch chains can be branched or unbranched, but unbranched chains wind into tight coils for compaction.
• Starch is adapted for energy storage in plants. It is insoluble, so doesn't impact water potential (water isn't drawn out of cells via osmosis). It is large and insoluble (doesn't diffuse out of cells). Starch is compact (lots in small space), and when hydrolysed forms alpha glucose monosaccharides, which are used in transportation and respiration. The branched form of starch has many ends, which creates a greater surface area for enzyme action, leading to faster release of glucose.
• Glycogen is found in animal and bacterial cells, and is similar in structure to starch, yet has shorter chains that are more highly branched.
• Glycogen is adapted to be a storage molecule. It is insoluble, so it doesn't affect water potential and it doesn't diffuse out of cells. It is compact, so lots can fit in a small space. Glycogen is more highly branched than starch, so has more ends and therefore has a higher rate of enzyme action (glucose monomers released more quickly). This is important since animals have a higher metabolic/respiratory rate than plants.
• Cellulose is a structural molecule (found in plant cell walls) made from beta glucose, and forms straight, unbranched chains. These chains run parallel to each other, and hydrogen bonds form cross-linkages between adjacent chains. The number of hydrogen bonds makes cellulose very strong.
• Cellulose molecules are grouped into microfibrils, which are arranged in parallel groups called fibrils. Cellulose provides rigidity in the plant cell wall. Also, cellulose prevents cells from bursting as water moves in via osmosis. This is because the cell wall exerts an inwards pressure to stop further influx of water, and keeps the cell turgid. This benefits stems/leaves of a plant, since turgid cells prevent wilting in the plant, creating max surface area for photosynthesis.
• Cellulose is adapted as a structural polysaccharide in plants. It is made of beta glucose monosaccharides, so forms long, straight, unbranched chains. These chains run parallel to each other and are cross-linked with hydrogen bonds for collective strength. Cellulose molecules are also grouped into microfibrils and fibrils, which also add strength.
• All lipids contain carbon, hydrogen and oxygen. They are insoluble in water, but soluble in organic solvents (e.g. alcohol).
• The main two types of lipid are triglycerides and phospholipids.
• One role of lipids is in cell membranes (phospholipids) to provide flexibility of membranes and transfer of lipid-soluble substances across them.
• One role of lipids is as a source of energy (when oxidised) and provide double the energy of carbohydrates.
• One role of lipids is in waterproofing because lipids are insoluble in water. Plants/insects have a waxy cuticle which preserve water. Mammals produce an oily secretion from skin glands.
• One role of lipids is in insulation, since lipids can retain body heat because they are slow conductors of heat, and can also provide electrical insulation in the myelin sheath surrounding nerve cells.
• One role of lipids is in protection of the delicate organs.
• A triglyceride is a form of lipid. Each one contains three fatty acids and a monoglyceride. Each fatty acid forms an ester bond with the monoglyceride.
• Triglycerides are formed through the condensation reaction of three fatty acids and a monoglyceride, and hydrolysis can break down the triglycerides.
• The differences in the properties of triglycerides arise from variations in the fatty acids that make them.