Topic 1 - Aspects of Biochemistry

TOPIC 1: ASPECTS OF BIOCHEMISTRY 1.1: Discuss how the structure and properties of water relate to the role that water plays as a medium of life. Your body is comprised of numerous elements, which make also combine to form molecules. These include macronutrients such as carbohydrates (such as starch and glucose, required for release of ATP), proteins (which are used for growth and repair of cells and also to form hormones) and fats (used for as an energy store). However, the molecule that comprises the majority of the human body (more than 70% of a cell‟s mass) is WATER. Why are water molecules attracted to each other? First, observe the molecular structure of water. Water consists of two hydrogen atoms COVALENTLY bonded to one oxygen atom. This means that electrons are shared between them. On the diagram, you will observe the symbol δ (delta), and a symbol for +ve or –ve charge. In this case, the OXYGEN has the negative charge and the HYDROGEN atoms have the positive charge. Water itself is electrically balanced or NEUTRAL. However, there is uneven distribution of these charges in the structure. This is called a DIPOLE. This allows weak electrical attraction between the water molecules, which results in COHESION and the ability to undego MASS FLOW. They also result in HYDROGEN BONDS, which are essential for many biological molecules. Property of Water What Allows This Temp. regulation Its high specific heat capacity and ability to evaporate easily. „Universal‟ solvent Its tiny charges attract other molecules or ions to form bonds. Allows mass flow Its H-bonds produce cohesion and surface tension. Suitable for excretion. Assists buffers Its neutral pH allows H ions or OH ions to be absorbed by proteins. Reactivity Used in hydrolysis reactions during digestion and in photosynthesis. 1.2/3: Explain the relationship between the structure and function of glucose and sucrose. What exactly are CARBOHYDRATES? Carbohydrates are organic molecules that comprise a ratio of carbon, hydrogen and oxygen. They are comprised of at least one sugar unit. However, they can be linked together to form increasingly complex molecules. Type of carbohydrate Number of units Examples MONOSACCHARIDE One Glucose, fructose, ribose, galactose, glyceraldehyde DISACCHARIDE Two Maltose, sucrose, lactose POLYSACCHARIDE More than two Starch, glycogen, cellulose, chitin MONOSACCHARIDES are the simplest carbohydrate and cannot be further hydrolysed. They are written with the general formula (CH2O)n. The „n‟ depends on the type of sugar. For example, a hexose sugar (e.g. glucose) has a value of „6‟, so glucose is written as C6H12O6. A pentose such as ribose has a value of „5‟ and is written as C5H10O5. However, modifications occur, such as deoxyribose having one less oxygen atom, so deoxyribose is written as C5H10O4 As previously mentioned, glucose is a HEXOSE, which means it has a six-membered ring consisting of five carbons and one oxygen. Observe the straight-chain and ring structures of glucose below. NOTE: Betaglucose’s ring structure is similar but H and OH are swapped on C-1. It can be observed that the 6th carbon atom in the ring structure does not exist as part of the ring structure. As a result of this, glucose tends to alternate between its ring and its chain form. This is why there are two different types of glucose (alpha and beta). How are DISACCHARIDES formed then? As previously mentioned, a DISACCHARIDE forms when two monosaccharide molecules are bonded. When this linkage occurs, it is known as a GLYCOSIDIC bond. These types of bonds are very strong. One common example of a disaccharide is SUCROSE, which we commonly know as the sugar that is sweet (such as in sugar cane). So, what two monosaccharides combine to form sucrose? That would be an ALPHA GLUCOSE and a BETA FRUCTOSE. What is notable about sucrose is that when it undergoes enzyme breakdown, sucrose yields two glucose molecules. However, one of those molecules has been reformed from fructose. Here are their ring structures: Carbon-1 of the alpha glucose will now bond with the Carbon-2 of beta-fructose. This is thus called a 1-2 glycosidic bond. A CONDENSATION reaction removes a water molecule in the process. Now observe the structure of SUCROSE: Sucrose is used for transport instead of glucose because it is much more complex, more energy-efficient and not as reactive as glucose. What is the difference between a REDUCING and NON-REDUCING sugar? In O‟ levels you would‟ve learned that Benedict‟s solution can be used to test for reducing sugar. However, the addition of HYDROCHLORIC ACID and then SODIUM HYDROXIDE was needed for non-reducing sugars. This is because disaccharides such as sucrose have a glycosidic bond that prevents Benedict‟s reageant from reacting with it. The HCl is needed to break that glycosidic bond and the NaOH is needed to neutralize the HCl.