Subject: Biology
Level: IGCSE

Biology 1


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a) The main elements found in living organisms. Some elements are needed in trace amounts (details not required). Key elements are present as inorganic ions: Mg2+, Fe2+, Ca2+, PO4 3– The importance of water in terms of its polarity, ability to form hydrogen bonds, surface tension, as a solvent, thermal properties, as a metabolite. **The most common elements in living organisms are carbon, hydrogen, oxygen and nitrogen (CHON). Carbon is the most important because of its ability to form long chains and ring structures which make up the skeletons of organic molecules. Some of the other key elements needed are:

  • Calcium (found as calcium phosphate) – used to strengthen bones and teeth, forms structure

  • Chlorine (Cl- in plants, NaCl in humans) – Helps to determine solute concentrations and balance within cells alongside Na+ and K+

  • Iron – Forms part of haemoglobin, myoglobin and cytochromes

  • Magnesium – Activates many enzymes, part of chlorophyll molecule

  • Phosphorus (phosphate) – part of plasma membrane, nucleic acids and nucleotides and ATP – required for successful photosynthesis.

There are micronutrients such as zinc and copper that are needed in very tiny amounts.

Water is a very important molecule that has many properties:

  • Water is a polar molecule, the oxygen end of the bond is slightly negative and the hydrogens are slightly positive. This means that it is good at dissolving polar molecules, which makes it a good solvent. Non-polar molecules are usually insoluble, which is advantageous, such as in the case of the hydrophobic cell membrane.

  • Water can form hydrogen bonds which gives it many of its properties. It makes it more difficult to separate water molecules, and gives many of its other properties.

  • Water molecules stick together (cohesion) and to surfaces (adhesion). Surface tension in water is caused by the intermolecular forces.

  • Water has a high latent heat of evaporation, so when it evaporates, it takes energy out of the surface from which it is evaporating. A large amount of heat must also be taken out of water before it will turn to a solid. As water gets cooler, it becomes less dense, and as a result, ice is denser than water.

  • Water is a metabolite – it is a medium in which reactants can dissolve and where metabolic reactions can take place.

**b) Structure, properties and functions of carbohydrates: monosaccharides (triose, pentose, hexose sugars); disaccharides (sucrose, lactose, maltose); polysaccharides (starch, glycogen, cellulose, chitin). Alpha and beta structural isomerism in glucose resulting in storage and structural carbohydrates as illustrated by starch, cellulose and chitin. Chemical properties enabling the use of starch and glycogen as storage and cellulose and chitin as structural compounds. **All carbohydrates contain C, H and O and are organic molecules. The general formula is Cx(H2O)y and there are three main groups, mono, di and polysaccharides.

Monosaccharides are sweet, single sugar molecules. They are soluble and are named in categories depending on how many C atoms they contain: trioses (3C), pentose (5C) and hexose (6C). They are used as a source of energy as they have many C-H bonds which can be broken down to release energy for formation of ATP. Two monosaccharide molecules are usually joined together by 1-4 glycosidic bonds, in a condensation reaction. Two monosaccharides such as glucose joined together form a disaccharide.

Disaccharides are still small enough to be soluble and sweet, and are formed from joining 2 hexose molecules. They are used for storage and transporting carbohydrates (e.g. sucrose in the phloem).

  • Sucrose – glucose + fructose

  • Maltose – glucose + glucose

  • Lactose – glucose + galactose

Monosaccharides are the monomers in polysaccharides (polymer chains). The monomers are joined by glycosidic bonds. Monosaccharides are changed into polysaccharides to prevent them from dissolving and also from reacting with normal cell chemistry. Some examples of polysaccharides are starch, glycogen and cellulose.

There are two isomers of glucose, alpha and beta glucose. This is caused by the fact that the OH group on C1 can be facing either up or down so that it is above or below the plane of the molecule. All human enzymes are based on alpha glucose – we cannot break down beta.

  • Starch (Amylose and amylopectin) – Amylose made from 1-4 glycosidic bonds, forms a long, unbranched chain. The chain coils into a helix which is held together by hydrogen bonds. Alpha glucose molecules can also make some 1-6 bonds, which forms a branched chain, which is known as amylopectin.

  • Glycogen – animal storage polysaccharide has very similar structure to amylopectin. More branched to form compact granules that can be broken back down into glucose fairly easily. Found mainly in liver and muscle cells.

  • Cellulose – structural carbohydrate rather than storage. Made of beta glucose, every other molecule is rotated so that the OH groups can join together with 1-4 linkages. Hydrogen bonding forms over large numbers of molecules, with 60-70 becoming cross-linked to form micro fibrils which form fibres. A cell wall is made up of several fibres.

You can test for starch using iodine, and for reducing sugars using benedict’s solution and heating.

**c) Structure, properties and functions of lipids as illustrated by triglycerides and phospholipids. Implications of saturated and unsaturated fat on human health. **Lipids are made up of carbon, hydrogen and oxygen, with the addition of phosphorus in phospholipids. The main types of lipids are fats and oils, which are classified by their different melting points (oils are liquid at room temperature, fats are solid). Lipids are immiscible with water, but dissolve in some organic solvents such as ethanol.

They are also used for insulation, protection, and waterproofing and energy storage. Lipids are used, rather than carbohydrates, as an energy store in seeds and animals because they have a high yield of energy per gram. Triglycerides also produce a lot of metabolic water when oxidised, which is important for desert animals such as camels.

Triglycerides are hydrolysed to fatty acids and glycerol, and it is the fatty acids that vary to give the different properties of triglycerides (melting point etc.) The hydrocarbon tail of a fatty acid is hydrophobic, which is what prevents triglycerides from dissolving in water, whereas glycerol is hydrophilic. A condensation reaction occurs when a fatty acid attaches to a glycerol molecule, forming an ester bond. Phospholipids are hydrolysed to fatty acids, glycerol and a phosphate group. The phosphate head is hydrophilic and the fatty acid tail is hydrophobic. Phospholipids are especially important in cell membranes.

A very high fat consumption, particularly of saturated fats, can be a contributing factor to heart disease. Fatty deposits build up in the coronary arteries, which can lead to a heart attack.

You can test for fats using the emulsion test: dissolve the substance in ethanol, and then add water. Ethanol dissolves in water, and lipids are insoluble in water, so they precipitate out, leaving a milky colour in the solution.

**d) Structure and role of amino acids and proteins. The peptide link. Relation of molecular structure to function. Primary, secondary, tertiary and quaternary structure of proteins. Globular and fibrous proteins. Candidates should be able to use given structural formulae (proteins, triglycerides and carbohydrates) to show how bonds are formed and broken by condensation and hydrolysis, including peptide, glycosidic and ester bonds. **

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