Ch 2 Molecules of life Think about (Ch 2, p.2) 1. Water is essential to life. If there is water on a planet, it is possible that life may exist on the planet. 2. Water makes up the largest percentage by weight of most organisms. It serves many vital functions in organisms. 3. Organisms usually need carbohydrates, lipids, proteins, nucleic acids and minerals to live. Quick practice (Ch 2, p.4) A Biomolecules refer to organic substances found in organisms. Water is inorganic, and hence not a biomolecule. Quick practice (Ch 2, p.6) C Sweat consists mainly of water. It takes away heat from the body when it evaporates. Quick practice (Ch 2, p.8) B Quick practice (Ch 2, p.13) B In green plants, glucose produced by photosynthesis is converted into starch for storage. Glucose is also used to produce cellulose which is the main component of plant cell walls. Glycogen is the storage form of carbohydrates in animals. Quick practice (Ch 2, p.16) C (a) is incorrect. The three fatty acids present in a triglyceride molecule can be the same or different. 1
Quick practice (Ch 2, p.19) D When the amino group ( NH 2 ) of one amino acid and the carboxyl group ( COOH) of another are joined by condensation, a dipeptide is formed. Quick practice (Ch 2, p.22) C The four nitrogenous bases in DNA are adenine, thymine, guanine or cytosine. Uracil is found in RNA. Section exercise (Ch 2, p.25) 1. T 2. F 3. F Some carbon-containing compounds such as carbon dioxide and carbon monoxide are inorganic. 4. (a) as a cooling agent 5. D 6. A (b) as a reactant (c) as a supporting agent (d) as a transport medium Some minerals (inorganic salts) are found in solid form in the human body. For example, bones contain a mixture of salts, including calcium phosphate and magnesium salts. 7. (a) carbon (b) hydrogen (c) oxygen 8. (d) Glucose 9. (e) starch (f) glycogen 10. (g) Cellulose 11. (h) condensation 2
Glycerol 12. (a) subcutaneous fat (b) insulating (c) shock 13. (d) Cholesterol (e) hormones 14. X: Glycerol Y: Fatty acid 15. Fatty acid Phosphate group Fatty acid 16. (a) amino acids (b) nitrogen 17. (c) carboyxl group (d) dipeptide (e) polypeptide 18. Statement DNA RNA (a) It contains a 5-carbon sugar. (b) It contains adenine. (c) It contains uracil. (d) It contains phosphate groups. (e) It has a double-helical structure. (f) It is single-stranded. 3
Revision exercise (Ch 2, p.27) 1. C 2. C Minerals are inorganic substances. Plants do not sweat, but instead transpire from their leaves. 3. B is the answer. Plants do not absorb nitrogen gas directly. Nitrogen is taken up by plants in the form of nitrates or ammonium. 4. D is the answer. 5. A 6. C 7. B 8. C 9. A 10. D Carbohydrates, lipids, proteins and nucleic acids are biomolecules. The general formula of carbohydrates is C x (H 2 O) y. Cellulose is the main component of plant cell walls. Glucose and fructose are monosaccharides. All monosaccharides are reducing sugars. B is incorrect. Monosaccharides are the simplest forms of carbohydrates which cannot be further hydrolysed. C is incorrect. The building block of cellulose is glucose. D is incorrect. Lactose is a disaccharide which must be broken down (hydrolysed) into monosaccharide before being absorbed by the intestine. Cellulose is the main component of plant cell walls, which are rigid to maintain the shape of the plant cells. A is incorrect. It is starch that stores energy for the plant cells. B is incorrect. It is starch that can be stained blue-black with iodine solution. D is incorrect. Cellulose cannot regulate the movement of water in and out of the plant cells. It is the function of the cell membrane. It is based on the fact that glycerol has three available OH groups for bonding to the fatty acids. Fat dissolves in alcohol. Thus, if the translucent spot is caused by the presence of fat, it would be washed away when rinsed with alcohol. Some proteins also contain sulphur. 4
11. D 12. C 13. (a) Proteins and phospholipids are the major components of cell membranes. Proteins are also used to make many important biological molecules, such as enzymes, antibodies and certain hormones. Both DNA and RNA are made up of carbon, hydrogen, oxygen and nitrogen. A is incorrect. Amino acids contain nitrogen while fatty acids do not. B is incorrect. Phospholipids contain phosphorus while triglycerides do not. D is incorrect. In addition to carbon, hydrogen and oxygen, proteins also contain nitrogen, and sometimes sulphur. Glucose Starch Glycogen Cellulose Contains bonds formed by condensation Soluble in water Found in plant cells 1 mark for correct answers in one row (1) 3 (b) Add iodine solution to the sample. (1) It turns blue-black in the presence of starch. (1) (Total: 5 marks) 14. (a) nitrogen, sulphur (1), (1) (b) (1) (c) dipeptide, water (1), (1) (d) peptide bond (1) (Total: 6 marks) 5
15. (a) hydrolysis (1) (b) C 12 H 12 O 11 (1) (c) (i) The positive result is due to the presence of sucrose, which is protein in nature. (1) (ii) Heat the sample with Benedict s solution. (1) The formation of a brick-red precipitate would show that the sample contained reducing sugars. (1) (Total: 5 marks) 16. (a) W: amino acid (1) X: glycerol (1) Y: phosphate group (1) Z: fatty acid (1) (b) triglyceride (1) 17. HKALE Biology 2005 Paper 2 Q1(a)(i) (Total: 5 marks) 18. (a) glucose (1) (b) (i) (ii) condensation (1) (c) (i) disaccharide / double sugar (1) (ii) sucrose / lactose (1) (d) Heat the reducing sugar with Benedict s solution (1) to from a brick-red precipitate. (1) (1) (Total: 7 marks) 19 (a) A: glycerol (1) B: fatty acid (1) (b) (i) Triglyceride (1) (ii) It acts as energy reserves / heat insulator / shock-absorber / waterproof layer to reduce water loss. (1) 6
(c) (i) Phospholipid (1) (ii) Cell membrane/ plasma membrane (1) (d) Add a small drop of the sample on a piece of filter paper. (1) Leave the piece of filter paper in the air for about 10 minutes and a translucent spot is observed. (1) Put the filter paper into an organic solvent. Take it out and let it dry. (1) The disappearance of the translucent spot indicates the presence of lipids. (1) (Total: 10 marks) 20. (a) A: amino acid (1) B: monosaccharide (1) C: DNA (1) (b) They are important for growth and repair of body tissues / component of the cytoplasm and the cell membrane. (1) They form enzymes, haemoglobin, antibodies and some hormones. (1) The can be broken down to provide energy if carbohydrates and lipids are used up. (1) (c) (i) glycogen (1) (ii) starch (1) (d) (i) hydrogen bond (1) (ii) double helix (1) (Total: 10 marks) 21. (a) Circle any NH 2 group (1) (b) Only protein in the sample contributes significantly to its nitrogen content. (1) (c) Any two of the following: (2) for growth and repair (1) as a structural component of various cellular structures (1) for making enzymes, antibodies and certain hormones (1) as a source of energy (1) (d) Meat, fish, eggs, peas and soya beans (any two) (2) (e) Milk is the main diet of infants, while adults can also consume other food substances for nutrition. (1) OR Milk is the main source of proteins in infants (1) 7
More rapid growth/ metabolism in infants than that in adults (1) (Total: 8 marks) 22. Knowledge on the importance of water (max. 7) As a solvent: Water is a main component of cells and body fluid. (1) Water provides a medium for chemical reactions to take place. (1) As a medium for transport: The human blood plasma consists of about 90% water. It carries nutrients, dissolved gases, wastes and hormones in blood vessels around the body. (1) As a reactant: For photosynthesis in plants (1) For hydrolysis, e.g. during digestion (1) As a cooling agent: During sweating, water in sweat evaporates and takes away heat. This helps cool down the body temperature. (1) Transpiration in land plants prevents the plants from overheating. (1) As a temperature regulator: The high specific heat capacity of water enables an organism to maintain a constant body temperature. (1) As a supporting agent: Provides a means of support in young seedlings and in some invertebrates such as earthworms. (1) Communication (max. 3) (Total: 10 marks) 23. Knowledge on the structure of carbohydrates (max. 5) Carbohydrates are organic substances made up of carbon, hydrogen and oxygen, in which the ratio of hydrogen to oxygen is 2:1. (1) Carbohydrates are classified into three types, monosaccharides, disaccharides and polysaccharides. (1) Monosaccharides are the simplest forms of carbohydrates. Glucose is an example of monosaccharides. (1) (May use other monosaccharides such as fructose and galactose as examples) Two monosaccharides are joined by condensation to form a disaccharide. Maltose is one of the examples of disaccharides. (1) (May use other disaccharides such as sucrose and lactose) 8
Polysaccharides are complex carbohydrates formed by condensation of a large number of monosaccharide molecules. Starch is an example of polysaccharides. (1) (May use other polysaccharides such as glycogen and cellulose) Knowledge on the functions of carbohydrates (max. 2) Glucose can be directly broken down in the body cells during respiration to release energy. (1) Starch and glycogen are the storage form of carbohydrates in plants and animals respectively. (1) Cellulose is the main component of plant cell walls. (1) Communication (max. 3) (Total: 8 marks) Article reading (Ch 2, p.31) 1. Sweat consists mainly of water. (1) When sweat evaporates, it takes away heat. This helps lower our body temperature. (1) 2. Sports drinks can quickly replenish the water and minerals lost during exercise and prevent dehydration. (1) 3. No. Sports drinks contain sodium and potassium. Sodium can enhance fluid absorption from the intestine, while potassium can assist with muscle contraction. (1) The energy-rich nutrients, like glucose and other carbohydrates, in sports drinks provide an energy source for the body. (1) Sports drinks contain carbohydrates which is an energy source. (1) If we drink too much sports drinks, we will obtain a lot of energy which will be stored as fat (1) if the amount exceeds our energy output. (1) 9