3.2: Carbohydrates, lipids and proteins
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What is the difference between organic and inorganic compounds?
Organic compounds contain the element carbon, usually hydrogen and are made by living organisms. Carbon compounds not included are carbon dioxide, carboon monoxide, hydrogencarbonates and carbonates. Inorganic compounds are all other compounds, e.g. of mineral origin and the very simple carbon compounds - carbon dioxide, carbon monoxide, hydrogencarbonates and carbonates.
List examples of monosaccharides, disaccharides and polysaccharides.
Identify the function of the listed carbohydrates in plants and animals.
- Carbohydrates in Animals
- Glucose: Source of energy - used in respiration to release energy in the form of ATP.
- Lactose: Found in the milk of mammals and is source of energy for growing infant.
- Glycogen: Stored energy in the liver.
- Carbohydrates in plants:
- Fructose: Sweetness in honey and flower nectar attracts pollinators. Source of energy.
- Sucrose: Main sugar transported in the phloem and is used to store energy.
- Cellulose: Part of the cell wall to give strength and support for plant cell.
Cellulose readily forms intramolecular hydrogen bonds within itself and intermolecular bonds with other cellulose chains. Suggest how this structure relates to its function.
The hydrogen bonding between cellulose and adjacent cellulose chains leads to the formation of microfibrils. The microfibrils give strength and form a mesh that is the basis of the cell wall.
Distinguish between condensation and hydrolysis of a polymer.
- Condensation: A condensation reaction is a dehydration reaction where a small molecule is eliminated, e.g. water is removed and two molecules combine covalently to form a larger one.
- Hydrolysis: A hydrolysis reaction is a reaction where water is added and molecules are split to form a substance with a new element-oxygen bond.
Complete the following equations to show the hydrolysis and condensation of monosaccharides and disaccharides.
Glucose + glucose
Glucose + fructose
Glucose + galactose
Glucose + glucose --(condensation)--> maltose + water
Glucose + fructose --(condensation)--> Sucrose + water
Glucose + galactose --(condensation)--> lactose + water
What are triglycerides and how are they formed?
Triglycerides are lipids that are found in vegetable oils and animal fats and are a major form of stored energy. They are found in very large particles called VLDL (very low density lipoprotein). They are formed when glycerol is esterified with three fatty acids. Water is formed making their formation a condensation reaction.
What is a dipeptide and how are dipeptides formed?
A dipeptide is a molecule formed by two amino acids combining with a single peptide bond in a condensation reaction. Dipeptides can be produced from polypeptides in reactions catalysed by hydrolase enzymes. Dipeptides are formed when dietary proteins are digested and are absorbed more rapidly than amino acids into the blood stream.
Outline how hydrolysis and condensation are involved in forming/separating a protein from amino acids.
Amino acids in a protein are joined so that the amino group of one amino acid links to the carboxyl group of its neighbour. In condensation, one water is removed and a peptide bond is formed. IN a hydrolysis reaction, one water is added and the amino acids are separated.
Identify three major functions of lipids.
- 1. Important food storage compound
- 2. Structural componnent of cells
- 3. Control the transfer of materials into and out of cells as part of phospholipid bilayer
- 4. Thermal insulation, e.g. subcutaneous fat layer reduces heat loss from animals.
- 5. Shock absorb er - adipose tissue around vital organs cushions movement.
For humans, there are two essential fatty acids - linoleic acid (LA) and alpha-linolenic acid (ALA). What is meant by an essential fatty acid and state the primary use of these fatty acids.
Essential fatty acids are the fatty acids that the human body can not produce and mus be present in dietary foods. These fatty acids are mainly used in the production of hormone-like substances that help regulate blood lipid levels, blood pressure, blood clotting, the immune response and the inflammatory response to injury.
'Free' fatty acids are important as a fuel source as they yield large quantities of ATP. Outline why they are called 'free' fatty acids.
Fats are insoluble in water and thus bound or attached to other molecules for transport around the body in the plasma, e.g. they are transported as triglycerides or phospholipids. 'Free' fatty acids are not attached to other molecules.
Compare the stored energy of a carbohydrate with fat.
One gram of fat stores more than twice as much energy as a gram of a polysacccharide. e.g. starch. One molecule of glucose yields 38 ATP. Stearic acid (a fatty acid) is a C18 compound and complete respiration yields 147 ATP. Since glucose is a C6 compound, a comparison of the energy released by 3 glucose molecules (114 ATP) with the energy released by stearic acid (147 ATP) shows that the fatty acid releases more energy than the carbohydrates.
Suggest a reason why fats are the main storage food in animals and why animal metabolism is usually fat-oriented compared with plants.
Energy stored as fat weighs less than carbohydrates or proteins. This means that an animal using fat for energy storage is less bulky and has less weight to move in locomotion. Several sessile animals or animals that move very little, e.g. clams, store food as carbohydrates. Plants store energy as carbohydrates, e.g. in roots, while sees which are mobile for dispersal often contain lipids.
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