Topic 3.2 Carbohydrates, Lipids and Proteins

3.2.1 Carbon Makes Organic Molecules Why Carbon? 1. Carbon is the second most abundant element in living organisms. Carbon can share four electrons, therefore it can bond to four additional atoms. Carbon establishes covalent bonds (stable, high energy bonds) Carbon molecules have strength, flexibility, and great versatility to chemically react with other atoms and molecules. Any molecule which contain carbon is defined as organic. There are few exceptions: Oxides of carbon (CO, CO2) and Hydrogencarbonates HCO3¯

Macromolecules Macromolecules are constituted by hydrocarbon backbones, which mainly provide structural stability, and by one or several functional groups. Functional groups are involved in many and diverse chemical reactions, establishing bonds with other atoms and molecules.

3.2.1 Structure of some important macromolecules

3.2.3/4 Examples of Monosaccharides and Function Monosaccharides are the simplest form of carbohydrate. They provide the building blocks for larger carbohydrate molecules. They also act as a respiratory substrate, providing cells with an energy source. Glucose is the main energy source for most living cells. It is one of the carbohydrates produced in photosynthesis and forms the building blocks of many carbohydrates. Fructose is a very sweet sugar. It is the main component of flower nectar and the sugar found in the fruits. Galactose is found in milk. It combines with glucose to form the disaccharide milk sugar molecule lactose

3.2.3 Examples of Disaccharides A disaccharide consists of two monosaccharides joined by a glycosidic linkage, a covalent bond formed between two monosaccharides through a condensation reaction. The hydroxyl group of one sugar and a hydroxyl of another sugar can join together, splitting out water to form a glycosidic bond: R-OH + HO-R' R-O-R' + H2O

3.2.3/4 Examples of Disaccharides and Function Disaccharides are relatively small molecules. They are water-solubes and taste sweet. Disaccharides are more suitable for transport and storage than monossacharides. Sucrose is stored in sugar beet and sugar cane. It is the main form in which carbohydrates are transported in the phloem tubes of plants. OH H Lactose 1 2 3 4 5 6 Sucrose

3.2.3 Examples of Polysaccharides The majority of sugars found in nature exist in the form of polysaccharides. Large chains of sugar units Plants store glucose as starch. The major constituent of plant cell walls, Cellulose, it consists of long linear chains of glucose with b(1  4) linkages.

3.2.3 Polysaccharides in animals: Glycogen Glycogen, the glucose storage polymer in animals, is similar in structure to starch, but glycogen has a(16) branches. The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise.

3.2.5 Lipids structure Lipids are organic molecules insoluble in water. They constitute the main reservoir of stored energy. The three main categories of lipids are: Fats (fatty acids and triglycerides), Phospholipids Steroids Fats also make cell membranes and coatings (i.e. fruit coats) The basic structure of fats is a hydrocarbon backbone with a carboxyl group attached

3.2.5 Lipids structure: Fatty acids • A fatty acid molecule: Hydrocarbon chain = Hydrophobic Carboxylic acid group =Hydrophilic Amphipathic Types of fatty acids: A fatty acid molecule has two distinct regions: a long, not very reactive, hydrophobic hydrocarbon chain, and a carboxylic acid group, extremely reactive and hydrophilic • Molecules such as fatty acids — with two distinct hydrophobic and hydrophilic regions — are termed amphipathic. Saturated: single C-C bond Unsaturated: Double C=C bond

3.2.5 Lipids structure: Triglycerides Fatty acids are very efficient sites of energy storage They are stored in cytoplasm of many cells as Triglycerides: 3 fatty acid chains bonded to a glycerol molecule. + H2O • Fatty acids are very efficient sites of energy storage; they are stored in the cytoplasm of many cells in the form of droplets of triacylglycerol molecules — compounds made of three fatty acid chains bonded to a glycerol molecule. • When a carboxylic acid and an alcohol react, a water molecule is removed, and an ester linkage is formed • Triglycerides make “the fat” of our bodies. In animals, they are stored as droplets in fat cells or adipocytes.

3.2.5 Lipids structure: Phospholipids Phospholipids are similar to triglycerides but one of the fatty acids molecules is replaced by a phosphate group (PO4³¯). The lipid part = hydrophobic The phosphate part = hydrophilic

3.2.5 Protein structure: peptide bond • Amino acids are joined together when a condensation reaction removes a hydroxyl group from the carboxyl end of one amino acid and a hydrogen from the amino group of another amino acid • The resulting covalent bond is called a peptide bond (C-N)

3.2.6 Function of lipids

3.2.7 Compare the use of carbohydrates and lipids in energy storage 1. Long time energy storage 2. Lipids contain twice as much energy per gram as carbohydrates 3. Lipids are insoluble in water – do not cause problems with osmosis cell. 1. Short time energy storage because are more easily digested so the energy stored can be released more rapidly 2. Soluble in water – easier to transport to and from the store.