Cambridge IGCSE Biology Coursebook - Carbohydrates, Fats, and Proteins PDF

# CAMBRIDGE IGCSET™ BIOLOGY: COURSEBOOK ## 4.1 Carbohydrates, Fats and Proteins * What is a human made of? * The most abundant compound in our bodies is **water** (62% of total body mass). * **Protein** makes up 16%. * **Fat** makes up 16%. * **Carbohydrate** makes up 1%. * **Calcium an...

# CAMBRIDGE IGCSET™ BIOLOGY: COURSEBOOK ## 4.1 Carbohydrates, Fats and Proteins * What is a human made of? * The most abundant compound in our bodies is **water** (62% of total body mass). * **Protein** makes up 16%. * **Fat** makes up 16%. * **Carbohydrate** makes up 1%. * **Calcium and phosphorus** are found in bones (mostly) and make up 6%. * **Other elements** such as potassium, sodium, phosphate and chloride ions make up the remaining 6%. * **DNA** makes up less than 1% of our mass. * In this chapter, you will find out about the three main organic compounds - **carbohydrates**, **fats** and **proteins** that make up the bodies of all living things. * If you are studying the Supplement, you also need to know about **DNA**. ## Carbohydrates * Carbohydrates include **starch**, **cellulose** and **sugar**. * Carbohydrate molecules are made from three elements - **carbon**, **hydrogen** and **oxygen**. * One molecule of carbohydrate contains about twice as many hydrogen atoms as carbon or oxygen atoms. ### The simplest types of carbohydrate, with the smallest molecules, are sugars. Sugars taste sweet. They are soluble in water. ### One type of sugar is glucose. * A glucose molecule is made of six carbon atoms, twelve hydrogen atoms and six oxygen atoms. * Its formula is $C_6H_{12}O_6$. * These atoms are arranged to form a hexagon shape (Figure 4.1). ## Glucose * Glucose is the way that carbohydrates are transported around the human body. * Glucose dissolves in blood plasma, which delivers it to every cell. * Cells need glucose for **energy**. * Glucose molecules can link together in **chains**, to form much larger molecules. * In animals, the large molecules that are formed are **glycogen**. * The **liver** helps to keep the concentration of glucose in the blood constant. * If there is too much glucose in the blood, the liver links glucose molecules together to make glycogen. * Glycogen is stored in the liver cells. * It can be broken down to form glucose again when the body needs it. * In plants, glucose molecules are linked together in a slightly different way, to make **starch** (Figure 4.2). * Starch is stored in plant cells. * It can be broken down to form glucose again when the plant needs it. ### KEY WORDS * **Carbohydrates**: substances that include sugars, starch and cellulose; they contain carbon, hydrogen and oxygen. * **Sugars**: carbohydrates that have relatively small molecules; they are soluble in water and they taste sweet. * **Glucose**: a sugar that is used in respiration to release energy. * **Glycogen**: a carbohydrate that is used as an energy store in animal cells. * **Starch**: a carbohydrate that is used as an energy store in plant cells. ## Plants also make another carbohydrate, called cellulose. * Cellulose molecules are also made of many glucose molecules linked in a chain, but they form straight lines instead of coiling into a spiral. * Cellulose is used for making plant cell walls. ## Fats and Oils * Fats and oils are also known as **lipids**. * A **fat** is a lipid that is solid at room temperature, and an **oil** is a lipid that is liquid at room temperature. * Fats and oils contain the three elements **carbon**, **hydrogen** and **oxygen**. * You may remember that carbohydrates also contain these elements. * However, in fats, each molecule contains much less oxygen than is found in a carbohydrate molecule. ## Fat molecules are made of two kinds of smaller molecule - glycerol and fatty acids. * Figure 4.6 shows how these are joined together to make a fat molecule. * Fats are important for making **cell membranes**. * They are also used as **energy stores**. * Mammals often have a layer of cells containing fat droplets beneath the skin. * This fat-containing tissue is both an energy store and a heat-insulating layer. ## Fats and oils do not dissolve in water. * However, they do dissolve in **ethanol**. * We can use this to detect their presence in food. * *First, the food is shaken with ethanol, to allow any fats in it to dissolve in the ethanol.* * *Next, the ethanol is poured into a clean tube containing water.* * *If there are fats in the ethanol, they form tiny droplets in the water, which give it a milky appearance (Figure 4.7).* * *The mixture of tiny droplets in water is called an emulsion, so this test is called the **ethanol emulsion test**.* ## Proteins * Protein molecules contain four elements - **carbon, hydrogen, oxygen** and **nitrogen**. * Some proteins, but not all, also contain a small quantity of the element **sulfur**. * A protein molecule is made of a long chain of smaller molecules, called **amino acids** (Figure 4.9). * There are 20 different kinds of **amino acid**. * Each kind of protein has different amino acids linked together in a precise order. * If even one amino acid is changed or moves into a different place in the protein molecule, then we get a different protein. * So, there is an almost infinite number of different kinds of protein that can be made. ## Proteins have many different functions in organisms. * All **enzymes** are proteins. (You will study enzymes in the next chapter.) * **Antibodies**, which help to protect the body against pathogens, are proteins. * So is **haemoglobin**, the red pigment that transports oxygen in mammalian blood. * Proteins are also important for forming **cell membranes** in all organisms. * In humans, hair and fingernails are made from a protein called **keratin**. ## KEY WORDS * **Protein**: a substance whose molecules are made of many amino acids linked together; each different protein has a different sequence of amino acids. * **Amino acids**: substances with molecules containing carbon, hydrogen, oxygen and nitrogen; there are 20 different amino acids found in organisms. * **Antibodies**: molecules secreted by white blood cells, which bind to pathogens and help to destroy them. * **Pathogens**: microorganisms that cause disease, such as bacteria. * **Keratin**: the protein that forms hair. * **Biuret reagent**: a blue solution that turns purple when mixed with amino acids or proteins. ## 4.2 The Structure of DNA * DNA stands for **deoxyribonucleic acid**. You do not need to remember this long name. * DNA is the material that makes up our **genes** and **chromosomes**. * The nucleus of every cell in the body (apart from red blood cells, which do not have a nucleus) contains DNA. * We inherit our DNA from our **parents**. * DNA is an amazing molecule. It carries a "code", which instructs the cell which amino acids to link together, in which sequence, to make proteins. * The sequence of bases in the DNA molecule **determines** the sequence of the amino acids that are used to build a protein. * You will find out more about this in Chapter 16. ## Because proteins have so many different functions in organisms, DNA determines almost everything about an organism's body structure, and how its metabolic reactions take place. * DNA is made of smaller molecules called **nucleotides**. * Each nucleotide contains a **base**. * There are four bases, A, C, G and T. * A DNA molecule contains two **chains** of nucleotides, coiled around one another (Figure 4.11). * This shape is called a **double helix**. * The two chains or strands are held together by bonds that form between the bases of opposite strands. * A always forms bonds with T, and C with G. * This is called **complementary base pairing**. ## KEY WORDS * **Nucleotides**: molecules that are linked together into long chains, to make up a DNA molecule. * **Base**: one of the components of DNA; there are four bases, A, C, G and T, and their sequence determines the proteins that are made in a cell. * **Complementary base pairing**: the way in which the bases of the two strands of DNA pair up; A always pairs with T, and C with G.