3.2 Carbohydrates, Lipids & Proteins 04/11/2010 08:20:00 Topic 3: Chemistry of Life 3.2 Carbohydrates, Lipids & Proteins Orange book: pg. 47-47 Green book: pg. 35 3.2.1 Distinguish between organic and inorganic compounds. (pg. 47, 35) 3.2.2 Identify amino acids, glucose, ribose and fatty acids from diagrams showing their structure. (pg. 48-51, 35-36) 3.2.3 List three examples of each of monosaccharides, disaccharides and polysaccharides. (pg. 48, 36) 3.2.4 State one function of glucose, lactose and glycogen in animals and of fructose, sucrose and cellulose in plants. (pg. 48, 36) 3.2.5 Outline the role of condensation and hydrolysis in the relationships between monosaccharides, disaccharides and polysaccharides; between fatty acids, glycerol and triglycerides; and between amino acids and polypeptides. (pg. 49-51, 36-38). 3.2.6 State three functions of lipids (pg. 50, 37) 3.2.7 Compare the use of carbohydrates and lipids in energy storage. (pg. 50, 37) 3.2.1 Organic vs Inorganic 04/11/2010 08:20:00 3.2.1 Distinguish between organic and inorganic compounds. Orange book pg. 47 Green book pg. 35 To do: Take a note of the IB syllabus definition of the term “organic” in your green exercise books. Complete the two questions in the recap. * The IB syllabus states that: “Compounds containing carbon that are found in living organisms (except hydrogencarbonates, carbonates and oxides of carbon) are regarded as organic”. So better to learn this definition rather than ‘containing carbon, hydrogen and oxygen. We are only interested in a few major compounds. Quick recap: What are the three major groups of organic compounds we are interested in? Name a few important inorganic compounds that are important to life on Earth. Background Reading Living things are composed of an amazing array of molecules. We can start to make sense of all these molecules by classifying these into a molecule type. Molecules of the same type have certain qualities in common and become fairly easy to recognize with a little practice. Molecules can be classified as being either inorganic or organic. All organic molecules contain the element carbon, although not all carbon-containing molecules are organic. Carbon dioxide is a common example of a molecules that contains carbon that is not organic. 3.2.2 Identifying Molecules 04/11/2010 08:20:00 3.2.2 Identify amino acids, glucose, ribose and fatty acids from diagrams showing their structure. Orange book pg. 48-51 Green book pg. 35-36 To do (in green exercise books): Under the title of “Amino Acid”: draw the structure of an amino acid label the H attached to the central carbon, the carboxylic acid group, the amino group and the functional ‘R’ group. In one sentence explain the importance of the ‘R’ group Under the title of “Glucose”: Draw the structure of -glucose In one sentence, describe the difference in -glucose and β-glucose In one sentence, explain the important of glucose to living organisms Under the title of “Ribose”: Draw the structures of ribose and deoxyribose Under the title of “Fatty Acids” Draw the structure of the following fatty acid CH3(CH2)4COO On your diagram label: the methyl group, hydrocarbon chain and carboxyl group. * Best way to learn this, is to draw them until you can draw them from memory without looking at a book. Amino Acids Proteins are made of amino acids. All amino acids contain the elements carbon, hydrogen, oxygen and nitrogen. A few amino acids contain sulphur too. The general structure of an amino acid molecule is shown below. There is a central carbon atom (called the "alpha carbon"), with four different chemical groups attached to it: a hydrogen atom an amino group a carboxylic acid group a variable "R" group (or side chain) Amino acids are so-called because they have both amino groups and acid groups, which have opposite charges. At neutral pH (found in most living organisms), the groups are ionised as shown above, so there is a positive charge at one end of the molecule and a negative charge at the other end. The overall net charge on the molecule is therefore zero. There are 20 different R groups, and so 20 different amino acids. Since each R group is slightly different, each amino acid has different properties, and this in turn means that proteins can have a wide range of properties. Glucose Glucose is the most widely distributed sugar in the plant and animal kingdoms and it is the sugar present in blood as "blood sugar". Glucose is the main sugar metabolised by the body for energy. The body digests carbohydrates in foods, transforming them into glucose, which serves as the primary fuel for the brain and muscles. Glucose is absorbed into the bloodstream through the intestinal wall. Only the monosaccharides glucose, fructose and galactose are absorbed in humans; these are the end-products of the digestion of carbohydrates. The diagrams below illustrate the difference between -glucose and β-glucose. Ribose DNA and its close relative RNA are perhaps the most important molecules in biology. They contain the instructions that make every single living organism on the planet, and yet it is only in the past 50 years that we have begun to understand them. DNA stands for deoxyribonucleic acid and RNA for ribonucleic acid, and they are called nucleic acids because they are weak acids, first found in the nuclei of cells. They are polymers, composed of monomers called nucleotides. Nucleotides have three parts to them: a phosphate group (PO42- ), a pentose sugar, which has 5 carbon atoms in it. a nitrogenous base. In this topic we are only interested in the pentose sugar. The sugar that is present in RNA is ribose and the sugar present in DNA is deoxyribose. Fatty Acids Fatty acids are long molecules with a polar, hydrophilic end and a non-polar, hydrophobic "tail". The hydrocarbon chain can be from 14 to 22 CH2 units long, but it is always an even number because of the way fatty acids are made. The formula for a fatty acid can be written CH3(CH2)nCOO 3.2.3 Saccharides 04/11/2010 08:20:00 3.2.3 List three examples of each of monosaccharides, disaccharides and polysaccharides. Orange book pg. 48 Green book pg. 35-36 To do (in green exercise books): Use your own knowledge and the green book to help you complete the table below. Copy the completed table into your greenbooks. Fill in the table below: Saccharide Definition Examples Monosaccharide 1. 2. 3. Dissacharide 1. 2. 3. Polysaccharide 1. 2. 3. Monomers 3.2.4 Functions of Carbohydrates 04/11/2010 08:20:00 3.2.4 State one function of glucose, lactose and glycogen in animals and of fructose, sucrose and cellulose in plants. Orange book pg. 48 Green book pg. 36 To do (in green exercise books): Use your own knowledge and the green book to help you complete the table below. Copy the completed table into your greenbooks. Background reading Carbohydrates are among the most commonly found biochemical molecules found in both animals and plants. Carbohydrates exist in different ‘sizes’ – monosaccharides, disaccharides and polysaccharides. All of these carbohydrates serve many functions in living organisms. Animal Carbohydrates Carbohydrate Type of Carbohydrate Function Glucose Lactose Glycogen Plant Carbohydrates Carbohydrate Type of Carbohydrate Fructose Sucrose Cellulose Function 3.2.5 Condensation & Hydrolysis 04/11/2010 08:20:00 3.2.5 Outline the role of condensation and hydrolysis in the relationships between monosaccharides, disaccharides and polysaccharides; between fatty acids, glycerol and triglycerides; and between amino acids and polypeptides. Orange book pg. 49-51 Green book pg. 36-38 To do: You will practice drawing condensation and hydrolysis reactions on scrap paper. Once you are confident in showing the reaction, copy the condensation and hydrolysis reactions into your books for: monosaccharides to/from disaccharides between fatty acids and glycerol to/from triglycerides amino acids to/from proteins Hydrolysis and Condensation in Carbohydrates Disaccharides Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond. The reaction involves the formation of a molecule of water (H2O): This shows two glucose molecules joining together to form the disaccharide maltose. Because this bond is between carbon 1 of one molecule and carbon 4 of the other molecule it is called a 1-4 glycosidic bond. Bonds between other carbon atoms are possible, leading to different shapes, and branched chains. This kind of reaction, where H2O is formed, is called a condensation polymerization reaction. The reverse process, when bonds are broken by the addition of water (e.g. in digestion), is called a hydrolysis reaction. In general: polymerisation reactions are condensation reactions. breakdown reactions are hydrolysis reactions. Hydrolysis and Condensation in Lipids (triglycerides) Triglycerides Triglycerides are commonly called fats or oils. They are made of glycerol and fatty acids. Glycerol is a small, 3-carbon molecule with three alcohol groups. Fatty acids are long molecules with a polar, hydrophilic end and a non-polar, hydrophobic "tail". The hydrocarbon chain can be from 14 to 22 CH2 units long, but it is always an even number because of the way fatty acids are made. The hydrocarbon chain is sometimes called an R group, so the formula of a fatty acid can be written as R-COO-. One molecule of glycerol joins together with three fatty acid molecules to form a triglyceride molecule, in another condensation polymerisation reaction: Hydrolysis and Condensation in Proteins Proteins are made of amino acids. All amino acids contain the elements carbon, hydrogen, oxygen and nitrogen. A few amino acids contain sulphur too. The general structure of an amino acid molecule is shown below. Peptide Bonds In suitable conditions amino acids polymerise. The α-amino group of one molecule joins to the carboxyl group of another in a condensation reaction, which results in the formation of a peptide bond. The reaction involves the formation of a molecule of water: When two amino acids join together a dipeptide is formed. Three amino acids form a tripeptide. Many amino acids form a polypeptide. e.g.: In a polypeptide there is always one end with a free amino (NH3) group, called the N terminus, and one end with a free carboxyl (CO2) group, called the Cterminus. In a protein the polypeptide chain may be hundreds of amino acids long. Amino acid polymerisation to form polypeptides is part of protein synthesis. It takes place in ribosomes, and is special because it requires an RNA template. The sequence of amino acids in a polypeptide chain is determined by the sequence of the genetic code in DNA. 3.2.6 Functions of Lipids 04/11/2010 08:20:00 3.2.6 State three functions of lipids. Orange book pg. 50 Green book pg. 37 To do: Read the section below and the textbook. Use the information to bullet point three uses of lipids in your green books. Role of Lipids Lipids are biochemically important molecules that serve many functions. We refer to triglyceride lipids in solid form as fats. In liquid form, triglycerides are called oils. Everyone is aware of the role fat plays in energy storage. If you eat more food than you burn, your body will store much of the excess as fat in adipose cells. Each adipose cell gets smaller or larger depending on how much lipid is being stored. People either gain or lose weight depending on how much lipid is being stored at any given time in a fairly fixed number of adipose cells. Lipids are very efficient molecules for storing energy. As seen earlier, carbohydrates are also used for storing energy in living organisms. Glycogen is a carbohydrate used by animals to store energy and starch is used by plants. Lipids are important for thermal insulation. A good reminder of this is to study the amount of blubber (fat) that cold-climate animals form in order to stay warm; 30% or more of the body mass of some seals may be due to the blubber layer beneath their skin. As you know a special category of lipid called phospholipid makes up the bilayer of all cell membranes. These phospholipid molecules have a polar head turned towards water and a non-polar tail which turns away from water. 3.2.7 Energy Storage 04/11/2010 08:20:00 3.2.7 Compare the use of carbohydrates and lipids in energy storage Orange book pg. 50 Green book pg. 37 To do: Read the section below and the textbook (it is important to read the green book for this objective). Summarise a paragraph in your greenbooks to compare the energy storage of carbohydrates and lipids. Introduction Animal cells obtain energy in the form of ATP by oxidizing food molecules through the process of respiration. The hydrolysis of ATP supplies energy needed for cellular processes, such as the transport of molecules or cellular movement. Respiration involves oxidation of an organic compound, the respiratory substrate. Carbohydrate, fats and proteins can all be used, however carbohydrates and fatty acids are the most important fuels for generating ATP in animal cells. Respiration in animal cells depends on oxygen. We can understand a great deal about animal metabolism by comparing the volume of oxygen consumed by an organism to the volume of carbon dioxide produced. These volumes will change depending on the energy source the animal is using. *The equations used to explain this are for illustration only – don’t try to learn this. Respiratory Quotient Consider the equation for respiration: C6H12O6 + 6O2 6CO2 + 6H2O (+ energy) From the equation for respiration we can see that given the time, the volume of carbon dioxide produced during respiration of carbohydrate is equal to the volume of oxygen consumed. The ratio of CO2:O2 is called the respiratory quotient and for respiration using carbohydrate its value is 1. RQ = volume of CO2given off volume of O2 consumed Therefore from the above equation: RQ = 6/6 = 1 Respiratory quotients: Carbohydrates (glucose) = 1.0 Protein = 0.9 fat (lipids) = 0.7 Carbohydrates Carbohydrates, when available will be used first by most cells. Polysaccharides such as starch (in plants) and glycogen (in animals) are hydrolysed to monosaccharides before they enter the respiratory pathway. As carbohydrates are generally broken down to monomers before they are respired we will look at the respiration of glucose. In general, carbohydrates contain approximately the same ratio of carbon, hydrogen and oxygen (although the equation indicates that there is always twice as much hydrogen, as oxygen, we say the ratios are similar when we are comparing them to lipids). Consider the equation for respiration: C6H12O6 + 6O2 6CO2 + 6H2O (+ energy) RQ = 6/6 = 1 Due to the number of C-H bonds, carbohydrates have a higher energy yield than proteins but lower than lipids. Lipids Although lipids release more energy than carbohydrates, they are mainly used in respiration when carbohydrate reserves have been exhausted. This is because carbohydrates are easier to respire and so will be used up first. Lipids must first be converted to glycerol and fatty acids and then the fatty acids can be respired. Generally lipids have a much higher carbon and hydrogen content compared to oxygen. This means there are plenty of C-H bonds to be broken and the hydrogens can be carried off to the electron transport chain to be used to make large amounts of ATP. To give an idea of this, below if the chemical formula for the fat tripalmitin (you do not need to learn this, it is simply to illustrate the point). Notice how few oxygens there are compared to the hydrogen and carbon. C51H96O6 If tripalmitin is respired we would get the equation: 2C51H98O8 + 145O2 102CO2 + 98H2O RQ = 102/ 145 = 0.7