class-2

Topic 1: Lifestyle, health and riskStudents will be assessed on their ability to:

Demonstrate knowledge and understanding of the practical and investigative skills identified in numbers 4 and 5 in the table of How Science Works on page 12 of this specification.

Explain the importance of water as a solvent in transport, including its dipole nature.

Distinguish between monosaccharides, disaccharides and polysaccharides (Glycogen and starch amylose and amylopectin) and relate their structures to their roles in providing and storing energy (-glucose and cellulose are not required in this topic).

Describe how monosaccharides join to form disaccharides (sucrose,lactose and maltose) and polysaccharides (glycogen and amylose) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions.

Describe the synthesis of a triglyceride by the formation of ester bonds during condensation reactions between glycerol and three fatty acids and recognise differences between saturated and unsaturated lipids.

Explain why many animals have a heart and circulation (mass transport to overcome limitations of diffusion in meeting the requirements of organisms).

Describe the cardiac cycle (atrial systole, ventricular systole and diastole) and relate the structure and operation of the mammalian heart to its function, including the major blood vessels.

Explain how the structures of blood vessels (capillaries, arteries and veins) relate to their functions.

Describe how the effect of caffeine on heart rate in Daphnia can be investigated practically, and discuss whether there are ethical issues in the use of invertebrates.

Describe the blood clotting process (thromboplastin release, conversion of prothrombin to thrombin and fibrinogen to fibrin) and its role in cardiovascular disease (CVD).

Lifestyle, Transport, Genes and Health Unit 1Concept approach

Explain the course of events that leads to atherosclerosis (endothelial damage, inflammatory response, plaque formation, raised blood pressure).

Describe the factors that increase the risk of CVD (genetic, diet, age,gender, high blood pressure, smoking and inactivity).

Describe the benefits and risks of treatments for CVD (antihypertensives, plant statins, anticoagulants and platelet inhibitory drugs).

Analyse and interpret data on the possible significance for health of blood cholesterol levels and levels of high-density lipoproteins (HDLs) and low-density lipoproteins (LDLs). Describe the evidence for a causal relationship between blood cholesterol levels (total cholesterol and LDL cholesterol) and CVD.

Discuss how people use scientific knowledge about the effects of diet (including obesity indicators), exercise and smoking to reduce their risk of coronary heart disease.

Describe how to investigate the vitamin C content of food and drink.

Analyse data on energy budgets and diet so as to be able to discuss the consequences of energy imbalance, including weight loss, weight gain, and development of obesity.

Analyse and interpret quantitative data on illness and mortality rates to determine health risks (including distinguishing between correlation and causation and recognising conflicting evidence).

Evaluate design of studies used to determine health risk factors (including sample selection and sample size used to collect data that is both valid and reliable).

Explain why peoples perceptions of risks are often different from the actual risks (including underestimating and overestimating the risks due to diet and other lifestyle factors in the development of heart disease).Unit 1 Lifestyle, Transport, Genes and Health1.Topic 2: Genes and health

Students will be assessed on their ability to:

Demonstrate knowledge and understanding of the practical and investigative skills identified in numbers 4 and 5 in the table of How Science Works on page 12 of this specification.

Explain how models such as the fluid mosaic model of cell membrane are interpretations of data used to develop scientific explanations of the structure and properties of cell membranes.

Explain what is meant by osmosis in terms of the movement of free water molecules through a partially permeable membrane (consideration of water potential is not required).

Explain what is meant by passive transport (diffusion, facilitated diffusion), active transport (including the role of ATP), endocytosis and exocytosis and describe the involvement of carrier and channel proteins in membrane transport.

Describe how membrane structure can be investigated practically, e.g. by the effect of alcohol concentration or temperature on membrane permeability.

Describe the properties of gas exchange surfaces in living organisms (large surface area to volume ratio, thickness of surface, difference in concentration) and explain how the structure of the mammalian lung is adapted for rapid gaseous exchange.

Describe the basic structure of an amino acid (structures of specific amino acids are not required) and the formation of polypeptides and proteins (as amino acid monomers linked by peptide bonds in condensation reactions) and explain the significance of a proteins primary structure in determining its three-dimensional structure and properties (globular and fibrous proteins and types of bonds involved in three-dimensional structure).

Explain the mechanism of action and specificity of enzymes in terms of their three-dimensional structure and explain that enzymes are biological catalysts that reduce activation energy, catalysing a wide range of intracellular and extracellular reactions. Lifestyle, Transport, Genes and Health Unit 1 Concept approach Describe how enzyme concentrations can affect the rates of reactions and how this can be investigated practically by measuring the initial rate of reaction.

Describe the basic structure of mononucleotides (as a deoxyribose or ribose linked to a phosphate and a base, i.e. thymine, uracil, cytosine, adenine or guanine) and the structures of DNA and RNA (as polynucleotides composed of mononucleotides linked through condensation reactions) and describe how complementary base pairing and the hydrogen bonding between two complementary strands are involved in the formation of the DNA double helix.

Describe DNA replication (including the role of DNA polymerase), and explain how Meselson and Stahls classic experiment provided new data that supported the accepted theory of replication of DNA and refuted competing theories.

Explain the nature of the genetic code (triplet code only; nonoverlapping and degenerate not required at IAS).

Describe a gene as being a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.

Outline the process of protein synthesis, including the role of transcription, translation, messenger RNA, transfer RNA and the template (antisense) DNA strand (details of the mechanism of protein synthesis on ribosomes are not required at IAS).

Explain how errors in DNA replication can give rise to mutations and explain how cystic fibrosis results from one of a number of possible gene mutations.

Explain the terms gene, allele, genotype, phenotype, recessive, dominant, homozygote and heterozygote, and explain monohybrid inheritance,including the interpretation of genetic pedigree diagrams, in the context of traits such as cystic fibrosis, albinism, thalassaemia, garden pea height and seed morphology.

Explain how the expression of a gene mutation in people with cystic fibrosis impairs the functioning of the gaseous exchange, digestive and reproductive systems.

Describe the principles of gene therapy and distinguish between somatic and germ line therapy.

Explain the uses of genetic screening: identification of carriers, preimplantation genetic diagnosis and prenatal testing (amniocentesis and chorionic villus sampling) and discuss the implications of prenatal genetic screening. Identify and discuss the social and ethical issues related to genetic screening from a range of ethical viewpoints.

Water:

It is the most important biochemical of all. About 80% of the body of an organism is water. It has unusual properties compared with other substances due to :-its dipolar nature and the hydrogen bonding that allows this.

Dipolar water molecule: A water molecule is made up of two atoms of hydrogen and one atom of oxygen Water is a dipolar molecule: Each water molecule has a small negative charge on the oxygen atom and a small positive charge on the hydrogen atoms. As the water molecule has both positive and negative poles, it is described as dipolar.

There is an attraction between the negative and positive parts of neighboring water molecules. The attractive force between these opposite charges is called a hydrogen bond. The hydrogen bonds form important forces that cause the water molecules to stick together, giving water its unusual properties.

ROLES OF WATER IN LIVING ORGANISMS AND AS AN ENVIRONMENT FOR ORGANISMS

Solvent properties of water: The dipoles on water molecules make water an excellent solvent for ions and polar molecules because the water molecules are attracted to them, collect around and separate them.

How this can be useful within living organisms? Materials can easily be transported in solution for example sap through the xylem and phloem of plants or the blood within animals, which contains hormones, glucose, amino acids and various other substances. Soluble substances are also free to move in water, enabling their particles to collide with others, resulting in metabolic reactions taking place.

Specific heat capacity:This is defined as the amount of energy required to raise the temperature of 1g of substance by 1oC. The hydrogen bonding between water molecules draw them in close together causing water to have a very high specific heat capacity. This means that a great deal of heat must be applied to water molecules to make them move about by even a small amount. This property of water can be extremely useful to the bodies of all organisms which contain large amounts of water which help them: -able to tolerate large temperature changes in the external environment without affecting the temperature of their cells by very much. Organisms are therefore able to regulate their body temperature much more effectively than is the case. This is an example of homeostasis. -Aquatic organisms which live completely surrounded by water and hence have a fairly stable environment immune to rapid temperature changes.

Latent heat of vaporizationThis is defined as the amount of heat energy required to turn a given quantity of liquid into a gas (i.e. a change of state is achieved). The attractions between water molecules make this property possible, allowing water to behave as an effective coolant. For example:-transpiration from the mesophyll layers inside leaves allows them to cool, as water molecules are able to draw a great deal of heat away from the leaf before evaporating from the leaf as a gas.-When we sweat, water evaporates from the surface of our skin, which in much the same way as in plant leaves, allows us to cool down.Density Liquid water becomes more dense as it cools, because the molecules loss kinetic energy and get closer together. But when it freezes, water becomes less dense than it was at 4C. Ice therefore floats on water.Importance of this property:- Bodies of water such as lakes start to freeze from the top down. Ice insulates the water below it, increasing the chance of survival of organisms in the water.

Surface tensionThe attractions between water molecules enable their surfaces to be able to behave as a sort of membrane or skin. This can be useful to many aquatic invertebrates such as pond skaters which use the surface to literally walk on the surface of water. Some invertebrate may also choose to lay eggs on the surface of water. Mosquito larvae also use the surface tension of water to cling to the surface and breathe air, through siphons.

High cohesion: the tendency of water molecules to stick together is known as cohesion.Importance of this property:-High cohesive forces allow water to be pulled up as long, unbroken columns through the xylem vessels in plants.

CARBOHYDRATES They are substances whose molecules contain the elements carbon, hydrogen and oxygen. The hydrogen and oxygen atoms are present in the ratio of 2:1(twice as many hydrogen atoms as carbon or oxygen). The general formula of a carbohydrate is written as Cx(H2O)y. Carbohydrates are divided in to three main groups-monosaccharides, disaccharides and polysaccharides.

Monosaccharides: The simplest carbohydrates and are called sugars. They dissolve easily in water to form sweet solutions. They have the general formula (CH2O)n and consist of a single sugar molecule. They include glucose, fructose and galactose. These three monosaccharides each have six carbon atoms so they are also known as hexose sugars. Their molecular formula is C6H12O6. Monosaccharide molecules exist in straight chain form as well as in a ring form made up of carbon atoms and one oxygen atom and may exist in different isomeric forms.

Structure of a glucose molecule Ring form

Glucose is the best known monosaccharideGlucose molecules can take up two different isomeric forms, called -glucose and -glucose. They have the same chemical formula but a slightly different arrangement of atoms in the molecule. In -glucose, the hydrogen on carbon 1 is above the plane of the ring but in -glucose, the hydrogen on carbon 1 is below the plane of the ring. Fructose and galactose are also isomers of glucose. You will notice that fructose has the same number of carbon, hydrogen and oxygen atoms as glucose but it has a keto group (C=O) instead of the aldehyde group (CHO). This gives fructose slightly different chemical properties, for example it is sweeter than glucose.

Disaccharides: A disaccharide is a carbohydrate formed when two monosaccharides join together by a process known as condensation reaction. The bond that joins them together is called a glycosidic bond. As the two monosaccharides react and the glycosidic bond forms, a molecule of water is released. This type of reaction is known as a condensation reaction. The most common disaccharides are:

DisaccharideMonosaccharides

MaltoseGlucose + Glucose

LactoseGlucose + Galactose

SucroseGlucose + Fructose

Formation of maltose by a condensation reaction

- Two OH groups line up alongside each other.- One OH group combines with a hydrogen atom from the other to form a water molecule.- This allows an oxygen bridge to form between the two molecules, holding them together and forming the disaccharide maltose.-The bridge is called a glycosidic bond.

Formation of maltose by a condensation reaction

- Two OH groups line up alongside each other.- One OH group combines with a hydrogen atom from the other to form a water molecule.- This allows an oxygen bridge to form between the two molecules, holding them together and forming the disaccharide maltose.-The bridge is called a glycosidic bond.

Formation of sucrose by a condensation reaction:

Formation of lactose by a condensation reaction

When water is added to a disaccharide under suitable conditions, it breaks the glycosidic bond into its constituent monosaccharides. This is called hydrolysis.

Breaking of glycosidic bond by addition of water (hydrolysis reaction)

All monosaccharides and some disaccharides act as reducing agents, and will reduce blue Benedicts solution to produce an orange-red precipitate. They are called reducing sugars. Sucrose is a non-reducing sugar.

Role of monosaccharides and disaccharides in living organisms: Function as respiratory substrates that are broken down to provide energy in the form of ATP for carrying out living processes. They are useful because they have a large number of C-H groups and these can be easily oxidized, yielding a lot of energy. Important as building blocks for larger molecules. E.g. Glucose is used to make polysaccharides like starch, glycogen and cellulose. As they are soluble, they are the form in which carbohydrates are transported through an organisms body. Eg. Sucrose is transported in phloem sap.

Polysaccharides: They are macromolecules or polymers formed by joining hundreds or thousands of monosaccharide molecules linked together by condensation reaction. Each successive monosaccharide is added by means of a glycosidic bond. Because their molecules are so enormous, the majority of them are insoluble in water to form sweet solutions. So they are not sugars. When they are hydrolysed, polysaccharides break down into monosaccharides or disaccharides. The most important polysaccharides are starch, cellulose and glycogen, all of which are polymers of glucose.

Types of polysaccharides: They are of two types- storage polysaccharides and structural polysaccharides.Storage polysaccharides: They are starch and glycogen.

Starch: It is the storage polysaccharide in plants. e.g. starch grains in chloroplasts. It is a mixture of two substances- amylose and amylopectin.

Amylose:

An amylose molecule is a very long, unbranching chain composed of between 200 and 5000 -glucose molecules which are joined in a straight chain by 1,4 glycosidic bonds(the monomers are linked between carbon atoms 1 and 4 of successive glucose units). This chain is then wound into a tight coil, making the final molecule very compact. The coil is held in shape by hydrogen bonds between small charges on some of the hydrogen and oxygen atoms in the glucose units.

Amylopectin:

An amylopectin molecule is made of between 5000 and 100000 -glucose units joined to each other by 1,4 glycosidic bonds. The chains are shorter than in amylose, and branch out to the sides. The branches are formed by 1,6 glycosidic linkages. It has up to twice as many glucose molecules as amylose. A suspension of amylose in water gives a blue-black colour with iodine-KIsolution but a suspension of amylopectin gives a red-violet colour. This forms the basis of the test for starch. Mixtures of amylose and amylopectin molecules build up into large starch grains which are found in chloroplasts of leaves and in storage organs such as potato tuber and in seeds of cereals and legumes.

Glycogen: It is the storage polysaccharide in animal cells and fungi. The structure of glycogen molecule is very similar to amylopectin. It is made of chains of 1,4 linked -glucose molecules with 1,6 linkages forming branches, but it has short chains and is more highly branched.

Glycogen molecules clump together to form granules which are visible in liver cells, muscle cells and fungi where they form an energy reserve.

Starch and glycogen are convenient storage molecules. Explain. Their large size makes them insoluble in water and therefore does not have any osmotic effects or chemical influence in the cell. Being insoluble, they do not easily diffuse out of the cells. They fold into compact shapes so a lot of them can be stored in a small space. They are easily converted back to sugars by hydrolysis when required for use in respiration.

Five ways in which the molecular structures of glycogen and amylopectin are similar: Macromolecules/polymers Polysaccharides Made from -glucose Glucose units held together by 1,4 links Branches formed by 1,6 links

Structural polysaccharides:Cellulose: It is a structural polysaccharide contained in plant cell walls. It is not part of the living cell but a non-living covering that encases the protoplast within. The cellulose cell wall is therefore freely permeable. It is a polymer of -glucose molecules linked by -glycosidic bonds between carbon 1 and carbon 4.

As the monomers of cellulose are -glucose molecules, when two such molecules line up, the OH group on carbon atom 1 can only line up along the OH group on carbon atom 4 if one of the molecules is rotated at 180 to the other. This is because the OH group on carbon atom 1 projects below the ring and the OH group on carbon atom 4 projects above the ring. This rotation of successive residue is the reason why cellulose has a different structure to starch.

Formation of 1,4 glycosidic bonds between three -glucose molecules

Functions of cellulose: it functions as a structural component of plant cell walls, making up about 20-40% of the wall on average. It is an important food source for some animals, bacteria and fungi

How the structure of cellulose molecule reveals its suitability for its role: It functions as a structural component of plant cell walls. Each cellulose molecule consists of long, straight chains of - glucose residues with about 10 000 monomers per chain. The 1,4 linkages cause the chains straight. -OH groups project outwards from each chain in all directions and form hydrogen bonds with neighboring chains. This cross linking binds the chains rigidly together. While each individual hydrogen bond adds very little to the strength of the molecule, the overall number of them makes a considerable contribution to strengthening cellulose and making it the valuable structural material it is. The cellulose molecules are grouped together, about 60 to 70 to form microfibrils, which are arranged in larger bundles to form macrofibrils which, in turn, are arranged in parallel groups called fibres. These have tremendous tensile strength. In cell walls, the macrofibrils are arranged in several layers, in a glue-like matrix made of other polysaccharides.

Comparison of amylose, amylopectin, glycogen and cellulose

CharacteristicAmylose Amylopectin Glycogen Cellulose

Found inplantsplantsAnimals and fungiplants

Found asgrainsgrainsTiny granulesFibres

Function Energy storeEnergy storeEnergy storeStructural support

Basic monomer unit-glucose-glucose-glucose-glucose

Type of bond between monomer units1,4 glycosidic1,4 and 1,6 glycosidic1,4 and 1,6 glycosidic1,4 glycosidic

Type of chainUnbranched and helical(coiled)Long, relatively few branches with some coilingShort, relatively many branches with some coilingLong, unbranched straight chains with no coiling

Identification of carbohydrates:1. Test for reducing sugarsIf you have a solution that you suspect contains reducing sugar, you can use Benedicts reagent to test it. Benedicts reagent is copper (II) sulfate in an alkaline solution and has a blue colour. If it is added to a reducing agent, its Cu2+ ions will be reduced to Cu+ resulting in a change of colour to the red of insoluble copper(I). Add 2cm3 of a solution of the reducing sugar to a test tube. Add an equal volume of Benedicts solution. Heat the test tube in a water bath at 90C for 2 minutes. If a reducing sugar is present, the solution will gradually turn from blue through green, yellow and orange to brick red precipitate. The intensity of the red colour is related to the concentration of the reducing sugar.

2. Test for non-reducing sugarsThe most common non-reducing sugar is sucrose, a disaccharide. If you test a non-reducing sugar using Benedicts reagent, you would get a negative result. You must therefore go on to a second stage of the test to be certain whether such a non-reducing sugar is present.

Steps: Add 2cm3 of sucrose solution to a test tube. Add 1 cm3 of dil.HCl. Boil for 1 minute. This is to hydrolyse any glycosidic bonds present. This will release the free monosaccharides, glucose and fructose. Carefully neutralize the solution with sodium hydrogen carbonate (check with pH paper). Purpose of this step is that Benedicts reagent needs alkaline conditions to work. Add Benedicts reagent and heat as before and look for the color change. If the solution goes red now but didnt in the first stage of the test, there is non-reducing sugar present. If there is no color change then there is no sugar of any kind present.

3. Test for starch:Iodine/potassium iodide test Add 2 cm3 1% starch solution to a test tube. Add a few drops of I2/KI solution. A blue black coloration shows the presence of starch

Lipids: They are a diverse group of water-insoluble organic chemicals that contain carbon, hydrogen and oxygen, like carbohydrates but the proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates. The main groups of lipids are triglycerides (fats and oils) and phospholipids.Triglycerides: The most common type of lipids. They are usually known as fats and oils. At room temperature (10-20C), fats are solid but oils are liquid.

Structure of a triglyceride: A triglycride molecule is made of a backbone of glycerol, to which three fatty acids are attached by ester bonds. Each of the three fatty acid molecules joins to glycerol by a condensation reaction.

Glycerol is a type of alcohol. As the glycerol molecule in all triglycerides is the same, the differences in the properties of different fats and oils come from variations in the fatty acids. Fatty acids are organic molecules which all have a COOH group attached to a long hydrocarbon tail, which is a long chain of carbon and hydrogen atoms. Many of the properties of lipids are determined by these tails. The tails are hydrophobic, meaning water-hating. This makes the lipids insoluble in water.

Unsaturated and saturated fatty acids:

Unsaturated fatty acidsSaturated fatty acids

Have one or more double bonds between neighboring carbon atoms, like C-C=C-CNo double bonds between neighboring carbon atoms.

Lower melting point, as double bonds make them melt more easily.Higher melting point

Liquid at room temperatureSolid at room temperature

Eg. Oleic acid, the main constituent of olive oil (all vegetable oils)Palmitic acid and stearic acid(animal fat)

Functions of triglycerides in living organisms: Used as energy reserve in plants, animals and fungi: lipids provide more than twice as much energy as carbohydrates when they are oxidized. This makes them excellent stores of energy. Being insoluble in water, lipids are not easily leached from cells. Advantages of storing triglycerides as energy reserves rather than starch:- More energy stored per gram- High calorific value: 37 kJ vs 17 kJ- Fats are highly reduced.- More CH bonds- Release more energy when oxidized

Act as an insulator: fats are slow conductors of heat so animals like mammals store extra fat below the skin in the form of adipose tissue. It acts as an insulator against loss of heat from the body. In mammals like whales, adipose tissue in the form of blubber also contributes to buoyancy, as lipids are less dense than water. Adipose tissue also forms a protective layer around delicate body organs like kidneys where it acts as packing material to protect the organ from physical damage. Metabolic source of water: When fats are oxidized, water is a product. This metabolic water can be very useful to some desert animals, such as kangaroo rats which never drinks water and stores fat for this purpose. Structural component of cell membranes, contributing their flexibility and transfer of lipid soluble substances across them.

Phospholipids: It is a special type of lipid. Each molecule is made of two fatty acid molecules, a phosphate group and a glycerol molecule.

Diagrammatic representation of a phospholipid molecule A phospholipid molecule is formed when one of the three OH groups of glycerol combines with a phosphoric acid. The other two OH groups combine with two fatty acids as in the formation of a triglyceride. The molecule consists of a phosphate head, with two hydrocarbon tails from the two fatty acids. The phosphate head carries an electrical charge and is therefore soluble in water (hydrophilic). The tails have no electrical charge and so are insoluble in water (hydrophobic). Thus one end of the molecule is soluble in water and the other end is not. This structure of phospholipids makes them important in the formation of cell membranes contributing to their flexibility and transfer of lipid soluble substances across them. Function: They are important components of cell membranes. Both the inside of a cell and the environment outside are watery, and the phospholipids in cell membranes form a double layer, with the hydrophilic heads of the molecules pointing into either the watery environment outside the membrane or the watery medium inside the cell. The hydrophobic tails point into the middle of the membranes. This bilayer arrangement makes cell membranes fluid and easily traversed by lipid-insoluble substances.

How phospholipid molecules are arranged in a cell surface membrane?

- they are arranged in to a bilayer.- hydrophilic heads facing outwards into the water/ outside the cell/cytoplasm - hydrophobic tails/ fatty acid chains, facing each other inwards, therefore avoiding contact with water.

Test for lipids:

Emulsion test:

Add 2cm2 fat or oil (olive oil) to a test tube containing 2 cm3of absolute alcohol. Dissolve the lipid by shaking vigorously. Add an equal volume of cold water. A cloudy white suspension shows the presence of lipid.Note: Lipids are immiscible with water. Adding water to a solution of the lipid in alcohol results in an emulsion of tiny lipid droplets in the water which reflect light and give a white, opalescent appearance. PROTEIN Amino acids are the basic units which combine to make up proteins.Amino acids: They are the basic units from which proteins are made. Over 170 amino acids are known to occur in cells and tissues. Of these only 20 amino acids are commonly found in proteins.

Structure of amino acid: All amino acids have the same basic structure.

There is a central carbon atom called carbon atom to which is always attached an acidic carboxyl group, -COOH, a basic amino group, -NH2 and a hydrogen atom. The fourth position is the only variable part of the molecule and is known as the R group. This group gives each amino acid its unique properties.

In the simplest amino acid, glycine, the R group is a single hydrogen atom. When R is CH3, the amino acid alanine is formed.

Peptide bond:

Amino acids combine to form proteins. In a protein molecule, two amino acids are joined together by a type of bond called peptide bond.

Formation of a peptide bond: This is formed when a water molecule is eliminated during a reaction between the amino group of one amino acid and the carboxyl group of another. It is a condensation reaction and the bond formed is a covalent bond called a peptide bond. The bond is formed between the carbon atom of one amino acid and the nitrogen atom of the other. The compound formed is a dipeptide.

The dipeptide can be broken down in a hydrolysis reaction, which breaks the peptide bond with the addition of a molecule of water, to give its two constituent amino acids.