Chapter 2: Biological Molecules PDF

Chapter 2: Biological molecules 1 describe and draw the ring forms of α-glucose and β- glucose 2 define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide 3 state the role of covalent bonds in joining smaller molecules together to form polymers 4 state t...

Chapter 2: Biological molecules 1 describe and draw the ring forms of α-glucose and β- glucose 2 define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide 3 state the role of covalent bonds in joining smaller molecules together to form polymers 4 state that glucose, fructose and maltose are reducing sugars and that sucrose is a non-reducing sugar 5 describe the formation of a glycosidic bond by 6 describe the breakage of a glycosidic bond in condensation, with reference to disaccharides, polysaccharides and disaccharides by hydrolysis, with including sucrose, and polysaccharides reference to the non-reducing sugar test Introductio n We will focus on (1) carbohydrates, (2) lipids and (3) proteins. - Monomer : made of 1 molecule. - Polymer: made of > 1 molecule. - Macromolecule: association of numerous molecules resulting in a large global molecule (ex: 1. Carbohydrate - s All carbohydrates contain the elements carbon, hydrogen and oxygen. - The ‘hydrate’ part of the name comes from the fact that hydrogen and oxygen atoms are present in the ratio of 2 : 1, as they are in water (‘hydrate’ refers to water). The general formula for a carbohydrate can be written as: Cx(H2O)y. - Carbohydrates are divided into three main groups, Monosaccharides (“mono” = 1), Disaccharides (“di” = 2), Polysaccharides (“poly” = x). 1.1. Monosaccharides - Monosaccharides are sugars that dissolve easily in water to form sweet-tasting solutions. - Monosaccharides have the general formula (CH2O)n and consist of a single sugar molecule (n = 1). - The main types of monosaccharides, if they are classified according to the number of carbon atoms in each molecule, are: Trioses (3C), Pentoses (5C) → two common pentoses are ribose and deoxyribose (DNA & RNA), Hexoses (6C) → common hexoses are glucose, fructose  The key molecules that are required to build structures that enable organisms to function are: Carbohydrates Proteins Lipids Nucleic Acids Water  Carbohydrates, proteins, lipids and nucleic acids contain the elements carbon (C) and hydrogen (H) making them organic compounds Carbon atoms are key to the organic compounds because: Each carbon atom can form four covalent bonds – this makes the compounds very stable (as covalent bonds are so strong they require a large input of energy to break them) Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur Carbon atoms can bond to form straight chains, branched chains or rings Carbon compounds can form small single subunits (monomers) that bond with many repeating subunits to form large molecules (polymers) by a process called polymerisation Macromolecules are very large molecules That contain 1000 or more atoms therefore having a high molecular Carbohydrates Types of carbohydrate Carbohydrates are one of table the main carbon-based compounds in living organisms All molecules in this group contain C, H and O As H and O atoms are always present in the ratio of 2:1 (eg. water H2O, which is where ‘hydrate’ comes from) they can be represented by the formula Cx (H2O)y The three types of carbohydrates are monosaccharides, di saccharides and polysac The Two Forms of Glucose The most well-known carbohydrate monomer is glucose Glucose has the molecular formula C6H12O6 Glucose is the most common monosaccharide and is of central importance to most forms of life There are different types of monosaccharide formed from molecules with varying numbers of carbon atom, for example: Trioses (3C) eg. glyceraldehyde Pentoses (5C) eg. ribose Hexoses (6C) eg. glucose Glucose exists in two structurally different forms – alpha (α) glucose and beta (β) glucose and is therefore known as an isomer This structural variety results in different functions between carbohydrates  Different polysaccharides are formed from the two isomers of glucose Structure of polysaccharides table Covalent Bonds in Polymers A covalent bond is the sharing of two or more electrons between two atoms The electrons can be shared equally forming a nonpolar covalent bond or unequally (where an atom can be more electronegative δ-) to form a polar covalent bond When two monomers are Generally each atom will form a certain close enough that their number of covalent bonds due to the outer orbitals overlap this number of free electrons in the outer results in their electrons orbital e.g. H = 1 bond, C = 4 bonds being shared and a covalent Covalent bonds are very stable as high bond forming. If more energies are required to break the bonds monomers are added Multiple pairs of electrons can be shared then polymerisation occur forming double bonds (e.g. unsaturated s (and / or a macromolecule Condensation Also known as dehydration synthesis (‘to put together while losing water’) A condensation reaction occurs when monomers combine together by covalent bonds to form polymers (polymerisation) or macromolecules (lipids) and water is removed Hydrolysis Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water) In the hydrolysis of polymers, covalent bonds are broken when water is added  Reducing & Non-Reducing Sugars Sugars can be classified as reducing or non-reducing; this classification is dependent on their ability to donate electrons Reducing sugars can donate electrons (the carbonyl group becomes oxidised), the sugars become the reducing agent Thus reducing sugars can be detected using the Benedict’s test as they reduce the soluble copper sulphate to insoluble brick-red copper oxide Examples: glucose, fructose, maltose Non-reducing sugars cannot donate electrons, therefore they cannot be oxidised To be detected non-reducing sugars must first be hydrolysed to break the disaccharide into its two monosaccharides before a Benedict’s test can be carried out Example: sucrose The mnemonic to remember the definitions for oxidation and reduction Forming the Glycosidic Bond To make monosaccharides more suitable for transport, storage and to have less influence on a cell’s osmolarity, they are bonded together to form disaccharides and polysaccharides Disaccharides and polysaccharid es are formed when two hydroxyl (-OH) groups (on different saccharides) interact to form a strong covalent bond called the glycosidic bond (the oxygen link that holds the two molecules together) Every glycosidic bond results in one water molecule  Each glycosidic bond is catalysed by enzymes specific to which OH groups are interacting As there are many different monosaccharides this results in different types of glycosidic bonds forming (e.g maltose has a α-1,4 glycosidic bond and sucrose has a α-1,2 glycosidic bond)The formation of a glycosidic bond by condensation between α-glucose and β-fructose to form a disaccharide (sucrose) Breaking the Glycosidic Bond The glycosidic bond is broken when water is added in a hydrolysis (meaning ‘hydro’ - with water and ‘lyse’ - to break) reaction Disaccharides and polysaccharide s are broken down in hydrolysis reactions Hydrolytic reactions are catalysed by enzymes, these are different to those present in condensation reactions Examples of hydrolytic reactions include the digestion of food in the alimentary tract and the breakdown of stored carbohydrates in muscle Glycosidic bonds are broken by and liver cells for use in cellular the addition of water in a respiration hydrolysis reaction  Sucrose is a non- reducing sugar which gives a negative result in a Benedict’s test. When sucrose is heated with hydrochloric acid this provides the water that hydrolyses the glycosidic bond resulting in two monosaccharides that will produce a A molecule of glucose and a molecule of fructose are positive Benedict's formed when one molecule of sucrose is hydrolysed; test the addition of water to the glycosidic bond breaks it 7 describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms 8 describe the molecular structure of the polysaccharide cellulose and outline how the arrangement of cellulose molecules contributes to the function of plant cell walls 9 state that triglycerides are non-polar hydrophobic molecules and describe the molecular structure of triglycerides with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds 10 relate the molecular structure of triglycerides to their functions in living organisms Starch & Glycogen: Structures & Functions Starch and glycogen are polysaccharides Polysaccharides are macromolecules that are polymers formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form chains. These chains may be: Branched or unbranched Folded (making the molecule compact which is ideal for storage eg. starch and glycogen) Straight (making the molecules suitable to construct cellular structures e.g. cellulose) or coiled Starch and glycogen are storage polysaccharides because they are: Compact (so large quantities can be stored) Insoluble (so will have no osmotic effect, unlike glucose which would lower the water potential of a cell causing water to move into cells, cells would then have to have thicker cell walls - plants or burst if they were animal cells) Starch Starch is the storage polysaccharide of plants. It is stored as granules in plastids (e.g. chloroplasts) Due to the many monomers in a starch molecule, it takes longer to digest than glucose Starch is constructed from two different polysaccharide s: Amylose (10 - 30% of starch) Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules The helix shape enables it to be more compact and  Amylopectin (70 - 90% of starch) 1,4 glycosidic bonds between α-glucose molecules but also 1,6 glycosidic bonds form between glucose molecules creating a branched molecule The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage Glycogen Glycogen is the storage polysaccharide of animals and fungi, it is highly branched and not coiled Liver and muscles cells have a high concentration of glycogen, present as visible granules, as the cellular respiration rate is high in these cells (due to animals being mobile) Glycogen is more branched than amylopectin making it more compact which helps animals store more The branching enables more free ends where glucose molecules can either be added or removed allowing for condensation and hydrolysis reactions to occur more rapidly – thus the storage or release of glucose can suit the demands of the cell Cellulose: Structure & Function Cellulose is a polysaccharide Polysaccharides are macromolecules that are polymers formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form chains. These chains may be: Branched or unbranched Folded (making the molecule compact which is ideal for storage, eg. starch and glycogen) Straight (making the molecules suitable to construct cellular structures, eg. cellulose) or coiled Polysaccharides are insoluble in water ​ Cellulose – structure​ Is a polymer consisting of long chains of β-glucose joined together by 1,4 glycosidic bonds​ As β-glucose is an isomer of α-glucose to form the 1,4 glycosidic bonds consecutive β-glucose molecules must be rotated 180° to each other​ ​ To form the 1,4 glycosidic bond between two β-glucose molecules, the glucose molecules must be rotated to 180° to each other  Due to the inversion of the β-glucose molecules many hydrogen bonds form between the long chains giving cellulose it’s strength Cellulose is used as a structural component due to the strength it has from the many hydrogen bonds that form between the long chains of β-glucose molecules Cellulose – function Cellulose is the main structural component of cell walls due to its strength which is a result of the many hydrogen bonds found between the parallel chains of microfibrils The high tensile strength of cellulose allows it to be stretched without breaking which makes it possible for cell walls to withstand turgor pressure The cellulose fibres and other molecules (eg. lignin) found in the cell wall form a matrix which increases the strength of the cell walls The strengthened cell walls provides support to the plant Cellulose fibres are freely permeable which allows water and solutes to leave or reach the cell surface membrane As few organisms have the enzyme (cellulase) to hydrolyse cellulose it is a source of fibre Lipids Macromolecules which contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates lipids contain a lower proportion of oxygen Non-polar and hydrophobic (insoluble in water) Different types: Fats and Oils (composed mainly of triglycerides) Phospholipids Steroids and waxes (considered lipids as they are hydrophobic thus insoluble in water) Triglycerides Are non-polar, hydrophobic molecules The monomers are glycerol and fatty acids Glycerol is an alcohol (an organic molecule that contains a hydroxyl group bonded to a carbon atom) Fatty acids contain a methyl group at one end of a hydrocarbon chain (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group Fatty acids can vary in two ways: Length of the hydrocarbon chain The fatty acid may be saturated (mainly in animal fat) or unsaturated (mainly vegetable oils, although there are exceptions e.g. coconut and palm oil) Unsaturated fatty acids can be mono or poly-unsaturated If H atoms are on the same side of the double bond they are cis-fatty acids and are metabolised by enzymes If H atoms are on opposite sides of the double bond they are trans-fatty acids and cannot form enzyme-substrate complexes, therefore, are not  Triglycerides are formed by esterification An ester bond forms when the hydroxyl group of the glycerol bonds with the carboxyl group of the fatty acid For each ester bond formed a water molecule is released Therefore, for one triglyceride to form three water molecules are released Triglycerides: Structure & Function Energy storage The long hydrocarbon chains contain many carbon-hydrogen bonds with little oxygen (triglycerides are highly reduced) So when triglycerides are oxidised during cellular respiration this causes these bonds to break releasing energy used to produce ATP Triglycerides therefore store more energy per gram than carbohydrates and proteins (37kJ compared to 17kJ) As triglycerides are hydrophobic they do not cause osmotic water uptake in cells so more can be stored Plants store triglycerides, in the form of oils, in their seeds and fruits. If extracted from seeds and fruits these are generally liquid at room temperature due to the presence of double bonds which add kinks to the fatty acid chains altering their properties Mammals store triglycerides as oil droplets in adipose tissue to help them survive when Thefood is scarce (e.g. oxidation hibernating of the bears) carbon-hydrogen bonds releases large numbers of water molecules (metabolic water) during cellular respiration Desert animals retain this water if there is no liquid water to drink Bird and reptile embryos in their shells also use this water Insulation Triglycerides are part of the composition of the myelin sheath that surrounds nerve fibresThis provides insulation which increases the speed of transmission of nerve impulses Triglycerides compose part of the adipose tissue layer below the skin which acts as insulation against heat loss (eg. blubber of whales) Buoyancy The low density of fat tissue increases the ability of animals to float more easily Protection The adipose tissue in mammals contains stored triglycerides and this tissue helps protect organs from the risk of damage  The Vital Role of Structure Phospholipids Phospholipids are a type of lipid, therefore they are formed from the monomer glycerol and fatty acids Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO43-) As the phosphate is polar it is soluble in Phospholipids are the major components of water (hydrophilic) cell surface membranes. They have fatty The fatty acid ‘tails’ acid tails that are hydrophobic and a are non-polar and phosphate head, that is hydrophilic,  Phospholipids are amphipathic (they have both hydrophobic and hydrophilic parts) As a result of having hydrophobic and hydrophilic parts phospholipid molecules form monolayers or bilayers in water Role The main component (building block) of cell membranes Due to the presence of hydrophobic fatty acid tails, a hydrophobic core is created when a phospholipid bilayer forms This acts as a barrier to water-soluble molecules The hydrophilic phosphate heads form H-bonds with water allowing the cell membrane to be used to compartmentalise This enables the cells to organise specific roles into organelles helping with efficiency Composition of phospholipids contributes to the fluidity of the cell membrane If there are mainly saturated fatty acid tails then the membrane will be less fluid If there are mainly unsaturated fatty acid tails then the membrane will be more fluid Phospholipids control membrane protein orientation Weak hydrophobic interactions between the phospholipids and membrane Amino Acids & the Peptide Bond Proteins Proteins are polymers (and macromolecules) made of monomers called amino acids The sequence, type and number of the amino acids within a protein determines its shape and therefore its function Proteins are extremely important in cells because they form all of the following: Enzymes Cell membrane proteins (eg. carrier) Hormones Immunoproteins (eg. immunoglobulins) Transport proteins (eg. haemoglobin) Structural proteins (eg. keratin, collagen) Contractile proteins (eg. myosin) Amino acid Amino acids are the monomers of proteins There are 20 amino acids found in proteins common to all living organisms The general structure of all amino acids is a central carbon atom bonded to: An amine group -NH2 A carboxylic acid group -COOH A hydrogen atom An R group (which is how each amino acid differs and why amino acid properties differ e.g. whether they are acidic or basic or whether they are polar or non-polar) Peptide bond In order to form a peptide bond a hydroxyl (-OH) is lost from a carboxylic group of one amino acid and a hydrogen atom is lost from an amine group of another amino acid The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid This is a condensation reaction so water is released. The resulting molecule is a dipeptide When many amino acids are bonded together by peptide bonds the molecule formed is called a polypeptide. A protein may have only one polypeptide chain or it may have multiple chains interacting with each other During hydrolysis reactions polypeptides are broken down to amino acids when the addition of water breaks the peptide bonds Proteins: Structures There are four levels of structure in proteins, three are related to a single polypeptide chain and the fourth level relates to a protein that has two or more polypeptide chains Polypeptide or protein molecules can have anywhere from 3 amino acids (Glutathione) to more than 34,000 amino acids (Titan) bonded together in chains Primary The sequence of amino acids bonded by covalent peptide bonds is the primary structure of a protein DNA of a cell determines the primary structure of a protein by instructing the cell to add certain amino acids in specific quantities in a certain sequence. This affects the shape and therefore the function of the protein The primary structure is specific for each protein (one alteration in the Secondary The secondary structure of a protein occurs when the weak negatively charged nitrogen and oxygen atoms interact with the weak positively charged hydrogen atoms to form hydrogen bonds There are two shapes that can form within proteins due to the hydrogen bonds: α-helix β-pleated sheet The α-helix shape occurs when the hydrogen bonds form between every fourth peptide bond (between the oxygen of the carboxyl group and the hydrogen of the amine group) The β-pleated sheet shape forms when the protein folds so that two parts of the polypeptide chain are parallel to each other enabling hydrogen bonds to form between parallel peptide bonds Most fibrous proteins have secondary structures (e.g. collagen and keratin) The secondary structure only relates to hydrogen bonds forming The secondary structure of a protein with the α-helix and β-pleated sheet shapes highlighted. The magnified regions illustrate how the hydrogen bonds form between the peptide bonds Tertiary Further conformational change of the secondary structure leads to additional bonds forming between the R groups (side chains) The additional bonds are: Hydrogen (these are between R groups) Disulphide (only occurs between cysteine amino acids) Ionic (occurs between charged R groups) Weak hydrophobic interactions (between non-polar R groups) This structure is common in globular proteins  A polypeptide chain will fold differently due to the interactions (and hence the bonds that form) between R groups. The three- dimensional configuration that forms is called the tertiary structure of a protein Each of the twenty amino acids that make up proteins has a unique R group and therefore many different interactions can occur creating a vast range of protein configurations and therefore functions Within tertiary structured proteins are the following bonds: Strong covalent disulphide Weak hydrophobic Disulphide Disulphide bonds are strong covalent bonds that form between two cysteine R groups (as this is the only amino acid with an available sulphur atom in its R group) These bonds are the strongest within a protein, but occur less frequently, and help stabilise the proteins These are also known as disulphide bridges Can be broken by oxidation Disulphide bonds are common in proteins secreted from cells eg. insulin Ionic Ionic bonds form between positively charged (amine group -NH3+) and negatively charged (carboxylic acid -COO-) R groups Ionic bonds are stronger than hydrogen bonds but they are not common These bonds are broken by pH changes Hydrogen​ Hydrogen bonds form between strongly polar R groups. These are the weakest bonds that form but the most common as they form between a wide variety of R groups​ ​ Hydrophobic interactions Hydrophobic interactions form between the non-polar (hydrophobic) R groups within the interior of proteins Summary of bonds in proteins table Quaternary Occurs in proteins that have more than one polypeptide chain working together as a functional macromolecule, for example, haemoglobin Each polypeptide chain in the quaternary structure is referred to as a subunit of the protein The quaternary structure of a protein. This is an example of haemoglobin which contains four subunits (polypeptide chains) The Molecular Structure of Haemoglobin Structure Haemoglobin is a globular protein which is an oxygen-carrying pigment found in vast quantities in red blood cells It has a quaternary structure as there are four polypeptide chains. These chains or subunits are globin proteins (two α– globins and two β–globins) and each subunit has a prosthetic haem group The four globin subunits are held together by disulphide bonds and arranged so that their hydrophobic R groups are facing inwards (helping preserve the three- dimensional spherical shape) and the hydrophilic R groups are facing outwards (helping maintain  The arrangements of the R groups is important to the functioning of haemoglobin. If changes occur to the sequence of amino acids in the subunits this can result in the properties of haemoglobin changing. This is what happens to cause sickle cell anaemia (where base substitution results in the amino acid valine (non-polar) replacing glutamic acid (polar) making haemoglobin less soluble) The prosthetic haem group contains an iron II ion (Fe2+) which is able to reversibly combine with an oxygen molecule forming oxyhaemoglobin and results in the haemoglobin appearing bright red Each haemoglobin with the four haem Function Haemoglobin is responsible for binding oxygen in the lung and transporting the oxygen to tissue to be used in aerobic metabolic pathways As oxygen is not very soluble in water and haemoglobin is, oxygen can be carried more efficiently around the body when bound to the haemoglobin The presence of the haem group (and Fe2+) enables small molecules like oxygen to be bound more easily because as each oxygen molecule binds it alters the quaternary structure (due to alterations in the tertiary structure) of the protein which causes haemoglobin to have a higher affinity for the subsequent oxygen molecules and they bind more easily The existence of the iron II ion (Fe2+) in the prosthetic haem group also allows oxygen to reversibly bind as none of the amino acids that make up the polypeptide chains in haemoglobin are well suited to binding with oxygen Proteins: Globular & Fibrous Globular Globular proteins are compact, roughly spherical (circular) in shape and soluble in water Globular proteins form a spherical shape when folding into their tertiary structure because: their non-polar hydrophobic R groups are orientated towards the centre of the protein away from the aqueous surroundings and their polar hydrophilic R groups orientate themselves on the outside of the protein  This orientation enables globular proteins to be (generally) soluble in water as the water molecules can surround the polar hydrophilic R groups​ The solubility of globular proteins in water means they play important physiological roles as they can be easily transported around organisms and be involved in metabolic reactions​ The folding of the protein due to the interactions between the R groups results in globular proteins having specific shapes. This also enables globular proteins to play physiological roles, for example, enzymes can catalyse specific reactions and immunoglobulins can respond to specific antigens​ Some globular proteins are conjugated proteins that contain a prosthetic group eg. haemoglobin which contains the prosthetic group called haem​ Fibrous Fibrous proteins are long strands of polypeptide chains that have cross- linkages due to hydrogen bonds They have little or no tertiary structure Due to the large number of hydrophobic R groups fibrous proteins are insoluble in water Fibrous proteins have a limited number of amino acids with the sequence usually being highly repetitive The highly repetitive sequence creates very organised structures that are strong and this along with their insolubility property, makes fibrous proteins very suitable for structural roles, for example, keratin that makes up hair, nails, horns and feathers and collagen which is a The Molecular Structure of Collagen Collagen is the most common structural protein found in vertebrates In vertebrates it is the component of connective tissue which forms: Tendons Cartilage Ligaments Bones Teeth Skin Walls of blood vessels Cornea of the eye Collagen is an insoluble fibrous protein Structure Collagen is formed from three polypeptide chains closely held together by hydrogen bonds to form a triple helix (known as tropocollagen) Each polypeptide chain is a helix shape (but not α-helix as the chain is not as tightly wound) and contains about 1000 amino acids with glycine, proline and hydroxyproline being the most common In the primary structure of collagen almost every third amino acid is glycine This is the smallest amino acid with a R group that contains a single hydrogen atom Glycine tends to be found on the inside of the polypeptide chains allowing the three chains to be arranged closely together forming a tight triple helix structure Along with hydrogen bonds forming between the three chains there are also covalent bonds present  Covalent bonds also form cross- links between R groups of amino acids in interacting triple helices when they are arranged parallel to each other. The cross- links hold the collagen molecules together to form fibrils The collagen molecules are positioned in the fibrils so that there are staggered ends (this gives the striated effect seen in electron micrographs) When many fibrils are arranged together they form collagen fibres Collagen fibres are positioned so that they are lined up with the forces they are withstanding Function Flexible structural protein forming connective tissues The presence of the many hydrogen bonds within the triple helix structure of collagen results in great tensile strength. This enables collagen to be able to withstand large pulling forces without stretching or breaking The staggered ends of the collagen molecules within the fibrils provide strength Collagen is a stable protein due to the high proportion of proline and hydroxyproline amino acids result in more stability as their R groups repel each other Length of collagen molecules means they take too long to dissolve in water (collagen is therefore insoluble in water) Water Molecules: Hydrogen Bonds Water is of great biological importance. It is the medium in which all metabolic reactions take place in cells. Between 70% to 95% of the mass of a cell is water As 71% of the Earth’s surface is covered in water it is a major habitat for organisms Water is composed of atoms of hydrogen and oxygen. One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding) Although water as a whole is electrically neutral the sharing of the electrons is uneven between the oxygen and hydrogen atoms The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a weak negatively charged region on the oxygen atom (δ-) and a weak positively charged region on the hydrogen atoms(δ+), this also results in the asymmetrical shape This separation of charge due to the electrons in the covalent bonds  Hydrogen bonds form between water molecules As a result of the polarity of water hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming. However when there are large numbers present they form a strong structure Hydrogen bonds contribute to the many properties water molecules have that make them so important to living organisms: An excellent solvent – many substances can dissolve in water A relatively high specific heat capacity A relatively high latent heat of vaporisation Water Molecules: In Living Organisms Water has many essential roles in living organisms due to its properties: The polarity of water molecules The presence and number of hydrogen bonds between water molecules Solvent As water is a polar molecule many ions (e.g. sodium chloride) and covalently bonded polar substances (e.g. glucose) will dissolve in it This allows chemical reactions to occur within cells (as the dissolved solutes are more Due to its polarity chemically reactive when they are free to move water is considered about) Metabolites can be transported efficiently a universal solvent High specific heat capacity The specific heat capacity of a substance is the amount of thermal energy required to raise the temperature of 1kg of that substance by 1°C. Water’s specific heat capacity is 4200 J/kg°C The high specific heat capacity is due to the many hydrogen bonds present in water. It takes a lot of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of water does not fluctuate greatly The advantage for living organisms is that it: Provides suitable habitats Allows for constant temperatures within bodies and cells to be maintained (this ensures enzymes have the optimal temperatures) This is because a large increase in energy is needed to increase the temperature of water Latent heat of vaporisation In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by water to break the hydrogen bonds and evaporate This is an advantage for living organisms as only a little water is required to evaporate for the organism to lose a great amount of heat This provides a cooling effect for living organisms, for example the transpiration from leaves or evaporation of water in sweat on the skin