CIE AS LEVEL Biology : Biological Molecules

Benedict's test for reducing sugars

Add Benedict's reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube
Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes
If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water
A positive test result is, therefore, a colour change somewhere along a colour scale from blue (no reducing sugar) to brown/brick-red (a high concentration of reducing sugar)
This test is semi-quantitative as the degree of the colour change can give an indication of how much (the concentration of) reducing sugar present

Iodine test for starch

To test for the presence of starch in a sample, add a few drops of orange/brown iodine in potassium iodide solution to the sample
The iodine is in potassium iodide solution as iodine is insoluble in water

If starch is present, iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black colour
This test is useful in experiments for showing that starch in a sample has been digested by enzymes

Emulsions test for lipids

Lipids are nonpolar molecules that do not dissolve in water but will dissolve in organic solvents such as ethanol
Add ethanol to the sample to be tested, shake to mix and then add the mixture to a test tube of water
If lipids are present, a milky emulsion will form (the solution appears 'cloudy'); the more lipid present, the more obvious the milky colour of the solution
If no lipid is present, the solution remains clear

Biuret test for proteins

A liquid solution of a sample is treated with sodium or potassium hydroxide to make the solution alkaline
A few drops of copper (II) sulfate solution (which is blue) is added to the sample
Biuret 'reagent' contains an alkali and copper (II) sulfate

If a colour change is observed from blue to lilac/purple, then protein is present.The colour change can be very subtle, it's wise to hold the test tubes up against a white tile when making observations)

If no colour change is observed, no protein is present
For this test to work, there must be at least two peptide bonds present in any protein molecules, so if the sample contains amino acids or dipeptides, the result will be negative

Test for non-reducing sugars

Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil
Neutralise the solution with sodium hydrogencarbonateUse a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline for the Benedict's test to work

Then carry out Benedict's test as normal; add Benedict's reagent to the sample and heat in a water bath that has been boiled - if a colour change occurs (orange-red precipitate), a non-reducing sugar is present

E xplaination for the test for non-reducing sugars

The addition of acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules. The resulting monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form

Key biological molecules

Carbohydrates
Proteins
Lipids
Nucleic Acids
Water

Carbohydrates

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 mass. Polymers can be macromolecules, however not all macromolecules are polymers as the subunits of polymers have to be the same repeating units

Types of carbohydrates

monosaccharides, disaccharides and polysaccharides

2 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

Covalent bond

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

Generally each atom will form a certain number of covalent bonds due to the number of free electrons in the outer orbital e.g. H = 1 bond, C = 4 bonds. Covalent bonds are very stable as high energies are required to break the bonds. Multiple pairs of electrons can be shared forming double bonds (e.g. unsaturated fats C=C) or triple bonds

Polymerization of monomers

When two monomers are close enough that their outer orbitals overlap this results in their electrons being shared and a covalent bond forming. If more monomers are added then polymerisation occurs (and / or a macromolecule forms)

Condensation of monomers

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 of polymers

Hydrolysis means 'lyse' (to break) and 'hydro' (with water)In the hydrolysis of polymers, covalent bonds are broken when water is added

Reducing sugars

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

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

Forming glycosodic bonds

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 polysaccharides 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 being removed, thus glycosidic bonds are formed by condensation
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)

Breaking the glycosodic bond

The glycosidic bond is broken when water is added in a hydrolysis (meaning 'hydro' - with water and 'lyse' - to break) reaction. Disaccharides and polysaccharides 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 and liver cells for use in cellular respiration
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 positive Benedict's test

What are starch and glycogen

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 polysaccharides:
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 thus it is more resistant to digestion

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

What is cellulose

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

Structure of cellulose

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
Due to the inversion of the β-glucose molecules many hydrogen bonds form between the long chains giving cellulose it's strength

Function of cellulose

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)

Phospholipids(structure)

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 water (hydrophilic) The fatty acid 'tails' are non-polar and therefore insoluble in water (hydrophobic)
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

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 chai, nThe 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 metabolised. They are linked with coronary heart disease.

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

Function of triglyceride (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 food is scarce (e.g. hibernating bears)

Functions of triglycerides(insulation, buoyancy, protection )

1-Insulation :
Triglycerides are part of the composition of the myelin sheath that surrounds nerve fibres
This 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)
2-Buoyancy:
The low density of fat tissue increases the ability of animals to float more easily
3-Protection:
The adipose tissue in mammals contains stored triglycerides and this tissue helps protect organs from the risk of damage

Functions of phospholipids

1.The main component (building block) of cell membranesDue 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
2.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
3.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
4. Phospholipids control membrane protein orientation, Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer

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

Importance of proteins in cells

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

Primary structure of a protein

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 sequence of amino acids can affect the function of the protein)

Secondary Structure of a protein

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-helixβ-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 bondsMost fibrous proteins have secondary structures (e.g. collagen and keratin)The secondary structure only relates to hydrogen bonds forming between the amino group and the carboxyl group (the 'protein backbone')The hydrogen bonds can be broken by high temperatures and pH changes

Tertiary structure of a protein

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

Quaternary structure of a protein

Occurs in proteins that have more than one polypeptide chain working together as a functional macromolecule, for example, haemoglobinEach polypeptide chain in the quaternary structure is referred to as a subunit of the protein

Bonds within tertiary structured proteins

Strong covalent
disulphide
Weak hydrophobic interactions
Weak hydrogenIonic

Bonds in a tertiary structured protein explained

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 proteinsThese are also known as disulphide bridgesCan be broken by oxidationDisulphide 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 groupsIonic bonds are stronger than hydrogen bonds but they are not commonThese 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

Globular proteins

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 andtheir 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 antigensSome globular proteins are conjugated proteins that contain a prosthetic group eg. haemoglobin which contains the prosthetic group called haem

Fibrous proteins

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 connective tissue found in skin, tendons and ligaments

Structure of haemoglobin

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 its solubility)
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 groups can therefore carry four oxygen molecules (eight oxygen atoms)

Function of haemoglobin

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 easilyThe 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

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
SkinWalls of blood vessels
Cornea of the eye

Collagen is an insoluble fibrous protein

Structure of collagen

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 atomGlycine 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 presentCovalent 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 fibrilsThe 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 fibresCollagen fibres are positioned so that they are lined up with the forces they are withstanding

Function of collagen

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

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 waterAs 71% of the Earth's surface is covered in water it is a major habitat for organismsWater 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 atomsThe 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 being unevenly shared is called a dipole. When a molecule has one end that is negatively charged and one end that is positively charged it is also a polar moleculeWater is a polar molecule

Hydrogen 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 is less dense when a solidWater has high surface tension and cohesion
It acts as a reagent

Why does waste have roles 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

Water as a 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 chemically reactive when they are free to move about) Metabolites can be transported efficiently (except non-polar molecules which are hydrophobic)

High specific heat capacity of water

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°CThe 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 greatlyThe advantage for living organisms is that it:Provides suitable habitatsAllows 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 vaporization of water

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 evaporateThis is an advantage for living organisms as only a little water is required to evaporate for the organism to lose a great amount of heatThis provides a cooling effect for living organisms, for example the transpiration from leaves or evaporation of water in sweat on the skin