AS LEVEL
BIOLOGICAL MOLECULES
By Ssemanda Jovan
Course outline
• Biochemistry
• The building blocks of life
• Monomers, polymers and macromolecules
• Carbohydrates
• Lipids
• Proteins
• water
Learning outcomes
In this chapter you will learn how to:
• describe how large biological molecules are made from smaller molecules
• describe the structure of carbohydrates, lipids and proteins and how their
structure relates to their functions
• describe and carry out biochemical tests to identify carbohydrates, lipids
and proteins
• explain some key properties of water that make life possible
Biochemistry
• The word “Biochemistry’’ was coined by the German chemist Carl Neuberg in
1930.
• Biological chemistry, often known as biochemistry, is a laboratory-based
branch of Biology that combines biology and chemistry. It explores chemical
processes that occur in and around living organisms and gives rise to the
complexity of life.
• Metabolism- The sum total of all the biochemical reactions in the body is
known as metabolism
• Metabolism is one of the most important processes taking place in all living
things.
• One example of metabolism is the process of digestion.
The building blocks of life
• The four most common elements in living organisms are, in order of
abundance, hydrogen, carbon, oxygen and nitrogen.
• They account for more than 99% of the atoms found in all living things.
• The building blocks of life are carbohydrates, proteins, lipids and nucleic
acids.
• Carbohydrates are building blocks for the most of the macromolecules
Covalent and hydrogen bonding
• A covalent bond is formed due to sharing of electrons between atoms.
• A hydrogen bond is formed due to the attraction between two atoms of
two different molecules.
These bonds are formed in covalent compounds
methane, CH4
water, H2O
ethanol, C2H5OH
a hydrocarbon, e.g. C3H8
ammonia, NH3
ethanoic acid, CH3COOH.
Uniqueness of carbon
• Ita a small molecule, does not take a lot of space
• It has ability to form bonds with itself- Catenation
• Can combine with a lot of other molecules
• Its not too reactive, very stable, able to join with upto 4 atoms
• Carbon is particularly important because carbon atoms can join
together to form long chains or ring structures- act as basic skeleton of
carbon molecules
Monomers, polymers and macromolecules
• Macromolecule- giant biological molecule
• Monomer- relatively simple molecule used as a building block for
polymers
• Polymers- a giant molecule made up of many subunits of monomers
joined together.
• Organic molecules-compounds containing carbon and hydrogen.
Synthesis of biological molecules
• The synthesis of biological molecules involves the formation of large
macromolecules essential for life, which include:
• Carbohydrates: Formed from monosaccharides linked by glycosidic bonds.
• Proteins: Synthesized from amino acids through peptide bonds.
• Lipids: Comprised of fatty acids and glycerol, forming structures like
triglycerides.
• Nucleic acids- built from nucleotides, include DNA and RNA
• Most macromolecules are made from single subunits, or building blocks,
called monomers.
• The monomers combine with each other using covalent bonds to form
larger molecules known as polymers.
Two types of reactions drive synthesis
Dehydration/ condensation reaction
• The reaction involves joining together two monomers by the removal of a
water molecule.
• the hydrogen of one monomer combines with the hydroxyl group of
another monomer, releasing a water molecule.
Hydrolysis
• addition of water to split a molecule
• Polymers break down into monomers during hydrolysis.
• During these reactions, the polymer breaks into two components: one part
gains a hydrogen atom (H+) and the other gains a hydroxyl molecule (OH–)
from a split water molecule.
carbohydrates
Learning outcomes Candidates should be able to:
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 condensation, with
reference to disaccharides, including sucrose, and polysaccharides
Learning outcomes cont..
6. describe the breakage of a glycosidic bond in polysaccharides and
disaccharides by hydrolysis, with reference to the non-reducing sugar
test
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
Structure of carbohydrates
• All carbohydrates contain the elements carbon,
hydrogen and oxygen.
• The ‘hydrate’ part of the name refers to water;
the hydrogen and oxygen atoms are present in
the ratio of 2 : 1 as in water.
• The general formula for a carbohydrate can be
written as Cx (H2O)y .
• Used as a source of energy in form of glucose
• Stored in form of starch/ glycogen
• Structure in for of cellulose, chitin ( Nacetylglucosamine), amylose, amylopectin,
lignin
C₆H₁₂O₆
Classes of carbohydrates
Carbohydrates are divided
into three main groups:
1. monosaccharides,
2. disaccharides
3. polysaccharides.
• The word ‘saccharide’
means a sugar or sweet
substance.
Monosaccharides
• Single unit sugars
molecule
• They test sweat
• Easily dissolve in water
• They have the general
formula (CH2O) n .
• Pentose and hexose
have long carbon chain
that can form stable
ring structures
Straight chain and ring structure of glucose
• Glucose is hexose sugar (C6)
• The molecular formula for a
hexose can be written as
C6H12O6
• The ring structure has two
isomers, α-glucose and βglucose
Glucose is long enough to
close up on itself to form a
more stable ring structure
Formation of ring structure
Alpha-glucose and Beta-glucose
• Carbon atom 1 has a hydroxyl
group
• The hydroxyl group could be
above or below the ring
• If above –its β-glucose
• If below- α-glucose
β-fructose
Question 1
• The formula for a hexose is C6H12O6 or (CH2O)6 .
• What would be the formula of:
1. a triose?
2. a pentose?
Functions of monosaccharides in living organisms
• Source of energy (ATP) during respiration (Glucose)
• Act as building blocks of large molecules. For example,
glucose is used to make the polysaccharides starch, glycogen and cellulose.
Ribose (a pentose) is one of the molecules used to make RNA (ribonucleic
acid) and ATP.
Deoxyribose (also a pentose) is one of the molecules used to make DNA
Disaccharides and the glycosidic bond
• Two sugar units
• They are formed by two
monosaccharides joining
together (‘di’ means two)
• The three most common
disaccharides are maltose,
sucrose and lactose.
• Sucrose is the transport
sugar in plants and the
sugar commonly sold in
shops.
• Lactose in sugar found in
milk.
Formation of disaccharides
• The process of joining two monosaccharides is an example of a
condensation reaction
• Joined by 1,4-glycosidic bonds.
• They can be split by adding water-Hydrolysis.
• For each condensation reaction, two hydroxyl (–OH) groups line up
alongside each other.
• One combines with a hydrogen atom from the other to form a water
molecule
glycosidic bond: a C–O–C
link between two sugar
molecules, formed by a
condensation reaction; it
is a covalent bond
Polysaccharides
• Polysaccharides are polymers made by joining many monosaccharide
molecules by condensation.
• Each successive monosaccharide is added by means of a glycosidic bond,
as in disaccharides.
• The final molecule may be several thousand monosaccharide units long,
forming a macromolecule.
• The most important polysaccharides are starch, glycogen and cellulose,
all of which are polymers of glucose. Polysaccharides are not sugars
Starch and glycogen
• The storage polysaccharide in plants is
starch; in animals, it is glycogen
• Starch is a mixture of two substances
– amylose and amylopectin
• Amylose is a straight chain made by
condensations between α-glucose
molecules
• Amylopectin is a branched chain, with
shorter chains than amylose- has 1-4
and 1-6 linkages
• Starch is commonly found in
chloroplast, tubers and cereals.
Amylose
• Amylose is a polysaccharide that forms a
linear chain of α-D-glucose units linked
by α(1→4) glycosidic bonds, making it a
crucial component of starch
alongside amylopectin. It contributes to
the structural property of starch,
affecting its digestibility and the texture
of food products.
• Its 20% in starch
• Less soluble in water
Amylopectin
• Amylopectin is also made of many
1,4 linked α-glucose molecules, but
the chains are shorter than in
amylose and also contain 1,6
linkages.
• These start branches out to the
sides of the chain
• More soluble in water
• 80% in starch
Suitability of starch as a storage compound
• Its un reactive/ inert
• Insoluble
• Compact
• Question: why glucose can't be stored in the cell.
Qn: State the differences between amylose and amylopectin (indicate in table
form)
Glycogen
• It is a polysaccharide made of many Alpha-glucose molecules linked
together, that acts as a glucose store in liver and muscle cells.
• Glycogen is the storage carbohydrate in animals. It has molecules
very like those of amylopectin because it is made of chains of 1,4
linked α-glucose with 1,6 linkages making branch points.
• It is made up single molecule of amylopectin.
• Glycogen molecules clamp together to make granules that are visible
in liver cells and in muscle cells where they form energy reserves
Glycogen
• Highly branchedmakes it efficient
energy store,
increased surface
area for enzyme
action, rapid
hydrolysis for quick
energy release,
Differences between glycogen and starch
1.Composition: Glycogen is made up of a single molecule, while starch is
made up of two molecules (amylose and amylopectin).
2.Structure: Glycogen forms a branched-chain structure, while starch
forms linear, spiral, and branch structures.
3.Usage: Starch is used for commercial purposes (e.g., paper and textile
industry), while glycogen is not used commercially
cellulose
• Its a polysaccharide made from beta glucose subunits; used as a
strengthening material in plant cell walls.
• It’s the most abundant molecule on planet
• This is due to its presence in plant cell walls and its slow rate of
breakdown in nature.
• It has a structural role because it is a mechanically strong molecule,
unlike starch and glycogen.
• The only difference is that cellulose is a polymer of β-glucose, and starch
and glycogen are polymers of α-glucose.
cellulose
• Cellulose is an example of a linear polysaccharide composed of
repeating glucose monomers linked by β-1,4-glycosidic bonds, unlike
starch, which is made of α-1,4-glycosidic bonds.
• The polymerization of glucose units forms long chains, which
combine to create the cellulose microfibrils, a critical structural
component in plant cell walls.
• The hydroxyl groups (-OH) on the glucose molecules form hydrogen
bonds with oxygen atoms.
• The linear structure allows cellulose molecules to form strong
hydrogen bonds with neighboring molecules, creating a highly stable
network.
cellulose
• Each glucose molecule is flipped 180 degrees
during joining, compared to its adjacent glucose
molecule in the chain. This alternating
orientation of glucose units results in a straight,
linear chain.
• Pure cellulose is odorless and flavorless. Due to
numerous hydroxyl groups, cellulose is
hydrophilic but insoluble in water.
• Enzymatic hydrolysis of cellulose forms glucose.
• Cellulose has both crystalline and amorphous
regions in its microfibrils. The highly ordered
crystalline parts provide strength and resistance
to deformation, while the amorphous regions
offer flexibility and allow for enzymatic
degradation.
Suitability of cellulose as a component of cell wall of
plants
High Tensile Strength – Cellulose is composed of β(1→4) linked
glucose molecules forming long, unbranched chains that aggregate
into microfibrils.
• These microfibrils provide mechanical strength and support,
essential for maintaining plant structure.
Rigidity and Flexibility – While providing rigidity, the arrangement of
cellulose fibers allows controlled flexibility, which is crucial for growth
and movement in response to external forces like wind.
Suitability of cellulose as a component of cell wall
of plants
Permeability – Cellulose forms a porous network that allows water,
nutrients, and gases to pass through, facilitating cellular communication
and metabolic exchange.
Resistance to Decomposition – The β(1→4) glycosidic bonds in cellulose
are resistant to enzymatic degradation by most organisms, except those
that produce cellulases (e.g., some bacteria and fungi).
This durability protects plants from microbial attack.
Interacts with Other Polysaccharides – Cellulose works with
hemicelluloses and pectins to form a dynamic and adaptable structure,
supporting plant growth and defense.
Lipids
• Lipids are a very varied group of chemicals.
• They are all organic molecules which are relatively insoluble in water.
• But can dissolve in non-polar solvents such as chloroform, benzene and
ester
• Lipids consist of repeating units of hydrocarbons known as fatty acids
• True lipids are esters formed by combination of a fatty acid and an alcohol.
• The alcohol part is known as a glycerol
• Lipids are involved in energy storage and are key building blocks of living
organisms-vital components of call membrane
• The most familiar lipids are fats and oils. Fats are solid at room
temperature and oils are liquid at room temperature, but chemically they
are very similar
Classification of lipids
Fatty acids
• Fatty acids are a series of acids, some of which are found in lipids.
• They contain the acidic group –COOH, known as a carboxyl group.
• The carboxyl group forms the ‘head’ of the fatty acid molecule.
• The common fatty acids have long hydrocarbon tails attached to the
carboxyl group.
• As the name suggests, the hydrocarbon tail consists of a chain of carbon
atoms combined with hydrogen.
• The chain is often 15 or 17 carbon atoms long
Types of fatty acids
Saturated fatty acids
Unsaturated fatty acids
• Carbon atoms forms either double
or triple bonds between otheratoms
• This causes chains to bend/ kink,
meaning they are less tightly packed
• Occur in plants e.g. sunflower, olive
oil
• Have low melting point
• This gives them a high melting point • Liquids at rtp
• Carbon atoms are bonded with
maximum number of hydrogen
bonds
• C-C interactions are single bonds
• They form straight chain- allows
tight packaging forming compact
energy stores in animals
• Are solids at rtp
Types of lipids
• Triglyceride
• Phospholipid
• cholesterol
Alcohols and esters
• Fatty acids are not always in free state, usually combine with glycerol to
form triglyceride
• Alcohols are a series of organic molecules which contain a hydroxyl group,
–OH, attached to a carbon atom.
• Glycerol is an alcohol with three hydroxyl groups
• The reaction between an acid and an alcohol known as condensation
produces a chemical known as an ester.
• The chemical link between the acid and the alcohol is called an ester bond
or an ester linkage
• The –COOH group on the acid reacts with the –OH group on the alcohol to
form the ester bond, –COO– . This is a condensation reaction because
water is formed as a product.
Triglyceride
• Most common lipid
• They are fats and oils
• It’s a type of lipid formed when three
fatty acid molecules combine with
glycerol, an alcohol with three
hydroxyl (−OH) groups
• Triglycerides are insoluble in water but
are soluble in certain organic solvents
such as ethanol.
• This is because the hydrocarbon tails
are non-polar
Formation of triglyceride
• A glyceride is an ester formed by a fatty acid combining with the
alcohol glycerol.
• As you have seen, glycerol has three hydroxyl groups.
• Each one is able to undergo a condensation reaction with a fatty
acid.
• When a triglyceride is made, the final molecule contains three fatty
acid tails and three ester bonds (‘tri’ means three).
• The tails can vary in length, depending on the fatty acids used.
Functions of triglycerides
• Excellent energy stores than carbohydrates- why?• It has a higher calorific value- adipose tissue.
• Act as insulators against heat loss below the skin of mammals- Cushion
the kidney, preventing damage- Subcutaneous fat.
• Blubber, a triglyceride found in sea mammals such as whales, has a
similar function, as well as providing buoyancy
• Acts as a metabolic source of water- when oxidizes releases water and
carbondioxide. Important for animals in dry habitats like Kangaroo,
camels
Phospholipids
• Phospholipids are a special type of lipid.
• Each molecule has the unusual property of having one end which is
soluble in water.
• This is because one of the three fatty acid molecules is replaced by a
phosphate group, which is polar and can therefore dissolve in water.
• The phosphate group is hydrophilic and makes the head of a
phospholipid molecule hydrophilic.
• The two remaining hydrocarbon
tails are still hydrophobic
Formation of a phospholipid
Functions of phospholipids
1.Structural unit: Phospholipids are the structural constituent of any cell
membrane which are made up of fatty acids, phosphate group, and
glycerol. These Phospholipids separates internal and external
environments.
2.Cell Signaling: Phospholipids that are essential to cell signaling pathways
include phosphatidylinositol (PI).
3.Recognition of cells: Phospholipids are involved in cell-to-cell
communications and aid in recognition of cells, by other cells.
Proteins
• Proteins are an extremely important class of macromolecule in living
organisms.
• More than 50% of the dry mass of most cells is protein.
• Macromolecules made up of polymers of amino acids (polypeptides)
• Amino acids are building blocks of proteins
Functions of Proteins
all enzymes are proteins
proteins are essential components of cell membranes
some hormones are proteins – for example, insulin and glucagon
Transport of oxygen-the oxygen-carrying pigments hemoglobin and myoglobin are
proteins
Body defense/ immunity-antibodies, which attack and destroy invading
microorganisms, are proteins
collagen is a protein that adds strength to many animal tissues – for example,
bone and the walls of arteries
Structural proteins-hair, nails and the surface layers of skin contain the protein
keratin
actin and myosin are the proteins responsible for muscle contraction
proteins may be storage products – for example, casein in milk and ovalbumin in
egg white.
Amino acids and general structure
• Building blocks of proteins (monomers)
• All amino acids have a central carbon atom.
• Attached/ bonded to the carbon atom is an amino group (–NH2) and a
carboxyl group –COOH.
• These two groups give amino acids their name.
• The third component that is always bonded to the carbon atom is a
hydrogen atom.
• The fourth group is a variable group, which distinguishes the various
amino acids.
• This is called the R group- specific to a specific amino acid
The peptide bond
• It’s a link formed when two amino acids join together.
• It is a C–N link between two amino acid molecules, formed by a
condensation reaction.
• A molecule formed when two amino acids are joined together via a
Peptide bond is called a Dipeptide.
• A molecule made up of many amino acids linked together by peptide
bonds is called a polypeptide.
• A polypeptide is another example of a polymer and a macromolecule,
like a polysaccharide.
• A protein may have just one polypeptide chain or it may have two or
more chains.
Formation of a peptide bond
• One loses a hydroxyl (–OH) group from its carboxylic acid group, while the
other loses a hydrogen atom from its amino group.
• This leaves a carbon atom of the first amino acid free to bond with the
nitrogen atom of the second.
• Protein synthesis takes place in Ribosomes of a living cell
Synthesis of proteins
• Protein synthesis takes place in Ribosomes of a living cell
• Proteins can be broken down to amino acids by breaking the peptide
bonds.
• This is a hydrolysis reaction, involving the addition of water It happens
naturally in the stomach and small intestine during digestion of proteins in
food.
• The amino acids released are absorbed into the blood
• After the polypeptide chain is synthesized, it undergoes multiple stages of
highly specific modifications
• These modifications results into the primary, secondary, tertially and
quaternary structures of proteins
Primary structure
• Initial linear sequence of amino
acids in a polypeptide chain.
• A polypeptide or protein molecule
may contain several hundred
amino acids linked into a long
chain.
• The particular amino acids
contained in the chain, and the
sequence in which they are joined,
is called the primary structure of
the protein
Secondary structure
• Amino acids near to each other in the chain may interact with each
other via hydrogen bonding.
• This secondary structure is due to hydrogen bonding between the
oxygen of the CO group of one amino acid and the hydrogen of the –
NH group of the amino acid four places ahead of it.
• The hydrogen bonds may occur between amino group of one amino
acid and carboxyl group of another amino acid within a polypeptide
chain.
• This interactions causes folding and irregular coiling of the Chain
resulting in either an α-helix or β-pleated sheet
Secondary structure
α-helix: a helical structure formed by
a polypeptide chain, held in place by
hydrogen bonds; an α-helix is an
example of secondary structure in a
protein .eg keratin, hemoglobin
β-pleated sheet: a loose, sheet-like
structure formed by hydrogen
bonding between parallel
polypeptide chains; a β-pleated
sheet is an example of secondary
structure in a protein eg fibroin (silk),
immunoglobins, porins
Tertially structure
• The compact structure of a protein molecule resulting from the threedimensional coiling of the chain of amino acids
• These arises from further coiling and folding of the polypeptide,
resulting from interactions between R groups.
• Tertiary Structure of Proteins refers to the overall 3D shape of a single
polypeptide chain, formed by the folding and interactions of
secondary structures (alpha-helices and beta-sheets).
• These interactions can be hydrogen bonds, disulphide bonds, ionic
bonds and hydrophobic interactions.
• Examples, insulin, Myoglobin, Lysozyme
Interactions in Tertially structure
Hydrogen bonds can form between a
wide variety of R groups. Hydrogen
bonds are weak in isolation but many
together can form a strong structure.
Disulfide bonds form between two
cysteine molecules. Cysteine molecules
contain sulfur atoms. The disulfide
bond forms when the sulfur atoms of
neighboring cysteines join together
with a covalent bond.
Interactions in Tertially structure
• Ionic bonds form between R
groups containing amino and
carboxyl groups. (Which amino
acids have R groups containing
amino or carboxyl groups?)
• Hydrophobic interactions occur
between R groups that are nonpolar. Such R groups are
hydrophobic so tend to avoid
water if possible
Quaternary structure
• The quaternary structure refers to the assembly of multiple polypeptide
chains (subunits) into a functional protein complex.
• These subunits are held together by non-covalent interactions (hydrogen
bonds, ionic bonds, hydrophobic interactions) and sometimes disulfide
bonds.
• Many protein molecules are made up of two or more polypeptide chains.
• The overall structure formed by the different polypeptide chains is called
the quaternary structure of the protein.
• Examples of proteins with quaternary structure include hemoglobin, DNA
polymerase, ribosomes, antibodies, and ion channels.
Globular and fibrous proteins
• Globular proteins are spherical proteins that play crucial roles in
biological processes.
• They have a tertiary structure where peptide chains fold into a
compact shape, making them water-soluble.
• Examples include hemoglobin, which carries oxygen in red blood
cells, and enzymes that catalyze biochemical reactions.
• Globular proteins are essential for various functions, including acting
as hormones and immunoglobulins.
• Denaturation of these proteins can lead to loss of function and
serious health issues.
• Many globular proteins have roles in metabolic reactions. Their
precise shape is the key to their functioning. Enzymes, for example,
are globular proteins
Hemoglobin as a globular protein
• Haemoglobin is the oxygen-carrying pigment found in red blood cells. It is a
globular protein.
• It is made up of four polypeptide chains.
• The main type of Haemoglobin in adults is made up of two subunits each of ‘𝜶
’ and ‘𝝱’ polypeptide chains.
• Each polypeptide chain is linked to a heme prosthetic group.
• 𝜶 subunit – It is made up of alpha polypeptide chain having 141 amino acid
residues.
• 𝝱 subunit – It is made up of beta polypeptide chain having 146 amino acid
residues.
• Heme group – It is an iron-containing prosthetic group, which is attached to
each polypeptide chain. It contains iron in the centre of the porphyrin ring.
Haemoglobin
• There are many types of globin – two types are used to make
haemoglobin, and these are known as α-globin (alpha-globin) and βglobin (beta-globin).
• Two of the haemoglobin chains, called α chains, are made from α-globin,
and the other two chains, called β chains, are made from β-globin.
• The haemoglobin molecule is nearly spherical.
• The four polypeptide chains pack closely together.
• Their hydrophobic R groups point in towards the center of the molecule,
and their hydrophilic ones point outwards.
Fibrous proteins
• Fibrous proteins are elongated, insoluble, and mainly serve structural
functions
• Unlike globular proteins, they have repetitive amino acid sequences
that form strong fibers or sheets
• Found in skin, hair, tendons, bones, and connective tissues.
• Fibrous proteins are not usually soluble in water and most have
structural roles.
Examples of fibrous proteins
1.Collagen – Triple-helix structure, found in skin, bones, tendons.
2.Keratin – Alpha-helix or beta-sheet, makes up hair, nails, feathers.
3.Elastin – Provides elasticity to skin, lungs, blood vessels.
4.Fibroin – Beta-sheet protein found in silk.
Functions of Fibrous Proteins
• Support & Structure – Collagen in bones and tendons.
• Protection – Keratin in skin, hair, and nails.
• Elasticity – Elastin in arteries and lungs.
• Strength & Flexibility – Fibroin in silk and spider webs.
water
• Water is arguably the most important biochemical of all.
• Without water, life would not exist on this planet.
• It is important for two reasons.
• First, it is a major component of cells, typically forming between 70%
and 95% of the mass of the cell.
• You are about 60% water.
• Second, it provides an environment for those organisms that live in
water.
• Three-quarters of the planet is covered in water
The structure and properties of water
Hydrogen bonds
can also form
between water
and other
electronegative
atoms (-OH or –
NH2)
Hydrogen bonding
responsible for many
unusual properties of
water
Water as a solvent
Water is polar
 This is mainly due to the difference in electron affinity of
the bonded atoms.
 It facilitates the formation of hydrogen bonds with other
polar substances.
 This responsible for cohesion and adhesion forces.
Water as a solvent
Universal solvent
• Water has the ability to dissolve a wide range of substances (solutes) because
of its small size and polar nature.
• This aids transportation of mineral salts and other polar substances in plants.
• The fact that molecules and ions dissolve in water also makes it ideal as a
transport medium, for example, in the blood and lymphatic systems in
animals,
High specific heat capacity
• The heat capacity of a substance is the amount of heat required to raise its
temperature by a given amount.
• The specific heat capacity of water is the amount of heat energy required
to raise the temperature of 1 kg of water by 1 °C.
• Water can absorb large amounts of heat energy with little change in actual
temperature.
• Water has a relatively high specific heat capacity.
• In order for the temperature of a liquid to be raised, the molecules must
gain energy and consequently move about more rapidly.
High specific heat capacity
• The hydrogen bonds that tend to make water molecules stick to each
other make it more difficult for the molecules to move about freely.
• The bonds must be broken to allow free movement.
• This explains why more energy is needed to raise the temperature of
water than would be the case if there were no hydrogen bonds.
• Hydrogen bonding, in effect, allows water to store more energy for a
given temperature rise than would otherwise be possible
Biological implications of High specific heat capacity of
water
• Makes water resistant to temperature changes.
• This means that the temperature within cells and within the bodies of
organisms tends to be more constant than that of the air around them.
• Allows biochemical reactions to operate at relatively constant rates and are
less likely to be adversely affected by extremes of temperature.
• Large bodies of water, like lakes and oceans, are slow to change
temperature with shifts in air temperature. This helps provide stable
habitats for aquatic organisms.
High latent heat of vaporisation
• Water has a relatively high latent heat of vaporisation.
• This is a consequence of its high specific heat capacity.
• Water molecules stick together through hydrogen bonds, requiring
significant energy to break these bonds.
• This energy is needed for vaporization, allowing molecules to escape
as gas.
Biological implications of High latent heat of
vaporisation
Vaporation requires a large amount of energy, making it an effective
cooling mechanism for living organisms, such as sweating or panting
in mammals.
This process allows a significant amount of heat to be lost with
minimal water loss, helping to reduce the risk of dehydration.
It can also be important in cooling leaves during transpiration
BIOLOGY
PRACTICAL
Food biochemical test
FOOD TEST
Food test is the test that can determine whether a
specific food substance is present or absent in a given
food sample solution.
The major food tested are;
Starch-Reducing sugar.
Non-reducing sugar.
Protein.
Lipids.
FOOD TEST
When carrying out food test the following things should
be noted
The reagent used for corresponding type of
food substance
Procedure to be followed during testing of a given type
of food substance.
Practical report should be in past tense
The appropriate colour should be observed.
The inference should be made
TESTING FOR THE PRESENCE OF STARCH
Starch is a complex carbohydrate found in a wide range of foods
such as potatoes, rice, corn, pasta and grains.
It is a mixture of the polysaccharides amylose and amylopectin,
which vary in concentration depending on the type of starch used
To test for starch, you use ‘iodine solution’.
Iodine doesn’t dissolve in water
Iodine solution is actually iodine in potassium iodide solution.
The starch–iodine complex that forms has a strong blue-black
colour.
The intensity of the colour produced when iodine solution is
added is related to the concentration of the starch solution.
Procedure
TESTING FOR THE PRESENCE OF SUGARS
Reducing sugars
The reducing sugars include all monosaccharides and
some disaccharides.
Reducing sugars are so called because they can carry
out a type of chemical reaction known as reduction.
The ability of some sugars to carry out reduction is the
basis of Benedict’s test for the presence of sugar.
The test uses Benedict’s reagent which is copper(II)
sulfate in an alkaline solution. It has a distinctive blue
colour.
Reducing sugars
Reducing sugars reduce the soluble blue copper
sulfate to insoluble brick-red copper oxide, containing
copper(I).
The copper oxide is seen as a brick-red precipitate.
reducing sugar + Cu2+ → oxidized sugar + Cu+
The intensity of the red colour is related to the
concentration of the reducing sugar.
The test can therefore be used as a semi-quantitative
test.
Procedure
Add Benedict’s reagent to the solution you are testing and
heat it in a water bath.
If a reducing sugar is present, the solution will gradually
turn through green, yellow and orange to red-brown as the
insoluble copper(I) oxide forms a precipitate.
Non-reducing
sugars
• Some disaccharides, such as sucrose, are not reducing sugars,
so you would get a negative result from Benedict’s test.
• In the non-reducing sugars test, the disaccharide is first broken
down into its two monosaccharide constituents.
• The chemical reaction is hydrolysis and can be brought about by
adding hydrochloric acid.
• The constituent monosaccharides will be reducing sugars and
their presence can be tested for using Benedict’s test after the
acid has been neutralized by sodium hydroxide.
• Benedict’s reagent needs alkaline conditions to work, so you
need to neutralise the test solution now by adding an alkali
such as sodium hydroxide
Procedure
Carry out Benedict’s test on the solution.
If you get a negative result, start again with a fresh sample of
the solution. Heat the solution with hydrochloric acid.
If a non-reducing sugar is present, it will break down to
monosaccharides.
Add Benedict’s reagent and heat as before and look for the
colour change.
If the solution now goes red, a non-reducing sugar is present.
If there is still no colour change, then there is no sugar of any
kind present.
TESTING FOR THE PRESENCE OF LIPIDS (ETHANOL
EMULSION TEST)
Lipids consist of fats and oils that are soluble in organic
solvents, such as ethanol, but insoluble in water.
Lipids are made up of fatty acids and glycerol.
Lipids are insoluble in water, but soluble in ethanol
(alcohol).
This fact is made use of in the emulsion test for lipids.
A vegetable oil sample is generally used in a school
science laboratory as a positive control
Procedure
• The substance that is thought to contain lipids is shaken
vigorously with some absolute ethanol (ethanol with little
or no water in it).
• This allows any lipids in the substance to dissolve in the
ethanol.
• The ethanol is then poured into a tube containing water.
• If lipid is present, a cloudy white suspension is formed.
• Any lipids in the sample precipitate in the water forming an
emulsion.
TESTING FOR THE PRESENCE OF PROTEINS (BIURET
TEST)
The Biuret test is used to detect the presence of peptide
bonds in proteins. It can be carried out in several ways:
1. Addition of a Biuret reagent.
2. Addition of sodium hydroxide and copper sulfate solutions.
If protein is present, the solution turns from blue to purple
due to the complexing of copper II ions with the peptide
bonds in the protein sample.
 The more peptide-copper complexes that are formed, the
deeper the purple colour.
Procedure
The biuret reagent is added to the solution to be
tested.
No heating is required.
A purple colour indicates that protein is present.
The colour develops slowly over several minutes.
Note: When testing foods, prepare a liquid sample by
crushing the food in some deionized/distilled water and
using only the liquid in the test procedure.
TESTING FOR VITAMIN C
Vitamin C also known as L-ascorbic or ascorbic acid is a watersoluble vitamin.
It is naturally present in a variety of fruits and vegetables and is
also a dietary supplement.
Sources include oranges, strawberries, red capsicums,
blackcurrants, broccoli and Brussel sprouts.
Vitamin C is also a reducing agent.
An indicator called DCPIP (2, 6-dichlorophenolindophenol) can
be used to test for the presence of vitamin C in foods.
DCPIP will change in colour from blue to red in the presence of
an acid but loses its blue colour in the presence of vitamin C.
Procedure
Add 2mL of 0.1% DCPIP solution to a test
tube.
 If using a liquid such as orange juice, add
it drop by drop to the DCPIP solution in a
test tube, mixing after each drop is added.
The colour will change from blue to red if
the sample is acid.
 Continue to add more of the test sample
and if the colour of the DCPIP disappears
then it shows that vitamin C is present.