BIOLOGY FORM 4 CHAPTER 4 - CHEMICAL COMPOSITION OF THE CELL PART 1

The Chemical Basis of Life
Living organisms
are composed of
about
25 chemical
elements

An element is a pure substance
containing only one kind of atom

C, H,O, N:
make up the
bulk of living
matter

Functions of elements
in animal cells and plant cells
ELEMENT FUNCTION
H, C, O, N Synthesis of organic
compounds – lipid,
protein
SULPHUR A component of some
protein
SODIUM (Na) Regulates osmotic
pressure in cells
Help transmission of
nerve impulses

Chemical elements
– Combine in fixed ratios to form
compounds
Sodium Chlorine
Sodium
chloride
(table salt)

Compounds are molecules
containing more than one
type of element.
C6H12O6

COMPOUNDS
 substance which consists of two or more
elements chemically combined in a fixed ratio
Compound
Organic compound Inorganic compound
IN LIVING MATTER
chemical compounds which
contain C and H
Eg. Carbohydrates, protein,
lipid
NON LIVING MATTER,
do not contain C
Eg. water, acids, bases, mineral
– not synthesised by cell
– obtained from external
environment

CHEMICAL COMPOUNDS IN THE CELL
Carbohydrates C, H, O
Protein C, H, O, N, S, P
Nucleic acid C, H, O, P, N
Water H, O

The importance of chemical compounds in the cell
Carbohydrates - Primary source of energy
1g – 17 kJ of energy
- Starch – food storage in plant cells
- Glycogen – food storage in animal and plant
tissues
- Cellulose – constituent of cell wall
Lipids - Fats and oil –source of energy
1g - 38 kJ of energy
- Layer of adipose tissue - insulation, protection
- Major constituent for plasma membrane
- Prevent water loss in plants (cuticle)
- Steroid – cholesterol, oestrogen, progesterone

The importance of chemical compounds in the cell
Protein - Build new cells, renew damaged tissues
- Synthesis of enzymes, antibodies,
hormones
- Component of plasma membrane
- Synthesis of haemoglobin
Nucleic
acids
- Store genetic information
- DNA, RNA

Nucleic Acids
 Nucleic acids - DNA (deoxyribonucleic acid)
and RNA (ribonucleic acid), are made
from monomers known as nucleotides.
 Each nucleotide has three components: a 5-carbon
sugar, a phosphate group, and a nitrogenous base.
 If the sugar is deoxyribose, the polymer is DNA.
 If the sugar is ribose, the polymer is RNA.
Nucleotide

4
nitrogenous
bases
A pairs with T
C pairs with G

Differences between DNA & RNA
DNA RNA
Double stranded
polynucleotide (double
helix twist)
Single stranded
polynucleotide
Found in the nucleus,
chloroplast &
mitochondrion.
Found in the cytoplasm,
ribosomes & nucleus
Contains genetic
information about an
organism.
Copies information in DNA
for protein synthesis.
Sugar - deoxyribose Sugar - ribose

Water and Life
The most abundant substance
in living systems
%
70
of earth’s surface
Human body = 65%
Plants = 80%
Fruits and Vegetables = 70-80%

Water is considered a polar molecule.
– It has a positive and negative end.

The hydrogen end of the water molecule
has a slight positive charge (delta +).
The oxygen end of the
water molecule has a
slight negative charge
(delta -).

Why is the water molecule said to be bipolar?
Due to the presence of a pair of electric charges
equal in magnitude but opposite polarity,
separated by some distance

All living organisms are dependent on
water.
The structure of water is the basis for its
unique properties.
The most important property of water is
the ability to form hydrogen bonds.

Importance of water
in the cell

WATER :
 a vital chemical of living cells
 makes up 60 - 95% of the fresh
mass of an organism
Fresh mass = mass of cells + water
Dry mass = mass of cells

WATER:
universal solvent for many
biological molecules
is a medium for
diffusion

WATER: Medium for biochemical reactions.
- breaking down proteins, lipids carbohydrates
- photosynthesis
photosynthesis

WATER:
- Transport medium in blood, lymphatic, excretory,
digestive systems and vascular tissue of plants.

Water : Aid lubrication
 mucus, synovial fluid consist of
water
 mucus – assist movement of food
substances – intestinal tract
 synovial fluid – ease movement of
joint

WATER:
- gives support in hydrostatic skeletons e.g. in
earthworms.
TS earthworm

WATER:
- is needed by plant cells for cell enlargement, the
guard cell mechanism , cell turgidity & support in
plants.

WATER:
- has a cooling effect - sweating and panting
in animals and transpiration in plants

WATER:
- needed for seed germination - testa
swells and splits after water is imbibed

Water: Maintaining osmotic balance
- dissolved inorganic salts in water maintain the
osmotic balance in animal blood and interstitial
fluid

Water: Providing moisture
- respiratory gases – dissolve in moist alveolus –
before diffusing – blood capillaries

Water: High
surface tension
and cohesion
- maintain a
continuous flow
of water up the
the stem to the
leaf

Animals get water from:-
1. Drinking
2. Eating
3. Chemical reactions e.g.
respiration
food
O2
H2O
ATP

Elements in macromolecules & the percentage
dry mass of each macromolecule in a cell
Macromolecule Elements
% dry mass of a
cell
Carbohydrates CHO 15
Lipids CHOP 10
Proteins CHONS 50
Nucleic acids CHONP 18

CARBOHYDRATES
 contain the elements:
 Carbon
 Hydrogen
 Oxygen
 Ratio H:O = 2:1
 Glucose:
 is the simplest carbohydrate (C6 H12 O6)

Easy to remember elements:
Food Elements
Carbohydrates C, H, O
Lipids C, H, O
Proteins C, H, O, N [sometimes
S and P]

Uses of carbohydrates:-
a) provide energy (17kJ/g)
Sugar in energy drink
equivalent to six pastries.

c) to build cell walls in
plants
Cell wall
b) to store energy
e.g. starch in
potatoes or roots
Storage
organs

d) fibre is important to prevent constipation
Food sources of fibre:
whole wheat, bran, fresh or
dried fruit & vegetables.
Constipated!!

Carbohydrates
Carbohydrates – 3 types
Monosaccharide Disaccharides
polysaccharides
Simple sugar
Glucose
Fructose
galactose
Maltose
Sucrose
lactose
starch cellulose glycogen

MONOSACCHARIDES
Simple sugar (C6H12O6)
Function : Main source of energy
SOURCE OF MONOSACCHARIDE:
Glucose – plants, fruits
Fructose - sweet fruits, honey
Galactose - milk
 All monosaccharides taste sweet, able to crystalline, water
soluble. Reducing sugars.

Disaccharides
- Consist of two monosaccharide joined together through
condensation
Condensation = process which involves removal of a water molecule
when a bond is formed between 2 molecules of monosaccharides
condensation
Glucose + glucose Maltose + water
condensation
Glucose + fructose Sucrose + water
condensation
Glucose + galactose Lactose + water
 All disaccharides taste sweet, able to crystalline, water soluble

FORMATION OF DISACCHARIDE
THROUGH CONDENSATION

Disaccharides
 Hydrolysis – chemical reaction that involves the breaking up
large molecules by adding water
hydrolysis
Maltose + water Glucose + glucose
hydrolysis
Sucrose + water Glucose + fructose
hydrolysis
Lactose + water Glucose + galactose
Sources of dissaccharides:
 Maltose – malt sugar
 Sucrose – cane sugar
 Lactose – milk sugar

BREAKING DOWN OF DISACCHARIDE
THROUGH HYDROLYSIS

Sweet & Soluble
& reducing sugar
(except sucrose)
Monosaccharides
Disaccharides
Polysaccharides
Not sweet & Insoluble
& non-reducing sugar

Polysaccharides
- Consist of hundreds of monosaccharides joined together
through condensation.
polysaccharides
starch cellulose glycogen
Found in Animals &
yeast
Main carbohydrate
reserve in plants
Component of cell
wall of plants
Found in Plants
Main carbohydrate reserve
in animals & yeast

In what form are excess carbohydrates stored?
1) in plants:
starch
2) in animals:
glycogen
Name two places
in animals where
glycogen is stored.
Liver & muscles

Cellulose: is the main source of dietary fibre
Explain why although humans cannot digest
dietary fibre, it is still important.

Functions of roughage:-
1. adds bulk to the contents of the intestine
and keeps food moving along the gut
Gut

Functions of roughage:-
2. prevents constipation as fibre retains water
so that the faeces are soft
3. prevents cancer of the colon (part of the
intestine)
faeces

Name the monosaccharide which forms
starch, glycogen and cellulose.
Glucose
Glycogen Cellulose Starch

How do starch, glycogen and cellulose
differ?
The way glucose units
are linked together
Glycogen Cellulose Starch

Polysaccharides
Polysaccharides + Water hydrolysis Monosaccharides
condensation
Hydrolysis : Adding diluted acid or through enzymatic
reaction

Question:
Where in a plant would you expect to find:
i) Cellulose (1)
ii) Starch (1)
i) Cellulose – found in cell walls of plant cell
ii) Starch – found stored in roots /
storage organs

Question:
Give biological explanations for each
of the following statements:
The diet of athletes is usually high in
carbohydrates. (5)
Athletes need a lot of energy. Carbohydrates are
the body’s main energy source. Carbohydrates
like starch in bread are digested into glucose.
Glucose is used in respiration to release energy. If
athletes take in monosaccharides, i.e. sugars e.g.
glucose, they are provided with energy very
quickly.

Why is the tired athlete choosing sugar
rather than starch?
Sugars, especially
monosaccharides can
be used for respiration
right away.
Starch needs to be
digested first.

LIPIDS
 a name for fats and oils
 contain the elements:- C, H, O
 Ratio H:O > 2:1
lard

Lipids are:
water-insoluble organic substances but
readily soluble in organic solutions e.g.
ether

Water is so attracted to other water molecules
that anything between them is squeezed out of
the way.
‘’;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;

Food rich in lipids:-
Nuts
Egg yolk

Types of Lipid
1. Triglycerides [Fats & Oils]
2.Waxes
3.Phospholipids
4. Steroids

Types of Lipid
1. Triglycerides
[Fats & Oils]
1. Waxes
2. Phospholipids
3. Steroids

 building blocks of a lipid molecule:
 1 Glycerol
 3 Fatty acids
A molecule of fat

Formation of Triglyceride
Formation – condensation
Breakdown - Hydrolysis

What is the difference between a fat and an oil?
 fats - solid at 20C
 oils - liquid at 20C

Animal fat solidifies at a low
temperature

 excess lipids are stored:-
1) in the ADIPOSE TISSUE under the skin
2) around organs
Fat around
heart.

Functions of Triglycerides
1. Store energy in animals +
Yield energy

 uses of lipid
a) provide energy (38 kJ/g)
b) store energy

Comparison of energy yield from:
Triglycerides: 37 kJ g-1
Carbohydrates: 17 kJ g-1
MORE energy from
triglycerides:
Due to more hydrogen.

Question:
Explain why one kilogram of starch crops
releases less energy when burnt, compared to
one kilogram of oil crops. (2)
Starch is a carbohydrate whilst oil is a lipid.
Carbohydrates have a lower energy content
than lipids.

2. Animals store extra fat when
hibernating: acts as an insulator.
Subcutaneous fat

2. Fat around organs:
 protect against bumps
 keeps them warm
Kidney surrounded
by fat

3. In aquatic mammals blubber contributes
to buoyancy.

Test for Triglycerides
Paper becomes
translucent.

Chemical Test for Triglycerides
Sudan III

Chemical Test for Triglycerides
Emulsion Test

Types of Lipid
2.Waxes
3. Phospholipids
4. Steroids

Waxes: not a food source
no enzymes to break them down

Feathers do not
get wet – WHY?
Raindrops on
feathers.

Waxes are mainly for
e.g.
cutin makes up the
waxy cuticle of leaves
fruits produce a
waxy coating to
keep from drying
out

wax in ears traps:
dust
sand
other foreign
particles
do not go deeper into the
ear and cause no damage

exoskeleton in insects
[contains wax]

waxes build elaborate structures such
as beehives

Types of Lipid
2.Waxes
3.Phospholipids
4. Steroids
5. Glycolipids
6. Lipoproteins
7. Terpenes

Phospholipids
• lipids with a phosphate group

Types of Lipid
2. Waxes
3. Phospholipids
4.Steroids

Steroids
are structurally different from all other
lipids
consist of a complex carbon ring
structure
Cholesterol is a steroid.

Cholesterol:
• is the steroid present in the
largest amount in humans
• an important constituent of
animal plasma membranes
made in the
liver

Why are fats and oils classified as
triglycerides?

C16H32O2
R
- fatty acids may be:
 saturated or
unsaturated

Saturated fatty acid
[Single bonds only]
Unsaturated fatty acid
[Double bonds]

Saturated & Unsaturated Fat
• Fats with saturated fatty acid = Saturated Fat
eg butter (solid at room temperature)
• Fats with unsaturated fatty acid = Unsaturated
Fat eg. Corn oil (liquid at room temperature)
Saturated Fat

Saturated & Unsaturated Fat
• Unsaturated fat with 1 double bond =
Monounsaturated fat
• Unsaturated fat with more than 1 double bond =
Polyunsaturated fat

Difference between saturated fats &
unsaturated fats
Saturated fats Differences Unsaturated fats
None The presence of
double bonds
between carbon
atoms in fatty acids
Minimum one
No Ability to react with
an additional
hydrogen atom
Yes
Solid Condition at room
temperature
Liquid
Increase Cholesterol level Decrease
Butter Examples Corn oil

Proteins are of primary importance
to the life of the cell
• by dry weight proteins are the major
components of an actively growing cell

PROTEINS
 contain the elements:-
 C, H, O, N (sometimes S
and P)
 food rich in proteins:-
 Meat
 Fish
 Egg white
 Cheese

 Uses of proteins:-
1. for growth
2. for cell repair and replacement
3. to make enzymes
4. to make antibodies

What are the building blocks
of proteins called?

How do we get the amino acids
needed to build proteins?
EATING Protein-Rich
Foods

Proteins ingested are digested by
enzymes called…p…r…ot…e…as…e…s …

From amino acids to proteins
two amino acids dipeptide
three amino acids tripeptide
more than 50 amino
polypeptide
acids
6 000-1000 000 protein

Amino Acid + Amino Acid --> Dipeptide
Amino Acid + Dipeptide --> Tripeptide
A.A. + A.A. + …..+ Tripeptide --> Polypeptide

Structure of an
amino acid
molecule

R = Side group/chain [varies]
What is an ‘amino acid’?
An organic molecule possessing both carboxyl
and amino groups

 Two amino acids are linked by a:
Peptide bond
 Polypeptide: many amino acids are linked

 A protein: many polypeptides joined up

different amino acids occur in proteins

Let us discover how
two amino acids
link together

H2N
Amino acids are joined together by a
H
condensation reaction
Carboxyl
group
C C
H
O
OH
N
H
C C
CH3
O
OH
H2N
H
C
H
O
H H
C N C C
CH3
O
OH
Peptide
bond
Amino
group
H
H
+
H2O
A peptide bond is formed by condensation
between the -NH2 of one amino acid and -COOH
of another

Many amino acids joined together =
N-terminus
H
H H O H H O H H O H H O H H O
N C C N C C N C C
H CH3
CH2
OH
N C C
CH2
C
O
OH
N C C
CH2
CH
CH3 H3C
C-terminus
CH2
OH
H H O
N C C
H H O
N C C
H H O
N C C
CH2
SH
OH
Polypeptide chain

A protein molecule:
 contains 100’s and 1000’s of amino acids joined
together by peptide links into one or more
chains
3 chains in collagen (in mouse tail)

Polypeptide chains can be folded
in various ways

Proteins are unbranched, not like
carbohydrates
Branched
molecule
Unbranched
Protein molecule

Many different types of proteins
exist. How can this be?
MILLIONS of
Antibodies exist
A LARGE NUMBER
OF ENYZMES

Because any of 20 different amino
acids might appear at any position
• E.g. a protein containing 100 amino acids
could form any of 20100 different amino acid
sequences
• this is 10130, i.e. 1 followed by 130
zeros

Number and Sequence of amino acids
determine the protein
6 amino acids
5 amino acids
7 amino acids
6 amino acids but in
a different
sequence

Test for Protein: Biuret Test
Protein
present

Test for Protein: Biuret Test
Cheese is rich
in protein.
Add an equal amount of
NaOH to the solution
followed by 1-2 drops of
CuSO4 solution
pestle
mortar

When a protein reacts with copper(II) sulfate
(blue), the positive test is the formation of a
violet colored complex.
Purple / Lilac:
Positive test

Proteins have many functions:
enzymes
hormones
structural
proteins
What dictates the function of each protein?
The exact sequence of amino acids.

DNA contains the information that
determines the sequence of amino acids

 Two types of amino acids:
1. Essential [8 amino acids]
must be eaten as they
cannot be made by the
body
2. non-essential
can be made by the
body

 contain all the essential amino acids
 Source: animals : eggs, meat, fish, cheese
 deficient in one or more essential amino acid
 Source: plants

 For example, corn is deficient in one
amino acid
 Individuals who eat only corn would show
symptoms of protein deficiency.
• This is true from any diet limited to a
single plant source, including:
 Rice
 Wheat
 or potatoes

How can protein deficiency from a vegetarian
diet be avoided?
By eating a
combination of plant
foods that complement
each other to supply all
essential amino acids.

 For example, beans supply the lysine that
is missing in corn, and corn provides
the methionine which is deficient
in beans.

Excess proteins:
 cannot be stored in the body
 are broken down in the liver by a process
called DEAMINATION
 the waste product produced is called UREA
liver

Structure of a Protein
• four levels of organisation exist:-
1) Primary structure
2) Secondary structure
3) Tertiary structure
4) Quaternary structure

Primary structure of a
protein:
 the number and sequence of
amino acids held together
by peptide bonds in a
polypeptide chain
 the primary structure of each
type of protein is unique

Secondary structure:
• the way in which the polypeptide is arranged
in space

Two common secondary structures are
the:
 -helix
 -pleated sheet
• bonds
present:
1. Peptide
2. Hydrogen

Tertiary structure:
• is when the polypeptide
chain bends and folds
extensively to form a
precise compact
• is a complex, three-dimensional shape that
determines the final configuration of the
polypeptide

The joining of more than one polypeptide chain
leads to the quaternary structure of proteins

Quaternary structure occurs in many highly
complex proteins
A huge variety of quaternary structures exist

The loss of the specific three-dimensional
conformation (secondary structure) of a protein
A protein spontaneously refolds into its original
structure under suitable conditions

How long can the change be?
Temporary or permanent.
Is the amino acid sequence affected?
Remains unaffected.

Why is denaturation of proteins
considered as harmful to an
organism?
The molecule unfolds and cannot
perform its normal biological
functions.

Denaturation agents can be:
i) Heat
ii) Strong acids & alkalis and high
concentrations of salts
iii) Heavy metals (e.g. mercury)
iv) Organic solvents and detergents

i) Heat
- weak hydrogen bonds and non
polar hydrophobic interactions
are disrupted
- Why?

Heat increases the
kinetic energy
Causes the molecules
to vibrate so rapidly
and violently that
bonds break

ii) Strong acids & alkalis + high
concentrations of salts
ionic bonds are
disrupted
the protein is
coagulated

Coagulation of milk by adding salts

Breakage of peptide bonds may occur if the
protein remains in the reagent for a long time

iii) Heavy metals
cause the protein to precipitate out of the
solution
Cations (+) form strong bonds with carboxylate anions
(COOH-) and often disrupt ionic bonds

iv) Organic solvents & detergents
disrupt hydrophobic
interactions
form bonds with non-polar
groups
this in turn disrupts
intramolecular H-bonding

Why does the solution become purple when
beetroot discs are placed in detergent?
1. Proteins in cell membrane & tonoplast are
denatured.
2. Phospholipid bilayer is damaged.

Why is the skin wiped with alcohol before an
injection is given?
Alcohol is used as a disinfectant.
It denatures the protein of any
bacteria present on the skin.

Food Test
Colour
change
Starch + iodine solution Yellow to blue
black
Iodine
solution
Starch

Look at this picture.
Is starch present in potato? Yes.

Food Test
Colour
change
Protein
(Biuret test)
+ sodium hydroxide solution +
1-2 drops of copper sulfate
solution
Blue to purple
1. Sodium
hydroxide
2. Copper solution
sulfate
solution
Egg white
[protein]

A positive test for
protein was obtained
for…a…lb…u…m…in…

Food Test
Colour
change
Oil
+ ethanol + shake + water +
shake
A white emulsion
forms

Food Test
Colour
change
Oil rub food onto a dry piece of
filter paper
A greasy spot
forms

Food Test
Colour
change
Glucose + Benedict’s solution or
Fehling’s solution + heat
Blue to brick red
or orange

Food Test
Colour
change
Starch + iodine solution Yellow to blue
black
Protein
(Biuret test)
+ sodium hydroxide solution +
1-2 drops of copper sulfate
solution
Blue to purple
Oil
1) + ethanol + shake + water +
shake
1) rub food onto a dry piece of
filter paper
A white emulsion
forms
A greasy spot
forms
Glucose + Benedict’s solution or
Fehling’s solution + heat
Blue to brick red
or orange

When the food to be tested is a solid:
1. Crush the food with some water using a
pestle and a mortar.
2. Filter.
3. Add the reagents to the filtrate.
pestle
mortar

Credits to Dr Marthese Azzopardi

BIOLOGY FORM 4 CHAPTER 4 - CHEMICAL COMPOSITION OF THE CELL PART 1

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