This document summarizes the key components of cells, including carbohydrates, proteins, lipids, nucleic acids, water, and enzymes. It describes the structures and functions of these biomolecules, such as how carbohydrates provide energy and structure, the roles of amino acids and protein structure, how lipids are used for energy storage and insulation, and how nucleic acids carry genetic information and regulate protein synthesis. It also outlines the importance of water and enzymes in biochemical reactions within cells.

1. Chapter 4: Chemical Composition of Cells
Carbohydrate
1. Consists of carbon, hydrogen and oxygen in the ratio CnH2nOn.
2. Functions:
a) Provide energy during respiration
b) Stored food in animals’ liver ( glycogen ) and in plants’ cell ( starch )
c) Build cell wall in plant cells
d) External skeleton of insects
3. Carbohydrate can be divided into 3 types:
a) Monosaccharide c) Polysaccharide
b) Disaccharide
4. Monosaccharide
a) Simplest form of carbohydrate
b) Some examples are glucose (most common monosaccharide), fructose (found in
fruits) and galactose (found in milk).
c) Monosaccharides are reducing sugar which turns light blue of Benedict’s solution to
red-brick precipitate upon heating.
5. Disaccharide
a) Monosaccharide + Monosaccharide  Disaccharide + water
b) The process of producing disaccharide is condensation
c) Some of the disaccharides are maltose (malt sugar), sucrose (cane sugar) and lactose
(milk sugar).
d) Glucose + Glucose  Maltose + Water
Glucose + Fructose  Sucrose + Water
Glucose + Galactose  Lactose + Water
e) Maltose is used for brewing beer while sucrose is used as sweetener in beverage and
cooking. Lactose is present in mammal’s milk, including human.
f) Only sucrose is non-reducing sugar.
6. Polysaccharide
a) Many glucose undergo condensation to form polysaccharide.
b) Polysaccharides are insoluble in water, do not crystallize and do not taste sweet.
c) 3 types of polysaccharides are starch, glycogen and cellulose.
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2. d) Starch is the main energy storage in plants and can be found in wheat, rice, potato
and bread.
e) Glycogen is the main energy storage in animals and yeast. They are stored in liver and
muscle.
f) Cellulose makes up the cell wall of plant cells, which provide support for plants.
g) Polysaccharide can be broken down via hydrolysis process.
Protein
1. Consists of carbon, hydrogen, oxygen and nitrogen. Some may have sulphur and
phosphorus.
2. The building block of protein is amino acid.
3. Amino acids are joined by peptide bond to form protein via condensation process.
Amino acid + Amino acid Dipeptide + Water
4. Many amino acids bind together to form polypeptides.
5. Breaking of polypeptide is known as hydrolysis.
6. Amino acids can be grouped into 2 types:
a) Essential amino acids (can’t be synthesized by body cells. Obtained from food.)
b) Non-essential amino acids (can be synthesized by body cells)
7. Structure of protein can be classified into 4 levels:
a) Primary structure (arranged in sequence forming long linear chain of polypeptide)
b) Secondary structure (coiled to form alpha-helix or beta-pleated sheet)
c) Tertiary structure (folded in various way to form globular protein)
d) Quaternary structure (folded polypeptide chains joined together forming a large
complex protein molecule)
Primary structure Secondary Tertiary structure Quaternary
structure (hormone, enzyme) structure
(haemoglobin in
red blood cell)
Beta-pleated Alpha-helix
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3. 8. Importance of protein:
a) Growth of new cells and replace dead cells
b) Synthesis of enzymes, antibodies and some hormones
c) Form keratin (skin) and collagen (bone)
d) Synthesis of haemoglobin ( quaternary structure)
Lipids
1. Consists of carbon, hydrogen and oxygen.
2. Building blocks are fatty acids and glycerol.
3. Triglycerides (a type of lipid) are made up of 1 glycerol and 3 fatty acids.
4. Triglycerides is formed when glycerol bind with fatty acids via condensation process.
Triglycerides can be broken down by hydrolysis
condensation
1 glycerol + 3 fatty acids triglycerides + water
hydrolysis
5. Fats and oils are triglycerides. Fats and oils can be divided into saturated and unsaturated
fat.
Aspect Saturated fats Unsaturated fats
Presence of double bond
at fatty acid No double bond at fatty Have at least 1 double bond
acids at fatty acids
Reaction with additional
No reaction with hydrogen
hydrogen bonds (give Can react with hydrogen as
as has maximum number of
reason for your answer) there are double bond
hydrogen
Cholesterol level Higher Lower
State at room temperature Solid Liquid
Example Animal fat (fatty meat),
Vegetable oil, margarine
butter
6. Importance of lipids:
a) Source of energy (twice of carbohydrate)
b) Heat insulator (keep organisms warm)
c) Protect major organs
3 © Gabriel Chua, 2013

4. Nucleic acids
1. Basic unit structure is nucleotides which consists of pentose sugar, nitrogenous base and
phosphate group.
2. Two types of nucleic acid:
a) DNA (deoxyribonucleic acid) which consists of 2 strands of polynucleotides, twisted
into alpha-helix structure.
b) RNA (ribonucleic acid) which consists of 1 strand of polynucleotide.
3. Importance of nucleic acids in cells:
a) Carry genetic information in all cells (DNA)
b) Regulate in protein synthesis (RNA)
c) Determine the traits inherited from parents (DNA)
Water
1. Importance of water:
a) As a solvent to dissolve substances such as gases (O2 & CO2) for respiration
b) As a medium for biochemical reaction
c) Maintain body temperature
d) As transport medium as blood plasma (90%) carries many biological molecules
4 © Gabriel Chua, 2013

5. e) Provide moisture such as in respiratory tract to allow diffusion of gas
f) Provide support such as in plants, especially when the cells are turgid
Enzymes
1. Enzymes are organic catalyst that increases the rate of biochemical reactions.
2. Enzyme + Substrate  Enzyme-substrate complex  Enzyme + Product
3. Works by the Lock and Key Hypothesis
Enzyme + Substrate Enzyme-substrate Enzyme + Product
complex
4. Characteristics:
a) Speed up biochemical reactions
b) Not changed / altered or destroyed after reaction
c) Needed in very small amount
d) Reaction is specific
e) Reversible reaction
f) Work with narrow range of temperature (350C – 400C) and sensitive to pH.
5. Enzyme synthesis is the same as protein synthesis because enzyme is a type of protein
DNA in nucleus carries information for protein synthesis
Information in DNA is transcribed into mRNA which carries information to
ribosome
The information is translated and protein is formed in ribosome
6. Enzymes can be divided into two types, ie. intracellular enzymes (use within the cell) and
extracellular enzymes (to be secreted outside the cell).
5 © Gabriel Chua, 2013

6. 7. For extracellular enzymes, after the enzyme protein is synthesized as shown in (5), it
follows a series of steps:
Proteins enter rough ER and packed as transport vesicle
Transport vesicle carries protein into Golgi apparatus where protein is
modified into enzymes
Enzymes are packed into secretory vesicle and transported to plasma
membrane
Secretory vesicle is fuses with plasma membrane and enzyme is released
I personally think that this flow chart is quite a good one. I only managed to sketch a
simple one in one of your relief class. So, here it is the complete one. You may copy it into
the notes I gave you.
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7. Rough ER
Nucleus
Secretory
vesicle
Ribosome
Transport
Protein vesicle
Golgi
apparatus
8. Factors that affect enzyme activities are temperature, pH, substrate concentration and
enzyme concentration.
Temperature pH
Substrate Enzyme
concentration concentration
7 © Gabriel Chua, 2013

8. Uses of enzymes
Applications Enzymes used
Detergent Enzyme protease, amylase and lipase to remove food and other
stains from clothes
Leather products Enzyme protease to remove animal hair from their skin.
Wine production Enzyme zymase
Tenderise meat Enzyme protease
I will not go into detail for this part. You can refer to your reference book, text book or
even internet for the usage of many different types of enzymes.
8 © Gabriel Chua, 2013