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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Beta-pleated Alpha-helix

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 structure

Tertiary structure (hormone, enzyme)

Quaternary structure (haemoglobin in red blood cell)

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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

5. Fats and oils are triglycerides. Fats and oils can be divided into saturated and unsaturated

fat.

Aspect Saturated fats Unsaturated fatsPresence of double bond at fatty acid No double bond at fatty

acidsHave at least 1 double bond

at fatty acids

Reaction with additional hydrogen bonds (give reason for your answer)

No reaction with hydrogen as has maximum number of

hydrogen

Can react with hydrogen as there are double bond

Cholesterol level Higher LowerState at room temperature Solid LiquidExample Animal fat (fatty meat),

butterVegetable oil, margarine

6. Importance of lipids:

a) Source of energy (twice of carbohydrate)

b) Heat insulator (keep organisms warm)

c) Protect major organs

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1 glycerol + 3 fatty acids triglycerides + watercondensation

hydrolysis

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

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Nucleotide

Enzyme + Substrate Enzyme-substrate complexEnzyme + Product

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

d) As transport medium as blood plasma (90%) carries many biological molecules

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

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

6. Enzymes can be divided into two types, ie. intracellular enzymes (use within the cell)

and extracellular enzymes (to be secreted outside the cell).

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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

7. For extracellular enzymes, after the enzyme protein is synthesized as shown in (5), it

follows a series of steps:

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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.

Enzyme

Intracellular

Extracellular

For use within the cell

To be secreted outside the cell

Synthesis of enzyme in ribosome

Synthesis of enzyme is the same as that of protein as enzyme is also a type of proteinProtein enters rough endoplasmic reticulum and packed as

transport vesicleTransport vesicle carries protein into Golgi apparatus where

protein is modified into enzymes

Secretory vesicle fuses with plasma membrane and enzyme is released

Enzymes are packed into secretory vesicle and transported to plasma membrane

Intracellular enzyme stays in cell while extracellular enzyme proceeds

8. Factors that affect enzyme activities are temperature, pH, substrate concentration and

enzyme concentration.

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Protein

Transport vesicle

Rough endoplasmic reticulum

Nucleus

Ribosomes

Golgi apparatus

Enzyme expelled

Temperature pH

Enzyme concentration

Nucleus

ProteinTransport vesicle

Golgi apparatus

Ribosome

Secretory vesicle

Rough ER

Uses of enzymes

Applications Enzymes usedDetergent 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.

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Limiting value

Limiting value

Enzyme concentration

d) Enzyme concentration

Substrate concentration