3.2 Carbohydrates, Lipids & Proteins
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Topic 3: Chemistry of Life
3.2 Carbohydrates, Lipids & Proteins
Orange book: pg. 47-47
Green book: pg. 35
3.2.1 Distinguish between organic and inorganic compounds. (pg. 47, 35)
3.2.2 Identify amino acids, glucose, ribose and fatty acids from diagrams
showing their structure. (pg. 48-51, 35-36)
3.2.3 List three examples of each of monosaccharides, disaccharides and
polysaccharides. (pg. 48, 36)
3.2.4 State one function of glucose, lactose and glycogen in animals and of
fructose, sucrose and cellulose in plants. (pg. 48, 36)
3.2.5 Outline the role of condensation and hydrolysis in the relationships
between monosaccharides, disaccharides and polysaccharides; between fatty
acids, glycerol and triglycerides; and between amino acids and polypeptides. (pg.
49-51, 36-38).
3.2.6 State three functions of lipids (pg. 50, 37)
3.2.7 Compare the use of carbohydrates and lipids in energy storage. (pg. 50,
37)
3.2.1 Organic vs Inorganic
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3.2.1 Distinguish between organic and inorganic compounds.
Orange book  pg. 47
Green book  pg. 35
To do:
Take a note of the IB syllabus definition of the term “organic” in your green
exercise books.
Complete the two questions in the recap.
* The IB syllabus states that:
“Compounds containing carbon that are found in living organisms (except
hydrogencarbonates, carbonates and oxides of carbon) are regarded as
organic”.
So better to learn this definition rather than ‘containing carbon, hydrogen and
oxygen.
We are only interested in a few major compounds.
Quick recap:
What are the three major groups of organic compounds we are interested in?
Name a few important inorganic compounds that are important to life on Earth.
Background Reading
Living things are composed of an amazing array of molecules. We can start to
make sense of all these molecules by classifying these into a molecule type.
Molecules of the same type have certain qualities in common and become fairly
easy to recognize with a little practice.
Molecules can be classified as being either inorganic or organic. All organic
molecules contain the element carbon, although not all carbon-containing
molecules are organic. Carbon dioxide is a common example of a molecules that
contains carbon that is not organic.
3.2.2 Identifying Molecules
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3.2.2 Identify amino acids, glucose, ribose and fatty acids from diagrams
showing their structure.
Orange book  pg. 48-51
Green book  pg. 35-36
To do (in green exercise books):
Under the title of “Amino Acid”:
 draw the structure of an amino acid
 label the H attached to the central carbon, the carboxylic acid group, the

amino group and the functional ‘R’ group.
In one sentence explain the importance of the ‘R’ group
Under the title of “Glucose”:



Draw the structure of -glucose
In one sentence, describe the difference in -glucose and β-glucose
In one sentence, explain the important of glucose to living organisms
Under the title of “Ribose”:

Draw the structures of ribose and deoxyribose
Under the title of “Fatty Acids”
 Draw the structure of the following fatty acid CH3(CH2)4COO
 On your diagram label: the methyl group, hydrocarbon chain and carboxyl
group.
* Best way to learn this, is to draw them until you can draw them from memory
without looking at a book.
Amino Acids
Proteins are made of amino acids. All amino acids contain the elements carbon,
hydrogen, oxygen and nitrogen. A few amino acids contain sulphur too. The
general structure of an amino acid molecule is shown below.
There is a central carbon atom (called the "alpha carbon"), with four different
chemical groups attached to it:




a hydrogen atom
an amino group
a carboxylic acid group
a variable "R" group (or side chain)
Amino acids are so-called because they have both amino groups and acid groups,
which have opposite charges. At neutral pH (found in most living organisms),
the groups are ionised as shown above, so there is a positive charge at one end
of the molecule and a negative charge at the other end. The overall net charge
on the molecule is therefore zero.
There are 20 different R groups, and so 20 different amino acids. Since each R
group is slightly different, each amino acid has different properties, and this in
turn means that proteins can have a wide range of properties.
Glucose
Glucose is the most widely distributed sugar in the plant and animal kingdoms
and it is the sugar present in blood as "blood sugar". Glucose is the main sugar
metabolised by the body for energy. The body digests carbohydrates in foods,
transforming them into glucose, which serves as the primary fuel for the brain
and muscles. Glucose is absorbed into the bloodstream through the intestinal
wall. Only the monosaccharides glucose, fructose and galactose are absorbed in
humans; these are the end-products of the digestion of carbohydrates.
The diagrams below illustrate the difference between -glucose and β-glucose.
Ribose
DNA and its close relative RNA are perhaps the most important molecules in
biology. They contain the instructions that make every single living organism on
the planet, and yet it is only in the past 50 years that we have begun to
understand them. DNA stands for deoxyribonucleic acid and RNA for
ribonucleic acid, and they are called nucleic acids because they are weak acids,
first found in the nuclei of cells. They are polymers, composed of monomers
called nucleotides. Nucleotides have three parts to them:
 a phosphate group (PO42- ),
 a pentose sugar, which has 5 carbon atoms in it.
 a nitrogenous base.
In this topic we are only interested in the pentose sugar. The sugar that is
present in RNA is ribose and the sugar present in DNA is deoxyribose.
Fatty Acids
Fatty acids are long molecules with a polar, hydrophilic end and a non-polar,
hydrophobic "tail". The hydrocarbon chain can be from 14 to 22 CH2 units long,
but it is always an even number because of the way fatty acids are made. The
formula for a fatty acid can be written CH3(CH2)nCOO
3.2.3 Saccharides
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3.2.3 List three examples of each of monosaccharides, disaccharides and
polysaccharides.
Orange book  pg. 48
Green book  pg. 35-36
To do (in green exercise books):
Use your own knowledge and the green book to help you complete the table
below.
Copy the completed table into your greenbooks.
Fill in the table below:
Saccharide
Definition
Examples
Monosaccharide
1.
2.
3.
Dissacharide
1.
2.
3.
Polysaccharide
1.
2.
3.
Monomers
3.2.4 Functions of Carbohydrates
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3.2.4 State one function of glucose, lactose and glycogen in animals and of
fructose, sucrose and cellulose in plants.
Orange book  pg. 48
Green book  pg. 36
To do (in green exercise books):
Use your own knowledge and the green book to help you complete the table
below.
Copy the completed table into your greenbooks.
Background reading
Carbohydrates are among the most commonly found biochemical molecules found
in both animals and plants. Carbohydrates exist in different ‘sizes’ –
monosaccharides, disaccharides and polysaccharides. All of these carbohydrates
serve many functions in living organisms.
Animal Carbohydrates
Carbohydrate
Type of
Carbohydrate
Function
Glucose
Lactose
Glycogen
Plant Carbohydrates
Carbohydrate
Type of
Carbohydrate
Fructose
Sucrose
Cellulose
Function
3.2.5 Condensation & Hydrolysis
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3.2.5 Outline the role of condensation and hydrolysis in the relationships
between monosaccharides, disaccharides and polysaccharides; between fatty
acids, glycerol and triglycerides; and between amino acids and polypeptides.
Orange book  pg. 49-51
Green book  pg. 36-38
To do:
You will practice drawing condensation and hydrolysis reactions on scrap paper.
Once you are confident in showing the reaction, copy the condensation and
hydrolysis reactions into your books for:
 monosaccharides to/from disaccharides
 between fatty acids and glycerol to/from triglycerides
 amino acids to/from proteins
Hydrolysis and Condensation in Carbohydrates
Disaccharides
Disaccharides are formed when two monosaccharides are joined together by a
glycosidic bond. The reaction involves the formation of a molecule of water
(H2O):
This shows two glucose molecules joining together to form the disaccharide
maltose. Because this bond is between carbon 1 of one molecule and carbon 4 of
the other molecule it is called a 1-4 glycosidic bond. Bonds between other
carbon atoms are possible, leading to different shapes, and branched chains.
This kind of reaction, where H2O is formed, is called a condensation
polymerization reaction. The reverse process, when bonds are broken by the
addition of water (e.g. in digestion), is called a hydrolysis reaction.
In general:
polymerisation reactions are condensation reactions.
breakdown reactions are hydrolysis reactions.
Hydrolysis and Condensation in Lipids (triglycerides)
Triglycerides
Triglycerides are commonly called fats or oils. They are made of glycerol and
fatty acids.
Glycerol is a small, 3-carbon molecule with three alcohol groups.
Fatty acids are long molecules with a polar, hydrophilic end and a non-polar,
hydrophobic "tail". The hydrocarbon chain can be from 14 to 22 CH2 units long,
but it is always an even number because of the way fatty acids are made. The
hydrocarbon chain is sometimes called an R group, so the formula of a fatty acid
can be written as R-COO-.
One molecule of glycerol joins together with three fatty acid molecules to form
a triglyceride molecule, in another condensation polymerisation reaction:
Hydrolysis and Condensation in Proteins
Proteins are made of amino acids. All amino acids contain the elements carbon,
hydrogen, oxygen and nitrogen. A few amino acids contain sulphur too. The
general structure of an amino acid molecule is shown below.
Peptide Bonds
In suitable conditions amino acids polymerise. The α-amino group of one
molecule joins to the carboxyl group of another in a condensation reaction,
which results in the formation of a peptide bond. The reaction involves the
formation of a molecule of water:
When two amino acids join together a dipeptide is formed. Three amino acids
form a tripeptide. Many amino acids form a polypeptide. e.g.:
In a polypeptide there is always one end with a free amino (NH3) group, called
the N terminus, and one end with a free carboxyl (CO2) group, called the Cterminus.
In a protein the polypeptide chain may be hundreds of amino acids long. Amino
acid polymerisation to form polypeptides is part of protein synthesis. It takes
place in ribosomes, and is special because it requires an RNA template. The
sequence of amino acids in a polypeptide chain is determined by the sequence of
the genetic code in DNA.
3.2.6 Functions of Lipids
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3.2.6 State three functions of lipids.
Orange book  pg. 50
Green book  pg. 37
To do:
Read the section below and the textbook.
Use the information to bullet point three uses of lipids in your green books.
Role of Lipids
Lipids are biochemically important molecules that serve many functions. We
refer to triglyceride lipids in solid form as fats. In liquid form, triglycerides are
called oils. Everyone is aware of the role fat plays in energy storage. If you eat
more food than you burn, your body will store much of the excess as fat in
adipose cells. Each adipose cell gets smaller or larger depending on how much
lipid is being stored. People either gain or lose weight depending on how much
lipid is being stored at any given time in a fairly fixed number of adipose cells.
Lipids are very efficient molecules for storing energy. As seen earlier,
carbohydrates are also used for storing energy in living organisms. Glycogen is a
carbohydrate used by animals to store energy and starch is used by plants.
Lipids are important for thermal insulation. A good reminder of this is to study
the amount of blubber (fat) that cold-climate animals form in order to stay
warm; 30% or more of the body mass of some seals may be due to the blubber
layer beneath their skin.
As you know a special category of lipid called phospholipid makes up the bilayer
of all cell membranes. These phospholipid molecules have a polar head turned
towards water and a non-polar tail which turns away from water.
3.2.7 Energy Storage
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3.2.7 Compare the use of carbohydrates and lipids in energy storage
Orange book  pg. 50
Green book  pg. 37
To do:
Read the section below and the textbook (it is important to read the green book
for this objective).
Summarise a paragraph in your greenbooks to compare the energy storage of
carbohydrates and lipids.
Introduction
Animal cells obtain energy in the form of ATP by oxidizing food molecules
through the process of respiration. The hydrolysis of ATP supplies energy
needed for cellular processes, such as the transport of molecules or cellular
movement. Respiration involves oxidation of an organic compound, the
respiratory substrate. Carbohydrate, fats and proteins can all be used, however
carbohydrates and fatty acids are the most important fuels for generating ATP
in animal cells. Respiration in animal cells depends on oxygen.
We can understand a great deal about animal metabolism by comparing the
volume of oxygen consumed by an organism to the volume of carbon dioxide
produced. These volumes will change depending on the energy source the animal
is using.
*The equations used to explain this are for illustration only – don’t try to learn
this.
Respiratory Quotient
Consider the equation for respiration:
C6H12O6 + 6O2  6CO2 + 6H2O (+ energy)
From the equation for respiration we can see that given the time, the volume of
carbon dioxide produced during respiration of carbohydrate is equal to the
volume of oxygen consumed.
The ratio of CO2:O2 is called the respiratory quotient and for respiration using
carbohydrate its value is 1.
RQ = volume of CO2given off
volume of O2 consumed
Therefore from the above equation:
RQ = 6/6 = 1
Respiratory quotients:
Carbohydrates (glucose) = 1.0
Protein = 0.9
fat (lipids) = 0.7
Carbohydrates
Carbohydrates, when available will be used first by most cells. Polysaccharides
such as starch (in plants) and glycogen (in animals) are hydrolysed to
monosaccharides before they enter the respiratory pathway.
As carbohydrates are generally broken down to monomers before they are
respired we will look at the respiration of glucose. In general, carbohydrates
contain approximately the same ratio of carbon, hydrogen and oxygen (although
the equation indicates that there is always twice as much hydrogen, as oxygen,
we say the ratios are similar when we are comparing them to lipids).
Consider the equation for respiration:
C6H12O6 + 6O2  6CO2 + 6H2O (+ energy)
RQ = 6/6 = 1
Due to the number of C-H bonds, carbohydrates have a higher energy yield than
proteins but lower than lipids.
Lipids
Although lipids release more energy than carbohydrates, they are mainly used in
respiration when carbohydrate reserves have been exhausted. This is because
carbohydrates are easier to respire and so will be used up first. Lipids must
first be converted to glycerol and fatty acids and then the fatty acids can be
respired.
Generally lipids have a much higher carbon and hydrogen content compared to
oxygen. This means there are plenty of C-H bonds to be broken and the
hydrogens can be carried off to the electron transport chain to be used to
make large amounts of ATP.
To give an idea of this, below if the chemical formula for the fat tripalmitin
(you do not need to learn this, it is simply to illustrate the point). Notice how
few oxygens there are compared to the hydrogen and carbon.
C51H96O6
If tripalmitin is respired we would get the equation:
2C51H98O8 + 145O2  102CO2 + 98H2O
RQ = 102/ 145 = 0.7