Topic 2.3 Complete Notes

2.3 Carbohydrates and lipids

Carbohydrates

All carbohydrates have the general formula CnH2nOn where n represents the number of atoms.
Carbohydrate molecules vary greatly in size but are easy to classify. There are three main
groups namely monosaccharides, disaccharides and polysaccharides. The monosaccharides
are simple sugars that have between 3 and 10 carbon atoms per molecule. Disaccharides are
described as ‘compound sugars’ and each molecule consists of two monosaccharide molecules
joined together. Polysaccharides are formed when many monosaccharides are joined together
in a chain

Monosaccharides are normally
classified according to the number of
carbon atoms they contain. The most
important types in living organisms are
3-carbon sugars called trioses, 5-
carbon sugars called pentoses and
6-carbon sugars called hexoses.

Many pentose and hexose sugars can
exist in two forms, a straight form and
a ring form. Usually in dry form they
adopt their straight form but when
dissolved in water they adopt their ring
form. Below are two versions of
glucose.

2.3 Carbohydrates and lipids

The carbon atoms tend to be
numbered so that they can be
identified when the molecule changes
shape.
When monosaccharides combine to
form disaccharides, a molecule of
water is released. This type of
reaction is called a condensation
reaction. This is an anabolic reaction,
and energy in the form of ATP must
be provided. The bond formed
between two monosaccharides is
called a glycosidic bond. To the right
is an example of two glucose
monomers joining to become the
disaccharide maltose. As this bond is between the 1st and 4th carbon of the adjacent α- glucose
molecules, this link is also termed an α-1,4 glycosidic bond.

Here are the reactions of other monomers combining to form disaccharides

2.3 Carbohydrates and lipids

These disaccharides can be converted back into monosaccharides in the process of digestion.
(The opposite reaction to a condensation reaction, is called a hydrolysis reaction. This involves
the addition of a water molecule to split the bond.) If further condensations are allowed to
happen between disaccharides then a longer chain (polymer) will form. These longer
saccharides are called polysaccharides. The most common examples of polysaccharides are
starch, cellulose and glycogen.

All three of these polysaccharides are made from glucose monomers. The difference between
each of these depends on:

● which type of glucose they use (α or β)
● and which carbon they bond to each other

Cellulose.

Cellulose is a

polymer of

β-glucose. The

adjacent molecules

are linked by bonds

between carbon 1

and carbon 4. They

are therefore linked

by β-1,4 glycosidic

bonds. To get these

bonds forming, each

glucose is at 180O to

the next one. This

results in long,

straight, unbranched chains of glucose molecules. Hydrogen attractions can form between the

chains which allows them to form bundles. These bundles are called cellulose microfibrils and

have a very high tensile strength. This is the reason that cellulose is used by plants to form the

cell walls, which prevents plant cells bursting when placed in hypertonic solutions.

2.3 Carbohydrates and lipids

Starch.

Starch is made by joining

α-glucose molecules

together with a 1,4

glycosidic bond. These

molecules are all

orientated the same way.

This diagram highlights the

differences in the sugar

type and orientation of

starch and cellulose.

There are two forms of starch depending on whether it is branched or not. Amylopectin is a
branched form of starch which results in a globular molecule, whereas amylose is a non-
branched form of starch.
Only plants make starch molecules, and both types are hydrophilic, but too large to be soluble in
water. Therefore starch is a good store of glucose as it has no osmotic effect on the
surroundings. It is used as a temporary store of sugars in leaves, long term storages such as
potatoes and as a ready energy source in seeds.

2.3 Carbohydrates and lipids

Glycogen.
Glycogen is similar to the
branching form of starch
but it is far more
branched leading to a
much more compact
globular molecule. It is
formed from α-glucose
linked by 1,4 bonds but
there are also 1,6 bonds
present too.
The main use of glycogen
is as an energy store for animals and some fungi for the same reasons that plants use starch
molecules.

2.3 Carbohydrates and lipids

Lipids

Lipids are a large group of organic molecules that are all insoluble in water. One of the principle
groups are the triglycerides. Triglycerides are formed from the condensation reaction of three
fatty acids with a molecule of glycerol. The bond formed is called an ester bond.

The fatty acids that are combined with the glycerol can be of three different forms as in the
table below.

Type Structure

Saturated
(no double bonds)

Monounsaturated
(one double bond)

Polyunsaturated
(two or more double bonds)

2.3 Carbohydrates and lipids

The difference in these fatty acids is whether or not there is an abundance of hydrogen atoms
bonded to the carbons. If the carbons have two hydrogens each, then it is called a saturated
fatty acid. If there is one less hydrogen, then a double bond will form (i.e. where the missing
hydrogen is) and a monounsaturated fatty acid will form. If there is even less hydrogen present,
then a polyunsaturated fatty acid forms with multiple double bonds. The double bonds usually
give the molecule a kink or bend.

Triglycerides do not need to have three identical
fatty acids attached to the glycerol. The diagram
to the right shows an unsaturated triglyceride with
three different fatty acids. This representation
(skeletal drawing) saves time by not drawing all of
the carbon, hydrogen and oxygen atoms in.

There is one additional piece of information
concerning unsaturated fatty acids that must be
considered. At the double bond, the hydrogens
can be on the same side (cis isomer) or across
the bond (trans isomer).

Cis configuration Trans configuration

2.3 Carbohydrates and lipids

Both carbohydrates and lipids

are used as energy stores in

organisms. However, lipids are

usually preferred for long-term

energy storage. In animals the

lipids are fats stored in adipose

tissue under the skin and

around organs.

The advantages of storing

lipids rather than

carbohydrates include:

● The amount of energy stored in lipids is about double that stored in carbohydrates (gram
for gram)

● The mass of lipids needed to produce the same amount of energy is therefore halved.
This is important especially for flying organisms.

● Stored lipids can provide secondary roles, such as thermal insulation and the shock
absorbing protection around organs.

For short term needs, carbohydrates such as glycogen can be broken down much more readily
to produce glucose molecules, making them a faster energy provider.

2.3 Carbohydrates and lipids

Lipids and health

There have been many claims about the risks
associated with eating different types of fats. The
biggest concerns are about coronary heart disease
(CHD). This is a condition that is marked by fatty
deposits being found in the coronary arteries that
increase the chances of having a heart attack.
A positive correlation has been found between
saturated fatty acids and CHD, and these are considered the ‘bad’ fats. However, a correlation
does not mean that saturated fatty acids cause CHD. There could be another factor at play.
Perhaps those people surveyed also had poor diets
in general.

A population who do not fit the pattern are the
Maasai of Kenya. These people consume a diet rich
in meat, fat, blood and milk (all high in saturated
fats) but have an extremely low occurrence of CHD.

Mediterranean meals are famous for the large
level of olive oil consumed. Olive oil contains cis-
monounsaturated fatty acids. Unsaturated fats
have been branded as ‘good’ fats. The level of
CHD in those areas is extremely small. Again,
other factors could come into play; genetics,
popular use of garlic, over- use of tomatoes etc.
The positive correlation between trans-fats
(almost all of these are man-made by
hydrogenating unsaturated fats to extend the shelf life) and CHD has been tested extensively,
and would seem to be a cause of CHD. Patients who have died from CHD all had high
percentages of trans-fats in the walls of their arteries.

2.3 Carbohydrates and lipids

Body mass index

The body mass index, usually abbreviated to BMI, was developed by a Belgian statistician,
Adolphe Quetelet in 1832. Two measurements are needed to calculate it: the mass of the
person in kilograms and their height in metres.

BMI is calculated using this formula:

BMI can also be found using a type of chart called a nomogram (see next page). A straight line
between the height on the right hand scale and the mass on the left hand scale intersects the
BMI on the central scale.

BMI is used to assess whether a person’s body mass is at a healthy level, or is too high or too
low.

2.3 Carbohydrates and lipids