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© Boardworks Ltd 2008
Water (H2O) consists of two hydrogen atoms
covalently bonded to one oxygen atom.
Each hydrogen shares a pair of
electrons with the oxygen. The oxygen
has a greater affinity for electrons than
the hydrogens, so it ‘pulls’ the
electrons closer.
δ–
δ+
104.5 °
δ+
This makes the oxygen slightly negative (indicated by δ–) and the hydrogens slightly
positive (indicated by δ+).
This creates different charged regions, making water a polar molecule. Because it has two
charged regions it is dipolar.
Hydrogen bonds
Many of the properties of water are due to its ability to form
hydrogen bonds.
The slight negative charge on the oxygen atom makes it
attract the slightly positive hydrogen atom of another water
molecule.
hydrogen
bond
The numerous hydrogen bonds in water make it a very stable structure.
Water as a solvent
Properties and biological roles of
water
Carbohydrates are a group of substances used as both
energy sources and structural materials in organisms.
All carbohydrates contain carbon, hydrogen and oxygen,
with the general formula: Cx(H2O)y.
There are three main groups of carbohydrates:
monosaccharides – these are simple sugars, with the
general formula (CH20)n, where n can be 3–7
disaccharides – these are ‘double sugars’, formed from
two monosaccharides
polysaccharides – these are large molecules formed
from many monosaccharides.
Glucose is an abundant and very important monosaccharide.
It contains six carbon atoms so it is a hexose sugar. Its
general formula is C6H12O6.
Glucose is the major energy source for most cells. It is highly
soluble and is the main form in which carbohydrates are
transported around the body of animals.
The structure of glucose can be represented in different ways:
straight chain
ring
ring (simplified)
Glucose exists in different forms called structural isomers.
Two common isomers are alpha glucose and beta glucose.
alpha
glucose
6
6
5
5
4
1
3
2
4
1
3
beta
glucose
2
The only difference between these two isomers is the position
of the –OH group attached to carbon 1. In alpha glucose it is
below the carbon and in beta glucose it is above the carbon.
This minor structural difference has a major effect on the
biological roles of alpha and beta glucose.
Two other important hexose monosaccharides are fructose
and galactose.
fructose
galactose
Fructose is very soluble and is the main sugar in fruits and
nectar. It is sweeter than glucose.
Galactose is not as soluble as glucose and has an important
role in the production of glycolipids and glycoproteins.
Pentose monosaccharides contain five carbon atoms. Like
hexoses, pentoses are long enough to form a ring.
Two important pentose molecules are the structural isomers
ribose and deoxyribose. These are important constituents
of RNA and DNA.
5
5
1
4
3
2
ribose
1
4
3
2
deoxyribose
The only difference between them is that ribose has one
H atom and one –OH group attached to carbon 2,
whereas deoxyribose has 2 H atoms and no –OH group.
The formation of disaccharides
Maltose (malt sugar) is
formed from two glucose
molecules joined by an
alpha 1–4 glycosidic bond.
Sucrose (table sugar) is
formed from glucose and
fructose joined by an
alpha 1–4 glycosidic bond.
Lactose (milk sugar) is
formed from galactose
and glucose joined by a
beta 1–4 glycosidic bond.
Benedict’s test for reducing sugars
Mono- and disaccharides
Polysaccharides are polymers containing many
monosaccharides linked by glycosidic bonds. Like
disaccharides, polysaccharides are formed by
condensation reactions.
Polysaccharides are mainly used as an energy store and as
structural components of cells.
The major polysaccharides are starch and cellulose in
plants, and glycogen in animals.
The structure of starch
Properties and uses of starch
Starch is the major carbohydrate storage molecule in plants.
It is usually stored as
intracellular starch
grains in organelles
called plastids.
Plastids include green
chloroplasts (e.g. in
leaves) and colourless
amyloplasts (e.g. in
potatoes).
Starch is produced from glucose made during photosynthesis.
It is broken down during respiration to provide energy and is
also a source of carbon for producing other molecules.
Iodine test for starch
Cellulose is another polysaccharide and is the main part of
plant cell walls. It is the most abundant organic polymer.
Unlike starch, cellulose is very strong, and prevents cells
from bursting when they take in excess water.
Cellulose consists of long
chains of beta glucose
molecules joined by beta
1–4 glycosidic bonds.
The glucose chains form
rope-like microfibrils,
which are layered to form
a network.
The structure of cellulose
Animals do not store carbohydrate as starch but as glycogen.
Glycogen has a similar
structure to amylopectin,
containing many alpha 1–6
glycosidic bonds that produce an
even more branched structure.
Glycogen is stored as small
granules, particularly in
muscles and liver.
Glycogen is less dense and more soluble than starch, and is
broken down more rapidly. This indicates the higher
metabolic requirements of animals compared with plants.
Polysaccharides: true or false?