Biological molecules
3.2 Water:
Water is a simple molecule that can form hydrogen bonds between its atoms. Hydrogen
bonds are weak forces of attraction and is due to different polarities
Why is water so important to living organisms?
• Water is a reactant in lots of chemical reactions
• Water is a solvent, this means substances can dissolve in it so; it can transport
substances and act as a medium for chemical reactions to occur in.
• Water acts as a coolant (helps with temperature control) because it has a high
specific heat capacity and latent heat of evaporation.
• Water is a habitat for many organisms
• Water is incompressible so it can be pressurised and pumped into transport systems
or used as support in hydrostatic skeletons
Properties of water:
1.Cohesion:
Cohesion is the attraction between molecules of the same type
Hydrogen bonds cause cohesion, they hold water molecules together loosely.
Cohesivity helps water flow, making it great for transporting substances and up plant stems.
2.Good Solvent:
A lot of substances in biological reactions are ionic. Because water is polar, the slightly
positive H is attracted to the negative ions and vice versa. This means the ions will be
surrounded by the water and they’ll dissolve. This allows the separated ions to react-so
water is a medium
3.Low density when solid:
When water freezes the water molecules are held further apart in ice than they are in
water, creating a lattice shape which makes ice less dense compared to water.
Ice floats which is useful to living organisms as it can form an insulating layer so the water
below doesn’t freeze.
4.High specific heat capacity:
Specific heat capacity is the energy needed to raise the temperature of 1 gram of a
substance by 1-degree Celsius.
Hydrogen bonds can absorb a lot of energy, so water has a high specific heat capacity.
This means that water doesn’t change temperature easily-useful for living organisms living
in water. Water can help to buffer temperature changes during chemical reactions
5.High latent heat of evaporation:
It takes a lot of energy to break the hydrogen bonds between molecules.
Coolant e.g. when we sweat a lot of energy is needed for the water to evaporate cooling us
down as it happens.
Monomers and polymers
Molecules are polymers, long chains that comprise of smaller molecules called monomers.
, There are 3 biologically important polymers found in living organisms.
1. Nucleic Acids
The monomers are nucleotides made up of a phosphate group, pentose sugar and an
organic base.
2. Polysaccharides
Complex carbohydrates such as starch and cellulose are polysaccharides-a group of
polymers made up of monosaccharides.
3. Proteins
The monomers are amino acids. There are 20 Amino Acids used to build up proteins.
Condensation and hydrolysis:
• The monomers in both complex carbohydrates and proteins are joined by covalent
bonds created by a condensation reaction.
• A condensation reaction is when two molecules join to become one molecule by the
formation of a covalent bond and the release of a water molecule.
• A hydrolysis reaction is where one large molecule is split into two by adding a water
molecule to bDefinitionsreak the covalent bond.
3.3 Carbohydrates
• Carbohydrates consist of Carbon, oxygen and hydrogen represented in the formula
(CH20) x.
• Carbohydrates can be divided into three groups: Monosaccharides, Disaccharides
and polysaccharides.
Monosaccharides:
• Single sugar units used as monomers to build up other carbohydrates.
• They are soluble and sweet reducing sugars.
• Glucose has 6 carbons and is a hexose monosaccharide. There are two types of
glucose:
• Alpha glucose and Beta glucose
• The difference between a-glucose and b-glucose is simply the position of the -H and -
OH groups on the first carbon atom. It has an effect on the way they bond together
and the polysaccharides produced.
• Pentose sugars contain five carbon e.g ribose and deoxyribose.
Disaccharides:
• These are two monosaccharides bonded together.
• They are bonded by a covalent bond called a glyosidic bond.
• Disaccharides are soluble and sweet.
• Most are reducing sugars but sucrose is not.
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