Biological Molecules
Polar Molecules:
Water consists of two hydrogen atoms, each covalently bonded to one oxygen atom
However, because the oxygen atom has a greater number of positive protons in its nucleus, this
exerts a stronger attraction for the shared electrons
This means the oxygen atom becomes slightly negative, and the hydrogen atoms become slightly
positive. When this happens, we say the molecule is POLAR
Properties of Water
Water is the medium in which all metabolic reactions take place in cells. Between 70-95% of the
mass of a cell is water
Water is composed of atoms of hydrogen and oxygen. One atom of oxygen combines with two atoms
of hydrogen by sharing electrons (covalent bonding)
Water is a polar molecule
- The oxygen atom attracts the electrons more strongly than the hydrogen atoms
- Weak negatively charged region on the oxygen atom
- Weak positively charged region on the hydrogen atoms
(the sharing of electrons is uneven between the oxygen and hydrogen atoms)
This separation of charge due to the electrons in the covalent bonds being unevenly shared is called
a dipole
o When a molecule has one end that is negatively charged and one end that is positively
charged, it is also a polar molecule
,Hydrogen bonds form between water molecules
o This is as a result of the polarity of the water – the hydrogen bonds form between the
positive and negatively charged regions of adjacent water molecules
Why are hydrogen bonds weak?
Charges are only slight and small, so bonds can be disrupted by other charges (acids or bases) or
temperature (more kinetic energy at higher temps so move and no longer an attraction)
When there are larger numbers of hydrogen bonds present, they form a strong structure
PROPERTIES OF WATER:
High heat capacity
Large latent heat of vaporisation
Strong cohesion between water molecules
Excellent solvent
A metabolite in many metabolic reactions
Water has many essential roles in living organisms due to its properties:
▪ The polarity of water molecules
▪ The presence and number of hydrogen bonds between water molecules
High specific heat capacity:
The specific heat capacity of a substance is the amount of thermal energy requires to raise the
temperature of 1kg of that substance by 1C
,Water’s specific heat capacity is 4200J/kgC so a relatively large amount of energy is required to raise
its temperature
The high specific heat capacity is due to the many hydrogen bonds present in water. It takes a lot of
thermal energy to break these bonds and a lot of energy to build them, thus the temperature of
water does not fluctuate greatly.
ADVANTAGE FOR LIVING ORGANISMS:
- Provides suitable habitats for aquatic organisms
- Is able to maintain a constant temperature as water is able to absorb a lot of heat without
big temperature fluctuations
o This is vital in maintaining temperatures that are optimal for enzyme activity
o The bodies of living organisms are mostly made of water
- Water in blood plasma is also vital in transferring heat around the body, helping to maintain
a fairly constant temperature
o As blood passes through the more active regions of the body, heat energy is
absorbed but the temperature remains fairly constant
o Water in tissue fluid also plays an important regulatory role in maintaining a constant
body temperature
Large latent heat of vaporisation
In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by
water to break the hydrogen bonds and evaporate
Latent heat of evaporation – the energy lost when some fast-moving molecules evaporate
ADVANTAGES TO LIVING ORGANISMS:
Only a little water is required to evaporate for the organism to lose a great amount of heat
This provides a cooling effect for living organisms
▪ For example, the transpiration from leaves
▪ Evaporation of water in sweat on the skin
Cohesive forces
Due to polarity of water molecules, they are attracted to each other and form hydrogen bonds
Hydrogen bonds between water molecules allows for strong cohesion between water molecules
o This allows columns of water to move through the xylem of plants (continuous stream) and
through blood vessels in animals (MASS FLOW)
, o Enable surface tension where a body of water meets the air (creates a sort of film/skin on
the water so that insects such as pond skaters can float)
Water is also able to hydrogen bond to other molecules, such as cellulose, which is known as
adhesion.
This enables water to move up the xylem (water is attracted to the side of vessels)
Solvent
A solvent is a liquid that other substances (solutes) can dissolve in.
The positive and negative charges of water attract other molecules causing them to separate from
each other (DISSOLVE)
When substances are dissolved in water, they are free to move around and react with other
molecules
As water is a polar molecule many ions (e.g., sodium chloride) and covalently bonded polar
substances (e.g., glucose) will dissolve in it – water molecules can bind to solute molecules.
- This allows for chemical reactions to occur within cells (as the dissolved solutes are more
chemically reactive when they are free to move about)
- Metabolites can be transported efficiently (except non-polar molecules, which are
hydrophobic)
IMPORTANCE TO LIVING ORGANISMS:
The metabolite reactions that happen inside all living organisms can only happen when the
reactants are dissolved in water
Substances being dissolved in water also allows them to be transported around the bodies of
living organisms.
- E.g., glucose, CO2, urea in blood plasma
- Mineral ions and sugars dissolved in water are transported in xylem and phloem in plants
- Excretion of urea/mineral/water in urine
- Minerals and water in sweat
A metabolite
Water molecules can also be involved in chemical reactions directly
IMPORTANCE TO LIVING ORGANISMS:
HYDROLYSIS:
Important in digestion of large molecules in animals
Polar Molecules:
Water consists of two hydrogen atoms, each covalently bonded to one oxygen atom
However, because the oxygen atom has a greater number of positive protons in its nucleus, this
exerts a stronger attraction for the shared electrons
This means the oxygen atom becomes slightly negative, and the hydrogen atoms become slightly
positive. When this happens, we say the molecule is POLAR
Properties of Water
Water is the medium in which all metabolic reactions take place in cells. Between 70-95% of the
mass of a cell is water
Water is composed of atoms of hydrogen and oxygen. One atom of oxygen combines with two atoms
of hydrogen by sharing electrons (covalent bonding)
Water is a polar molecule
- The oxygen atom attracts the electrons more strongly than the hydrogen atoms
- Weak negatively charged region on the oxygen atom
- Weak positively charged region on the hydrogen atoms
(the sharing of electrons is uneven between the oxygen and hydrogen atoms)
This separation of charge due to the electrons in the covalent bonds being unevenly shared is called
a dipole
o When a molecule has one end that is negatively charged and one end that is positively
charged, it is also a polar molecule
,Hydrogen bonds form between water molecules
o This is as a result of the polarity of the water – the hydrogen bonds form between the
positive and negatively charged regions of adjacent water molecules
Why are hydrogen bonds weak?
Charges are only slight and small, so bonds can be disrupted by other charges (acids or bases) or
temperature (more kinetic energy at higher temps so move and no longer an attraction)
When there are larger numbers of hydrogen bonds present, they form a strong structure
PROPERTIES OF WATER:
High heat capacity
Large latent heat of vaporisation
Strong cohesion between water molecules
Excellent solvent
A metabolite in many metabolic reactions
Water has many essential roles in living organisms due to its properties:
▪ The polarity of water molecules
▪ The presence and number of hydrogen bonds between water molecules
High specific heat capacity:
The specific heat capacity of a substance is the amount of thermal energy requires to raise the
temperature of 1kg of that substance by 1C
,Water’s specific heat capacity is 4200J/kgC so a relatively large amount of energy is required to raise
its temperature
The high specific heat capacity is due to the many hydrogen bonds present in water. It takes a lot of
thermal energy to break these bonds and a lot of energy to build them, thus the temperature of
water does not fluctuate greatly.
ADVANTAGE FOR LIVING ORGANISMS:
- Provides suitable habitats for aquatic organisms
- Is able to maintain a constant temperature as water is able to absorb a lot of heat without
big temperature fluctuations
o This is vital in maintaining temperatures that are optimal for enzyme activity
o The bodies of living organisms are mostly made of water
- Water in blood plasma is also vital in transferring heat around the body, helping to maintain
a fairly constant temperature
o As blood passes through the more active regions of the body, heat energy is
absorbed but the temperature remains fairly constant
o Water in tissue fluid also plays an important regulatory role in maintaining a constant
body temperature
Large latent heat of vaporisation
In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by
water to break the hydrogen bonds and evaporate
Latent heat of evaporation – the energy lost when some fast-moving molecules evaporate
ADVANTAGES TO LIVING ORGANISMS:
Only a little water is required to evaporate for the organism to lose a great amount of heat
This provides a cooling effect for living organisms
▪ For example, the transpiration from leaves
▪ Evaporation of water in sweat on the skin
Cohesive forces
Due to polarity of water molecules, they are attracted to each other and form hydrogen bonds
Hydrogen bonds between water molecules allows for strong cohesion between water molecules
o This allows columns of water to move through the xylem of plants (continuous stream) and
through blood vessels in animals (MASS FLOW)
, o Enable surface tension where a body of water meets the air (creates a sort of film/skin on
the water so that insects such as pond skaters can float)
Water is also able to hydrogen bond to other molecules, such as cellulose, which is known as
adhesion.
This enables water to move up the xylem (water is attracted to the side of vessels)
Solvent
A solvent is a liquid that other substances (solutes) can dissolve in.
The positive and negative charges of water attract other molecules causing them to separate from
each other (DISSOLVE)
When substances are dissolved in water, they are free to move around and react with other
molecules
As water is a polar molecule many ions (e.g., sodium chloride) and covalently bonded polar
substances (e.g., glucose) will dissolve in it – water molecules can bind to solute molecules.
- This allows for chemical reactions to occur within cells (as the dissolved solutes are more
chemically reactive when they are free to move about)
- Metabolites can be transported efficiently (except non-polar molecules, which are
hydrophobic)
IMPORTANCE TO LIVING ORGANISMS:
The metabolite reactions that happen inside all living organisms can only happen when the
reactants are dissolved in water
Substances being dissolved in water also allows them to be transported around the bodies of
living organisms.
- E.g., glucose, CO2, urea in blood plasma
- Mineral ions and sugars dissolved in water are transported in xylem and phloem in plants
- Excretion of urea/mineral/water in urine
- Minerals and water in sweat
A metabolite
Water molecules can also be involved in chemical reactions directly
IMPORTANCE TO LIVING ORGANISMS:
HYDROLYSIS:
Important in digestion of large molecules in animals
