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Biology OCR A level Biological molecules Summary Notes
Biology OCR A level Biological molecules Summary Notes
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1 of 15B3- Biological molecules
2.1.2 Biological molecules
(a) How hydrogen bonding occurs between water molecules, and relate this, and other properties
of water, to the roles of water for living organisms.
Key words:
Polar- a partial delta negative charge and a partial delta positive charge of a molecule.
Cohesion- the attraction between molecules of the same type.
Capillary action- the process by which water can rise up a narrow tube against the force of
gravity.
Speci c heat capacity- is the energy needed to raise the temperature of 1 gram of a substance
by 1ºC.
• Water consists of two hydrogen molecules covalently bonded together to an oxygen molecule.
• Water is a polar molecule and a strong permanent dipole.
• The slightly negatively charged oxygen atoms attract the slightly positively-charged hydrogen
atoms of other water molecules- this attraction is called hydrogen bonding.
• Shared negative hydrogen electrons are pulled towards the oxygen atom.
Roles of water
• Good solvent: polar water molecules attract and dissolve other polar molecules and ions. The
slightly positive end of a water molecule will be attracted to the negative ion, and the slightly
negative end of a water molecule will be attracted to the positive ion, meaning that ions will get
totally surrounded by water molecules and dissolve. E.g. cytosol of prokaryotes and eukaryotes
are mainly water.
Importance- water transports dissolved solutes, chemical reactions occur in water.
• Transport medium: cohesion of hydrogen bonds cause water molecules to be attracted to each
other and ow together. Adhesion occurs between water molecules and other polar molecules
and surfaces- their joint impact results in capillary action. This makes it great for transporting
substances. E.g. allows transport of dissolved solutes in blood to travel around the body.
Importance- water movement up xylem vessels (capillary action), surface tension allows small
organisms to move on the water surface.
• Coolant/high speci c heat capacity: helps to bu er temperature changes during reactions as
large amounts of energy is required to overcome hydrogen bonds. This maintains constant
temperatures in cellular environments as hydrogen bonds between water molecules can absorb
a lot of energy so it doesn’t experience rapid temperature changes.
Importance- enzymes conditions are maintained at optimum levels due to the stable environment.
• Habitat: crystalline structure in ice is less dense than liquid water as the molecules are held
further apart, forming a lattice shape.
Importance- provides an insulating layer for aquatic habitats in cold climates preventing the water
below from freezing providing a habitat for some organisms.
• High latent heat of evaporation: takes a lot of energy to break the hydrogen bonds between
water molecules, so it has high latent heat of evaporation, a lot energy is used up when water
evaporates. E.g. when sweat evaporates, it cools the surface of the skin.
Importance- sweating and panting can cool an organism when water on the body is evaporated
(thermoregulation).
(b) The concept of monomers and polymers and the importance of condensation and hydrolysis
reactions in a range of biological molecules.
Key words:
Monomers- building blocks of polymers.
Polymers- chains of monomers.
Condensation reaction- a reaction in which two molecules become covalently bonded to each
other through the loss of a small molecule, usually water; also called dehydration reaction.
Hydrolysis reaction- a chemical reaction that breaks apart a larger molecule by adding a
molecule of water.
Biochemistry- the chemistry of all biological reactions taking place in an organism.
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Metabolism- is the total of all chemical reactions the take place in an organism. The reactions are
usually catabolic or anabolic.
• Biological molecules are normally polymers and are formed by either condensation or hydrolysis
reactions.
• Monomers are building blocks of polymers.
• Metabolism is the total of all chemical reactions that take place in an organism. The reactions
are usually catabolic or anabolic.
• Condensation reactions where molecules are joined together by a covalent bond to make a
larger, more complex molecule, with the loss of a small molecule.
• Hydrolysis reactions where water is used to break down the bonds of a particular substance.
(c) The chemical elements that make up biological molecules.
• Carbohydrates- C,H,O
• Lipids- C,H,O
• Proteins- C,H,O,N,S
• Nucleic acids- C,H,O,N,P
- Carbon atoms can form 4 bonds with other atoms.
- Nitrogen atoms can form 3 bonds with other atoms.
- Oxygen atoms can form 2 bonds with other atoms.
- Hydrogen atoms can form 1 bond with other atoms.
(d) The ring structure and properties of glucose as an example of a hexose monosaccharide and
the structure of ribose as an example of a pentose monosaccharide.
Key words:
Disaccharide- a double sugar, consisting of two monosaccharides joined by dehydration
synthesis.
Monosaccharide- a single sugar unit.
Reducing sugar- all monosaccharides and some disaccharides. This means that they can donate
electrons, or reduce another molecule or chemical.
Glucose
• Glucose is a monosaccharide containing six
carbon atoms in each molecule. Has two isomers-
alpha and beta glucose.
• Glucose molecules are polar and soluble in water
(allows it to dissolve in the cytosol), due to
hydrogen bonds that form between the hydroxyl
groups and water molecules.
• 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.
• Glucose has two structural isomers- alpha and beta glucose.
• Alpha glucose is a substrate in respiration.
• Beta glucose polymerises to form cellulose.
• The only di erence between these 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.
• Animals and plants have enzymes that will only break down alpha-glucose
because they only work with chemicals of speci c shapes and beta-glucose
doesn’t t the active site of the enzyme.
Ribose
• Ribose is a monosaccharide containing ve carbon atoms in each molecule.
C,H,O
• Used as the sugar in RNA nucleotides.
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