Summary
Summary Full set of notes on topic 2.2
- Module
- Biological molecules
- Institution
- OCR
These notes summarise all of the content in topic 2.2 of OCR biological Molecules.
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Biological MoleculesAlways contain, carbon, hydrogen and oxygen atoms.
Proteins can contain nitrogen and sulfur.
Nucleic acids can contain nitrogen and phosphorus.
Lipids do not form polymers but do contain, C, H, O
Covalent bonds:
- When atoms share electrons to gain a full outer shell.
- This is drawn by a single line.
Condensation reaction:
- When two molecules are joined together with the removal of water.
- Two OH groups react together.
hydrolysis reaction:
- When two molecules are split apart with the addition of water.
Monomers and Polymers
- Condensation and hydrolysis reactions are responsible for linking and splitting apart
biological molecules
- The small units that are joined together are monomers
- When two Monomers join together they form a dimer
- When lots of Monomers join together they form a Polymer
Type of Molecule Monomer Polymer
Carbohydrates (C,H and O) Monosaccharides (e.g. Polysaccharides (e.g. Starch)
Glucose)
Protein (C,H,O,N and S) Amino Acids Polypeptides and Proteins
Nucleic Acids (C,H,O,N and Nucleotides DNA and RNA
P)
Hydrogen Bonds
- Water consists of two Hydrogen Atoms, covalently bonded to one oxygen
- Oxygen has a greater number of positive protons, so has a stronger attraction to the
electrons in the covalent bond
- Therefore it gains a partial negative charge, and the two hydrogen gain a partial positive
charge.
- When this happens, the molecule is polar.
, - A hydrogen Bond is a weak interaction which happens
between a slightly positively charged hydrogen atom
and a slightly negatively charged other atom.
- Its weaker than a covalent bond.
- But in some polymers where there are thousands of the
bonds between the monomers which helps make the
molecule more stable and strong.
Properties of water
Liquid at room temperature
- Water is a liquid at room temperature, because of the constant making and breaking of
hydrogen bonds, unlike in other liquids.
- The hydrogen bonds between the molecules make it more difficult for them to escape to
become a gas. Compared to similar sized molecules that are gasses at room temp.
- Even with the hydrogen bonds, water has a low viscosity, and flows easily.
Because it is a liquid at room temperature, water can:
- Provide Habits for living things in rivers, lakes and seas
- form a major component of the tissues in living organisms
- Provide a reaction medium for chemical reactions
- Provide an effective transport medium, eg, in blood and vascular tissue
Density
- The density of water provides an ideal habitat for living things
- If water was less dense, aquatic organisms would find it very difficult to float
- As water gets colder until about 4 degrees, it gets more dense.
- As it goes from 4 degrees to freezing, the water molecules align themselves in a
structure which is less dense than liquid water.
Because Ice is less Dense than water:
- Means that a layer of ice forms on the top of water, and organisms underneath have a
stable environment.
- The bodies of water are insulated.
- The layer of ice reduces rates of heat loss.
Solvent - ability to dissolve other substances.
- Water is a very good solvent for many substances, due to its polar nature.
, - The positive and negative parts of the water molecules are attracted to the negative and
positive parts of the solute (thing being dissolved)
- Water molecules cluster around these charged
parts of the solute molecules and help to seperate
them and keep them apart.
- At this point they will dissolve.
- Only polar substances dissolve in water
Because water is such a good solvent:
- Molecules and ions can move around and react
together in water, many such reaction
happen in the cytoplasm in the cell which is over
70%
- Molecules and Ions can be transported around living things whilst dissolved in water
Cohesion and surface tension
- A drop of water on a flat surface does not spread out, because Hydrogen bonding
between the molecules pulls them together
- water molecules demonstrate cohesion (pulling)
- This happens at the surface of the water as well, the molecules at the surface are
hydrogen bonded to the molecules beneath them.
- This means they are more attracted to the water molecules beneath them to the air
molecules above them.
- Meaning the surface of the water can contract as it is being pulled inwards, and gives the
surface of the water an ability to resist the force applied to it.
- Known as surface tension.
Because of the cohesion and surface tension
- Column of water in plant tissue are pulled up the xylem together from the roots
- Insects like pond skaters can walk on water.
High specific heat capacity
- Water temperature is a measure of kinetic energy of the water molecules
- The molecules are held together tightly by hydrogen bonds
- Meaning a lot of heat energy is needed to increase the kinetic energy and temperature.
- The amount of heat energy needed is known as specific heat capacity ( 4.2KJ of energy
to raise the temperature of 1kg of water by 1 degrees celsius)
- it takes a lot of energy to heat up or cool down water - meaning that the temperature of
water is mainly stable.
High specific heat capacity is important for:
- Living things need a stable temperature for enzyme- controlled reaction to happen
properly.
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