• Revision:Molecules and Cells

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Contents

Molecules

Properties and Functions of Important Molecules
Molecule Building Blocks Elements Present Functions!
Polysaccharides Monosaccharides
  • Carbon
  • Hydrogen
  • Oxygen
Starch/Glycogen:
  • Energy storage

Celluose:

  • Used in Cell Walls
Protein Amino Acids
  • Carbon
  • Hydrogen
  • Oxygen
  • Nitrogen
  • Possibly Sulphur
  • Act as Receptors
  • Form Antibodies
  • Enzymes
  • Structure
Lipids Fatty Acids and Glycerol
  • Carbon
  • Hydrogen
  • Oxygen
  • Energy Storage
  • Can be respired
Nucleic Acids

(DNA & RNA)

Nucleotides
  • Carbon
  • Hydrogen
  • Oxygen
  • Phosphorus
  • Nitrogen
  • Carry Genetic Code
  • Protein Synthesis
Biochemical Tests
Substance Name of Test Details Result
Protein Biuret Test

Add Biuret solution

Solution turns lilac
Reducing Sugars Benedict's Test Heat sample with Benedict's reagent (copper sulphate) Red-brown precipitate (copper oxide)
Non-reducing sugars
  1. Check not a reducing sugar
  2. Add hydrochloric acid
  3. Neutralise with sodium hydrogencarbonate
  4. Heat sample with Benedict's reagent
Red-brown precipitate (copper oxide)
Starch Iodine Test Add iodine solution Turns blue-black
Lipids Emulsion Test
  1. Dissolve by shaking with ethanol
  2. Pour into water
White "milk-like" emulsion formed

Water

Water is vital to all living organisms.

It has...

  • dipolar molecules (one end Oδ- and one with the two Hδ+).
    • causes them to group together
    • attractive forces form hydrogen bonds
    • is a good solvent for hydrophilic substances
  • a high specific heat capacity - that's to say, it takes a lot of energy to raise its temperature.
    • important inside cells to maintain enzyme-controlled reactions
    • provides a constant environment for aquatic organisms
  • a high latent heat of vaporisation - it takes a lot of energy to change it from a liquid to a gas
    • important in sweating; a lot of energy is taken away from the body when the water in sweat evaporates
  • its maximum density at 4°C
    • ice is less dense than cold water, so floats on top
    • allows fish and other aquatic organisms to survive in frozen ponds
  • high surface tension, high cohesion and adhesion
    • certain small organisms rely on water's surface tension, e.g. water boatmen, pond skaters, to walk on water
    • vascular plants rely on cohesion for transpiration to occur, pulling the water through the roots and stems because of the attraction between the water molecules, and powered by water leaving the plant through stomata as water vapour

Carbohydrates

Monosaccharides

These contain carbon, hydrogen and oxygen in the ratio 1:2:1.

The general formula for a monosaccharide is (CH2O)n, where 3 ≤ n ≤ 9.

Each monosaccharide contains:

  • a carbonyl group ( C=O )
  • at least 2 hydroxyl groups ( OH )

All monosaccharides are reducing sugars

Pentoses

These have 5 carbon atoms. Examples are ribose and deoxyribose. They can exist in either ring or chain form.

Hexoses

These have 6 carbon atoms. Can exist in either ring or chain form.

Glucose

Glucose can exist in 2 different forms.

Because the arrangment is spatial, they are called stereoisomers

Disaccharides

2 monosaccharide units form a disaccharide. The reaction is called a condensation reaction, and involves the removal of a water molecule.

Maltose = Glucose + Glucose

Sucrose = Glucose + Fructose

Lactose = Glucose + Galactose

The bond between the molecules is called a glycosidic bond. When this breaks, it is called a hydrolysis reaction

Polysaccharides

These are many monosaccharide units joined together.

Starch (in plants) and Glycogen (in animals)

Starch:

  • α-glucose monomers
  • storage role
  • insoluble and compact

Cellulose:

  • long chains of β-glucose monomers, cross linked by hydrogen bonds
  • chains grouped into microfibrils
  • structural role

Lipids

Fats, oils and waxes are all lipids. There are two important kinds of lipids - triglycerides and phospolipids

Triglicerides

Triglycerides consist of a glycerol molecule and three fatty acids, joined by ester bonds formed by condensation reactions. They are non-polar and insoluble.

Important functions include:

  • storage
  • heat and electrical insulation
  • protection of internal organs
  • waterproofing

Unsaturated fatty acids contain one or more double bonds.

Saturated fatty acids do not have any double bonds.

Phospholipids

Phospholipids are lipids in which:

  • one of the fatty acid chains is replaced by a phosphate group
  • the phosphate group is polar (hydrophylic)
  • the rest is non-polar (hydrophobic)

They are important in the structure of cell membranes

Proteins

Proteins are built up from linear sequences of amino acids. There are about 20 of these.

Proteins contain carbon, hydrogen, oxygen and sometimes sulphur and/or phosphorus. They posses an amino group ( NH2 ) at one end, and a carboxyl group ( COOH ) at the other.

Proteins are crystalline, colourless and amphotenic, so can act as buffers.

Peptide bonds are formed by condesation reactions, and broken down by hydrolysis reactions.

There are four levels of protein structure:

1. Primary Structure
This is the sequence of amino acids in the polypetide chain
2. Secondary Structure
This is the shape that the polypetide chain forms as a result of hydrogen bonding. It is most often an α-helix or a β-pleated sheet
3. Tertiary Structure
This is the 3-D shape of the helix. It is maintaind by disulphide, ::ionic and hygrogen bonds
4. Quaternary Structure
This is where more than one polypetide chains are joined together to form the protein.

Nucleic Acids

Nucleic acids are built up of nucleotides. These are made from three parts, joined by condensation reactions:

  • phosphate
  • pentose sugar (either ribose or deoxyribose)
  • nitrogenous base

<insert diagram>

There are five nitrogenous bases, devided into two categories:

Pyrimidines Purines
  • Cytosine
  • Thymine (only in DNA)
  • Uracil (only in RNA)
  • Guanine
  • Adenine

RNA is a single strand.

DNA is double stranded and forms a double helix, the shape of which is maintained by hydrogen bonding. The base pairs are also joined together with hydrogen bonds. The bases pair up according to complementary base pairing:

  • adenine joins with thymine (DNA) or uracil (RNA)
  • cytosine joins with guanine.

There are three types of RNA:

  • ribosomal RNA (rRNA)
  • transfer RNA (tRNA)
  • messenger RNA (mRNA)

DNA

DNA Replication

DNA replication is semi-conservative:

  1. The double helix unwinds, and as the strands separate, DNA polymerase catalyses the addition of free nucleotides to their exposed complimentary bases.
  2. Each chain acts as a template for the formation of a new double helix
  3. The end result is two new chains, each containing half the original - hence "semi-conservative".

The Genetic Code

The codes carried by DNA determine an organism's characteristics, by controlling the production of enzymes and other proteins.

Each code is carried on a particular length of DNA, called a gene, and determines the sequence of amino acids in a polypeptide.

Each amino acid is coded for by three bases, called a codon. There are 64 possible codons.

Each codon is read separately - there's no overlapping.

Protein Synthesis

There are four main stages:

  1. synthesis of amino acids
  2. transcription
  3. amino acid activation
  4. translation

DNA in the nucleus acts as a template for the production of mRNA, which conveys the instructions to the cytoplasm. The ribosomes provide a suitable surface for the attachment of mRNA and the assembly of proteins.

Steps in Protein Synthesis

  1. A very short section of the DNA molecule carries the code for the synthesis of a polypeptide chain. The double-stranded DNA first untwists, then unzips in the relevant region.
  2. Free RNA molecules then align themselves opposite one of the two strands, according to complementary base pairing.
  3. RNA polymerase moves along the DNA adding one complimentary RNA nucleotide at a time. This is called transcription.
  4. The resulting mRNA moves out through the nuclear pores, and attaches itself to a ribosome

Cell Components

Centrioles

The centrioles are small and cylindrical, and lie adjacent to the nucleus. There are two of them, and during mitosis and meiosos they migrate to opposite ends of the cell, forming a spindle between them. The spindle is an array of micro-tubules, which pull the chromatids into each daughter cell.

Celluose cell wall

The cell wall is made up of three layers. The outermost layer is called the middle lamella, and is made of pectins. Working in towards the cell surface membrane, the other two layers are the primary cell wall and the secondary cell wall. Both of these contain pectins, but mixed with other polysaccharides. The polysaccharides form a gell-like matrix in which fibres of cellulose are embedded.

The function of the cell wall is to maintain the shape of the cell and protect it from damage. It allows plants to keep their form even when cells are flaccid.

Chloroplasts

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