Edexcel B Biology - All AS topics.

• Carbohydrates are molecules only made of carbon, hydrogen and oxygen. They are long chains of sugar units called saccharides.
• Monosaccharide = single sugar monomer.
• Disaccharide = two monsaccharides.
• Polysaccharide = many monosaccharides.
• Glucose is a monosaccharide made of six carbon atoms and is the main substrate for respiration. It has two isomers - alpha and beta glucose.
• Beta glucose has an OH group on the left hand side of the ring structure.
• Alpha glucose has an OH group on the right hand side of the ring structure.
• Ribose is a monosaccharide containing five carbon atoms. It is a pentose sugar and a component of RNA.
• Maltose is a disaccharide formed by the condensation of two glucose molecules.
• Sucrose is a disaccharide formed by the condensation of glucose and fructose.
• Lactose is a disaccharide formed by the condensation of glucose and galactose.
• Glycogen and starch are formed by the condensation of alpha glucose.
• Cellulose is formed by the condensation of beta glucose.
• Glycogen is the main energy store in animals. Its made of alpha glucose molecules joined by 1,4 and 1,6 glyosidic bonds. It has a large number of branches meaning that it can by hydrolyzed quickly and energy can be released quickly. Its large and compact.
• • Starch stores energy in plants and comes in two forms: Amylose and Amylopectin.
  • Amylose is unbranched chain of glucose molecules joined by 1,4 glycosidic bonds - it is coiled and very compact.
  • Amylopectin is a branched glucose chain, joined by 1,4 and 1,6 glycosidic bonds. It is rapidly digested by enzymes and energy is released quickly. It is also not as compact as amylose.
• • Cellulose is a component of cell walls in plants and is made up of long unbranched chains of beta glucose which are joined by 1,4 glycosidic bonds.
  • Microfibers and microfibrils are strong threads that join together these chains by hydrogen bonds so they provide structural support to plant cells.
• • Lipids are molecules which are only soluble in organic solvents.
  • Saturated lipids only contain single carbon bonds.
  • Unsaturated lipids contain double carbon bonds and melt at lower temperatures than saturated lipids.
  • Intermolecular forces are weaker in unsaturated lipids and therefore they have a lower melting point.
  • Saturated lipids are more compact because there are no kinks in the carbon chain.
• The properties of lipids include:

  • Waterproof - fatty tail is hydrophobic.
  • Very compact because more C-O bonds are hydrolyzed.
  • Non polar and insoluble in water, they dont interfere with water based reactions in the cytoplasm - good for storage.
  • Conduct heat slowly, thermal insulation.
• Triglycerides are lipids made up of one molecule of glycerol and three fatty acids joined by ester bonds formed in condensation reactions. They are usually used as energy reserves in plant and animal cells.
• In phospholipids, one fatty acid in a triglyceride is substitued for a phosphate containing group. Phosphate heads are hydrophilic and the tails are hydrophobic. They form a bilayer in the cell membrane, with the head pointing outward and the tail inward.
• Amino acids contain an amino group, NH2, a carboxyl group, COOH and a variable R group. Amino acids are joined by peptide bonds formed in condensation reactions,
• A dipeptide contains two amino acids and polypeptides contain three or more amino acids.
• The primary structure of a protein is the linear sequence of amino acids in the polypeptide chain, held by peptide bonds.
• The secondary structure is formed by the folding of the polypeptide chain into an alpha helix or a beta pleated sheet. Only contains hydrogen bonds.
• The tertiary structure of a protein is the 3D folding of the secondary structure into a complex shape. Shape is determined by the bonds present, such as - hydrogen bonds, ionic bonding and disulphide bridges.
• The quaternary structure of a protein is the 3D arrangement of more than one polypeptide.
• Fibrous protein:

  • Long parallel polypeptides.
  • Mainly secondary structure.
  • Occasional cross linkages which form microfibres for tensile strength.
  • Insoluble.
• Globular proteins:

  • Complex tertiary/quaternary structures.
  • Form colloids in water.
  • Many uses.
• Collagen is an example of a fibrous protein. It has a high tensile strength due to the large number of hydrogen bonds. Collagen molecules are made up of three alpha chains which form a triple gamma helix. Multiple helices link together to form fibrils and strong collagen fibers.
• Nucleotides consist of pentose, a nitrogen containing organic base and a phosphate group.
• The DNA bases are: Adenine, Cytosine, Guanine and thymine.
• The components of RNA are ribose, a phosphate group and one of the bases (A,C,G,U).
• Nucleotides join together by phosphodiester bonds formed in condensation reactions.
• Semi conservative replication of DNA ensure genetic continuity between generations of cells meaning that genetic information is passed on from generations.
• The steps of semi conservative replication:

  • DNA double helix unwinds as hydrogen bonds are broken. DNA helicase catalyzes the unraveling.
  • One strand is used as a template. Free nucleotides and complementary base pairing occurs between the base strand and the free nucleotides.
  • Adjacent nucleotides are joined by phosphodiester bonds, catalyzed by DNA polymerase.
  • The new DNA molecule folds into a double helix as hydrogen bonds are reformed.
• The genetic code consists of triplet bases called codons.
• Features of the genetic code:

  • Non overlapping - each triplet is read once and triplets don't share bases.
  • Degenerate - one triplet codes for the same amino acid.
  • Contains start and stop codons which either start of stop protein synthesis.
  • Universal - its the same for all species.
• Protein Synthesis:
• Transcription:

  1. Hydrogen bonds break and DNA uncoils, catalyzed by DNA helicase.
  2. One strand is used as a template, the antisense strand.
  3. Free nucleotides line up on the antisense strand by complimentary base pairing and are joined together by phosphodiester bonds, catalyzed by RNA polymerase.
  4. mRNA then moves out the nucleus by a nuclear pore and attaches to a ribosome for translation.
• Translation:

  1. mRNA attaches to a ribosome on the rough endoplasmic reticulum. A tRNA molecule attaches to its amino acid binding site, binds to the mRNA via its anticodon.
  2. Hydrogen bonds form between the anticodon of the tRNA and the codon of mRNA.
  3. A second tRNA molecule binds to the next codon of the mRNA and the two amino acids form a peptide bond.
  4. A third tRNA molecule joins and the first one leaves.
  5. This process is repeated leading to a polypeptide chain being formed until a stop codon is reached.