S-Cool Revision Summary

S-Cool Revision Summary

Contain 3 elements:

  1. Carbon (C)
  2. Hydrogen (H)
  3. Oxygen (O)

Carbohydrates are found in one of three forms:

  1. Monosaccharides
  2. Disaccharides (both sugars)
  3. Polysaccharides

These are formed when two monosaccharides are condensed together. One monosaccharide loses an H atom from carbon atom number 1 and the other loses an OH group from carbon 4 to form the bond.

The reaction, which is called a condensation reaction, involves the loss of water (H2O) and the formation of a 1,4-glycosidic bond.

Examples of Disaccharides

Sucrose: glucose + fructose,

Lactose: glucose + galactose,

Maltose: glucose + glucose.


  1. Substrate for respiration (glucose is essential for cardiac tissues).
  2. Intermediate in respiration (e.g. glyceraldehydes).
  3. Energy stores (e.g. starch, glycogen).
  4. Structural (e.g. cellulose, chitin in arthropod exoskeletons and fungal walls).
  5. Transport (e.g. sucrose is transported in the phloem of a plant).
  6. Recognition of molecules outside a cell (e.g. attached to proteins or lipids on cell surface membrane).

Lipids are made up of the elements carbon, hydrogen and oxygen but in different proportions to carbohydrates. The most common type of lipid is the triglyceride


Lipids can exist as fats, oils and waxes. Fats and oils are very similar in structure (triglycerides).

These are made up of 3 fatty acid chains attached to a glycerol molecule.

  1. Storage - lipids are non-polar and so are insoluble in water.
  2. High-energy store - they have a high proportion of H atoms relative to O atoms and so yield more energy than the same mass of carbohydrate.
  3. Production of metabolic water - some water is produced as a final result of respiration.
  4. Thermal insulation - fat conducts heat very slowly so having a layer under the skin keeps metabolic heat in.
  5. Electrical insulation - the myelin sheath around axons prevents ion leakage.
  6. Waterproofing - waxy cuticles are useful, for example, to prevent excess evaporation from the surface of a leaf.
  7. Hormone production - steroid hormones. Oestrogen requires lipids for its formation, as do other substances such as plant growth hormones.
  8. Buoyancy - as lipids float on water, they can have a role in maintaining buoyancy in organisms

A phosphate-base group replaces one fatty acid chain. It makes this part of the molecule (the head) soluble in water whilst the fatty acid chains remain insoluble in water.

Due to this arrangement, phospholipids form bilayers (the main component of cell and organelle membranes).

Different proteins can appear very different and perform diverse functions (e.g. the water-soluble antibodies involved in the immune system and the water-insoluble keratin of hair, hooves and feathers). Despite this, each one is made up of amino acid subunits.

There about 20 different amino acids that all have a similar chemical structure but behave in very different ways because they have different side groups. Hence, stringing them together in different combinations produces very different proteins.

When 2 amino acids are joined together (condensation) the amino group from one and the acid group from another form a bond, producing one molecule of water. The bond formed is called a peptide bond.

Hydrolysis is the opposite of condensation and is the breaking of a peptide bond using a molecule of water.

Fibrous proteins are made of long molecules arranged to form fibres (e.g. in keratin). Several helices may be wound around each other to form very strong fibres.

Globular proteins are made of chains folded into a compact structure. One of the most important classes are the enzymes. Although these folds are less regular than in a helix, they are highly specific and a particular protein will always be folded in the same way. If the structure is disrupted, the protein ceases to function properly and is said to be denatured.

If a protein is made up of several polypeptide chains, the way they are arranged is called the quaternary structure. Again, each protein formed has a precise and specific shape (e.g. haemoglobin)

  1. Virtually all enzymes are proteins.
  2. Structural: e.g. collagen and elastin in connective tissue, keratin in skin, hair and nails.
  3. Contractile proteins: actin and myosin in muscles allow contraction and therefore movement.
  4. Hormones: many hormones have a protein structure (e.g. insulin, glucagon, growth hormone).
  5. Transport: for example, haemoglobin facilitates the transport of oxygen around the body, a type of albumin in the blood transports fatty acids.
  6. Transport into and out of cells: carrier and channel proteins in the cell membrane regulate movement across it.
  7. Defence: immunoglobulins (antibodies) protect the body against foreign invaders; fibrinogen in the blood is vital for the clotting process.

The majority of the reactions that occur in living organisms are enzyme-controlled. Enzymes are proteins and thus have a specific shape. They are therefore specific in the reactions that they catalyse - one enzyme will react with molecules of one substrate.

The site of the reaction occurs in an area on the surface of the protein called the active site.

Reactions proceed because the products have less energy than the substrates.

However, most substrates require an input of energy to get the reaction going, (the reaction is not spontaneous).

The energy required to initiate the reaction is called the activation energy.

When the substrate(s) react, they need to form a complex called the transition state before the reaction actually occurs. This transition state has a higher energy level than either the substrates or the product.

  1. Temperature
  2. pH
  3. Enzume Concentration
  4. Substrate Concentration

Most enzymes require additional help from cofactors, of which there are 2 main types:

  1. Coenzymes - these are organic compounds, often containing a vitamin molecule as part of their structure.
  2. Metal ions - most speed up the formation of the enzyme-substrate complex by altering the charge in the active site e.g. amylase requires chloride ions, catalase requires iron.

Inhibitors slow down the rate of a reaction. Sometimes this is a necessary way of making sure that the reaction does not proceed too fast, at other times, it is undesirable.

Reversible Inhibitors:

Competitive reversible inhibitors

Non-competitive reversible inhibitors

Irreversible Inhibitors:

These molecules bind permanently with the enzyme molecule and so effectively reduce the enzyme concentration, thus limiting the rate of reaction, for example, cyanide irreversibly inhibits the enzyme cytochrome oxidase found in the electron transport chain used in respiration. If this cannot be used, death will occur.

This technique separates out mixtures of chemicals by using their different solubilities in certain solvents.