Biological Molecules

Carbohydrates

A naturally made compound found in all living things, made up of Carbon, Hydrogen and Oxygen.

A monosaccharide is a simple sugar e.g. glucose (C6H12O6) or fructose

Glucose molecules contain lots of energy which can be released in respiration by breaking the bonds between the carbon atoms

A disaccharide is made when two monosaccharides join together

  • Maltose is formed from two glucose molecules
  • Sucrose is formed from one glucose and one fructose molecule

A polysaccharide is formed when lots of monosaccharides join together

  • Starch, glycogen or cellulose are all formed when lots of glucose molecules join together
  • Polysaccharides are insoluble and therefore useful as storage molecules

Lipids

Contain elements of Carbon,Hydrogen and Oxygen.

Most fats (lipids) in the body are made up of triglycerides. Triglycerides are composed of one glycerol molecule chemically bonded to three fatty acid chains; the fatty acids vary in size and structure.

Proteins

  • Proteins are formed from long chains of amino acids.
  • Proteins contain elements carbon,hydrogen,oxygen and nitrogen. ( sulphur)
  • There are 20 different amino acids
  • When amino acids are joined together a protein is formed.
  • Amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins.
  • Examples of proteins include enzymes, haemoglobin, ligaments and keratin.

Food Tests

Iodine Test for Starch

  • Place one spatula of the food sample on a dish or 1 cm3 if the sample is liquid.
  • Using a dropper, place a few drops of iodine solution onto the food.
  • Record any change in the colour of the solution.
  • Negative test: will stay orangey - brown
  • Positive Test: will change from orange to blue-black

Benedict's Test for Glucose

  • Add food sample to test tube
  • Add a few drops of Benedict’s solution
  • Heat in a boiling water bath for 5 minutes
  • Take the test tube out of the water bath and observe the colour
  • Negative : will stay blue
  • Positive test : will change from blue to orange or brick red

Biuret's Test for Protein

  • Add food sample to test tube
  • Add a few drops of Biuret solution
  • Observe the colour
  • Negative test : changes from blue to purple
  • Positive Test: will remain blue

Enzymes

Definitions

Enzyme - Proteins that act as biological catalysts, meaning they speed up reactions without being used up themselves.
Catalyst - A substance that speeds up reactions without being used up themselves.
Substrate - A substance on which enzymes act.
Active Site - Region of an enzyme where the substrate attaches.

  • Enzymes are proteins that act as biological catalysts to speed up the rate of a chemical reaction without being changed or used up in the reaction.
  • They are biological because they are made in living cells.
  • Enzymes are necessary to all living organisms as they maintain reaction speeds of all metabolic reactions at a rate that can sustain life.
  • For example, if we did not produce digestive enzymes, it would take around 2 – 3 weeks to digest one meal; with enzymes, it takes around 4 hours.
  • Often the products of one reaction are the reactants for another (and so on).

The mechanism of enzyme action - lock and key model

  • Enzymes are specific to one particular substrate(s) .
  • The active site of the enzyme, where the substrate attaches, is a complementary shape to the substrate.
  • When the substrate moves into the enzyme’s active site they become known as the enzyme-substrate complex.
  • After the reaction has occurred, the products leave the enzyme’s active site as they no longer fit it and it is free to take up another substrate.

Factors Affecting Enzymes

Temperature

  • Enzymes are proteins and have a specific shape, determined by the amino acids that make the enzyme and held in place by bonds.
  • This is extremely important around the active site as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed.
  • Enzymes work fastest at their ‘optimum temperature’ - in the human body, the optimum temperature is 37⁰C.
  • Heating enzymes to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and it will lose its shape. This is known as denaturation.
  • Substrates cannot fit into denatured enzymes as the shape of their active site has been lost.
  • Denaturation is irreversible.

Describe- Example
As temperature increases, the enzyme activity starts to increase. This continues until temperature A is reached, when enzyme activity is highest. Once the temperature increases above temperature A, enzyme activity rapidly decreases.

Explain - Example
Initially, increasing temperature increases the kinetic energy of enzyme and substrate, leading to more frequent collisions (and formation of enzyme-substrate complexes) and a higher rate of activity At a certain temperature an optimum rate of reaction is achieved (temperature A).
Above this temperature the enzyme starts to denature due to breaking of bonds and the rate of activity decrease

pH

  • The optimum pH for most enzymes is 7
  • Enzymes produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2).
  • Enzymes produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
  • If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be disrupted/destroyed. This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity.
  • Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop.

Describe - Example
As pH of the solution increases from 2, the rate of enzyme activity rapidly increases. Enzyme activity peaks at ~pH 6 and then decreases. Enzyme activity has stopped by pH 10. Enzyme B follows a similar pattern, but at a higher pH, with an optimum of 10 and a range of 6 - 14.


Explain - Example
Enzymes have an optimal pH for activity. At a higher or lower pH enzyme activity will decrease. This is because changing pH can alter the charge, shape and solubility of the protein molecule making it non-functional.


Ethanol Test for Lipids

  • Add food sample to test tube
  • Add 4cm3 of ethanol and shake to mix. Allow time for the sample to dissolve
  • Strain the ethanol into a new test tube
  • Add an equal volume (4cm3) of distilled water
  • Observe colour changes
  • Negative Test: will stay clear
  • Positive Test: will change from clear to cloudy