Biological Molecules

Biological Molecules

Metabolism is the sum total of all the biochemical reactions (anabolic and catabolic) taking place in the cells of an organism.

  • Anabolic Reactions: building smaller molecules into larger ones e.g. muscle growth
  • A condensation reaction is where bonds are formed and water is removed
  • Catabolic Reactions: breaking larger molecules into smaller ones e.g. digestion
  • A hydrolysis reaction is where bonds are broken by the addition of water


Carbon can form so many different molecules because it has a valency of 4 which means it can form 4 single covalent bonds with 4 other atoms


  • describe how hydrogen bonding occurs between water molecules, and relate this and other properties of water, to the roles of water in living organisms


The shared electrons between the oxygen and hydrogen atoms in water are not shared evenly. The oxygen atom has a greater pull on the shared electrons than the hydrogen atoms so the oxygen atom becomes slightly negatively charged and the hydrogen atom becomes slightly positively charged so that there is an uneven charge distribution across the molecule, making water polar.


b) describe, with the aid of diagrams, the structure of an amino acid


c) describe, with the aid of diagrams, the formation and breakage of peptide bonds in the synthesis and hydrolysis of dipeptides and polypeptides

The quaternary structure is made up of more than one polypeptide chains joined together.

Haemoglobin is a globular protein made of 4 polypeptide chains (2 alpha chains and 2 beta chains), bonded together. They have a prosthetic group which is the haem group – they have 4 haem groups.


Haemoglobin’s function is to carry oxygen from the lungs to the tissues. It binds oxygen in the lungs and releases it in the tissues.


(g) explain, with the aid of diagrams, the term quaternary structure, with reference to the structure of haemoglobin












Heating a protein increases the kinetic energy in the molecules. This causes the molecules to vibrate and break the bonds holding the tertiary structure in place as most of the bonds holding the tertiary structure in place are quite weak (not covalent – hydrogen, ionic, hydrophilic or hydrophobic bonds), so they are easily broken.


If enough heat is applied, the whole tertiary structure can unravel and the protein will no longer function – this is called denaturation.


(h) describe, with the aid of diagrams, the structure of a collagen molecule

Collagen is a fibrous protein which is an important structural component in cell walls as it’s very strong. It can be found in skin, tendons, cartilages, bones and teeth.


A collagen molecule is made up of three polypeptide chains wound around each other to form a twisted rope. Hydrogen bonds form between the chains, which gives the structure strength. Covalent bonds, called cross-links, form between other collagen molecules, adds to the strength.


Collagen can give strength in the walls of arteries to withstand high pressure as well as the tendons to connect skeletal muscles to bonds. Collagen can form bones, cartilage and connective tissue.



(i) compare the structure and function of haemoglobin (as an example of a globular protein) and collagen (as an example of a fibrous protein)

Functions of Lipids in Living Organisms:

  • Source of energy – respire to release energy
  • Energy storage – stored as adipose cells
  • Membranes – phospholipid bilayer
  • Insulation – e.g. blubber in whales
  • Protection – e.g. surface of plant protected against drying out
  • Hormones – e.g. steroid hormones

(s) describe how the concentration of glucose in a solution may be determined using colorimetry

The Benedict’s Test reveals the presence of reducing sugars by producing an orange-red precipitate. The more reducing sugar there is present, the more precipitate will be formed, and the more Benedict’s solution (copper sulfate) will be ‘used up’. If the precipitate is filtered out, then the concentration of the remaining solution can be measured. This will tell you how much Benedict’s solution has been used up.



The calorimeter is a device that shines a beam of light through a sample. A photoelectric cell picks up the light that is passed through the sample and gives a reading on how much light has passed through.

  • Use water to calibrate the colorimeter (for 100% transmission/ 0% absorption)
  • Place the solution in a sample chamber between the light and the photoelectric cell in a cuvette.
  • The more copper sulfate that is used up in Benedict’s Test in a sample, the less light will be blocked out and the more light will be transmitted.
  • Plot these readings on a graph to show light getting though (transmission) against reducing sugar concentration, creating a calibration curve.
  • If there is an unknown sample, use the graph to find the equivalent reducing sugar concentration for the reading on the calorimeter.
  • Colour filters are often used for greater accuracy – in this case, a red filter would be used.


The higher the glucose concentration, the higher the transmission and the lower the absorption.