2.2 – Carbohydrates, Lipids and Proteins

2.2 – Carbohydrates, Lipids, and Proteins

2.2.1 – Distinguish between organic and inorganic compounds

Organic compounds are based on carbon and can be found in living things. Exceptions include HCO₃, CO₂ and CO. These are classed as non-organic carbon. Three types of organic compounds widely found in living organisms are lipids, proteins and carbohydrates. Inorganic compounds are any compounds that do not fall into the category of organic compounds.


2.2.2 – Identify amino acids, glucose, ribose and fatty acids from diagrams showing their structure

Amino acids
There are 20 common amino acids found in the protein structures of living things. These are monomers, and combine to form larger polypeptides, which in turn form proteins. These are the basis of enzymes, as well as many cellular and extracellular components. All amino acids are soluble. Each common amino acid has the same structure, except for the R group.



Glycine (below) is the smallest amino acid. A common source of it is sugar cane. Glycine has an amino group, a carboxylic acid group and an R group (H).





Alanine (below) is a common amino acid, similar to glycine, but the R group is CH₃. The R group of each amino acid is different, and the amino acids have very different characteristics as a result (and consequently the proteins containing them).





Monosaccharides are carbohydrates with relatively small molecules. They taste sweet and are soluble in water. All the bonds in these molecules are covalent. Glucose is an important monosaccharide as:

  • All green leaves manufacture glucose using light energy
  • Our bodies transport glucose in the blood
  • All cells use glucose in respiration – it is called one of the respiratory substrates
  • Glucose is the building block for many larger molecules in cells and organisms




The molecular formula of glucose is C₆H₁₂O₆



Ribose is an example of a pentose, or 5-carbon sugars. Deoxyribose is a modified version of

ribose, and is known for its role in DNA as part of the sugar phosphate backbone. Its chemical
properties are very different to ribose.




Fatty acids

These are the basis of triglycerides and many other types of lipid. They are also the basis of the phospholipid molecules of the phospholipid bilayer of the cell membrane. Lipids are insoluble in water, often described as hydrophobic. This is a basic saturated (no double bonds) fatty acid. There is a methyl group (CH₃) at one end of the chain. The chains are made up of covalently bonded carbons, saturated with hydrogens. The chain is non-polar and hydrophobic. These are typically made up of 16-18 carbon atoms, but can be anywhere from 14-22. The carboxyl group is polar, making the end of the molecules hydrophilic.

In water, fatty acid molecules arrange into spheres called micelles. The tails are at the centre, away from water. This is important to fat digestion and membrane structure.


2.2.3 – List three examples each of monosaccharides, disaccharides, and polysaccharides



Glucose – [animal] transported to cells in the blood plasma, and used as a respiratory substrate. In plants, it is a first product of photosynthesis

Galactose – [animal] used in the production of lactose

Fructose – [plant] this is produced in cellular respiration, and in the production of sucrose



Lactose – [animal] produced in mammary glands and secreted into the milk as an important component in the diet of very young mammals

Sucrose – [plant] produced in green leaves from glucose and fructose. It is transported in the plant in solution, in the vascular bundles

Maltose – [plant] this is a breakdown product in the hydrolysis of starch

Glycogen – [animal] this is a storage carbohydrate formed from glucose in the liver and other cells (except brain cells) when glucose is not immediately required for cell respiration

Cellulose – [plant] this is manufactured in cells and laid down externally, in bundles of fibres, as the main component of the cell walls

Starch – [plant] this is a storage carbohydrate


2.2.4 – State one function of glucose, lactose and glycogen in animals, and of fructose, sucrose and cellulose in plants



  • Respiratory substrate

   Lactose Dietary component for young mammals, secreted from the mammary glands in

  • Dietary component for young mammals, secreted from the mammary glands in the milk


  • Storage carbohydrate for when glucose is not immediately needed


  • Used in the production sucrose, and also an intermediate of glucose breakdown


  • Produced from glucose and fructose, and transported in the plant in solution


  • The main component of cell walls, laid down in bundles of fibres


2.2.5 – Outline the role of condensation and hydrolysis in the relationships between monosaccharides, disaccharides, and polysaccharides; between fatty acids, glycerol and triglycerides; and between amino acids and polypeptides

A polymer consists of large molecules made up of a linked series of repeated simple monomersA

A monomer is a simple molecular unit.

Condensation is the process of two monomers into a dimer or polymer. A molecule of water will also be formed as a product. This is a condensation reaction. The link between them after the removal of H₂O is called a glycosidic linkage, which comprises of strong, covalent
bonds. This reaction is brought about by an enzyme.

The reverse, a hydrolysis reaction, is when a molecule of water is added and the glycosidic linkage is split. It is also catalysed by an enzyme, but a different one from in the condensation reaction.





Below is a condensation reaction between two molecules of glucose
to form maltose. This can be reversed in a hydrolysis reaction.
To form polysaccharides, many monosaccharides are joined.
Polysaccharides form in the same way as disaccharides.




Fats and oils

Fats and oils are triglycerides (simple lipids). At 20oC,  fats are solid and oils are liquid. Oils have a lower density and melting point due to bends in their tails and unsaturated bonds. Fats tend to have longer fatty acid tails and saturated bonds. This makes them denser and raises the melting point. Triglycerides are not formed as in above. Instead, the chains are bonded to the molecule glycerol. The triglyceride formed is insoluble.

Phospholipids are the principle molecules in the cell membrane that form the bilayer. Their structure is similar to the triglyceride, except one of the fatty acids chains are replaced by a phosphate group.


+ → +  

Amino Acids

In the formation of a dipeptide or polypeptide, two amino acid monomers will align to form peptide bonds by condensation reactions. This bond can form between the carboxyl group of the
first amino acid and the amino group of the second amino acid. Again, water is removed in the reaction. A dipeptide is formed when there is a bond between C-N. This pattern is true for all polypeptides.

→ +

Polypeptide chains fold into the complex, specific shapes of the protein seen below. The shape is determined by the hydrogen bonding and some covalent bonding between R groups. Polypeptides can be hydrolysed in the same way as polysaccharides by incubating with acids. They are digested into amino acids by peptidases.

2.2.6 – State three functions of lipids

  • Energy store – Fats and oils transfer twice as much energy as carbohydrates. They are also insoluble, so their presence does not cause osmotic water uptake
  • Metabolic water source – Energy and water are released when fats are used as a substrate in respiration.
  • Buoyancy aid – fat is not as dense as muscle or bone, so [for example, the blubber in whales] it will give buoyancy to the body.
  • Thermal insulation – Heat can be retained in the body through fat insulation
  • Water-proofing for hairs and feathers – this oil acts as a water repellent cuticle, and prevents hair and feathers from becoming waterlogged when wet.
  • Electrical insulation – Myelin lipid forms sheaths around the long fibres of nerve cells, electrically isolates the cell plasma membrane and facilitates the conduction of nerve impulses.
  • Hormones – Steroids can act as hormones in the body, examples include progesterone and testosterone
  • Cell receptors – Glycolipids on the surface of cells can act as receptors for hormones and other substances
  • Structure – Lipids like cholesterol are essential for maintaining the structure of cell membranes.


2.2.7 – Compare the use of carbohydrates and lipids in energy storage