Biological Molecules- WJEC AS

Carbohydrates, Lipids, Proteins, water and inorganic ions

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  • Created by: ava.scott
  • Created on: 05-05-14 12:25

Carbohydrates : Monosaccharides

Monosaccharides are the monomer for carbohydrates. They all have same general formula, CH20)n. The 'n' is determined by how many carbon atoms there are. Glucose is a hexose sugar and has 6 carbon atoms.

Glucose exists as two isomers: alpha and beta.


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Carbohydrates : Monosaccharides

Monosaccharides have two main functions:

  • used as a source of energy for respiration
  • as building blocks for larger molecules.

Glucose + glucose >>> maltose

Glucose + galactose >>> lactose

Glucose + fructose >>> sucrose

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Carbohydrates: Disaccharides

These consist of two monosaccharise units join by a glycosidic bond, by a condensation reaction.

glucose + glucose >>> maltose and water

They are used for storage and transport e.g. carbohydrate is carried as sucrose in plants.


  • Reducing sugars: 

e.g. glucose. Add benedicts reagent and heat in a water bath. If a reducing sugar is present, the reagent will go from blue to brick red.

  • Non reducing sugars:

e.g. sucrose. Add HCl to break down the disaccharide into its constituent monomers. Then you must add an alkali as Benedicts only works in neutral conditions. Then add Benedicts and heat as before.

This test is a qualitative or semi quantitative. It shows the volume of precipitate formed. To get a quantitative reading, you could use a biosensor.

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Carbohydrates: Polysaccharides: Starch

These are long chains of monosaccharides joined together by glycosidic bonds.


Made up from two polysaccharides made from alpha glucose; amylopectin and amylose. Amylose is a linear molecule (1,4 glycosidic bonds) and winds up into a single helix. Amylopectin is branched (1,4 and 1,6 glycosidic bonds) and fits inside the amylose helix.

  • Insoluble, so doesn't effect osmosis in the cell.
  • compact, doens't take up much room.
  • formes starch grains , which are found in seeds and storage organs such as potato tubers.

Glycogen has a very similar structure to amylopecton, but wth more branches. Both are very easily hydrolysed for energy.

Test for Starch:

Iodine will turn blue- black from orange-brown when in contact with Starch.

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Polysaccharides: Cellulose and Chitin


Made from beta glucose. It forms long parallel chains of beta glucose, with crosslinking hydorgen bonds. These are allowed to form as each beta gluocse monomer has to rotate by 180° to match the hydroxyl groups. These hydrogen bonds give cellulose its strength.

The chains then form microfibrils, which then bundle together as fibres. A cell wall has several layers of fibres running in different directions. It is freely permeable but extremely strong and tensile, allowing plant cells to become turgid.


This has a very similar structure to cellulose, but has amino acids joined onto it. This makes it waterproof, lightweight and durable. It is used in insect's exoskeletons.

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These ae non-polar compounds so do not dissolve in water. They do dissolve in rganic solvents such as acetone and alcohols.


These contain a glycerol molecule and three fatty acids. The fatty acids are joined to the glycerol with an ester bond, by a condensation reaction.

Properties of Lipid

Saturated or Nonsaturated

Saturated fatty acids have no double bonds between carbons. This means every carbon has the maxiumum amount of hydrogens attached to it. Non-saturated fatty acids have carbon double bonds.

Animal lipids are often saturated and plants = nonsaturated.

A high intake of non-saturated fats in a contributor to heart disease.

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Roles of Lipids and Phospholipids

  • Great energy resources; they provide 2x the energy per gram compared to carbohydrates when metabolised.
  • Good source of metabolic water.
  • Good insulator and protector of delicate organs.
  • Waterproofing- lipids often act as barrrier to water loss e.g. leaves and insects waxy cuticle.


Similar structure to trigylcerides but instead of the third fatty acid, they have phosphate group. This makes phospholipids polar, as the phospahte head is hydrophilic, and the fatty acids are hydrophobics.

They form the basis of plasma membranes in cells.

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Proteins: Amino Acids


Amino acids are the monomer of proteins. They always contain nitrogen.

There are about 20 different amino acids, and the combination of them decide the shape and function of a protein.

They each contain a amino group, a carboxyl group and a R-group (this distinguishes each amino acid.)

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Structure of a polypeptide

The carboxyl group of one amino acid reacts with the amino group of another, eliminating water and making  petide bond.

Primary Structure

This is sequence of amino acids in the protein, joined by peptide bonds.

Secondary Structure

This is when a polypeptide chain forms either a alpha helix or a beta pleated sheet. These shaps are defined by hydrogen bonds.

Tertiary Structure

This is when a polypetide fold up into a 3D complex structure, maintained by ionic, disulphide and hydrogen bonds.

Quaternary Structure

This is when there is more than one polypeptide within a protein e.g. haemoglobin joined by haem groups.

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Types of proteins


  • Soluble
  • Spherical
  • Many functions e.g. hormones, enzymes, antibodies
  • Travel in blood stream


  • Waterproof
  • Strong and flexible
  • Structural function
  • e.g. Collagen (three polypepides intertwined like plait wth many crosslinkages.)
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  • Solvency- due to its polarity as a molecule, means charged particles are attracted to each dipole of the water molecule.
  • Relatively high boiling point- hydrogen bonds form between water molecules, and tehse have to be broken before water can boil. This leads to a high latent and specific heat.
  • LATENT HEAT- This means it requires a lot of energy to change water from one state (liquid) to another (gas). This is useful for heat loss in organisms e.g. sweating because the water carries away alot of heat energy when it evaporates.
  • SPECIFIC HEAT- This means it requires a lot of heat to change the temperature of water by 1 degree celcius. This is useful in keeping environments stable for aquatic organisms, and also for enzymes to work well.
  • Cohesive properties- Hydrogen bonds meean water molecules stick together; useful in the xylem vessels in trees.
  • Surface tension- The surafce of water is very strong( H-bonds) useful for pond skaters.
  • Ice Density- Ice is less dense that water, so it floats. This means water beneath it remains liquid, so aquatic organisms can survive. Water is most dense at 4 degrees celcius.
  • Transparent- Water is see through, so aquatic plants can still photosynthesise using light from the sun.
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Mineral Ions

Magnesium- key constituent in chlorophyll and in photosynthesis.

Calcium- Used in bones for strength and tenacity.

Phosphorus- Key part of ATP and Phospholipids

Iron- In haemoglobin haem groups to help oxygen bind and be transported around the body.

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