There are three main groups of biological molecules: proteins, carbohydrates and lipids. Each of these molecules has different properties, making them suitable for different purposes within the body.
These are long chains made up of amino acids. All amino acids have a similar structure, with only the R group changing (it can be one of 20 chemical groups). The amino acids with a protein molecule are held together by covalent bonds, which are very strong. It is the order of amino acids in a protein which determine its function.
Protein structure can be organised into four levels:
Occasionally a protein will have a quaternary structure. This is when multiple proteins interact or when the protein has other (prosthetic) groups attached. An example of this is haemoglobin, with 4 protein chains together and 4 haem groups that contain iron.
There are three elements present in carbohydrates: carbon, hydrogen and oxygen. Sugars are simple carbohydrates, and are grouped into monosaccharides (single carbon rings) and disaccharides (2 monosaccharides that join by a condensation reaction - a water molecule is produced). Glucose and fructose are monosaccharides, maltose (2 glucose molecules) and sucrose (glucose + fructose) are disaccharides.
Starch and cellulose are polysaccharides that are found in plant cells. Both are insoluble molecules made up of α-glucose, but starch is a storage molecule and cellulose is structural. Starch molecules are branched, consisting of two polysaccharides: amylase and amylopectin. Cellulose forms microfibrils, which are long polysaccharide chains that are held together by hydrogen bonds.
These are fats and oils. Most lipids found in plants and animals are triglycerides, made up of one molecule of glycerol with three fatty acid chains. Fatty acids are long chain molecules with a carboxylic acid (-COOH) group at one end. If the chain has one or more double bonds between carbon atoms, it is unsaturated.
Phospholipids are a particular type of lipid present in cell membranes. In these molecules, one of the fatty acid chains is replaced with a phosphate group.
There are two types of microscope used to study cell structure: light microscopes and electron microscopes.
Light microscopes use a series of lenses to focus light on or through a specimen that has been stained. Not all organelles can be viewed with a light microscope - the nucleus, mitochondria, chloroplasts and vacuole can be viewed. The cell membrane and cell wall can also be seen. The best light microscopes can magnify objects up to 1,500 times.
Electron microscopes allow the study of smaller organelles that are found in the cytoplasm (such as ribosomes and the endoplasmic reticulum), as well as showing the larger organelles in much more detail. Electron microscopes can magnify an object up to 1,500,000 times
You should know what diagrams of both animal & plant cells look like, and the functions of key organelles:
The cell membrane is also known as the plasma membrane. It is a double layer (bilayer) of phospholipid molecules tightly packed together, with protein molecules embedded.
Molecules pass through the cell membrane in one of four ways: