Biology Unit 1 - Biological Molecules

Water is a SOLVENT which means some substances dissolve in it. 

Water transports substances. Substances can be transported more easily is they're dissolved in a solvent. So because water is a liquid and a solvent it means it can easily transport all sorts of materials, like glucose and oxygen, around plants and animals. 

Water's Dipole Nature Makes it Very Cohesive

Cohesion is the attraction between molecules of the same type (e.g two water molecules).

Water molecules are VERY COHESIVE (they tend to stick together) because they're dipolar.  

This helps water flow, making it great for transporting substances. 

Water's Dipole Nature Also Makes it a Good Solvent 

A lot of important substances in biological reactions are ionic (like salt). This means they are made from one positively charged atom/molecule and one negatively charged atom/molecule. 

Because water is dipolar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to the positive ion. This means the ions will get totally surrounded and dissolve. 

Cells get energy from glucose. Plants store excess glucose as starch (when a plant needs more glucose for energy is breaks down starch to release the glucose) 

Starch is a mixture of two polysaccharides of alpha-glucose - amylose and amylopectin.

Amylose - a long, unbranched chain of glucose joined together with 1-4 glycosidic bonds. The angles of the glycosidic bonds give it a coiled structure. The makes it compact, so it's great for storage. 

Amylopectin - a long, branched chain of glucose that contains 1-4 and 1-6 glycosidic bonds. Its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. Glucose can therefore be released quickly. 

Starch is insoluble in water, so it doesn't cause water to enter cells by osmosis. This makes it god for storage. 

The main energy storage material in animals. 

Animals get energy from glucose too. But animals store excess glucose as glycogen. 

Its structure is similar to amylopectin because it has 1-4 glycosidic bonds. Except it has loads more side branches. Loads of side branches means that stored glucose can be release quickly, which is important for energy release in animals. 

It's also a very compact molecule, so good for storage. 

Like starch, glycogen's insoluble in water, so it doesn't cause cells to swell by osmosis. 

It's a large molecule, so can store lots of energy.