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Biological Molecules


Water is a solvent – means that some substances dissolve in it. Most biological reactions take place in solution, so water is very useful.

Water transports substances. Substances can be transported more easily if they are dissolved in a solvent. So the fact that water’s a liquid and a solvent means that it can easily transport all sorts of materials, like glucose and oxygen around plants and animals.

Water molecules structure in regards to properties and function:

·         A water molecule is one atom of oxygen joined to two atoms of hydrogen by shared electrons

·         Because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge.

·         The unshared negative electrons on the oxygen atom give it a slight negative charge.

·         This makes water a dipolar molecule – it has a negative charge on one side and a positive charge on the other

·         The negatively charged oxygen atoms of water attract the positively charged hydrogen atoms of other water molecules. This attraction is called hydrogen bonding.

Waters 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 are dipolar. This helps water to 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 (e.g. salt). This means they’re made from one positively charged atom or molecule and one negatively charged atom or molecule (e.g. salt is made from a positive sodium ion and a negative chloride ion)

·         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 completely surrounded by water molecules – they will dissolve.

·         So waters dipole nature makes it useful as a solvent for other polar molecules.


Carbohydrates are largecomplexmolecules made of longchains of monosaccharide’s (e.g. starch is a large carbohydrate composed of long chains of glucose)

Glucose is a monosaccharide with six carbon atoms in each molecule – there are two types alpha and beta

Glucose’s structure is related to its function as the main energy source in animals and plants. Its structure makes it soluble so it can be easilytransported, and its chemicalbonds contain lotsofenergy.

Monosaccharide’s join together to form disaccharides and polysaccharides

Monosaccharides are joined together by glycosidicbonds in a condensationreaction

A hydrogenatom on one monosaccharide bonds to a hydroxyl (OH) group on the other releasing a molecule of water

The reverse of this is a hydrolysisreaction – a molecule of waterreacts with the glycosidicbond, breakingit apart

When the twomonosaccharide’s join together, they form a disaccharide

Two a glucose molecules…


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