Organic compounds, containing carbon, oxygen and hydrogen.
Made up of a chain of individual molecules- monomer.
Polymers are longer chains of repeating monomer units.
In carbohydrates, the basic monomer unit is called a monosaccharide. Two monosaccharides combine to form a disaccharide. Many monosaccharide units combine to form a polysaccharide
Relatively small organic molecules with the general formula (CH20)n and their name is determined by the number of carbon atoms in the molecule (n).
- A triose sugar has 3 carbon atoms
- A pentose sugar has 5 carbon atoms
- Glucose is a hexose sugar.
All sugars shre the formula C6H12O6, but they differ in their structure.
Monosaccharides usually exist as ring structures when dissolved in water.
Glucose exists as two isomers, the a- form and the B-form.
Consist of 2 monosaccharide units linked together with the formation of a glycosidc bond and the elimination of water (condensation reaction).
Disacchrides can be formed by the joining of two similar monosaccharides or by the joining of 2 different monosaccharides.
- Glucose joined to glucose forms maltose
- Glucose joined to fructose forms sucrose
- glucose joined to galactose forms lactose
Large, complex molecules called polymers. They are formed from very large numbers of monosaccharide units linked together by glycosidic bonds.
Starch is a storage polysaccharide in plants. Starch is made up of many a-glucose molecules held together. It is compact and can be stored in a small space; is insoluble and doesn't draw water towards it by osmosis.
Starch is made up of 2 polymers, amylose and amylopectin. Amylose is linear and coils into a helix, whereas amylopectin is branched and fits inside the amylose.
Glycogen is the main storage product in animals. It is very similar to amylopectin. Both starch and glycogen are readily hydrolysed to alpha-glucose, which is soluble and can then be transported to areas where energy is needed.
Cellulose is a structural polysaccharide and is a major component of plant cell walls. It consists of many long parallel chains of B-glucose cross-linked via hydrogen bonds, each molecule is rotated 180'. These chains are grouped together into microfibrils. The large number of hydrogen bonds present contribute to the strenght and rigidity of plant cell walls.
Chitin is the exoskeleton of insects. Has amino acids and is strong, waterproof and lightweight.
Contain carbon, hydrogen and oxygen, but less oxygen. They are polar compounds and so are insoluble in water.
Triglycerides are formed by condensation reactions between glycerol and 3 fatty acid molecules. The glycerol molecule is always the same, but the fatty acids vary. In the reaction, water is removed and an ester bond is formed.
There are two main kinds of fatty acids:
- Saturated: carbon atoms are joined via a single bond
- Un-saturated: contain one or more double bonds and therefore have fewer hydrogen atoms.
A high intake of fat is a contributory factor in heart disease.
Chemical properties and Functions
They are insoluble in water but dissolve in organic solvents such as acetone.
Fats are solid at room temperature, whereas oils are liquids.
- Energy storage. Fats are an efficient energy store in plants and animals. 1g of fat, when oxidiseed yields approximately twice as much energy as 1g of carbohydrate
- Triglycerides produce a lot of metabolic water when oxidised, which is important in desert animals.
- Protection of delicate internal organs
- Insulation. Fats are poor conductors.
Important in the formation and functioning of membranes in cells. They are similar to triglycerides with one of the fatty acid groups replaced by a phosphate group.
The lipid part is non-polar and insoluble in water (hydrophobic)
The phosphate group is polar and dissolves in water (hydrophilic)
Phospholipids allow lipid-soluble substances to enter and leave a cell and prevent water-soluble substances entering and leaving the cell.
Contain carbon, oxygen, hydrogen and nitrogen.
Proteins are large compounds built up of sub-units called amino acids, there are about 20 different amino acids used to make up proteins.
All amino acids have the same basic structure in that each possesses an amino group (NH2), at one end of the molecule, and a carboxyl group (-COOH) at the other end. It is the R group which differs from one amino acid to another.
The peptide bond
Proteins are built up of a linear sequence of amino acids. The amino group of one amino acid reacts with the carboxyl group of another amino acid, with the elimination of water. The bond formed is called the peptide bond, and the resulting compound is a dipeptide. A number of amino acids joined in this way is called a polypeptide.
There are four levels of proteins.
- Primary strucure is the sequence of amino acids in a polypeptide chain
- Secondary structure is the shape the primary structure forms as a result of hydrogen bonding. a-helix
- Tertiary structure is the bending and twisting of the polypeptide helix into a compact structure. This gives the molecule its 3D shape. The shape is maintained by; disulphide, ionic and hydrogen bonds.
- Quaternary structure arises from a combination of two or more polypeptide chains in tertiary form. E.G. Haemoglobin
Classification of proteins
Proteins can be divided into 2 groups depending on their structure
- Fibrous proteins perform structural functions. They consist of polypeptides in parallel chains or sheets with numerous cross-linkages to form long fibres. They are insoluble in water, strong and tough. A single fibre consists of three polypeptide chains twisting around each other. They are linked via cross-bridges making a verys table molecule.
- Globular proteins carry out a number of functions- enzymes/anitbodies/plasma proteins/hormones. They are compact and are folded as spherical molecules. They are soluble in water. Haemoglobin consists of 4 folded polypeptide chains, at the centre is an iron-containing group called haem.
These play important roles in organisms. In plants, minerals are transported dissolved in water.
They can be divided into 2 groups
Macronutrients which are needed in small amounts, which include:
- Magnesium: a constituent of chlorophyll in leaves
- Iron: a constituent of haemoglobin in blood
- Phosphate: found in the plasma membrane, nucleic acids, ATP
- Calcium: a constituent of bones and teeth.
Micronutrients which are needed in trace amounts. E.g. copper/ zinc
Water is transparent, allowing light to pass through, enabling aquatic plants to phosynthesise effectively.
Water acts as a medium for metabolic reactions. Water makes up between 65% and 95% by mass of most plants and animals. It is an important constituent of cells. The hydrophobic property of lipids is important in cell membranes.
Cohesion and surface tension
Water is a polar molecule and has no overall charge. When two water molecules are in contact, their opposing charges attract each other, forming a hydrogen bond.
Individually, the hydrogen bonds are weak, but because there are so many of them, they stick together in a strong lattice framework. This sticking together of water molecules is called cohesion. This means that tall collums of water can be drawn up xylem vessels in tall trees.
At ordinary temperatures, water has the highest surface tension of any liquid except mercury. In a pond the cohesion between water molecules produces a surface tension so that the body of an insect is supported.
Water as a solvent
Because water is a polar molecule, it will attract other charged particles, such as ions and other polar molecules, such as glucose. This allows chemical reactions to take place in solution and since these chemicals dissolve in water, it acts as a transport medium. Non-polar molecules, such as lipids will not dissolve in water.
Water has a high specific heat. A large amount of energy is needed to raise the temperature of water. This is because the hydrogen bonds between the waer molecules restrict their movement. This prevents large fluctations in the temperature of water and this is particularly important in keeping the temperature of aquatic habitats stable so that organisms do not have to endure extremes of temperature. This also allows enzymes within the cells to work effectively.
Water has a high latent heat, i.e. a great deal of heat energy is needed to change it from a liquid to a vapour state. This is important, for example, in temperature control where heat is used for vaporisation of water when sweating. That is, the evaporation of water from a surface results in cooling.
Water has a maximum density at 4'C. Water in its solid form is less dense than water and so floats on the surface. Ice forms an insulating layer and allows organisms to survive beneath it.