- Proteins- they play a central role in the structure and metabolism of all living organisms.
- Protein molecules are relatively large compared to the structure of carbohydrates and lipids.
- The structure of proteins allows them to be very versatile and functional.
- Proteins contain Carbon, Hydrogen, Oxygen and Nitrogen and sometimes Sulphur.
Examples of proteins
- Keratin- gives strength to your skin, hair, claws,hooves etc.
- Antibodies- play a central role in defence against disease.
- Hormones- insulin and protein are proteins.
- Acton and Myosin- are found in the fibres of the muscles.
- Enzymes- perhaps the most important as it takes part the metabolic activity, and controls it.
Classes of proteins
- Fibrous- have a structural function (hairs and tendons), such as collagen and keratin. They are long and thin molecules in shape, and have no complex folding. Fibrous proteins are insoluble to water and have resistance to change by pH and temperature.
- Globular- plays role in the cell chemistry and metabolic function. Some examples are hormones and enzymes (insulin). They are spherical and globular in shape and have complex folding. Globular proteins are soluble in water and are easily damaged by changes in pH and temperature.
The general formula of the amino acids
An amino acid consists of a central carbon atom attached to a carboxyl group (–COOH), an amino group (–NH2), a hydrogen atom, and a side group (–R), giving the general formula R-CH-NH2-COOH. Only the side group differs from one amino acid to another.
Amino acids and bonds
- Amino acids are the building blocks of proteins.
- Amino acids are linked linearly trough peptide bonds.
- These bonds are formed via a dehydratation synthesis reaction the carboxyl group of the first amino acid with the amino group of the second amino acid.
- On forming the peptide bond, water is released.
Complexity in proteins- primary structure
Amino acids in a simple chain formation form a primary structure.
Complexity in proteins- secondary structure
- The most common secondary structures are alpha helices and beta sheets.
- The straight chain amino acids will be able to twist into helical shape, which will be held in place by hydrogen bonds. These structures are called alpha helices.
- The amino acid chain can form an even more compact shape, where the chains are folded into a sheet called a beta-plated sheet and is held in place by hydrogen bonds.
Complexity in proteins- tertiary structure
- The tertiary structure of a protein or any other macromolecule is its three-dimensional structure. This is produced because of the sequence of amino acids, which produces alpha helices, beta plated sheets and bends at particular places along the chain.
- Hydrophobic nature- globular proteins are surrounded by water, so the hydrophobic chains tend to point to the outside of the structure.
Complexity in proteins- quaternary structure
Quaternary structure only present if there is more than polypeptide chain. Nonocovalent forces hold this structure together.
- Lipids are a mixed group of hydrophobic compounds composed of the carbon, hydrogen and oxygen. They share the one physical property of not mixing with water.
- Lipids can be complex and simple. Complex lipids are triglycerides, phospholipids and waxes. Simple lipids are steroids and terpenes.
Triglycerides are commonly called fats or oils. They are formed from a single molecule of glycerol, combined with three fatty acids on each of the OH groups in a condensation reaction.
Triglycerides- properties and formation
Triglycerides are insoluble in water and are used for storage and insulation. In addition, they are used for protection in fatty and adipose tissues.
Saturated fatty acids
Saturated fatty acids have no double carbon bonds. These acids are form straight chains and have a high melting point.
Unsaturated fatty acids
Unsaturated fatty acids have double carbon bonds and forms bent chains. They have a low melting point.
Glycerol C3H5 (OH) 3 is a small three-carbon molecule with three alcohol groups. The glycerol substructure is a central component of many lipids.
Properties/roles of saturated triglycerides
Triglycerides containing saturated fatty acids have a high melting point and tend to be found in warm-blooded animals. At room temperature, they are solids e.g. - butter, lard.
Properties/roles of unsaturated triglycerides
Triglycerides containing unsaturated fatty acids have a low melting point and tend to be found in cold-blooded animals and plants. At room temperature, they are liquids. E.g. fish oil, vegetable oils.
Phospholipids are the main components of the cell membrane. They have a similar structure to triglycerides. Phospholipids have a polar hydrophilic ‘head’, and two non-polar hydrophilic ‘tails’.
A disaccharide is the carbohydrate formed when two monosaccharides undergo a condensation reaction. Like monosaccharide’s, they also dissolve in water, taste sweet, and are called sugars.
Common disaccharide- maltose
Maltose (malt sugar) is glucose 1-4 glucose. It is formed on digestion of starch by amilose, because this enzyme breaks down starch into two glucose units.
Common disaccharide- sucrose
Sucrose (cane sugar) is glucose 1-2 fructose. This disaccharide is common in plants because it is less reactive than glucose, and it is their main transport sugar.
Common disaccharide- lactose
Lactose (milk sugar) is galactose 1-4 glucose. It is found only in mammalian milk (it gives energy for infants).
Polysaccharides are long chains of many monosaccharides joined together by glycosidic bonds.
Three important polysaccharides
Starch- the formation of starch is the way that plants store glucose. Glycogen- found in animals. It is stored in liver and muscles. Cellulose- the structural component of plants are formed primary from cellulose.
Other glycosidic bonds
Other glycosidic linkages are possible and forms different shapes and branched chains (condensation and hydrolysis reactions).
Condensation reaction is this kind of reaction, where water is formed.
The reverse process when bonds are broken by the addition of water (e.g. in digestion), is called a hydrolysis reaction.
Waxes are formed from fatty acids and long chain alcohols. They are commonly found wherever waterproofing is needed (e.g. in leaf cuticles, insect exoskeletons, birds feathers and mammals).
Steroids are small hydrophobic molecules found mainly in animals. They include cholesterol- that is found in animals cell membranes to increase stiffness; bile salts, which help to emulsify dietary fats; steroid hormones such as testosterone, oestrogen; progesterone and cortisol; vitamin D, which aids Ca2+ uptake by bonds.
Terpenes are widespread in nature, mainly plants. Many terpenes are hydrophobic. They include vitamin A, carotene and plant oil such as camphor and menthol.
Glucose (C6 H12 O6) is a monosaccharide and forms six-sided ring. The six carbon atoms are numbered, so we can refer to individual carbon atoms in it. In the animals, glucose is the main blood sugar. There are many isomers of glucose with the same chemical formula but different structural formulae (includes fructose and galactose).
Five carbon sugars
Common five (C5 H10 O5)- carbon, or pentose sugars include ribose and deoxyribose (found in nucleic acids and ATP), ribulose (which occurs in photosynthesis).
Carbohydrates contain only the elements carbon, hydrogen and oxygen. They can be monosaccharides- monomers (glucose, fructose and ribose), disaccharides- dimmers (sucrose, maltose, lactose) and polysaccharides- polymers (starch, cellulose and glycogen).
Classification of the carbohydrates
Carbohydrates can be classified as simple sugars (formed from one or two sugar units, sweet and soluble in water) and complex sugars (formed from long chains of monomers, not sweet, insoluble in water, chains are held together by weak hydrogen bonds).
Role and structure of starch
Starch is the plant storage polysaccharide. It is insoluble in water and forms starch granules inside many plant sells. Being insoluble in water means that starch does not change the water potential of cells so does not cause the cells to take up water by osmosis. It is not a pure substance, but a mixture of amylase and amylopectin.
Structure and role of cellulose
Cellulose is only found in plants where it is the main component of sell walls. It is poly 1-4 glucose but with different isomer of glucose. Cellulose contains beta glucose, in which the hydroxyl group on carbon 1 sticks up. This means that in a chain alternate glucose molecules ale inverted.
The beta 1-4 glucose polymer in cellulose forms straight chains. Hundreds of these chains are linked together by hydrogen bonds to form cellulose microfibrils. These microfibrils are very strong and rigid. They give strength to the plant cells, and therefore to young plants and also to amterials such as paper, cotton and sellotape.