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What are Biological Molecules?
The Key Biological Molecules
The key biological molecules are carbohydrates, lipids, proteins and nucleic acids. These molecules either become part of our bodies or are used to provide us with energy. We are - literally - what we eat.
Humans also require:
- Fibre - aids in the removal of indigestible material from the body (it eases the flow of materials through the gut by giving gut muscles a bulk to push against)
- Vitamins and Minerals - these form parts of some larger molecules and take part in some metabolic reactions.
- Water - the closest thing to a universal solvent! Most of our chemical reactions take place in water; water is also heavily involved in transport of other important molecules around the body.
The structure of the key biological molecules is closely related to their function within living organisms. Below the structure and function of each class of biomolecule will be examined more closely.
Carbohydrates are made of carbon hydrogen and oxygen atoms only. In food science they are often further classified into 'sugars' and 'starches'.
The most basic carbohydrates are monosaccharides such as glucose (C6H12O6), the main fuel for metabolism, and ribose (C5H10O5), which forms the backbone of nucleic acids (see below).
When two monosaccharides join together, a dissacharide is formed. The most recognisable of these is sucrose - table sugar, which is formed from fructose and glucose. When more than two monosaccharides join together, huge, complicated 'polysaccharides' such as starch are formed. These more molecules can be used as:
- Structural units, e.g. cellulose
- Energy storage, e.g. glycogen (animals) or starch (plants)
Proteins are large, complex molecules that play crucial roles in cells:
- Catalysts, e.g. Enzymes
- Transport, e.g. Haemoglobin
- Immunity, e.g. Antibodies
- Structural, e.g. Collagen
Proteins are made of long chains of repeating subunits called amino acids. Some of these amino acids cannot be made by the body and have to be eaten in our diet - so-called 'essential amino acids.' When two amino acids join together, a dipeptide is formed.
In plants and animals, all proteins are made from different sequences of 20 amino acids. The sequence of amino acids in the chain is the primary structure, the beta-pleats and alpha-helices that this chain folds into is called the secondary structure. These kinks, folds and spirals form the final 3-D structure: the tertiary structure.
These final 3-D shapes are closely related to the function the protein performs in the organism. Any slight change to primary structure can change the twists and folds which in turn changes the tertiary structure. This is why slight mutations to amino acid sequence can have huge implications on the health of the organism.
Lipids are a huge family of molecules that even include some vitamins (A, D, E and K) that repel water - they are 'hydrophobic' (water-hating) - and only dissolve in organic solvents such as acetone.
As lipids are so diverse, they have a wide range of functions in biology. Lipids:
- Store energy, e.g. fats.
- Are part of cell membranes, e.g. phospholipids.
- Act as Hormones, e.g. steroids.
The most common types of lipids are fats, steroids and phospholipids. Each type has a very distinct shape that helps it perform its' function.
Crash Course in Biological Molecules
Nucleic Acids are molecules that encode the information of life. They are made from chains of nucleotides joined together in a condensation reaction (water is released).They are broken down into two main categories: RNA and DNA. These are categorised according to the sugar that forms the backbone, either ribose or deoxyribose. DNA is usually double stranded, and RNA is usually single stranded.
The information in Nucleic acids is stored in organic bases called:
- Adenine (A)
- Thymine (T) - only found in DNA
- Guanine (G)
- Cytosine (C)
- Uracil (U) - only found in RNA
These follow strict bonding rules.
- In DNA, A bonds with T, and G bonds with C
- In RNA, A bonds with U, and G bonds with C.
DNA can be thought of as the master blueprint which must be kept securely locked away in the nucleus at all times. RNA is a shrunk-down photocopy of the instructions contained in DNA to make a single protein.