9.12.1 Chemical Reactions
Chemical compounds undergo two types of changes.
- A physical change simply refers to a change in shape without breaking of bonds. This is a physical process.
- Another physical process is a change in state of matter: when ice melts into water, or when water becomes a vapour. These are physical processes.
However, when bonds are broken and new bonds are formed during transformation, this will be called a chemical reaction.
For example: Ba(OH)2 + H2SO4 → BaSO4 + 2H2O is an inorganic chemical reaction.
Similarly, hydrolysis of starch into glucose is an organic chemical reaction. Rate of a physical or chemical process refers to the amount of product formed per unit time.
It can be expressed as: rate = δ δ/ δ t
Rate can also be called velocity if the direction is specified. Rates of physical and chemical processes are influenced by temperature among other factors. A general rule of thumb is that rate doubles or decreases by half for every 10°C change in either direction. Catalysed reactions proceed at rates vastly higher than that of uncatalysed ones.
When enzyme catalysed reactions are observed, the rate would be vastly higher than the same but uncatalysed reaction.
For example: CO2 + H2O ⎯⎯⎯⎯⎯⎯⎯ (Carbonic anhydrase) ⎯⎯⎯> H2CO3 (carbonic acid)
In the absence of any enzyme this reaction is very slow, with about 200 molecules of H2CO3 being formed in an hour. However, by using the enzyme present within the cytoplasm called carbonic anhydrase, the reaction speeds dramatically with about 600,000 molecules being formed every second.
The enzyme has accelerated the reaction rate by about 10 million times. The power of enzymes is incredible indeed! There are thousands of types of enzymes each catalysing a unique chemical or metabolic reaction.
A multistep chemical reaction, when each of the steps is catalysed by the same enzyme complex or different enzymes, is called a metabolic pathway.
For example, Glucose → 2 Pyruvic acid
C6H12O6 + O2 → 2C3H4 O3 + 2H2O is actually a metabolic pathway in which glucose becomes pyruvic acid through ten different enzyme catalysed metabolic reactions. At this stage you should know that this very metabolic pathway with one or two additional reactions gives rise to a variety of metabolic end products.
In our skeletal muscle, under anaerobic conditions, lactic acid is formed. Under normal aerobic conditions, pyruvic acid is formed. In yeast, during fermentation, the same pathway leads to the production of ethanol (alcohol). Hence, in different conditions different products are possible.