Symmetry can be tricky, especially when you’re a chemist. Grab some plasticine and discover why a mirror doesn’t always make a perfect copy.
Replace a red ball on each model with a ball of another colour. Make sure they are still mirror images.
The 2 models might look the same, but they’re slightly different. Even though they are made of the same components, they are not identical. This property is called chirality, from the Greek word for ‘hand’. This is because hands show a similar property – they are mirror-images of each other, but, keeping the palms facing the same direction, they can’t be placed exactly over each other. To make these models, you need to have 4 different balls arranged around the centre ball. If any 2 balls around the centre are the same colour, the models are identical mirror-images.
In chemistry, some molecules come as chiral pairs that are mirror images of each other. As they have the same number and types of atoms, chiral molecules share most of the same chemical properties. However, they have some differences.
The geometry of a molecule is important. Receptors in the mouth and nose are themselves chiral. They can interact with some molecules, provided the shapes of the receptor and molecule can fit together. These interactions give rise to the sensations of taste and smell.
Think of receptors as being like your hands, and chiral molecules as gloves. You can fit your right hand into a both a right- and left-handed glove. The interactions between your hand (the receptor) and the different gloves (the molecules) produce different responses – each glove fits, but feels different.
The slight difference in the geometry of chiral molecules means they can taste or smell different from each other. For example, with amino acids (the building blocks of proteins), one form will tend to taste sweet, while its mirror-image may be bitter.
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