Like all fruits, the grape is really a 'package' which serves to protect the seeds (the pips) until they are fully developed.
The grape is composed of the pedicel or stalk (which connects it to the bunchstem and so to the shoot); the base of the stalk penetrates into the grape through the brush. The small swelling at the base of the grape is called the stylar remnant. On the outside is the skin (or epicarp) which is composed of an external layer, the cuticle, which is generally covered with a waxy substance (pruin) and an internal part comprising the epidermis and 6 to 10 layers of hypodermic cells. This is where many of the pigments and aromatic substances are found.
Stripping away the skin, we arrive to the pulp, also known as the sarcocarp, composed of the mesocarp, which is sugary and fleshy, and the endocarp, which is thin and soft and contains the pips. The pips are rich in anthocyans and flavones (important for the colour of the wine). They are also rich in tannins, but are not crushed because too much tannin would come out and so would an oily substance which is bad for the wine. The pulp contains the substances which will end up in the must (grape juice) and in the wine.
It is composed by 70-80% of water, sugars, acids, nitrogen compounds, vitamins, minerals and colorant substances. In the innermost part of the pulp, near the pips, there is less sugar and less acidity. In the middle part there is medium acidity and a lot of sugars and the outermost part has a medium sugar level and few acids.