Yeast is a fungus and needs a supply of energy for its living and growth. Sugar supplies this energy. (Your body also gets much of its energy from sugar and other carbohydrates.). Yeast can use oxygen to release the energy from sugar (like you can) in the process called respiration. So, the more sugar there is, the more active the yeast will be and the faster its growth (up to a certain point-even yeast cannot grow in very strong sugar-such as honey).
Molecules are held together by either covalent bonding or ionic bonding. Several types of non-metal elements exist only as molecules in the environment. For example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements.
The atoms in a molecule are held together because they share or exchange electrons. Molecules are made from atoms of one or more elements. Some molecules are made of only one type of atom. For instance, two oxygen atoms bond together to form O 2, the part of the air that we need to breath to survive.