Endergonic reactions must have an input of energy in order to take place. In living systems exergonic reactions and endergonic reactions are often coupled. These endergonic reactions involve phosphorylation - the addition of a phosphate group. This is how sugars are formed during photosynthesis and glycolysis. ATP is recycled.
Enzymes lower the activation energy needed to start a reaction.
There are over 2,000 known enzymes. Each catalyzes a specific reaction. No cell contains all enzymes. Different types of cells manufacture different enzymes. This is the major factor in determining the biological activities and functions of that cell.