Carbohydrates

An energy source
The pentose phosphate pathway, or the hexose monophosphate (HMP) shunt, is a way to produce energy in the form of NADPH from the oxidation of six carbon sugars. It is an alternative to glycolysis, but its primary role is anabolic rather than catabolic: the transformation of sugars with the release of CO₂ produces the five carbon ribose, which can be used for DNA and RNA synthesis. Also, if there is a need for NADH, FAD or CoA, ribose can be converted to glyceraldehyde 3-phosphate used in glycolysis.

The PPP is found throughout the body, in fat and liver cells, which use NADPH for fatty acid synthesis and (in liver) for cholesterol synthesis. Red blood cells lacking mitochondria use this pathway as an important source of energy.

Carbohydrate storage / Glycogen breakdown
Steps of glycogen formation:

1. Glucose 
2. Glucose 6-P
3. Glucose 1-P
4. UDP-glucose (addition of ribose sugar UTP)
5. Amylose (unbranched)
6. Glycogen (branched)

Enzymes of glycogen breakdown:

1. Phosphorylase breaks apart glucose units, but cannot break 1-6 branching. Glucose units are phosphorylated to become glucose 1-P. 
2. Debranching enzyme breaks down branched residue, producing more glucose 1-P. 
3. Salivary and pancreatic amylase can break down starch directly to maltose, which can then split to glucose by maltase. 
4. Lysosomal alpha-glucosidase, found throughout the body, converts glycogen directly to glucose. 

Note that glycogen can only be broken down to glucose or glucose 1-P. Unlike glucose, glucose 1-P does not easily cross membranes, so it is preferred by cells in the muscle or liver.