There are many pathways of metabolism involved in the catabolic (break down molecules to produce energy) and anabolic (biosynthesis) processes. What lies in the central hub of the metabolic network is glucose metabolism. A cell can take up glucose from the environment by glucose transporters located at its plasma membrane. After entering a cell, glucose is processed through a series of biochemical reactions in the glycolysis pathway, and becomes pyruvate. Pyruvate has two different branch pathways to go: it can be further converted to lactate by lactate dehydrogenase and secreted out of the cell, or it can be imported into mitochondria for further oxidation in the TCA cycle. ATP is produced from both glycolysis and mitochondrial oxidative phosphorylation (OXPHOS) apparatus, with glycolysis producing less ATP per molecule of glucose but in a higher turnover rate.
Not only energy comes from these pathways, biosynthesis "building blocks" are also provided during these processes. For example, ribose and NADPH from a detour of glycolysis-pentose phosphate shunt (PPS) - is required for nucleotide biosynthesis and lipid biosynthesis, and many TCA cycle intermediates are indispensable for non-essential amino acid biosynthesis.