Why are cells always in need of glucose and oxygen?
ATP is the source of energy for all cells. Cells need glucose for this energy. A good analogy would be burning coal (glucose) to generate electricity (ATP).
They first break it down into two molecules of pyruvate, using the energy to power phosphorylation and make ATP. (This takes place in the cytosol)
The pyruvate molecules go into the citric acid cycle, which I won't explain in depth, but it's used to make more ATP. (This and the next step take place in the mitochondria)
Both of these processes provide electrons which are used in oxidative phosphorylation, which also makes ATP.
ATP from all of these processes are used to power cellular functions like making proteins, allowing substances in and out of the cell through channels, etc. Cells need ATP as much as we need electricity to turn on the lights.
Oxygen. We use oxygen as an acceptor for electrons from hydrogen ions in the oxidative phosphorylation process. This process includes reduced molecules NADH and FADH2, both of which need to donate an electron to be useful. Oxygen is an acceptor for these molecules, accepting hydrogen ions to form water.
I won't go too much farther into detail, but this is a key component of creating a proton gradient between the cytosol and the mitochondrial matrix, which is important for the function of ATP synthase phosphorylating ADP to get ATP.