Can someone explain the roles of the following in aerobic respiration; NAD, oxygen, FAD.?

Answer 1

Glycolysis is the first step in cellular respiration and all cells regardless of the type of cellular respiration they do are able to carry out glycolysis. Because of this we believe that glycolysis probably arose very early in the evolution of life on the planet. In glycolysis glucose is partially oxidized and broken down into two 3 carbon molecules called pyruvate or pyruvic acid. In the process, glycolysis produced 4 ATP for a net gain of two ATP and two molecules of NADH. Each NADH is carrying two energy rich electrons away from the glucose and these electrons can be used by the cell to do work.

After glycolysis the pyruvate is processed to harvest 2 more NADH molecules and remove one carbon per pyruvate. The carbon and two oxygens is removed since it no longer has any useful energy. So it is waste. This little step is the source of some of the carbon dioxide we produce.

Note that glycolysis itself is anaerobic, in that oxygen is not required.

The remaining two carbons from the pyruvate feed into a complicated set of reactions called the Kreb's cycle. The Kreb's cycle produces 8 more NADH molecules and two molecules of FADH2. Again both of these are carrying energy rich electrons.

Most of the NADH and FADH2 travel to special membranes in the cell which have a series of molecules called the electron transport system that harvest the energy rich electrons from the NADH and FADH2 and use that energy to male lots of ATP by a process called electron transport phosphosphorylation. If we are dealing with aerobic respiration this is where the oxygen becomes important.

As the energy rich electrons from food are used to make ATP by electron transport phosphorylation they loose energy and once they are no longer useful they have to be removed. Oxygen is a great electron acceptor and so the electrons are combined with hydrogen ions and oxygen to make water. This prevents electrons from building up in the electron transport system.

Look also here:
http://www.purchon.com/biology/aerobic.htm

Answer 2

They are high energy electron carriers that transport electrons from the enzymatic reactions in glycolysis and the Kreb's cycle to the electron transport chain.