Which one of the following processes can occur in the absence of oxygen?
a. glycolysis
b. krebs cycle
c. the generation of carbon dioxide from pyruvic acid
d. the electron transport chain

Sugar + oxygen > carbon  dioxide + water + energy  c6h1206 + 6o2 > 6co2 +6h2o + energy  and the numbers after the element symbols are small.     

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.

The direction a chemical reaction occurs is represented by an arrow 

Streaks on the face of wave rock