Your friend is having difficulty keeping track of the energy flow from glucose through glycolysis, the krebs cycle and electron transport. your best advice would be to

Split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). Recall that glycolysis produces two molecules of pyruvate (pyruvic acid). These molecules enter the matrix of a mitochondrion, where they start the Krebs cycle. Electron transport is the final stage of aerobic respiration. In this stage, energy from NADH and FADH2, which result from the Krebs cycle, is transferred to ATP. 

Interacting group of various species in a common location. for example, a forest of trees and undergrowth plants, inhabited by animals and rooted in soil containing bacteria and fungi, constitutes a biological community.