The  goal of this biological process is to produce as much ATP (universal form of biological  energy, recognized   by  all life  forms)  from  an  easily transportable and  used  form  of  stored   chemical energy--GLUCOSE. Using  people as a frame of reference, various other forms of  available  energy (i.e.,  various types of food--complex carbohydrates, fats, proteins) are ultimately  digested, absorbed and  transported by the blood plasma. The process of releasing the energy stored in glucose  (686,000 calories/mole)  is  one of a step-wise, enzymatically controlled sequence  of oxidation/reduction  chemical reactions. Recall, any one of these steps may be considered a METABOLIC REACTION.

Cell  respiration is essentially a three part (stage) process. The end products of cell respiration  are:  CO₂ H₂O,  and energy in the form of ATP and body heat. However, various intermediate products are  formed as  part of each of the three stages. Each stage is well illustrated in the Chapter 8 of the text, pgs. 123-138. I will present the important part of each stage--you should read the text carefully and study the illustrations and captions.

GLYCOLYSIS:

The first stage respiration within any type of organism begins with Glycolysis. This is the beginning point of oxidation of glucose and the release of the energy bound within this simple sugar molecule.  Glycolysis takes  place  in the CYTOPLASM OF THE CELL, and requires an input of some energy (just  to  get the process  started) in the form of ATP. Examine Fig. 8.3 and 8.4 and you will note that the reaction with  glucose begins  with using two (2) molecules (moles) of ATP. Following this, various Rx's take place,  ultimately providing  the end product of glycolysis--PYRUVIC ACID. As you will observe in Fig. 8.4, (jump to image page.) as Pyruvic Acid  is being produced, ATP is also produced. In fact, four (4) molecules (moles) of ATP  are  produced, resulting  in a net gain of 2 ATP's for each molecule(mole) of glucose. In addition, note that an  electron (hydrogen) carrier  molecule,  NADH  is  produced  from NAD⁺ by the oxidation  (electron  donation) process.  This electron (Hydrogen) carrier molecule will be of importance later when we discuss  the  last stage  of  cell respiration--the ELECTRON TRANSPORT SYSTEM, so just remember that two  (2) units (moles) of NADH are produced during Glycolysis.

Take place in the cell CYTOPLASM. Start oxidation process of glucose with input (addition)  of  two  ATP's. End products of glycolysis = 2 Pyruvic Acids, 4 ATP's (for a net  gain  of  2 ATP's) and 2 NADH's.

In  the  absence  of  oxygen,  an  organism  respiring  (oxidizing)  glucose  will do so  as   ANAEROBIC RESPIRATION  (FERMENTATION). Two types of fermentation are referenced in your text  (pg.  121-122):

Alcoholic  Fermentation--occurs  in the cytoplasm of single celled organisms such as  certain  bacteria  and yeast.  The  end product of Glycolysis--Pyruvic Acid--is further oxidized to Ethyl Alcohol. Note  in  Fig. 8.9 & 8.10 (jump to image page.) that the NADH looses and electron (hydrogen) and produces NAD⁺. The only net gain in terms  of ATP energy is the two (2) produced in Glycolysis.

Lactic  Acid Fermentation--occurs in muscle tissue (like in people) when under extreme oxygen debt,  i.e., heavy  exercise.  The  muscle tissue reverts to anaerobic respiration in the  absence  of  sufficient  oxygen supply,  with the end product of glycolysis (Pyruvic Acid) being converted to lactic acid. Again,  the only net gain in terms of ATP energy are the two (2) units produced in Glycolysis.

AEROBIC RESPIRATION--occurs in many organisms in the presence of oxygen. In this process, the end product  of  Glycolysis  (Pyruvic  Acid)  is further oxidized  to  carbon  dioxide  and  water.  As  this  is accomplished, significant amounts of ATP are produced.

The  second  stage  of cell respiration--The Kreb's;  it  is  also know  and the TCA cycle)--takes place within the Mitochondrion of the cell (see Fig. 8.5). (jump to image page.)This process is  illustrated in Fig. 8.6. (jump to image page.)Essentially, it can be viewed as a cyclic sequence of Rx's where Pyruvic  Acid is further oxidized (burned) to produce Carbon Dioxide (CO²). The more important part of this process  is the production of electron (hydrogen) carrier molecules—FADH₂ and NADH. In addition, a small amount of ATP is directly produced in this process.

The importance of the two types of electron (hydrogen) carrier molecules is evidenced in the third stage of cell  respiration--the  ELECTRON TRANSPORT SYSTEM (Electron Transport Phosphorylation = your  text).  This system  is represented  as a sequence of interlocking molecules (see Fig. 8.7 & 8.8) (jump to image page.) which have the  ability  to transfer electrons along a specific pathway (one direction). As electrons (hydrogen) are passed through  the ETS,  there are specific points where the drop in electron potential (energy) is sufficient to  produce  ATP. Note that NADH enters the ETS at the first step, and thus will generate three (3) ATP's for each  NADH. FADH₂  on  the other hand, enters the sequence one step later, and thus only produces two (2)  ATP's  for each FADH₂_. The end point of this electron (hydrogen) transfer is the production water. This happens  as hydrogen are transferred to the end of the ETS, oxygen accepts these hydrogen (electrons). Therefore, we state  that  oxygen  is  the  final  electron acceptor in the  cell  respiration  (biological  oxidation)  process involving glucose.

For each molecule of glucose completely oxidized in a cell respiration process, 36-38 molecules (moles) of ATP are produced. In that 2 ATP's are used to get the Pyruvic Acid across the mitochondrial  membrane, we generally use the number 36 ATP's per glucose molecule. These 36 ATP's represent 266,000 calories of   free   energy--potential   energy   available  to  do  work  in  the  cell. Recalling   the   1st   Law   of Thermodynamics,  the  remainder of the energy once contained in the glucose molecule  is  represented  as body heat.

In terms of efficiency of types of respiration, anaerobic respiration (fermentation) is extremely  inefficient. It produces only a net gain of 2 ATP's per glucose, and the total amount of energy available to do work  is only  14,000  calories,  about  a 2 % efficiency. In contrast,  aerobic  respiration, a  complete  biological oxidation of glucose is about 40 % efficient (266,000/686,000 calories)

Figure 8.8   (jump to image page.)summarizes the production of ATP and two types of electron carriers during  cell  respiration. You need to study this carefully.