Glucose in the cells:Glucose provides the cells with fuel. Glucose is obtained from the food we eat or from glycogen which is the form that glucose takes when it’s stored in the liver. If glucose or glycogen are not available, the cells look to stored fats and protein for fuel. But one glucose molecule can provide enough energy to make 36 ATP molecules. There are 4 steps involved:*glycolysis*the preparatory step*the citric acid cycle*the electron transport system

GLYCOLYSIS – this step occurs in the cell’s cytoplasm, not in the mitochondria. Glucose is a 6-carbon molecule C6H12O6

During glycolysis, glucose is broken down into 2 three-carbon pyruvate molecules. At the end of glycolysis, the net gain of energy is 2 molecules of ATP.

PREP STEP – this step prepares the molecules for the citric acid cycle. So, the pyruvate molecules enter the mitochondria. In this step, each pyruvate molecule forms a two-carbon acetyl group and a carbon dioxide waste molecule. Each acetyl group joins with another coenzyme to form acetyl CoA, which takes the acetyl group to the next step, which is the citric acid cycle.

CITRIC ACID CYCLE – this is also called the KREBS CYCLE. In this step, each acetyl group is broken apart to form carbon dioxide waste and energy in the form of hydrogen ions. At the end of the citric acid cycle, there is a gain of 4 ATPs. A series of eight sequential steps occur in the citric acid cycle. In the first step,

acetyl CoA from the Prep Step combines with a 4-carbon fragment to form CITRIC ACID, for which the cycle is named. Citric acid is then the substance (or starting molecule) for the remaining 7 reactions. Each reaction is run by a different enzyme. By the end of this cycle, glucose has been broken down to form carbon dioxide waster and high energy hydrogen ions and electrons. These hydrogen ions and electrons are carried by 3 molecules of NADH and a molecule of FADH2. These molecules are important in the final step to produce ATP.NADH = high energy molecule nicotinamide adenineFADH2 = high energy molecule flavin adenine

ELECTRON TRANSPORT SYSTEM – in this step, the NADH and FADH2 move to the inner membrane of the mitochondria and release their hydrogen ions and high-energy electrons to a carrier protein of the electron transport system. Energy is both lost and gained and finally the carrier protein takes the hydrogen ions from the inner membrane of the mitochondria to the outer membrane. Now there becomes a higher concentration of hydrogen ions in the outer membrane than in the inner membrane. The hydrogen ions try to diffuse back to the inside membrane of the mitochondria but the hydrogen ions can only diffuse through special channels which help produce ATP from ADP and inorganic phosphate (Pi).

The process of producing ATP from ADP and Pi by using energy from transferred electrons is called OXIDATIVE PHOSPHORYLATION. The molecules of NAD and FAD and ADP are low energy molecules because they lack hydrogen and electrons. However, they are recycled and used again to keep the process going. This process is efficient because NAD, FAD and ADP do not have to be made again; they just recycle.

The net result of electron transport and oxidative phosphorylation is about 34 ATPs.

So… what is CELLULAR RESPIRATION?It is metabolism that uses oxygen and produces carbon dioxide in order to make ATP. Cellular respiration occurs in the mitochondria. Remember that the first step (glycolysis) did not occur in the mitochondria and did not use oxygen. The remaining 3 steps of the cycle are considered to be cellular respiration. But without glycolysis, the other steps would not run or be efficient.

Once glycogen is depleted, the body looks to fats and proteins to provide energy. Here’s what happens… after we eat a meal, the body has plenty of glucose, lipids and amino acids to use for energy. The body

first uses available glucose. The excess energy is stored as glycogen in the liver and the rest is converted to fat and is stored in fat tissue. Between meals, the body then uses the stored glycogen, fats, and sometimes protein for energy.

Fats actually carry more energy than glucose. The body stores fats as triglycerides, which can be broken down into glycerol and fatty acids. Glycerol can be converted either into glucose or pyruvic acid, which will then become part of the citric acid cycle. Fatty acids can enter the citric acid cycle because they break down into acetyl groups (in the Prep Step). Because triglycerides have 16-18 carbon tails, they can make lots of ATP.

Proteins can produce as much energy as glycogen. Proteins are broken into amino acids. The carbon portion of the amino acids enters the citric acid cycle. Even though proteins can provide the body with an energy source, proteins are used mostly as enzymes and for structure. However, proteins can be used by the body for energy when the body is starving.

Finally we see that oxygen is necessary to keep the citric acid cycle and the electron

transport chain going. These systems are called aerobic metabolism because they use oxygen. There is one system in the body that does not use oxygen… that is glycolysis, which we mentioned earlier. Glycolysis is anaerobic respiration. Without oxygen, glucose is broken down into pyruvate, which is then converted into lactic acid. When lactic acid builds up, the body gets the burning sensation and cramping due to muscle fatigue. This happens because there is not enough oxygen in the body. This process makes only 2ATPs and it is not efficient.