1. Cellular Respiration: How do your cells get energy?
Through the process of cellular respiration, the energy in food is changed into energy that can be used by the body's cells. In other words, glucose (and oxygen) is converted into ATP (and carbon dioxide and water).
ATP is the molecule that provides energy for your cells to perform work, such as contracting your muscles as you walk down the street or performing active transport.
Cellular respiration is simply a process that changes one type of chemical energy, the energy stored in sugar, into another type, ATP.
How does the food you eat provide energy? When you need a quick boost of energy, you might reach for an apple or a candy bar. Although foods with sugars can give you a quick boost of energy, they cannot be used for energy directly by your cells. Energy is simply stored in these foods.

2. Overview of Cellular Respiration
Cellular respiration can be divided into three phases.
Glycolysis: the breakdown of glucose.
The Kreb Cycle (citric acid cycle): the formation of electron carriers.
The electron transport chain: use of oxygen to make ATP.
In eukaryotic cells, the first phase takes place in the cytoplasm of the cell, while the other phases are carried out in the mitochondria. This organelle is known as the “powerhouse” of the cell because this is the organelle where the ATP that powers the cell is produced.
Most often, cellular respiration proceeds by breaking down glucose into carbon dioxide and water. As this breakdown of glucose occurs, energy is released. The process of cellular respiration includes the conversion of this stored energy into ATP. The overall reaction for cellular respiration is as follows: C6H12O6 + 6O2 → 6CO2 + 6H2O.
Notice that the equation for cellular respiration is the direct opposite of photosynthesis. While water was broken down to free hydrogen and oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water. While photosynthesis requires carbon dioxide and releases oxygen, cellular respiration requires oxygen and releases carbon dioxide. This cycle of carbon dioxide and oxygen in all the organisms that use photosynthesis and/or cellular respiration worldwide, helps to balance atmospheric oxygen and carbon dioxide.
Cellular respiration doesn’t happen all at once. Glucose is broken down slowly so that cells convert as much sugar as possible into the usable form of energy, ATP. Still, some energy is lost in the process in the form of heat. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. Although the process is not 100% efficient, it is much more efficient than, for instance, a car engine obtaining energy from gasoline.

4. Glycolysis
The first step of cellular respiration is glycolysis. During glycolysis, glucose is converted into pyruvate and energy.
After glycolysis, the pyruvate can go down several different paths. If there is oxygen available, the pyruvate moves inside the mitochondrion to produce more ATP during further break-down stages. In the absence of oxygen, the fermentation process begins.

5. The Krebs Cycle
If oxygen is available, the next step of cellular respiration is moving the pyruvate into the mitochondria. The mitochondria’s unique shape makes it possible to carry out the Krebs Cycle.
The Kreb cycle starts with pyruvic acid (pyruvate) and produces energy. What are the final products of the Kreb cycle?
Within the mitochondria the Kreb’s Cycle or citric acid cycle occurs. The citric acid cycle is a series of steps that produce Carbon dioxide, ATP, NADH and FADH2

6. The Electron Transport Chain
In the final steps of cellular respiration, the electron transport chain converts NADH and FADH2 into ATP. During this energy conversion oxygen, which is necessary, bonds with hydrogens and becomes water. That is the key reason why this process only occurs in the presence of oxygen. This is known as aerobic respiration
The Electron Transport Chain takes place in the Mitochondria and changes NADH and FADH in to ATP. What is the final product of the ETC?
However, there is not always enough oxygen present for aerobic respiration to occur. In this case, the next step after glycolysis will be fermentation instead of the citric acid cycle.

7. Anaerobic Respiration: Fermentation
In the process of fermentation, the NAD+ is recycled so that is can be reused in the glycolysis process. No additional ATP is produced during fermentation, so the organism only obtains the two net ATP molecules per glucose from glycolysis.
Sometimes cellular respiration is anaerobic, occurring in the absence of oxygen. Yeasts (single-celled eukaryotic organisms) carry on alcoholic fermentation in the absence of oxygen, making ethyl alcohol (drinking alcohol) and carbon dioxide. Alcoholic fermentation is central to bread baking. The carbon dioxide bubbles allow the bread to rise, and the alcohol evaporates. In winemaking, the sugars of grapes are fermented to produce the wine.
Animals and some bacteria and fungi carry out lactic acid fermentation. Lactate (lactic acid) is a waste product of this process. Our muscles undergo lactic acid fermentation during heavy exercise, when oxygen cannot be delivered to the muscles quickly enough. The buildup of lactate is what makes your muscles sore after vigorous exercise. Bacteria that produce lactate are used to make cheese and yogurt. Tooth decay is also accelerated by lactate from the bacteria that use the sugars in your mouth. In all these types of fermentation, the goal is the same: to recycle NAD+ for glycolysis.
Products of fermentation include cheese (lactic acid fermentation) and wine (alcoholic fermentation).