Cellular Respiration (Metaboism) for Nurses

This article about respiration is written for nurses who are in their year of Professional training. Understanding metabolism at a cellular level is very important because each cell is a unit of life and contributes to the health of the whole individual. For more details please see here.

The following topics are discussed in this lecture: Metabolism, Respiration, External Respiration, Adenisone diphosphate, adenosine triphosphate, creatine phosphate, adenosine monophosphate, phosphagen system, glycogen-lactic system, aerobic, anaerobic system.

Respiration: It is defined as the metabolic processes whereby living things obtain energy from organic molecules; processes that take place in the cells and tissues during which energy is released and carbon dioxide is produced and absorbed by the blood to be transported to the lungs.

In mammals respiration is a single complete act of breathing in and out; “eighteen respirations per minute”

Other definitions of Respiration:

1. the process where food is oxidized (burned) to release energy.
2. The interchange of gases between a cell and its environment, or between an animal and it’s environ ment (known as breathing).

Diagram A. External Respiration mainly takes place in the lungs.

The process often referred to as breathing. External respirations are of many types and differ with our health status. For more details please see here.

In some literature breathing is called respiration: In this situation, respiration is defined as the bodily process of inhalation and exhalation; the process of taking in oxygen from inhaled air and releasing carbon dioxide by exhalation.

As seen above, respiration is of two types. External Respiration has already been discussed and is found here.

In this article I am going to discuss Internal Respiration. Internal Respiration is the metabolic processes whereby living things obtain energy from organic molecules; processes that take place in the cells and tissues during which energy is released and carbon dioxide is produced and absorbed by the blood to be transported to the lungs. It is necessary for life.

Other terms for Internal Respiration: cellular respiration, respiration, metabolism and metabolic process.

Overall reaction for Respiration is: a six-Carbon sugar is converted to C02 (six molecules), Water, and Energy.

WHAT IS ATP? ATP is adenosine Triphosphate, it is energy which is needed by all our muscles. In fact, for all of our cells in our body — the source of energy which is available is ATP. The structure is as given above and it is the biochemical way biochemical way to store and use energy. The whole process is very complex but we as nurses need to understand the following:

Chemically, ATP is an adenine nucleotide bound to three phosphates. Review the diagrams to understand the structure and energy levels of AMP, ADP and ATP.

There is a lot of energy stored in the bond between the second and third phosphate groups that can be used to fuel chemical reactions.

Diagram B. Biochemical structure of (Adenosine Monophosphate) AMP. Image from Wikipedia). Chemical formula: C10H14N5O7P

When a cell needs energy, it breaks this bond to form adenosine diphosphate (ADP) and a free phosphate molecule.

In some instances, the second phosphate group can also be broken to form adenosine monophosphate (AMP).

When the cell has excess energy, it stores this energy by forming ATP from ADP and phosphate.

Diagram C. Biochemical structure of (Adenosine Diphosphate) DMP. Image from Wikipedia). Chemical formula: AMP =C10H14N5O7P, ADP=C10H15N5O10P2

ATP is required for all muscle contraction and cellular activity. The amount of ATP used up is directly proportional to the amount of work done. As ATP gets consumed it must be replaced in order for the muscle to keep moving.

Diagram D. Biochemical structure of (Adenosine Diphosphate) DMP. Image from Wikipedia). Chemical formula: AMP =C10H14N5O7P, ADP=C10H15N5O10P2, ATP=C10H16N5O13P3

Since ATP is an important source of energy for the body, it has three different pathways to create ATP. The three systems work together in phases. It is interesting to note that different forms of exercise use different systems, so a sprinter is getting ATP in a completely different way from a marathon runner!

Please not that energy is needed to add on phosphate and is released when the phosphate group is detached from adenosine.

AMP =C10H14N5O7P (one phosphate, monophosphate)

ADP=C10H15N5O10P2 (two phosphates, diphosphate)

ATP=C10H16N5O13P3 (three phosphates, triphosphate)

ATP comes from three different biochemical systems in the muscle, in this order:

1st. System. Phosphagen system: Used when one is sprinting. There is a certain amount of ATP floating around the cells for immediate use by the cells for all activities. The amount is very small and gets used up completely in about three seconds. For example, when an athlete is sprinting large amounts of energy are needed. In this situation the ATP is replenished from a high-energy phosphate compound called creatine phosphate. Cretine phosphate is available in larger amounts. The phosphate group is removed from creatine phosphate by an enzyme called creatine kinase, The enzyme creatine kinase quickly converts ADP to ATP. The muscles turn ATP to ADP. The phosphagen rapidly turns the ADP back into ATP. As the muscle continues to work, the creatine phosphate levels begin to decrease. Together, the ATP levels and creatine phosphate levels are called the phosphagen system. The phosphagen system can supply the energy needs of working muscle at a high rate, but only for 8 to 10 seconds.

2nd. System. The Glycogen-Lactic system. In this situation muscles must have big reserves of a complex a carbohydrate called glycogen which is manufactured from carbohydrates which we eat, in the liver. Glycogen consists of glucose molecules. In the cell, glycogen (stored form of sugar) is split into glucose. In anaerobic metabolism (without oxygen) to make ATP. When this happens, fairly large amounts of Lactic Acid are produced and stored in muscles. Athletes will often feel pain, the following day in the muscles which have been working anaerobically.

This system is slower than the phosphagen system because there are about 12 biochemical reactions which take place in order make ATP by this method. Consequently, it supplies ATP at a slower rate than the phosphagen system. This system can still act rapidly and produce enough ATP to last about 90 seconds. This system does not need oxygen, which is handy because it takes the heart and lungs some time to get more oxygen to the muscles. It is also handy because the rapidly contracting muscle squeezes off its own blood vessels, depriving itself of oxygen-rich blood.

3rd. System. The Aerobic system. This system takes about two minutes of exercise to become activated. In the presence of oxygen, glucose is completely broken down into carbon dioxide and water in a process called aerobic respiration. The glucose can come from three different places:

1. Stored glycogen supplies in the muscles
2. Stored glycogen is broken is broken into glucose, and is supplied to working muscle through the circulatory system (blood vessels and heart)
3. Consumption of glucose from food in the intestine, which gets to working muscle through the circulatory system.

Aerobic respiration can also use the body’s stores of fat and fatty acids from to produce ATP. In extreme cases (like starvation), proteins can also be broken down into simple amino acids and eventually used to make ATP. Aerobic respiration would use carbohydrates first, then fats and finally proteins, if necessary. Aerobic respiration takes even more chemical reactions to produce ATP than either of the above systems. Aerobic respiration produces ATP at the slowest rate of the three systems, but it can continue to supply ATP for several hours or longer, so long as the fuel supply lasts.

This concludes the lecture. Cellular respiration has implications when we are looking after diabetic patients. As you are aware, insulin is adjusted to help glucose metabolism in the diabetic patient. When the diabetic patient cannot use glucose / glycogen he/she ends up using the phosphagen system. In that system body fats and proteins are used for metabolism. The sweet smell which we get is due to the ketones / keto acids which are toxic to the patient. The accumulated lactic acid in these patients also alters the patinets blood gases.

If you are interested in the normal blood gases please see here.