תרגום אישי לספר פיזיולוגיה של המאמץ

Chapter A: the cell and its purpose

The science of Physiology studies the manner in which the living organism functions (regarding natural systems), which is unique because it can be explained using the rules of other sciences: Physics, Mathematics and Chemistry.

In physiology there are several fields that possess three common properties:

1. A match between the cell's structure and its function.
2. The movement of matter.
3. The existence, control and direction of energy.

These fields include:

1. General physiology of the cell, which describes the most basic of occurrences that take place in the living creature: metabolism, diffusion, transportation by carriers, osmosis and electric conductivity.
2. Physiology of systems, which studies groups of cells or organs that play similar roles.
3. Comparative physiology, which compares the physiological responses of animals to those of humans.
4. Environmental physiology, which studies the interactions between an organism and the environment it has to adapt to, for example heat or cold.
5. Physiology of effort, which is the transformation of chemical energy into mechanical energy. The uniqueness of muscles is their ability to change very drastically in levels of metabolism compared to any other organ in the body. For example, working skeletal muscles can increase their oxygen intake up to 20 times more than in their relaxed state. However, such a function could cause problems for a working cell under different states of muscle metabolism. The amount of heat, carbon dioxide (CO2), and other byproducts created from the rapid rise in metabolism need to be reduced relative to the body's increased consumption of energy, fuel (fats/sugars), and oxygen.    

Even though the chemical structure and the membranes' qualities differ between organs, all membranes share common attributes. They consist mostly of lipids (fats), and proteins. The main fat in this structure is the phospholipid, a molecule that has a head made of a phosphate and a tail consisting of two fatty chains.

Illustration 2 is a simple display of the phospholipids. The ovals show the hydrophilic side (the part exposed to water); the tail coming out of the hydrophilic side depicts the hydrophobic side which does not come into contact with water. The phospholipid's head, the hydrophilic part, has an electric charge and the ability to dissolve in water. However, the tail of the phospholipid, the hydrophobic part, is located on the inside of the membrane because it cannot dissolve in water.

When the phospholipids are exposed to water a double layer is formed. On the outside is the hydrophilic part that comes into contact with water and will block liquid from entering the cell. Meanwhile, the hydrophobic part is facing inside the membrane in a manner that the two phospholipid tails avoid coming into contact with water.

In animals that are more evolved such as dogs and horses, the cells have developed and transmuted specifically to the organs' different roles. This is why there is no typical cell in the body. Nevertheless, all cells are wrapped by a membrane (the cell's coating). Within it there are a number of corpuscles also enclosed by membranes: the mitochondria, the Golgi apparatus, the endoplasmic reticulum, lysosomes, ribosomes and the nucleus.

The term cytoplasm refers to anything in the space between the cell's membrane and the nucleus envelope.

The cytoplasm contains mostly water, salts, dissolved gasses and essential nutrients. The cell's membrane consists of a semi-penetrable barrier which allows certain molecules to enter the cell and blocks other molecules from access. This property of semi-penetrability changes in response to fluctuations near the membrane, such as temperature and hormones. The membrane's main roles are to protect the cell from the entrance of foreign bodies and to maintain the cell's structure.

Round proteins of different kinds are located inside the membrane. Some are stationary and some are not, instead moving within the phospholipids located in the membrane. These proteins enable active transportation of a molecule outside the cell and release it into the cell through a point in the membrane.

The main roles of the proteins of the cell membrane are:

* Regulating the entry and exit of different matter in the cell.

* Using the receptors located on the membrane to receive signals from outside the cell (such as hormones), which can change the cell's function.

* Allowing the identification and differentiation of other cells in order to permit or prevent their entry into the cell.

* Providing a stable working environment for enzymes.

The cell nucleus is central to the cell and is unique. It holds the genetic information that is inside the DNA, (a molecule that contains the genetic code), which renews and reviews the cell's functionality.

 

The cell's nucleus is characterized by two membranes that create the nucleus envelope, which separates the content of the nucleus from the rest of the cell. Specific gateways in the double membrane allow the entry and exit of cell nuclei in order to renew different cell parts.

The cell's nucleus functions are:

* The storage of genes and chromosomes.

* Turning genes into chromosomes which is imperative for cell division.

* Transporting regulative components and gene products through the nucleus' membrane gateways.

* Transporting signals from the DNA to produce proteins via the messenger RNA (mRNA).

* The creation of ribosomes.

* Duplicating major genes from the existing DNA in the cell.

The mitochondria differ among different cells, however every mitochondria has a sausage-like shape. Its structure consists of an outer membrane and an inner membrane called crista. The crista has folds that increase its surface area. The mitochondria is the cell's power station, creating all the energy the cell needs. The chemical processes which take place in the mitochondria will be specified in the chapter that discusses metabolism. The mitochondria's outer layer holds enzymes that play a role in biological oxidation. This oxidation process creates molecules that are necessary for other processes that occur inside the crista. Inside the mitochondria are enzymes related to the lemon acid chain (or the carabas chain) and the breathing chain. The mitochondria also contains DNA, however, the genetic information held within the mitochondria is not enough to create all of its contents.

There are unique large structures in the cell's cytoplasm. These corpuscles are:

* The lysosome, which contains different enzymes that would digest other cells if they were not wrapped and sealed by the lysosome's membrane. The lysosome's main purpose is the devouring and digestion of foreign bodies such as bacteria and dead cells.

* The endoplasmic reticulum, which is comprised of a series of tubes and is located in the cell's cytoplasm. The walls of these tubes are made of a membrane. This is where the cell's detoxification process takes place, and where steroids are formed in cells specific to steroid production. In muscle cells the endoplasmic reticulum plays a major role in regulating the intensity of heart and skeletal muscle contraction by releasing calcium, which permits connections between muscles and proteins. The construction of proteins is done by the ribosomes located on the endoplasmic reticulum. These ribosomes transmute amino acids into proteins according to the code passed from the DNA to the mRNA.

* The golgi corpuscles are in charge of storing proteins for the cell. When proteins are made by the endoplasmic reticulum, they are immediately transferred into carriers that constitute most of the golgi corpuscles. Later on, the proteins are pushed outside the golgi corpuscles into the cytoplasm.