Cellular Biology

Eukaryotic Cell Structure and Function



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Eukaryotic cells differ than prokaryotic cells in that the latter do not possess nucleus while eukaryotes do possess nucleus. The structure of eukaryotes is comprised of a membrane which surrounds the cell in addition to organelles suspended inside the membrane.

The organelles are suspended within a fluid that is called the cytoplasm. The eukaryotic cell has many organelles within its cytoplasm but we will discuss here the most important ones.

These organelles are the cellular membrane and the mitochondria and the nucleus. The cellular membrane is composed of phospholipid bilayer. This layer is not permeable to every compound but has a selective permeability for most compounds.

Water is an example of a compound that enters and exits the cell by a diffusional pathway. Diffusion of compounds into the cell depends on the osmotic concentration inside and outside the cell.

The diffusion of water outside the cell depends on the osmotic concentration in the extracellular space and in the blood plasma. When the osmotic concentration in the extracellular space is greater than that inside the cell as occurs after an infusion of a hypertonic saline solution, water begins to diffuse from the inner side of the cell to the extracellular space. Thus leading to its contracture.

If the osmotic concentration outside the cell is lower than that inside the cell, as occurs after an infusion of a hypotonic saline solution, water begins to flow inside the cell from the extracellular space. Thus leading to its inflammation and burst. Subsequently cell death occurs.

Therefore it is important when doing an infusion of a saline solution to a dehydrated patient to make sure that the solution is isotonic.

The passage across the cell membrane or lipid bilayer is not always possible by diffusion. In most cases there is active transport such as occurs with ions that maintain the cell electric potential.

For example, sodium ions are predominant ouside the cell while little is found inside the cell. This concentration gradient is made possible due to active transport across the cellular membrane of sodium ions using special pumps that use ATPase enzymes. This process requires energy which is made possible using ATP molecules.

The second important structure of eukaryotic cells and which I discuss here is the mitochondria or energy generating molecular apparatus. The mitochondria in eukaryotic cells is very important due to its role in generating energy for the cell and the body.

The energy currency of the cell is the ATP molecule. It is generated by a cellular process that is called glycolysis. This process does not require oxygen and does not produce much energy in the form of ATP molecules.

Two other processes which are called krebs cycle and oxidative phosphorylation are the major producers of energy in the cell. Oxidative phosphorylation occurs in the mitochondria. It is a chain of oxidation / reduction reactions that use Fe-S clusters in addition to cytochromes. Cyanide poisoning interferes with the function of one of the cytochromes in this pathway.

The third structure that I will discuss here is how the genetic code is processed to synthesize proteins. This process is done in the nucleus and in the ribosomes. Proteins are very important group of compounds in the body. Especially important are the group of enzymes which catalyze the metabolic pathways in the body.

In the nucleus there is the DNA template which carries the genetic material for protein synthesis. DNA template is composed of chains of chemically connected bases which are called DNA bases.

The translation of DNA structure into proteinis done using a code that is made of three DNA bases. These three DNA bases are called codon and code for one amino acid.

Transfer RNA transfers one amino acid at a time to the ribosome where the protein structure is synthesized. Cancer is manifested by abnormal protein synthesis due to the defective genetic code.

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