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Category: Medical

Topic: Cellular Physiology

Level: Paramedic

12 minute read

Cell Theory

Cell theory is the concept that cells are the smallest functioning part of a living thing and that the living thing is a combination of all of the properties of all the cells. There is no such thing as a "typical" cell, but all cells have many chemical reactions and physical features in common.

The physiology of cells and the chemical reactions that take place follow the laws of chemistry and physics.

Chemical elements found in the body are 99% made up from six elements:

  1. oxygen,
  2. carbon,
  3. hydrogen
  4. nitrogen,
  5. calcium, and
  6. phosphorus.

Only about 0.85% is made up of another five elements:

  1. potassium,
  2. sulfur,
  3. sodium,
  4. chlorine, and
  5. magnesium.

At the chemical level, the body uses carbohydrates, lipids (fats), and proteins to create the structure of each cell.

  • Common types of carbohydrates used by the body include monosaccharides, disaccharides, oligosaccharides, polysaccharides, starches, glycogen, cellulose, and fiber.
  • The lipids used by the body include true fats, triglycerides, phospholipids, and steroids.
  • The proteins used by the body include amino acids, peptide bonds, and polypeptides.

Nucleic acids: the DNA, RNA, and ATP within the cells of the body.

  • Deoxyribonucleic acid (DNA) is the double-helix-structured self-replicating material that carries all of the chromosomes or genetic information.
  • Ribonucleic acid (RNA) is the single strand of material that acts as a messenger carrying instructions from DNA to control the synthesis of proteins.
  • Adenosine triphosphate (ATP) is an organic compound that is the energy currency of life, found in every cell. Manufactured by mitochondria, each cell then stores and supplies it for energy. This energy is released when it is broken down to ADP by cell metabolism when needed.

In every cell, DNA acts like a recipe book full of instructions for making proteins. These proteins are the workers that do most of the tasks in our body. But, DNA doesn’t make the proteins directly. First, a copy of the recipe, called RNA, is made. This RNA copy then moves to a part of the cell called a ribosome, which reads the RNA and puts together the proteins following the instructions.

So, you can think of DNA as keeping the master copy safe, while RNA makes a working copy and helps make the proteins. Also, when cells need to make new cells, DNA makes sure the new cells get a complete set of instructions. This is how DNA helps with both making proteins and creating new cells.

Also included at the chemical level are enzymes and trace elements of other materials.


Cellular Dysfunction

Impaired Oxygen and Glucose Metabolism

Cellular metabolism impairment occurs with:

  • oxygen impairment, and
  • impaired glucose use.


Impaired Oxygen Metabolism

The Krebs Cylce is the citric acid cycle (or the tricarboxylic acid--TCA) cycle which generates energy via the production of ATP during aerobic respiration.

With hypoxia and subsequent oxygen impairment, anaerobic metabolism causes increases in lactate that lead to metabolic acidosis. Decreased oxygen affinity for hemoglobin leads to decreased ATP and changes in cellular electrolytes, causing cellular edema and release of lysosomal enzymes that destroy the cell.

Glucose metabolic impairment is when an increased serum glucose causes catecholamine, cortisol, and growth hormone release.

There is increased gluconeogenesis, which by-passes the TCA cycle, resulting in generation of glucose for energy from non-carbohydrate carbon substrates (such as lactate). This is an anaerobic response to hypoxia. Gluconeolysis and lipolysis destroy the remaining glucose and lipids.

The back-up plan for the TCA cycle for when oxygenation fails is to generate glucose anaerobically via gluconeogenesis. This alternate method is meant to be temporary, because the accumulating by-products (lactate) become toxic. 

Cell Death

Cells die through either apoptosis or necrosis:

  • Apoptosis is a normal and controlled part of an organism’s growth or development. It is a pre-programmed lifespan stored genetically in each cell.
  • Necrosis is the process by which cell injury results in premature death of cells by autolysis.

Autolysis is the destruction of cells or tissues by their own enzymes, especially those released by lysosomes.


The Active Site Theory

The active site theory states that enzymes can only undergo chemical reactions under certain constraints and at specific parts of a substrate's structure.

An enzyme's active site consists of temporary bonds at the substrate (binding site) and residues that catalyze a reaction with that substrate (catalytic site).

Although the active site is small relative to the whole enzyme--10-20% of total volume--it is considered the "business end" of the enzyme and therefore the most important part. (It's only 3-4 amino acids, and the rest of the amino acids are used to maintain the protein's 3D structure.)