Secret Life of a Cell (Part 2)

Part 2 of this series continues describing the organelles, the basic tiny structures of eukaryotic cells: lysosomes (which contain over 50 different enzymes), endoplasmic reticulum, ribosomes, Golgi apparatus, and mitochondria (the main energy source for the majority of cellular functions).

Another organelle, the lysosome, is a vesicle that uses hydrolytic enzymes to break down all kinds of biomolecules in the cell. The lysosomes act like the waste disposal system of the cell by digesting unwanted materials in the cytoplasm and materials from outside the cell. In addition to being the digestive stomach of the cell, the lysosomes also get involved in secretion, membrane repair, cell signaling and energy metabolism. The size of lysosomes varies and the digestive enzymes inside these purifying vesicles require a steady acidic environment to be able to function properly.

There is a complex process of pumping in hydrogen ions across the vesicle membrane to maintain the acidic pH levels necessary for optimal enzyme activity. Enzymes are biological molecules that act as catalysts, helping complex reactions occur everywhere in life. Like all catalysts, enzymes increase the rate of a reaction by lowering it’s activation energy. Almost all metabolic processes in the cell need enzymes in order to occur at rates fast enough to sustain life. Lysosomes are known to contain over 50 different enzymes.

These digestive enzymes are synthesized in the rough endoplasmic reticulum, an interconnected network of membrane sacs, continuous with the nuclear membrane. There are two varieties of endoplasmic reticulum, smooth ER and rough ER. Smooth endoplasmic reticulum is responsible for synthesizing lipids, phospholipids used in plasma membranes and steroids. Smooth ER functions in several cellular processes, such as carbohydrate and steroid metabolism. Smooth ER serves different functions in each type of cell and contributes to detoxification.

The rough endoplasmic reticulum is studded with protein manufacturing ribosomes. A ribosome is the part of the cell where proteins are created. A sequence of DNA becomes transcribed into a sequence of amino acids and the ribosomes compile these amino acids in the correct order to form proteins. This is a process known as translation. Once a protein is translated, it folds into a functional three dimensional structure and becomes inserted directly into the endoplasmic reticulum and transported into the secretory pathway where it will reach it’s destination.

An organelle closely related to endoplasmic reticulum is the Golgi apparatus, or the Golgi body, a complex of flattened disk like structures located close to the endoplasmic reticulum. The Golgi apparatus receives proteins and lipids from the the rough ER, modifies and sorts them and then packs them into vesicles to be sent to their final destinations within the cell. These vesicle packages will either be sent to lysosomes, the plasma membrane of the cell or they can be transported across the cell membrane and sent outside of the cell.

When a vesicle is secreted from the cell, it fuses with the plasma membrane and releases it’s content in regulated bursts. All of these complex molecular processes require energy. The mitochondria are the powerhouse of the cell. The organelle where most of the cell’s supply of adenosine triphosphate, or ATP, is generated. ATP is often regarded as the molecular unit of currency, the main energy source for the majority of cellular functions.

In the next episode we’ll discuss the nucleus.

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