Cellular Structures Enclosed in Membranes

Cellular Structures Enclosed in Membranes

Cellular Membrane-Bound Organelles: Classification and Functions

Cellular organelles, permanent components of cells, are the site of numerous essential cellular processes. This post delves into the membrane-bound organelles of the cell, presenting a revised classification below and additional information on non-membrane-bound organelles by following this link.

Single Membrane-Bound Organelles

1. Golgi Apparatus/Golgi Complex: A membrane-bound organelle named after its discoverer Camilo Golgi, it consists of 2 to 20 vesicles, sacs, or tubules known as cisternae. Each cell typically has only one Golgi complex, although some may contain more. Red blood cells lack Golgi bodies. The Golgi apparatus processes incoming proteins, adding carbohydrate to form glycoproteins and lipids to form lipoproteins, as well as sorting and packaging proteins for transport to their destinations, making it often referred to as the cell's post office.
2. Structure: The Golgi apparatus is situated near the nucleus, with space between its entry and exit faces known as medial cisternae. The curved cisternae give the organelle a cup-like shape, and the opposite ends differ in size, shape, content, and enzymatic activities. The entry face processes materials and receives proteins from the rough endoplasmic reticulum (RER), while the exit face further modifies protein molecules.
3. Functions: The Golgi apparatus processes, packages, labels, and delivers proteins, lipids, carbohydrates, and other molecules to various parts of the cell, thereby playing a critical role in protein modification and cellular communication.
4. Rough Endoplasmic Reticulum (RER): A network of interconnected tubules that are connected to the nuclear outer membrane and extend throughout the cell cytoplasm, the RER is responsible for protein synthesis. Two types of rough endoplasmic reticulum have been identified:
5. Rough endoplasmic reticulum synthesizes proteins by having ribosomes attached to its outer surface, arranging amino acid chains into small protein units that are transported into the RER for further processing and sorting. Cells that are actively engaged in protein synthesis show a large number of RER, such as acinar cells of the pancreas, nerve cells, and plasma cells (Russell's bodies).
6. The rough endoplasmic reticulum also plays a role in breaking down exhausted organelles by a process known as autophagy, wrapping itself around worn-out organelles and enzymes to form a vacuole for digestion.
7. Smooth Endoplasmic Reticulum (SER): Extending from the rough endoplasmic reticulum, the smooth endoplasmic reticulum (SER) contains no ribosomes on its outer surface and is formed by a network of many connected tubules. The SER is functionally more diverse than RER due to its unique enzymes and performs the following functions:
8. Synthesis of non-protein substances: The SER synthesizes fatty acids and steroids like estrogen and testosterone. It is well-developed in Leydig cells and cells of the adrenal cortex.
9. Role in various metabolic processes: The outer surface of the smooth endoplasmic reticulum contains many enzymes that play an important role in cellular metabolism, such as by separating the phosphate group from glucose-6-phosphate in cells of the liver, kidneys, and intestine, allowing free glucose molecules to enter the bloodstream.
10. Storage and metabolism of calcium ions: The smooth endoplasmic reticulum serves as a major site of calcium ion storage and metabolism. In skeletal muscle cells, calcium ions are released to trigger muscular contraction.
11. Catabolism and detoxification: Enzymes in the smooth endoplasmic reticulum inactivate or detoxify lipid-soluble or harmful drug substances.

Double Membrane-Bound Organelle

One double-membrane bound organelle, the mitochondria, is responsible for energy production (ATP synthesis) in the cell and is described in a separate post, Mitochondria: The Powerhouse of the Cell. (Link to the Mitochondria post here.)

In the realm of physiology, the Golgi apparatus, a significant membrane-bound organelle, plays a pivotal role in the science of cellular-conditions, especially within health-and-wellness. It processes incoming proteins, adding carbohydrates to form glycoproteins and lipids to form lipoproteins, acting as the cell's post office. Additionally, the smooth endoplasmic reticulum, another membrane-bound organelle,synthesizes non-protein substances like fatty acids and steroids, contributing to various metabolic processes and hormone production, and serves as a major site of calcium ion storage and metabolism.

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