Introducing the Endomembrane System- Structure and Function 

Think about a cell whose job is to secrete an enzyme. Eventually, the enzyme will be useful for another far-away organ that needs it to perform a specific function. How will that cell possibly ship this enzyme from inside the cell (site of synthesis) to outside the cell (site of action)?

The thought of some postal service comes to mind. Well, yes! There is a postal system found inside the cell that is scientifically termed an endomembrane system! Let us dive in to explore this wonderful portal system of our body cells!

Breaking Down the Endomembrane System 

In eukaryotic cells, a network of membranes and organelles known as the endomembrane system (endo = “within”) functions to change, pack, and move lipids and proteins. A system of suspended membrane organelles is found in the cytoplasm of the cell. Despite having distinct shapes and roles, the organelles are referred to as a system since they are necessary for the cell to function. These organelles collaborate with one another. 

The endomembrane system allows a higher degree of cell specialization by allowing different eukaryotic cell functions to be compartmentalized (protein breakdown, for example, takes place in the lysosome). 

Compartmentalization: 

The nuclear envelope, ER, Golgi apparatus, endosomes, and lysosomes are characteristics of the compartmentalization that occurs within the endomembrane system.  Proteins that are produced on ribosomes connected to the endoplasmic reticulum are processed there and subsequently transported to the Golgi apparatus by membrane carriers. Proteins are modified further in the Golgi apparatus before being sorted and delivered to distinct cellular destinations.

Membrane Trafficking:

Membrane trafficking in the endomembrane system requires dynamic interactions between compartments through vesicle transport. Proteins and protein complexes are responsible for vesicle production, transport, and fusion with target membranes.

Some Facts: 

• Gruber and Rosario (1979) investigated the Golgi apparatus, vacuoles, and cytoplasmic vesicles that make up the endomembrane system of C. Reinhardtian.
• Cell scientists see the Golgi as a machine, having cisternae at its core (Nicholson, 2019). 
• However, Golgi cisternae experience fast turnover and modification, while human-made machines are constructed from stable parts that function in a set manner (Pantazopoulou & Glick, 2019).

Different Parts of the Endomembrane System

Let us discuss the different parts of the endomembrane system, along with their function in the proteins and lipids transport and their structures.

Endoplasmic Reticulum 

The endoplasmic reticulum (ER) comprises a network of membranous tubules and flattened sacs. The lumen is the hollow space found inside the ER’s sacs and tubules. It plays a major part in the modification of proteins and the synthesis of lipids. 

Smooth Endoplasmic Reticulum

Structure: The endoplasmic reticulum, with interconnected membrane-bound sacs and tubules studded with ribosomes on its cytoplasmic surface, gives it a rough appearance. 

Functions: The following are the functions and activities of SER:

1. Protein Synthesis:
   • Ribosomes synthesize proteins, feeding them into the lumen for further processing.
   • Proteins undergo folding and modifications, such as the addition of carbohydrate side chains.
2. Phospholipid Synthesis:
   • Produces phospholipids for cellular membranes, including those of the ER itself.
3. Protein Sorting:
   • Sorts modified proteins destined for secretion or incorporation into cellular membranes.
   • Packages proteins into vesicles for transport to the Golgi apparatus

Rough Endoplasmic Reticulum

Structure: The endoplasmic reticulum, with a network of membrane-bound tubules that lack ribosomes on their cytoplasmic surface, gives it a smooth appearance.

Functions: The following are the functions and activities of RER:

1. Lipid and Carbohydrate Synthesis:
   • Synthesizes lipids, carbohydrates, and steroid hormones.
2. Detoxification:
   • Metabolizes and detoxifies drugs and toxins, aiding in their elimination from the cell.
3. Calcium Ion Storage:
   • Stores calcium ions, regulating their levels within the cell.
4. Muscle Contraction:
   • The specialized smooth ER in muscle cells called the sarcoplasmic reticulum, stores calcium ions that are essential for muscle contraction.
5. Vesicle Formation:
   • Contains transitional ER patches serving as exit sites for vesicle budding from the rough ER.

Golgi Apparatus 

What happens to the vesicles when they break off from the ER? The proteins and lipids formed are now packaged, tagged, and sorted in the transport vesicles before they can arrive at their destination. The Golgi apparatus/Golgi body is where this distribution, packing, labeling, and sorting happens.

Structure:

The Golgi body/apparatus is a single membrane-bound organelle within the endomembrane system. It consists of membrane sacs called cisternae, which are around four to eight in a Golgi stack. The Cis (entry) face and Trans (exit) face are the proximal and distal ends of the Golgi stack that face towards and away from the ER, respectively.

Functions: The following are the functions and activities of the Golgi body:

1. Protein Packaging:
   • Modifies, sorts, and packages proteins received from the ER for transport to various cellular destinations.
2. Secretion:
   • Plays a crucial role in the secretion of proteins from the cell, including hormones and enzymes.

Lysosomes 

The famous ‘suicidal bags of cells’ are lysosomes that are membrane-bound cell organelles within the endomembrane system packed with digestive enzymes in abundance.

Structure:

These are single membrane-bound organelles containing digestive enzymes that are part of the endomembrane system, which receive vesicles from the Golgi.

Functions: The following are the functions and activities of the Lysosomes:

1. Digestive Role:
   • Break down old cellular components and foreign particles.
   • Participate in organelle recycling within the cell.
2. Immune Response:
   • Engulf pathogens through phagocytosis.
   • Phagosomes fuse with lysosomes, forming combined compartments for pathogen degradation.
3. Cellular Defense:
   • Destroy disease-causing organisms that enter the cell. For example, Macrophages in the immune system use lysosomes for pathogen destruction during phagocytosis.

Vacuoles 

The lysosome-like cell organelles present in the plant endomembrane systems are known as vacuoles. These organelles mostly occur in plants and in large numbers, and rarely do they occur in animal cells.

Structure:

These organelles are membrane-bound and covered in a tonoplast membrane and are found in plant cells. The cell sap within them is unique from the cytoplasm that surrounds them.

Functions: The following are the functions and activities of the vacuoles:

1. Storage Function:
   • Stores water, wastes, toxins, and pigments.
2. Digestive Function:
   • Contains enzymes to break down macromolecules and cellular components, akin to lysosomes.
3. Water Balance:
   • Regulates osmotic balance and turgor pressure within the cell.
4. Membrane Transport:
   • Phospholipids form layers in the membrane.
   • Proteins aid in transporting molecules across the membrane.
5. Specialized Combinations:
   • Proteins facilitate vacuoles’ ability to hold different types of matter.

To Conclude

There is a suspended membrane organelle system in the cytoplasm of the cell. Even though the organelles have different shapes and functions, they are nevertheless talked about as part of a system since they are required for the cell to function correctly. 

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