Cell Biology

Endomembrane system- and its components

The Membranous organelle in eukaryotes varies in terms of structure and function. Many of them with other membranes form the co-ordinated system to perform a particular function. 

The Endomembrane system is composed of the membrane inside the cell. Dividing cells into the cellular compartments is also called cell organelle.

The system is defined more accurately as the set of membranes that form a single functional and developmental unit, either being connected directly or exchanging material through vesicle transport.

Components of the Endomembrane System

  • Endoplasmic Reticulum
  • Golgi body
  • Vacuoles
  • Lysosomes
  • Vesicles
  • Nuclear membrane, and
  • Cell Membrane 

Note that the membranes of the Mitochondria, Plastids, and Peroxysomes are not included in the Endomembrane system.

As the function of these is not associated with the other components of the system.

There is another theory behind it that is called the Endo-symbiosis theory.

According to this theory Mitochondria and Plastids were Prokaryotes at a time. But, during evolution, they were engulfed by the Eukaryotes and remain in Symbiosis (dependency state) with each other.

So, the membrane of Mitochondria and Plastids are different from Eukaryotes. So, their function.

Let’s take a quick look at its components.

Nuclear Membrane

The nuclear membrane separates the nucleoplasm (fluid of the nucleus) and cytoplasm.

It also acts as the barrier to prevent the entry of the undesired substance to enter inside the Nucleus. And interrupt the Transcription (synthesis of RNA from DNA).

It is 2 in amount and is connected to some spots. Thus forming a pore called the Nuclear Pore.

This 2 membrane is also referred to as the Nuclear Envelope. The space between these 2 membranes is called the Peri-nuclear space.

The nuclear envelope continues with the ER.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a membranous synthesis and transport organelle that is an extension of the nuclear envelope. 

More than half the total membrane in eukaryotic cells is accounted for by the ER. 

The ER is made up of flattened sacs and branching tubules that are interconnected. So that the ER membrane forms a continuous sheet enclosing a single internal space. 

This highly convoluted space is called the ER lumen, also called the ER cisternal space. The lumen takes up about ten percent of the entire cell volume. 

The endoplasmic reticulum membrane allows the transferring of only selected molecules between the lumen and the cytoplasm.

Since it is connected to the nuclear envelope, it provides a channel between the nucleus and the cytoplasm.

ER is divided into 2 types based on their structure-

RER (Rough Endoplasmic Reticulum)

Its outer surface is Rough (as the main suggests) due to the presence of the Ribosome. Their main function is to perform Protein synthesis.

SER (Smooth Endoplasmic Reticulum)

Its outer surface is smooth and shiny. It is responsible for the Lipid synthesis.

So, majorly found in the cells actively engaged in Lipid Synthesis. Like Hepatic Cells (cells of the Liver).

Vesicles

Vesicles are small membrane-enclosed transport units that can transfer molecules between different components.

Most Vesicles transport substance from the Endoplasmic reticulum to the Golgi apparatus, and then from the Golgi apparatus to various locations.

There are various types of vesicles each with a different protein configuration. Most of them contains the specific regions on the membranes. This are made of protein helps in identifying the target cell (like a navigation in Google maps).

Each vesicle travels to place that contains a marker on its Cytosolic surface complimentary to that of the Vesicle (like a Key and Lock).

Once the vesicle finds the membrane, they fuse and secrete their substance.

Golgi Apparatus

Also called Golgi Complex or Golgi Body, as it the complex arrangement of the Tube like structure interconnect with each other.

 Getting the vibe of the ER. 

But it differs from the ER in structure and function.

Golgi bodies receive the secretion of the ER with the help of the Vesicles.

Modifies these substances and releases them again with the help of Vesicles mostly outside the cell or sometimes inside.

Function- So, as you may had understood. Their function is the packaging and transport of the substance.

Vacuole

A big membranous organelle occurring only in the Plant cell.

It occupies up to 90 percent of the space in the mature Plant cell. The membrane of the Vacuole is called the Tonoplast.

Its main function is to store the waste material of the plant cell and maintain the Ionic balance and Osmolarity of the cell.

It’s grown in size as the cell matures. You may guess why?

As plants don’t have their excretory system. So, all the waste is stored in the vacuole.

You might be guessing where this goes then?

No, where it remains in the cell until the cell dies and also after that until the plant is alive and again after that also.

It is this waste that makes the wood more durable from moisture and insects.

Lysosome

Also called the Suicidal Bag of the cell.

A lysosome is derived from the Latin word Lyso and some. In which lyso means ‘Lytic’ and some means ‘Body’.

So, lysosomes are the Lytic Body containing Lytic Enzymes (breaking or destroying).

Lysosomes are formed by the Golgi Body. They are the single membranous organelle having approx 50 types of enzymes.

All these enzymes are combinedly called as the Acidhydrolases. As they are activated by acidic pH.

Their main function is to digest unused parts of the cell or sometimes the whole cell if it is infected by the Virus or Cancerous Cell.

Plasma Membrane

Also called the Cell Membrane / Bio-membrane / Plasma Lemma.

It is the covering of the Cytosol. And is the only protection for the animal cell.

The plasma membrane is made of Phospho-lipid (formed by Golgi body) bilayer, containing some embedded proteins.

In this bilayer, the hydrophilic head is present outside and the hydrophilic tail is present between the heads (so as to be protected by the water).

It’s like the Glacier (protein) in the Ocean (phospho-lipid).

The cell membrane is quasi-fluid in nature. Means not that solid and nor that liquid.

This nature is essential for the metabolic of the cell. Like cell growth, cell division, and permeability of cell (precisely selective permeability).

Function of the Endomembrane System

The membranous component of the cell comes together to perform the basic metabolism of the cell effectively in a coordinated fashion.

This is the only function of the Endomembrane system.


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