Endomembrane System: Explanation, Components, and Functions

The endomembrane system consists of several membranes (endomembranes) suspended in the cytoplasm of a eukaryotic cell.

• These membranes separate the cell into functional and structural compartments called organelles.
• In eukaryotes, endomembrane system organelles include the nuclear membrane, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, endosomes, and plasma (cell) membrane, among others.
• The system is more accurately characterized as a set of membranes that function and develop as a single unit.
• They are either directly connected or exchanging material through vesicle transfer.
• Importantly, the endomembrane system does not comprise the membranes of plastids or mitochondria.
• Although it may have developed largely from the activities of the latter.

Key Terms: Endomembrane system, Organelles, Endoplasmic reticulum, Lysosomes, Enzymes, Cell, Vacuoles, Cytoplasm, Plant cell

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Endomembrane System

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The endomembrane system (endo- = "within") is a collection of membranes and organelles found in eukaryotic cells that works to modify, package, and transport lipids and proteins.

• The cytoplasm of the cell comprises a suspended form of membrane organelles.
• The organelles are referred to as a system despite the fact that they have various forms and functions since they are necessary for the cell to function.
• The cell membrane, vacuoles, nuclear membrane, lysosomes, Golgi complex, vesicles, and endoplasmic reticulum (ER) are all coordinated organelles.
• Other organelles, such as mitochondria and chloroplasts, are not considered part of the system since their functions do not coordinate with those of the above. 

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Components of the Endomembrane System

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The components of the Endomembrane system which play an essential  role are

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a network of interconnected membranous sacs and tubules whose sole function is to modify proteins and synthesize lipids.

• These two functions, however, are carried out in separate areas of the ER: the rough ER and the smooth ER, respectively.
• The lumen or cisternal space is the hollow portion of the ER tubules.
• The ER membrane is a phospholipid bilayer embedded with proteins that are continuous with the nuclear envelope.

Golgi Bodies

Golgi bodies, named after Camilo Golgi, the scientist who discovered them, are membraneous disc-shaped sacs or cisternae with varied diameters between 0.5 - 1.0 µm stacked parallel like pancakes.

• Interconnected cis and trans faces or developing and mature cisternae faces, are placed concentrically around the nucleus.
• It primarily functions as the packaging center for endomembrane secretions such as proteins.

Lysosomes

A lysosome is a membrane-bound cell structure containing digestive enzymes.

• The function of digestive enzymes is to digest worn-out organelles, misfolded proteins, engulfed viruses or prokaryotes, and food particles.
• Lysosomes are referred to as “suicidal bags” because if the cell cannot be repaired, the lysosomes will “self-destruct”.
• The contents are highly acidic and hydrolytic enzymes break down complex molecules into smaller, simpler molecules.

Vacuole

A vacuole is a membrane-bound organelle found in all plant, fungal, and animal cells, as well as some protist, animal, and bacterial cells.

• It is a fluid-filled vacuole and is the most visible component in a plant cell.
• The type of cell determines the significance and function of vacuoles.
• The plant, fungi, and certain protists have much greater importance than animals and bacteria.
• A single cell may contain multiple vacuoles.
• A single-unit membrane called the tonoplast separates each vacuole from the cytoplasm.
• In general, they take up more than 30% of the cell volume, but this can range from 5% to 90% depending on the cell type.

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Things To Remember

• The endomembrane system consists of several membranes (endomembranes) suspended in the cytoplasm of a eukaryotic cell.
• The endomembrane system does not comprise the membranes of plastids or mitochondria.
• The endoplasmic reticulum (ER) is a network of interconnected membranous sacs and tubules.
• Rough Endoplasmic Reticulum made up of interconnected flattened sacs with ribosomes bound to its surface.
• Smooth Endoplasmic Reticulum composed of interconnected flattened sacs with no ribosomes bound to its surface.
• Golgi bodies are membraneous disc-shaped sacs or cisternae with varied diameters between 0.5 - 1.0 µm stacked parallel like pancakes.
• A lysosome is a membrane-bound cell structure containing digestive enzymes.
• A vacuole is a membrane-bound organelle found in all plant, fungal, and animal cells, as well as some protist, animal, and bacterial cells.

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