The endomembrane system is a collection of interconnected and functionally related membrane-bound organelles found within eukaryotic cells. These organelles work together to modify, package, and transport proteins and lipids throughout the cell and to the cell’s exterior. Endoplasmic reticulum (ER), Golgoi apparatus, Lysosomes and Vacuoles are considered part of the endomembrane system as they work coordinated manner.
Question- Why are mitochondria, chloroplasts and peroxisomes not considered part of the endomembrane system?
Answer- The functions of mitochondria, chloroplasts and peroxisomes are not coordinated with the endoplasmic reticulum, Golgi bodies, lysosomes and vacuoles, hence these are not considered as part of the endomembrane system.
Components of Endomembrane System
The main organelles and membranes that make up the endomembrane system include:
In eukaryotic cells, the presence of a network or reticulum of tiny tubular structures is seen that is scattered in the cytoplasm, called the endoplasmic reticulum. Endoplasmic reticulum divides the intracellular space into two distinct compartments- luminal (inside ER) and extraluminal (cytoplasm) compartments.
Endoplasmic reticulum is in two forms- Rough Endoplasmic Reticulum (RER), which bears ribosomes on its surface. They are extensive and continuous with the outer membrane of the nucleus. Smooth Endoplasmic Reticulum (SER), which bears no ribosomes and is smooth in appearance.
RER is actively involved in protein synthesis, secretion and modification; while SER is the major site for lipid synthesis and detoxification. In animal cells lipid-like steroidal hormones are synthesised by SER.
: In 1898, Camillo Golgi first observed densely stained reticular structure near the nucleus. These were later named Golgi bodies. They consist of of many flat, disc-shaped sacs or cisternae of 0.5 um to 1.0 um diameter. These are stacked parallel to each other. Varied numbers of cisternae are present in a Golgi complex. The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face. The cis and trans faces of the organelle are entirely different, but interconnected.
The main function of Golgi apparatus is to packaging of materials, and either to deliver them to the intra-cellular targets or to secrete outside the cell.
Question- Why the Golgi apparatus remains in close association with the endoplasmic reticulum?
Answer- To receive materials that produces by ER through cis face of the golgi apparatus, it remains close to the ER. Later, the collected materials are moved towards the trans or maturing site of the golgi apparatus.
Several proteins synthesized by ribosomes on the endoplasmic reticulum are modified in the cisternae of the Golgi apparatus before they are released from its trans face.
Golgi apparatus is an important site of formation of glycoproteins and glycolipids.
: These are membrane bound vesicular structures formed by the process of packaging in the apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrophytic enzymes like- hydrolases- lipases, proteases, carbohydrases etc. optimally active at the acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.
: The vacuole is a membrane-bound space present in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell. The vacuole is bound by a single membrane called tonoplast. In plant cells, vacuole can occupy about 90% of the total volume of the cell.
In plant cell, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole than in the cytoplasm.
In Amoeba, the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food materials.
Functions of Endomembrane System
The endomembrane system is essential for:
Synthesising, folding, and modifying proteins and lipids.
Packaging and transporting these molecules to specific destinations inside or outside the cell.
Digesting and recycling cellular waste.
Maintaining the structural integrity and compartmentalisation within the cell.
