Endomembrane system – Definition, Structure and Examples
The eukaryotic cell is composed of many membrane bound organelles. Each of the membrane -bound organelles has a distinct function and structure.
The organelles included in the endomembrane system are endoplasmic reticulum, Golgi complex, Lysosomes and vacuoles.
Structure of Endomembrane System
Types of Endomembrane System
1. Endoplasmic Reticulum (ER)
The presence of a network of reticulum of tiny tubular structure scattered in the cytoplasm that is called the endoplasmic reticulum (ER).
Types of Endoplasmic Reticulum
On the basis of presence or absence of ribosomes on the surface of endoplasmic reticulum, it is two types
(i) Smooth Endoplasmic Reticulum (SER) : The endoplasmic reticulum which is free of robosomes is known as SER. When it is observed under the electron microscopes it appears as smooth tubular structures. The muscles cells are also rich in smooth type of endoplasmic reticulum which is known as sarcoplasmic reticulum.
(ii) Rough Endoplasmic Reticulum (RER) : The endoplasmic reticulum bearing ribosomes on its surface is called RER, its gives a rough granular appearance under the electron microscope.
2. Golgi Apparatus
Golgi apparatus was first observed by Camillo Golgi in 1898. He described it as a densely stained reticulum structures present near the nucleus of the cell. Therefore, these were given the name Golgi bodies, after his name .it is present in eukaryotic cells, except in mature sieve tubes of plants, mature RBCs of mammals, sperm cells. In plants, it is called dictyosomes as Golgi apparatus is made up of unconnected units.
- The important function of Golgi apparatus is to process, package and transport the materials for secretions The packaged material is delivered either to the intracellular targets ie., within the cell or secreted to extracellular targets i.e., outside the cell. The material to be secreted moves from ER to the Golgi apparatus in the form of transitional vesicles. These vesicles then fuse with the cis face and move towards the maturing face of the golgi apparatus. Therefore, Golgi apparatus is closely associated with ER in structural as well as functional aspects.
- A number of proteins synthesised by ribosomes present on the ER are transferred to golgi apparatus. These proteins are then modified in the cistermae of Golgi apparatus before they are released from its trans face.
- Golgi apparatus is the important site of formation of glycoproteins (glycosylation of proteins) and glycolipids (glycosidation of lipids).
- Root cap cells are rich in Golgi bodies which secrete mucilage for the lubrication of root tip.
- Acrosome of the sperm is modified Golgi apparatus.
- Formation of plasma membrane during cytokinesis.
Lysosomes are very simple tiny spherical sac – like structure evenly distributed in the cytoplasm. These are formed by process of packaging in the golgi apparatus. They are bounded by a single membrane. It contains hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases).
This organelle shows polymorphism : On the bases of morphology, their contents & functions lysosomes are divided into following four forms :
- Primary lysosomes
- Secondary lysosomes
- Residual bodies
- Autophagic vacuoles
(i) Vacuoles is the membrane – bound space found in the cytoplasm. Vacuoles contains water, sap, excretory products & other materials not useful for the cell. These are also called Sap vacuoles.
(ii) In pant cells, the vacuoles can occupy upto 90% of the volume of the cell. They are bounded by a single, semipermeable membrane called Tonoplast. This membrane facilities the transport of a numbers of ions & others materials against concentration gradients into the vacuole. Thus, their concentration is significantly higher in the vacuole than in the cytoplasm.
Types of vacuoles :
- Contractile vacuole : In Amoeba, it helps in excretion.
- Food vacuoles : In many cells, as in protists, food vacuoles are formed by engulfing the food particles.
- Gas vacuoles (Pseudovacuoles): These are membraneless vacuoles found in prokaryotes to provide buoyancy.