What is an organ system? It is a system composed of different organs which work together in a coordinated manner to achieve a common goal for the body of an organism, right? Did you know that even at the cellular level such a system or coordinated organelles exist? This system is known as the endomembrane system and is the characteristic feature of eukaryotic cells.
This system consists of four single membrane bound organelles which function in coordinated harmony for packaging and transport of materials within and outside the cell, intracellular digestion and storage of substances. These organelles include - the endoplasmic reticulum, the Golgi apparatus, the lysosomes and the vacuoles.
Are you aware of the role played by each of these organelles individually? Come let us take a tour of the cell and learn more about the endomembrane system.
Table of contents
It was first observed by Garnier under an electron microscope. ‘Endo’ means inside, ‘plasmic’ means cytoplasm, ‘reticulum’ means network. Thus, the name, endoplasmic reticulum was proposed by Porter as this organelle forms a network of small tubular structures in the cytoplasm.
It is present in all eukaryotic cells except mature RBCs, embryonic cells and ovum. Prokaryotic cells lack ER.. It occupies most of the cytoplasmic area.
The endoplasmic reticulum is composed of long, flattened units called cisternae that are arranged in stacks and irregularly branched tubules which may be free or associated with cisternae. The cisternae and the tubules are hollow and possess a lumen within. Thus, ER divides the intracellular space into two compartments -
1) luminal (inside ER)
2) extra luminal (cytoplasm) compartments.
Endoplasmic reticulum can be divided into two types
It mainly consists of cisternae and is continuous with the outer membrane of the nucleus. Ribosomes are attached to the surface of the ER which makes the surface look rough and irregular. It is involved in protein synthesis. RER is more abundant in cells which are actively engaged in protein synthesis and secretion, for example, liver cells, pancreatic cells, etc. Proteins synthesised on the surface of the ER are transferred to the ER lumen
It mainly consists of tubules and is continuous with the rough endoplasmic reticulum. No ribosomes are attached to its surface thus it looks smooth from outside. Involved in lipid synthesis. In animal cells lipid-like steroidal hormones are synthesised in SER. SER is more abundant in cells of adipose tissue, glycogen-storing liver cells, cells of adrenal cortex, etc.
The ER helps in the transport of material from one part of the cell to another. RER provides a site for ribosome attachment and acts as a surface for protein synthesis. SER acts as a surface for the synthesis of lipids and steroids. ER acts as a supporting framework in the cytoplasm which maintains the form of the cell. Thus it is a part of the ‘cytoskeleton’.
Golgi apparatus was first discovered by Camillo Golgi in 1898under an electron microscope. It is also known as Golgi complex or Golgi bodies. Their size may vary from 0.5μm to 1.0μm in diameter.
Found in all eukaryotes except mature RBC. Extensively present in secretory cells. In plants and invertebrates small golgi bodies are scattered throughout the cytoplasm, called dictyosomes.
Golgi bodies have three main structural components
Many flat, disc-shaped sacs or cisternae of 0.5μm to 1.0μm diameter are stacked parallel to each other and are concentrically arranged near the nucleus. Their number may vary cell to cell. Cisternae of opposite sides differ in thickness and composition,which divide the golgi into two poles - cis face and trans face.
The cis face of the Golgi apparatus facestowards the ER. It is of convex shape as the cisternae fold away from the ER. It is the receiving face and receives proteins and lipids from the ER.
Trans face of the Golgi apparatus faces away from the ER and towards cytoplasm.It has a concave shape as the cisternae fold towards the cytoplasm. It is also known as the maturing face as the modified materials are packed and released from this region.
Fig: Cis and Trans face of Golgi appartaus
These are short tubules arising from the periphery of cisternae. Some of the tubules get enlarged at their ends to form vesicles.
They generally pinch off from the tubules. These vesicles are involved in packaging and transport of material. They can either fuse with other vesicles and be stored in the cell or be secreted out of the cell. Vesicles might also pinch off from the Golgi bodies to form lysosomes.
The main function of the Golgi complex is packaging of material. It is also the primary site for glycolipid and glycoprotein synthesis within the cell. Vesicles contain proteins or lipids that are to be delivered within the cell or secreted outside. When signalled by the cell, these vesicles fuse with the plasma membrane and release their contents into the extracellular space. Many proteins synthesised by ribosomes on the endoplasmic reticulum get modified in the cisternae of the golgi apparatus before they are released from its trans face.The transport vesicles budded off from the Golgi may contain external molecules on their surface.
Lysosomes can be observed under electron microscope only. The Greek word ‘lyso’ means ‘digestive’ and ‘soma’ means ‘body’. As these single membrane bound organelles contain digestive enzymes, they are named as lysosomes. These are also known as ‘suicidal bags’ of the cell and were discovered by Christian de Duve (1955).
It is present in all animal cells and protozoans . They are absent in RBCs and prokaryotes and are ess commonly found in plants.
These are generally spherical in the form and are formed by the pinching off of vesicles from the Golgi apparatus. They are tiny sacs bound by a single unit membrane made of lipoprotein. The cytoplasm of lysosomes contains hydrolytic enzymes of acidic nature known as ‘acid hydrolases’.
Lysosomes help in the digestion of foreign particles engulfed by the cell by the process of phagocytosis. The food vacuoles in which the foreign particles are internalised are known as phagosomes and the primary lysosomes that pinch off from the Golgi apparatus fuse with the phagosomes to form secondary lysosomes or phagolysosomes. The action of hydrolytic enzymes within the phagolysosome helps in intracellular digestion of the foreign particles. This process is also known as heterophagy.
The lysosomes are capable of digesting the cell’s own organelles which are damaged or dead. This process is known as autophagy and the lysosomes are known as autophagosomes.
Digestion of materials outside the cell is called extracellular digestion. In certain occasions lysosomes release enzymes outside the cell by exocytosis and bring about digestion.
For example, osteoclast cells release their lysosomal content on the surface of the bone to bring about the extracellular digestion of bones. This process also helps regulate the level of blood calcium.
Lysosomes can digest the whole cell, the one that they belong to, if the cell is starved, damaged or worn out. This is the reason lysosomes are known as the suicide bags of the cell.
Vacuole is a single membrane-bound organelle which is large and prominent in plant cells and occupies almost 90% of the cell. Vacuoles might also be present in animal cells but they are smaller and very few in number.
The vacuoles are bubble-like sacs bound with a single unit membrane known as tonoplast. These are filled with water, sap, excretory products and other waste materials of the cell. The fluid within the vacuole is called ‘cell sap'. Concentration of sap is higher than that of cytoplasm. Hence concentration of ions and minerals is significantly higher in vacuoles than in the cytoplasm.
Vacuoles are mainly of two types
It is important for osmoregulation and excretion. Osmoregulation is the maintenance of constant osmotic pressure in the fluids of an organism by the control of water and salt concentrations. If water enters into a unicellular organism such as Amoeba by osmosis, then the excess water is internalised by the contractile vacuole. The contractile vacuole swells up, moves to the edge of the cell.
These are found in protozoan protists and lower animals like sponges and coelenterates. They contain food particles and break them down with the help of lysosomes.
Plant vacuoles maintain turgor pressure of plant cells. In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole. They are the food reserves in seeds and other plant parts.
Contractile vacuoles in Amoeba can help in osmoregulation and excretion of wastes.
Question 1. The main organelle involved in modification of newly synthesised proteins is
(a) Golgi bodies
Answer: Proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the Golgi apparatus before they are released from its trans face. Thus, the correct option is (a).
Question 2. Which of the following statements is incorrect?
(a) Golgi bodies are the primary sites of glycolipid and glycoprotein synthesis.
(b) Autophagosomes help in digestion of cellular organelles.
(c) The packaged proteins are released from the cis face of the Golgi body.
(d) Vacuoles in plant cells help to maintain turgidity.
Answer: The cis face of the Golgi apparatus faces the endoplasmic reticulum and is the receiving face which receives proteins and lipids from the ER. The trans face is also known as the maturing face as the modified materials are packed and released from this region. It faces away from the ER. Thus, the correct answer is (c).
Question 3. What are the differences between rough ER and smooth ER?
Answer: The differences between rough ER and smooth ER are as follows -
|Rough ER||Smooth ER|
|Mainly composed of cisternae||Mainly composed of tubules|
|Have ribosomes attached on the surface||Do not have ribosomes attached on their surface|
|Involved in protein synthesis||Involved in lipid metabolism|
Question 4. How do contractile vacuoles assist in osmoregulation?
Answer: Osmoregulation is the maintenance of constant osmotic pressure in the fluids of an organism by the control of water and salt concentrations. If water enters into a unicellular organism such as Amoeba by osmosis, then the excess water is internalised by the contractile vacuole. The contractile vacuole swells up, moves to the edge of the cell.
Question 1. Do bacterial cells have an endomembrane system?
Answer: The endomembrane system is made up of four single membrane bound organelles which work in a coordinated fashion - endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles. Bacterial cells are prokaryotic in nature and hence lack membrane bound organelles. Thus, bacterial cells do not have an endomembrane system.
Question 2. Why do mature RBCs lack the endoplasmic reticulum?
Answer: RBCs transport oxygen to all parts of the body and hence need to have a flexible structure that can flow through the capillaries with very narrow lumen. Thus they lose the ER which serves as a skeletal framework for other eukaryotic cells. Lack of ER also results in increased surface area and allows these cells to carry more oxygen more efficiently.
Question 3. What is the need for an endomembrane system within the cell?
Answer: The endomembrane system provides better compartmentalisation and specialisation for various functions within the cell. For example, the proteins and lipids are synthesised in the ER, packaged within the Golgi apparatus, degraded within lysosomes and stored in vacuoles when necessary.
Question 4. Do all eukaryotic cells have an endomembrane system?
Answer: Endomembrane system is a characteristic feature of eukaryotic cells but some eukaryotic cells such as the mammalian red blood cells which lack an endomembrane system.
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