#2 Cell Structure and Functions (Cytoplasm: Cytosol, Cell Organelles)
Introduction
In my previous article, I explained about two basic components of cell structure, Cell membrane or plasma membrane and Nucleus. I am happy that my previous article liked by a lot of people. In this article, I will explain you about Cytoplasm. I wish this article also helps you to add knowledge.
Cytoplasm
Cytoplasm is a fluid-like substance of viscous nature filled in the cell. In eukaryotic it can also be defined as the part of the cell present between the cell membrane and the nuclear envelope.
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'Protoplasm', which is the living content of a cell that is surrounded by a plasma membrane. It is basically composed of a mixture of small molecules such as nucleic acids, proteins, lipids, and polysaccharides.
In eukaryotes, the protoplasm surrounding the cell nucleus is known as the cytoplasm and that inside the nucleus as the nucleoplasm. While in prokaryotes, the material inside the plasma membrane is the bacterial cytoplasm.
Cytoplasm contains the following components
(1) Cytosol
Cytosol is the cytoplasmic matrix (liquid) found inside the cells. It is separated into compartments by membranes. e.g., the mitochondrial matrix separates the mitochondrion into compartments i.e., outer and inner compartments.
(2) Cell Organelles
Each cell has certain specific components within it, known as cell organelles. Various cell organelles perform special functions, such as making new material in the cell, cleaning up the waste material from the cell, etc. Cell organelles constitute various components like endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids, and vacuoles, which perform various functions in the cell.
These organelles are described below in detail
(a) Endoplasmic Reticulum (ER)
It was discovered by Albert Claude and Keith Porter in 1945. ER is basically structured as a network of tubules or flattened sac, called vesicles or oblong bags. These are all interconnected by a single continuous membrane.
There are two types of endoplasmic reticulum
Rough Endoplasmic Reticulum (RER)
Synthesises and secretes serum proteins (e.g., albumin), hormones (e.g., insulin) and other substances (e.g., milk) in the glands because it possesses ribosomes at the cytoplasmic side of the membrane.
Smooth Endoplasmic Reticulum (SER)
Synthesises lipids and steroids, metabolises carbohydrates and helps in regulating calcium concentration, drug detoxification and attachment of receptors on the cell membrane.
The SER helps in the manufacturing of fat molecules, or lipids that are essential for the functioning of the cell. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis.
ER carries out various functions like transport of materials between various parts of the cell. It acts as waste disposal and cell cleansing system by digesting foreign or unwanted materials. It has some digestive enzymes in the membrane-bound sacs that break down foreign material entering the cell. They also break down worn-out cellular components.
Microbodies
These are small cell organelles bounded by a single membrane that absorb oxygen and also take part in oxidation other than those involved in respiration
These are of following two types
(i) Peroxisomes These are developed from endoplasmic reticulum. They have special docking proteins called peroxins for obtaining materials from cytosol and endoplasmic reticulum
(ii) Glyoxysomes They are considered to be special peroxisomes as they are microbodies that contain enzymes for fi-oxidation of fatty acids and for glyoxylate pathway.
(b) Golgi Apparatus
(Golgi Body) It was first discovered in the nerve cells of barn owl and cat. It is basically composed of flat sacs known as cisternae. The sacs are attached/stacked in a bent or semi-circular shape. Each stacked grouping has a membrane that separates it inside from the cell cytoplasm. It is also composed of tubules (arising from the periphery of cisternae) and vesicles. These membranes get, link to the membrane of ER and forms another complex membrane system
Golgi apparatus is to carry out the processing of proteins generated in ER. It helps in the transportation of lipids around the cells. In Golgi bodies. some complex sugars are formed from the simple sugars. It also helps in the formation of cell wall (in plant cells) and lysosomes.
(c) Lysosomes
These are also known as suicidal bags (digestive bags) or atom bomb of the cell, due to the presence of powerful hydrolytic enzymes which are able to digest every organic substance in the cell. It has complexly arranged lumen (internal space) in the form of packet that can serve a variety of function in a cell. Lysosomes occur practically in all animal cells and Protozoans.
Intracellular and extracellular digestion is performed by lysosomes. i.e., in intracellular digestion, individual cells may obtain food through phagocytosis and extracellular digestion occurs when lysosomes release enzymes in the external environment through exocytosis. It helps in disposing useless cells and replace them with new cell. i.e_. breakdown aging and dead cell. It helps in body defense system.
(d) Mitochondria
It was named as ‘bioblast’. It acts as the cell's power producer and also called as power house or Power plant of the cell.
Mitochondria is elliptical in shape and bounded by two membrane coverings, i.e., an outer one which is porous and smooth and an inner one, which is deeply gold. The inner membrane form finger-shaped matrix called tubuli (cristae) at various places.
Mitochondria have tier own DNA and ribosomes, therefore, they can make some of their proteins
It has the ability to convert energy into such forms that are usable by the cell, i.e., in the form of ATP (Adenosine Triphosphate). It acts as the major site of cellular respiration, which ultimately generates fuel for the cell’s activities.
Mitochondria is also involved in other cell processes such as cell division and growth as well as in cell death (apoptosis).
However, mitochondria are not fully autonomous as both structure and function are partially controlled by nucleus of the cell and they depend upon the availability of material from cytoplasm. Hence, they are considered as semiautonomous in nature.
It provides energy for energy-requiring processes of the cell like muscle contraction, nerve impulse, conduction etc. It may store and lease calcium when required.
(e)Plastids
It is a major organelle found in the cells of plants and algae. It has numerous membrane layers embedded in a part called stroma. Plastids are the site of manufacture and storage of important chemical compound used by the cell. Like mitochondria plastids also have their own DNA and ribosomes.
Based on the types of pigments present, plastids can be classified into following three types
Types of Plastids
(i) Leucoplast
Colorless, Occurs near the nucleus, in non-green cells like roots, underground stems etc They may be amyloplast (store starch) elaioplast (store fat) or proteinoptast (store proteins)
(ii) Chloroplast
Greenish plastids due to the presence of chlorophyll and carotenoids, takes part in photosynthesis. Chloroplasts of algae other than green ones are called chromatophores.
(iii) Chromoplast
Yellow or reddish in color due to the presence of carotenoid pigments, found in colored parts like flowers, trusts, etc. various forms are found in tomato (lycopene), carrot (carotene), beet root (betamin) etc.
(f) Ribosomes
These are granular structures first observed under the electron microscope as dense particles by George Palade (1953). These are basically composed of ribonucleic acid (RNA) and proteins. Ribosomes are not surrounded by any membrane. It is the smallest cell organelle and was earlier known as microsomes before the contribution of Palade, who later called it ribosomes The eukaryotic ribosomes are 80 S type while prokaryotic are 70 S type.
Its major function is to synthesise proteins in the cell, due to which it is also known as factory of protein in the cell.
(g) Centrosomes
It is an organelle usually containing two cylindrical structures called centrioles. Both centrioles lie perpendicular to each other forming an organisation like that of a cartwheel. Centrosome is mainly found in animal cells. The function of Centrosomes It is helpful in cell division and formation of spindle fiber.
(h) Vacuoles
It is a membrane-bound organelle present in all plants, fungal cells, some protists, animals and bacterial cells. They enclose compartments, which are filled with water containing inorganic and organic molecules including enzymes in solution. In certain cases, they may also contain solids, which have been engulfed e.g., in Amoeba.
Vacuoles are formed mainly by the fusion of multiple membrane vesicles.
The organelle has no basic shape and size, its structure varies according to the needs of the cell. It is generally large in plants (confined within the nucleus) to provide turgidity and rigidity to its cell but are less in number and smaller in size in animals.
In some unicellular organisms, specialised vacuoles are present for releasing excess water and some waste from the cell.
Sap vacuoles They are fluid filled vacuoles which are separated from the cytoplasm by a semi-permeable membrane called tonoplast.
Gas vacuoles They store gases and provide buoyancy, mechanical strength and protection from harmful radiation.
The main function of vacuole is to collect the food stuffs in which sugar, 02, CO2, mineral salts, organic acids and residual substances are dissolved. It helps to maintain turgidity of the cells. It is also helpful in maintaining original shape of the cell and in the transportation of various cellular substances in the cell.
(3) Inclusions
These are basically considered as the non-living components present in the cell that do not possess metabolic activities and are not bounded by membranes as well. The most common inclusions are glycogen, lipid droplets, crystals and pigments.
Cytoplasm contains all the contents within the cell membrane and outside the nucleus. It is gell like appearance. Cytoplasm contains cytosol, cell organelles, and Inclusions. In next topic, I will explain you the diffrence between Prokaryotic cell and Eukaryotic cell plus difference between Animal Cell and Plant Cell.
Thanks for reading!
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Cytosol
Cytosol
Endoplasmic reticulum
Golgi apparatus
Lysosome
Mitochondrion
Ribosome
Centrosome
Vacuole