Cell organelles are a specialized entity present inside a particular type of cell that performs a specific function. There are various cell organelles, out of which, some are common in most types of cells like cell membranes, nucleus, and cytoplasm. However, some organelles are specific to one particular type of cell-like plastids and cell walls in plant cells.
Functions of Cell Organelles
- The cell membrane provides mechanical support that facilities the shape of the cell while enclosing the cell and its components from the external environment.
- It regulates what can be allowed to enter and exit the cell through channels, acting as a semi-permeable membrane, which facilities the exchange of essential compounds required for the survival of the cell.
- It generates and distributes signals in and outside of the cell for the proper functioning of the cell and all the organelles.
- It allows the interaction between cells required during tissue formation and cell fusion.
Cell Organelles and their Functions
The plasma membrane is also termed as a Cell Membrane or Cytoplasmic Membrane. It is a selectively permeable membrane of the cell, which is composed of a lipid bilayer and proteins. The plasma membrane is present both in plant and animal cell, which functions as the selectively permeable membrane, by permitting the entry of selective materials in and out of the cell according to the requirement. In an animal cell, the cell membrane functions by providing shape and protects the inner contents of the cell. Based on the structure of the plasma membrane, it is regarded as the fluid mosaic model. According to the fluid mosaic model, the plasma membranes are subcellular structures, made of a lipid bilayer in which the protein molecules are embedded.
An additional non-living layer present outside the cell membrane in some cells that provides structure, protection and filtering mechanism to the cell is the cell wall.
- In a plant cell, the cell wall is made up of cellulose, hemicellulose, and proteins while in a fungal cell, it is composed of chitin.
- A cell wall is multilayered with a middle lamina, a primary cell wall, and a secondary cell wall.
- Middle lamina contains polysaccharides that provide adhesion and allows binding of the cells to one another.
- After the middle lamina is the primary cell wall which is composed of cellulose. The last layer, which is not always present, is the secondary cell wall made of cellulose and
- The critical function of the cell wall is protecting and maintaining the shape of the cell. It also helps the cell withstand the turgor pressure of the cell.
- It initiates cell division by providing signals to the cell and allows the passage of some molecules into the cell while blocking others.
Centrioles are tubular structures mostly found in eukaryotic cells which are composed mainly of the protein tubulin.
- A centriole consists of a cylindrical structure made with nine triplets microtubules that surround the periphery of the centriole while the center has a Y-shaped linker and a barrel-like structure that stabilizes the centriole.
- Another structure called a cartwheel is present in a centriole which is made up of a central hub with nine spokes/filaments radiating from it. Each of these filaments/spokes is connected to the microtubules through a pinhead.
- During cell division, centrioles have a crucial role in forming spindle fibers which assist the movement of chromatids towards their respective sides.
- They are involved in the formation of cilia and flagella.
Cilia and Flagella
Cilia and Flagella are tiny hair-like projections from the cell made of microtubules and covered by the plasma membrane.
- Cilia are hair-like projections that have a 9+2 arrangement of microtubules with a radial pattern of 9 outer microtubule doublet that surrounds two singlet microtubules. This arrangement is attached to the bottom with a basal body.
- Flagella is a filamentous organelle, the structure of which, is different in prokaryotes and eukaryotes.
- In prokaryotes, it is made up of the protein called flagellin wrapped around in a helical manner creating a hollow structure at the center throughout the length.
- In eukaryotes, however, the protein is absent and the structure is replaced with microtubules.
- The most critical role of cilia and flagella is movement. These are responsible for the movement of the organisms as well as for the movement of various particles present around the organisms.
- Some cilia present in some particular organs may have the function of sense. The cilium in the blood vessels, which helps in controlling the flow of blood is an example.
A chloroplast is a type of plastic that is involved in photosynthesis in plants and algae. Chloroplast contains an essential pigment called chlorophyll necessary to trap sunlight for the production of glucose.
- It is a double-membraned structure with its own DNA which is inherited from the previous chloroplast.
- These are usually lens-shaped with shape and number varying according to cells. They have an outer membrane, an inner membrane, and a thylakoid membrane that enclosed the gel-like matric called the stroma.
- The outer and inner membrane is porous and allows transport of materials while the stroma contains DNA, chloroplast ribosomes, proteins, and starch granules.
- The chloroplast is the primary center for light-dependent and light-independent reactions during photosynthesis.
- Different proteins present in chlorophyll are involved in the regulation of photorespiration.
Cytoplasm refers to everything present inside the cell except the nucleus.
- The cytoplasm consists of a cytosol; a gel-like substance that contains other matter; cell organelles; smaller cell-like bodies bound by separate membranes; and cytoplasmic inclusions; insoluble molecules that store energy and are not surrounded by any layer.
- The cytoplasm is colorless and has about 80% water along with various nutrients required for the cell.
- It is known to have the properties of both viscous matters as well as elastic matter. Under its elasticity, cytoplasm helps in the movement of materials inside the cell by a process termed cytoplasmic streaming.
- Most of the vital cellular and enzymatic reactions like cellular respiration and translation of mRNA into proteins occur in the cytoplasm.
- It acts as a buffer and protects genetic materials as well as other organelles from damage due to collision or change in the pH of the cytosol.
- The process called cytoplasmic streaming helps in the distribution of various nutrients and facilitates the movement of cell organelles within the cell.
Microfilaments are a part of the cytoskeleton of a cell made up of actin protein in the form of parallel polymers. These are the smallest filaments of the cytoskeleton with high rigidity and flexibility, providing strength and movement to the cell.
- The filaments are present either in cross-linked forming networks or as bundles. The chains of protein remain twisted around each other in a helical arrangement.
- One of the polar ends of the filament is positively charged and barbed, whereas the other end is negatively charged and pointed.
- It generates the strength for the structure and movement of the cell in association with myosin protein.
- They help in cell division and are involved in the products of various cell surface projections.