Cell Structure-what are organelles definition

By | July 5, 2021

Cell Organelles definition

  • Cell organelle is a specialized entity present inside a particular type of cell that performs a specific function.
  • There are various cell organelles, out if 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.
Cell Organelles- Structure and Functions with diagram

Cell membrane (Plasma membrane/ Plasmalemma)

  • A plasma membrane is composed of lipids and proteins where the composition might fluctuate based on fluidity, external environment, and the different stages of development of the cell.

Structure

  • Structurally, it consists of a phospholipid bilayer along with two types of proteins viz. embedded proteins and peripheral proteins that function in providing shape and allowing the movement of particles in and out of the cell.
  • The most abundant lipid which is present in the cell membrane is a phospholipid which contains a polar head group attached to two hydrophobic fatty acid tails.
  • The embedded proteins act as channels for the transfer of particles across the cell with some proteins acting as receptors for the binding of various components.
  • The peripheral proteins function as to provide fluidity as well as mechanical support to the structure of the cell.
Plasma membrane (Cell membrane) Diagram

Functions

  • 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.
  • 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.

Structure

  • 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 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 hemicellulose.
Cell Wall (Plant, Fungal, Bacterial)

Functions

  • 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.

Structure

  • 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 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.
Centrioles Diagram

Functions

  • 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 are tiny hair-like projections from the cell made of microtubules and covered by the plasma membrane.

Structure

  • 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.
Cilia and Flagella Diagram

Functions

  • 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.Read More

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