Cytoplasm

Introduction

The cytoplasm is the semi-fluid material of a cell enclosed by the plasma membrane. It is made up of 70-80% water and appears colourless. The cytoplasm contains all kinds of molecules i.e. carbohydrates, proteins, amino acids, and nucleotides. There is a difference between the cytoplasm of eukaryotic and prokaryotic cells. In a eukaryotic cell, all the material inner to the plasma membrane and outer to the nuclear membrane is termed as the cytoplasm. In the prokaryotic cell, the nuclear material is not surrounded by a nuclear membrane, so it is also included in the cytoplasm. 

The cytoplasm is usually colorless and contains all the cellular organelles. In the cytoplasm, all the cellular activities like glycolysis and processes like mitosis and meiosis take place. The cytoplasm is sometimes called the substance of life. There is another term “Protoplasm” which includes all the substances of a cell included nucleus and nucleoplasm but excluding the cell membrane and the cell wall.

The cytoplasm has three main components. These are cytosol, organelles, and cytoplasmic inclusions. The cytoplasm is usually divided into two parts i.e. the ectoplasm and the endoplasm. 

Ectoplasm 

The name ectoplasm comes from the Greek word ektos which means outer. It is also called exoplasm and it refers to the outer part of the cytoplasm which is non-granulated. It is usually clear and watery and contains a large number of microfilaments. These microfilaments are made of actin and myosin and provide elastic support to the outer cell membrane. Ectoplasm is also known to protect the constituents of the cell and it helps in the transport of different materials within the cell.

Endoplasm

Endoplasm is the inner part of cytoplasm which is dense and often granulated as opposed to the ectoplasm which is non-granulated. It is the site where all the organelles are present and important cellular processes take place. It is called granulated because it contains many small particles or secretory vesicles that contain enzymes, hormones, and other materials. Endoplasm helps in the degradation and synthesis of materials. The important process of cell respiration and protein synthesis also takes place in the endoplasm. 

Physical Properties

There are many different theories related to the physical nature of cytoplasm. According to the sol-gel theory, the cytoplasm sometimes behaves like a colloidal solution which is disordered and watery and sometimes as a solid material like gel. Being in either solid or fluid phase depends on the interactions between components of the cytoplasm. According to research, cytoplasm acts like a liquid on a smaller length scale and acts as a solid on a larger length scale.

According to a recently proposed theory, the cytoplasm is just like a glass-forming liquid. It behaves like a solid when there is a greater concentration of components of cytoplasm and it behaves like a liquid when there is a low concentration of cytoplasmic components. Being in a solid phase is beneficial for the cytoplasm as it prevents damage by freezing larger cytoplasmic components and allowing the transport of only small particles and proteins. That is how it helps in the revival of the cell from dormancy.

Cytoskeleton

In the cytoplasm, many filaments make a dynamic and complex network which is called the cytoskeleton. It extends from the nuclear membrane to the plasma membrane. The three main components of the cytoskeleton are microfilaments, intermediate filaments, and microtubules. 

Microfilaments

Microfilaments are linear polymers of actin protein. That is why they are also known as Actin filaments. These are the smallest filaments in the cytoplasm having a structure resembling a double helix. Their main functions include cell movement, muscle contraction, and transport within the cell. 

Intermediate filaments

Intermediate filaments are larger than actin filaments but smaller than microtubules. They have an average of 10 nm in diameter. The structure of the intermediate filaments is just like two anti-parallel helices, forming dimers. These filaments maintain and organize the internal structure of the cell and also anchor the cellular organelles. 

Microtubules 

Microtubules are the largest of all filaments of the cytoskeleton. They have a cylindrical shape with an average outer diameter of about 23 nm to 27 nm and an inner diameter of about 15 nm. Microtubules are composed of polymers of alpha and beta tubulin. They take part in maintaining the structure and shape of the cell and also help in intracellular transport and cell wall synthesis. 

Constituents

The cytoplasm has three main constituents; the cytosol, cellular organelles, and cytoplasmic inclusions. 

Cytosol

Cytosol refers to the inner liquid portion of the cell. It is the fluid around the cellular organelles in which different chemical reactions take place. The cytosol is composed of water, ions, different micro molecules, and macromolecules. The water present in the cytosol mostly acts like pure water. The cell nucleus is separated from the cytosol by the nuclear membrane and the extracellular fluid is separated by the plasma membrane.

The ionic concentration of cytosol or the intracellular fluid is different from the extracellular fluid. The concentration of sodium and potassium ions is higher in the cytosol as compare to the extracellular fluid. This difference is responsible for the osmoregulation of the cell. The cytosol of prokaryotes is different as it also contains nuclear material.

The cytosol plays an important part in the signal transduction within the cell. It is the site of cytokinesis that takes place after the rupture of the nuclear membrane during mitosis. Transportation of metabolites from their site of production to their site of use is also an important function of cytosol. In prokaryotes, the cytosol is the site where most of the metabolic processes take place.

Organelle

Cellular organelles are the subcellular structures having a specific function. They may be membrane-bound organelles (enclosed within lipid bilayer) or non-membrane bound organelles (without surrounding lipid bilayer). The prokaryotic organelles are not enclosed by any lipid membranes. Some important cellular organelles are mitochondria, ER, Golgi apparatus, and vacuoles, etc.

Mitochondria

Mitochondria is present in almost all eukaryotes. It has a double membranous structure which divides it into two compartments. Mitochondria have their own DNA. The function of mitochondria is to produce energy in the form of ATP (Adenosine triphosphate) from the oxidation of glucose molecules.

Endoplasmic reticulum 

The endoplasmic reticulum is a single membranous structure present in all eukaryotes. It has two types; rough endoplasmic reticulum which has ribosomes on its surface and the smooth endoplasmic reticulum that has a tubular structure. The rough endoplasmic reticulum takes part in the translation of new proteins while the smooth endoplasmic reticulum plays important role in the expression of lipids. 

Golgi apparatus

Golgi apparatus is a single membranous structure present in all eukaryotic cells. It has a cis-face (convex) and a trance-face (concave). Golgi apparatus plays an important role in the sorting, packaging, processing, and modification of different types of proteins. 

Vacuoles

Animal cells have many small single membranous vacuoles, but the plant cells have a large single vacuole. Vacuoles do the function of storage, transportation and in plant cells, they provide shape to the cell. Vacuoles also help in maintaining homeostasis. 

Chloroplast

The chloroplast is a double-membranous structure that is present only in plants. It also has its own DNA which is theorized to have been engulfed by the parent cell. The main function of the chloroplast is the production of glucose by trapping solar energy, a process that is called photosynthesis. 

Cytoplasmic Inclusions

Cytoplasmic inclusions are the insoluble, non-living substances and molecules suspended in the cytosol. These inclusions are not bounded by any type of membranes. Cytoplasmic inclusions include glycogen granules, lipid droplets, pigments, and different types of crystals. Glycogen granules are present near the endoplasmic reticulum and they appear as clusters. These granules go through the process of glycogenolysis which produces glucose molecules. These glucose molecules are then used by mitochondria to produce energy.

Lipid droplets are present in the form of triglycerides mainly in adipocytes and hepatocytes. Lipids are an important source of energy as they yield twice as many calories per gram than does any form of carbohydrates. Important pigments that are present in the body are hemoglobin and melanin. Melanin is produced by melanocytes and also by some specialized nerve cells. A brown pigment called lipofuscin is also present in some cardiac tissues.

Functions

Cytoplasm along with its organelles does a lot of important functions. It is the place where the growth of cells happens, and the cell expands. Most of the enzymatic reactions need a medium that is provided by the cytoplasm. The cytoplasm also acts as a buffer and protect the organelles and nucleus from the damage caused by collisions. 

Glycolysis is an important metabolic process that produces energy in the form of ATPs. It takes place in the cytoplasm because all the enzymes necessary for the process are present in the cytoplasm. The cytoplasm also provides support to the cell and maintains its shape through filaments which makes the cytoskeleton. The cytoplasm is the medium through which intracellular signal transduction and transport of different molecules takes place.

The production of proteins by the translation process of messenger RNA also takes place in the cytoplasm. This is done by small non-membranous organelles called ribosomes which are present on the rough endoplasmic reticulum. 

Cytoplasmic streaming

Cytoplasmic streaming refers to the movement and flow of cytoplasm inside the cell. It is more evident in large animal and plant cells. The movement of chloroplasts to a suitable position for light absorption is an example of this phenomenon. Studies have shown that the intracellular temperature and pH greatly affect cytoplasmic streaming. Cytoplasmic streaming is also important in the distribution of different nutrients throughout the cell.

Cytoplasmic Inheritance

Cytoplasmic inheritance is the transmission of genetic material that does not involve the nucleus. That is why it is also sometimes called extranuclear inheritance. It happens in almost all eukaryotic cells. Mitochondria and chloroplast are the two organelles that have their own DNA. These two organelles are not inherited from both parents but only from the mother through cytoplasmic inheritance. 

The replication of the genomic material of mitochondria and chloroplast is not dependent on the cell division and it takes place according to the needs of the cell. The phenomenon of cytoplasmic inheritance also shows that mitochondrial diseases are inherited from the mother.

Summary

The cytoplasm is a sol-gel or semi-fluid type of material that is present in a cell bounded externally by the plasma membrane and internally by the nuclear membrane. The outer, non-granulated, clear, watery area is called ectoplasm while the inner, dense, granulated, and gel-like area is called endoplasm. All the organelles are present in the endoplasm. Cytoplasm behaves like a solid material when there is a greater concentration of cytoplasmic components and it behaves like liquid when there is a low concentration of cytoplasmic components. The physical nature of the cytoplasm depends upon the interactions between various cytoplasmic structures.

There are three types of filaments that make the internal skeleton of the cytoplasm that is also called the cytoskeleton. The smallest are called microfilaments that are made of actin protein. Intermediate filaments are somewhat bigger than microfilaments and they help in anchoring the cellular organelles. Microtubules are the biggest filaments of the cytoskeleton and they are made of alpha and beta tubulin. 

Three main constituents of cytoplasm include cytosol, organelles, and cytoplasmic inclusions. Cytosol refers to the inner fluid of the cytoplasm. Organelles are the special subunits within the cytoplasm which are designed by nature to perform specific functions. Important cellular organelles include mitochondria, chloroplast, Golgi apparatus, endoplasmic reticulum, and vacuoles. Cytoplasmic inclusions are small, insoluble particles and pigments which are present in the cytoplasm. For example, glycogen granules and melanin pigments.

Cytoplasm provides the medium for enzymatic reactions and metabolic processes of the cell. Glycolysis, translation of mRNA, and other important processes take place in the cytoplasm. The cytoplasm also protects and anchor the cellular organelles. Cytoplasmic streaming is the movement of cytoplasm inside the cell which is necessary for the positioning of some cellular organelles. The cytoplasmic inheritance is the inheritance of genetic material through the cytoplasm that is shown by mitochondria and chloroplast.

References

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