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What is a Cell?

Solitary cells are the most basic forms of life. Evolutionarily evolved organisms, such as humans and other vertebrates are made up of communities of cells that arose from a single founder cell through growth and division. Every animal or plant is made up of a large number of distinct cells, each of which has a unique function that is controlled by complex cell-to-cell communication systems. Thus it can be said that every function is coordinated by the cellular mechanism. In simpler terms, the cell is termed as the basic fundamental unit of life. 


The branch of science that deals with the study of cell and cell structure and function is known as cell biology, the branch also deals with the study of cellular communication. The article deals with the discussion of cells which is focused on what is cells, cell structure and function and the types of cells. The article also depicts the cell diagram for a better understanding of the concept.


What Defines a Cell?

As we have discussed earlier a cell is a cytoplasmic mass that is outwardly attached by a cell membrane. Cells are the smallest structural units of living matter and make up all living things. Since all cells are enclosed by a layer known as the cell membrane, it acts as a distinct barrier between both the cell's internal and exterior environments, just like the walls of a house. The plasma membrane is a term used to describe the cell membrane. 


The cell membrane, nucleus, and cytoplasm are the three primary components of a cell. The cell membrane encloses the cell and regulates the molecules that enter and exit it. The nuclei are a cell substructure that includes the nucleolus as well as the majority of the cell's DNA. It's also where the majority of RNA is produced. The cytoplasm is the liquid that fills the inside of the cell. The Golgi complex, mitochondria, and endoplasmic reticulum are only a few of the microscopic cell sections that have distinct tasks. The cytoplasm is the site of the majority of chemical processes and the production of most proteins.


Cell Theory

The scientific theory that defines that all living entities are made up of cells, the smallest functional unit, is known as cell theory. Botanist Matthias Schleiden and zoologist Theodor Schwann were researching tissues in late 1839 when they suggested integrated cell theory. According to the unified cell theory, all living things are made up of one or more cells, and the cell is the basic structural and functional unit of life. Later, Rudolf Virchow contributed significantly to this notion by claiming that new cells emerge from existing cells. The following are the important points  of contemporary cell theory that are widely accepted:


  • All cellular organisms are made up of one or more cells.

  • The cell is a structural and functional unit of life.

  • All cells arise from pre-existing cells.

  • A cell contains hereditary information in the form of DNA which is passed on from generation to generation.

  • All cells have basically the same chemical compositions.

  • Energy flow occurs within the cell. The energy flow is the term used to define metabolism or cellular metabolism.


The cell definition can be better understood by understanding the cell diagram mentioned below in the article.


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Types of Cells: Prokaryotic Cells and Eukaryotic Cells

Since we have understood what is a cell, cell theory led to the development of a comprehensive understanding of cell biology. Let us now look into what are the types of cells, the broadest classification of cells includes grouping into two categories, prokaryotes and eukaryotes. Scientists usually classify cells based on how their genetic material is packaged, rather than their size or shape. A prokaryotic cell does not have DNA surrounded by the nuclear envelope rather it is continuous with the cytoplasm. Single cells make up all known prokaryotes, such as bacteria and archaea.


A eukaryote, on the other hand, has its DNA separated from the cytoplasm by its own membrane-bound compartment called the nucleus. Amoebae, for example, are free-living, single-celled eukaryotes. Multicellular organisms contain other eukaryotic cells. Eukaryotic cells, for example, are found in all plants and animals.


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Cell Structure: Plasma Membrane

Since we have understood what is a cell, let us look briefly into the cell structure and function of the human cells to develop an overall understanding. Cell membranes are composed up of phospholipids, which are fat-based compounds that effectively inhibit liquid, or hydrophilic, compounds from entering and exiting the cell. Proteins that provide distinct activities are also embedded within those membranes. A few of these proteins serve as gatekeepers, determining which chemicals are allowed to pass across the membrane and which are not. Others serve as markers, indicating whether the cell belongs to a certain entity or is external. Others act as anchors, connecting cells together so that they can work as a unit. Other membrane proteins function as communicators, transmitting and receiving information from neighbouring cells and the environment.


Fluid Mosaic Model of Plasma Membrane

The plasma membrane structure is described as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates in the fluid mosaic model. S.J. Singer and Garth L. Nicolson proposed the fluid mosaic concept in 1972 to describe the organization of plasma membranes. The fluid mosaic model defines the plasma membrane's composition as a mosaic of molecules, including phospholipids, cholesterol, proteins, and carbohydrates, which offer it fluidity. Cell membranes have a width of 5 to 10 nm. Proteins, lipids, and carbohydrates make up different amounts in the plasma membrane depending on the cell type. Myelin, for example, is 18 per cent protein and 76 per cent lipid. The inner membrane of the mitochondria includes 76 per cent protein and 24 per cent lipid.


Phospholipids, cholesterol, proteins, and carbohydrates are connected to some of the lipids and proteins they serve as the main components of a plasma membrane. The membrane structure is mainly composed of the amphiphilic molecules, that is the hydrophobic parts of the phospholipids are found in the interior of the membrane, while the hydrophilic head groups are found in the external sides of the cytoplasmic and ECF matrix. Thus the plasma membrane is defined as a lipid bilayer, in which the proteins are embedded.


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Phospholipid molecules are composed of two fatty acid molecules attached to carbons 1 and 2 and a 3-carbon glycerol skeleton with a phosphate-containing group attached to the third carbon. In water, hydrophobic molecules tend to form spheres or clusters. The hydrophilic regions of phospholipids tend to form hydrogen bonds with water and other polar molecules, both inside and outside the cell. Therefore, the membrane surface facing the inside and outside of the cell is hydrophilic. In contrast, the centre of the cell membrane is hydrophobic and does not react with water.


Plasma membranes are made up of two major components: proteins and fats. Integral proteins (some of which are referred to as integrins) are entirely embedded into the cell membrane, and its hydrophobic surface portions interact with the phospholipid bilayer's hydrophobic region. A hydrophobic transmembrane region of 20–25 amino acids is generally seen in single-pass integral membrane proteins. Some merely cover a portion of the envelope and are associated with a single layer, while others reach from one side to the other and are exposed on both sides. Many intricate proteins are made up of up to 12 segments, each of which is coiled excessively and embedded inside the bilayer. This type of protein has one or more modestly hydrophobic regions and a hydrophilic area. The organization of protein sections tends to align the protein alongside the phospholipids, with the hydrophobic region next to the phospholipid tails and the hydrophilic region or parts extending out from membranes and in conjunction with the cytosol or extracellular fluid.


Cell Structure: Cytoplasm

The interior environment of a cell is water-based within this membrane. This liquid environment, known as cytoplasm, is densely packed with cellular machinery and structural elements. In reality, whether the outside is ocean water (as in the case of a single-celled alga) or blood serum, the concentrations of proteins inside a cell significantly outnumber those on the outside (as in the case of a red blood cell). Despite the fact that cell membranes act as natural barriers in watery environments, a cell must waste a significant amount of energy to sustain the high intracellular constituent concentrations required for survival. Indeed, cells may expend up to 30% of their energy simply to maintain their composition.


In conclusion of the article, we have learnt about what is a cell, human cell structure and function. We have also learnt about the types of cells, based on the nucleus diagram. We have also learnt about the structure of the plasma membranes.

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FAQs on Cell

1. Name two phospholipids found on the plasma membrane?

Phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine and phosphatidic acid are some of the phospholipids found in the plasma membrane. Phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine, are found in the inner leaflet of the plasma membrane while phosphatidylcholine and phosphatidyl myelin are found on the outer leaflet of the plasma membrane.

2. What is the function of mitochondria?

Mitochondria are a cellular organelle found in an animal cell, it is responsible for the generation of ATP.  It is a semi-autonomous organelle. It also plays an important role in the metabolism and apoptosis of the cell. Mitochondria also play an important role in cellular differentiation, cell signalling, cell senescence.