Cell - the fundamental unit of life
In this article you will find a detailed description about the cell which is the fundamental unit of life. You will also get to know about the different cell organelles and the functions of these organelles which help in the existence of the cell.
Introduction
All living organisms are made of up small basic structures called cells. These were first observed by a scientist named Robert Hooke in 1665 while he was observing a cork slice under primitive microscope. Later on Leeuwenhoek observed the living cells in pond water with the help of an improvised microscope in 1674. Various other scientists later on discovered the other organelles of the cells in addition to the main discovery. Thus Robert Brown discovered the nucleus in 1831 and in 1839 a scientist named Purkinje discovered protoplasm and coined the term to the fluid content inside the cell.
Later on two biologists named Schleiden in 1838 and Schwann in 1839 presented the cell theory which states that all the plants and animals are made up of cells which are the basic units of life. Later on in the year 1855 another scientist named Virchow gave an expansion to the cell theory stating that all cells arise from their pre-existing cells. The invention of electron microscope in 1940 made it possible to observe and understand the complex structure of the cell and its organelles. Cell - the fundamental unit of life
A single cell is smallest structure in a living organism that capable of performing the basic life processes like taking in food, digesting the food, respiring, excreting the wastes, reproducing etc that are the main characteristic features of all living organisms. Hence cells are considered as the basic structural and functional units of life. As a single cell which is the smallest structural unit that is capable of performing all the functions, it is considered as the fundamental unit of life. A cell is a very complex structure of the living organism which is marvelously designed to carry out the various biological processes.Number of cells in an organism
The number of cells present in an organism varies. There are some organisms whose entire body is made up of a single cell while there are other organisms whose body is made up of millions of cells. Basing on the number of cells in the organisms, these are broadly classified into two categories:
These are the single-celled organisms whose entire body is made of only one cell. Hence such organisms are called unicellular ('uni' means one, 'cellular' means cell). Examples of such unicellular organisms are Amoeba, Paramecium, Bacteria and Chlymadomonas
Multi-cellular ('multi' means 'many') organisms are the organisms whose body consists of a number of cells. For e.g. hydra, plants and animals.
Multi-cellular organisms are further divided into two groups basing on the grouping of cells:
These are the simple or lower organisms like hydra whose body is made of many cells that are not well organised. But these cells are formed in groups called tissues which perform the basic functions. Such organisms are called the simple multi-cellular organisms.
The organisms like plants and animals are termed the higher organisms as the body which is a complex structure in which the cells are perfectly organised into groups of tissues, organs and organ systems to form the body of the organism.
All the basic functions like nutrition, digestion, excretion and reproduction are performed by the single cell in the unicellular organisms like amoeba. While there is a division of labor in higher multi-cellular organisms where these functions are performed by specialized organ systems. However as per the cell theory it is said that every multi-cellular organism has evolved from a single cell and as the cells divide to produce their own kind, it is found that all the cells come from pre-existing cells. Shape and size of the cells
Cells are of different sizes and different shapes. Mostly the shape of a cell is determined by its function. Cells considerably vary in their size. There are cells that are as small as 0.00001 mm (0.000004 inches) in diameter like a bacterium called mycoplasm which is the smallest known cell. While there are cells as long as 3 m (9.7 ft) like the nerve cells in the giraffe's neck that are the largest known cells.
Cells in the human body display a variety of sizes like the red blood cells which are 0.00076 small and on the other hand the liver cells are almost 10 times larger than the RBC.
There is a wide variety in the shapes of the cells. Generally, the cells get their shape basing on the function they perform. Accordingly there are cells that are round while there are others that are spherical or elongated. Yet other cells are long and pointed at both the ends. There are even some cells which have no specific shape but the shape keeps changing basing on the function it performs. Example of such a shapeless cell is amoeba which keeps changing its shape depending on the function it does. So also the White Blood Cells (WBC) in the human body are irregularly shaped cells which keeps changing.
Each living cell is capable of performing certain basic functions which is possible due to the various components present inside a cell. These specific components which are responsible for performing specific functions inside a cell are called the cell organelles. Thus due to the specific functions of these various organelles, a cell on the whole is able to live and perform various functions of the organism to survive. All the organelles together in a cell form the fundamental and basic unit of life.Structural organization of a cell
Each cell is composed of three basic components viz. the plasma membrane also called the cell membrane, the nucleus and the cytoplasm. All the life processes inside a cell and the interactions of a cell with its external environment are possible due to these main components. In addition to these basic components, the cell consists of other components which are called the cell organelles and each organelle is responsible for carrying out specific function in the cell.Plasma membrane or cell membrane
This is the outermost covering of the cell which protects all the internal organelles of the cell from the external environment. The membrane allows the entry and exit of some selective materials only and prevents the entry or exit of some other materials. Hence the membrane acts as a semi permeable membrane and is called selectively permeable due to this function.
Gaseous substances like carbon dioxide and oxygen move in and out of the cell through a process called diffusion. For instance when carbon dioxide which is a cellular waste gets accumulated inside the cell thus raising its concentration and when the concentration of CO2 outside the cell is comparatively lower, then due to the difference in concentration of CO2, it starts moving from inside the cell to outside i.e from a region of higher concentration to a region of lower concentration till it becomes equal in concentration. And the cell membrane plays an important role in diffusion of materials.
So also water molecules move from a region of higher concentration to a region of lower concentration through cell membrane. And such a movement of water molecules from a region of high water concentration to a region of low water concentration through a semi permeable membrane is called Osmosis.Cell Wall
Cell wall is present only in the plant cells in addition to the plasma membrane. The cell wall which lies outside the the plasma membrane in plant cells are mainly composed of cellulose. Cellulose is a complex substance which provides strength to the plant cells.Nucleus
Nucleus is the important organelle of a cell which is generally placed at the center of the cell and it has a double-layered porous covering called the nuclear membrane. This membrane too like the cell membrane allows the entry and exit of certain materials from inside the nucleus to its outside and the other way.
Nucleus is the organelle which contains chromosomes that are composed of DNA (Deoxyribo Nucleic Acid) molecules and protein molecules. These DNA molecules in the nucleus contain the information necessary for constructing and organizing the cells. The functional segments of DNA are called genes which play a key role in the inheritance of features from parents to the next generation.
Chromosomes are visible as rod-shaped structures at the time of cell division. In a cell that is not undergoing cell division, this DNA is present as part of chromatin material which appears as entangled mass of thread like structures. At the time of cell division, this chromatin material gets organised into chromosomes. Thus nucleus plays a key role during cell division.
In primitive organisms like bacteria and blue green algae, the nucleus lacks a nuclear membrane. Such cells are called the prokaryotic cells and the organisms with prokaryotic cells are called prokaryotes. The meaning of pro is primitive and karyon means nucleus. On the other hand the cells that have a nucleus with a definite nuclear membrane are called the eukaryotic cells and the organisms with eukaryotic cells are called eukaryotes. Almost all the cells in higher organisms are eukaryotic cells.Cytoplasm
The fluid content inside the plasma membrane of the cell is called the cytoplasm which contains many specialized structures called the cell organelles. Each organelle performs a specific function for the cell. These organelles are enclosed by membranes.Endoplasmic Reticulum (ER)
In a cell, the endoplasmic reticulum (ER) for the largest network and these look like long tubules. The membrane of ER is similar in structure to the plasma membrane. There are two types of ER:
Ribosomes are the sites of protein manufacture in a cell. The proteins that are manufactured here are sent to different places of the cell depending on its protein requirement. The SER helps in manufacturing fat molecules / lipids that are important for the functioning of cell. Some of the proteins and lipids help in the building of the cell membrane. Some other proteins function as enzymes and hormones. The ER serves as a network of channels which help in the transport of materials between various regions of the cytoplasm or between the cytoplasm and the nucleus.Golgi Apparatus
Golgi apparatus is involved in the formation of lysosomes. It consists of a system of membrane-bound vesicles that are arranged in parallel rows called cisterns. These membranes often are connected to the membranes of ER. The material that is synthesized near the ER is packaged and dispatched to various places inside and outside the cell through the Golgi apparatus. The functions of Golgi Apparatus are storage, modification and packaging of products in vesicles. In some cells, complex sugars may be made from simple sugars in the Golgi apparatus.Lysosomes
Lysosomes are the membrane-bound sacs that are filled with powerful digestive enzymes which are made by Rough Endoplasmic Reticulum. Lysosomes help in the disposal of wastes from the cell and thus helps in keeping the cell clean by digesting the foreign materials and worn-out organelles in the cell. Lysosomes contain powerful digestive enzymes that are capable of breaking down all organic material into small pieces.
When the cell gets damaged, the lysosomes may burst and the enzymes in the lysomes digest their own cell. Hence, lysosomes are called the suicide bags of a cell.Mitochondria
This is the most important organelle of the cell and is known as the powerhouse of the cell as it releases energy in the form of ATP (Adeonsine Tri Phosphates)molecules that is required for various activities in the cell. Each mitochondrion (singular form of mitochondria) is covered with two membrane. The outer porous membrane and the inner folded membrane. These inner folds create a large surface area for the chemical reactions to generate ATP molecules. Mitochondria have their own DNA and ribosomes as well and hence these are able to make some of their own proteins.Vacuoles
Vacuoles are the sacs that help in the storage of solid or liquid content inside the cell. Animal cells have smaller vacuoles while plant cells have larger vacuoles.
Vacuoles in plant cell are full of cell sap and these provide rigidity and turgidity to the cell. Many important substances like amino acids, sugars and other organic acids are stored in these vacuoles.
In unicellular organisms like amoeba, the food vacuole contains the food materials while in other unicellular organisms these vacuoles help in expelling wastes and excess water from the cell.Plastids
Plastids are the organelles that are present only in the plant cells. These are of two types:
Plastids that contain the green colored pigment (chlorophyll) are called the chloroplasts which are very important for the process of photosynthesis. There are even chloroplasts that contain yellow or orange colored pigments in addition to chlorophyll. While leucoplasts help in the storage of materials like starch, oils and proteins.
The inside of plastids have various membrane layers which are embedded in a material called stroma. Plastids look like mitochondria and these have their own DNA and ribosomes.Conclusion
Thus, each cell with its organelles and membrane is capable of performing specific function. And thus the cell is the fundamental unit of living organisms and it is also the basic functional unit of life.
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