Cell: Fundamental Unit of Life

Fundamental Unit of Life 

Cell : The functional and structural unit of life is called cell. 

·         This is fundamental unit of all living things.

·         All living things are made up of cells.

·         For example - A brick works as structural unit to build a building similarly construction of a living thing body to add one by one cells to each others. 

Cells

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Tissues 

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Organs

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Body system

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Body

Cell is structural unit:

Cell provides structure to our body therefore it is structural unit of the body.

Cell is function unit:

All functions of the body take place at cell level, therefore It is called the functional unit of body.  

Discovery of cell: Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. 

Robert Brown in 1831 who discovered the nucleus in the cell.

Cell theory: 

All the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells.  

Type of organisms on the basis of Cell:

Unicellular Organisms: The organisms that are made up of single cell and may constitute a whole organism. such organisms are called unicellular organisms. Example- Amoeba, Chlamydomonas, Paramecium and bacteria etc.

(B) Multicellular Organisms: Those organisms in which many cells group together in a single body and assume different functions in it to form various body parts. Such organisms are called multicellular organisms. Example - fungi, plants, animals etc.

* Every multi-cellular organism has come from a single cell.

* Cells divide to produce cells of their own kind. 

* All cells thus come from pre-existing cells.

* Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms.

The shape and size of cells are related to the specific function they perform.

(i) Some cells like Amoeba have changing shapes.

(ii) In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell; for example, nerve cells have a typical shape.

Name of some cell present in human body:

(i) Nerve cell 

(ii) Blood cell

(iii) Fat cell

(iv) Bone cell

(v) Muscular Cell

(vi) Reproductive Cell   (a) sperm (b) Ovum

Parts Of Cell:

(i) Plasma Membrane Or Cell Membrane: It is outer most covering of the cell that separates the contents of the cell forms its external environment. It is flexible and made up of doubled layer organic molecules like lipids and protein. 

Flexibility of Cell Membrane:  

The flexibility of cell membrane helps to obtain foods and other materials from their our environment in unicellular organisms like Amoeba. Due to this flexibility cells able to change their shapes and they phagocyte the food materials with the help of pseudopodia.

The process of taking food in amoeba or other unicellular organisms by using flexibility of cell membrane is known as endocytosis or phagocytosis. 

Functions of cell membrane:  

(i) It separates the cytoplasm from outer environment.  

(ii) It protects cell from foreign elements. 

(iii) It allows or permits the entry and exit of some selected materials like water, CO₂ and O₂ in and out of the cell. 

(iv) It also prevents movement of some other materials. 

(v) The processes of diffusion and osmosis also take place through cell membrane.  

Plasma membrane is selectively permeable membrane: 

Plasma membrane allows or permits the entry and exit of some selected materials like water, CO₂ and O₂ in and out of the cell and it also prevents movement of some other materials, therefore is called a selectively permeable membrane. 

Some selected materials like carbon dioxide and oxygen may transport  in and out through cell membrane by process of diffusion and water substances by osmosis. 

The law of movement of materials: Materials move high concentration to low concentration of material.  

Diffusion:  Diffusion is a process that takes place in cells in which the transportation of gaseous molecules like carbon dioxide and oxygen take place through selectively permeable membrane. This process is known as diffusion. 

The proccess of diffusion in cells: some cellular wastes substance like CO₂ (which is cellular waste and requires to be excreted out by the cell) accumulates in high concentrations inside the cell. In the cell’s external environment, the concentration of CO₂ is low as compared to that inside the cell. As soon as there is a difference of concentration of CO₂ inside and outside a cell, CO₂ moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process of diffusion. Similarly, O₂ enters the cell by the process of diffusion when the level or concentration of O₂ inside the cell Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.

Osmosis: The movement of water molecules takes place through selectively permeable membrane such process is known as osmosis.

As gaseous exchanges occurs by the process of diffusion similarly osmosis follows the same rule. The movement of water molecules takes place through selectively permeable membrane from high concentration to low concentration.  

(ii) Nucleus: 

Nucleus is the largest organelle of cell which locates inside the cell. The nucleus has a double layered covering called nuclear membrane. 

Nuclear Membrane: This is a outer and double layer covering of nucleus. Nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside. 

Chromosomes: The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribose Nucleic Acid) molecules.  

·         Chromosomes are composed of DNA and protein.

·         DNA molecules contain the information necessary for constructing and organizing cells.

·         Functional segments of DNA are called genes.

·         In a cell which is not dividing, this DNA is present as part of chromatin material.

Chromatin: Chromatin material is visible as entangled mass of thread
like structures. Whenever the cell is about to divide, the chromatin material gets organized into chromosomes.

Cell Division: 

Cell division is a process in which a  single cell divides to form two new cells. 

The role of nucleus in cellular generation:

·         Nucleus divides into two parts to form new cells during cell division. 

·         New cells have remain all characteristics of parent cell. 

·         It determines the development and maturity of a cell.  

·         It also detects chemical activities of the cell. 

·         The nucleus plays a central role in cellular reproduction. 

Nucleoid: In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

Functions of nucleus: 

(i) The nucleus plays a central role in cellular reproduction.

(ii) It divides itself to form new cells during cell division.

(iii) It carries chromosomes, DNA as well as gene with all information’s in order to synthesize protein . 

(iv) It controls all activities of cell therefore it is called the brain of cell. 

(A) Prokaryotes: Such organisms, whose cells lack a nuclear membrane, are called prokaryotes. E.g. - bacteria etc.  

(B) Eukaryotes: Organisms with cells having a nuclear membrane are called eukaryotes. e.g. all multicellular organisms.  

There are two types of cell on the presence of nuclear membrane: 

(I) Prokaryotic cell: Cells those do not have nuclear membrane are called prokaryotic cells. 

(II) Eukaryotic cell: Cells those have clear nuclear membrane are called eukaryotic cells. Eg. Algee and cells of other all multicellular organisms.  

Prokaryotic Cells	Eukaryotic Cells
1. It is often small in size.   2. Nuclear membrane is absent. |  3. It has often single chromosomes.   4. Many of organelles do not exits.  5. The chlorophyll is found with membranous vesicles in organisms of photosynthesis.    6. There is no membrane-bound organelles as well as nuclear membrane.	1. It is often large in size.   2. Nuclear membrane is present.   3. It has often more than one chromosomes.   4. There exits about all organelles.    5. Chlorophyll is in plastid.   6. There is present nuclear membrane as well as membrane-enclosed organelles.

There are two types of cell on the basis of organisms:

Plant cell	Animal cell
1.      It has cell wall. 2.      Chloroplast is present. 3.      Vacuoles are large in size. 4.      The process of photosynthesis occurs in it.	1.      It has not cell wall. 2.      Chloroplast is not present. 3.      Vacuoles are mostly absent or present is very small size. 4.      The process of photosynthesis doesn’t occur in it.

(iii) Cytoplasm: A large  region of each cell enclosed by the cell membrane and filled with a special kind of fluid. It is called the cytoplasm. This region contains all the organelles. 

organelles: Cytoplasm of each cell contains many specialized microscopic structures. Each of these microscopic structure performs a specific function for the cell. These are known as cell organelles. e.g. - Mitochondria, Golgi apparatus, Ribosomes, Lysosomes and vacuoles.

Protoplasm: The region joining cytoplasm and nucleus together is called protoplasm.

Protoplasm contains all the organelles of a cell. 

Functions of Cell Organelles: 

(i) Building up new substances.

(ii) Cleaning up waste materials and removing other materials.

(iii) Storing energy for cell.  

There are the same organelles in various functioned cell whether it is any cell. 

The significance/utility of membranes : 

Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.

CELL WALL :

Cell wall is only found in plant cells, which is mainly made up of cellulose. It provides structural hardness to the plants due to cellulose.

Cellulose: It is a special kind of complex carbohydrate, which is only found in plants and it provides structural hardness to the plants. All herbivores digest cellulose easily while human intestine does not do so. This is so that human intestine is shorter than other herbivores animals.  

Plasmolysis: When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

Type of solutions on the basis of concentration:

 

(I) Hypotonic solution: If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution.

The Consequence of this the cell is likely to swell up.

(II) Isotonic Solution: If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution.

There is no overall movement of water in this solution, so the cell will stay the same size.

(III) Hypertonic Solution: If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.

The consequence of this solution water moves from inside to outside causes cell will shrink. 

The Absorption of water by plant's roots: Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.

Significance of diffusion in the life of cell: 

(i) Diffusion is important in exchange of gases and water in the life of a cell.

(ii) The diffusion also helps to cell to obtain nutrients from its environment. 

(iii) Different molecules move in and out of the cell through a type of transport that is diffusion. 

(iv) Absorption of water by plant roots also takes place by another similar process osmosis.

Plants Cells can easily withstand much greater changes in the surrounding medium than animal cells.

Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting. In such media the cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell. Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells. 

Cell Organelles: 

1. Endoplasmic Reticulum (ER):

The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane.

There are two types of ER:

(I) Rough endoplasmic reticulum (RER): RER looks rough under a microscope because it has particles called ribosomes attached to its surface.

(a) The manufactured proteins are then sent to various places in the cell by RER. 

(b) 

(c) Ribosomes are present in RER.  

(II) Smooth endoplasmic reticulum (SER): 

(a) SER helps in the manufacturing of fat molecules and lipids. 

(b) Some of these proteins and lipids help in building the cell membrane.

(c) Ribosomes are not present in SER. 

Function of Endoplasmic Reticulum: 

(i)  It creates a network systems between cytoplasm and nucleus. 

(ii) It works as a pipeline for transportation of protein between cytoplasm and nucleus. 

(iii) ER also works as cytoplasmic framework providing a surface for some cellular biochemical process. 

(iv) SER plays a crucial role in detoxifying many poisons and drugs in liver cells. 

(v) SER helps in the manufacturing of fat molecules and lipids. 

Membrane biogenesis: Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis.

2. Golgi apparatus : 

The first Camillo Golgi described this cell organelle. This is a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns.

Functions Of Golgi apparatus: 

(i) This forms another portion of a complex cellular membrane system to connect with ER. 

(ii) ER functions include the storage, modification and packaging of products in
vesicles.

(iii) Complex sugars may be made from simple sugars in the Golgi apparatus.

(iv) The Golgi apparatus is also involved in the formation of lysosomes. 

Black Reaction: Camillo Golgi carried out was a revolutionary method of staining individual nerve and cell structures. This method is referred to as the ‘black reaction’. This method uses a weak solution of silver nitrate and is particularly valuable in tracing the processes and most delicate ramifications of cells. 

3. Ribosome:

 

Ribosomes are rounded structure which are mainly locate freely in cytoplasm or may be attach with Endoplasmic reticulum. It is also called the protein factory of cell as they manufacture protein.

Functions of Ribosome:

(i) It is made up of RNA (Ribonucleic-acid).

(ii) It forms protein from amino-acid.

(iii) It provided surfaces for cellular biochemical activities.

4. Lysosome: 

Lysosomes are a kind of waste disposal system of the cell. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces. Lysosomes are able to do this because they contain powerful digestive
enzymes capable of breaking down all organic material.

The suicide bags:

During the disturbance in cellular metabolism, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.

Functions of Lysosomes:  

(i) It cleans cells to digest cellular wastes.

(ii) Its highly strong enzymes digest such as bacteria or food, as well as old organelles end up in the lysosomes.

(iii) It removes dead and damaged cells from body. 

5. Mitochondria:

Mitochondria have two membrane coverings instead of just one. The outer membrane is very porous while the inner membrane is deeply folded. Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.

The Powerhouses: 

The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphosphate) molecules. ATP known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work. Hence, ATP is a cellular energy that is manufactured and stored in mitochondria therefore Mitochondria is called powerhouses of the cell. 

Adenosine triphosphate (ATP): ATP is a cellular energy, which is used in various biochemical process to synthesize new molecules. 

Functions of mitochondria: 

(i) It provide energy to the cell in the form of ATP. 

(ii) There are many important enzymes for cellular respirations. 

(iii) It makes some of their own proteins.

(iv) The manufacturing and storage of cellular enrgy is also taken place by mitochondria.  

6. Plastids:  

Plastids are present only in plant cells. The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma. Plastids are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.

There are three types of plastid: 

(I) Chromoplasts (coloured plastids): Chlorophyll is not present in it and it does not take part in the process of photosynthesis. Its main work is to make beautiful to the plant. It mainly locates in fruits and petals of flowers.

(II) Leucoplasts ( White or Colorless plastids): Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored. They locate in those parts of plant where the photosynthesis does not occur, because they have not green pigments chlorophyll.  

(III) Chloroplasts: Plastids containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.

Chlorophyll: The green pigment which is mainly found in leaves of plants is known as chlorophyll. It takes part in the photosynthesis.

Functions of plastids:

(i) There are various colors in the various parts of plants due to various types of plastids.

(ii) The process of photosynthesis takes place in the presence of having green pigment plastid named chlorophyll.

(iii) Leucoplasts store starch, oils and protein granules in the form of stored products. 

7. Vacuoles: 

Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.

In plant cells vacuoles are full of cell sap and provide turgidity (swollen) and rigidity (hardness) to the cell.

Functions of Vacuoles:

(i) They provide turgidity and rigidity to the cell. 

(ii) They store many important substances for plants like amino acids, sugars, various organic acids and some proteins. 

(iii) Specialized vacuoles also play important roles in expelling excess water and some wastes from the cell in unicellular organisms.