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Cell - Fundamental Unit of Life

IMPORTANT TERMS

Prokaryotic Cell: - It lacks a membrane bound nucleus. Membrane bound cell organelles are also absent. Their chromosomes are composed of only nucleic acid, and they have only very small ribosomes as organelles.

Eukaryotic Cell: - It has nucleus and other organelles enclosed within membranes. Their chromosomes are composed of nucleic acid and histone proteins

Animal Cell: - They do not have a cell wall and plastids. Plasma membrane is the outermost layer of animal cell.

Plant cell: - It has a cell wall composed of cellulose. Presence of a large central vacuole and plastids in the cytoplasm of plant cell, distinguishes it with other type of cell.

Plasmolysis: - Shrinkage or contraction of the contents of the cell away from the cell wall due to loss of water through osmosis is called plasmolysis.

Diffusion: - It is a process in which the movement of a substance always occurs from higher concentration to lower concentration.

Osmosis: - It is a special kind of diffusion in which water molecule move from the low concentration solution to high concentration solution through the semi permeable membrane. 

Semipermeable: - Plasma membrane that allows passage to some selected substances. It is also called a selectively permeable membrane.

                                          

Gene: - It is functional unit of DNA which is transferred from a parent to offspring.

Plastids: - The plastid is a double-membrane cell organelle found only in the plant cell. Plastid containing Chlorophyll is called chloroplast.

Cell

  • A cell is capable of carrying out all the functions such as nutrition, respiration, excretion, transportation and reproduction the way an individual organism does. These functions are necessary for survival of a living being. Hence a cell is called the functional unit of life.
  • Cell is the smallest unit of life and all the living beings are made up of cells. Therefore, a cell is considered as the structural unit of life.

Cell theory

Matthias Schleiden and Theodor Schwann formulated the cell theory in 1839 which is    described as below:

  • All living organisms are composed of one or more cells. (However, this is not true for virus as it has non-cellular life).
  • The cell is the basic unit of structure and organization in organisms.
  • The cell theory was further expanded by Rudolf Virchow (1855) by suggesting that all cells arise from pre-existing cells

Types of organisms on the basis of no. of cells

On the basis of number of cells, organisms are classified into two types:

  • Unicellular organisms: The organism that are made up of single cell and may constitute a whole organism are called unicellular organisms E.g., Amoeba, Paramecium, bacteria etc.

 

  • Multicellular organisms: Multicellular organisms are organisms that consist of more than one cell.  In such organisms, cells are usually specialized. All the cells with the same function work together in a coordinated member. E.g., Plants, Animals, Human beings etc.

 

Structure of cell

  • Cells vary in shape and size. While most of the cells are spherical in shape, cells of various other shapes like oval, rectangular, spindle shape are also found.
  • The size of the cell also varies in different organisms. Most of the cells are microscopic in size like red blood cell. Some cells are fairly large like nerve cells.

Types of cell

There are two basic types of organisms based on cell type

  1. Prokaryotic cell:
  • They are always unicellular, primitive and incomplete cell.
  • They are generally small and its size vary from one to ten microns.
  • Their Nuclear region is poorly defined due to absence of nuclear membrane.
  • This nuclear region contains a single chromosome and called nucleoid.
  • Membrane bound cell organelles are absent.
  • The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles but not with plastids as in eukaryotic cells.
  • Examples of prokaryotic cells are: Archaebacteria, blue green algae, bacteria.

 (B)  Eukaryotic cell

  • They may be either unicellular or multicellular.
  • They are generally large and its size vary from five to hundred microns.
  • Nuclear region is well defined and surrounded by a nuclear membrane and it is called nucleus.
  • More than one chromosome is present in the nucleus.
  • Membrane bound cell organelles are present.
  • All organisms except blue green algae, bacteria, and Archaebacteria.

Cell walls

  • Cell walls are present in almost all prokaryotes (except mycoplasma bacteria), in algae, plants and fungi. 
  • Cell wall of plants is mainly composed of the polysaccharides cellulose whereas bacterial cell wall is composed of peptidoglycan.
  • The cell wall is rigid and it lies outside the cell membrane (plasma membrane) in plant cells, fungi, bacteria.
  • The cell wall of most plant cells is freely permeable to small molecules including small proteins.
  • The functions of the cell wall:
  • The cell wall provides mechanical strength and support to the cell.
  •  Cell wall withstand turgor pressure. Turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture.
  • 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.
  • Cell wall protects cell membrane and the internal structures of the cell.
  • It provides rigidity and determines the shape of the plant cell.
  • It prevents drying of the cell and helps it to bear unfavorable conditions.

 

 

Plasma membrane (Cell membrane)

  • In plant cells, it is present just below the cell wall while in animal cell, it forms the outer cover. It encloses entire cell and its organelles.
  • It is a very thin, delicate, elastic and selectively permeable, flexible layer of the cell.
  • It is composed of bilayer of lipid and protein.
  • The structure of the plasma membrane can be seen only through an electron microscope.
  • Functions of Plasma Membrane: 
  • Plasma membrane provides a container to the cytoplasm. As it is selectively permeable membrane, it facilitates passage of limited substances in and out of the cell.
  • The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes.

Protoplasm

  • It is the fluid substance of the cell surrounded by plasma membrane.
  • Living content of protoplasm includes cytoplasm and nucleus.

Nucleus

  • It is covered by double membrane called nuclear membrane and the fluid present inside the nucleus is called nucleoplasm.
  •  It serves as the information processing and administrative center of the cell. Nucleus contains chromosomes which are composed of deoxyribonucleic acid (DNA) and proteins.
  • DNA exists in chromatin form in the nucleus. The chromatin condenses to form Chromosomes during cell division.
  • Chromosomes are visible as rod-shaped structures during cell division.
  • Chromatin material is visible as entangled mass of thread.
  • Gene is the functional unit of DNA which is transferred from a parent to offspring.
  • Nucleus coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).

Cytoplasm

  • It is the material within a living cell without the cell nucleus.
  • It comprises cytosol (A gel-like substance enclosed within the cell membrane) and the organelles (the cell's internal sub-structures).

 

 

Cell Organelle

  • Cell organelle is a specialized sub-unit within a cell that has a specific function.
  • Important cell organelles are endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles. They are important because they carry out some very crucial functions in cells.
  • Presence of these cell organelles in the eukaryotic cells distinguish them from prokaryotic cells.

Endoplasmic Reticulum (ER)

  • Endoplasmic reticulum is a mesh-like structure which is composed of numerous tubes.
  • It is attached to nucleus and extend thereof like being suspended in the cytoplasm.
  • There are two kinds of endoplasmic reticulum, viz. smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER).
  • Rough ER has ribosomes on its surface which gives it the rough appearance. The ribosomes are the sites of protein manufacture.
  • The SER helps in the manufacture of fat molecules or lipid which is important for cell function.
  • Some of the proteins and lipids synthesizing in the RER and SER respectively is used in building the cell membrane. This process is known as membrane biogenesis.
  • SER of liver cell plays a crucial role in detoxifying many poisons and drugs.
  •  Function of ER:
  • ER serves the role of packaging many substances in the cell.
  • It serves as the transport channel of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
  • The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

Golgi apparatus

  • It is also known as the Golgi complex, Golgi body or simply the Golgi.
  • Golgi Complex was discovered by Camillo Golgi.
  • It consists of a set of membrane bound, fluid filled vesicles, vacuoles and flattened cisternae (closed sacs). Cisternae are usually arranged parallel to each other.
  • Function of Golgi appratus:
  • Its main function is to store, modify, package and dispatch the substances.
  • It is also involved in the formation of lysosomes.

 

Ribosome

These are tiny dot like structures interspersed in the cytoplasm and also on the surface of Rough ER. It is made up of ribonucleic acid (RNA) and protein. It is responsible for protein synthesis.

Lysosomes

  • It is a small sac-like structures and it is derived from Golgi complex.
  • Lysosome contains hydrolytic enzymes made by RER that can break down virtually all kinds of biomolecules.
  •  Functions of lysosome:
  • It helps in digestion of foreign substances and worn out cell organelles.
  • It provides protection against bacteria. It is the last compartment of endocytic pathway in which foreign matter is digested by its own enzymes.
  • Lysosome digests the cell itself when it becomes unstable and damage, due to this specific function, it is called suicidal bag of the cell.

Mitochondria

  • It is a rod-shaped organelle.
  • It is a double membrane structure. Outer membrane is smooth and porous whereas its inner membrane is projected into numerous finger-like structures; called cristae.
  • It has its own DNA and ribosomes and thus are able to make some of their own proteins.
  • Functions of mitochondria:- 
  • It is the sites of cellular respiration and thus provide energy for the vital activities of the cell.
  • Energy produced during cellular respiration is stored in the form of ATP (Adenosine triphosphate). ATP is known as the energy currency of the cell. Therefore, mitochondria are also known as the ‘powerhouse of the cell’.

Plastids

  • The term ‘plastid’ was given by Haeckel in 1866.
  • Plastids occur in most plant cells and are absent in animal cells.
  • Plastids have their own DNA and ribosome like mitochondria.
  • On the basis of colour, plastids are of following three types: Chloroplasts, Chromoplasts and Leucoplasts.
  • Structure of chloroplasts:
  • These are spherical or discoid in shape and are enclosed in double membrane.
  • Two definite regions are clearly visible- grana and stroma inside a chloroplast. Grana are stacks of membrane-bounded, flatterened discoid sacs containing the molecules of chlorophyll. Stroma is the homogeneous matrix in which grana are embedded.
  • Functions of plastids:
  • Chloroplasts are photosynthetic organelles. The chlorophyll present in them trap solar energy for the purpose of synthesizing food for the plant. Hence chloroplasts are also called ‘Kitchens of the cells’.
  • Leucoplasts help in storing food products like starch, protein and lipids.
  • Chromoplasts provide colour to flower which in turn attracts insects for pollination

Vacuoles

  • These are fluid-filled or solid-filled and membrane bound spaces in the cytoplasm.
  • Plant cells have very large vacuoles while in animal cells the size of vacuoles is small.
  • It is bounded by a membrane, called tonoplast.
  • It is filled with cell sap which is watery solution rich in sugar, amino acids, proteins, minerals and metabolic wastes.
  • The central vacuole of some plant cells may occupy 50-90% of the cell volume.
  • Functions of vacuoles:
  • These are meant for the storage of food, water and other substances.
  • They help to maintain the osmotic pressure in a cell (osmoregulation). They   provide turgidity and rigidity to the plant cells.

Transport of substance through plasma membrane

  • Substance can pass through plasma membrane by two processes:

(A) Diffusion:

  • Diffusion is spontaneous movement of a substance from a region of its high concentration to a region of low.
  • It plays an important role in gaseous exchange (oxygen and carbon dioxide) between the cells as well as the cell and its external environment which always occurs from a region of high concentration to a region of lower concentration.

(B) Osmosis:

  • It is the passage of water from the region of high water concentration to region of low water concentration through a semi-permeable membrane is called osmosis.

 

  • If the medium surrounding the cell has a higher water concentration (Hypotonic solution) than the water concentration in the cell, the cell will gain water by osmosis and the cell is likely to swell up.
  • If the medium has exactly the same water concentration as the water concentration in the cell (Isotonic solution), no net movement of water across the cell membrane take place. The cell will remain in the same size.
  • If the medium surrounding the cell has a lower water concentration (Hypertonic solution) than the water concentration in the cell, the cell will lose water by osmosis and as a result the cell will shrink.

History of discovery of cells

  • Robert Hooke was the first to discover cell (1665).
  • Leeuwenhoek was the first to discover free living cells in pond water (1674).
  • Robert Brown discovered the nucleus (1831).
  • Purkinje coined the term ‘protoplasm (1839).
  • Schleiden (1838) and Schwann (1839) proposed the Cell Theory.
  • Virchow (1855) made further addition to the cell theory.
  • The discovery of electron microscope (1940) made it possible to study the structures of cell organelles.

 

Interesting Facts:

  • Almost four billion years ago, the first known cell originated on earth which was a prokaryote.
  • The mitochondria can quickly change shape and move around the cell when needed.
  • The mitochondria can reproduce by growing larger and then dividing, when the cell needs more energy.
  • Red Blood cell lack a cell nucleus and most organelles, in order to accommodate maximum space for hemoglobin.
  • The largest cell in the human body is the female egg, (ovum); it's 1000 micrometers in diameter.
  • The longest cells in the human body are neurons (nerve cells).
  • The largest cells found in nature are ostrich eggs which can weigh up to 5 pounds.
  • Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large (50S) subunit. Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit.