What is Cell?
Cell is called a structural and functional unit of life of all living organisms capable of independent existence and can perform all functions of life.To see cells clearly we need a microscope. Larger cells can be seen through simple microscope but to see smaller cells we require compound microscope.
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Simple microscope can magnify image 50 to 10 times but a compound microscope can do so 1000 times or more. In the microscope we use in the laboratory, a beam of light is used to make things visible hence it is light microscope. To see interior of cell we need electron microscope. It can magnify image 500000 times.
Some Basic information about cell
There is no typical shape of a cell. Cells may be spherical, rectangular, flattened, polygonal, oval, triangular, conical, columnar, etc.Cell size varies greatly in various plants and animals. Some of them are not visible to naked eye.
Some are barely visible while some are macroscopic. The smallest cell size can be seen in mycoplasma (0.3 µm in length), bacterial cell size is 3 to 5 µm, while the largest size of cell is seen in Ostrich egg (nearly 15cms). Longest cells are nerve cells. You already know that cell theory was proposed by Schwann and Schleiden.
However, in this theory, there was no explanation about formation of new cells. It was Rudolf Virchow (1855) who explained for the first time that new cells are formed by cell division from pre existing cells (Omnis cellula- e- cellulla).In later years, advanced research in cytology led to modification in cell theory, which is now known as Modern Cell Theory.
Totipotency
It is the capacity or the potential of living nucleated cell to differentiate and divide to form any other type of cell and thereby a complete new organism.
A cell is totipotent because it has the entire genetic information of the organism in its nucleus. Embryonic animal cells are totipotent and termed as stem cells. Stem cells have great medical applications including cure for diseases.
Postulates of modern cell theory
• All living organisms are made up of cells.
• Living cells arise from pre-existing cells.
• A cell is the structural and functional unit of life.
• Total activities of cells are responsible for
activity of an organism.
• Cells show transformation of energy.
• Cells contain nucleic acids; DNA and RNA in the nucleus and cytoplasm.
Prokaryotic cells
The cell in prokaryotes show following main features. It has chemically complex protective cell envelop. However, it does not have well-defined nucleus and other membrane bound cell organelles. Cell envelop is a three-layered structure with outer glycocalyx, middle cell wall and inner plasma membrane. Glycocalyx is present as either slime layer (loose sheath) or capsule (tough).
Bacteria are better observed when stained. The most followed staining method is ‘Gram staining’ developed by Danish bacteriologist Hans Christian Gram. The cell wall is made up of peptidoglycan (in Gram positive bacteria) and murein (in Gram negative bacteria). It gives mechanical strength to the cell. Cell membrane is a phospholipid bilayer. All these structures give protection to the cell and also help in inter-cellular transport.
In motile bacteria either cilia or flagella are found. Both are driven by rotatory movement produced by basal body (which works as motor). Other parts are filament and hook. Some other surface projections are the tubular pili (which help in inter-cellular communication) and fimbriae (for clinging to support). The cell membrane shows infoldings called mesosomes, which help in cell wall formation and DNA replication.
Some bacteria especially photosynthetic cyanobacteria show more longer extensions called chromatophores. They carry photosynthetic pigments. The cytoplasm contains dense particles called ribosomes helping in protein synthesis.
Ribosomes are described by their sedimentation rate in Svedberg units. Bacterial ribosome are 70S (composed of a larger subunit 50S + smaller subunit 30S).
Important Points about Prokaryotic Cell
• The term cell was first used by Robert Hooke (1665) in his book ‘‘Micrographic’’.
• Purkinje and Mohl (1835- 37) discovered protoplasm.
• Camillo Golgi (1838) discovered the Golgi apparatus.
• Robert Brown (1881) discovered the Nucleus.
• Balbiani (1881) discovered chromosomes in salivary glands of Chironomus larva.
• Flemming (1882) studied cell division in detail and coined the term Mitosis.
• Porter (1945) discovered Endoplasmic Reticulum.
• C. Benda gave the name Mitochondria.
• C. de Duve (1955) discovered Lysosomes.
Eukaryotic Cell
Cells in which the nucleus has a definite nuclear membrane are known as Eukaryotic cells. These cells exhibit presence of membrane bound cell organelles. e.g. Cells of Protists, Plants, Animals and Fungi.
The eukaryotic cells have different shape, size and physiology but all the cells are typically composed of plasma membrane, cytoplasm and its organelles viz. Mitochondria, Endoplasmic Reticulum, Ribosomes, Golgi complex, etc. and a true nucleus.
1. Cell wall
It is rigid, supportive and protective outer covering of plasma membrane of plant cells, fungi and some protists. Algae show presence of cellulose, galactans, mannans and minerals like calcium carbonate in cell wall. In other plants, it is made up of hemicelluloses, pectin, lipids and protein.
Microfibrils of plant cell wall show presence of cellulose which is responsible for rigidity. Some of the depositions of cell wall are silica (grass stem), cutin (epidermal walls of land plants), suberin (endodermal cells of root), wax, lignin. It gives shape to the cell and protects from mechanical injury and infections.In plants, cell wall shows middle lamella, primary wall and secondary wall.
2. Middle lamella
It is thin and lies between two adjacent cells. It is the first structure formed from cell plate during cytokinesis. It is mainly made up of pectin, calcium and magnesium pectate. Softening of ripe fruit is due to solubilization of pectin.
3. Primary wall
In young plant cell, it is capable of growth. It is laid inside to middle lamella. It is the only wall seen in meristematic tissue, mesophyll, pith, etc.
4. Secondary wall
It is present inner to primary wall. Once the growth of primary wall stops, secondary wall is laid. At some places thickening is absent which leads to formation of pits. Plasmodesmata are cytoplasmic bridges between neighbouring cells. It shows pores between cell wall and middle lamella.
5. Cell membrane
It is thin, quasifluid structure present both extracellularly and intracellularly. Extracellularly, it is present around protoplast and intracellularly, it is present around most of the cell organelles in eukaryotic cell. It separates cell organelles from cytosol. Thickness of biomembrane is about 75mm.
Under electron microscope, cell membrane appears trilaminar (made up of three layers). It shows presence of lipids (mostly phospholipids) arranged in bilayer. Lipids posses one hydrophilic polar head and two hydrophobic non-polar tails.
So phospholipids are amphipathic. Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwitched in between heads. Due to this, tails never come in direct contact with aqueous surrounding. Cell membrane also shows presence of proteins and carbohydrates. Ratio of proteins and lipids varies in different cells. For example- in human beings, RBCs show approximately 52% protein and 40% lipids.
6. Fluid mosaic model
It is most accepted model of cell membrane. It was proposed by Singer and Nicholson in 1972. According to this model, it is made up of phospholipid bilayer and proteins. Proteins are like icebergs in the sea of lipids. Proteins can change their position. Some proteins are intrinsic i.e. occur at different depths of bilayer.
They span the entire thickness of the membrane. So, they are called transmembrane proteins. They form channels for passage of Water. Extrinsic or peripheral proteins are found on two surfaces of the membrane. Quasi fluid nature of lipid enables lateral movement of proteins.
This ability to move within the membrane is measured as fluidity.Main function of plasma membrane is transport of molecules across it. This membrane is selectively permeable. During passive transport, many molecules move across the membrane without spending energy. Some solutes move by simple diffusion along the concentration gradient (from higher to lower concentration).
Neutral solutes may move across the membrane by the process of simple diffusion This is called the passive transport.
Water may also move by osmosis.During active transport, few ions or molecules are transported against concentration gradient (from lower to higher concentration). It requires energy. So, ATP is utilized. As such a transport is an energy dependent process in which ATP is utilized, it is called Active transport e.g. Na+ /K+ pump. Polar molecules cannot pass through non-polar lipid bilayer. So, they require carrier proteins.
7. Golgi complex
Golgi complex or Golgi apparatus or Golgi body; various terms are used to denote this assembly, manufacturing cum packaging and transport unit of cell. Golgi complex essentially consists of stacks of membranous sacs called cisternae.
Diameter of cisternae varies from 0.5 to 1 µm. A cell may have few to several cisternae depending on its function. The thickness and molecular composition of two membranes of a Golgi sac differ from each other. The Golgi sacs show specific orientation in the cell.
Each cisterna has a forming or ‘cis’ face (cis: on the same side) and maturing or trans’ face (trans: the opposite side). Transport vesicles that pinch off from transitional ER merge with cis face of Golgi cisterna and add its contents into the lumen.
This explains why Golgi bodies are usually located near ER. Modified and condensed secretions leave Golgi through transface again as membrane bound vesicles.
Golgi body carries out two types of functions,
modification of secretions of ER and production of its own secretions. Cisternae contain specific enzymes for specific functions. Refining of product takes place in an orderly manner. For example, glycolipids and glycoproteins that are brought from ER loose certain sugars and regain other, thus forming a variety of products.
Golgi bodies also manufacture their own products. Golgi bodies in many plant cells produce non-cellulose polysaccharides like pectin. Manufactured or modified, all products of Golgi complex leave cisternae from transface as transport vesicles.
