FUNDAMENTAL UNIT OF LIFE
- All living forms are composed of microscopic units called as “Cells”.
- A cell is the basic structural and functional unit of all life forms.
- Study of structure and composition of cell is called as “Cytology”.
- Cell was first observed by “Robert Hooke” in a dead cork slice in the year 1665. He described about
This in his book “Micrographic”.
- The word cell was derived from a Greek word “Cellulae” which means small room.
- First living cell was discovered by A.V. Leeuwenhoek.
- The term protoplasm was coined by purkinje in 1839.
- Protoplasm was discovered by “Felix Dujardin” and named as sarcode.
- It’s consistency differs under different condition. It exists in sol-gel states.
- Protoplasm is an aggregate of various chemicals such as water, ions, salts and other organic molecules like proteins, carbohydrates, fats, nucleic acids, vitamins etc.
Two biologists, “Schleiden and Schwann” gave the “Cell theory” which was later on expanded by “Rudolf Virchow”. Cell theory states that
(i) All plants and animals are composed of cells.
(ii) Cell is the basic unit of life.
(iii) All cells arise from pre-existing cells.
Viruses are the exceptions of cell theory.
TYPES OF CELL & ORGANISM
(a) On the Basis of Number of Cells Organisms can be categorized as:
- Unicellular organisms: These are organisms which made up of single cell only. This single cell Performs all the vital body functions of an organism. e.g. Amoeba
- Multicellular organisms: These are the organisms which made up of numerous cells. These cells then combine to form an organ and group of organs performing different functions forms an organ system which further forms an organism. e.g. plants and animals
(b) On the basis of type of organization, cells are two types:
- Prokaryotic cells: these are primitive and incomplete cells. They have less developed nucleus without nuclear membrane & nucleolus. g. Bacteria.
- Eukaryotic cells: these are well developed cells. They have advanced nucleus with unclear membrane and nucleolus. eg. Plants & animals.
Cells are of variable shapes and sizes. Their shape is according to the function. Generally cells are spherical but they may be elongated (nerve cell), branched (pigmented), discoidal (RBC), spindle shaped (muscle cell) etc.
Size of cell is variable depending upon the type of organism. Some are microscopic while some are visible with naked eyes.
Their size may vary from 0.2 µm to18 cm.
- Size of a typical cell in a Multicellular organism ranges from 20-30 mn.
- The largest cell is ostrich egg(15 cm. in dia with shell & 8 cm. in dia without shell)
- The longest cell is nerve cell.(upto 1m. or more)
- Smallest cells so far known are PPLOs e.g. mycoplasma (0.1 µm in dia.)
- Human egg is 0.1 mm. in dia.
COMPONENTS OF CELL
There is an occurrence of division of labour within a cell as they all got certain specific components called “Cell organelles” each of them perform a specific function.
The three basic components of all the cells are
(i) 1M (Plasma Membrane) (ii) Nucleus (iii) Cytoplasm
(a) Cell Membrane:
- Cell membrane is also called as plasma Membrane or Plasma lemma.
- It is the limiting boundary of each cell which separates the cytoplasm from its surroundings.
- It is found in both plant as well as animal cells.
- It is the outer most covering of a cell in case of animals and lies below the cell wall in case of plants.
- It is made up of proteins and lipids where proteins are sandwiched between bilayer of lipids.
- Plasma membrane name was given by Nageli.
- Plasma membrane is selectively permeable in nature. It allows or permits the entry and exit of some materials in and out of the cell.
- Singer and Nicholson gave the fluid mosaic model of plasma membrane according to him it consists of a protein layer sandwiched between two layers of lipids. It is in quasifluid state. It is 75A thick.
- It is flexible and can be folded, broken and reunited.
Function of plasma membrane:
(A) It regulates the movement of molecules inside and outside the cell.
(B) It helps in maintaining the distinct composition of the cell.
(ii) Transportation of molecules across the plasma membrane: This can be done by following ways:
(A) Diffusion: Movement of solutes or ions from higher concentration to lower concentration is called as diffusion. It does not require energy therefore it is called as passive transport.
(B) Osmosis: The movement of solvent or water from higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called as osmosis. Or The movement of solvent or water from lower concentration to higher concentration of solution through a semipermeable membrane is called as osmosis. Osmosis can also be called as diffusion of solvents”.
- Endosmosis: Movement of solvent into the cell is called as Endosmosis.
- Endosmosis: Movement of solvent outside the cell is called as Endosmosis.
(iii) Types of solution on the basis of concentration:
(A)Isotonic solution: When the concent, ation of the solution outside the equal to the
Concentration of cytoplasm of the cell it is called as isotonic solution.
(B) Hypertonic solution: When the of concentration of the solution outside the cell is more than that inside the cell. Due to this cell looses water and becomes plasmolysed.
(C) Hypotonic solution: When the of concentration of the solution outside the cell is lesser than that of cytoplasm of cell. Due to this cell swells up and bursts.
(b) Cell Wall:
- It is the outermost covering of the plant cells.
- It is absent in animal cells.
- Cell wall is rigid, strong, thick, porous and non living structure. It is made up of cellulose and hemicelluloses. Cell walls of two adjacent cells are joined by a layer called middle lamellae. It is made up of calcium and magnesium pectate.
- Functions of cell wall:
- It provides definite shape to the cell.
- It provides strength to the cell.
- It is permeable and allows entry of molecules of different sizes.
- It is antigen specific.
- It has the characteristics of repair and regeneration.
- Nucleus is the most important cell organelle which directs and controls all its cellular activities.
- It is called as “Headquarter of the cell”.
- It was discovered by “Robert Brown in 1831”.
- In eukaryotes a well defined nucleus is present while in prokaryotes a well defined nucleus is absent.
- Prokaryotes contain a primitive nucleus.
- It has double layered covering called as nuclear membrane.
- Nuclear membrane has pores which regulate the movement of materials of materials in & out of the cell.
- Besides nuclear membrane nucleus also contains nucleolus and chromatin material and the substance filled inside the nucleus is nucleolus or karyolymph.
- Chromosomes or chromatin material consists of DNA which stores and transmits hereditary information for the cell to function, grow and reproduce.
(i) Function of the nucleus:
(A) It controls all the metabolic activities of the cell and regulates the cell cycle.
(B) It helps in transmission of hereditary characters from parents to off springs
- Cytoplasm was discovered by Kolliker in 1862.
- It is the site of both biosynthetic and catabolic pathways.
- It can be divided into two parts:
(i) Cytosol: Aqueous soluble part contains various fibrous proteins forming cytoskeleton.
(ii) Cell organelles: Living part of the cells having definite shape, structure and function bounded By Plasma membrane.
- It is the network of membranes present in the cytoplasm.
- It was discovered by Porter, Claude and Fullam.
- These are present in all cells except prokaryotes and mammalian erythrocytes.
- They are made up of three components:
- Cistemae: These are long, flattened, parallely arranged, unbranched tubules. These from Successive layers of nucleus. These are found in cells which are active in protein synthesis and are 40-50 µm in diameter.
- Vesicles: These are around or spherical they are founded in synthetically active cells.
Function of ER:
- It is the only organelle which can move within a cell so it serves as a channel for the transport of materials between various regions of cytoplasm and between cytoplasm and nucleus.
- It also function as a cytoplasmic framework to provide space for some of the biochemical activities. It forms endoskeleton of cell.
- It helps in synthesis of fats, steroids, cholesterol etc.
- It contains secretory proteins.
- SER plays a crucial role in detoxification of drugs and poisonous by-products.
- Golgi apparatus consists of a system of membrane bounded vesicles arranged parallel to each other in stacks called Cisternae along with some large and spherical vacuoles.
- It was discovered by Camilo Golgi.
- In plants Golgi membrane bounded.
- It is single membrane bounded.
- It is absent in prokaryotes, mammalian RBC’s & sieve cells.
- It helps in formation of lipids
- It helps in formation of middle lamellae
- It is secretary in nature.
- It helps in melanin synthesis
- Lipids and proteins synthesized in endoplasmic reticulum are packed at Golgi complex. They provide the site for assembly of new membrane material.
- It is a rod shaped structure found in cytoplasm of all eukaryotic cells except mammalian RBC’s.
- These are also absent in prokaryotes.
- It was first seen by Kolliker in insect cells.
- Maximum mitochondria are found in metabolically active cells.
- It is also called as “Power House of the Cell” or the “Storage Battery”.
- It is double membranous structure where outer membrane has specific proteins While inner membrane is folded inside to from chambers called Cristae. ”Cristae” are the infoldings of inner mitochondrial membrane that possess enzymes for respiratory cycles like Kreb Cycle. ATP synthesizing units are called Oxysomes or F1 particles.
- Space between inner and outer mitochondrial membranes is called as Perimitochondrial space. The fluid present in mitochondrial is called as matrix.
- Its main function is to produce and store the energy in the form of ATP.
- It is the site of Kreb cycle of respiration.
- Oxysome contains enzymes for ATP production.
- Matrix contains enzymes for Kreb cycle.
Ribosomes are the sites of protein synthesis.
All structural and functional proteins (enzymes) coded by the nuclear DNA, are synthesized upon cytoplasmic ribosomes. The DNA codes are transcripted into messenger RNA (mRNA) molecules in the chromosomes of the nucleus. mRNA molecules diffuse out into the cytoplasm and each becomes attached to several ribosomes which thus from a group called polyribosome or polyribosomes. In this way each mRNA molecule brings about polymerization of specific protein molecules, with the help of ribosomes from amino acid molecules found in the Cytosol.
- It is double membranous discoidal structure, fond only in plant cells.
- Term plastid was given by Haeckel.
- Chloroplast was discount by A.V. Leeuwenhoek and named by Schimper.
- Besides being discoidal of rhombic in plant cells they occur in variable shapes like in algae they can be ‘U’ shaped , spiral , coiled , ribbon shaped etc.
Depending upon the type of pigment present in them they are of following three types.
- Grana: It constitutes the lamellar system. These are found layered on top of each other, these stacks are called as Grana. Each granum of the chloroplast is formed by superimposed closed compartments called Thylakoids.
- Chloroplast have Following Two Parts :
- Function: They are the sites of light reaction of photosynthesis as they contain photosynthetic pigment chlorophyll. In each thylakoid Quantasomes are present which are called as Photosynthetic units. Each quantansome possesses 230 chlorophyll molecules.
- Stroma: It is a granular transparent substance also called as matrix. Grana are embedded in it. Besides Grana they also contain lipid droplets, starch grains, ribosomes etc.
Function: This is the site of dark reaction of photosynthesis. Also helps in protein synthesis due to presence of ribosomes.
- These are membrane bounded regions in the cytoplasm containing water and other substances.
- They are bounded by a single membrane called Tonoplast.
- In animal cells vacuoles are smaller in size and numerous while in plant cells a single large vacuole Is found which occupies about 90% of the volume of cell.
- It helps in maintaining osmotic pressure in a cell.
- It stores toxic metabolic products of plant cell.
- It contains various Coloured pigments like anthocyanins.
(Discovery: Christian de Duve) (Lyso = digestive, some = body)
- These are tiny sac like granules containing enzymes of intracellular digestion.
- They are bounded by a single membrane.
- They occur in animal cells and a few plant cells.
- They do not have a definite shape or size.
- They contain hydrolyzing enzymes called acid hydrolyses.
- Their main function is phagy = digestion
- They are kind of waste disposal system.
- They help in digesting foreign materials and wom out cells.
- During disturbances in cellular metabolism i.e. in case of cell damage lysosomes burst and their enzymes are released into the cytoplasm and they digest their own cell so they are also called as “Suicidal Bags”.
- These structures were first described from liver and kidney cells by Rodhin(1954)
- In plant cells, they were first observed in germinating seeds by Tolbert(1969)
- The term ‘peroxysomes’ was first used by de Duve and also called as uricosomes.
- Peroxysomes are ovoid or granular structures, limited by a single unit membrane and have a diameter of 0.5 to µm.
- In green leaves of C3 plants, peroxysomes carry out photorespiration.
- In animal cells they carry out lipid metabolism.
- They contain important enzymes as oxidases (peroxide producing enzyme), peroxidases and catalases (which break down toxic peroxides to water and oxygen).
- A beaver (1961) was the first person to discover these organelles and were described later by R.W. Briedenbach (1967).
- They are about 0.5 to 1 µm in size and are surrounded by a single unit membrane.
- They are found in plant cells, particularly, in germinating fatty seeds e.g. Ricinus (castor) and groundnut where fat is being converted into carbohydrates by a process called glyoxylate cycle.
- Glyoxysomes contain important enzymes, isocitrate, lyase, maltase and Synthelast along with several others.
- Structure of glyoxysomes is similar to peroxisome.
DIFFERENCES BETWEENA PLANT CELL AND ANANIMAL CELL
SOME IMPORTANT POINTS
- Centrosome :
- It is found in animal cell.
- It consists of two centrioles.
- It is non membranous.
- It is made up of microtubules.
- It helps in cell division and spindle formation.
- Cilia and flagella: These are thread like appendages used for locomotion and emerge from basa body.
- Chromosomes: These are thread like structures containing hereditary information in from of genes.
- There are four different types of membranes on the basis of permeability:
(A)Permeable: They allow diffusion of both solvent and solute molecules and icon through them. e.g. Cell wall of plant cell.
(B) Impermeable: They prohibit the diffusion of both solvent and solute particles through them. e.g. Cutinized cell wall, Lignified cell wall.
(C) Semipermeable: Allows diffusion of solvent molecules but do not allow the passage of solute molecules. e.g. Parchment paper, Kidney membrane.
(D) Differentially Permeable: It allow some solutes to pass through them along with the solvent molecules. E.g. Plasmalemma, Tonoplast.
- Cell organelles can be separated by the method of differential centrifugation.
- First cell that developed in laboratory by Sydney Fox and A.I. Oparin was called as Coacervate.
- Smallest cell is Mycoplasma (PPLO)(0.1 µ dia)
- Largest cell is Ostrich egg (15 cm.dia.)
- Longest Animal Cell is Nerve fibre (1m.)
- Lysosomes exhibit polymorphism i.e. they occur in different forms.
- Mitochondria, plastid and centrioles have their own DNA molecules so they are called as “Semiautonomous Cell Organelles ”.
- Connection through which cells communicate chemically with each other through their thick walls are called as “Plasmodesmata”.
- Cell wall of fungi is made up of chitin instead of cellulose.
- Protoplasm was called as “Physical Basis of Life” by Huxley
- DNA is called as “Chemical Basis of life”.
- Euglena is the connecting link between plants and animals as it lacks cell wall but has plastids.
- Mesosomes in bacteria are analogous to mitochondria as they both help in cellular respiration.
- Centre for cellular and molecular biology is situated at Hyderabad.
- In animal cell the ‘Cell Coat’ is present instead of cell wall which provides them protection. It is made up to glycocalyx.
- ATP (Adenosine Triphosphate) is the energy currency of the cell i.e. energy in cell is stored in from of ATP molecules.