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Cell: The Fundamental Unit of Life

Cell: The Fundamental Unit of Life Synopsis

Synopsis

  • Cell is the basic structural and functional unit of all living organisms. It is the smallest part of the body of an organism capable of independent existence and able to perform the essential functions of life. 

Cell Theory 

  • In 1838, Matthias Schleiden and Theodor Schwann proposed a basic cell theory. In 1858, another scientist, Virchow made an addition to the existing cell theory.
  • The postulates of the modern cell theory are
    • The cell is the smallest unit of structure of all living things.
    • The cell is the unit of function of all living things.
    • All cells arise from pre-existing cells.

Contribution of Scientists in the Study of Cells

Variety in Cells 

On the Basis of Number of Cells 

  • Single-celled: Organisms made of a single cell. 
    Examples: Bacteria, yeast, Chlamydomonas, Amoeba, Paramoecium
  • Few-celled: Organisms made of few hundred or few thousand cells. 
    Examples: Spirogyra, Volvox 
  • Multi-celled: Organisms made of millions and billions of cells.
    Examples: Man, cow, mango tree, crow

On the Basis of Size of Cells 

  • Smallest cell: Bacteria (0.3–5.0 µm), red blood cells (7 µm)
  • Longest cell: Nerve cell in the neck of a giraffe (˃3 m long) 
  • Largest cell: Ostrich egg (170 mm × 130 mm)

On the Basis of Shape of Cells 

  • Columnar - Epithelial cells, Spherical - Human ovum, Oval - Fat cells
  • Spherical, biconcave - Red blood cells, Rectangular - Spirogyra
  • Spiral - Sperm cell, Rod-shaped – Bacteria, C-shaped - Cartilage cells
  • Branched - Nerve cells, Spindle-shaped - Smooth muscle cells

Prokaryotic and Eukaryotic Cells 

Prokaryotic Cell 
  • Prokaryotic cells may vary greatly in their shape and size, but their cell organisation remains similar.
Structure of Prokaryotic Cell
  • Cell wall: The cell wall of bacteria is made of peptidoglycan. It is absent in mycoplasma and PPLO.
  • Cytoplasm: It is the matrix present in the cell.
  • Nuclear material: DNA is naked, i.e. it is not enclosed by the nuclear envelop. In addition to DNA, a single molecule of circular DNA called a plasmid is present.
  • Mesosome: It is the extension of the plasma membrane present in the cytoplasm in the form of vesicles, tubules or lamellae.
  • Cell organelles: Membrane-bound cell organelles are absent.
  • Flagella: In motile bacteria, flagella are present as extensions of their cell walls.
  • Fimbriae: In non-motile bacteria, fimbriae or pili are present.
  • Ribosomes: 70S ribosomes are present which are made of 50S and 30S subunits.
  • Inclusion bodies: They are suspended in the cytoplasm. They store reserve food material.
  • Gas vacuoles: Gas vacuoles are found in cyanobacteria, purple bacteria and green photosynthetic bacteria.
Eukaryotic Cells
  • Eukaryotic cells include all protists, fungi, plants and animals.
  • These cells show the presence of membrane-bound cell organelles and a well-developed organised nucleus.
Eukaryotic Cells – Cell Membrane
  • The cell or plasma membrane is thin, elastic, semi-fluid, living protective membrane of the cell.
  • The term cell membrane was coined by Nageli and Cramer in 1855.
  • The advanced model of the structure of the cell membrane called the fluid mosaic model was proposed by Singer and Nicolson in 1972.
Postulates of the Fluid Mosaic Model
  • The membrane does not have a uniform disposition of lipids and proteins. There is a mosaic of two.
  • The membrane is not a solid but a quasi-fluid.
  • Lipids are arranged in a viscous bilayer.
  • Two kinds of proteins are present - integral and peripheral proteins.
  • Some lipids and peripheral proteins possess small carbohydrate molecules called glycolipids and glycoproteins.
Functions of the Cell Membrane 
  • It protects the cell from mechanical shocks and injuries.
  • It forms various types of cell junctions to keep the cells together.
  • Selectively permeable, i.e. they allow only selected substances to move across the membrane.
  • Many molecules move across the membrane without any energy requirement.
  • Neutral solutes move across the membrane from a region of high concentration to a region of low concentration.
  • Water moves from a higher to lower water potential across the membrane.
Transport of Materials through the Cell Membrane 
  • Different processes such as diffusion and osmosis are responsible for the transport of materials in and out of the cell. 
Passive Transport
  • Passive transport is a kind of diffusion in which an ion or a molecule crossing the cell membrane moves against its electrochemical or concentration gradient.
  • In passive transport, no metabolic energy is consumed. 
  • Passive transport occurs by three processes—simple diffusion, facilitated diffusion and osmosis.  
Simple Diffusion
  • In simple diffusion, molecules of gases such as oxygen and carbon dioxide enter the cell without the help of transport proteins such as permeases.
Facilitated Diffusion
  • In facilitated diffusion, ions or molecules cross the membrane rapidly by using specific proteins called transport proteins or permeases which are present in the membrane. The transport proteins form a small passageway through the membrane allowing the solute molecules to cross the phospholipid bilayer.
  • Facilitated diffusion occurs only in the direction of the concentration gradient. It does not require any metabolic energy.  
  • In several animal cells, facilitated diffusion helps in the transport of glucose or blood sugar into the body cells. The carrier system or mediated transport is responsible for an increase in the inward flow of glucose. 
Osmosis 
  • The spontaneous passage of water molecules from a region of high water concentration through a selectively permeable membrane to a region of low water concentration is called osmosis.
  • Osmosis occurs due to a difference in the concentration of solutes on either side of the membrane.
     
  • If the medium surrounding the cell has exactly the same water concentration as that of the cell (isotonic solution), then there will be no overall movement of water molecules across the plasma membrane. The cell remains the same size.
  • If the medium surrounding the cell has higher water concentration than that of the cell (hypotonic solution), then water molecules will freely pass across the plasma membrane in both the directions. The cell will gain water by osmosis. It will swell up and become inflated or turgid.
  • If the medium surrounding the cell has lower water concentration than that of the cell (hypertonic solution), then water will pass across the membrane in both the directions. Water leaving the cell will be more than that entering it. The cell will shrink.
Endosmosis
  • The process by which water molecules enter a cell is called endosmosis.  
Exosmosis
  • The process by which water molecules move out of the cell is called exosmosis. 
  • In plant cells, when excess of exosmosis occurs, the cytoplasm and plasma membrane shrink away from the cell wall. This is known as plasmolysis.
Active Transport
  • Active transport is the movement or transport of substances through a biological membrane such as the cell membrane. This process requires energy. 
  • The molecules move uphill against the forces of passive diffusion. They always move in a direction opposite to that of the concentration gradient, i.e. from a lower concentration to a higher concentration. Because materials are pumped against the concentration gradient, the process always involves the expenditure of energy in the form of ATP
     
  • Certain membrane proteins act as carrier molecules and transport the solute to the other side of the membrane.
  • Active transport occurs in only one direction. This results in accumulation of materials within the cell.
  • The important active transport systems in animals maintain sodium potassium gradients between cells 
Eukaryotic Cells – Cell Wall
  • The cell wall is the outer, rigid protective structure of the cell.
  • It is non-living.
  • It is found in plant cells, fungi and algae. 
Layers of the Cell Wall 
  • Primary Cell Wall: 
    • It is the first formed wall.
    • It is composed of pectates, cellulose, hemicellulose and some polysaccharides.
    • It is capable of growth.
  • Secondary Cell Wall:
    • It appears inner to the primary wall.
    • It is composed of cellulose, hemicellulose, lignin, suberin, cutin, silica, calcium and magnesium salts, waxes, resins, tannins, gums etc.
  • Middle Lamella:
    • It is a thin, common layer between two adjacent cells.
    • It is composed of calcium pectate and magnesium. 
Functions of the Cell Wall
  • It provides definite shape to the cell.
  • It provides mechanical support to the cell.
  • It protects the cell from mechanical injuries and infections. 
Endomembrane System
  • Membranous cell organelles whose functions are coordinated are collectively called the endomembrane system.
  • These organelles are the endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles.  
Endoplasmic Reticulum
  • It is the network or reticulum of tiny tubular structures scattered in the cytoplasm.
  • The endoplasmic reticulum (ER) divides the intracellular space into a luminal compartment (inside ER) and an extraluminal compartment (cytoplasm). 
Kinds of Endoplasmic Reticulum
 
 
 
Functions of the Endoplasmic Reticulum
  • Endoplasmic reticulum Helps in transport and exchange of materials.
  • SER helps in the synthesis of fat and steroid hormones.
  • RER provides a surface for protein synthesis. 
Golgi Apparatus
  • The Golgi apparatus or complex was discovered by Camillo Golgi in 1898.
  • The Golgi apparatus is found in eukaryotic cells except in mature sieve tube cells and mature erythrocytes. 
Ultrastructure of the Golgi Apparatus
  • The Golgi apparatus is composed of four parts—cisternae, tubules, vesicles and vacuoles.
  • Cisternae:
    • They are flat, curved, smooth-surfaced membranes with swollen ends.
    • One face of the cisternae is the cis face which is the forming face associated with the nuclear envelop, while the other face is the trans face which is the maturing face.
    • The cis and trans faces are interconnected.
  • Tubules:
    • They are small, flat, interconnecting.
    • They arise from the periphery of cisternae.
  • Vesicles:
    • They are large, round sacs.
    • They are present in clusters at the edge of the cisternae.
  • Vacuoles
    • They are large, spherical.
    • They are expanded parts of cisternae modified to form vacuoles which develop from the matured face.
       
Functions of the Golgi Apparatus
  • It is the site for the formation of glycoproteins and glycolipids.
  • It is involved in the processing, packaging, transport and release of secretory proteins.
  • Proteins synthesised by the endoplasmic reticulum are modified in the Golgi complex.  
Lysosomes
  • Lysosomes are formed by the Golgi apparatus from packaging.
  • They are single membrane, large vesicles.
  • Their vacuoles contain about 50 hydrolytic enzymes.
  • These hydrolytic enzymes are acid hydrolases such as lipases, proteases and carbohydrases. 
Functions of Lysosomes
  • They carry out the lysis of organelles or other cellular content, lysosomes secrete enzymes in the external environment.
  • Lysosomes remove carcinogens by engulfing them. 
Vacuoles
  • Vacuoles are non-cytoplasmic spaces present in the cytoplasm. 
  • A vacuole is covered by a thin membrane called tonoplast.
  • Inside the vacuole is water, cell sap, excretory products and other cell material which are not useful.
  • In plant cells, vacuoles are permanent and occupy about 90% of the cell.
  • In animal cells, vacuoles are small and temporary.
  • Different kinds of vacuoles are sap vacuole, food vacuole and contractile vacuole. 
Functions of Vacuoles
  • Vacuoles act as food reserve.
  • Solutes present in the cell sap help to maintain the osmotic pressure inside the cell.
  • Sap vacuoles store and concentrate waste products. 
Mitochondria
  • Mitochondria are present in aerobic eukaryotes.
  • The number of mitochondria varies from cell to cell, and it depends on the cell’s physiological activities.
  • Mitochondria are cylindrical or sausage-shaped. In yeast, they are spherical. 
Ultrastructure of the Mitochondrion
  • It is covered with a double membrane.
  • The outer membrane is smooth; however, its inner membrane projects into the space in the mitochondrial cavity in the form of folds called cristae.
  • Both membranes have their specific enzymes involved in mitochondrial function.
  • Thus, the lumen of mitochondria is divided into two distinct compartments—outer compartment (intermembrane space) and inner compartment.
     
Functions of Mitochondria
  • Mitochondria are the sites of aerobic respiration.
  • Mitochondria bring about the oxidation of carbohydrates, proteins and fats. 
Plastids
  • Plastids are found in plant cells and euglenoids.
  • They bear pigments which impart them with colours.
  • There two kinds of plastids: 
    • Chromoplasts: They are coloured plastids. Because of chromoplasts, the plants or parts of plants appear red, green, yellow or purple.
    • Leucoplasts: They are colourless plastids.
    • There are three kinds of leucoplasts: 
    • Amyloplasts: Starch-containing leucoplasts
    • Elaidoplasts: Store fats
    • Aleuroplasts: Store proteins 
Ultrastructure of Chloroplasts
  • Chloroplasts are lens-shaped, oval, spherical, discoid or ribbon-like in shape.
  • They are 5–10 mm in length and 2–4 mm in width.

     
  • Chloroplast is bounded by a double membrane.
  • The space enclosed by the inner membrane is filled with stroma.
Functions of Chloroplasts 
  • Leucoplasts act as storage organs.
  • Chloroplasts are the sites of photosynthesis.
  • Chromoplasts provide colour to flower, fruits and leaves. 
Ribosomes
  • Ribosomes are granular structures and occur in all cells.
  • Ribosomes are composed of two subunits. One unit is larger in size and the other is smaller in size.
  • Eukaryotic ribosomes are 70S while prokaryotic ribosomes are 80S.

      
Functions of Ribosomes
  • Ribosomes are the site of protein synthesis.
  • Ribosomes which are free in the cell synthesise structural proteins for use within the cell. 
Cytoskeleton
  • Cytoskeleton is an elaborate network of filamentous proteinaceous structures present in the cytoplasm.
  • Microtubules, microfilaments and intermediate filaments constitute the cytoskeleton.
  • Cytoskeleton provides mechanical support to the cell.
  • They help cells in maintain their shape. 
Cilia and Flagella
  • Cilia and flagella are the cell-like outgrowths of the cell membrane.
  • They emerge from the cytoskeleton. 
Ultrastructure of Cilia and Flagella 
  • Cilia and flagella are covered by the unit membrane which resembles the plasma membrane.
  • The space or the core bounded by the unit membrane is called an axoneme which is filled with the matrix.
  • Nine pairs of doublets of microtubules are arranged peripherally and a pair is located centrally.
  • Such an arrangement of microtubules is called a 9 + 2 array.
  • The microtubules are located centrally by a bridge and are enclosed in a central sheath.
  • The central sheath is connected to one of the microtubules of peripherally arranged doublets by a radial spoke.
  • The peripheral microtubules are also connected by linkers.

Functions of Cilia and Flagella 

  • Cilia and flagella function as organs of locomotion. 

Centrosome and Centrioles

  • Centrioles are sub-microscopic microtubular sub-cylinders which occur in pairs.
  • A pair of centrioles is surrounded by a clear, fibrous, cytoplasm called the centrosphere.
  • Centrioles and centrospheres are collectively called a centrosome.
  • Centrioles lie perpendicular to each other in the centrosome. 

Structure of Centrioles

  • Each centriole is made of nine evenly spaced microtubule fibrils.
  • The central part of the proximal region of the centriole is proteinaceous and is called a hub.
  • The hub is connected to tubules of the peripheral triplets by radial spokes made of protein.
  • Such organisation gives a cart-wheel-like appearance to centrioles.
     
Functions of Centrioles
  • Centrioles polymerise microtubules for the formation of spindle fibres and astral rays during cell division.
  • They also determine the poles during cell division. 
Nucleus 
  • The nucleus is present in all eukaryotic cells except erythrocytes in mammals and sieve tubes in plants. 
Structure of a Nucleus
  • Four parts which form the nucleus are nuclear membrane, nucleoplasm, nucleolus and chromatin.

     
Nuclear membrane
  • It is a double membrane.
  • The space between two membranes is called the perinuclear space.
  • The outer membrane is continuous with the endoplasmic reticulum, while the inner membrane is smooth.
  • The outer membrane bears ribosomes.
  • The nuclear membrane is interrupted by minute pores called nuclear pores.  
Nucleoplasm
  • It is the nuclear matrix. It is a transparent, semi-fluid, granular substance.
  • It is mainly made of nucleic acids, proteins, enzymes, lipids and minerals. 
Nucleolus
  • The nucleolus is the spherical structure present in the nucleus.
  • The nucleolus remains attached to a special type of chromosome having a specific region called the nucleolar organising region. 
Chromatin
  • The chromatin is the loose network of nucleoprotein fibres.
  • It is visible during the interphase of the cell cycle.
  • It is the hereditary material of the nucleus.
  • The chromatin consists of DNA and histones, non-histone proteins and RNA. 
Structure of Chromosome
  • A chromosome is made of the following four parts:
  • Centromere: It is a primary constriction on the chromosome. The two chromatids are attached to each other at the centromere.
  • Kinetochore: It is a disc-shaped protein structure attached on either side of the centromere.
  • Chromatids: They are the chromosomal arms. Both arms are symmetrical.

     
Classification of Chromosomes
  • Based on the position of the centromere, there are four types of chromosomes:
  • Metacentric: Centromere is located in the median position.
  • Sub-metacentric: Centromere is slightly away from the median position.
  • Acrocentric: Centromere is located close to one end.
  • Telocentric: Centromere is located terminally. 
Functions of Chromatin
  • Chromosomes carry genetic information from one generation to the next generation.
Microbodies
  • Microbodies are membrane-bound vesicles usually present in the cytoplasm.
  • The peroxisome is a microbody which contains enzymes for peroxide biosynthesis.
  • Glyoxysomes are microbodies which contain enzymes required for the β-oxidation.
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