DS

Cell: The Unit of Life

What is a cell?

  • Unicellular organisms are capable of:
    • independent existence
    • performing the essential functions of life.
  • Anything less than a complete structure of a cell does not ensure independent living.
    • Hence, the cell is the fundamental structural and functional unit of all living organisms.
  • ^^Anton Von Leeuwenhoek^^ first saw and described a live cell.
  • ^^Robert Brown^^ later discovered the nucleus.
    • The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.

Cell Theory:

  • In 1838, ^^Matthias Schleiden, a German botanist^^, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant.
  • At about the same time, ^^Theodore Schwann (1839), a British Zoologist,^^ studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’.
    • He also concluded, based on his studies on plant tissues, that ^^the presence of cell walls is a unique character of the plant cells.^^
    • On the basis of this, Schwann proposed the hypothesis that the ^^bodies of animals and plants are composed of cells and products of cells.^^
  • Schleiden and Schwann together formulated the cell theory.
    • This theory, however, ^^did not explain how new cells were formed.^^
  • ^^Rudolf Virchow (1855)^^ first explained that ^^cells divided and new cells are formed from pre-existing cells^^ (Omnis cellula-e cellula).
    • He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.
  • Cell theory as understood today is
    • all living organisms are composed of cells and products of cells.
    • all cells arise from pre-existing cells.

An Overview of Cell

  • Inside each cell is a dense membrane-bound structure called the nucleus.

    • This nucleus contains the chromosomes which in turn ^^contain the genetic material, DNA.^^

  • Cells that have membrane-bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus are prokaryotic.

    • In both prokaryotic and eukaryotic cells, a ^^semi-fluid matrix called cytoplasm occupies the volume of the cell.^^

  • The cytoplasm is the ^^main arena of cellular activities^^ in both plant and animal cells.

    • Various chemical reactions occur in it to keep the cell in its ‘living state’.

  • Besides the nucleus, the eukaryotic cells have other membrane-bound distinct structures called organelles like the ^^endoplasmic reticulum (ER), the Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.^^

  • The prokaryotic cells ^^lack such membrane-bound organelles.^^

    • Ribosomes are ^^non-membrane bound organelles found in all cells – both eukaryotic as well as prokaryotic.^^
    • Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles – chloroplasts (in plants) and mitochondria and on rough ER.
    • Animal cells contain another ^^non-membrane bound organelle called centrosome which helps in cell division.^^

  • Cells differ greatly in size, shape, and activities.

    • For example:
    • Mycoplasmas, the ^^smallest cells^^, are only 0.3 µm in length while bacteria could be 3 to 5 µm.
    • The ^^largest isolated single cell^^ is the egg of an ostrich.
    • Among multicellular organisms, human red blood cells are about 7.0 µm in diameter.
    • ^^Nerve cells are some of the longest cells^^.
      • Cells also vary greatly in their shape.
    • They may be disc-like, polygonal, columnar, cuboid, thread-like, or even irregular.
      • The shape of the cell may vary with the function they perform.

Prokaryotic Cell:

  • The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).
    • They are generally smaller and multiply more rapidly than eukaryotic cells.
    • They may vary greatly in shape and size.
  • The four basic shapes of bacteria are
    • bacillus (rod-like)
    • coccus (spherical)
    • vibrio (comma-shaped)
    • spirillum (spiral).
  • The organization of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions.
    • All prokaryotes have a cell wall surrounding the cell membrane ^^except in mycoplasma.^^
    • The fluid matrix filling the cell is the cytoplasm.
    • There is ^^no well-defined nucleus.^^
  • The genetic material is basically naked, not enveloped by a nuclear membrane.
    • In addition to the ^^genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA.^^
    • This smaller DNA are called plasmids.
      • The plasmid DNA confers certain unique phenotypic characters to such bacteria.
      • One such characteristic is resistance to antibiotics.
      • Plasmid DNA is used to monitor bacterial transformation with foreign DNA.
    • The nuclear membrane is found in eukaryotes.
    • No organelles, like the ones in eukaryotes, are found in prokaryotic cells ^^except for ribosomes.^^
    • Prokaryotes have something unique in the form of inclusions.
    • A specialized differentiated form of the cell membrane called ^^mesosome^^ ^^is the characteristic of prokaryotes.^^
      • They are essentially ^^infoldings of the cell membranes^^.

Cell Envelope and its Modifications:

  • Most prokaryotic cells, particularly bacterial cells, have a chemically complex cell envelope.
    • The cell envelope consists of a tightly bound three-layered structure i.e.,
    • the outermost glycocalyx
    • the cell wall
    • the plasma membrane
  • Although each layer of the envelope performs a distinct function, they act together as a ^^single protective unit.^^
    • Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the ^^staining procedure^^ developed by Gram viz.,
    • those that take up the gram stain are Gram-positive
    • the others that do not are called Gram-negative bacteria.
    • Glycocalyx differs in composition and thickness among different bacteria.
    • It could be a ^^loose sheath^^ called the slime layer in some, while in others it may be ^^thick and tough^^, called the capsule.
    • The cell wall determines the shape of the cell and provides strong structural support to prevent the bacterium from bursting or collapsing.
    • The plasma membrane is selectively permeable in nature and interacts with the outside world.
    • This membrane is similar structurally to that of the eukaryotes.
    • A special membranous structure is a mesosome which is formed by the extensions of the plasma membrane into the cell.
    • These extensions are in the form of vesicles, tubules, and lamellae.
      • They help with cell wall formation, DNA replication, and distribution to daughter cells.
      • They also help in respiration, and secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
    • In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.
    • Bacterial cells may be motile or non-motile.
    • If motile, they have thin filamentous extensions from their cell wall called flagella.
    • Bacteria show a range in the number and arrangement of flagella.
      • The bacterial flagellum is composed of three parts – filament, hook, and basal body.
      • The filament is the longest portion and extends from the cell surface to the outside.
    • Besides flagella, Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility.
      • The pili are ^^elongated tubular structures made of a special protein.^^
      • The fimbriae are ^^small bristle-like fibers sprouting out of the cell.^^
    • Some bacteria are known to help attach bacteria to rocks in streams and also to the host tissues.

Ribosomes and Inclusion Bodies:

  • In prokaryotes, ribosomes are associated with the plasma membrane of the cell.
    • They are about 15 nm by 20 nm in size and are made of two subunits - ^^50S and 30S units which when present together form^^ 70S prokaryotic ribosomes.
  • Ribosomes are the site of protein synthesis.
    • Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome.
    • The ribosomes of a polysome ^^translate the mRNA into proteins.^^
  • Inclusion bodies: ^^Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies.^^
    • These are not bound by any membrane system and lie free in the cytoplasm,
    • Examples: phosphate granules, cyanophycean granules, and glycogen granules.
    • Gas vacuoles are found in blue-green and purple and green photosynthetic bacteria.

Eukaryotic Cell:

  • The eukaryotes include all ^^protists, plants, animals, and fungi.^^

    • In eukaryotic cells, there is extensive compartmentalization of cytoplasm through the presence of membrane-bound organelles.
    • ^^Eukaryotic cells possess an organized nucleus with a nuclear envelope.^^
    • In addition, eukaryotic cells have a variety of complex locomotory and cytoskeletal structures.
      • Their genetic material is organized into chromosomes.
    • All eukaryotic cells are not identical.
    • Plant and animal cells are different as the ^^former possess cell walls, plastids, and a large central vacuole which are absent in animal cells.^^
    • On the other hand, ^^animal cells have centrioles which are absent in almost all plant cells.^^

Cell Membrane:

  • Chemical studies on the cell membrane, especially in ^^human red blood cells (RBCs),^^ enabled scientists to deduce the possible structure of the plasma membrane.

    • These studies showed that the cell membrane is mainly composed of lipids and proteins.

  • The major lipids are ^^phospholipids that are arranged in a bilayer.^^

    • Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.
    • This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment.
    • In addition to ^^phospholipids membrane also contains cholesterol^^.
    • Later, biochemical investigation clearly revealed that ^^the cell membranes also possess protein and carbohydrates.^^
    • The ^^ratio of protein and lipid varies considerably in different cell types.^^
    • In human beings, the membrane of the erythrocyte has approximately ^^52 percent protein and 40 percent lipids.^^
      • Depending on the ease of extraction, membrane proteins can be classified as integral and peripheral.
    • ^^Peripheral proteins lie on the surface of the membrane while the integral proteins are partially or totally buried in the membrane.^^

  • An improved model of the structure of cell membranes was proposed by Singer and Nicolson (1972) widely accepted as a fluid mosaic model.

    • According to this, the quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer.
    • This ability to move within the membrane is measured as its fluidity.
    • The fluid nature of the membrane is also important from the point of view of ^^functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.^^
    • One of the most important functions of the ^^plasma membrane is the transport of molecules across it.^^
    • The membrane is selectively permeable to some molecules present on either side of it.
      • Many molecules can move briefly across the membrane without any requirement of energy and this is called passive transport.
    • Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to lower.
    • Water may also move across this membrane from higher to lower concentrations.
      • The movement of water by diffusion is called osmosis.
    • As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.
      • A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to higher concentration.
      • Such a means of transport is an energy-dependent process, in which ATP is utilized and is called active transport, ^^e.g., Na+/K+ Pump.^^

Cell Wall:

  • Cell wall not only gives ^^shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier^^ to undesirable macromolecules.
    • Algae have cell wall, made of ^^cellulose, galactans, mannans and minerals like calcium carbonate^^, while in other plants it consists of ^^cellulose, hemicellulose, pectins and proteins^^.
    • The cell wall of a young plant cell, the primary wall is ^^capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side^^ of the cell.
      • The middle lamella is a layer mainly of ^^calcium pectate^^ which holds or glues the different neighbouring cells together.
      • The cell wall and middle lamellae may be traversed by plasmodesmata which ^^connect the cytoplasm of neighbouring cells.^^

Endomembrane System:

  • While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated.
    • The endomembrane system include ^^endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles.^^
    • Since the functions of the ^^mitochondria, chloroplast and peroxisomes are not coordinated^^ with the above components, these are not considered as part of the endomembrane system.

Endoplasmic Reticulum:

  • Electron microscopic studies of eukaryotic cells reveal the presence of a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER).

    • Hence, ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
    • The ER often shows ribosomes attached to their outer surface.
      • The endoplasmic reticulun bearing ribosomes on their surface is called rough endoplasmic reticulum (RER).
      • RER is frequently observed in the cells actively involved in ^^protein synthesis and secretion.^^
      • They are extensive and continuous with the ^^outer membrane of the nucleus.^^
      • In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER).
      • The smooth endoplasmic reticulum is the major site for ^^synthesis of lipid.^^
      • In animal cells ^^lipid-like steroidal hormones are synthesised in SER.^^

Golgi Apparatus:

  • ^^Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus.^^

    • These were later named Golgi bodies after him.
    • They consist of many ^^flat, disc-shaped sacs or cisternae^^ of 0.5µm to 1.0µm diameter.
    • These are stacked parallel to each other.
      • ^^Varied number of cisternae are present in a Golgi complex.^^
      • The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face.
        • The cis and the trans faces of the organelle are entirely different, but interconnected.
    • The golgi apparatus principally ^^performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell.^^
      • Materials to be packaged in the ^^form of vesicles^^ from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face.
      • This explains, why the golgi apparatus remains in close association with the endoplasmic reticulum.
      • A ^^number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae^^ of the golgi apparatus before they are released from its trans face.
    • ^^Golgi apparatus is the important site of formation of glycoproteins and glycolipids.^^

Lysosomes:

  • These are ^^membrane bound vesicular structures formed by the process of packaging in the golgi apparatus.^^
    • The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH.
    • These ^^enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.^^

Vacuoles:

  • The vacuole is the ^^membrane-bound space found in the cytoplasm.^^
    • It contains water, sap, excretory product and other materials not useful for the cell.
    • The vacuole is bound by a ^^single membrane^^ called tonoplast.
    • In plant cells the ^^vacuoles can occupy up to 90 per cent of the volume of the cell.^^
    • In plants, the tonoplast ^^facilitates the transport of a number of ions and other materials against concentration gradients^^ into the vacuole, hence their ^^concentration is significantly higher in the vacuole than in the cytoplasm.^^
    • In Amoeba the ^^contractile vacuole is important for osmoregulation and excretion.^^
    • In many cells, as in protists, food vacuoles are formed by engulfing the food particles

Mitochondria:

  • Mitochondria, unless specifically stained, are not easily visible under the microscope.

    • The number of mitochondria per cell is variable depending on the physiological activity of the cells.
    • In terms of shape and size also, considerable degree of variability is observed.
    • Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0µm (average 0.5µm) and length 1.0-4.1µm.

  • Each mitochondrion is a ^^double membrane-bound structure^^ with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment.

    • The inner compartment is filled with a ^^dense homogeneous substance^^ called the matrix.
    • The inner membrane forms a ^^number of infoldings^^ called the cristae towards the matrix.
    • The cristae ^^increase the surface area.^^
    • The outer membrane forms the continuous limiting boundary of the organelle.
    • The two membranes have their own specific enzymes associated with the mitochondrial function.

  • Mitochondria are the sites of aerobic respiration.

    • They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell.

  • The matrix also possesses ^^single circular DNA molecule, a few RNA molecules, ribosomes^^ (70S) and the components required for the synthesis of proteins.

  • The mitochondria divide by fission.

Plastids:

  • Plastids are found in all ^^plant cells and in euglenoides.^^

    • These are easily observed under the microscope as they are large.
    • They bear some ^^specific pigments,^^ thus imparting specific colours to the plants.

  • Based on the type of pigments plastids can be classified into:

    • Chloroplasts:
    • The chloroplasts contain chlorophyll and carotenoid pigments which are ^^responsible for trapping light energy essential for photosynthesis.^^
    • Chromoplasts:
    • In the chromoplasts ^^fat soluble carotenoid pigments^^ like ^^carotene, xanthophylls and others are present.^^
      • This gives the part of the plant a yellow, orange or red colour.
    • Leucoplasts.
    • The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients:
      • Amyloplasts store carbohydrates (starch)
      • Example: potato;
      • Elaioplasts store oils and fats.
      • Aleuroplasts store proteins.

  • Majority of the chloroplasts of the green plants are found in the ^^mesophyll cells of the leaves.^^

    • These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10µm) and width (2-4µm).
    • Their number varies from ^^1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll.^^
    • Like mitochondria, the chloroplasts are also ^^double membrane bound^^.
    • Of the two, the ^^inner chloroplast membrane is relatively less permeable.^^
      • The space limited by the inner membrane of the chloroplast is called the stroma.
      • A number of organised ^^flattened membranous sacs^^ called the thylakoids, are present in the stroma.
        • Thylakoids are arranged in stacks like the ^^piles of coins called grana^^ or the intergranal thylakoids.
        • In addition, there are flat membranous tubules called the ^^stroma lamellae connecting the thylakoids^^ of the different grana.
        • The membrane of the thylakoids enclose a space called a lumen.
        • The stroma of the chloroplast contains enzymes required for the ^^synthesis of carbohydrates and proteins^^.
        • It also contains small, ^^doublestranded circular DNA^^ molecules and ribosomes.
        • ^^Chlorophyll pigments are present in the thylakoids.^^
      • The ^^ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).^^

Ribosomes:

  • Ribosomes are the ^^granular structures^^ first observed under the electron microscope as dense particles by ^^George Palade (1953).^^

    • They are composed of ^^ribonucleic acid (RNA) and proteins^^ and are not surrounded by any membrane.
    • The ^^eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.^^
    • Each ribosome has two subunits, larger and smaller subunits.
    • The two subunits of ^^80S ribosomes are 60S and 40S while that of 70S ribosomes are 50S and 30S.^^
    • Here ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size.
    • Both 70S and 80S ribosomes are composed of two subunits.

Cytoskeleton:

  • An elaborate network of filamentous proteinaceous structures consisting of ^^microtubules, microfilaments and intermediate filaments^^ present in the cytoplasm is collectively referred to as the cytoskeleton.

    • The cytoskeleton in a cell are involved in many functions such as ^^mechanical support, motility, maintenance of the shape of the cell.^^

Cilia and Flagella:

  • ^^Cilia and flagella are hair-like outgrowths of the cell membrane.^^
    • Cilia are small structures which work like oars, causing the ^^movement of either the cell or the surrounding fluid.^^
    • ^^Flagella^^ ^^are comparatively longer and responsible for cell movement.^^
    • The prokaryotic bacteria also possess flagella but these are structurally different from that of the ^^eukaryotic flagella.^^
  • The electron microscopic study of a ^^cilium or the flagellum^^ show that they are covered with plasma membrane.
    • Their core called ^^the axoneme, possesses a number of microtubules running parallel to the long axis.^^
    • The axoneme usually has ^^nine doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules.^^
    • Such an arrangement of axonemal microtubules is referred to as the 9+2 array.
    • The central tubules are connected by bridges and is also enclosed by ^^a central sheath, which is connected to one of the tubules of each peripheral doublets^^ by a radial spoke.
    • Thus, ^^there are nine radial spokes.^^
    • The ^^peripheral doublets are also interconnected by linkers.^^
  • Both the cilium and flagellum emerge from ^^centriole-like structure^^ called the basal bodies.

Centrosome and Centrioles:

  • Centrosome is an organelle usually containing ^^two cylindrical structures^^ called centrioles.
    • They are surrounded by ^^amorphous pericentriolar materials.^^
    • Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.
    • They are made up of ^^nine evenly spaced peripheral fibrils^^ of tubulin protein.
      • Each of the ^^peripheral fibril is a triplet.^^
      • The adjacent triplets are also ^^linked.^^
    • The central part of the proximal region of the centriole is also ^^proteinaceous and called the hub^^, which is connected with tubules of the ^^peripheral triplets by radial spokes made of protein^^.
    • The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

Nucleus:

  • Nucleus as a cell organelle was first described by ^^Robert Brown as early as 1831.^^

    • Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.

  • The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called ^^chromatin, nuclear matrix and one or more spherical bodies called nucleoli.^^

    • Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.
    • The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
    • At a number of places the nuclear envelope is interrupted by minute pores, which are formed by ^^the fusion of its two membranes.^^
    • These nuclear pores are the passages through which ^^movement of RNA and protein molecules^^ takes place in both directions between the nucleus and the cytoplasm.

  • Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed.

    • Some mature cells even lack nucleus, e.g., ^^erythrocytes of many mammals and sieve tube cells of vascular plants.^^

  • The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.

    • The nucleoli are spherical structures present in the nucleoplasm.
    • The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure.
    • It is a ^^site for active ribosomal RNA synthesis.^^
    • Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.