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.