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Cells are very small in size and cannot be seen by the naked eye. For observing a minute cell one needs a microscope (see Figure 4.1). Leeuwenhoek prepared a light microscope which is comparable to today’s compound microscope. But compound microscope is much advanced from the microscope of Robert Hooke and Leeuwenhoek.

The object on a glass slide is kept on a stage bearing a central hole under an objective lens. Light is reflected through the specimen with the help of a mirror and a condenser from below the stage. Through eye piece one can see the magnified image of the object. Eye piece is placed on the top as seen in the figure. The specimen is focused by adjustors (coarse and fine) fitted in the microscope. The eye piece and objectives of high and low powers are available. If a microscope has 10× objective len and 10× ocular len, then the object is magnified 100 times.

The invention of the electron microscope which was first designed by Knoll and Ruska added further to the unknown facts about the cell. It can give a magnification upto 200,000 times. The ordinary compound microscope uses light which is bent by glass lenses to magnify the object while the electron microscope uses beams of electronswhich are bent by magnets.


Compound Microscope

Cell Diversity


Not all cells are alike. Even cells within the same organism show enormous diversity in size, shape and internal organisation. Your body contains around 1013 to 1014 cells of about 300 different cell types, which are broadly classified under the following four groups, viz., epithelial, muscular, connective and nervous.

Cell Size and Volume


A few types of cells are large enough to be seen by the unaided eye. The human egg (ovum) is the largest cell in the body. The sciatic nerve cell is the longest cell in the human body. These two can be seen without the aid of a microscope.

In multicellular organisms, cell size varies from 0.1µ to several metres. Most cells are small due to the following reasons:

  • The cell’s nucleus can only control a certain volume of active cytoplasm.
  • Cells are limited in size by their surface area-to-volume ratio. A group of small cells has a relatively larger surface area than a single large cell of the same volume. This is important because the nutrients, oxygen and other materials that a cell requires must enter through its surface. As the cell grows larger, at some point its surface area becomes too small to allow these materials to enter the cell quickly enough to meet the requirement of the cell.


Cell Number

The cell number in an organism varies with the size of organism. Unicellular organisms have a single cell. In multicellular organisms, it is indefinite. In man, the number of cells is about 100 trillion (104).

Cell Shape


Cells come in a variety of shapes depending on their function.

  • The neurons are long and thin to transmit the impulses faster from distant parts of the body to brains and vice versa
  • Blood cells are circular and biconcave disks, hence they can flow smoothly to transport oxygen
  • White blood cells are amoeboid that can squeeze out through capillary walls
  • Guard cells of stomatal pore in the leaves are bean-shaped to open and close the pore

Internal Organisation Of A Generalised Cell


In a generalised cell, the protoplasm is enclosed within a limiting membrane, which is called the cell membrane or plasma membrane. The protoplasm is differentiated into (i) cytoplasm lying outside the nucleus and (ii) nucleoplasm present within the nucleus bounded by nuclear membrane. Thus, a generalised cell can be differentiated into the following three major regions:

  • Plasma membrane
  • Cytoplasm bearing organelles
  • Nucleus

Based on the presence or absence of true nucleus, cells are classified as follows:

  • Prokaryotic cell
  • Eukaryotic cell

Structure of a Prokaryotic Cell

The structure of prokaryotic cell is shown in the below Figure.

  • Cytoplasm: It contains all the enzymes needed for all metabolic reactions, since there are no organelles.
  • Ribosome: It contains only non-membranous organelles. It is 70S type (‘S’ refers to Svedberg unit). 70S ribosomes are comparatively smaller in size with two subunits (30S + 50S).
  • Nucleoid: It is the region of the cytoplasm that contains deoxyribonucleic acid (DNA). It is not surrounded by a nuclear membrane and nucleoplasm is absent.
  • DNA: It is always circular and not associated with any proteins to form chromatin.
  • Plasmid: It is a small loop of DNA used to exchange DNA between bacterial cells. It is used in genetic engineering.
  • Cell membrane: It is made of phospholipids and proteins, such as eukaryotic membranes.
  • Meosome: It is a tightly folded region of one cell membrane containing all the membrane-bound proteins required for respiration, and photosynthesis can also be associated with the nucleoid.
  • Cell wall: It is made of murein, which is a glycoprotein (also called peptidoglycan). There are two kinds of cell, which can be distinguished by Gram’s stain.
  • Gram +ve bacteria: They have a thick cell wall, may have spores and are sensitive to penicillin and lysozyme (antibacterial enzyme found in tears and saliva).
  • Gram –ve bacteria: They have a thin cell wall with an outer lipid layer, have no spores and stain pink.
  • Capsule: It is a thick polysaccharide layer outside the cell wall. It is used for sticking cells together, as a food reserve, as protection against desiccation and chemicals.
  • Flagellum: It is a rigid rotating helical-shaped tail used for propulsion.
  • Pilus: Hair-like structure found on the surface of the bacterium. It is used for bacterial conjugation.

Structure of an Eukaryotic Cell

Cell Membrane A cell cannot survive if it is totally isolated from its environment. The cell membrane is a complex barrier separating every cell from its external environment.

The cell membrane is a fluid mosaic of proteins floating in a phospholipid bilayer.

J. Singer and G.L. Nicolson proposed fluid mosaic model to explain the structure of plasma membrane. According to this model, the molecules of proteins are incorporated in a bilayer of phospholipids. The phospholipids spontaneously arrange themselves into a bilayer. The hydrophilic (water loving) polar heads of the phospholipid face the outside and inside of the cell. The hydrophobic non-polar tails face each other. In addition to phospholipids, there are two types of lipids in the membrane. Glycolipids have a structure similar to phospholipids except that the hydrophilic head is joined by different types of sugars to form a straight or branching carbohydrate chain. Cholesterol is a lipid that is found in animal’s plasma membrane, and related steroids are found in the plasma membrane of plants. Cholesterol reduces the permeability of the membrane to most biological molecules.

Some proteins are attached at the polar surface of the lipid (i.e. extrinsic or peripheral proteins), while others (i.e. intrinsic or integral proteins) either partially penetrate the bilayer or span the membrane entirely to stick out on both sides (called transmembrane proteins).

Functions of cell membrane or plasma membrane are as follows:

  • It maintains the individuality and form of the cell
  • It protects the cell from injury
  • Its junctions keep the cells together
  • Its oligosaccharide molecules help in recognising self from non-self
  • It controls cellular interactions necessary for tissue formation and defence against microbes
  • It helps certain cells in movement by forming pseudopodia as in Amoeba and leucocytes
  • It is also called selectively permeable membrane. This membrane regulates what passes into and out of the cell. The cell membrane functions like a gate, controlling which molecules can enter and leave the cell. The cell membrane controls which substances pass into and out of the cell. Transmembrane proteins (carrier proteins) found in a biological membrane are specific, i.e., it acts as channels that move specific molecules into and out of the membrane. For instance, it allows b-glucose whereas restricts a-glucose to enter. Many molecules cannot cross at all. For this reason, the cell membrane is said to be selectively permeable.

Transport Across Membranes


Passive Transport It is a type of diffusion in which an ion or molecule crossing a membrane moves down its electrochemical or concentration gradient. No metabolic energy is consumed in passive transport. The types of passive transport are diffusion, osmosis and facilitated transport.

  • Movement of gases such as oxygen and CO2 in and out of the cell occur by diffusion. Diffusion takes place from higher concentration to a region of lower concentration. Within the cell, the concentration of CO2 is higher due to metabolic activities. Outside the cells, the concentration of CO2 is lower and whereas oxygen concentration remains higher. Thus, CO2 moves out and oxygen enters the cell through plasma membrane by diffusion. Thus, diffusion plays an important role in exchange of gases between the cells and between the cell and its external environment.
  • Similarly, water molecules also diffuse through plasma membrane and this is called osmosis. It involves movement of water molecules from a region of higher concentration to a region of lower concentration through a semi-permeable membrane.
  • Tonicity refers to the strength of a solution in relationship to osmosis. If a cell is placed in isotonic solution, where the solute concentration is same on both sides of the membrane, there is no net gain or loss of water. If a cell is placed in hypotonic solution, the cell swells or even bursts due to intake of water (endosmosis). If a plant cell is placed in hypotonic solution, we observe the expansion of cytoplasm because the large central vacuole gains water and the plasma membrane pushes against the rigid cell wall. Osmotic pressure or turgor pressure is extremely important to maintain the plant’s erect position. If you forget to water the plants, they wilt due to decreased turgor pressure. If a cell is placed in hypertonic solution, it shrinks or shrivels due to loss of water (exosmosis). When a plant cell is placed in hypertonic solution, the plasma membrane pulls away from the cell wall as the large central vacuole loses water. This is an example of plasmolysis, a shrinking of the cytoplasm due to osmosis.
  • Facilitated transport or facilitated diffusion is the spontaneous passage of molecules or ions across a biological membranes passing through specific transmembrane integral proteins. Glucose, sodium ions and chloride ions are few examples of molecules and ions that must efficiently cross the plasma membrane but to which the lipid bilayer of the membrane is virtually impermeable. Their transport must therefore be facilitated by proteins that span the membrane and provide an alternate route or bypass.

Active Transport It is the movement of any substance through the cell membrane that requires energy. The energy is provided by adenosine triphosphate (ATP) produced by oxidative phosphorylation in the mitochondria. Active transport is a rapid process and is usually unidirectional. Some membrane proteins act as carrier molecules and transport the substance to the other side of the membrane.

Exocytosis and Endocytosis 
The transportation of macromolecules such as polypeptides, polysaccharides or polynucleotides that are too large to be transported by carrier proteins are transported in and out of the cell by vesicle formation.

  • Exocytosis is a process in which an intracellular vesicle formed by Golgi Apparatus moves to the plasma membrane and subsequent fusion of the vesicular membrane and plasma membrane ensues. For example, a few of the processes that use exocytosis are secretions of proteins such as enzymes, peptide hormones and antibodies from the cells, release of neurotransmitter from presynaptic neurons, etc.
  • Endocytosis is the process by which the cell takes in substances by vesicle formation, a portion of the plasma membrane invaginates to envelope the substance and then the membrane pinches off to form an intracellular vesicle.
  • The process of ingestion of large-sized solid substances by a cell is known as phagocytosis (in Greek, phagein = to eat, kytos = cell), i.e., cell eating. The term ‘phagocytosis’ was coined by Metchnikoff in 1893.
  • When the ingestion of fluid material in bulk takes place by the cell (i.e., through the plasma membrane), the process is known as pinocytosis (in Greek, pinein = to drink), i.e., cell drinking.

Cell Wall


One of the most important features of all plants is the presence of a cellulosic cell wall. Fungi such as mushrooms and yeast also have cell walls, but these are made of chitin.

The cell wall is freely permeable (porous), and so has no direct effect on the movement of molecules in or out of the cell. The rigidity of their cell walls helps both to support and to protect the plant.

Plant cell walls are classified into the following two types:

  • Primary (cellulose) cell wall—While a plant cell is being formed, a middle lamella made of pectin, is formed and the cellulose cell wall develops between the middle lamella and the cell membrane. As the cell expands in length, more cellulose is added, enlarging the cell wall. When the cell reaches full size, a secondary cell wall may be formed. Proteins interweave through cellulose and pectin networks. Extensin protein provides structural support and may form a barrier invading microbes. Expansin protein loosens the cell wall for cell expansion by the addition of cellulose molecules of the cellulose microfibrils in mature cells that otherwise cannot grow.
  • Secondary (lignified) cell wall—The secondary cell wall is formed only in woody tissue (mainly xylem). The secondary cell wall is stronger and waterproof, and hence once a secondary cell wall forms, a cell can grow no more.
  • At certain places, secondary wall is not laid down. Such areas are called simple pits. The pits in the walls of adjacent cells are often opposite to each other. These pits are separated by a pit membrane composed of middle lamella and primary walls. These pits are separated by a pit membrane composed of middle lamella and primary walls. In the tracheids of gymnosperms, the secondary wall party overhangs the pits. Such pits are known as the bordered pits.


Everything within the cell membrane which is not the nucleus is known as the cytoplasm.

Cytosol is the jelly-like mixture in which the other organelles are suspended. Hence, cytosol + organelles = cytoplasm.

Role of Cytoplasm

  1. It is a site of metabolic processes such as biosynthesis of fatty acids, sugars, proteins, etc.
  2. It is also a storehouse for the raw materials needed for metabolism in both the cytoplasm and nucleus.
  3. It distributes nutrients, metabolites, and enzymes in the cell.
  4. It brings about the exchange of materials between the cell organelles.
  5. It also exchanges materials with the environment or extracellular fluid.

Organelles carry out specific functions within the cell. In eukaryotic cells, most organelles are surrounded by a membrane, but in prokaryotic cells there are no membrane-bound organelles. There are two types of organelles, namely membranous and non-membranous organelles.

Membranous Organelles
Mitrochondria: Mitochondria were first observed by Kolliker in 1880 who teased them out of muscle cells of insects. The present name, mitochondria, was assigned by Benda.

Mitochondria are found scattered throughout the cytosol, and are relatively large organelles (second only to the nucleus and chloroplasts). The average size of mitochondria is 0.2 nm to 2n. They are more numerous in cells that have a high energy requirement—our muscle cells contain a large number of mitochondria, as do liver, heart and sperm cells.

  • A mitochondrion consists of outer limiting membrane and inner mass or matrix.
  • It is surrounded by two membranes, indicating that it was once free-living organisms that have become mutualistic and then a part of almost every eukaryotic cell (not RBCs and xylem vessels). The smooth outer membrane serves as a boundary between the mitochondria and the cytosol. The inner membrane has many long folds, known ascristae, which greatly increase the surface area of the inner membrane, providing more space for ATP synthesis to occur.
  • Matrix is dense and contains gel-like substance which fills the internal cavity of the mitochondria.
  • Mitochondria have their own DNA, and new mitochondria arise only when existing ones grow and divide. Mitochondrial ribosomes are of 70S types. Mitochondrial DNA (mtDNA) contains the genetic information for the synthesis of the mitochondrion. However, synthesis of the entire mitochondrion is not coded by mtDNA. Certain mitochondrial proteins are synthesised under the control of nuclear DNA. They are thus semi-autonomous organelles. mtDNA has a unique pattern of inheritance. It is passed down directly from mother to child, and it accumulates changes much more slowly than other types of DNA. Because of its unique characteristics, mtDNA has provided important clues about evolutionary history. For example, differences in mtDNA are examined to estimate how one species is closely related to another.

Functions of Mitochondria: Mitochondria are the sites of aerobic respiration, in which energy from organic compounds (carbohydrates and fats) is transferred to ATP. Hence, they are sometimes referred to as the ‘powerhouse’ of the cell. ATP is the molecule that most cells use as their main energy ‘currency’.

 The term plastid was introduced by E. Haeckel in 1866. It was used by A.W.F. Schimper in 1885. The plastids are the cytoplasmic organelles that are found only in plant cells and some unicellular organisms such as Euglena. Schimper classified plastids into following three types with respect to their structure, pigments and functions:

  1. Leucoplasts
  2. Chromoplasts
  3. Chloroplasts

Leucoplasts are colourless plastids found in cells which are not exposed to light (cells of roots). They are devoid of pigments. They are involved in the synthesis and storage of various kinds of carbohydrates (starch), oil and protein granules.

Chromoplasts are coloured plastids containing carotenoids and other pigments. Chromoplasts impart colours to flowers and fruits which attract insects for pollination.

 are most widely occurring plastids of the plants. They occur mostly in green algae and higher plants.

Chloroplasts are the most important plastids of all types because they carry out photosynthesis. Like mitochondria, they also have double membrane but no cristae. The space enclosed within the chloroplast is filled with a jelly-like fluid called stroma or matrix. Located in the stroma are present numerous double-membrane layers calledthylakoids. The photosynthetic pigments such as chlorophyll a, chlorophyll b, carotenes and xanthophylls are present along the inner side of the thylakoids. These pigments occurring as photosynthetic units are called photosystems. A number of thylakoids are organised like a pile of coins to form a granum. About 40–60 grana are formed at frequent intervals by packed stack of thylakoids. In these thalykoids, the light reaction of photosynthesis takes place. The grana lying adjacent to each other are connected by stromal lamellae or frets. Plastids, like mitochondria, have their own DNA and ribosomes.

Functions of Plastids

  • Photosynthesis is one of the most fundamental biological functions. Due to the presence of chlorophyll, the green plants trap the energy of sunlight and transform it into chemical energy. This energy is stored in the chemical bonds produced during synthesis of various food stuffs, such as starch.
  • Production of NAPDH2 and release of oxygen through the process of photolysis of water.
  • Production of ATP by photophosphorylation. NADPH2 and ATP are the assimilatory powers of photosynthesis.
  • Fixation of CO2 from the air to five carbon sugar in the stroma during dark reaction
  • Breaking of six-carbon atom compound into two molecules of phosphoglyceric acid (PGA) by the utilisation of assimilatory powers.
  • Conversion of PGA into different sugars and store as starch. The chloroplast is very important as it is the cooking place for all the green plants. All heterotrophs also depend on plants for this food.
  • Chromoplasts impart colour to petals and fruits which attract insects for pollination.
  • Leucoplasts store starch, lipids or proteins.

Endosymbiosis theory suggests that mitochondria and chloroplasts have striking similarities to bacteria cells. They have their own DNA, which is separate from the DNA found in the nucleus of the cell. And both organelles use their DNA to produce many proteins and enzymes required for their function. A double membrane surrounds both mitochondria and chloroplasts, further evidence that each was ingested by a primitive host. The two organelles also reproduce like bacteria, replicating their own DNA and directing their own division.

Mitochondria and chloroplasts are the semiautonomous organelles because they have their own DNA, Mitochondria Ribonucleic Acid (mRNA), Transfer Ribonucleic Acid (tRNA) and ribosomes and they replicate by binary fission so that they are said to be self-governing. Semi-autonomous means that they want to leave but they are in a symbiotic relationship with the cell and have evolved to become a part of it.

Endoplasmic Reticulum
 The cytoplasmic matrix is traversed by a complex network of interconnecting membrane-bound vacuoles or cavities. These vacuoles or cavities often remain concentrated in the endoplasmic (granulated) portion of the cytoplasm, therefore, known as endoplasmic reticulum (ER). The term ‘endoplasmic reticulum’ (ER) was coined by Keith Porter in 1953. It communicates with the plasma membrane and also with the nuclear envelope.

The ultrastructure of ER reveals that they are formed of three types of elements: cisternae, vesicles, and tubules.
  • Cisternae are narrow, two-layered, unbranched elements found near the nucleus. They are piled one upon the other and are interconnected.
  • Vesicles are oval or rounded elements found scattered in the cytoplasm. Both cisternae and vesicles are studded with ribosomes and are mainly found in protein-forming cells.
  • Tubules are wide, branched elements found mainly near the cell membrane. These are without ribosomes and are more in lipid-forming cells.

Types of ER

  • The rough ER is studded with 80s ribosomes and is the site of protein synthesis. It is an extension of the outer membrane of the nuclear envelope, hence allowing mRNA to be transported swiftly to the 80s ribosomes, where they are translated in protein synthesis.
  • The smooth ER is where polypeptides are converted into functional proteins and where proteins are prepared for secretion. It is also the site of lipid and steroid synthesis, and is associated with the Golgi Apparatus. Smooth ER has no 80s ribosomes and is also involved in the regulation of calcium levels in muscle cells, and the breakdown of toxins by liver cells.

Functions of ER:

  • The primary function of the ER is to act as an internal transport system, allowing molecules to move from one part of the cell to another.
  • The quantity of ER inside a cell fluctuates, depending on the cell’s activity. Cells with a lot of ER include secretory cells and liver cells. Both types of ER transport materials throughout the cell.

Golgi Apparatus

Golgi Apparatus
Golgi Apparatus was discovered by Camillo Golgi in 1898 in the cytoplasm of nerve cells. Morphologically, it is similar in plant and animal cells. It is a system of membranes, made of flattened sac-like structures called cisternae and associated secretory vesicles. In plant cells, they are called dictyosomes.

Functions of Golgi Apparatus

  • The Golgi Apparatus is the processing, packaging and secreting organelle of the cell. Hence, it is much more common in glandular cells.
  • It works closely with the smooth ER, to modify proteins for export by the cell.
  • During cell division in plants, Golgi Complex forms the cell plate.
  • Synthesis of cell wall, plasma membrane and peroxisomes occur from Golgi Apparatus.
  • Acrosome of sperm is formed by the Golgi Apparatus.

Lysosomes Lysosomes were first reported by Christian de Duve in 1955. They occur in most animal cells and in the meristematic cells of a few plants. They are absent in bacteria and mature mammalian erythrocytes.

Lysosomes are small spherical organelles that enclose hydrolytic enzymes within a single membrane (see Figure 4.3). They are formed from pieces of the Golgi Apparatus that break off. They are common in the cells of animals, protista and even fungi, but rare in plants.

They are polymorphic (i.e., of four types)—primary lysosomes, secondary lysosomes, residual bodies and autophagic vacuoles.

  • Primary lysosomes are newly formed lysosomes. They are formed as vesicles by pinching off from Golgi Complex.
  • Secondary lysosomes are known as heterophagosomes or digestive vacuoles. Membrane bound vesicles are formed by phagocytosis or pinocytosis. These vesicles called phagosomes or pinosomes, respectively, fuse with primary lysosomes to form secondary lysosomes. The breakdown of ingested material takes place in them.
  • Autophagic vacuoles: Lysosomes not only bring about digestion of extracellular materials, but also the digestion of intracellular substances. This latter process is called autophagy. This process is responsible for the turnover of various organelles. It is initiated by the formation of a covering by smooth-surfaced membrane around the organelle concerned. Lysosomes fuse with such vesicles to form autophagic vacuoles. The digestion inside these vacuoles releases the breakdown products into cytoplasm, which are utilised as raw material for remodelling of the organelle.
  • Residual bodies: The process of digestion by lysosomal enzymes releases simpler molecules into cytoplasm. However, this process is not complete as some undigestible materials still remain inside and these are called residual bodies.

Forms and Functions of Lysosome


Functions of Lysosomes

Functions of Lysosomes:
 Lysosomes are the site of protein digestion—thus allowing enzymes to be re-cycled when they are no longer required. They are also the site of food digestion in the cell, and of bacterial digestion in phagocytes. Thus, they are called digestive bags of the cell.

Sometimes, they also help in the digestion of the cell itself by autolysis. Hence, lysosomes are called suicidal bags of the cell.

Several diseases are caused by lysosomal disorder. A particularly devastating is a nervous disorder known as Tay-Sachs diseases in which lysosomes lack a particular lipid digesting enzyme. On the fusion of these enzymes-deficient lysosomes with lipid-containing vesicles, they do not fully digest their content. The resulting defective lysosomes accumulate and lack the long thin parts of nerve cells responsible for transmitting nerve impulses, leading eventually to death.

 The most prominent structure in plant cells is the large vacuole. It is a large membrane-bound sac that fills up much of most plant cells. In plants, one or more vacuoles are present in the cytoplasm. The watery fluid of the vacuoles is called cell sap and is surrounded by a membrane called tonoplast. Cell sap contains several minerals, sugars, amino acids and waste products dissolved in water. While the cells of other organisms may also contain vacuoles, they are much smaller.

Functions of Vacoules
: Vacuoles help in maintaining the osmotic pressure of the cell. They store metabolic wastes of the plant cells. The vacuoles of some plants contain poisons (e.g. tannins) that discourage animals from eating their tissues. In protozoans, small and temporary vacuoles are present, which function asosmoregulators (excretory organs).

A microbody is an organelle bound by a single-boundary membrane. Its matrix, or intracellular material, is electron dense and contains enzymes and other proteins. Four different types of microbodies include peroxisomes, glyoxysomes, glycosomes and hydrogenosomes.

Microbodies are found in cells of plants, protozoa and animals. There are many types of microbodies (see Function below) found in eukaryotic cells. In vertebrates, microbodies are especially prevalent in the liver and kidney organs.

Functions of Microbodies: 
The function of microbodies is specific to the cell type. However, across the board, all microbodies contain enzymes which participate in the preparatory or intermediate stages of biochemical reactions within the cell. Specifically, microbodies allow for the breakdown of fats, alcohols and amino acids to take place. Microbodies in plants convert oils and/or fats to sugars that are used in energy-releasing reactions in the mitochondria. They also help breakdown about half of the ethyl alcohol which we consume. Often, hydrogen peroxide is a by-product of these deconstructive reactions. Hydrogen Peroxide itself is then broken down into water and oxygen.

Non-membranous Organelles


Ribosomes Ribosomes were first isolated from cell cytoplasm by Claude in 1943. Ribosomes were first observed in plant cells by Robinson and Brown in 1953 in bean roots. They were observed by George Palade in animal cells in 1953.

Ribosomes are found both in prokaryotes and eukaryotes with the exception of mature sperm and RBCs. They also occur inside the cell organelles such as mitochondria and chloroplasts.

Structure of Ribosomes:
 Ribosomes are extremely small bodies found either in the free state in the cytoplasm or attached to the surface of the ER. They are composed of ribonucleoprotein (ribonucleic acid and protein). According to the size and sedimentation coefficient (expressed in Svedberg unit, S), two types of ribosomes have been recognised:

  1. 70S ribosomes are comparatively smaller in size with two subunits (30S + 50S). These are found in prokaryotic cells as well as in the mitochondria and plastids of eukaryotic cells. (mtDNA is now found to be 55S.)
  2. 80S ribosomes have two subunits (40S + 60S). They occur in the eukaryotic cells of plants and animals. The two subunits of both types of ribosomes contain different types of rRNA and proteins.

Several ribosomes may become attached to a single messenger-RNA for simultaneous synthesis of variable protein molecules. Such complexes between messenger RNA and ribosomes are known as polyribosomes or polysomes.

Functions of Ribosomes
: Ribosomes are the sites of protein synthesis. They contain protein and RNA.

Centrioles (Centrosome)
 Centriole (in Greek, centrum = centre) was discovered by Van Beneden in 1887 and its structure was elaborated by Boveri in 1888. Centrioles are found in all animal cells except the mature mammalian RBCs. They are absent in prokaryotes, fungi, and higher plants such as gymnosperms and angiosperms.

Centrioles are barrel-shaped organelles found in the cells of animals and protists. They occur in pairs, usually at right angles to each other near the nucleus. The region surrounding the pair of centrioles is known as centrosphere. Each centriole is a short cylinder with a 9+0 pattern of microtubule triplets, i.e., a ring having nine sets of peripheral triplets with none in the middle.

Functions of Centriole:

  • At the time of cell division, centrioles move to the poles and form asters which organise the spindle fibres during the process of cell division.
  • Centrioles give rise to cilia and flagella in animal cells.

Cytoskeleton The cytoplasm of all eukaryotic cells is criss-crossed by a network of protein fibres called cytoskeleton (in Greek kytos = cells, skeleton = dried body). The cytoskeleton extends from the nucleus to the plasma membrane. Eukaryotic cells may contain three types of cytoskeletal fibres, namely microfilaments (actin filaments), microtubules (tubulin) and intermediate filaments, each formed from different kinds of protein subunits. These cytoskeletal fibres apart from maintaining the cell shape are also involved in cell movements.

Cilia and Flagella
 Cilia and flagella are structures that project from the cell, where they assist in movement. Cilia (singl. cilium) are short, and numerous and hair-like. Flagella (singl. flagellum) are much longer, fewer, and are whip-like.

The cilia and flagellae of all eukaryotes are always in a ‘9+2’ arrangement that is characteristic of the core. Protoctista commonly use cilia and flagellae to move through water. Sperm uses flagellae (many, all fused together) to swim to the egg. Cilia line our trachea and bronchi, moving dust particles and bacteria away from the lungs.

 The nucleus is the most conspicuous and largest organelle controlling all the vital activities of eukaryotic cells. The nucleus was discovered and named by Robert Brown in 1831.

In a young cell, it occupies a central position. In mature plant cells, with the formation of the vacuole, it is shifted to one side. Usually, a single nucleus is present in each cell, but some cells may have more than one nucleus. In bacteria and blue-green algae, a true nucleus is absent, but nuclear material is present. The nucleus is absent in mature mammalian RBCs and in the sieve tube cells in phloem tissue of plants. The size of nuclei varies from 5 to 25 μm.

Structure of Nucleus:

  • The content of the nucleus is enveloped by a double membranous structure called nuclear envelope or karyotheca. It separates the nucleus from the surrounding cytoplasm.
  • Numerous pores are present in this membrane to allow the transport of materials between the nucleus and the cytoplasm. These pores are called nuclear pores.
  • The colourless dense sap present inside the nuclear envelope is known as nuclear sap or karyolymph or nucleoplasm.
  • Inside the nucleus is a tangled mass of thread-like structures called chromatin. It is formed of DNA and proteins. When a cell starts to divide, the tangled mass of chromatin condenses into threads and finally rod-like bodies called chromosomes. The chromosomes contain stretches of DNA which carry information for protein synthesis. These stretches of DNA are called genes. Genes are passed from parents to children through sperms and eggs. This is why a gene is called the hereditary unit and DNA is called the hereditary material.
  • Nucleolus is a spherical organelle inside the nucleus. It is not covered by membrane. More than one nucleolus may be found in a cell. Nucleoli are very large in cells that are active in protein synthesis. Their role is to synthesise and assemble RNA molecules and numerous proteins that make up the ribosome.

The differences between the plant and animal cells are clearly shown in Table.


Differences between Plant and Animal Cells


Plant Cells

Animal Cells

Usually larger with distinct boundaries.

Usually smaller, with less distinct boundaries.

Cell wall made up of cellulose is present.

Cell wall absent.

Plasma membrane is present internal to the cell wall.

Plasma membrane forms the boundary of the cell.

Cytoplasm is not so dense and only a thin lining mostly pushed to the periphery.

Cytoplasm denser and granular and fills almost the entire cell.

Vacuoles prominent, one or more.

Vacuoles, if any, are small and temporary, concerned with excretion or secretion.

Plastids are usually present.

Plastids are absent.

Centrosome is absent.

Centrosome is present.

Microvilli and desmosomes are absent.

Microvilli and desmosomes are present.

General Organisation of Eukaryotic Cell

  • Usually large, 5-100 nm.
  • Cellulosic cell wall is present only in plants.
  • Cell organelles such as mitochondria, endoplasmic reticulum, Golgi Apparatus, lysosomes, peroxisomes, plastids etc. are present.
  • Nucleus is prominent and DNA is surrounded by nuclear membrane. Nucleolus is present.
  • Presence of cytoskeleton.
  • Cell division takes place by mitosis or meiosis.

General Organisation of Prokaryotic Cell

  • Generally small, 1-10 nm.
  • Cell wall is non-cellulosic.
  • Cell organelles except ribosomes are absent.
  • Nucleus is absent. Nucleoid or nuclear region is not surrounded by nuclear membrane, i.e. DNA is naked.
  • Cytoskeleton is absent.
  • Cell division takes place by fission or budding, i.e., amitosis.

Chromosomes and the DNA


Structures of Chromosomes Chromosomes are thread-like darkly staining bodies in the nucleus of the cell (see Figure 4.4). It was discovered in 1882 by Walter Fleming. These are the messengers of heredity.


General Structure of a Chromosome

  • The elastic-contractile structures in chromosomes are known as chromatin threads. This condition is found in interphase of the cell.
  • Two similar spirally coiled threads, called chromonemata, are found embedded in the chromosomal matrix.
  • Each chromosome contains two symmetrical chromatids. These two chromatids are held together at a point called centromere.
  • The centromere appears as a narrow region, called primary constriction of the chromosome. 

A chromosome may have an additional constriction termed secondary constriction near its tip.

  • The part of the chromosome beyond the secondary constriction is termed satellite. The secondary constriction is always constant in its position.
  • The chromosomes having satellites are known as SAT chromosomes. The prefix SAT stands for sine acido thymidine which means that the DNA present in satellite does not contain thymidylic acid.
  • Certain secondary constrictions are sites for the formation of nucleoli and are known as nucleolar organisers (NOR). The secondary constrictions are always constant in their positions and hence can be used as mark chromosomes.
  • A chromatid contains a single fine chromatin fibre, the chromonema. It is very long and greatly coiled to be accommodated in a short chromatid. The chromonema is composed of DNA, combined with proteins.
  • The terminal parts of linear chromosome are called telomeres. They are regions of highly repetitive DNA. Telomere prevents the adhesion of one chromosome to another at the telomeric ends. When a chromosome breaks, the telomeric ends ensure that the broken chromosomes are reunited at the broken ends only.
  • The electron microscope shows a chromatin fibre as a chain of repeating units called nucleosomes.
  • The central part or core of a nucleosome is made up of histone proteins which are of four types, namely H2A, H2B, H3 and H4 (H for histones). There are two molecules of each of these, which together form an octamer (eight molecules) in the centre.
  • DNA surrounds the histone octamer by 13/4 turns and is 2 nm in diameter. This part of DNA consists of 146 base pairs.
  • There is another histone protein H1 which binds to the DNA of the nucleosome where it enters and leaves. H1 helps in DNA packing (a process of condensing a chromatin into chromosomes). This process enables a very large amount of DNA to fit into the nucleus of a cell.
  • The part of the DNA that connects two adjacent nucleosomes is called linker DNA or spacer DNA.

Types of Chromatin: The chromatin is observed only in the interphase and is differentiated into two regions, namely euchromatin and heterochromatin.

  1. Euchromatin: The lightly stained and diffused region of the chromatin is known as euchromatin. It contains comparatively large amount of DNA.
  2. Heterochromatin: The darkly stained, tightly packed, or condensed region of the chromatin is known as heterochromatin.

Types of Chromosomes: A chromosome may have either equal or unequal arms depending on the position of the centromere. Accordingly, the chromosomes are of the following four types:

  1. Metacentric chromosome: Here, the centromere is near the middle and the two arms are almost equal in length.
  2. Submetacentric chromosome: The centromere is slightly away from the middle point and consequently its one arm is slightly shorter than the other arm.
  3. Acrocentric chromosome: Here, the centromere is near the end and consequently its one arm is very short and the other arm very long.
  4. Telocentric chromosome: The centromere is at the tip of chromosome and the arm is on one side only.

Nucleic Acids We know that chromosomes are popularly known as ‘vehicles or carriers of heredity’ and genes are the ‘units of heredity’. These genes consist of DNA, which is a polymer that belongs to the class of compounds known as nucleic acids.

There are two types of nucleic acids, namely DNA and RNA. These are the molecules that enable living organisms to reproduce their complex components from one generation to the next.

Deoxyribonucleic Acid
 DNA shape was studied by Rosalind Franklin in 1953 and the structure was finally worked out by Watson and Crick in the same year and was called the Watson–Crick model or double helix model of DNA.

  • The structure of a DNA molecule resembles a twisted ladder, and hence called ‘double helix’.
  • Each strand of DNA contains a large number of small units of nucleotides which are made up of a pentose sugar, a phosphate unit and a nitrogen base. The pentose sugar in DNA is the deoxyribose sugar (C6H10O4).
  • Deoxyribose sugar and phosphate units are arranged alternately and the nitrogen bases connect the two opposite strands like the rungs of a ladder.
  • Adjacent nucleotides are joined by covalent bonds called phosphodiester linkages between the –OH group on the 3′-carbon of one nucleotide and the phosphate on the 5′-carbon of the next. This bonding results in a backbone with a repeating pattern of sugar phosphate units. The two free ends of the polymer are distinctly different from each other. One end has a phosphate attached to 5′-carbon and the other end has a hydroxyl group on a 3′-carbon, we refer to these as the 5′-end and the 3′-end, respectively. Thus, we say that the DNA strand has a built-in directionality along its sugar phosphate backbone, from 5′ to 3′. Thus, the two strands of DNA have an opposite chemical polarity, that is the 5′-ends of the two strands are at the opposite ends.
  • The two chains run in opposite directions and their 3′-and 5′-phosphodiester links are antiparallel.
  • The diameter of DNA double helix is 20 Å.
  • One full turn of a helix is called a gyre. It measures 34 Å in length.
  • Each gyre accommodates 10 pairs of nucleotides, and therefore the distance between the two successive nucleotides is 3.4 Å.
  • Nitrogen bases are of two types—purines and pyrimidines. Purines are of two types, namely adenine and guanine. Similarly pyrimidines are also of two types, namely cytosine and thymine. A purine on one chain always pairs with a pyrimidine on other chain. It means adenine pairs with thymine with two hydrogen bonds and guanine pairs with cytosine with three hydrogen bonds. Thus, the two strands of DNA are complementary to each other.
  • Significance of DNA: DNA controls all the activities of the cell both directly and indirectly. The unique feature of the DNA is its property of duplicating itself during cell division (replication).

DNA is the hereditary material but where it is absent, RNA functions as the hereditary material.

Ribonucleic Acid
 Ribonucleic Acid (RNA) occurs in all living cells and in viruses. In eukaryotic cells, more than 90% of the RNA is found in the cytoplasm and the rest in the nucleus. RNA has been found to be the genetic material in some plants, animals, viruses, and bacteriophages. Such RNA is called the genetic RNA. In other organisms, where the genetic information is contained in the DNA, RNA occurs in the nucleus, cytoplasm, ribosomes, chloroplasts and mitochondria. Such an RNA is called non-genetic RNA.

Like DNA, RNA is a macromolecule and is a polynucleotide chain. But it is a single-stranded molecule and is shorter than the DNA molecule. It may twist by itself so as to form loops and helical regions. The pentose sugar in RNA is ribose. Nitrogenous bases of RNA are adenine and guanine (purines) and cytosine and uracil (pyrimidines).

Types of RNA:
 Three types of RNA are distinguished based on structure and functions. They are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).

All types of RNA are synthesised on one of the strands of DNA by a process called transcription.

  • Messenger RNA (mRNA) accounts for about 5–10% of the total cellular RNA. Because it carries information from the DNA for protein synthesis, it is known as messenger RNA.
  • Transfer RNA (tRNA) or adopter RNA consists of about 70–90 nucleotides and forms about 10–12% of the total RNA in the cytoplasm. tRNA molecules transfer a specific amino acid from the amino acid pool to the ribosome during protein synthesis. Hence, it is called transfer RNA (tRNA).
  • Ribosomal RNA (rRNA) forms about 80% of the total RNA in a cell. rRNA helps in binding the mRNA and tRNA to the ribosomal surface. It coordinates the process of protein synthesis, and acts as an enzyme ribozyme.

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