THE CELL

 

            THE CELL

WHAT IS A CELL?

Cells are the basic unit of life. It is present in all living organisms as no living organism can have life without being cellular because cell is a unit of both its structure and function. Life begins as a single cell Cells are the building blocks of the body or act as the basic unit of body structure.

Important

Branch of biology that deals with various aspects of chemistry, structure development genetics and functioning of the cell is called cell biology. But the study of forms composition and structure Is called cytology.

On the basis of number of cells present, organisms can be divided into:

(1) Unicellular organisms: These are made up of only a single cell and it Is capable of independent existence. It is able to perform all the essential functions. Examples: Amoeba, yeast, etc.

(2) Multicellular organisms: These are made up of many cells. In multicellular organisms, cells are building blocks of the body or basic units of body structure but, the cell becomes specialised for performing different functions. Examples: Man, higher plants, etc.

Anything less than a complete cell cannot perform the essential functions of life as it has no independent existence. Cell is the structural and functional unit of an organism.

Important

Besides structural units the cells are also the functional unit of life. The activities of an organism are actually the sum total of the activities of its cells. Each cell consists of various organelles which perform different functions. Thus the cell is the basic unit and also the structural and functional unit of life.

Discovery of Cell

First time the word cell was referred red to tiny units of life in 1665 by a British Scientist named Robert Hooke. Hooke was one of the earliest scientists to study living things under the microscope. The microscopes of those days were not very strong, but Hooke was still able to make an important discovery. When he looked at a thin slice of cork under his microscope, he was surprised to see a honeycomblike structure in that slide.

Soon after Robert Hooke discovered a cell in the cork cell, Anton van Leeuwenhoek in Holland made other important discoveries using a microscope Leeuwenhoek made his own microscope lenses, and his microscope was almost as strong as modern light microscopes. Leeuwenhoek was the first person to observe human cells and bacterial cells.

Robert Brown discovered the presence of a nucleus inside the cell. With the invention of the electron microscope, the structural details of the cells had been revealed.

Cell Theory

This theory was put forward by both Schleiden and Schwann Observations were started by Matthias Schleiden, a German botanist in 1837. He found that all plants are made up of different types of cells which form the tissues of plants. Theodore Schwann (1838). a British zoologist also studied different types of animal cells. He found that all the cells were covered by a thin membrane or outer layer, now called plasma membrane. He also concluded that the presence of a cell wall is a unique feature of plant cells.

Schwann proposed a cell hypothesis-bodies of animals and plants are made up of cells and their products. Schleiden and Schwann together proposed the cell theory. However, one of the limitations of this theory was that it could not explain the formation of new cells.

Rudolf Virchow (1855) observed that new cells arise from pre-existing cells (Omnis cellula-e-cellula) and he gave the final shape to the cell theory as:

(1) All living organisms are formed from the cells and products of cells.

(2) All cells arise from pre-existing cells.

An Overview of Cell

When we observe the onion peel cells or human cheek cells under a microscope, we conclude certain things. There is a cell wall as its outer boundary in an onion cell which is a typical plant cell.

Inside the cell wall there is a cell membrane. In the human cheek cell the outer membrane acts as the delimiting structure of the cell.

Each cell contains a nucleus. Nucleus is a dense membrane-bound structure. It contains the chromosome which contains the genetic material DNA.

Genetic material is something which passes on from parents to their children and is responsible for hereditary characteristics.

On the basis of the presence of a membrane-bound nucleus, the cell is of two types-prokaryotic cell and eukaryotic cell.

(1) Prokaryotic cell: Cells that lack a membrane bound nucleus are called prokaryotic cells. e.g. bacteria, Mycoplasma, etc.

(2) Eukaryotic cell: Cells that have membrane -bound nuclei e.g. plants, animals, fungi, etc.

Inside each cell there is a semi-fluid matrix called cytoplasm which occupies the main volume of the cell Many cellular processes occur in cytoplasm as it is the main area of cellular activities. In eukaryotic cells, various other organelles are also present like endoplasmic reticulum, Golgi apparatus, chloroplast. lysosomes, mitochondria, microbodies, vacuoles, etc.

In both prokaryotic and eukaryotic cells, ribosomes are present Ribosomes are non-membrane-bound organelles. They are present in the cytoplasm and they are also found within the two double membrane bound cell organelles-chloroplast (in plants) and mitochondria and they are also found on rough ER.

Centrosomes are present only in animal cells. It is a nonmembrane-bound organelle. It helps in cell division.

The shape of a cell can vary according to its function They may be disc-like, polygonal, columnar, cuboid, thread-like or even irregular in shape sometimes.

 

 

Size and Shapes of Cell

Cells differ greatly in size and shape.

(1) Mycoplasma is only 0.3 micrometers.

(2) Bacteria are 3 to 5 micrometers long.

(3) Largest single cell is present in the egg of an ostrich.

(4) Human red blood cell is about 7.0 micrometers.

(5) In multicellular organisms, nerve cells are the longest cells.

PROKARYOTIC CELLS

Prokaryotic cells are primitive type of cell in which the genetic material is not organised in the form of a nucleus but instead lies freely in the cytoplasm in a naked supercoiled state and known as nucleoid.

Prokaryotic cells are smaller than eukaryotic cells and they are known for their rapid division capacity.

Defining characteristics of Prokaryotic organisms/ Prokaryotes:

(1) It is represented by bacteria, blue-green algae. Mycoplasma and PPLO (Pleuropneumonia-like Organisms).

(2) Lack a membrane-bound nucleus, thus nucleus is not well defined and DNA (genetic material) lies naked in the cytoplasm.

(3) Cell wall is present, except in Mycoplasma.

(4) Ribosomes are present.

(5) Some bacterial cells have a self-replicating. extrachromosomal segment of double-stranded. circular naked DNA called Plasmid.

(6) They have something unique in the form of inclusions.

(7) Mesosome is present, which is a specialised differentiated form of cell membrane. characteristic of prokaryotes Mesosomes are infoldings of cell membranes.

(8) In shape, the bacterial cell is of four types Bacillus (rod-like), Coccus (spherical), Vibrio (comma-shaped) and Spirillum (spiral).

Components of Bacterial cell

A bacterial cell consists of a cell envelope, cytoplasm. nucleoid, plasmids, inclusion bodies, flagella, pili and fimbriae.

Cell envelope and its modifications

It is the outer covering of protoplasm of bacterial cell.

Cell envelope consists of 3 components:

(1) Glycocalyx: It is the outermost mucilage layer of the cell envelope. Glycocalyx can occur in the form of a loose sheath, then it is called slime layer. If it is thick and tough, the mucilage covering is called capsule. Glycocalyx provides protection from phagocytosis, toxic chemicals and drugs, viruses and prevents the bacteria from desiccation. It also helps in attachment

(2) Cell wall: It is a rigid solid covering which generally provides shape and structural support to the cell. Cell wall lies between plasma membrane and glycocalyx.

(3) Plasma membrane: It is selectively permeable covering of the cytoplasm that forms the innermost component of cell envelope.

Important

In Gram-negative bacteria, cell wall Is 8-12 nm thick complex wavy and double-layered. The outer layer is consist of lipopolysaccharides, lipids and proteins. In Gram-positive bacteria, the cell wall is 20-80 nm thick, it is single-layered and smooth. The single layer of Gram-positive bacteria and inner layer of Gram-negative bacteria is made up of peptidoglycan, proteins, non-cellulosic carbohydrates. lipids, amino acids, etc.

Gram-positive bacteria remain blue or purple after the Gram staining but Gram-negative bacteria do not retain the stain due to the high lipid content of cell wall which gets dissolved in organic solvents like acetone.

Cytoplasm

It is a crystalloid colloidal complex that forms the protoplasm excluding its nucleoid. Cytoplasm is a little granular in structure due to the presence of ribosomes in large quantities. Membrane-bound organelles are absent in prokaryotic cells but found in eukaryotic cells. All the biochemical pathways found in prokaryotes are similar to eukaryotes. Cytoplasmic streaming is absent. Sap vacuoles are absent. Instead. they have some gas vacuoles. Various structures present in the cytoplasm are as follows:

(1) Mesosome: Mesosome is the extension of plasma membrane into the cell It consists of vesicles, tubules and lamellae. They aid in the development of cell walls, DNA replication, and distribution to daughter cells. They also aid respiration, secretion, and the expansion of the plasma membrane's surface area and enzymatic content. In cyanobacteria, chromatophores are present. It is also the membranous extension into the cytoplasm which possesses photosynthetic pigments.

(2) Ribosomes: Ribosomes are small, non-membranous bound structures. Mainly. they are attached to the plasma membrane of a prokaryotic cell. They are 15 nm to 20 nm in size. The ribosomes of prokaryotes are 70S, which is made up of 30S (smaller) and 50S (larger) subunits. Each ribosome has two subunits, larger and smaller. They act as the site of protein synthesis. Polyribosomes or polysomes are formed when many ribosomes are attached to a single strand of messenger or mRNA. They translate the mRNA into proteins.

Nucleoids

These represent the genetic material of prokaryotes. Nucleoid consists of a single circular strand of DNA duplex which is supercoiled with the help of RNA and polyamines to form a nearly oval or spherical complex. Polyamines or nucleoid proteins are different from histone proteins and absence of nuclear envelope around them. Nucleoid is embedded freely in cytoplasm It is equivalent to a single chromosome of eukaryotes because nucleoid consists of a single DNA double-stranded. Nucleoids may be directly attached to the plasma membrane through the mesosomes.

Plasmids

They are self-replicating, extracellular chromosomal segments of double-stranded, circular, naked DNA Plasmids provide unique phenotype characters to the bacteria. Most of the time they are independent of nucleoids but if they are temporarily associated with nucleoids then known as episomes. Some of the plasmids contain important genes like fertility genes, nifgenes, resistance genes, etc. Plasmids are used as a vector in genetic engineering.

Inclusion bodies

They are non-membranous, non-living structures lying freely in the cytoplasm Reserve material is stored in the form of inclusion bodies. Examples: Phosphate granules, cyanophycean granules, and glycogen granules. Gas vacuoles are found in blue green, purple, and green photosynthetic bacteria.

Flagella

Bacterial cells can be motile or non-motile. If motile, they have flagella. Flagella is a thin membranous extensions from their cell wall Bacterial flagellum is made up of three parts filament, hook and basal body. Filament extends from the cell surface to the outside. It is a tubular structure that is the longest among the three.

Pili and fimbriae

These are the structures present on the surface of bacteria but are not involved in locomotion Pili are elongated with fewer and thicker tubular outgrowth. They are made up of a special protein, pilin. Fimbriae are small bristle-like fibres sprouting from the cell surface in large numbers. They are involved in attaching bacteria to solid surfaces like rocks in streams and also to the host tissues.

 

EUKARYOTIC CELLS

STRUCTURE AND COMPONENTS OF EUKARYOTIC CELLS

Eukaryotic cells have a nucleus with a nuclear envelope and the cells also have locomotory and cytoskeletal structures. Chromosomes contain genetic material in eukaryotic cells. Plant and animal cells are different cell types as plant cells contain cell wall and plastids, a large central vacuole but these are absent in animal cells and animal cells have centrioles that are absent in plant cells. Let's have a look at cell organelles of eukaryotic cells.

 

Cell Membrane

In eukaryotic cells, the plasma membrane surrounds a cytoplasm filled with ribosomes and organelles. Organelles are structures that are themselves encased in membranes. Some organelles (nuclei. mitochondria and chloroplasts) are even surrounded by double membranes. All cellular membranes are composed of two layers of phospholipids embedded with proteins. All are selectively permeable (semi-permeable), allowing only certain substances to cross the membrane.

Biochemical investigations that were conducted later proved that plasma membranes also possess protein and carbohydrates. The ratio of protein and lipid varies in different cell types. The membranes of erythrocytes in human beings, for example, have 52 per cent protein and 40 per cent lipids.

The structure of plasma membrane as proved by various studies is:

(1) It is made up of lipids and proteins.

(2) Lipids-mainly phospholipids, arranged in a bilayer with polar heads of lipid molecules on the outer side and hydrophobic tails are inside. This arrangement protects the nonpolar tails from aqueous environment.

(3) Phospholipid membrane contains cholesterol and phosphoglycerides.

(4) Membrane proteins can be integral (buried partially or completely in the membrane) or peripheral (present on the surface).

Fluid Mosaic Model

(1) It is the most recent model of a biomembrane given by Singer and Nicolson in the year 1972.

(2) According to this model, the membrane does not have a uniform deposition of lipids and proteins but is instead, a mosaic of two layers.

(3) The membrane in the fluid mosaic model is stated as quasi-fluid.

(4) The quasi-fluid nature of the biomembranes is shown by their properties of quick repair, ability to fuse, dynamic nature, contract and expand, cell growth and cell division, secretion, formation of intercellular junctions and endocytosis.

Functions of plasma membrane

(1) It transports molecules across it and is selectively permeable to some molecules.

(2) Plasma membrane protects the cell from injury.

(3) This membrane allows the flow of material and information between different organelles of the same cell as well as between one cell and another.

(4) As plasmodesmata or gap junctions, the membranes provide organic connections between adjacent cells.

(5) Membrane infolds are used for bulk intake of material by endocytosis.

(6) Secretory, excretory and waste products are thrown out by plasma membrane through exocytosis.

(7) Transport in Membrane: The transport of molecules occurs in the following ways:

(i) Passive transport: When molecules move across the membrane without the requirement of energy. It occurs as:

(a) Simple diffusion: Neutral solutes may move by simple diffusion from higher to lower concentration and water also moves in a similar fashion across the membrane.

(b) Osmosis: The movement of water from higher to lower concentration through a semi-permeable membrane by diffusion is called osmosis.

(ii) Active transport: It is difficult for polar molecules to move across the non-polar bilayer membrane so, they require a carrier protein. When ions are transported against their concentration gradient from lower to higher concentration then ATP is utilised (energy-dependent process) and it is called active transport. Example Na*/K* pump.

Important

The Components of the Plasma Membrane:

COMPONENTS	LOCATION
Phospholipids	Main component of the membrane.
Cholesterol	Tucked between the hydrophobic tails of the membrane phospholipid.
Integral Proteins	Embedded in the phospholipid bilayer. may or may not extend through both layers.
Peripheral Protein	On the inner or outer surface of the phospholipid bilayer, but not embedded in its hydrophobic core.
Carbohydrates	Attached to proteins or lipids on the extracellular side of the membrane. (forming glycoproteins and glycolipids).

 

Cell Wall

(1) The plasma membrane of plants and fungi is protected by an additional covering called as cell wall.

(2) Cell wall is a non-living, solid structure which aids cell-to-cell communication, acts as a barrier against unwanted macromolecules, gives the cell structure, and protects the cell from damage and infection.

(3) Algae have cellulose, galactians, mannans, and minerals (calcium carbonate) in their cell walls, whereas plants have cellulose, pectins, hemicellulose, and proteins in their cell walls.

(4) The cell wall of immature plant cells is termed the main wall and it is capable of expansion. As the cell matures, the primary wall becomes less capable of growth. The secondary wall is then created on the cell's inner side. The middle lamella, which is composed of calcium pectate, connects the adjacent cells. Furthermore, the plasmodesmata may traverse the cell which connects the cytoplasm of neighbouring cells.

Important

A cell wall can have three parts

(1) Middle lamella: It is a thin, amorphous and cementing layer between two adjacent cells. Middle lamella is made up of calcium and magnesium pectate.

(2) Primary Wall: It is the first layer produced inside the middle lamella. Primary wall consists of a number of microfibrils embedded in the amorphous gel-like matrix. In plants, Microfibrils made up of cellulose

(3) Secondary wall: It is laid inner to primary wall It grows when the cell stop growing. Cellulose content is generally high than the primary wall.

Endomembrane System

Endomembrane system is a group of some membrane-bound cell organelles which function in a close coordination with one another. These organelles are distinct with respect to structure and function but when they work together, they form an endomembrane system. For example, the endoplasmic reticulum, Golgi complex, lysosomes, and vacuoles form an endomembrane system.

 

Endoplasmic Reticulum

(1) It is a network of tiny tube-like structures that are scattered in the cytoplasm, as revealed by studying eukaryotic cells with an electron microscope.

(2) The ER divides the intracellular space into two comportments: (i)Luminal (inside ER): Means space within the ER (ii) Extra-luminal (rest of cytoplasm): Meaning in the cytoplasm.

(3) Sometimes ribosomes are attached on the surface of ER and then it is called the rough endoplasmic reticulum (RER). Without ribosomes, the ER appears smooth and is called the smooth endoplasmic reticulum (SER).

(4) RER is found in the cells that are actively involved in protein synthesis and secretion and it is continuous with the outer nuclear membrane. On the other hand, lipid-like steroidal hormones are produced by SER and it is a site of lipid synthesis in animal cells.

Important

RER provides a larger surface area to ribosome. Proteins and enzymes synthesised by ribosomes enter the channels of RER both for intercellular use as well as secretion.

SER can develop from RER by discarding ribosomes.

Golgi Apparatus

(1) Discovered by Camillo Golgi in 1898 and named Golgi bodies after him.

(2) They are densely stained reticular structures, found near the nucleus and consist of flat disc-shaped sacs or cisternae of 0.5 to 1.0 µm diameter.

(3) Cisternae are stacked parallel to each other and a varied number of cisternae are present in a Golgi complex. The cisternae of the Golgi apparatus are concentrically arranged near the nucleus. They have a convex or (cis or forming face) and a concave (trans or maturing face), and although these faces are entirely different, they are interconnected.

Functions:

1. To wrap/package materials, which are either carried into the intracellular domain or secreted extracellularly.

2. The material to be packaged is released from the ER in the form of vesicles, from which it fuses with the Golgi’s cis face and travels towards the trans face. This explains why the Golgi apparatus is so close to the ER

3.  Many proteins made in the ribosomes of endoplasmic reticulum are modified by the Golgi apparatus, which is an important synthetic site for glycoproteins and glycolipids.

 

Lysosomes

(1) Membrane-bound vesicular structures, generated by packaging in the Golgi apparatus.

(2) Lysosomal vesicles are abundant in nearly all types of hydrolytic enzymes (hydrolases-lipases. proteases, carbohydrases) that are ideally active at an acidic pH. Carbohydrates, proteins, lipids, and nucleic acids can all be digested by the enzymes found in lysosomes.

(3) Lysosomes are called suicide bags of the cell because of the presence of large number of different digestive enzymes or acid hydrolases in them.

Vacuoles

(1) Membrane-bound spaces found in the cytoplasm containing water, sap, excretory products, and other materials not used by the cell.

(2) Vacuoles have a single membrane called tonoplast and in plant cells, they occupy 90 per cent of the volume of the cell.

(3) Tonoplast transports ions and other materials against their concentration gradients into the vacuole and this makes their concentration higher in the vacuole.

(4) In Amoeba, there is a contractile vacuole for excretion and in many cells, like protists, there are food vacuoles formed by the intake of food particles.

(5) Plants, fungi, algae, and other organisms store potent secondary metabolites like tannins or other biological pigments in their vacuoles to protect themselves against self-toxicity.

Mitochondria

(1) The number of mitochondria is variable in each cell depending on their physiological activity and they are not visible under the microscope unless stained specifically.

(2) Their shape and size are also variable but typically, they are sausage-shaped or cylindrical having a length of 1.0-4.1 µm and a diameter of 0.2-1.0 µm.

(3) Double membrane-bound structure with a lumen having two aqueous compartments, Le an outer compartment and an inner compartment divided by the outer membrane and the inner membrane.

(4) The outer membrane forms the limiting boundary of the mitochondrion while the inner compartment has a dense homogeneous substance called the matrix.

(5) The inner membrane also forms many infoldings called cristae into the matrix and the cristae increase the surface area.

(6) The two membranes have their own enzymes for mitochondrial function and mitochondria are the site of aerobic respiration. They produce energy in the form of ATP so they are called the 'powerhouse of the cell'.

(7) Mitochondrial matrix has ribosomes (70S). a single circular DNA, a few RNA molecules, and some components for protein synthesis.

(8) The division of mitochondria occurs by fission.

Important

Mitochondria are not fully autonomous. Both their structure and functioning are partially controlled by nucleus of the cell and availability of material from cytoplasm. Mitochondria are believed to be symbionts in eukaryotic cells which become associated with them quite early in the evolution.

Plastids

Mostly found in plant cells and euglenoids and because of their large size they are easily observed under the microscope.

Plastids contain different types of pigments that give the plant certain colours. Based on the kind of pigment, they are classified as chloroplasts, chromoplasts and leucoplasts.

Chromoplast: Chromoplasts have fat-soluble carotenoid pigments like carotene and xanthophylls giving yellow, orange. or red colour to the part of the plant

Leucoplasts: Leucoplasts are colourless plastids of varied shapes or sizes that store nutrients. Now leucoplasts can be further divided into three categories on the basis of stored substances in them. They are amyloplasts that store carbohydrates (e.g. potato); elaioplasts that store oils and fats and lastly, aleuroplasts that store proteins.

Chloroplast: Chloroplasts contain chlorophyll that traps light energy for photosynthesis. Chloroplasts of green plants are usually found in the mesophyll cells of leaves and they can be oval, spherical, lens-shaped, discoid, or ribbon-like with variable length or width.

Their number per cell also varies ranging from 1 per cell (Chlamydomonas) to 20-40 per cell in the mesophyll (green alga).

Structure of chloroplasts

They are double membrane-bound with the inner membrane being relatively less permeable of the two membranes. The space bound by the inner membrane is called stroma and the stroma has many flattened membranous sac-like structures called thylakoids. When thylakoids are organised in stacks like piles of coins, they are called grana (intergranular thylakoids). and additionally, there can be flat, membranous tubules that connect thylakoids of different grana. The thylakoids enclose a space called a lumen and chlorophyll pigments are present in the thylakoids. The stroma contains enzymes for the synthesis of carbohydrates and proteins and it also has double stranded circular DNA and ribosome (70S) molecules. Ribosomes are 70S and they are smaller than the ribosomes found in the cytoplasm that are 80S.

Ribosomes

(1) Ribosomes were discovered by Robinson and Brown in plant cells in the year 1953 while George Palade discovered ribosomes in 1955 in animals and also termed the granular structure as ribosomes.

(2) Made up of ribonucleic acids (RNA) and proteins.

(3) Ribosomes lack a membrane and eukaryotic ribosomes are 80S while prokaryotic ribosomes are 70S.

(4) Ribosomes are popularly known as protein factories.

(5) Every ribosome has two subunits (larger and smaller) while the two subunits of 80S ribosomes are 60S and 40S, the two subunits of 70S ribosomes are 50S and 30S. Here, S is an indirect measure of density or size and it is the Svedberg's unit that stands for sedimentation coefficient.

70S Ribosomes	80S Ribosomes
A small 30S subunit and a large 50S subunit.	A large 60S subunit and a small 40S subunit.
They are found in both prokaryotes and eukaryotes.	They are found exclusively in eukaryotes.
Prokaryotes have a lot of ribosomes in their cytoplasm. It is also found in eukaryotic cell organelles like mitochondria and chloroplasts.	In eukaryotes, ribosomes are found freely inside the cytoplasm or they can be found attached to RER.
They are made in the cytoplasm of prokaryotes.	They are formed inside the nucleolus.

 

Cytoskeleton

(1) Cytoskeleton is extremely minute, fibrous and tubular structure which forms the structural framework inside the cell.

(2) Cytoskeleton is only found in eukaryotic cells.

(3) It is a network of filamentous, proteinaceous structures present in the cytoplasm and it has many functions like motility, mechanical support maintenance of cell shape, etc.

(4) They are of the following three types:

(i) Microfilaments: They are ultramicroscopic long, narrow, cylindrical rods and protein filaments which occur in eukaryotic plant and animal cells. They are made up of Actin protein.

(ii) Intermediate filament: They are almost solid unbranched filaments of about 10 nm thickness which are formed by a variety of proteins.

(iii) Microtubules: They are unbranched hollow sub microscopic tubules of protein tubulin which develop on a specific region.

Cilia and Flagella

(1) Hair-like outgrowths of the cell membrane.

(2) Cilia (sing: cilium) are small structures that work like oars and cause the movement of the cell or surrounding fluid while flagella (sing: flagellum) are comparatively longer and cause cell movement.

(3) Prokaryotic and eukaryotic flagella are structurally different.

(4) Both cilia and flagella have a plasma membrane and their core is called an axoneme that contains several microtubules running parallel to the long axis. In the axoneme, there are nine pairs of doublets of radially arranged peripheral microtubules together with a pair of centrally located microtubules, and such an arrangement is called the 9+2 or 11 stranded. The central tubules are enclosed by a sheath and connected by bridges and they are connected by radial spokes to one of the tubules of each peripheral doublet. Also, the peripheral doublets are connected by linkers and there are a total of nine radial spokes. The centriole-like structures are called basal bodies which give rise to the cilium and flagellum.

 

Important

Cilla and flagella help in cell movement and they are outgrowths of the cell membrane. Cilla and flagella create current to obtain their food in aquatic medium.

Differences between Cilia and Flagella:

Cilia	Flagella
The number of cilia is higher in comparison (typically ranges in the thousands).	In comparison, the number of flagella is lower (usually ranges from 1 to 8).
They can be found in only eukaryotic cells.	Flagella are found in both eukaryotic and prokaryotic cells.
They are shorter in length than the flagella.	The flagella are comparatively longer than the cilia.
The cilia beating pattern is quite complex - Can move in a variety of ways.	Flagella has a circular, wave-like, or propeller-like beating pattern.

 

Centrosome and Centrioles

(1) The centrosome is an organelle with two cylindrical structures called centrioles that are surrounded by pericentriolar material.

(2) The centrosome's centrioles are organised in a cartwheel pattern, and both centrioles are perpendicular to each other. They are made up of nine tubulin protein peripheral fibrils, each of which is made up of three triplets. The adjacent triplets are interconnected, and radial spokes of protein connect the central part of the proximal region of the centriole to the tubules of the peripheral triplets. Hub refers to the centre proteinaceous portion.

(3) During cell division in animal cells, centrioles give rise to the spindle apparatus and serve as the foundation for cilia, flagella, and spindle fibres.

Nucleus

(1) Robert Brown first identified nucleus in 1831. and Flemming subsequently named it chromatin after staining the nucleus' components with basic dyes.

(2) Nucleus is a specialised double-membrane bound protoplasmic body which contains all the genetic material for controlling cellular metabolism and transmission of information.

(3) Usually, there is one nucleus per cell but their number per cell is variable and some mature cells like erythrocytes (in mammals) and sieve tube cells (in vascular plants) even lack a nucleus.

(4) The nucleolus and chromatin are present in the nuclear matrix or nucleoplasm and the nucleoli are spherical structures. The nucleolus is not membrane-bound and their content is continuous with the nucleoplasm. Ribosomal RNA synthesis occurs in the nucleolus and in cells actively carrying out protein synthesis, there are larger and more numerous nucleoli.

Structure of nucleus

(1) It has elaborate and extensive nucleoprotein fibres termed as chromatin.

(2) It contains one or more spherical structures called nucleoli (sing: nucleolus).

(3) Nuclear envelope contains large number of pores or perforations. The nuclear envelope has two parallel membranes with a space between them termed perinuclear space, which forms a barrier between the cytoplasm and the content of the nucleus, as revealed by the electron microscope.

(4) The outer membrane is continuous with the endoplasmic reticulum and also has ribosomes on it and at a number of places, there are minute pores on the nuclear envelope formed by the fusion of its two membranes. The movement of RNA and protein molecules between the nucleus and cytoplasm take place through these nuclear pores.

During interphase, chromatin is a loose, indistinct network of fibres, but during cell division, the cell displays organised chromosomes in their place. In a single human cell, chromatin contains RNA, histone proteins, non-histone proteins, and DNA, which is dispersed throughout forty-six chromosomes and is approximately two metres long.

The centromere is the major constriction on chromosomes, while kinetochores, which are disc shaped, are found on the sides.

The centromere holds the chromosome's two chromatids, and chromosomes are classified into four categories based on where the centromere is located:

(1) Metacentric chromosomes: They have two equal chromosome arms formed by the middle centromere.

(2) Sub-metacentric chromosomes: They have one long and one short arm because the centromere is slightly moved away from the middle.

(3) Acrocentric chromosomes: The centromere is situated close to one end forming one very long arm and one extremely short arm.

(4) Telocentric chromosomes: There is a terminal centromere.

Microbodies

They are minute vesicles that are membrane-bound containing various enzymes and they are found in both plant and animal cells.

Differences between Prokaryotic and Eukaryotic cell

Prokaryotic cell	Eukaryotic cell
Cell size is usually small between (1.0 – 5.0 μm).	The cell is comparatively larger from (5 - 100 µm).
Cell wall possesses muramic acid if present	Cell wall, if present, without muramic acid.
DNA is naked. that is, without histones.	Nuclear DNA is associated with histones.
DNA is circular.	Nuclear DNA is cellular but extra nuclear DNA is circular.
DNA lies freely in the cytoplasm. It is not associated with organelle.	Most of the cell DNA lies in the nucleus. A small quantity is also found in the cell organelles like mitochondria and plastids.
Protein synthesis occurs only in cytoplasm.	Protein synthesis takes place in cytoplasm, mitochondria and plastids.
Ribosomes are 70S types.	Ribosomes are 80S type. 70S ribosomes. However. It occur in plastid and mitochondria.
Membrane bound organelles like ER mitochondria, Golgi apparatus, centrioles, lysosomes and other microbodies are absent.	Mitochondria, ER, Golgi apparatus, microbodies including lysosomes and centrioles are present in the cell of eukaryotic organisms.