What is a cell ?

Chapter 8: Cell the unit of life

1. What is a Cell?

All living organisms are composed of cells. Cells are the building blocks of any living body. It is the basic structural as well as the functional unit of life. Some living organisms are made up of only a single cell and are thus known as unicellular organisms (e.g. bacteria), while many other organisms are composed of numerous cells and are therefore called multicellular organisms (e.g. plants and animals).

The invention of the microscope in the 17th century paved the way for the discovery of cells. Thecell was firstdiscovered by Dutch scientist Antonie van Leeuwenhoek. He was the first person to observe and describe a live bacterial cell. He is also known as the father of microbiology. Later, the discovery of the nucleus in a cell, by Robert Brown was another significant milestone in the field of microbiology.

What is a cell ?


Unicellular organisms are capable of (i) independent existence and (ii) performing the essential functions of life. Hence, cell is the fundamental structural and functional unit of all living organisms.


Aristotle proposed that all animals and plants, however complicated but are constituted of a few elements which are repeated in each of them.

The simple microscope was invented by Galileo.

The first compound microscope was made by Robert Hooke (1665).

He examined thin slices of cork under his microscope and observed the honey comb like structures composed of box like compartments which were termed as the cellulae (cells). 

His work was published in his book "Micrographia". Cells were observed prior to Hooke by Malpighi (1661), who called them saccules and utricles.

Leeuwenhoek observed few living cells capable of moving, such as bacteria. protozoa, spermatozoa and red blood corpuscles under his own designed microscope.

Lamarck remarked that "no living being can have life if its constituent parts are not formed by cells". 
Dutrochet concluded that plants and animals were made up of globular cells and the cells are held together by cohesion. 

In 1831, Robert Brown discovered the presence of nucleus in the cells of orchid root. 
Fontana discovered nucleolus in the skin cell of Eel. 

The term nucleolus was given by Bowman.

Colloidal theory of protoplasm explains the nature of protoplasm in the best manner. It is most acceptable theory.

It was proposed by Fischer, according to which conversion of solution into gel and vice versa is due to colloidal nature of cytoplasm.

Presently, this can be better explained as "Multiphasic colloidal system of life".

Protoplasm theory was proposed by Max Shultze (1861).

According to it "cell is an accumulation of living substances which is limited by an outer membrane & possesses a nucleus".

M.J. Schleiden, a German botanist in 1838 stated that "All plants are formed of one or more cells".

Theodore Schwann, a German Zoologist in 1839 stated that "All animals are formed of cells, have nuclei and are enclosed by thin cell membrane instead of thick cell wall as found in plant cells".

Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and their products.

Rudolf Virchow in 1858 observed that new cells arise from pre-existing cells by division i.e., Omnis cellula e cellula.

Cell Theory

Cell Theory

In the year 1838, a German botanist called Matthias Schleiden discovered that all plant tissues are made up of different types of cells. A British Zoologist, Theodore Schwann, who was a contemporary of Schleiden, reported the presence of athin outer layer in the animal cells which is now known as the plasma membrane.  Schwann also discovered that along with plasma membrane,an additional unique outer layer is also found exclusively in plant cells i.e. the cell wall. Both Schleiden and Schwann together postulated the Cell Theory, based on their findings. However, this theory did not give any indication of the genesis of cells.

In 1855, a German biologist named Rudolf Virchow first explained the origin of cells from pre-existing cells. This led to the modification of the original cell theory proposed by Schleiden and Schwann which is today understood as:

  1. All living organisms are made up of cells and cell products.
  2. Every cell is made up of pre-existing cells.

Cell Theory


The theory was formulated by Schleiden and Schwann.

The various points of cell theory are:

1. Each cell is made of a small mass of protoplasm having a nucleus and bounded by a cell membrane with or without cell wall.

2. All cells are basically alike in structure and metabolism.

3. Organisms are composed of cells and their products.

4. The functions of an organism is an outcome of activities and interactions of its constituent cells.

But, it did not explain as to how new cells are formed.

Cell theory was first modified in the light of Virchow's findings that cells develop from pre-existing cells i.e.,"Omnis cellula e cellula". It is known as law of cell lineage. Number of other modifications were carried out in cell theory. The modern cell theory is known as cell principle or cell doctrine.

Drawbacks of Cell Theory

Important drawbacks of cell theory are given below:

1. Viruses cannot be explained using this theory.

2. Bacteria and blue-green algae do not have an organized nucleus.

3. Certain fungi, such as Rhizopus, have hyphae composed of a multi-nucleated mass of cytoplasm (coenocyte).

4. Acetabularia (unicelled, marine green algae) has a uninucleated differentiated body (acellular).

5. Sieve tube and mature RBC lack nuclei.

6. Volume of the cell is occupied by a semi-fluid matrix called cytoplasm, and is main arena of cellular activities in both pro and eukaryota.

7. Cells differ greatly in size, shape and activities e.g., Mycoplasma -the smallest cells (0.3 µm length), Bacteria (3 -5 µm), Human RBC (~7.0 µm)

Cell Structure : (a) Plant
Cell Structure :(b) Animal


An overview of cell

An overview of the cell

If we observe an onion peel or human cheek cells under a microscope, we will find that the cells in an onion peel show a distinct cell wall as its outermost boundary.  The cell membrane lies just inwards to the cell wall.  On the other hand, in a human cheek cell, the outermost delimiting structure is the cell membrane. In both, plant and animal cells, a centrally located dense membrane-bound structure called the nucleus is present. This nucleus is known to contain hereditary material i.e. DNA. 
Cells that contain a membrane-bound nucleus are known as Eukaryotic cells, whereas the cells in which the nucleus is not defined by a membrane are called Prokaryotic cells. In both prokaryotic as well as eukaryotic cells, the volume of the cell is composed of a semi-fluid matrix called the Cytoplasm. This cytoplasm is the primary site of cellular activities which are important for a living cell.


    Figure 1: Basic structure of a cell.

Along with the nucleus, a eukaryotic cell also contains cell organelles which are distinct membrane-bound structures. These organelles are responsible for carrying out different metabolic functions.  These cell organelles are absent in prokaryotic cells. Some important organelles of a cell are:
a)    Mitochondria: are responsible for generating the majority of the chemical energy required by the cell's metabolic activities.
b)    Endoplasmic Reticulum: is responsible for protein synthesis.
c)    Golgi complex: aids in the processing and packaging of proteins and lipid molecules, particularly proteins destined for cell export.
d)    Lysosomes: act as the digestive system of a cell.
e)    Vacuoles: help in the storage and disposal of various waste substances in a cell.
f)    Ribosomes: are the site for protein synthesis in the cell. They are not bound by a membrane.
g)    Centrosome: is also not bound by a membrane. It is present in animal cells and helps in cell division.

Cells can be differentiated based on size, shape, and activities. The smallest cell i.e. Mycoplasma is only 0.3µm long and is even smaller than bacteria. On the other hand, an Ostrich egg, the largest known single cell, is about 15cm to 18 cm long and wide. Cells can be of varying shapes such asdisc-shaped (RBCs), columnar (Goblet cells in the intestine), polygonal (Hydra), cuboid (kidneys), branched (Neuron), elongated (tracheid),and even irregular (Amoeba and WBCs).

An overview of cell

A typical cell possesses three major elements — outer envelope, genetic material and cytoplasm.

Outer Envelope: A cell is surrounded by an outer membrane called plasma membrane or plasmalemma. It isolates the cell interior. A distinct cell wall lies on its outer side in plant cells. Cell wall provides protection, rigidity and shape to cells.

Genetic Material: It represents hereditary material that not only controls the functioning of the cell but also contains information for forming the whole organism. Genetic material is DNA. In eukaryotes it is enclosed inside the nucleus as chromatin material. The latter appears as chromosomes during cell division. In prokaryotes, the genetic material lies freely inside the cytoplasm as coiled structure called nucleoid.

Cytoplasm: It is semifluid matrix that occupies the interior of cell between nuclear region and outer envelope. Cytoplasm is the area of major cellular or life activities which keep the cell in living state. Certain functions are associated with special cytoplasmic structures called organelles. Organelles are of three types (i) Membrane less, e.g., ribosomes, centrioles, (ii) Single Membranous, e.g., endoplasmic reticulum, Golgi complex, lysosomes, microbodies, sphaerosomes. (iii) Double Membranous, e.g., mitochondria, plastids (in plant cells).

Size and Shape

Cells differ greatly in size, shape and activities. For example, Mycoplasma, the smallest cell, are only 0.3μm in length while bacteria could be 3 to 5μm. The largest isolated single cell is the egg of an ostrich, Acetabularia, a unicellular green alga is about 10 cm in length.

Cell of alga Caulerpa may be upto one metre. Among multicellular organisms, human red blood cells are about 7.0 μm in diameter, nerve fi bres are the longest, upto 90 cm to few metres.

The upper limit or cell size or cell volume is determined by number of factors like :

(i) Metabolic Activity : Metabolically active cells are small in size while less active ones are large, e.g., sperm (active) and egg (passive).

(ii) Nucleocytoplasmic Ratio: Nucleus controls the metabolic activities of the cytoplasm. A higher nucleocytoplasmic ratio provides more efficient metabolic working. (iii) Surface

Volume Ratio : Active cells possess a higher surface : volume ratio. This occurs in small cells, elongated cells and cells with surface invaginations or ingrowths like microvilli of absorptive 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. e.g., RBCs are biconcave to pass through capillaries and carry O2; WBCs are irregular to perform phagocytosis, nerve cells are long to conduct impulses, sperms have tail for motility etc.

Diagram showing different shape of cells 

Types of cells

There are two basic types of cells i.e., prokaryotic cells and eukaryotic cells. They are differentiated based on organisation of bio membranes, variety of cytoplasmic organelle and complexity of nuclear material.

Prokaryotic Cells

Prokaryotic cells

The best-known examples of prokaryotic cells are bacteria, blue-green algae, PPLO, and Mycoplasma. Prokaryotic cells multiply more rapidly than eukaryotic cells and are also smaller in size. Their basic shapes are rod-like (bacillus), comma-shaped (vibrio), spherical (coccus), and spiral (spirillum). Despite exhibiting great diversity in shapes, their basic organization remains similar. Except for Mycoplasma, all prokaryotes have a cell wall surrounding their cell membrane. The cytoplasm contains a non-membrane-bound nucleus, containing naked genetic material. Apart from genomic DNA additional circular DNA is also present in prokaryotes known as Plasmid DNA. This plasmid DNA is responsible for imparting unique phenotypes to bacteria. The plasmid DNA also offers antibiotic resistance to the bacterium, which is helpful in case of bacterial transformation.  Except for Ribosomes, no other organelle is seen in prokaryotes. 

A prokaryotic cell envelope is composed of a tightly bound triple-layered structure. The outermost part of this triple layer is the glycocalyx followed by a cell wall and then the plasma membrane. Each layer has a specific function. Based on the distinctions in the cell envelopes, bacteria are classified into 2 groups namely Gram-positive and Gram-negative. Due to the differences in cell envelops, the bacteria which respond to the Gram stain are regarded as positive whereas the ones that do not take up the stain, are known as Gram-negative.

The composition of glycocalyx differs among different bacterial groups. In some groups, it appears as a loose slimy layer while in others it is present as a thick and sturdy capsule. The cell wall maintains the shape of the cell and provides resistance to the bacterium against disintegration or bursting. The plasma membrane establishes the connection of cells with the outside world. It is selectively permeable in both prokaryotes and eukaryotes.

A unique characteristic of Prokaryotes is the presence of Mesosomes, which are the infoldings of the plasma membrane into the cell. These infoldings can be tubular, lamellar, or vesicular in appearance. They are responsible for cell wall formation, DNA replication, respiration, and secretion processes. In cyanobacteria, other cell membrane modifications are seen which extend into the cytoplasm. They contain pigments and are known as Chromatophores.

Motile bacterial cells possess thin filamentous extensions arising from the cell wall, known as Flagella. A single flagellum consists of a filament, a hook, and a basal body.  Apart from flagella, Pili and Fimbriae are also found on the bacterial surface although they have no role to play in motility. Pili are elongated structures made up of proteins and Fimbriae are bristle-like structures erupting from the cell surface. They are known to help the bacteria attach to host tissues or surfaces. 

The prokaryotic plasma membrane encloses ribosomes. These ribosomes consist of 2 subunits namely 50s (15nm) and 30s (20nm). Together they form the 70s ribosomal subunit. Ribosomes are known as the site of protein synthesis. They often attach themselves to a single mRNA and form a chain called polyribosome which eventually translates mRNA into proteins.

Prokaryotic cells store their unused or reserved material in specialized non-membranous structures called inclusion bodies which are present in the cytoplasm. Examples of inclusion bodies include glycogen granules, phosphate granules, and cyanophycean granules.  In blue-green and purple photosynthetic bacteria, gas vacuoles are also present. 

Figure 2: Features of a Prokaryotic cell.

Prokaryotic Cells

Prokaryotic Cells

1. Cell wall present (bacteria) or absent (mycoplasma)

2. A prokaryotic cell is a single membrane system.

3. Cell membrane bears respiratory enzymes.

4. Mesosomes are formed by infolding of cell membrane.

5. Cytoplasm lacks membrane bound organelles

6. Ribosomes (non membrane bound organelle) are70S, lie free in cytoplasm.

7. There are no streaming movements of cytoplasm.

8. Photosynthetic lamellae i.e., thylakoids (if present) occur free in the cytoplasm.

9. Sap vacuoles are lacking. Gas vacuoles may occur.

10. Transcription and translation occur in the cytoplasm.

11. Protein synthesis takes place in cytoplasm only.

12. Cytoskeleton absent.

13. Nuclear material is not enclosed by nuclear envelope and lies directly in cytoplasm. It is called nucleoid.

14. There is no nucleolus.

15. DNA is closed and circular and without histone core (Polyamines may be present in place of histones)

16. DNA occurs in the cytoplasm only.

17. Plasmids and pili occur in many prokaryotic cells.

18. Flagella, if present, are singlet fibres (9 + 0) and are formed of a protein flagellin.

19. Mitotic spindle is not formed in cell division (Amitotic).

20. Sexual reproduction absent (recombination is present in bacteria)

21. e.g., Bacteria, blue-green algae and mycoplasmas.

Eukaryotic Cells

Eukaryotic Cells
Eukaryotes refer to any cell or organism having an identifiable nucleus. The nucleus of a eukaryotic cell is surrounded by a nuclear membrane, which contains well-defined chromosomes. Plants, animals, protists, and fungi are examples of eukaryotic cells. Chromosomes contain their genetic material. Eukaryotic cells have organelles such as the Golgi apparatus, Mitochondria, Ribosomes, and Nucleus. They have intricate locomotory and cytoskeletal features, as well.
Among eukaryotes, plant and animal cells show structural dissimilarities. Plant cells possess a cell wall, plastids, and a large central vacuole. An animal cell, on the other hand, possesses a centriole that is absent in the plant cell. 

Figure 3: A plant cell.

Figure 4: An Animal Cell.
(A)Cell Membrane:
The cell membrane, also known as the plasma membrane, separates the interior of the cell from the outside environment and is found in all cells. A semipermeable lipid bilayer makes up the cell membrane. The transfer of materials into and out of the cell is controlled by the cell membrane. In the late 1600s, Robert Hooke discovered the cell membrane.
Lipids and proteins make up the majority of the cell membrane. Phospholipids, which are organized in a bilayer, are the most important lipids. The lipids are also organized within the membrane with the polar heads on the outside and the hydrophobic tails on the inside, with the polar heads on the outside and the hydrophobic tails on the inside. This guarantees that saturated hydrocarbons' nonpolar tails are protected from the aqueous environment. Membranes include cholesterol in addition to phospholipids.
Biochemical analysis later revealed that cell membranes contain protein and carbohydrates as well. In different cell types, the protein-to-lipid ratio varies greatly. For example, the erythrocyte membrane contains around 52 percent protein and 40 percent lipids in humans.Membrane proteins are divided into two categories: integral and peripheral. Integral proteins are partially or completely buried in the membrane, whereas peripheral proteins are on the surface.

Singer and Nicolson (1972) provided an improved model of cell membrane construction that is now widely accepted as the fluid mosaic model. The quasi-fluid property of lipid, according to this, allows for lateral mobility of proteins within the total bilayer. The fluidity of a membrane is a measure of its capacity for internal movements. The fluid nature of the membrane is also critical for tasks such as cell proliferation, intercellular connection creation, secretion, endocytosis, and cell division.

Figure 5: Fluid Mosaic Model of the plasma membrane.

The transport of molecules across the plasma membrane is one of the most significant tasks performed by this structure. Some molecules on each side of the membrane are selectively permeable through the membrane. The term "passive transport" refers to the ability of several molecules to travel across a membrane without requiring any energy. Simple diffusion along a concentration gradient, i.e. from a higher to a lower concentration, can transport neutral solutes across the membrane. Water can also travel from a greater to a lower concentration across this membrane. Osmosis is the movement of water by diffusion. As polar molecules cannot pass through the nonpolar lipid bilayer, they must be transported across the membrane by a membrane carrier protein.A few ions or molecules are transported across the membrane in the opposite direction of their concentration gradient, that is, from lower to higher concentration. Active transport, for example, is an energy-dependent activity that uses ATP as observed in a sodium-potassium pump.

(B) Cell Wall:
The cell wall is a non-living inflexible structure that is only found in plants and fungi. A cell wall not only gives the cell structure and protects it from mechanical damage and infection, but it also aids cell-to-cell communication and acts as a barrier to undesirable macromolecules. Algae have cellulose, galactans, mannans, and minerals like calcium carbonate in their cell walls, whereas other plants have cellulose, hemicellulose, pectins, and proteins. The primary wall of a young plant cell can develop, but as the cell grows, the primary wall shrinks, and the secondary wall forms on the inner (towards membrane) side of the cell.The middle lamella is a calcium pectate-based layer that binds or glues the surrounding cells together. Plasmodesmata, which connect the cytoplasm of neighboring cells, can pass through the cell wall and middle lamellae.

Figure 6: Structure of a Cell Wall.
Each membranous organelle has its structure and function, however, those organelles whose functions are coordinated, are grouped as an endomembrane system. Endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles are all part of the endomembrane system. The mitochondria, chloroplast, and peroxisomes are not considered part of the endomembrane system because their functions are not coordinated with those of the above organelles.

(C) Endoplasmic Reticulum (ER):
Upon observing a eukaryotic cell under an electron microscope, it was discovered that there is a network or reticulum of microscopic tubular structures dispersed throughout the cytoplasm.This network was identified as Endoplasmic Reticulum (ER). It divides the intracellular cavity into 2 parts namely, luminal (within ER) and extraluminal (cytoplasm). Ribosomes are frequently seen attached to the outer surface of ER. This is known as Rough ER (RER). RER is found in a lot of cells that are involved in protein synthesis and secretion. They are long and contiguous with the nucleus's outer membrane. Smooth ER (SER) refers to the absence of ribosomes on ER surface.  The SER is the primary location for lipid synthesis. SER produces lipid-like steroidal hormones in animal cells.

Figure 7: Endoplasmic Reticulum.

(D) Golgi apparatus:
In the year 1898, Camillo Golgi, an Italian researcher, first identified the heavily pigmented reticular structures near the nucleus. These entities were given the name Golgi bodies. They are made up of a large number of flat, disc-shaped sacs or cisternae with a diameter of 0.5m to 1.0m. These are piled one on top of the other. In a Golgi complex, there are a variety of cisternae. The Golgi cisternae are organized in a concentric pattern near the nucleus, with distinct convex Cis (forming face) and concave Trans (maturing face) face. The organelle's Cis and Trans faces are completely different, although they are linked. The Golgiapparatus is primarily responsible for packing materials for delivery to intracellular targets or secretion outside the cell.

Figure 8: The Golgi apparatus.

(E) Lysosomes: 
Lysosomes are membrane-bound vesicular organelles generated during the Golgi apparatus packaging process. Almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) were observed to be abundant in the isolated lysosomal vesicles, which are best active at acidic pH. Carbohydrates, proteins, lipids, and nucleic acids can all be digested by these enzymes. They are spheres made up of a lipid bilayer that encloses fluid containing a range of hydrolytic enzymes and have a simple structure.

In the cytoplasm, the vacuole is a membrane-bound compartment. It contains water, sap, excretory product, and other non-cellular elements. Tonoplast is a single membrane that separates the vacuole from the rest of the cell. Plant cells have vacuoles that can take up to 90% of the cell's volume. The tonoplast in plants accelerates the transport of ions and other materials over concentration gradients into the vacuole, resulting in a substantially higher concentration in the vacuole than in the cytoplasm. The contractile vacuole is crucial for osmoregulation and excretion in Amoeba. Food vacuoles are generated by engulfing food particles in various organisms, including protists.

Figure 9: Lysosomes

Figure 10: Vacuole.

(F) Mitochondria:
Mitochondria (plural: mitochondrion) are membrane-bound cell organelles that provide the majority of the chemical energy required to fuel the cell's metabolic activities. Adenosine triphosphate (ATP) is a tiny molecule that stores the chemical energy created by mitochondria. Each mitochondrion is a double membrane-bound structure, with the outer and inner membranes partitioning the lumen into two different aqueous compartments, the outer and inner compartments, respectively. The matrix is a solid, homogeneous substance that fills the inner compartment. The organelle's continuous limiting boundary is formed by the outer membrane. Towards the matrix, the inner membrane creates a series of infoldings called cristae (sing.: crista). The surface area is increased by the cristae. The enzymes linked with the two membranes are different.The two membranes each contain their own set of mitochondrial function-related enzymes. Aerobic respiration takes place in mitochondria. They are known as the 'power houses' of the cell because they produce cellular energy in the form of ATP. A single circular DNA molecule, a few RNA molecules, ribosomes (the 70S), and the components required for protein synthesis are also present in the mitochondrial matrix. Fission is the process through which mitochondria divide.
Mitochondria are difficult to see under a microscope unless they are stained specifically. The quantity of mitochondria per cell varies according to the cells' physiological activity. There is a great deal of variation in terms of shape and size as well. 

Figure 11: Mitochondria.

(G) Plastids:
All plant cells and euglenoids contain plasmids. Because they are big, they are easily visible under a microscope. They contain unique pigments, which give the plants their colors Plastids are divided into three types based on the pigments they contain: chloroplasts, chromoplasts, and leucoplasts. Chlorophyll and carotenoid pigments are found in chloroplasts, and they are important for capturing the light energy required for photosynthesis. Carotene, xanthophylls, and other fat-soluble carotenoid pigments are found in the chromoplasts. The plant's portion turns yellow, orange, or red as a result of this. Amyloplasts store carbohydrates (starch) inpotatoes, whereas elaioplasts store oils and lipids, and aleuroplasts store proteins.
The mesophyll cells of the leaves contain the majority of the chloroplasts in green plants. These organelles can be lens-shaped, oval, spherical, discoid, or even ribbon-like, with varying lengths (5-10m) and widths (2-4m). Their numbers range from one per cell in the green alga Chlamydomonas to 20-40 per cell in the mesophyll.
The chloroplasts, like mitochondria, a membrane-boundary bound. The inner chloroplast membrane is the less permeable of the two. The stroma is the compartment enclosed by the chloroplast's inner membrane. The stroma contains several thylakoids, which are flattened membrane sacs that are structured. Thylakoids are placed in stacks similar to grana (singular: granum) or intergranal thylakoids, which are coin piles. In addition, the stroma lamellae are flat membranous tubules that connect the thylakoids of the various grana. Thylakoids have a lumen that is enclosed by their membrane. The thylakoids contain chlorophyll pigments.Enzymes necessary for glucose and protein synthesis are found in the stroma of chloroplasts. It also contains ribosomes and tiny double-stranded circular DNA molecules.The chloroplast ribosomes (the 70S) are smaller than the cytoplasmic ribosomes (80S). 

Figure 12: Plastids

(H) Ribosomes:
Ribosomes are granular structures that were first discovered as dense particles by George Palade (1953) through electron microscopy. They are made up of ribonucleic acid (RNA) and proteins and do not have a membrane surrounding them. The ribosomes of eukaryotes are, while those of prokaryotes are the 70S. There are two subunits in each ribosome: bigger and smaller subunits (Fig 8.9). The 60S and 40S are the two subunits of 80S ribosomes, while 50S and 30S are the subunits of 70S ribosomes. 

 Figure 13: A Ribosome.

(I) Cytoskeleton:
The cytoskeleton is an intricate network of filamentous proteinaceous structures found in the cytoplasm that includes microtubules, microfilaments, and intermediate filaments. The cytoskeleton in a cell is engaged in a variety of tasks, including mechanical support, motility, and cell shape maintenance.

Figure 14: Cytoskeleton
(J) Cilia and Flagella:
Cilia (singular: cilium) and flagella (singular: flagellum) are hair-like protrusions from the cell membrane. Cilia are tiny structures that act like oars, causing the cell or the surrounding fluid to move. Flagella, on the other hand, are longer and are in charge of cell motility. Flagella are also seen in prokaryotic bacteria, however, they differ structurally from eukaryotic flagella.
An electron microscope examination of a cilium or flagellum reveals that they are encased in a plasma membrane. The axoneme, or core, severalseveralrotubules that run parallel to the long axis. Nine doublets of radially oriented peripheral microtubules and a pair of centrally positioned microtubules make up the axoneme. The 9+2 array refers to a specific arrangement of axonemal microtubules. 

Figure 15: Difference between Cilia and Flagella.

 Figure 16: Structural arrangement in Cilia and Fagella.
The central tubules are linked by bridges and encased by a central sheath, which is linked to one of the tubules of each peripheral doublet by a radial spoke. There are nine radial spokes as a result. Linkers are also used to connect the peripheral doublets. The cilium and flagellum both emerge from the basal bodies, which are centriole-like structures.
(K) Centrosome and Centrioles:
A centrosome is an organelle that normally has two centrioles, which are cylindrical structures. Pericentriolar materials, which are amorphous, surround them. Both centrioles in a centrosome are perpendicular to one other and have a cartwheel-like arrangement. They are made up of nine tubulin protein peripheral fibrils that are uniformly spaced. Each peripheral fibril is made up of three triplets. The triplets next to it are also related. The hub, which is connected to tubules of the peripheral triplets by radial spokes comprised of protein, is located in the center section of the proximal region of the centriole. During cell division in animal cells, the centrioles create the basal body of cilia or flagella, as well as spindle fibers that give rise to the spindle apparatus.

Figure 17: A centrosome and a centriole.

(L) Nucleus:
A nucleus is a membrane-bound organelle that governs and regulates the cell's functions (such as development and metabolism) and houses the structures that contain hereditary information, such as genes. Robert Brown was the first to characterize the nucleus in 1831. The nucleus is made up of a nuclear envelope, which is made up of two parallel membranes with a space between them termed the perinuclear space, according to electron microscopy. The nuclear membrane serves as a barrier between the contents of the nucleus and the cytoplasm. The outer membrane is normally connected to the endoplasmic reticulum and contains ribosomes. The nuclear envelope is punctured at several points by minute pores created by the merging of its two membranes.

Figure 18: Nucleus
These nuclear pores are the conduits through which RNA and protein molecules travel between the nucleus and the cytoplasm in both directions. Although each cell normally has only one nucleus, variations in the number of nuclei are common. Nucleolus and chromatin are found in the nuclear matrix, also known as nucleoplasm. In the nucleoplasm, there are spherical structures called nucleoli. Because the nucleolus is not a membrane-bound structure, its contents are continuous with the rest of the nucleoplasm. It's where active ribosomal RNA synthesis happens. In cells that are actively synthesizing proteins, nucleoli are larger and more numerous.

Figure 19: Types of Chromosomes.
The interphase nucleus is chromatin, which is a loose and unclear network of nucleoprotein fibers. Cells, on the other hand, reveal organized chromosomes in place of the nucleus at various phases of cell division. DNA, histones (basic proteins), non-histone proteins, and RNA are all found in chromatin. Each of the forty-six (twenty-three pairs) chromosomes in a single human cell has a two-meter-long strand of DNA. 
Every chromosome (visible only in dividing cells) has a central constriction called the centromere, which is flanked by disc-shaped structures called kinetochores. A chromosome's centromere holds two chromatids.The chromosomes can be divided into four categories based on the position of the centromere. The central centromere of the metacentric chromosome divides the chromosome into two equal arms. The centromere of the sub-metacentric chromosome is located somewhat distant from the chromosome's center resulting in one shorter and one long arm. The centromere of an acrocentric chromosome is at the end, forming one extremely short and one extremely long arm, whereas a telocentric chromosome contains a terminal centromere.Occasionally, a few chromosomes will have non-staining secondary constrictions at the same spot. This appears to be a little component known as the satellite.
Microbodies are membrane-bound minute vesicles that contain a variety of enzymes. Both plant and animal cells have them. Peroxisomes, glyoxysomes, glycosomes, and hydrogenosomes are all microbody organelles. Microbodies are particularly common in the liver and kidney of vertebrates.

Figure 20.

Eukaryotic Cells

Eukaryotic cells

1. Cellulosic cell wall (Plants) or absent (Animals)

2. A eukaryotic cell is a double membrane system.

3. Cell membrane lacks respiratory enzymes.

4. Mesosomes are absent.

5. Cytoplasm contains membrane bound organelles, (endoplasmic reticulum, mitochondria, golgi apparatus, lysosomes and centrosome.

6. Ribosomes are 80 S, may lie free or bound to E.R. and nuclear envelope. (70 S ribosomes are found within mitochondria and chloroplast)

7. Cytoplasm shows streaming movements (cyclosis).

8. Photosynthetic lamellae if present, occur within

9. the chloroplasts.

10. Sap vacuoles are common.

11. Transcription and translation occur in nucleus and cytoplasm respectively.

12. Protein synthesis occurs in the cytoplasm,

13. mitochondria and plastids.

14. Cytoskeleton (microtubule, microfilament and

15. intermediate filaments) present.

16. Nuclear material is enclosed by nuclear envelope to form a nucleus distinct from cytoplasm.

17. One or more nucleoli occur within the nucleus.

18. Nuclear DNA is linear with a histone protein core.

19. DNA occurs in the nucleus as well as in mitochondria and chloroplasts.

20. There are no plasmids and pili in eukaryotic cells.

21. Flagella, if present, are complex, have 9 + 2 pattern of microtubules formed of a protein tubulin.

22.Mitotic spindle is formed in cell division.

23. Sexual reproduction occurs.

e.g., Algae other than blue-green algae, protists, fungi, plants and animals.