The cell is the functional and structural unit of all living organisms. The eukaryotic cells are present both in the plants and animals. The cells have various shapes, sizes, and physiology. All the cells are typically composed of cell-covering, cytoplasm, cytoplasmic organelles, ergastic substances, and a true nucleus.
The cytoplasm is the live, semi-transparent, vacuolated, granular, colloidal, semi-fluid, colorless, flexible and highly viscous (gel-like) substance which surrounds the nucleus and is bounded peripherally by the cell membrane or plasma membrane.
The main and matrix component of the cell is the cytoplasm where most of the metabolic activities occur. In eukaryotic cells, the cytoplasm is confined to the space between the nuclear envelope and plasma membrane/cell membrane, surrounding the cytoplasmic organelles. A membrane-bound nucleus separates the nucleus from the other parts of the cell. Cytoplasm makes up nine-tenths of the entire cell and contains all the cell organelles, solid non-living materials, salts, stored foods, pigments, organic acids, water, etc.
The cytoplasm is composed of three types of structures:
1. The cytoplasmic matrix: The homogeneous transparent colloidal fluid that remains after the removal of all the cytoplasmic organelles and inclusions is known as a cytoplasmic matrix. It forms the most essential part of the cell because it provides the space for all biosynthetic and bioenergetic functions due to the presence of enzymes. The peripheral part of the matrix is comparatively non-granular, less viscous, clear and elastic in nature which is known as ectoplasm or ectoplast or plasmagel. The inner part of the matrix is granular and viscous, is known as endoplasm or endoplast. The thin layer of matrix present around the large vacuoles is known as tonoplasm or tonoplast.
2. The cytoplasmic organelles: Some specific small living organs are found in the cytoplasmic matrix of all eukaryotic cells that perform important specific functions in the cell metabolism, are known as cytoplasmic organelles.
Types of cytoplasmic organelles: There are different types of organelles, such as:
3. The cytoplasmic inclusions or ergastic substances: The cytoplasm of the cells contains a variety of non-living substances are known as ergastic substances or cytoplasmic inclusions. These substances are of three main groups:
It is the intracellular fluid of the cytoplasm which is largely composed of over 70% water and surrounds all organelles. It is a water-based solution which also contains the varying size of soluble molecules, proteins, amino acids, mRNA, ribosomes, sugars, dissolved ions, etc.
Cytoplasm is the gelatin-like, semi-transparent fluid which fills the cell.
Cytosol is the intra-cellular fluid which is placed inside the cells.
Cytoplasm is present between the cell membrane and nuclear envelope.
Cytosol is also found within the cell membrane and nuclear envelope.
It is the total content within the cell membrane and nuclear envelope.
Cytosol is the part of the cytoplasm that does not contain any of the organelles of the cell.
It contains all the cell organelles such as mitochondrion, golgi apparatus, vacuoles, plastids, cell wall and the endoplasmic reticulum
The major components of cytosol are: concentration gradients, protein complexes, protein compartments and cytoskeletal sieving, etc.
Cytoplasm is composed of three chief elements such as cytosol, cell organelles and the cytoplasmic inclusions.
The fundamental compositions of cytosol are water, dissolved ions, smaller minute molecules, large water soluble molecules, and proteins.
Some important cellular activities such as cell division, glycolysis occur in cytoplasm.
In prokaryotic cells, all types of chemical reaction occur in cytosol.
Signal transduction, cytokinesis, nuclear division occur in the cytoplasm.
Molecules transportation and signal transduction occur in the cytosol.
Generally, cytosol is the watery part of the cytoplasm while cytoplasm contains cytosol and other components between plasma membrane and nuclear envelope. The term cytosol was first coined by H.A. Lardy in 1965. The main and matrix component of the cell is the cytoplasm. The cytoplasm is found inside the cell membrane which surrounds the nuclear envelope and the cytoplasmic organelles. It contains all the cell organelles, solid non-living materials, stored foods, organic acids, water, etc. The cell organelle less part of the cytoplasm is called cytomatrix or matrix or hyaloplasm. The central, granular mass in the cytoplasm is the endoplasm while the surrounding clear layer is known as the cell cortex or the ectoplasm.
The cell is the fundamental structural, biological and functional unit of living things. The typical cell contains the most outstanding visual and functional feature, the nucleus. The nucleus is the most prominent organelle which occupies about 10 % of the volume of the cell.
The nucleus is the double membrane-bounded dense round cellular component which contains a genetic material DNA in chromosome and generally, it is located at the center of the cell.
Microbiologist Antonie van Leeuwenhoek first observed the nucleus in the red blood cells of salmon fish while in 1804, Franz Bauer also described the nucleus but Scottish Botanist Robert Brown (1831) observed the dense circular region inside the plant cell and he called it nucleus and described more details about the nucleus.
The nucleus is the biggest cell organelle found in the cytoplasm of all eukaryotic cells of plants and animals. The size of the nucleus varies from cell to cell. The size is directly proportional to that of cytoplasm. The average size of the nucleus in mammalian cells is approximately 6 µm in diameter. The mammalian matured erythrocytes and matured sieve tubes of higher plants contain no nucleus. The contents of the nucleus are present in the nucleoplasm which is similar to the cytoplasm.
The shape of the nucleus normally remains related to the shape of the cell. The shape of the nucleus also varies which may be circular, oval, disc-shaped, elongated, lobed in WBC (white blood cell), C-shaped in Vorticella, pyriform in Paramecium, moniliform in Spirostomum, spindle or elliptical in sperm.
The eukaryotic cells usually have a single nucleus (mono-nucleated cell), sometimes, the cell may contain two nuclei (bi-nucleated cell) and the osteoclasts bear many nuclei (poly-nucleated cell) while the mammalian red blood cells (RBC) have no nuclei.
The nucleus consists of the following structure:
It is a double-layered membrane (outer and inner membrane) which envelops the nucleus and separates the nucleoplasm from the cytoplasm. It is also known as nuclear envelope, nucleolemma or karyotheka. It is derived from the membrane of the endoplasmic reticulum.
Each membrane is 75-90 Å thick and is made up of lipoprotein. Two layers (outer and inner membrane) are mainly separated by a fluid-filled space of 100-150 Å which is also known as perinuclear cisternae. The outer layer of the membrane is attached to the (ER) endoplasmic reticulum at several points. Ribosomes remain attached to the outer surface of the outer membrane and hence it looks rough. At certain points, the nuclear envelope is interrupted by the structures called pores. Both membranes are in continuity around the margins of these pores. Nuclear pores act as selective diffusion barriers between the nucleus and cytoplasm.
Nuclear Pores: The apertures (50-80 µm in diameter) present in the nuclear envelope are known as pores, within each pore octagonal shaped electron dense ring is present which is called annulus (60 nm in diameter). It consists of eight granules that present at both the nuclear and cytoplasmic surface. The pores and annulus together are referred to as the pore complexes.
Nucleoplasm is a relatively clear granulated semi-liquid, slightly acidic gelatinous substance which is present in the space between the nuclear membrane and nucleolus. This semi-aqueous material is similar to the cytoplasm. It is also called nuclear sap or karyolymph. It contains a thread-like elongated structure which is known as chromosomes.
It also contains different types of chemical substances such as:
The nucleolus is a discrete densely stained membrane-less spherical body which is found in the nucleoplasm of the nucleus. It is composed of RNA and proteins. It is attached to the special regions of particular chromosomes, known as nucleolar organizer regions which help to synthesis ribosome by transcribing and assembling ribosomal RNA subunits.
During the karyokinesis or first phase of cell division, it disappears completely and again reappears after cell division. It is composed of a large amount of ribosomal protein and ribosomal RNA (rRNA).
Structurally, it is made up of four zones:
1. Granular Zone: It is the peripheral part of the nucleolus which contains granules of ribonucleoprotein.
2. Fibrillar Zone: It is composed of fibrils of ribonucleoprotein.
3. Amorphous Zone: It has low electron density and it is found only in a certain nucleolus.
4. Chromatic Zone: Surrounding the nucleolus, a shell-like structure is present which is known as perinuclear chromatin. This may be in the form of a thick continuous wall as an exocrine pancreatic cell. From perinuclear chromatin, the intranuclear chromatins arise.
They are elongated thread-like coiled structures within the nucleoplasm of the nucleolus. It is also known as nuclear reticulum. They got the name chromatin (Gr. Chroma=color) due to their colorful nature during cell staining when it is viewed under microscope. They are twisted, fine anastomosed chromatin fibers and are uniformly distributed in the nucleoplasm. These chromatin fibers are observed only during a resting state of cell division. During cell division, chromatin fibers become thick ribbon-like structures, called chromosomes. Chromatin fibers are of two types: Euchromatin and Heterochromatin. Actually, chromatins are composed of histone protein, DNA and trace amount of RNA. In this case, DNA and histone protein exist in chromatin in 1:1 ratio.
Also read: Plasma Membrane: Structure and Functions
The cell wall is the semi-rigid, laminated, semi-permeable non-living outermost cellulose coat which is positioned next to the cell membrane of the plant cells, Fungi, Algae, Bacteria, and some Archaea. The protoplasm of the plant cells is separated from the external world by the cell wall which is entirely lacking in animal cells. Generally, the composition of the cell wall varies from organisms to organisms. The cell wall of the plant cell is composed of cellulose (Carbohydrates), bacterial cell wall contains sugar and amino acid polymer which is known as peptidoglycan while fungal cell wall is composed of chitin, glucans, and proteins. The cell wall performs lots of functions such as structure, protection, and support.
The cytoplasmic organelles like Endoplasmic reticulum, Golgi bodies, etc. take part in the formation of the cell wall. During cytokinesis process, small vesicles of endoplasmic reticulum migrate to the equator of the dividing cell and fused with one another to form barrel-shaped discontinuous cell plate or phragmoplast. Golgi bodies synthesize all the polysaccharides as pectin, hemicelluloses of α-cellulose, microfibril of the cell wall of plant cells. All these polysaccharides along with cell plate help in the cell wall formation.
The plant cell wall is multi-layered and it is complex in nature. There are three distinct layers in the cell wall. From the outermost layer of the cell wall, these are the middle lamella, the primary cell wall, and the secondary cell wall. Occasionally tertiary cell wall may also be present. All plant cells contain a middle lamella and primary cell wall but not all have a secondary cell wall.
It is comparatively thin viscous and jelly-like intercellular cementing material which is present in between the primary cell walls of two adjacent cells. It is first formed after cell division (Cytokinesis). It is mainly composed of pectin (polysaccharides), calcium and magnesium, etc. Pectin acts as intercellular cement which helps in cell adhesion to bind the adjacent cells together.
It is the outermost thin elastic permeable membrane which is situated between the middle lamella and plasma membrane in growing plant cells. It is formed during the early stages of growth and development. It is composed of cellulose, hemicelluloses, pectin, and lignin, etc. It is 1-3 µm thick. The primary cell wall provides the flexibility and strength which is essential for cell growth.
It is thick permeable cell wall which is present in between the primary cell wall and plasma membrane in some plant cells. As the cell matures and differentiates, the secondary cell wall is deposited. Thus, further cell expansion ceases. The secondary cell wall is composed of organic compounds like cellulose, hemicelluloses, lignin, suberin, cutin, wax, etc. and inorganic salts like calcium carbonate, oxalate, silica, etc. It is about 6-10 µm thick. The secondary cell wall commonly has three layers, such as a thin outer layer, thick middle layer, and a thin inner layer. This rigid layer makes strength, gives support to the cell and helps in water conductivity in plant vascular tissue cells.
In some cells, a very thin tertiary cell wall is formed on the inner surface of the secondary cell wall. It is made up of xylan instead of cellulose.
Electron microscope revealed that the cell wall is composed of two main parts such as microfibrils and matrix.
Microfibrils: Each microfibril is 0.5 µm in width and about 250 Å in diameter. Each microfibril contains a bundle of micelles of elementary fibrils. Each micelle is about 100 Å in diameter and contains about 100 cellulose chains. Cellulose is a polysaccharide made of glucose arranged side by side.
Matrix: Matrix consists of largely of polysaccharide. In it, the microfibril of cellulose and other polysaccharides are embedded.
The cell wall does not form a continuous layer but has many small openings or pores. In the cell wall at many places, the cellulose layers are absent and at these places, small pits are formed. These places are called primary pit fields. These pits are always opposite in between the walls of adjacent cells. In such a case, the middle lamella is called a pit membrane. There are many minute pores in the middle lamella. The cytoplasm of one cell communicates with the cytoplasm of adjacent cells through these pores together by microscopic fibrils. Each fibril is called plasmodesma and the group of these fibrils is called plasmodesmata (singular-plasmodesma).
Secondary wall is thicker and more rigid than the primary cell wall due to lignin deposition. These depositions occur in such a way that various many peculiar patterns of ornamentation are formed on the cell wall such as:
1. Annular: The materials of the secondary cell wall are deposited inside the primary cell wall. This deposition forms the ring like thickening at regular intervals and the rest of the wall is thin. This type of structure is present in the protoxylem vessels.
2. Spiral: The matrix of the secondary cell wall is deposited inside the primary cell wall. This deposition occurs in such a way that forms the spiral or helical thickening. This is also found in the protoxylem vessels.
3. Scalariform: The secondary wall matters are deposited irregularly in such a way that forms a staircase-like structure.
4. Reticular: The matters of the secondary wall are deposited irregularly on the primary cell in such a way that looks network-like structure.
5. Pitted: The matters of the secondary cell wall are deposited uniformly all over the primary cell wall except certain areas. These areas look like cavities which are known as pits.
Generally, pits occur in pair, called pit pairs, i.e., when a pit is formed on the cell a complimentary pit will be formed just on the opposite side of the adjacent cell. Pits are usually found in non-living cells like tracheids and fibers. Pits are of two types: Simple and branched pits.
Simple pits: They have no projecting margin or border around the aperture. Simple pits are found in sclerids, xylem parenchyma, ordinary parenchyma cells, vessels, etc.
Branched pits: These types of pits are provided with projecting margin or border. They consist of (i) pit chamber which is the area within the pit and (ii) Pit aperture which is the aperture through which the pit chamber opens outside. (iii) Pit membrane- adjacent pits are separated by the middle lamella and the primary cell wall together form pit membrane. It may have a thickening area, called the torus which is formed by the circular deposition of microfibrils.
Also read: Plasma Membrane: Structure and Functions
All types of cells are bounded by a thin membrane which is known as the plasma membrane. It is also known as the cell membrane, cytoplasmic membrane or plasmalemma. It is a living ultra-thin, elastic porous selectively permeable membrane that separates the cell content from the external environment.
The term cell membrane was first coined by C. Nageli and C. Cramer in 1855 while the term plasmalemma has been given by J.Q. Plowe in 1931. According to some Scientists, cell membrane originated from the endoplasmic reticulum. Some hold that the cell membrane is formed by the modification of the outer surface of the cytoplasm. Plasma membrane lies between the cell wall and cytoplasm in the bacteria and plant cells and it is the outer limiting membrane of most animal cells.
The plasma membrane or cell membrane is porous, thin and invisible. Sometimes the membrane may be distinguished because it is either folded to form brush border or sac-like structure which is called pinocytic vacuoles. When it is examined under an electron microscope, brush border looks finger-like processes, called microvilli. In between two adjacent cells, the plasma membranes become thicker in certain regions. From these areas, many fine filaments are seen, known as tonofilaments radiate towards the interior of the cell. Such thickened areas of the plasma membrane are known as desmosomes.
According to Dannielli and Davson (1935), the plasma membrane is about 75-80 Å in thick. They also observed that the membrane is made up of trilaminar (triple-layered) structure. J.D. Roberson (1959) described this basic trilaminar structure of all cell membrane as a unit membrane model. At high magnification with an electron microscope, the cell membrane consists of double layers of lipid molecules of 35 Å thick. They are sandwiched within the two densely stained protein layers. The thickness of each protein layer is 20 Å. The lipid layer is mostly phospholipids of which the head end contains the water-soluble and positively charged phosphate group called polar ends. The tail end contains the water-insoluble and negatively charged lipid group, called non-polar ends remain side by side.
The polar ends remain attached to protein layers probably by the hydrogen bonds, ion bonds or electroscopic forces. In some eukaryotic cells (animal cells) beside the plasma membrane, there is another covering is present which is known as glycocalyx or cell coat. It is made up of protein and carbohydrate.
Also read: Cell Wall: Structure and Functions
The lipid-globular protein mosaic model suggests, as the name implies, that instead of a continuous layer of protein on the surface of the membrane there is discontinuous mosaic globular protein. They remain partially embedded in and partially protruding from the phospholipid bilayer. There are also some discontinuous peripheral globular proteins arranged just outside and along the surface of the phospholipid bilayer.
This model was observed by English Scientists S. J. Singer and Garth Nicolson in 1972. This model is also known as Singer – Nicolson’s fluid mosaic model. According to this model, the plasma membrane looks like a mosaic which contains some components like phospholipids, cholesterol, proteins, and carbohydrates, etc. which gives the membrane a fluid character. Generally, the percentages of proteins, carbohydrates, and lipids in the plasma membrane vary with cell type. In myelin, the proportion of proteins and lipid are 18% and 76% respectively while the inner membrane of mitochondrial contains 76% protein and 24% lipid.
According to this theory, the main component of the cell membrane is a bimolecular lipid layer which actually consists of two rows of amphiphilic phospholipids molecules. Each phospholipid molecule contains three-carbon glycerol backbone with two fatty acid molecules which are attached to carbons 1 and 2 and a phosphate-containing group that is attached to the third carbon.
Each phospholipid molecule has a water-soluble polar head and two fat-soluble non-polar tails. The polar head of phospholipids is hydrophilic (hydro=water, philic=loving) that is attracted to water and the non-polar tail is hydrophobic (hydro=water, phobic(=fearing) that always try to avoid water. Within the phospholipid bilayer, different types of protein and cholesterol molecules are embedded which gives the plasma membrane look of a mosaic. In this way, phospholipids build up a magnificent lipid bilayer cell membrane that isolates fluid inside the cell from the fluid outside of the cell.
The plasma membrane is illustrated to be fluid of its hydrophobic basic parts; for example, lipids and membrane proteins that move along the side or sideways all through the membrane. That implies the membrane isn't solid however more like a fluid. That is the reason the plasma membrane is depicted utilizing the fluid mosaic model.
Chemically, the second major component of plasma membranes is proteins. Some protein molecules exist outside the lipid layer; called peripheral protein molecule and some are partially or entirely pass across the lipid layer, called integral protein molecules. Integral protein molecules create an ion channel through the cell membrane for passing water-soluble molecules. A single integral protein usually consists of 20–25 amino acids.
The third major component of plasma membranes is oligosaccharide molecules (carbohydrates). These oligosaccharide molecules attached to some protein and lipid molecules of the outer side of the cell membrane to form glycoprotein and glycolipid respectively. Generally, these carbohydrate chains contain 2–60 monosaccharide units which can be either branched or straight.
Chemically cell membrane is made up of the following components:
Also read: Cell Structure and Functions
A cell contains different types of cell organelles which perform different roles within the cell and they help in the survival of living organisms. As cell organelles, lysosomes and ribosomes carry out different functions in the cell. Lysosomes are found only in eukaryotic cells but ribosomes are present in both prokaryotic and eukaryotic cells.
Lysosomes are the membrane-bounded cell organelle which contains different types of digestive enzymes. It is also known as the garbage disposal system of cells which destroy unwanted garbage materials from the cell. Lysosomes are capable of degrading all types of polymers of the cell because they contain an array of digestive enzymes.
Ribosomes are small in the structure which is made up of ribosomal RNA and proteins. It is considered as the protein-synthesizing machines of the cell that synthesize proteins from RNA molecules. The ribosome has two subunits. Prokaryotic cells contain 70S ribosomes which comprise of 50S and 30S subunits. In a eukaryotic cell, ribosomes are the 80S which contain the 40S and 60S subunits. The difference between ribosome and lysosome are stated in the following table:
The lysosome is present in the cytoplasm of the eukaryotic cell only.
The ribosome is present in the cytoplasm of both prokaryotic and eukaryotic cell.
The lysosome is larger in size than ribosome which ranges from 0.1 to 1.2 µm.
Ribosomes are typically smaller in size than lysosome which ranges 20 to 30 nm.
It is covered by a unit membrane i.e., it is membrane-bound organelle.
It is not covered by the membrane i.e., it is non-membranous organelle.
These are evenly and freely distributed throughout the cytoplasm.
These are freely distributed throughout the cytoplasm as well as these remain attached with the membrane of the endoplasmic reticulum etc.
It consists of a single part.
It consists of mainly two parts.
It is a minute spherical vesicle filled with hydrolytic enzymes and small vesicles.
It is a minute spherical structure of ribonucleoprotein and contains no vacuoles.
It always remains separate from each other.
Ribosomes are grouped together during protein synthesis and forms polyribosome.
Functionally it is of four types; Primary lysosome, secondary lysosome, residual body, and autophagosome.
According to size, the ribosomes are two types; 80S and 70S.
It is responsible for digestion of protein along with other substances of the cell.
It is responsible for the digestion of protein synthesis.
It is discovered by Cristian De Duve
It is discovered by George E. Palade.
It is made up of digestive enzymes and membrane proteins.
It made up of ribosomal RNA and ribosomal proteins.
The lysosome contains a vast array of enzymes.
It does not contain an enzyme.
The cell is the fundamental structural, biological and functional unit of all living organisms. It is also known as building block of life. The numbers of cells vary from species to species. Each cell is enclosed by a membrane and composes of proteins and nucleic acids. In bacteria, fungi, algae and plant cell, the outer most covering of the cell is known as cell wall which covers the cell membrane. The cell wall is absent in all types of animal cell. The cell wall makes the outermost boundary of the cell (if present) over the plasma membrane. The cell membrane is also known as biological membrane, plasma membrane or plasma lemma. It is present in almost all types of cells. The cell membrane is semi permeable which allows the passage of certain substances through them.
In most of the plant cells, the cell wall is composed of the following components:
It is a delicate and thin structure which is about 5-10 nm wide.
It is a thick and rigid structure which is 4-20 µm wide.
It is observable under electron microscope.
It is observable under light microscope.
It helps in the passage of molecules and protects the protoplasm.
It helps in providing shape and rigidity to the cell.
It is present both in animal and plant cells.
It is found in bacteria and fungus and plant cell.
It is present outside the protoplasm (cytoplasm) of all living cells.
It is present outside the plasma membrane of a plant cell.
It is a living material of a cell.
It is a non-living material of a cell.
It is composed of lipid, protein and carbohydrates.
It is composed of cellulose(plant), chitin(fungi) and peptidoglycan (bacteria).
It is thin and elastic.
It is comparatively thick and rigid.
It has some specialized structures like micrivilli and desmosomes.
Between the adjacent cells there are cytoplasmic bridges known as plasmodesmata.
It is semi permeable.
It is completely permeable.
It has phagocytic and pinocytic functions.
It has no phagocytic or pinocytic functions.
Cell membrane shows no pattern of ornamentation.
Cell wall shows different patterns of ornamentation.
It needs proper nutrition to survive.
It doesn't need any nutrition from cell but deposition instead.
The plasma membrane is non-elastic.
The cell wall is elastic.
It is metabolically active and living.
It is metabolically inactive and non-living.
It protects the cell from the external environment.
It guards and maintains the internal environment of the cell.
Plasma membrane has receptors which help to make communication between cells to cell to receive signals from external chemicals.
Cell wall does not have any receptors.
It remains same thickness throughout its life.
Thickness does not remain constant; it grows with life.
On the basis of internal structure, cells are of two types: Prokaryotic and Eukaryotic cells. Prokaryotic cells are simple and small in size while eukaryotic cells are complex and large in size. Eukaryotic cells have true nucleus and nucleolus with different types of membrane-bound cell organelles such as Endoplasmic reticulum, Golgi body, Mitochondria etc. Well defined nucleus is absent in prokaryotic cell but it contains DNA molecule which is placed in the cell, known as nucleoid. Differences between prokaryotic and eukaryotic cells are stated in the following table:
Simple, usually unicellular;
Complex, usually multi-cellular, some cyanobacteria may be multicellular);
Smaller; 0.5 –10 µm in diameter;
Larger; 10.0-100 µm in diameter;
3. Plasma membrane or Plasmalemma
Present, generally contains no sterols;
Present, contains sterols;
4. Cell wall
Generally present, composes of peptidoglycan or mucopeptide (polysaccharide);
Absent in animal cells, present in plant cells; composes of cellulose (polysaccharide).
Present, made up of mucopolysachharide;
7. Endoplasmic reticulum
9. Golgy body
Absent; in some prokaryotic cell, chromatophores are present;
Present in plant cell;
Present; two parts but both are small;
Present; two parts, large and small;
12. Lysosomes and peroxisomes
Absent; but contains central component known as nucleoid;
Present; contains true nucleus;
14. Nuclear membrane
16. Chromosomes and its number
Present; single in the nucleiod;
Present; multiple in number;
Present but naked;
Present but complex with protein;
18. Shape of DNA
Circular, double-stranded DNA;
Linear, double-stranded DNA;
19. Exocytosis and endocytosis
20. Cell division
Amitosis, simple fission;
Mitosis and meiosis;
21. Transcription and Translation
They occur together;
Transcription occurs in the nucleus while translation occurs in cytosol (intracellular fluid or cytoplasmic matrix);
It is simple in structure and composes of protein, flagellin.
Complex with 9+2 structure; composes of tubulin and other protein;
Absent or rare;
Animals and plants
The cell is the functional, structural and biological unit of all living organisms. It is of two types such as prokaryotic and eukaryotic cells. The cell contains many types of organelles, among them, some are typically solitary such as the nucleus and golgi complex, while others can be numerous such as mitochondria, lysosomes, chloroplasts, etc. Animal cell does not contain chloroplast while plant cells contain both mitochondria and chloroplast. Mitochondria can produce energy in the form of ATP using oxygen and nutrients for the cell while chloroplast provides space for the process of photosynthesis during production of glucose.
The difference between mitochondria and chloroplast are stated in the following table.
1. Mitochondria are found in all eukaryotic cells (animal and plant).
2. It is a double membrane-bounded organelle; It does not contain any pigments;
2. It is also double membrane-bounded organelle which contains pigments;
3. It is bean-shaped organelle;
3. It is disc-shaped organelle;
4. It has two chambers: matrix and the cristae;
4. It also has two chambers: stroma and thylakoid.
5. The inner membrane remains folded which forms critae;
5. Some portion of the inner membrane remains folded into the matrix and transformed into disc-shaped thylakoid; They pile closely to form grana;
6. The inner chamber is divided into incomplete compartments by cristae;
6. The inner chamber is not divided;
7. Presence of particles on the inner membrane;
7. Absence of particles on the inner chamber;
8. 8. It forms respiratory parts of the cell;
8.It forms the part for synthesis and metabolism of carbohydrate;
9. It produce energy; hence it is known as energy producer or power house of the cell;
9. It converts one kind of energy into another and thus it is known as energy transducer;
10. It does not take part in photosynthesis;
10. It takes part in photosynthesis;
11. O2 is used for oxidative phosphorylation;
11. O2 is not used for oxidative phosphorylation;
12. Carbon dioxide(CO2) and water are produce during energy production;
12. It stores energy and uses carbon dioxide(CO2) and water to produce glucose
13. It is site for electron transport chain, beta oxidation, oxidative phosphorylation, photorespiration;
13. It is site for photosynthesis and photorespiration;
Also read: Difference between Lysosome and Ribosome
The ribosome is one of the essential membrane-bound organelles of the cells. It is a minute spherical structure that contains RNA and protein and serves as the site of protein synthesis. It occurs either freely in the cytoplasm or in the matrix of mitochondria and chloroplast or attached on the outer membranes of the endoplasmic reticulum (ER) and nucleus. The word ribosome comes from ‘ribo’ meaning ribonucleic acid and Greek ‘soma’, meaning body.
Albert Claude first observed the tiny particles or granules of ribonucleoprotein and lipid under an electron microscope and he called them as microsomes in 1943. But Romanian born American Cell Biologist George E. Palade discovered it as dense particles or granules of the cytoplasm in 1955. In 1958, Richard B. Roberts proposed the name “Ribosome”. George E. Palade and other scientists discovered that ribosomes are the protein synthesizer in the cells and for this reason, in 1974; Palade was awarded the Nobel Prize for his great job.
Ribosomes are remarkably plentiful in cells. A mammalian cell contains about 10 billion protein molecules and most of them are produced from the ribosomes. One active eukaryotic cell contains about 10 million ribosomes while the prokaryotic cell (Escherichia coli) contain about 15000-20000 ribosomes which are equal about 25% of the total cell mass. The size of the ribosomes varies within cells which depend on the cell types. The average size of the ribosomes of the Escherichia coli is 200 Å in diameter.
Ribosomes are of two types on the basis of the size and sedimentation coefficient (S) such as the 70S and 80S. Here, ‘S’ refers to Svedberg unit. This is a sedimentation coefficient which shows how fast a cell organelle sediments in an ultracentrifuge.
70S Ribosome: This type of ribosome is comparatively smaller and has sedimentation coefficient 70S which consists of two subunits such as large 50S subunit and small 30S subunit and they are linked together. The molecular weight is 2.7×106 Daltons. They are found in all prokaryotic cells, in mitochondria and chloroplast of eukaryotic cells.
80S Ribosome: This type of ribosome is comparatively larger and has sedimentation coefficient 80S which consists of large circular 60S subunit and small elliptical 40S subunit. The molecular weight is 40×106 Daltons. They are found in all eukaryotic cells.
Each ribosome contains two subunits, such as a larger one and a smaller one. Ribosomal subunits are made in the nucleolus from the proteins and nucleic acids. Then, through the nuclear pores, they are exported into the cytoplasm. The sizes of the two subunits are unequal and exist in this state until required for use. The larger sub-unit is about two times larger than smaller one
The 40S and 60S are the small and large subunits of eukaryotic cells, respectively, while 30S and 50S are the small and large subunit, respectively of prokaryotic cells. The smaller subunit occurs above the larger subunit-like a cap. The ribosomal subunits occur individually when ribosomes are not involved in protein synthesis. The two subunits join together when protein synthesis starts and undergo dissociation when protein synthesis stops. During protein synthesis, two subunits are grouped and form polyribosome or polysome.
Chemically, ribosomes consist of ribosomal proteins and rRNA(ribosomal RNA). In the prokaryotic cells, ribosomes have 40% protein and 60% rRNA while in eukaryotic cells, ribosomes are made up of about 50% protein and 50% rRNA. Besides these, ribosomes also contain trace amounts of Magnesium (Mg), Calcium(Ca) and manganese(Mn).
Lysosomes are single membrane-bound cytoplasmic organelles of most cells filled with a wide variety of hydrolytic enzymes that are involved in intracellular digestion. The term “Lysosome” comes from the Greek word ‘lysis’, to separate and ‘soma’ body. Sometimes it can be described as the stomach of the cell. In 1950, Belgian Cytologist Christian Rene de Duve isolated a type of particles from the animal cell and named it lysosome. In 1965, Novikoff clearly identified the lysosome in rat live cells through the electron microscope.
It is a small round or irregular vesicle which is found in most animal cells except in RBC and a few plant cells. They are abundant in epithelial cells of intestine, liver, and kidney and usually remain distributed evenly in the cytoplasm.
Generally, Lysosomes are rounded in shape but may be irregular. The size of the lysosome usually ranges from 0.1-1.2 µm but they may be up to 5.0 µm or more. The number varies in different cells. The secretary cells like liver, pancreas, spleen, etc. contain more lysosomes. Lysosomes originate from the endoplasmic reticulum (ER) and Golgi complex.
The membrane of lysosome is made up of protein, lipid, and trace element of carbohydrate. They contain vacuolated, granular, dense homogeneous materials which rich in acid phosphatase and other hydrolytic enzymes (about 60 different enzymes) with more than 60 membrane proteins. They also contain some stack of membranous structure which is known as myelin figure. These hydrolytic enzymes are able to digest most biological substances. Lysosomes can separate the enzymes from the cellular contents. These enzymes are so powerful and sometimes they can cause severe damage to the cell. For this reason, lysosome is called “suicidal bags or suicidal sacs”.
Lysosomes are four types on the basis of their functions such as:
1. Primary Lysosome: It is also known as storage granule. It is a small sac-like body which is formed from the maturing face of the Golgi bodies. This type of lysosome also occurs as monocytes or granulocytes. They are surrounded by a single layer of a phospholipid which contains acid hydrolases. Generally, the majority of the primary granules combines with phagosomes and finally forms secondary lysosomes.
2. Heterophagosome: It is also known as digestive vacuole or secondary lysosome. They are formed by the fusion of a primary lysosome with phagosomes. The phagosomes are membrane-bound vesicles which contain foreign materials. It is formed by the fusion of food containing phagosome with lysosome during the phagocytosis or pinocytosis. Ultimately, heterophagosome is left with undigested food.
3. Residual body: Lysosome with undigested food materials or debris are known as the residual body. The undigested material is ejected from the cell and combines with the plasma membrane by exocytosis or ephagy into the external environment. Sometimes, these types of lysosome left inside the cells due to malfunction of exocytosis and lack of hydrolytic enzymes which lead to many diseases such as hepatitis, Tay-Sachs disease, polynephritis, Pompe’s disease, and Hurler’s disease.
4. Autophagosome: It is also known as autophagic vacuole. An autophagosome is a sphere-shaped body which is covered by a double layer membrane. It is a special type of lysosome which contains some part of the cell in the process of auto-digestion such as various cell organelles like mitochondria or portion of the endoplasmic reticulum(ER). After forming autophagosomes, they release cytoplasmic components to the lysosomes. The outside membrane of the autophagosome combines with a lysosome to develop an autolysosome. Autophagosome`s formation is controlled by genes. The size of the autophagosome varies from species to species. In yeast, the size ranges from 500-900 nm while in mammals; it is larger in size which ranges from 500-1500 nm. In the embryonic fibroblasts, hepatocytes and embryonic stem cells, autophagosomes are ring-shaped structures and seen under a light microscope.