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3.1.1: Characteristics of Prokaryotic Cells - Biology

3.1.1: Characteristics of Prokaryotic Cells - Biology



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A prokaryote is a simple, unicellular organism that lacks an organized nucleus or other membrane-bound organelle.

Learning Objectives

  • Describe the structure of prokaryotic cells

Key Points

  • Prokaryotes lack an organized nucleus and other membrane-bound organelles.
  • Prokaryotic DNA is found in a central part of the cell called the nucleoid.
  • The cell wall of a prokaryote acts as an extra layer of protection, helps maintain cell shape, and prevents dehydration.
  • Prokaryotic cell size ranges from 0.1 to 5.0 μm in diameter.
  • The small size of prokaryotes allows quick entry and diffusion of ions and molecules to other parts of the cell while also allowing fast removal of waste products out of the cell.

Key Terms

  • eukaryotic: Having complex cells in which the genetic material is organized into membrane-bound nuclei.
  • prokaryotic: Of cells, lacking a nucleus.
  • nucleoid: the irregularly-shaped region within a prokaryote cell where the genetic material is localized

Components of Prokaryotic Cells

All cells share four common components:

  1. a plasma membrane: an outer covering that separates the cell’s interior from its surrounding environment.
  2. cytoplasm: a jelly-like cytosol within the cell in which other cellular components are found
  3. DNA: the genetic material of the cell
  4. ribosomes: where protein synthesis occurs

However, prokaryotes differ from eukaryotic cells in several ways.

A prokaryote is a simple, single-celled (unicellular) organism that lacks an organized nucleus or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid.

Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule. The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Fimbriae are used by bacteria to attach to a host cell.

Cell Size

At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm. The small size of prokaryotes allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport.

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr2, while the formula for its volume is 4/3πr3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had the shape of a cube. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide; another way is to develop organelles that perform specific tasks. These adaptations led to the development of more sophisticated cells called eukaryotic cells.


Ultra-structure of Prokaryotic Cells

Some of the major cell organs involved in ultra-structure of prokaryotic cell are as follows:

1. Cell envelope 2. Cytoplasm 3. Nucleoid 4. Appendages.

Prokaryotic cells are the simplest of most primitive cells. The records of microfossils suggest that they have evolved 2.5 billion years ago and existed as the only organisms on earth for the next one billion years until eukaryotes evolved about 1.5 billion years ago. Earlier claims that oldest prokaryotic microfossils, the stromatolites (i.e., giant colonies of ancient cyanobacteria or BGA) of 3.5 billion years ago are actually lifeless mineral artifacts.

The Prokaryotic ceil is the structural unit of two microbial groups: the archaebacteria and the eubacteria. Despite variations in shape and size, the fundamental structures of prokaryotic cells are same. Each prokaryotic cell is essentially a one envelop system that consists of protoplasm encased within cell envelope. The ultrastructure of a prokaryotic cell, particularly a typical bacterial cell consists of cell envelope, cytoplasm, nucleoid, plasmids and surface appendage.

1. Cell envelope:

It is the protective covering of bacterial cell that has three basic layers: the outermost glvcocalyx, middle cell wall and innermost cell membrane (plasma membrane),

It is the outermost layer of cell envelope which chemically composed of polysaccharides with or without proteins. When glycocalyxis thick and tough, it is called capsule, and when it forms a loose sheath it is called slime layer.

Though not essential for bacterial survival, glycocalyx has many functions:

(a) Protects cell from desiccation, toxins and phagocytes,

(b) Helps in adhesion, immunogenicity and virulence.

It is the rigid middle layer of cell envelope that provides shape and prevents a bacterium from osmotic bursting in a hypotonic solution. In Gram-positive bacteria, cell wall is single layered and almost uniform in thickness (10 to 80nm).It is composed of peptidoglycan (murein or mucopeptide), which consists of a three dimensional network of glyean strands cross linked by peptide chains. Each glycan strand is 20-130 units long consists of two alternating amino sugars, N-acetylglucosamine (NAG) and N-acetylmuramicacid (NAM).

Certain antibiotics (penicillin) lysozyme prevents cross linking and kills bacteria. The walls of Gram- positive bacteria also contain teichoic acids (Polyphosphate Polymery but proteins are almost absent. Teichoic acids act as surface antigen. In Gram-negative bacteria cell wall is two- layered and only 7.5-12 nm thick. The inner layer in a thin peptidoglycan and the outer layer is a unit membrane called outer membrane. The outer membrane is a lipid bilayer consists of phospholipids, proteins and a unique lipid, lipopolysaccharide (LPS).

The LPS molecules present only in the outer face of the outer membrane. The outer membrane also contains channel like proteins called porins, through which low molecular weight sub­stances enter or exit. In some Gram-negative bacteria (.Mycobacterium, Noccardia) the wall contains long chain fatty acids called mycolic acids. In the walls of Gram- negative bacteria, a fluid- filled space present on either side of the peptidoglycan layer i.e. between the outer membrane of cell wall and the cell membrane. This space is called periplasm or periplasmic gel. It acts like bacterial lysosome.

Many species of bacteria are wall-less, either develop spontaneously or induced by cell wall degrading agents. These bacteria are called L-forms, after the Lister Institute, London. The L- forms of bacteria may develop cell wall but mollicutes will never develop cell wall.

It is the innermost layer of cell envelope. It is a semi-permeable, quasi-fluid, dynamic membrane similar to that of eukaryotic membrane. But the only difference is that, in bacteria they lack sterols instead hopanoids present. The hopanoids are pentacyclic sterol-like molecules that stabilize the bacterial cell membrane. In Gram-negative bacteria at certain places, the outer face of plasma membrane is continuous with the inner face of the outer membrane to form Bayer’s junctions. There are about 200-400 Layer’s junctions present in a Gram- negative cell.

In a bacterial cell, plasma membrane performs many functions:

(a) It retains the cytoplasm

(b) Prevent loss of essential components through leakage

(c) Aids in the movement of nutrients, wastes and secretions across the membrane

(d) Holds receptor molecules that detect and respond chemicals in their surroundings. Such as respiration, photosynthesis, synthesis of lipids and cell wall constituents

(f) It invaginates to form mesosome and thylakoids of cyanobacteria.

2. Cytoplasm:

It is granular, crystallo-colloidal complex that fills the whole prokaryotic cell excluding nucleoid. The cytoplasm contains mesosome, chromatophores, ribosomes, inclusion bodies and plasmids.

(i) Mesosome (or chondrioids):

It is a convoluted membranous infolding of the plasma membrane. Mesosome when connected with nucleoid is called septal mesosome and when not connected called lateral mesosome.

(b) Chromosome replication and distribution to daughter cells

(d) Increase the surface area of plasma membrane

(e) Mesosome can be considered analogous to mitochondria of eukaryotic cells as these are the sites of respiratory enzymes. Hence, mososomes are also called as “mitochondria of prokaryotic cells” or “bacterial mitochondria”.

(ii) Chromatophores:

These are internal membrane systems of prokaryotic cells. These are very extensive and complex in photosynthetic forms like cyanobacteria and purple bacteria where they are called as thylakoids. In nitrifying bacteria the chromatophores increase metabolic area.

Prokaryotic ribosomes are 70S in nature that consist of larger 5OS and smaller 30S subunits. During protein synthesis, about 4-8 ribosomes attach to a single mRNA to form polyribosomes or polysomes. Non functional ribosomes present in separated subunits.

(iv) Inclusion bodies:

These are non-living structures present freely in the cytoplasm. Inclusion bodies may be organic or inorganic. They include mainly food reserve and special prokaryotic organelles like gas vacuoles, chromosomes, carboxysomes, and magnetosomes. Except food reserve other inclusion bodies are surrounded by a single layer non-unit membrane which is 2-5 nm thick.

These are reserve materials or storage granules which are not bounded by any membrane system. Generally, a given bacterial species store only one kind of reserve material. Further, the cellular content of reserve material is low in actively growing cells but them increase when cells are short of nitrogen.

The volutin granules (= polyphosphate granules) and sulphur granules are inorganic inclusion bodies which store phosphate and sulphur respectively. These granules are also called meta-chromatic granules because of their ability to take different colours to basic dyes. The organic- food reserves present in some bacteria are glycogen granules, protein granules (=cyanophycin), cyanophycean granules (= starch granules) and PHB granules. The PHB (Poly β-hydroxybutyrate) granules are storage reservoir of fatty acids in case of Pseudomonas, Bacillus, Azotobacter species. The PHB is commercially used to prepare biodegradable plastics.

These are the organic inclusions of most aquatic, free floating forms. Each gas vacuole is an aggregate of variable number of hollow, cylindrical gas vesicles. Gas vacuoles help in floating, for proper positioning in water to trap sunlight for photosynthesis and protect against harmful radiations.

These are cigar-shaped vesicles that enclose photosynthetic pigments like bacteriochlorophyll c, d, or e. Chlorosomes are distinct structures found just below the plasma membrane but tightly joint to it by a basal plate. These are found in the green bacteria.

These are principal sites of CO2 fixation in case of autotrophic prokaryotes like cyanobacteria, purple bacteria, nitrifying bacteria etc.

These are the vesicles filled with crystals of magnetite (Fe3O4). Magneto-somes help the bacteria to orient themselves in a magnetic field and determine the direction of swimming.

Laderberg and Hays (1952) introduced the term ‘plasmid’ to those ring-like self replicating extra chromosomal double stranded DNA that are found in the cytoplasm of prokaryotes. They are also found in eukaryotes (yeast) and their organelles. Plasmids are used as in ideal vector for indirect gene transfer in recombinant DNA technology (Genetic Engineering).

Plasmids are generally double stranded closed circles of D.N A with sizes vary from 1-3 00 kilobase pairs (1 Kbp = 1000bp) and carries 5-100 genes. Hence, plasmids are often called as minichromosomes. They are also known as dispensable autonomous elements because the genes they carry have no role in viability and bacterial growth.

The average number of plasmids per bacterial cell is called copy number. Plasmids with low copy number (1-2) are called single copy plasmids, while those with high copy number (10-30) are called multi-copy plasmids. Some plasmids can temporarily integrate or can detach from main chromosome and are called as episomes, e.g., F-plasmid. It is to be noted that all episomes are plasmids but all plasmids are not episomes. The term episome was coined by Jacob and Wollman 1958).

On the basis of function, plasmids are of following types –

It carries fertility factor (F-factor) responsible for the formation of sex-pili and conjugation. Hence, often called F-plasmid.

It carries resistance (R) factor which provide resistance again si antibiotics, heavy metals, UV-radiation etc., e.g. Rl.R4G etc.

It carries colicinogenic factor that, produce colicins (bacteriocins) to kill other bacterial.

(d) Degradative plasmid:

It decompose hydracarbon in petroleum, e g. present in Pseudomonas putida [A genetically engineered bacterium which would degrade all the four types of substrates i.e. OCT (Octane, Hexane, Dcene), XYL (Xylene, Toluene), CAM (camphor) and NAH (naphthalene)>

These are tumour including plasmids carried by the Agrabacterium tumefaciens. Ti-plasmid carries

T-DNA (transforming DNA) which is 200 Kbp long and causes crown gall disease in plants, T- DNA is an ideal vector for gene transfer in plants.

A subground of Ti-plasmids inducing the hairy root tumours e.g.,-A-rhizogene.

Plasmid can be categorised on the basis of number of copies per cell

(a) Released Plasmid: It normally maintain multiple copies per cell

(b) Stringent Plasmid: It has limited number of copies per cell

3. Nucleoid

Nucleoid is the genetic material of a prokaryotic cell that occupies up to 1/5 of the interior of the bacterial cell. It is represented by a single circular naked ds DNA which is highly looped and super coiled with the help of polyatnines (nucleoid proteins) and RNA. Nucleoid is a compact structure hat nuclear envelope and nucleolus, and therefore it is not an organized nucleus rather an incipent nucleus. In Escherichia coli the compact DNA is 1.2 mm in length which is about 250-700 times the length of the cell. The nucleoid is attached to the plasma membrane directly or by mesosomes.

4. Appendages:

The surface appendages present on bacterial cell may be motile flagellum or non-motile pili and fimbriae.

(a) Flagella (sing. Flagellum):

These are long (1-71m) fine hairy locomotary appendages present on bacterial surface for swimming. Their number and arrangement is called flagellation which is characteristic features of different genera of bacteria

Some bacteria bear sheath flagella surrounded by extension of cell membrane. In Vibrio, flagellation is mixed type where polar sheathed flagellum present along with many peritrichously arranged unsheathed flagella.

The ultrastructure of each flagellum shows 3 parts – basal body, hook and filament. The Basal body consists of a central root with one two pairs of rings. In Gram positive bacteria the basal body possesses two pairs of rings the outer pair (L and P rings) remains attached to the outer membrane, whereas the inner pair (S and M rings) remains connected to the cell membrane. In Gram- positive bacteria, on the other hand, only the inner pair (Sand M) is present and the remains attached to the cell membrane (fig. 2.4).

The hook is slightly wider and curved structure about45 nm long that connects, basal body with the filament. The filament is hollow cylindrical structure about 1-70 nm long and 20 nm in diameter. Filament composed of 3-8 spiral of flagellian proteins. But hook is made up of a different kind of proteins.

The bacterial flageilum rotate by 360″ rather than a whip like back and forth movement. As a result the bacterial cell spins in the opposite direction and pushes the bacterium in forward direction.

(b) Pili (sing. Pilus) and fimbriae:

These two terms have often been interchangeably used. The pili are tubular outgrowths of about 18-20 nm made up of pilin protein. They are reported only in donor cell Gram negative bacteria where they help in conjugation. Hence, pili are also called sex-pili or F-pili. Their number is 1 -4 per cell. Fimbriae are bristle like surface appendages help in adhesion and mutual clinging length varies from 6.1 – 7.5 nm and diameter 3-10 nm.


Prokaryotic Cell: 18 Main Characteristics (With Diagram)

DNA is naked and lies variously coiled in the cytoplasm. It is often called gonophore, nuclear body or nucleoid. It is equivalent to a single naked chromosome and is, therefore, also called prochromosome. Many prokaryotes also have additional small circular DNA entities called plasmids. Plasmids carry additional specific factors like nitrogen fixation, resistance, fertility, etc.

Characteristic # 2. Nuclear Components:

Nuclear envelope, nucleoplasm, nucleolus and histone cov­ering of chromatin are absent. In eukaryote (= eukaryotic) cells, a typical nucleus is found.

Characteristic # 3. Types:

Prokaryote contains organisms like blue-green algae (BGA = cyanobacteria, e.g., Nostoc), bacteria, pleuropneumonia-like organisms or PPLO (e.g., Mycoplasma), archaebacteria, spirochaetes, rickettsiae and chlamydiae. PPLOs are the smallest free living organisms.

Characteristic # 4. Cell wall:

It is present in bacteria and cyanobacteria. A cell wall is absent in mycoplasma or PPLO.

Characteristic # 5. Flagella and Fimbriae:

Flagella are present in some bacteria only (Fig. 8.6). The bacterial fla­gella are single-stranded as compared to 11-stranded flagella of eukaryotes. In some bacteria, non-motile appendages called pili or fimbriae also occur. They take part in attachment (e.g., Neisseria gonorrhoeae) and conjugation (e.g. Escherichia coli).

Characteristic # 6. Photosynthetic Thylakoids:

Blue – green algae and some bacteria are photo-autotrophic. Their photosynthetic thylakoids lie freely in the cyto­plasm. They are not organised into chloroplasts.

Characteristic # 7. Membrane-lined Cell Organelles:

The prokaryotic (= prokaryotic) cells lack mitochon­dria, endoplasmic reticulum, Golgi apparatus, lysosomes, microtubules, microfilaments and cen­trioles.

Characteristic # 8. Vacuoles:

Typical vacuoles are doubtful. Instead complex gas vacuoles are found.

Characteristic # 9. Ribosomes:

Ribosomes are 70S as com­pared to 80S. Similar 70S ribosomes occur inside chloroplasts and mitochondria of eukaryotes.

Characteristic # 10. One-Envelope System:

In prokaryotic cells, membrane bound cell organelles are absent so that there is a single membrane that surrounds the cell. Hence, prokaryotes have a single membrane or one-envelope system. In eukaryotes many organelles are surrounded by their own covering membranes in addition to the cell membrane that covers the whole cell. Therefore, eukaryotes have a double membrane or two-envelope system of organisation.

Characteristic # 11. Cyclosis:

Cytoplasm does not show streaming movements or cyclosis.

Characteristic # 12. Spindle:

Mitotic spindle is not formed during cell division.

Characteristic # 13. Sexual Reproduction:

It is absent. Therefore, meiosis and gamete formation are unknown. They multiply very rapidly by asexual means like binary fission, sporulation etc.

Characteristic # 14. DNA Content:

Characteristic # 15. Transcription and Translation:


The Nucleoid

All cellular life has a DNA genome organized into one or more chromosomes. Prokaryotic chromosomes are typically circular, haploid (unpaired), and not bound by a complex nuclear membrane. Prokaryotic DNA and DNA-associated proteins are concentrated within the nucleoid region of the cell (Figure (PageIndex<6>)). In general, prokaryotic DNA interacts with nucleoid-associated proteins (NAPs) that assist in the organization and packaging of the chromosome. In bacteria, NAPs function similar to histones, which are the DNA-organizing proteins found in eukaryotic cells. In archaea, the nucleoid is organized by either NAPs or histone-like DNA organizing proteins.

Figure (PageIndex<6>): The nucleoid region (the area enclosed by the green dashed line) is a condensed area of DNA found within prokaryotic cells. Because of the density of the area, it does not readily stain and appears lighter in color when viewed with a transmission electron microscope.


Examples of Prokaryotes:

Prokaryotes include bacteria and archaea, two of the three domains of life. This two domains are discussed bellow-

Bacterial cells

  • Bacteria are single-celled organisms that occur in all ecosystems throughout the world.
  • The cell wall of the bacterial cell is composed of peptidoglycan that invents it tough and thick.
  • Capsules are identical to some bacteria and thus might not be present in other prokaryotic cells.
  • The genetic material of bacteria is found in the form of circular coils of chromosomes.
  • E. coli , Streptomyces spp, Pseudomonas spp, etc., are examples of prokaryotes.

Archaeal cell (Archaea)

  • Archaeal cells are similar to bacterial cells as they, too, are primitive single-celled organisms.
  • Archaeal cells mostly occur in extreme environments like hot springs, oceans, and marshlands.
  • The capsule is absent in archaeal cells, and the cell wall is made up of pseudopeptidoglycan, proteins.
  • Similarly, the cell membrane of archaeal cells has a single layer of phospholipid that saves the cell against severe environments.
  • Examples of archaeal cells are Halobacterium spp, Thermoplasma spp, Sulfolobus spp, etc.

3.1.1: Characteristics of Prokaryotic Cells - Biology

A prokaryote is a simple, unicellular organism that lacks an organized nucleus or other membrane-bound organelle.

Learning Objectives

Describe the structure of prokaryotic cells

Key Takeaways

Key Points

  • Prokaryotes lack an organized nucleus and other membrane-bound organelles.
  • Prokaryotic DNA is found in a central part of the cell called the nucleoid.
  • The cell wall of a prokaryote acts as an extra layer of protection, helps maintain cell shape, and prevents dehydration.
  • Prokaryotic cell size ranges from 0.1 to 5.0 μm in diameter.
  • The small size of prokaryotes allows quick entry and diffusion of ions and molecules to other parts of the cell while also allowing fast removal of waste products out of the cell.

Key Terms

  • eukaryotic: Having complex cells in which the genetic material is organized into membrane-bound nuclei.
  • prokaryotic: Of cells, lacking a nucleus.
  • nucleoid: the irregularly-shaped region within a prokaryote cell where the genetic material is localized

Components of Prokaryotic Cells

All cells share four common components:

  1. a plasma membrane: an outer covering that separates the cell’s interior from its surrounding environment.
  2. cytoplasm: a jelly-like cytosol within the cell in which other cellular components are found
  3. DNA: the genetic material of the cell
  4. ribosomes: where protein synthesis occurs

However, prokaryotes differ from eukaryotic cells in several ways.

A prokaryote is a simple, single-celled (unicellular) organism that lacks an organized nucleus or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid.

Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule. The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Fimbriae are used by bacteria to attach to a host cell.

General Structure of a Prokaryotic Cell: This figure shows the generalized structure of a prokaryotic cell.All prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane, and a cell wall.The other structures shown are present in some, but not all, bacteria.

Cell Size

At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm. The small size of prokaryotes allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport.

Microbial Size: This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured).

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr 2 , while the formula for its volume is 4/3πr 3 . Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had the shape of a cube. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide another way is to develop organelles that perform specific tasks. These adaptations led to the development of more sophisticated cells called eukaryotic cells.


Characteristics of Prokaryotic Cells

Prokaryotic cell

  • Prokaryotic cell type is called prokaryotes.
  • Prokaryotic cell lacks membrane-bound organelles.
  • The size range of prokaryotic cell is usually smaller than 3 micrometre.
  • The genetic material of the prokaryotic cell is located in nuclear region or nucleoid.
  • Nucleoid contains a single chromosome and a piece of circular double-stranded DNA.
  • Further prokaryotic cells are divided into two types on basis of composition of cell wall into gram positive and gram negative cell.
  • Here nuclear membrane, nucleolus, histone proteins are absent.
  • Genes may be present in the cluster.
  • Plasma membrane lacks sterols.
  • The ribosome is of 70 S structure and evenly dispersed in the cytoplasm.
  • Chloroplast, Golgi body, Mitochondria, Pinacocytes and endoplasmic reticulum is absent.
  • Mesosomes and gas vacuole may be present in some prokaryotes.
  • The cell wall of the prokaryotic cell is mainly composed of peptidoglycan, polysaccharides and glycoprotein molecules.
  • It may show the presence of endospore in some prokaryotes.
  • The respiratory system is located in the plasma membrane.
  • The lysosome is absent in prokaryotes.
  • It shows a wide variety of anaerobic energy-yielding mechanism.
  • A prokaryotic cell has the ability to fix atmospheric nitrogen.
  • It shows the presence of magnetosome.
  • The G+C content is around 28 to 70 %
  • Protein synthesis is carried out in the cytoplasm.
  • It may contain flagella, pili and capsule on its surface.
  • A chromosome is circular in shape.
  • Mitosis and Meiosis mechanism is absent in a prokaryotic cell.
  • Reproduction of prokaryotic cell is carried out binary fission mechanism.
  • Magnetogenesis is absent in these cells.
  • Examples of prokaryotic cells are Bacteria, Cyanobacteria, Eubacteria, and Archaebacteria.

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3.1.1: Characteristics of Prokaryotic Cells - Biology

There are three types of cells: prokaryotic, eukaryotic, and mesokaryotic. Prokaryotic cells are generally single -celled organisms, eukaryotic cells are generally found in multi-cellular organisms, and mesokaryotic cells exhibit characteristics of both prokaryotic and eukaryotic cells.

Prokarytotic Cells
Prokaryotic Cells are the simplest cells and scientists believe they were the first cells to inhabit our world. They are much smaller than eukaryote cells, as they lack a defined nucleus and most other organelles that are found in eukaryotes. In fact, the word "prokaryote" means "before the nucleus." There are two types of prokaryotes - bacteria and archaea.

The nuclear substance of prokaryotes consists on a single chromosome which is in direct contact with the cytoplasm of the cell. There is no defined membrane surrounding the nuclear region, which is called a "nucleoid" in these cells.

Three primary features found in prokaryotic cells are:

1. Flagella and Pili - protein structures which project from the cell's surface which are used primarily for movement. They are also used to help cells attach to one another. Flagella and pili are not found in all prokaryotes.

2. Envelope - a cell wall which covers a plasma membrane. The envelope of the cells is the containing structure of the cell, separating the interior of the cells from the outer environment. Some bacteria also have an additional outside later of cells, called a capsule.

3. Cytoplasmic Region - the area contained inside the cell envelope or capsule. The cell DNA and ribosomes are found in this area. The DNA of bacteria is generally circular in shape. Some prokarotes carry additional extrachromosomal DNA inclusions called "plasmids." Plasmids are also circular in shape, and the generally carry out additional functions within the cell, such as antibiotic resistance.


Eukaryotic Cells
Eukaryotic cells, found in plants and animals, are more advanced than prokaryotic cells, and are more advanced in structure. They are about 15 times wider than the average prokaryote, and can have a cell volume as much as 1000 times greater.

The primary difference between peukaryotes and prokaryotes is that the eukaryotic cell has a defined cell nucleus, which contains the cell's DNA. The word "eukaryote" means "true nucleus."

Additional primary features of the eukaryotic cell are:

1. Cell Membrane - the plasma membrane that makes up the outer boundary of the cell. In addition to the cell membrane, plant cells also have a cell wall.

2. Chromosomes - the DNA in eukaryotes is organized into linear molecules called chromosomes. These chromosomes are stored in the nucleus in the cell.

3. Primary Cilia - protein structures on the outside of the cell which serve as sensory organs. Eukaryotes use these cilia to sense temperatures, movement and chemical makeup of their environment.

4. Flagella or Motile Cilia - more complete than those found in prokaryotes, but which perform similar functions in controlling the mobility of the cell.

5. Organelles - eukaryotic cells may also contain various "small organs" which perform specific cell functions. Various organelles include: nucleolus, ribosomes, vesicles, rough endoplasmic reticulum, Golgi body (or "apparatus"), cytoskeleton, smooth endoplasmic reticulum, mitochondrion, microtubules, vacuole, cytosol, lysosome, and centriole.


Mesokaryotic Cells
Mesokaryotic cells share characteristics of both prokaryotic and eukaryotic cells. The first part of the word, 'meso,' means 'in between', while 'karyon' means 'nucleus.'

Mesokaryotic cells exhibit a well-organized nucleus, as eukaryotes do, but it's nucleus divides through a process called amitosis, which more closely resembles the behavior of prokaryotic cells. The nucleus of a mesokaryotic cell duplicates itself with one nucleus going with each cell half when the rest of the cell divides.

A group of organisms known as "dinoflagellates", marine plankton and algae, are generally considered to be examples of mesokaryotes.

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3.1.1: Characteristics of Prokaryotic Cells - Biology

From prokaryotes to eukaryotes

The complex eukaryotic cell ushered in a whole new era for life on Earth, because these cells evolved into multicellular organisms. But how did the eukaryotic cell itself evolve? How did a humble bacterium make this evolutionary leap from a simple prokaryotic cell to a more complex eukaryotic cell? The answer seems to be symbiosis — in other words, teamwork.

Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union. In fact, the mitochondrion itself seems to be the "great-great-great-great-great-great-great-great-great granddaughter" of a free-living bacterium that was engulfed by another cell, perhaps as a meal, and ended up staying as a sort of permanent houseguest. The host cell profited from the chemical energy the mitochondrion produced, and the mitochondrion benefited from the protected, nutrient-rich environment surrounding it. This kind of "internal" symbiosis — one organism taking up permanent residence inside another and eventually evolving into a single lineage — is called endosymbiosis.


Characteristics of Living Things: Homeostasis

Homeostasis denotes a constant internal environment. To survive, cells must maintain a stable environment within itself, regardless of changes outside the cell. Cell membranes allow cells to regulate the situation within cells. Certain substances must stay inside, whereas other substances must stay outside the boundaries.

Cells control the amount of water coming in and going out, to preserve the equilibrium of water inside the cell with respect to the quantity outside the cell. In the same vein, certain vital cellular processes only take place under very specific pH and temperature conditions. pH is the measure of the acidity of a substance.

Cells maintain such stability with the aid of feedback loops. In a feedback loop, a cell detects changes in the concentration of certain substances, such as sodium, and then alters the amount of these substances entering and exiting the cell by tweaking components embedded in the cell membrane.


4. Prokaryotic cells can also carry small molecules of DNA called plasmids.

Plasmids are small, circular DNA molecules that contain the cell’s nonessential genes. Although plasmids can occur in a variety of sizes (ranging from around a thousand base pairs to hundreds of thousands), they usually only have a small number of genes. Antibiotic resistance is a trait that is frequently attributed to genes on plasmids.

The genetic material of plasmids is separate from that of the cell’s main chromosome, and they can replicate independently of that chromosome. When a prokaryotic cell with a plasmid divides, the daughter cells each receive a copy of the plasmid, along with its regular chromosome.


Watch the video: Introduction to Prokaryotic Cells. A-level Biology. OCR, AQA, Edexcel (August 2022).