molecular & cell biology prokaryotes and eukaryotes...

All living organisms are classified into two broad categories, prokaryotes & eukaryote. Prokaryotes are those organisms whose cells lack a cell nucleus while eukaryotes possess a well-defined, membrane bound nucleus.

Prokaryotes and Eukaryotes

In this learning object, the learner will be able to:

Recall the biochemical unity & biological diversity.Recall classification of living organism.Define Prokarytic cell structure.Define Eukaryotic cell structure.

Learning Objective

Prokaryotes and EukaryotesMolecular & Cell Biology

All living organisms from various periods of evolution have been found to exhibit remarkable similarity at the biochemical level. Genetic information is stored in the form of DNA or RNA , the same set of 20 amino acids form the structural elements of proteins, similar metabolic pathways and several proteins with structural similarity have been found to have similar roles in different organisms. All of these point towards the existence of a common ancestor from which various organisms evolved at different points of time.

Biochemical unity & biological diversity


Several proteins have been identified that possess similar three dimensional structures and perform very closely related functions in organisms that are separated in evolution over billions of years. One such protein is the TATA-box binding protein, which plays an important role in gene regulation.

Biochemical unity & biological diversity


Archaea are a group of prokaryotic organisms that are distantly related to bacteria-like organisms. They are however more similar to eukaryotes than bacteria. Both archaea and eukaryotic genomes encode homologous histone proteins, which are not present in bacteria. The ribosomal RNA and proteins or archaea closely resemble those of eukaryotes. However, archaea are capable of growing in extreme environmental conditions such as high temperatures, salt concentrations etc.

Classification of living organisms


Classification of living organismsOne of the most recent classification of living organisms is the three-domain system consisting of bacteria, eukarya and archae. Although archae were originally considered as bacteria, they were later classified into their own domain due to several differences in their metabolic pathways and genetics. Eukaryotes are believed to have evolved through several endosymbiotic relationships between various bacteria and archae.


Prokaryotes are simple, unicellular organisms that lack a well-defined nucleus for carrying their genetic material. They are usually a few microns is size and are one of the most ancient life forms known from which eukaryotes are believed to have evolved.

Prokaryotic cell structure


Bacteria can be divided into two major groups based on the structure of their cell-wall & thereby their response to Gram staining. The cell wall of Gram positive bacteria is composed of mainly polysaccharides and glycosylated molecules. It is made up of a single 20-80 nm thick homogenous layer of peptidoglycan. In addition cell wall usually contains teichoic acid, which is covalently connected to either peptidoglycans itself or to plasma membrane lipids(lipoteichoic acids)Plasma membrane is composed of a bilayer sheet of phospholipid molecules with their polar heads on the surface and their fatty -acyl chains forming the interior.



Gram negative bacteria have a more complex cell wall. They have a relatively thin peptidoglycan layer, around 2-7nm, covered by a 7-8 nm thick outer membrane made up of lipopolysaccharides. Porin proteins are present in the outer membrane which allow passage of small molecules across the membrane.



Nuclear material in the bacterial cell is not separated from the cytosol by a distinct nuclear membrane. However, it is usually concentrated in a specific clear region of the cytoplasm called the nucleoid. The genetic material usually contains a single, circular DNA molecule.



Ribosomes are composed of proteins and ribosomal RNA. The prokaryotic 70S ribosome is made up of a 50S large subunit and a 30S small subunit, where S refers to the Svedberg coefficient, which provides an indication about rate of sedimentation of the particle.



A motile bacteria propels itself from one place to another within the medium by rotating its flagella. A bacterial flagellum is made up of the protein flagellin. It has a helical structure with a sharp bend called the hook just outside the membrane, and a basal body containing the motor just below the membrane. To “swim” forward, the flagella rotates in counterclockwise direction. However, when flagellar rotation abruptly changes to clockwise direction, the bacterium "tumbles" in its place and seems incapable of moving. It then begins swimming again in another new, random direction.



Typical animal cell lacks a cell wall and contains several membrane-bound organelles, such as nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and peroxisomes.

Eukaryotic cell structure


An electron micrograph image of a plasma cell is shown here, clearly depicting the nucleus, golgi material, lysosome and mitochrondrion.



Plant cells have a rigid cell wall and membrane-bound organelles, such as nucleus, mitochondria, chloroplast, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles and peroxisomes.



Mitochondria, commonly referred to as “power house of the cell”, are membrane-bound organelles found in eukaryotic cells. They are responsible for generation of ATP to satisfy the body’s energy requirements and are also involved in other processes such as cell signalling, cell cycle control & cell gorwth. The organelle is made up of several compartments that carry out specialized functions and also contains its own independent genome that codes for mitochondrial proteins.



Ribosomes, which are composed of proteins and ribonucleic acids (RNAs), play a central role in protein biosynthesis. They “read” the nucleic acid information from messenger RNA and convert this into the corresponding amino acid code of proteins. Eukaryotic 80S ribosomes are composed of a large 40S subunit, which binds to tRNA and amino acids, and a small 28S subunit which binds to mRNA during protein synthesis. The subunit structure of prokaryotic and eukaryotic ribosomes differ from one another.



The endoplasmic reticulum and golgi apparatus are involved in synthesis, packaging & transport of various biomolecules. The ribosome-studded rough ER is a major site for protein synthesis while the smooth ER synthesizes lipids, steroids, metabolizes carbohydrates & steroids and regulates calcium concentration in muscles. The Golgi complex functions to process & package macromolecules such as proteins & lipids for their export to various other cellular organelles or outside the cell.



The nucleus is a membrane-bound organelle found in eukaryotic cells that is often considered as the “control centre” of the cell. It houses the genetic material of the cell in the form of chromosomes containing DNA molecules complexed with proteins known as histones. The nucleus is responsible for maintaining this genetic information by replication and for expression of genes performing various functions. The nucleolus is mainly involved in ribosome assembly, after which the ribosomes are exported to the cytoplasm for protein synthesis.



Lysosomes are found exclusively in animal cells for degrading any intracellular debris. They contain hydrolytic enzymes within sacs, which can digest and degrade any unwanted material when released.



Peroxisome is an organelle containing enzymes like catalase that are responsible for protecting the cell against free radicals and peroxides. They also play a role in metabolism of very long chain fatty acids. The have a single membrane and no independent genetic system.



Unity & Diversity

1. Evolution: The process by which various populations of organisms acquire and pass on their novel traits, in addition to other existing traits, from one generation to the next. This explains the origin of new species of organisms and the vast diversity that is observed in the biological world. However, it is believed that the origin of all organisms can be traced back to one common ancestor due to several underlying biochemical similarities.


1. Bacteria: These are a group of unicellular, prokaryotic microorganisms that are present in every habitat on earth. They are usually a few micrometers in length and have a range of shapes such as rods, spheres, and spirals. Bacteria are one of the most ancient life forms, believed to have been present over 3 billion years ago. These organisms lack a defined cell-nucleus and other cellular organelles, due to which they are classified as prokaryotes.

2. Archae: Archae are a group of single-celled microorganisms that are also classified as prokaryotes due to their lack of cellular organelles and a nucleus. They were originally classified as bacteria but now form their own separate domain due to several genetic and metabolic differences, in which they more closely resemble eukaryotes.

Unlike eukaryotes however, they are capable of obtaining their energy from various sources such as organic compounds, sugars, ammonia etc.

3. Eukarya: These are multicellular organisms whose cells contain complex structures including a well-defined, membrane-bound nucleus carrying the genetic material. Eukaryotic cells are typically larger than prokaryotes and contain several other membrane-bound organelles which carry out complex metabolic & cell division processes.

Classification


1. Cell envelope: This encompasses the plasma membrane and layers outside it which confer the cell with rigidity. The composition of the cell envelope varies with organism.

2. Plasma membrane: The bacterial protoplast is bound by a living ultrathin and dynamic plasma membrane. It chemically comprises molecules lipids and proteins which are arranged in fluid mosaic pattern.

3. Cell wall: The plasma membrane is covered with a strong and rigid cell wall.

a) Peptidoglycan: Peptidoglycan is an enormous mesh like polymer composed of many identical subunits, lying outside the plasma membrane of bacteria. The polymer contains two sugar derivative, N- acetylglucoseamine and N- acetlymuramic acid and a chain of three to five amino acids

Prokaryotic cell

attached to the N- acetlymuramic acid .

b) Periplasmic space: A space observed between the plasma membrane and the outer membrane of Gram negative bacteria or equivalent space between the plasma membrane and wall of Gram positive bacteria.

c) Teichoic acid: They are polymers of glycerol or ribitol joined by phosphate group, covalently linked to either the petidoglycan or plasma membrane of Gram positive bacteria. Amino acids such as D- alanine or sugars like glucose are attached to the glycerol and ribitol groups.

d) Lipoteichoic acid: Teichoic acids covalently connected to plasma membrane of Gram positive bacteria


e) Integral protein: Proteins attached to the bacterial plasma membrane.

f) Outer membrane: It is 7 – 8 nm thick membrane lying outside the peptidoglycan layer and is linked to the cell either by a lipoprotein or via many adhesion sites.

g) Porins: Porins are proteins composed of beta sheets , spans the outer membrane of Gram negative bacteria and is more or less tube shaped. Its narrow channels allows passage of molecules smaller than 600 – 700 Daltons.

h) Lipopolysachharide(LPS): LPS is the major component of the outer membrane of Gram-negative bacteria. These large, complex molecules contain both lipid and carbohydrate and consists of three parts:1) lipid A, (2) the core polysachharide and (3) the O side chain or O antigen.

4. Capsule: In some bacteria, the cell wall is surrounded by an additional slime or gel layer called capsule.

5. Nucleoid: In bacteria the nuclear material is not separated from the cytosol by the nuclear membrane. However, the nuclear material is usually concentrated in a specific clear region of the cytoplasm, called nucleoid, Contains a single, simple, long circular DNA molecule.

6. Ribosomes: Ribosomes are tiny spheroidal dense particles that contain approximately equal amount of RNA and proteins. Ribosomes have a sedimentation coefficient of about 70S and are composed of two subunits namely 50S and 30S.

Prokaryotic cell


7. Flagella: Many bacteria are motile and one or more flagella for the cellular locomotion that propel cell through its surroundings.

8. Pili: Some bacteria contain non-flagellar, extremely fine, appendages called fimbriae or pilli that provide points of adhesion to surface of other cells.

9. Gram staining: It is a method of differentiating bacterial species into two broad categories, gram-positive and gram-negative, based on the composition of their cell walls.

Prokaryotic cell


1. Cell wall: Present only in plant cells, these are responsible for providing the cell with structure and rigidity. They also protect the cell from swelling due to osmosis.

2. Cytoskeleton: Provides the cell with required support and also plays a role in movement of cellular organelles.

3. Plasmodesmata: Helps in interaction between adjacent plant cells by providing a path between them.

4. Plasma membrane: Provides a protective layer around the cell by separating it from the environment and monitoring the movement of molecules into and out of it.

5. Lysosome: These are found exclusively in animal cells for degrading any intracellular debris.

6. Peroxisome: Organelle containing enzymes that are responsible for protecting the cell against free radicals and peroxide. They also play a role in metabolism of very long chain fatty acids.

7. Smooth endoplasmic reticulum (SER): Biosynthesis of lipids and metabolism of several drugs occurs at the SER.

8. Rough endoplasmic reticulum (RER): This is a site where much of the protein synthesis occurs as it is studded with ribosomes.

9. Vacuole: These are essential for storage of metabolites and also for degrading and recycling macromolecules.

Eukaryotic cell


10. Ribosomes: The function of ribosomes is common to prokaryotes and eukaryotes. They serve to synthesize proteins from the corresponding RNA.

11. Golgi complex: It is a cup shaped organelle located near the nucleus. It acts as a downstream processing centre for packaging proteins and then targeting them for distribution to other organelles or for export outside the cell.

12. Mitochondria: Commonly referred to as the “power house of the cell”, this organelle is responsible for energy production in the form of ATP by oxidation of nutrients.

13. Glyoxysome: Site at which the glyoxylate cycle occurs for energy production from acetate precursors in plants and certain bacteria.

14. Nucleus: The defining feature of all eukaryotes, nucleus is the critical organelle housing all genetic information in the form of DNA.

15. Nucleolus: This is present within the nucleus and is responsible for synthesis of ribosomal RNA (rRNA).

16. Nuclear envelope: Surrounds the nucleus and functions to segregate the chromatin i.e. DNA and proteins, from the external cytoplasm.

17. Chloroplasts: Organelles that are responsible for the major difference between plant and animals. They allow the synthesis of carbohydrates and produce energy by harvesting sunlight.

Eukaryotic cell


18. Thylakoids: They are stacked organelles present within the chloroplast that are essential for ATP production using light energy.

19. Starch granules: These are also found within the chloroplast and are necessary for temporary storage of carbohydrates.

Eukaryotic cell


molecular & cell biology prokaryotes and eukaryotes...

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