Comparative study of typical prokaryotic cell and

eukaryotes

ByDR SAMUEL AGUAZIM

STRUCTURE, FUNCTION & GROWTH OF PROKARYOTIC & EUKARYOTIC CELLS

• Prokaryotic and Eukaryotic Cells .

• All living creatures are made up of CELLS , small membrane bound units filled with aqueous solutions of chemicals, which have the ability to create copies of themselves by growing and dividing.

• Living organisms can be classified into 3 major domains:

• Prokaryotes• *Bacteria • *Archaea • Eukaryotes• *Plant cells • *Animal cells • Prokaryotes and Eukaryotes are 2 distinct cell types

with STRUCTURAL differences

The Prokaryotic Cell

• The Prokaryotic Cell Simply stated, prokaryotes are molecules surrounded by a membrane and cell wall.

• Prokaryotes Lack a membrane bound nucleus enclosing the DNA • DNA is present as a single circular molecule called a BACTERIAL

CHROMOSOME • DNA is naked having no associated histone proteins • No membrane bound organelles • Apart from the DNA nucleoid, there is little internal structure apart

from dissolved substances and a large number of RIBOSOMES essential for PROTEIN SYNTHESIS

• The cytosol is an effective site for bacterial cell metabolism. This allows bacteria to adapt quickly to changing nutritional conditions, but means the regulation of genetic and metabolic activity has to be tightly regulated.

• Divide by BINARY FISSION • Some prokaryotic cells have external whip-like FLAGELLA for

locomotion or hair like PILI for adhesion. • Prokaryotic cells come in multiple shapes: cocci (round), baccilli

(rods), and spirilla or spirochetes (helical cells).

External Prokaryotic Structures

• Cell wall Contains PEPTIDOGLYCAN (only found in bacteria). Large complex molecule consisting of polysaccharide polymers cross-linked by short chains of amino acids

• Capsules • Sometimes the cell wall is further surrounded

by a gelatinous polysaccharide sheath called an attach CAPSULE , GLYCOCALYX or SLIME LAYER

• Plasma Membrane • Basic structure of the phospholipid bilayer is

the same for all bacteria • Flagella Motile bacteria usually have long,

thin appendages called FLAGELLA . These protein sub-units are used to propel bacteria through liquids

• Pili or Fimbrae • A pilus ( Latin ; plural : pili )

is a hairlike protein structure on the surface of a bacterial cell, required for bacterial conjugation (transfer of genetic material)

• A fimbrium (Latin; plural: fimbria ) is a short pilus that is used to attach the cell to a surface. Mutant bacteria that lack fimbria cannot adhere to their usual target surfaces and, thus, cannot cause diseases

• Spores & Cysts • These are produced by some bacteria to survive unfavourable

environmental conditions. Dormant forms are metabolically inactive and only germinate under suitable conditions

• ENDOSPORES : a dormant , tough, non-reproductive structure produced by a small number of bacteria . The primary function of most endospores is to ensure the survival of a bacterium through periods of environmental stress. They are therefore resistant to ultraviolet and gamma radiation , desiccation , lysozyme , temperature , starvation , and chemical disinfectants . Endospores are commonly found in soil and water, where they may survive for long periods of time e.g. Clostridium (tetanus, gas gangrene), Bacillus (anthrax)

• CYSTS : also dormant, but unlike endospores are not resistant to heating at high temperatures

Structure Chemical Composition Function

Cell wallPeptidoglycan

Sugar backbone with peptide side chains that are cross-linked

Gives rigid support, protects against osmotic pressure;is the site of action of penicillins andcephalosporins and is degraded by lysozyme.

Outer membrane of gram- negative bacteria

Lipid A Toxic component of endotoxin.

Polysaccharide Surface fibers of gram- Teichoic acid positive bacteria Gives rigid support, protects against osmotic pressure; is the site of action of penicillins and cephalosporins and is degraded by lysozyme. Toxic component of endotoxin.Major surface antigen used frequently in laboratorydiagnosis.

Cytoplasmic membrane Lipoprotein bilayer without sterols

Site of oxidative and transport enzymes.

Ribosome RNA and protein in 50S and 30S Protein subunits

Protein synthesis; site of action of aminoglycosides,subunits erythromycin, tetracyclines, and chloramphenicol.

Nucleoid DNA Genetic material.

Mesosome lnvagination of plasma membrane

Participates in cell division and secretion.

Periplasm Space between plasmamembrane and outer membrane

Contains many hydrolytic enzymes, including P-lactamases.

Non-essential componentsCapsule Polysaccharide Protects against

phagocytosisPilus or fimbria Glycoprotein Two types: (1) mediates

attachment to cell surfaces;(2) sex pilus mediates attachment of two bacteriaduring conjugation

Flagellum Protein Motility.

Spore Keratinlike coat, dipicolinic acid

Provides resistance to dehydration, heat, andchemicals.

Plasmid DNA Contains a variety of genes for antibiotic resistanceand toxins.

Granule Glycogen, lipids, polyphosphates.

Site of nutrients in cytoplasm.

Glycocalyx Polysaccharide Mediates adherence to surfaces.

Classifying Prokarotes• Main method is using the GRAM’S STAIN • This separates bacteria into GRAM-POSITIVE

(purple) and GRAM-NEGATIVE (red) depending on the percentage of PEPTIDOGLYCAN in the cell walls

• - GRAM-POSITIVE bacteria have a cell wall only 1 layer thick

• - GRAM-NEGATIVE bacteria have a cell wall several layers thick

Differences between cell wall of Gram positive and Gram negative bacteria

Character Gram positive Gram negative

Thickness Thicker Thinner

Periplasmic space Absent Present

Lipids Absent or small Present

Teichoic acid Present Absent

Peptidoglycan 16- 80nm 2nm

Eukaryotes• More complex multicellular organisms e.g. plants, animals,

fungi and also many single-celled organisms e.g. amoeba, yeast • Possess an NUCLEUS and other organelles all of which are

surrounded by a MEMBRANE , which divided the cell up into compartments

• COMPARTMENTALISATION: very important ! • ADVANTAGES: • Molecules are ‘concentrated’ together, increases

rate of reactions • Keeps reactive molecules away from other parts of

the cell that may be affected by them • Large work surface area … many enzymes are

bound in membranes

Eukaryotes• The basic eukaryotic cell contains the following: • - membrane-bound nucleus • - plasma membrane • - glycocalyx (components external to the plasma • membrane) • - cytoplasm (semifluid) • - cytoskeleton – microfilaments, intermediate filaments and

microtubules that suspend organelles, give shape, and allow motion .

• - presence of characteristic membrane • enclosed subcellular organelles e.g. mitochondria, golgi, rER,

sER etc

• Plant & Animal Cells • For ANIMAL CELLS only:

– Peroxisomes & Lysosomes often present – Some have microvilli on their surface – Centrioles organise spindle fibres during cell division

• For PLANT CELLS only: – Cell walls made from cellulose – Communication with neighbouring cells occurs through

plasmodesmata – Usually a large central vacuole – Photosynthesis occurs in cells containing chloroplasts

Plasma Membrane

• Plasma Membrane • A lipid/protein/carbohydrate

complex, providing a barrier and containing transport and signalling systems.

Nucleus

• Nucleus • Double membrane

surrounding the chromosomes and the nucleolus. Pores allow specific communication with the cytoplasm. The nucleolus is a site for synthesis of RNA making up the ribosome

Mitochondria

• Mitochondria • Surrounded by a double

membrane with a series of folds called cristae.

• Functions in energy production through metabolism.

• Contains its own DNA, and is believed to have originated as a captured bacterium.

Rough endoplasmic reticulum (RER)

• Rough endoplasmic reticulum (RER) • A network of interconnected

membranes forming channels within the cell.

• Covered with ribosomes (causing the rough appearance) which are in the process of synthesizing proteins for secretion or localization in membranes.

• Ribosomes • Protein and RNA complex

responsible for protein synthesis

Golgi Apparatus• Golgi apparatus • *A series of stacked membranes.

Vesicles (small membrane surrounded bags) carry materials from the RER to the Golgi apparatus.

• *Vesicles move between the stacks while the proteins are processedto a mature form.

• *Vesicles then carry newly formed membrane and secreted proteins to their final destinations including secretion or membrane localisation.

Centrioles

• Centrioles • Centrioles are found only in

animal cells. They function in cell division.

Lysosymes

• Lysosymes • A membrane bound

organelle that is responsible for degrading proteins and membranes in the cell, and also helps degrade materials ingested by the cell.

Peroxisomes

• Peroxisomes or Microbodies • Produce and degrade hydrogen

peroxide, a toxic compound that can be produced during metabolism

Chloroplasts

• Chloroplasts • Surrounded by a double

membrane, containing stacked thylakoid membranes.

• Responsible for photosynthesis, the trapping of light energy for the synthesis of sugars.

• Contains DNA, and like mitochondria is believed to have originated as a captured bacterium.

Vacuoles

• Vacuoles • Membrane surrounded “bags” that contain

water and storage materials in plants.

• Cell wall • Plants have a rigid

cell wall in addition to their cell membranes. They provide support for the plant.

• Similarities between P & E cells • Prokaryotes & Eukaryotes are CHEMICALLY &

METABOLICALLY similar: – Both have genetic material – Both have a cell membrane – Both have a cytosol – Both have ribosomes – Both contain nucleic acids, proteins, carbohydrates &

lipids – Both use similar reactions for storing energy and

metabolic activities e.g. building proteins

Differences between P & E cellsmain differences are structural

• Main differences are STRUCTURAL : Prokaryotes eukaryotes

No membrane bound organelles nuclues Membrane bound nucleus

Cell walls made of peptidoglycan (Thickness of wall depends on whether the cell is Gram +ve or –ve)

Cell walls, if present, made of cellulose (chitin in fungi)

No membrane bound organelles Membrane bound organelles (compartmentalisation)

Have pili & fimbriae (for adhesion) and flagella (for propulsion)

Have cilia or flagella (for movement)

Mucilaginous capsule No mucilaginous capsule present (numerous internal structures present including microtubules, ER, Golgi, secretory vesicles etc)

ranges from 0.5um to 100um bound nucleus Cell size ranges from 10 – 150um Cell size

Comparison of prokaryotic and eukaryotic cells

Prokaryotes Eukaryotes

Organisms Monera: Eubacteria & archebacteria

Protists, fungi, plants and animals

Level of organization Single celled Single celled (protists mostly) or multicellular usually with tissues and organs

Typical cell size Small(1-10microns) Large(10-100 microns)

Celll wall Almost all have cell walls(murein)

Fungi and plants(cellulose and chitin): none in animlas

Organelles Usually none many different ones with specialized functions

Metabolism Anaerobic and aerobic: diverse

Mostly aerobic

Comparison of prokaryotic and eukaryotic cells

Genetic material Single circular double stranded DNA

Complex chromosomes usually in pairs: each with a single double stranded DNA molecule and associatied proteins contained in a nucleus

Mode of division Binary fission mostly: budding

Mitosis and meiosis using a spindle: followed by cytokinesis