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.
[The sizes of cells and organelles]
Living organisms can be classified into 3 major domains:
Prokaryotes and Eukaryotes are 2 distinct cell types with STRUCTURAL differences
The Prokaryotic Cell
Simply stated, prokaryotes are molecules surrounded by a membrane and cell wall.
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
Contains PEPTIDOGLYCAN (only found in bacteria). Large complex molecule consisting of polysaccharide polymers cross-linked by short chains of amino acids
Sometimes the cell wall is further surrounded by a gelatinous polysaccharide sheath called an attach CAPSULE , GLYCOCALYX or SLIME LAYER
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
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
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 !
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
The basic eukaryotic cell contains the following:
- membrane-bound nucleus
- plasma membrane
- glycocalyx (components external to the plasma
- 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
[Stick in & label plant & animal cell diags]
A lipid/protein/carbohydrate complex, providing a barrier and containing transport and signalling systems.
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
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.
Protein and RNA complex responsible for protein synthesis
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 "processed" to a mature form.
Vesicles then carry newly formed membrane and secreted proteins to their final destinations including secretion or membrane localisation.
Centrioles are found only in animal cells. They function in cell division.
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 or Microbodies
Produce and degrade hydrogen peroxide, a toxic compound that can be produced during metabolism
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.
Membrane surrounded "bags" that contain water and storage materials in plants.
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 cells
Main differences are STRUCTURAL :
Cell size ranges from 10 – 150um Cell size ranges from 0.5um to 100um No mucilaginous capsule present (numerous internal structures present including microtubules, ER, Golgi, secretory vesicles etc) Mucilaginous capsule Have cilia or flagella (for movement) Have pili & fimbriae (for adhesion) and flagella (for propulsion) Membrane bound organelles (compartmentalisation) No membrane bound organelles Cell walls, if present, made of cellulose (chitin in fungi) Cell walls made of peptidoglycan (Thickness of wall depends on whether the cell is Gram +ve or –ve) Membrane bound nucleus No membrane bound nucleus EUKARYOTES PROKARYOTES