Prokaryote Morphology and Size Quiz

Prokaryote Morphology

• Prokaryotes exhibit a wide variety of shapes and structures.
• Cell shape is genetically determined, influenced by proteins.
• Shape is determined as the cell wall material is being synthesized.
• Example: Escherichia coli RodA protein; if the rodA gene is missing, the cells become spherical.

Geeking out on Morphology

• Scientists have studied bacterial curvature versus length.

• This analysis reveals relationships between morphology and function relating to how they swim and perform chemotaxis.

• Studies show trade-offs between properties like swimming efficiency, chemotaxis sensitivity, and construction/formation ease.

Relative Sizes of Prokaryotes

• Prokaryotes vary greatly in size.
• Size differences strongly impact the surface area to volume ratio.
• Higher surface area to volume ratios allow for more efficient transport of nutrients and waste products, which aids quicker growth and larger population size.
• Example organisms and their sizes/volumes.

The Importance of Being Small

• Transport rates in and out of cells are directly related to the cell's surface area.
• Cells must balance transport efficiency with maintaining a large enough nutrient concentration to support their needs.
• Smaller cells have larger surface area to volume ratios.
• This makes nutrient/waste transport efficient and promotes faster growth and larger populations.

Prokaryotic Cell Walls

• The cell wall provides structure and shape and protects cells from osmotic forces.
• Bacteria are divided into Gram-positive and Gram-negative groups based on their cell wall structures.
• Gram-positive cells have a thick peptidoglycan layer surrounding their cytoplasmic membrane.
• Gram-negative cells have a thin peptidoglycan layer surrounded by an outer membrane.
• Bacteria and Archaea have different cell wall structures.

Cell Wall Functions

• A crucial function of the cell wall is to protect the cell from lysis due to osmotic forces.
• Lysozyme, an enzyme found in bodily secretions like tears and saliva, can break down the cell walls of some bacteria, leading to bacterial lysis by osmotic forces.

Cells Without Walls

• Some bacteria (like Mycoplasma) and some archaea (like Thermoplasma) lack cell walls.
• They are only surrounded by cell membranes and are, therefore, particularly vulnerable to osmotic pressure.

Bacterial Cell Walls

• Gram-positive cells contain thick peptidoglycan layers outside their cytoplasmic membranes.
• Gram-negative cells have thin peptidoglycan layers which are further surrounded by an outer membrane.

Peptidoglycan

• Peptidoglycan is a unique polymer in bacterial cell walls; a mesh-like structure made of sugars (NAG and NAM) and a short peptide.
• Neighboring chains in the peptidoglycan are connected by covalent attachments between peptide chains.
• Different Gram-positive and Gram-negative bacteria use different peptides and cross-linking.

The Periplasm

• The periplasm is the space between the inner and outer membranes in Gram-negative bacteria.
• It plays a vital role in cell function, containing peptidoglycans, water, proteins involved in transport, sensors, binding proteins of ABC transporters, and enzymes involved in peptidoglycan synthesis.

Lipopolysaccharide (LPS)

• Found in the outer leaflet of the outer membrane in Gram-negative bacteria.
• LPS consists of lipids and sugars.
• Lipid A is a toxin (endotoxin) that can cause fever, inflammation, shock, and blood clotting when it enters tissues or the bloodstream.
• Lipid A can also result in vomiting and diarrhea if it enters the gastrointestinal tract.

Porins

• Outer membranes of Gram-negative cells are highly permeable due to porins.
• Porins are protein channels allowing small molecules to diffuse into and out of the cell.

Cell Walls in Archaea

• Archaea possess diverse cell wall structures.
• Some archaea have pseudopeptidoglycan, which is similar to peptidoglycan, but contains different glycans, bonds, and peptides, making it resistant to lysozyme.

Endospores

• Endospores are formed by some Gram+ bacteria (e.g., Bacillus and Clostridium) when they experience nutrient depletion or stress.
• Endospores are highly resistant to drying, heat, radiation, and toxic chemicals.
• Spores can exist for an indefinite period and germinate when they reenter favorable conditions.

Endospores Versus Other Spores

• Endospore formation is unique to certain Gram-positive bacteria.
• Some bacteria produce other types of spores for different purposes than bacterial endospores.

Eukaryotic Cells

• Eukaryotic cells can have highly complex structures, differing from prokaryotic cells.
• Eukaryotic flagella and cilia are made of microtubules.
• Cilia resemble fimbriae but function more like oars.

Eukaryotic Structures

• Mitochondria evolved from bacteria and are the sites of respiration.
• Chloroplasts in phototrophic eukaryotes evolved from cyanobacteria.
• They use light energy to energize electrons to produce ATP and convert CO2 to glucose.

Movement with Intracellular Structures: Gas Vesicles

• Some aquatic cells use gas vesicles to regulate their buoyancy, changing their depth without active swimming.

Movement Using Magnets

• Magnetosomes—made of magnetite and surrounded by a membrane—allow bacteria to sense the Earth's magnetic field and migrate accordingly via magnetotaxis.

Endospores, continued

• Endospore formation and development are restricted to specific Gram-positive bacteria.
• Some other bacteria form spores, but their function might be for dispersal instead of survival in harsh conditions.

Other Intracellular Structures

• Bacteria and other cells can produce poly-hydroxyalkanoates (PHAs), lipid-like polymers, as energy reserves.
• Polyphosphate is another example of an intracellular structure used for energy storage and other various functions.
• Sulfur that is stored inside the cytoplasm can also be used as an energy or carbon source during energy production.

Taxis- Movement with a purpose

• Cell movements often move toward or away from stimuli rather than just in a random fashion.
• Chemotaxis is the movement toward or away from chemicals such as nutrients and toxins, which are detected by chemoreceptors.
• Phototaxis is the movement toward or away from light, which many cells detect through photoreceptors.

Beyond the Wall

• Capsules are tight, rigid layers of polysaccharides that surround certain cells, offering protection from desiccation and immune recognition.
• Slime layers are less structured and loosely attached layers of polysaccharides that enhance attachment and offer protection from desiccation.
• Fimbriae are proteinaceous extensions aiding in bacterial adherence but not motility, playing a role in biofilm formation and pathogenesis.
• Pili are protein filaments used for cell movement, attachment to surfaces, and gene exchange.
• Hami are grappling hook-like structures that are used for attachment to surfaces and forming biofilms.
• Flagella are complex, whip-like protein components involved in motility through rotating motion rather than through extension. Flagella are important for swimming through a medium, with different numbers and arrangements based on different species.
• The flagellar machinery is complex and uses ATP for rotation and proper function.
• Cells can use flagellum-type rotation to swim forward through a medium, but can also rotate in the opposite directional by pulsing and switching to change direction.

Cytoplasmic Membrane Functions

• The cytoplasmic membrane acts as a permeability barrier, preventing leakage and controlling nutrient/waste transport.
• It anchors proteins involved in transport, bioenergetics, and chemotaxis.
• It plays a central part in the generation and dissipation of the proton motive force, conserving energy for cell functions.

Transport Across Membranes

• Simple transport uses energy from the proton motive force to transport substances across the membrane.
• ABC transporters utilize ATP energy for specific transport and often utilize binding proteins in the periplasm to help with transport.
• Group translocation alters the transported substance as it moves through the membrane.

Test your knowledge on the diverse shapes and sizes of prokaryotes. Understand how genetic factors influence cell morphology and the implications of size on nutrient transport efficiency. This quiz will cover essential concepts related to prokaryotic morphology and size variations.