Prokaryotic and Eukaryotic Cells Anatomy

Lecture 4: Functional Anatomy of Prokaryotic and Eukaryotic Cells

The Structure of a Prokaryotic Cell

• Typical structures may be found in bacteria
• Prokaryotic cells usually lack membrane-enclosed organelles
• Every bacterium contains cytoplasm, ribosomes, a plasma membrane, and a nucleoid
• Almost all bacteria have cell walls
• Some structures play specific roles
  • Capsule (glycocalyx) is for bacterial virulence
  • Cell wall or flagella for bacterial identification
  • Cell wall works as a target of antimicrobial agents
• Plasmids encode information for resistance to antibiotics or the production of toxins
• Plasmids may be shared between bacteria

Composition of Glycocalyx

• Most bacteria are found sticking to solid surfaces, like other cells, rather than free-floating
• Glycocalyx is secreted by many bacteria on their surface outside of the cell wall
  • It is viscous and gelatinous
  • Made of polysaccharide or polypeptide
• Glycocalyx protects cells and allows attachment of cells to surfaces
• Two types of glycocalyx exist: capsule is neatly organized and firmly attached; slime layer is unorganized and loose

Functions of Glycocalyx

• Contributes to virulence with varying severity
• Prevents phagocytosis by host cells, preventing ingestion or elimination of bacteria
• Helps microbes adhere to body surfaces
  • Bacillus anthracis causes anthrax only when encapsulated
  • Streptococcus pneumoniae causes pneumonia only when encapsulated
• Helps form bacterial communities, otherwise known as biofilms
  • Streptococcus mutans uses glycocalyx to attach to teeth
• Glycocalyx protect cells against dehydration
• Viscosity may inhibit nutrient loss

Flagella

• Filamentous appendages on the cell surface
• Propel bacteria found in motile bacteria, allowing them to move themselves
• Three basic parts:
  • Filament is the outermost region and is made of the protein flagellin
  • Hook attaches to the filament
  • Basal body anchors the flagellum to the cell
• Bacteria without flagella are called atrichous, meaning "without projections"

Flagellar Rotation

• Flagellar rotation is either clockwise or counterclockwise
• "Run" or "swim" describes when a bacterium moves in one direction for a length of time, counterclockwise
• "Tumble" describes when a bacterium changes direction periodically, randomly, and abruptly, rotating clockwise
• Tumbles interrupt runs
• Tumbles are caused by a reversal of flagellar rotation

Flagella Arrangement

• Peritrichous flagella are spread over the entire cell surface
• Monotrichous flagella are a single flagella
• Lophotrichous flagella have two or more flagella at one end
• Amphitrichous flagella have flagella at each end

Advantage of Motility

• Enables a bacterium to both move toward a favorable environment and away from adverse areas
• Taxis describes the movement of a bacterium toward or away from a particular stimulus
• Chemotaxis describes movement in response to the presence of a chemical
• Phototaxis describes movement in response to the presence of light
• Receptors in motile bacteria pick up stimuli like oxygen and glucose
• Attractants create a positive chemotactic signal, causing bacteria to move toward the stimulus, resulting in runs
• Repellents create a negative chemotactic signal, causing bacteria to move away from the stimulus, resulting in tumbles

Axial Filaments

• Found in spirochetes, the bacteria causing Syphilis and Lyme Disease
• Anchored at one end of a cell
• Filament rotation causes movement in an outer sheath leading to movement like a corkscrew

Fimbriae and Pili

• Hairlike appendages that are shorter, straighter, and thinner than flagella
• Consist of a protein called pilin
• Divided into two types with different functions, fimbriae and pili

Fimbriae

• Allow for attachment
• Involved in the formation of biofilms
• Enable some bacteria to adhere to body surfaces
  • E. coli 0157 adheres to the lining of the intestine, causing severe diarrhea
• Colonization and disease do not occur in the absence of fimbriae

Pili

• Involved in gliding and twitching motility
• Twitching occurs when a pilus extends by the addition of subunits of pilin, making contact with a surface or another cell
• DNA transfer between cells occurs by conjugation through sex pili
• Confer antibiotic resistance

The Cell Wall

• The cell wall of a bacterial cell is a semirigid structure, responsible for the shape of the cell
• Almost all prokaryotes have a cell wall that surrounds the underlying, fragile plasma membrane (cytoplasmic membrane) (cell membrane)
• It protects the interior of the cell from adverse changes in the outside environment, like changes in water pressure
• Made of peptidoglycan in bacteria
• Contributes to the ability to cause disease, or pathogenicity
• It is the site of action of some antibiotics
• Chemical composition can be used to differentiate major groups of bacteria

Composition of Bacterial Cell Walls

• Made of a network called peptidoglycan, also known as murein
• Peptidoglycan consists of a repeating disaccharide in rows
  • Rows are made of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
• Rows of repeating disaccharides, also known as glycans, are linked by polypeptides, or proteins
• Together these form a lattice structure that surrounds and protects the entire cell

Peptidoglycan

• Glycan includes NAM and NAG disaccharide
• Peptido refers to peptide links
• Tetrapeptide side chains consist of four amino acids attached to NAMs in the backbone
• Peptide cross bridges link the tetrapeptide to an amino group of a tetrapeptide on a neighboring NAM

Crosslinking of Peptidoglycan

• Gram-negative bacteria have tetrapeptides connected with direct cross-linking, forming a thin peptidoglycan layer
• Gram-positive bacteria have tetrapeptides connected with a peptide interbridge, forming a thick peptidoglycan layer

Penicillin

• Interferes with the final linking of the peptidoglycan rows by peptide cross-bridges
• This weakens the cell wall, leading the cell to undergo lysis, or burst

Components of Gram-Positive Cell Walls

• Contain thick peptidoglycan
• Teichoic acids thread through multi-layers of peptidoglycan, reinforcing the cell wall
• Lipoteichoic acid spans the peptidoglycan layer, linking the cell wall to the plasma membrane
• Wall teichoic acid is linked to the peptidoglycan layer
• It is recognized by the human immune system, serving as an antigenic

Gram-Positive Cell Wall Overview

• Periplasmic space exists between the cell wall and plasma membrane

Gram-Negative Cell Walls

• Gram-negative cell walls have thin peptidoglycan and an outer membrane
• Peptidoglycan is bonded to lipoproteins in the outer membrane and sits in the periplasm
• Also contain a periplasmic space
  • The periplasm sits between the outer membrane and the plasma membrane
• The outer membrane is made of lipopolysaccharide (LPS), lipoproteins, and phospholipids
  • Provides a barrier to substances like Penicillin
• Contain porin proteins for channels allowing for permeability, facilitating the entrance of nutrients to the cell
• Do not contain teichoic acids

Lipopolysaccharide (LPS)

• Complex molecule containing lipids and carbohydrates comprising three components
• Lipid A is embedded in the top layer of the outer membrane
  • Is released when gram negative (-) bacteria die, functioning as an endotoxin
  • Responsible for infection symptoms like fever and shock
• The core polysaccharide is attached to lipid A with a structural role
• O polysaccharide extends from the core polysaccharide and is composed of sugars
  • Recognized by the human immune system and has an antigenic role
  • This antigenic role is comparable to teichoic acids in gram positive (+) cells

Mycoplasmas- Atypical Cell Walls

• Smallest known bacteria able to grow and reproduce outside a living host cell
• Pass through most bacterial filters due to their size and the lack of cell walls
• Have unique plasma membranes, containing lipids called sterols
• Important human pathogen amongst the mycoplasmas is Mycoplasma pneumoniae, the common form of mild pneumonia

Atypical Cell Walls

• Acid-fast cell walls
  • Contain peptidoglycan with a waxy lipid called mycolic acid bound to the peptidoglycan that prevents the uptake of dyes
• An arabinogalactan polysaccharide holds together acid and peptidoglycan
• Acid-fast genera are Mycobacterium and Nocardia

Damage to the Cell Wall

• The outer membrane of gram-negative cell walls blocks access of penicillin to its target on the cell membrane surface, making them less susceptible
• Enzymes can break up the glycan portion of peptidoglycan, which weakens the cell wall of gram-positive bacteria

The Plasma (Cytoplasmic) Membrane Structure

• Have a similar structure in eukaryotes and prokaryotes
  • Phospholipid bilayer enclosing the cytoplasm with integral and peripheral proteins
• Differences in structure from Eukaryotes
  • Eukaryotic plasma membranes also contain carbohydrates and sterols, such as cholesterol, for attachment and cell-to-cell recognition

Plasma Membrane

• Peripheral proteins are on the inner or outer surface of the plasma membrane
• Integral and transmembrane proteins penetrate the membrane
  • Some transmembrane proteins form channels
• Influenza and toxins from cholera and botulism enter target eukaryotic cells by first binding to glycoproteins on their plasma membranes

Fluid Mosaic Model of the Plasma Membrane

• Plasma membrane is as viscous as olive oil
• Proteins move freely for various functions
• Phospholipids rotate and move laterally
• Exhibits self-sealing

The Plasma (Cytoplasmic) Membrane Function

• Selective permeability allowing only certain molecules to pass through
• Contains enzymes for ATP production
• Some bacteria have chromatophores with photosynthetic pigments that allow them to create ATP from sunlight through the infoldings of the plasma membrane
• Disinfectants and some antibiotics damage the plasma membrane
  • Alcohols and quaternary ammonium compounds (detergents)
• Damage can cause leakage of cell contents

Transport

• The movement of materials to cells is done through transport

The Movement of Materials across Membranes

• Passive processes cause substances to move with the concentration gradient, from an area of high concentration to low concentration, without any energy requirement
  • Includes simple diffusion, facilitated diffusion and osmosis
• Active processes cause substances to move against the concentration gradient, from an area of low concentration to high concentration, requiring energy
  • Applies to ions- Na+, K+, amino acids, sugars

Passive Processes: Simple Diffusion

• Solute movement from an area of high concentration to an area of low concentration
• Continues until molecules are evenly distributed and have reached equilibrium
• Oxygen and carbon dioxide move through membranes this way

Passive Processes: Facilitated Diffusion

• Integral membrane proteins serving as channels or carriers, otherwise known as transporters or permeases, across a plasma membrane
• Helps ions or larger molecules move across the membrane
• Substances move with the concentration gradient
• Requires no energy
• Involves Transporter proteins
  • Some are nonspecific
  • Others are more specialized

Passive Processes: Osmosis

• The net movement of water across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration
• Requires no energy
• Through lipid layer with difficulty
• Through aquaporins, the integral membrane proteins functioning as water channels

Active Processes

• Used by a bacterial cell to move substances across the plasma membrane if in an environment where nutrients are low
• Active transport utilizes energy, usually from ATP
  • Involves coupled transportation and group translocation
• Coupled Transportation requires a transporter protein where a transported substance moves against gradient, from lower to higher concentration and is not altered
• Group translocation requires a transporter protein and energy provided by phosphoenolpyruvic acid (PEP) where a substance is altered as it crosses the membrane
  • Involves Active processes that can move a variety of ions as well as amino acids and sugars against the gradient
  • Altered substances remain inside the cell, and the plasma membrane then becomes impermeable to it
  • Exclusive to prokaryotes

Active Transport: Coupled Transportation

• Symport
  • Energy releases as one substrate moves down its concentration gradient
  • Second substrate moves against the gradient and into the cell
• Antiport
  • Transports the same way as symport

Active Transport: Group Translocation

• Substrates enter the cells by facilitated diffusion through specific transporter proteins
• Substrates are modified during the transportation process, so the gradient is maintained for the parent molecules
• Energy comes from phosphoenolpyruvate (PEP), serving as a phosphate donor
• Phosphate attaches to specific sugars during transportation

Cytoplasm

• Prokaryotic cell cytoplasm is the thick, aqueous, elastic, semitransparent substance inside the plasma membrane
• 80% water plus proteins, carbohydrates, lipids, and ions
• Includes DNA in the form of a nucleoid, ribosomes, and inclusions
• Contains a prokaryotic cytoskeleton that participates in cell division, maintaining cell shape, growth, and DNA movement

The Nucleoid

• Bacterial chromosome is typically a circular thread of doublestranded DNA that contains the cell's genetic information
  • The genetic information is not enclosed within a nuclear envelope, or membrane
  • Does not associate with histones
• Plasmids are small extrachromosomal circles of DNA
  • They carry non crucial genes, encode antibiotic resistance, or production of toxins
  • They replicate independently of the chromosome
  • They are often transferred to other bacteria
  • Used in biotechnology for gene manipulation

Ribosomes

• Present in all eukaryotic and prokaryotic cells, where protein synthesis takes place
• Made of protein and ribosomal RNA (rRNA)
• Prokaryotic ribosomes (70S) differ from eukaryotic ribosomes (80S) in the number of proteins and rRNA molecules they contain

Eukaryotic and Prokaryotic Ribosomes

• Prokaryotic ribosome is a 70S total consisting of 50S large subunit and 30S small subunits
• Eukaryotic ribosome is an 80S total
  • 60S large subunit and 40S small subunit
  • Some membrane-bound, attached to endoplasmic reticulum
  • Others are free: in cytoplasm
• Found inside Chloroplasts and mitochondria as well

Ribosomes: Antibiotic Targets

• Antibiotics specifically interfere with prokaryotic ribosomal function, inhibiting protein synthesis
  • Streptomycin and Gentamicin attach to 30S interfering with protein synthesis
  • Erythromycin and Chloramphenicol attach to 50S interfering with protein synthesis

Inclusions

• Contain reserve deposits of certain nutrients
  • Polysaccharide granules store energy reserves
  • Sulfur granules store energy reserves
• Carboxysomes contain enzyme for CO2 fixation during photosynthesis
• Gas vacuoles maintain buoyancy
• Magnetosomes align bacteria along Earth's magnetic field to move downward toward a suitable environment and destroy H2O2

Endospores

• Vegetative growth occurs through binary fission, which increases cell number
• Endospore formation mostly takes place from gram-positive bacteria as a survival mechanism in unfavorable conditions, and is not a reproductive process
  • Caused by nutrient depletion
• Are highly durable dehydrated cells extremely resistant to harsh conditions like desiccation, heat, chemicals, and radiation
• Able to survive in a dormant state for thousands of years, 7500-year-old spores have been germinated
• Produced by members of the genera, Bacillus and Clostridium
• Sporulation is the process of endospore formation
• Germination indicates the endospore returns to vegetative state
• Important to the food industry

Formation of Endospores by Sporulation

• Takes place in a vegetative cell
• Spore septum isolates a Chromosome and cytoplasm behind plasma membrane
• Spore septum double layered, creating a forespore
• Proteins then surround the outside membrane forming spore coat
  • Responsible for the resistance of endospore to harsh chemicals
• Original cell degrades as the endospore is released
• Under favorable conditions an endospore's enzymes break down extra layers, water enters, and metabolism resumes at a 1:1ratio, which is not reproduction

Eukaryotic Cells

• Some structures are unique to plants
• Other structures are unique to animals
• Plant and animal cells share similar properties

Flagella and Cilia

• Projections used for movement or to move substances along the cell surface
• Flagella are the longer projection with fewer amounts
• Cilia are short projection, but more numerous
• Both consist of microtubules of the protein called tubulin
• Microtubules are organized as 9 pairs in a ring, plus 2 microtubules in the enter (9+2 array)
• Flagella moves in a wavelike manner

The Cell Wall and Glycocalyx

• Cell wall found in plants, algae, and fungi
  • Made of cellulose-plants and algae and chitin-fungi
• Glycocalyx consists of carbohydrates bonded to proteins and lipids in the plasma membrane, found in animal cells
  • Strengthens the cell surface and helps attached cells each other

The Nucleus

• Has a double membrane structure (nuclear envelope) encloses the cell's DNA
• DNA is complexed with histone- DNA packaging and gene regulation

Endoplasmic Reticulum

• Folded transport network
• Rough ER: studded with ribosomes; sits of protein synthesis
• Smooth ER: no ribosomes; synthesizes cell membranes, fats, and hormones

Golgi Complex

• Transport organelle
• Modifies proteins from the ER
• Transports modified proteins using vesicles to the plasma membrane

Mitochondria

• Contain inner folds of the inner mitochondrial membrane and matrix
• Is involved in cellular respiration/ATP production
• Have 70S Ribosomes and circular DNA
• Can reproduce on their own

Chloroplasts

• Serves as the location of photosynthesis
• Contain flattened membranes called thylakoids that contain chlorophyll
• Contains 70S Ribosomes and circular DNA
• Can reproduce on their own