Biology of Membranes and Transport Processes

Introduction

• Most bacteria share similar fundamental traits
• Thick, complex outer region
• Compact genome, efficient use of space
• Tightly coordinated cell functions
• Archaea are prokaryotes, but not bacteria
• Unique membrane and envelope structures
• Eukaryotes have a nucleus and extensive membranous organelles

The Bacterial Cell: An Overview

• Early 20th century view of cells as a "soup" of floating ribosomes and enzymes
• Modern research shows that cell parts fit together in an ordered, but flexible, structure

Structural Differences between Prokaryotes and Eukaryotes

• Prokaryotes lack defined organelles
• Prokaryotic DNA is not membrane-bound (nucleoid)
• Prokaryotes are significantly smaller than eukaryotes, comparable in size to eukaryotic mitochondria

Cell Structures

• Prokaryotic cells have smaller structure than eukaryotic cells
• Prokaryotic key features include cytoplasm, nucleoid, ribosomes, and cell wall
• Eukaryotic key structures are cytoplasm, membrane-bound organelles (eg. nucleus, mitochondria, chloroplasts)

Bacterial Cells are Smaller than Eukaryotes

• Microscopy and X-ray crystallography provide different levels of resolution for viewing cells
• The range of resolution from human eye to atomic force microscopy

Surface Area vs Volume

• Larger surface area to volume ratios allow for faster growth rates
• This principle is fundamental to biological systems
• Rapid transport rates, nutrient import, and waste removal are facilitated
• Cell membrane location and larger area optimizes active metabolism

Bacterial Cell Shapes

• Cocci (spherical)
• Bacilli (rod)
• Vibrios (bent rod)
• Spirochetes (helical)
• Irregular (various)

Bacterial Cellular Morphology

• Bacterial cells can exist in pairs, clusters, or chains. 
• Cocci can exist as diplococci, staphylococci, streptococci, and sarcina.
• Bacilli can exist as diplobacilli, streptobacilli, and coccobacilli in specific, varying shapes

The Bacterial Cell

• Cytoplasm is surrounded by the cell envelope
• DNA resides in the non-membrane-bound nucleoid
• Cell membrane encloses the cytoplasm

The Cell Envelope

• The cell envelope includes cell membrane, cell wall, and outer membrane (if present)

Bacterial Cytoskeleton

• Shape-determining proteins
• FtsZ forms a Z-ring in spherical cells
• MreB forms a coil in rod-shaped cells
• CreS (crescentin) forms a polymer along the inner side of crescent-shaped bacteria

The Bacterial Nucleoid

• Single loop of double-stranded DNA
• Compact, supercoiled DNA (~4M base pairs)
• Non-membrane-bound structure, not separated from the cytoplasm
• Replicates once per cell division
• Forms loops (domains) of DNA compacted by DNA-binding proteins

What's happening in the Nucleoid?

• Nucleoid is not separate from the rest of the cell
• RNA polymerase transcribes DNA to mRNA within the nucleoid
• Ribosomes translate mRNA to protein within the nucleoid

Cell Division and Replication

• Cell elongation precedes cell division
• Cell wall is formed at the cell equator
• DNA replicates bidirectionally
• DNA replication can begin before the cell divides into two cells

Cell Membrane

• Double layer (bilayer) of phospholipids separating the cytoplasm from the environment
• Fluid mosaic model with integral proteins embedded in the membrane
• Important roles in transport, energy generation, and environmental sensing

Phospholipid Bilayer

• General structure of biological membranes
• Composed of fatty acid (hydrophobic) and glycerol-phosphate (hydrophilic) components
• Hopanoids strengthen the membrane in certain bacteria, Cholesterol strengthen membrane in eukaryotes
• Possibly the most abundant organic compounds on earth

Transport Across the Cell Membrane

• Transporters move materials into and out of cell
• Passive transport follows concentration gradients.
• Active transport uses energy.
• Pumps use energy to move materials against gradients (ATP or PMF)

Transport Across the Cell Membrane (Proton Pumps)

• Proton pumps push protons out of the cell
• These pumps generate the proton motive force (PMF). 
• PMF pushes protons against their concentration gradient
• Protons move back through ATP synthase, producing ATP

Bacterial Cell Wall (Sacculus)

• Rigid structure outside the cytoplasm membrane supporting cell shape and structure
• Allows bacteria to withstand turgor pressure from dissolved substances.
• Gram positive wall is thicker compared to Gram negative one

Bacterial Cell Wall (Sacculus)

• Peptidoglycan, a unique bacterial polymer forms the sacculus. 
• Sugar chains form circles around the cell
• Sugar chains are interlinked by short peptides

Bacteria Having Two Different Ways To Crosslink Peptidoglycan Units

• Gram negative bacteria have direct crosslinking
• Gram positive bacteria have peptide interbridge

Overall Structure of Gram Positive Peptidoglycan Layers

• layers composed of N-Acetylmuramic acid and N-Acetylglucosamine, connected by pentaglycine interbridge

Gram-Positive Cell Wall

• Thick peptidoglycan layer
• Teichoic acids reinforce the peptidoglycan layer
• Lipoteichoic acid links the cell wall to the cell membrane
• Absent in Gram-negative bacteria

The Gram-Positive Envelope

• Capsule (not in all species)—polysaccharide coat
• S Layer—protein coat
• Thick cell wall—peptidoglycan with amino acid crosslinks
• Teichoic acids for strength, located in peptidoglycan layers
• Plasma membrane

Gram-Negative Cell Wall

• Thin peptidoglycan layer
• Surrounded by an outer membrane

Gram-Negative Envelope

• Capsule (not in all species)—polysaccharide coat
• Outer membrane
• Thin cell wall
• 4-amino acid crosslinks in peptidoglycan
• Thick periplasm
• Plasma membrane

Gram-Negative Outer Membrane

• Outer membrane—protection and permeability barrier
• Composed of a phospholipid monolayer and lipopolysaccharide (LPS)
• Integral proteins in outer membrane facilitating transport (porins and murein lipoprotein)

Lipopolysaccharide (LPS)

• Conserved sequence of major components: lipid A - core polysaccharide- O-specific polysaccharide
• Core polysaccharide ( about 5 sugars)
• O-specific polysaccharide (can include up to 200 sugars), highly variable between species

Other Outer Coverings

• S-layers—protein or glycoprotein layers providing protection
• Glycocalyx—network of polysaccharides extending from the cell surface, important for survival and attachment; Capsule a well-organized glycocalyx that is difficult to remove
• These covers protect bacteria

Gram Stain

• Bacterial cell wall composition is categorized by the Gram stain technique. 
• Developed by Hans Christian Gram
• Stains cells differently according to differences in cell wall composition
• Gram-positive cells stain purple
• Gram-negative cells stain pink

The Gram Stain Procedure

• Cells fixed to a slide.
• Crystal violet and iodine added
• Alcohol wash removes stain from Gram-negative cells
• Safranin counterstain colors Gram-negative cells pink.

The Gram Stain Procedure

• Cells fixed to a slide.
• Crystal violet and iodine added
• Alcohol wash removes stain from Gram-negative cells
• Safranin counterstain colors Gram-negative cells pink.

The Gram Stain Procedure

• Cells are fixed to a slide. 
• Crystal violet and iodine added, which stains peptidoglycan purple.
• Alcohol wash removes stain from Gram-negative cells.
• Safranin counterstain colors Gram-negative cells pink. Cells which had retained initial stain appear purple.

Cell Division

• Binary fission used to divide bacterial cells
• Cells elongate and adds new cell wall at cell equator, DNA replicates before the cell divides into two cells
• DNA replicated bidirectionally, starting from the origin of replication.
• The septum, formed during binary fission, divides the cell into two daughter cells each having the same structure.

Cell-Surface Structures: Pili and Fimbriae vs Flagella

• Rigid structure outside cytoplasm membrane to withstanding pressure from dissolved solutes
• Two types (Gram Positive and Gram negative): Gram negative (thinner), and Gram positive walls (thicker).

Cell-Surface Structures: Pili and Fimbriae vs Flagella

• Appendages extending from the cell surface. 
• Flagella—long protein filaments facilitating motility, swimming, and swarming
• Pili and fimbriae—shorter structures for attachment and motility. 

Specialized Cell Attachment Structures

• Fimbriae and Pili are thin, hair-like protein filaments (pili longer and more prominent, fimbriae numerous)
• Twitching—individual movement on surfaces
• Secretion systems—attaching cells to prey
• Conjugation (mating)—transferring DNA between cells via sex pili.

Bacterial Motors (Flagella)

• Flagella are long protein filaments vital for motility (swimming and swarming).
• Individual movements occur in liquid, and multicellular movements occur on surfaces

Flagella Structure

• Whip-like appendage on bacterial cell.
• A motor for motility involved.
• Flagellin monomers forms the whip structure.
• Flagellin protein monomers make up the structure.
• The hook connects flagellin filaments to ring structures.
• Rings anchor the flagella. 
• Flagella rotate to propel cells

Flagella Types

• Monotrichous (Polar): single flagellum at one end
• Lophotrichous: multiple flagella at one or both ends
• Peritrichous: flagella distributed all over the cell surface

Movement of Polar Flagella

• Polar flagella move reversibly or unidirectionally
• Cell stops, reorients (tumble), then restarts rotation and movement. 

Movement of Peritrichous Flagella

• Peritrichous flagella bundle for smooth swimming,
• Disaggregation for tumbling and reorientation

Directing Movement (Chemotaxis)

• Chemotaxis—movement in response to chemical gradients
• Attractants—chemicals that cause movement towards the source (e.g., nutrients)
• Repellents—chemicals that cause movement away from the source (e.g., toxic waste)
• Runs and tumbles, a random walk, allow movement towards attractants.

Directing Movement (Chemotaxis)

• Attractant concentration increases, and the run is prolonged.
• Biased random walk.
• Net movement towards attractants.

This quiz covers essential concepts related to biological membranes and transport mechanisms in cells. Topics include hopanoids, the proton pump, and characteristics of passive transport. Test your knowledge on bacterial structures and their functionalities, including flagellar arrangements and chemotaxis.