Prokaryote cell structure

Bacterial Structure and Function shapes and arrangements

 • Bacilli (s., bacillus)—rods • The length-to-width ratio differ. Vibrios—comma shaped Spirilla—rigid spiral-shaped Spirochetes—flexible spiral-shaped Mycelium (Fungi) —network of long filaments (hyphae). Pleomorphic—organisms that are variable in shape.

Cell Membrane: A living boundary between the cell and the environment. Function: Controlling what enters and leaves the cell. Very Dynamic – Remember the Fluid Mosaic Model. Made up of phospholipids, and integrated proteins and carbohydrates.

Cell Membrane In bacteria the cell membrane is more involved than eukaryotic CM. In bacteria the CM synthesizes cell wall components, helps in DNA replication, secretes proteins, carries out respiration, captures energy (ATP). Cell transport.

Cell Wall: Structural boundary outside the CM, gives cell its shape and prevents the cell from bursting under osmotic pressure. Made up of peptidoglycan (Murein), components are referred to as NAG and NAM, has peptide cross-links. Use the cell wall to classify bacteria into Gram-positive and Gram- negative groups.

Bacterial Stains Simple Stain – Only one stain • Does not specifically stain an organelle • Used to determine cell size, shape and arrangement • Examples – iodine, methylene blue Differential Stain –more than one stain • More complex – gives specific information about the cell • Used to compare cell structures and different cells • Examples – gram stain

Gram Stain

1. Primary stain: Crystal violet    • Stains both gram-positive & gram-negative bacteria
2. Mordant: Gram's iodine    • Enters bacterial cell & forms iodine-crystal violet complexes
3. Decolorizer: Acetone or Alcohol    • Gram-positive bacteria; iodine-crystal violet dye complex is trapped in cells → gram-positive    cells remain purple    • Disrupts thin cell wall of gram-negative bacteria; iodine-crystal violet dye complex escapes cells    → gram-negative cells become clear

Counterstain: Safranin • Stains cell wall of cells • Purple of gram-positive cells masks pink color & remain purple • Unstained gram-negative cells become pink- Gram-positive bacteria

Gram Positive bacteria have a thick layer of peptidoglycan in their cell walls. Gram Negative bacteria have a small amount of peptidoglycan, but have an outer membrane, much more complex cell wall. Mycobacteria or Acid Fast bacteria have barely any peptidoglycan but have a thick wall made up of lipids.

Components Outside the Cell Wall Outermost layer in the cell envelope. Capsules—well-organized layers made of polysaccharides. Slime layers—polysaccharide layers that are unorganized Glycocalyx—polysaccharide extension that aids in attachment to solid surfaces. S-layers—geometric pattern made of protein that aid in protecting from ion and pH fluctuations.

Glycocalyx: a layer outside the cell wall mostly made up of polysaccharides. Can be as thin as a layer of slime, or very thick and called a Capsule. Capsule: a thick layer secreted by the cell over the cell wall that serves a protective (desiccation, chemicals) as well as an attachment function.

Bacterial cytoplasm: Makes up the largest part of the inside of the cell. Made up of 4/5 water, and 1/5 dissolved substances including enzymes, proteins, carbohydrate, lipids and inorganic ions. Many metabolic reactions will take place in the cytoplasm.

Cytoskeleton – made up of actin filaments, microtubules, and intermediate filaments Functions are similar as in eukaryotes • Participate in cell division. • Localize proteins. • Maintain cell shape.

Intracytoplasmic Membranes – plasma membrane invaginations Plasma membrane infoldings. • Observed in many photosynthetic bacteria. • Observed in many bacteria with high respiratory activity. • May be aggregates of spherical vesicles.

Ribosomes: non-membrane bound organelles, the site of protein synthesis. They are found throughout the cytoplasm of bacteria. • smaller than eukaryotic ribosomes but perform the same function. • Referred to as 70s ribosomes, based on sedimentation rates – Svedberg units (s).

Nucleoid: The area in the cytoplasm where the mostly DNA, some RNA and some proteins are located. • NOT membrane-bound. • circular • usually only 1 strand, but there may be exceptions.

Plasmids: Small extrachromosomal DNA molecules. • Carry genes that are not essential for survival - nonessential cellular functions. • can be exchanged between cells during a process called conjugation which allows for genetic transfer. • may give a selective advantage in some situations – antibiotic resitance.

Flagella: Long, thin helical appendage used for mobility. • About half of known bacteria are motile. Arrangement of flagella can be used to classify bacteria Different structurally than eukaryotic flagella, much simpler.

Ultrastructure composed of 3 parts: • Filament—extends from cell surface to the tip. • Basal body—embedded in cell envelope. • Hook—short curved segment.

Bacteria move in response to environmental conditions Flagellar movement • Swimming • Swarming • Spirochete motility Twitching and gliding motility Chemotaxis

Types of Bacterial Movement: Swarming • Occurs on when cells move in unison across a moist surfaces. • Most swarmers have peritrichous flagella. • Commonly, the cell produces a molecule that lowers surface tension.

Spirochete Motility • Multiple flagella form axial fibril which winds around the cell. • Flagella remain in inside the cell wall. • Corkscrew shape exhibits flexing and spinning movements.

Twitching & Gliding Motility Occurs on solid surface. Does not involve flagella. May involve Type IV pili and slime. Twitching motility • Pili at ends of cell. • Short, intermittent, jerky motions. • Cells are in contact with each other and surface. Gliding • Smooth movements that do not require appendages.

Chemotaxis Movement toward a chemical attractant or away from a chemical repellent. Chemoreceptors that transmit signals throughout the chemosensing system to signal movement. Behavior of bacterium altered by changing concentration of chemical

Pili (Pilus): Tiny hollow projections that help the bacteria adhere (Stick) to surfaces • NOT involved in movement. • found on Gram -ve bacteria only Sex pili: Long, modified for transfer of genes between cells in a process called conjugation. Fimbriae: Short attachment pili. (Pathogenicity). Gram + & - bacteria

Endospores: A resting stage • a way of survival when conditions are not favorable. • NOT the same as fungal spores. Allows bacteria to survive very harsh conditions, high temps, radiation, dehydration • very difficult to kill spores. • can survive for thousands of years.

Spore Formation Organized process that occurs over several hours. Normally starts when growth slows du to lack of nutrients. • Produces a dormant cell that can persist until nutrients are available and growth resumes.

Endospore Formation Spore surrounded by thin covering called exosporium. Thick layers of protein form the spore coat. Cortex, beneath the coat, thick peptidoglycan. Core has nucleoid and ribosomes.