Bacterial Cell structure- Plasma Membrane & Cytoplasm

  The Plasma Membrane

•        Most widely accepted model for membrane structure is the fluid mosaic model of S. Jonathan Singer and Garth Nicholson

 

•        Membranes contain lipid bilayers with floating proteins- both proteins and lipids; higher proportion of protein

 

•        Membrane-associated lipids (mostly phospholipids) with polar and nonpolar ends (amphipathic); organized in two layers, or sheets

•        The outer surfaces are hydrophilic, whereas hydrophobic ends are buried in the interior away from the surrounding water

 

•        Bacterial membranes lack sterols such as cholesterol ; contain pentacyclic sterol-like molecules called hopanoids ; synthesized from the same precursors as steroids; probably stabilize the bacterial membrane.

 

•        Two types of membrane proteins.

•        Peripheral proteins are loosely connected to the membrane, easily removable, soluble in aqueous solutions -about 20 to 30% of total membrane protein.

•        Integral proteins- About 70 to 80% of membrane proteins; not easily extracted from membranes and are insoluble in aqueous solutions when freed of lipids.

•        The plasma membrane retains the cytoplasm, and separates it from the surroundings

•        Selectively permeable barrier: it allows particular ions and molecules to pass, either into or out of the cell, while preventing the movement of others.

•        Prevents the loss of essential components through leakage

•        Many substances cannot cross the plasma membrane without assistance; Transport systems for nutrient uptake, waste excretion, and protein secretion

•        Location of a variety of crucial metabolic processes: respiration, photosynthesis, the synthesis of lipids and cell wall constituents, and chromosome segregation

•         Membrane contains special receptor molecules that help procaryotes detect and respond to chemicals in their surroundings

 

Cytoplasm

•        The plasma membrane and everything within is called the protoplast; thus the cytoplasmic matrix is a major part of the protoplast

•        Procaryotes have few well defined internal structures; so main component is semifluid cytoplasm

•        4/5th water and 1/5th substances dissolved/suspended in water (Enzymes and other proteins, carbohydrates, lipids and a variety of inorganic ions)

•        Site of Anabolic and catabolic chemical reactions

Ribosomes

•         Site of protein synthesis

•        Made of both protein and ribonucleic acid (RNA)

•        Large and small subunits

•        Abundant in cytoplasm; as long chains (Polyribosomes)

•        Size expressed in Swedberg Units (Sedimentation rates –rate at which they sediment when centrifuged); vary with molecular weight

•        Bacterial ribosomes – 70S (30S + 50S)

•        Streptomycin & Erythromycin bind to 70S ribosomes- inhibit protein synthesis

Nucleoid

•        No  well-defined nucleus & nuclear membrane

•        Central nuclear region/nucleoid- mainly DNA, some RNA and protein

•        DNA arranged in one or two large circular chromosomes (Rhodobacter)/one circular and one linear chromosome (Agrobacterium)/One large circular and one small circular (Vibrio)

•        Some bacteria have plasmids -extrachromosomal, genetic elements

No mitochondria/chloroplasts

 

Internal Membrane Systems

•        Photosynthetic bacteria & Cyanobacteria – internal membrane systems called Chromatophores; Membranes derived from cell membrane- house pigments to capture light for synthesis of sugars

•        Nitrifying bacteria- internal membranes to house enzymes for oxidation of nitrogen compounds

•        Mesosomes -invaginations of the plasma membrane in the shape of vesicles, tubules, or lamellae- involved in cell wall formation during division or play a role in chromosome replication and distribution to daughter cells (Artefacts!?)