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!?)
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