CH.3.2

in bacteria

phospholipids are built from generally linear fatty acids. while archaea use long branched lipids called isoprenoids instead of fatty acids

polar molecules

tend to interact with water while nonpolar molecules avoid water.

warmer temperatures increase membrane fluidity while cooler temperatures reduce it

a higher proportion of unsaturated fats in the fatty acid portion of phospholipids improves membrane fluidity in cold temperatures by preventing tight packing of the fatty acids in the bilayer. in contrast saturated fatty acids are more rigid at low temperatures because they tightly pack within the bilayer.

the gram stain technique

allows us to classify cells based on cell wall structure.

all structures outside of the plasma membrane

are considered extracellular structures while structures that lie withing the boundary defined by the plasma membrane are intracellular structures.

glycolipids

are fats that have added sugar groups. (glyco means sugar; lipid means fat)

bacteria use peptidoglycan (or murein)

as a core component of their cell walls. this meshlike molecule includes protein and sugar units. drugs from the penicillin family work by interfering with peptidoglycan construction. in general archaea tend to exhibit a lot more diversity in their cell walls than bacteria do. archaea lack peptidoglycan instead their cell walls tend to consist of one or more layers of pseudopeptidoglycan (or pseudomurein)

certain archaea that live in extreme heat build lipid monolayers

as opposed to lipid bilayers. these monolayer membranes contain unique lipids(called tetrather lipids) that are basically long lipid molecules capped on either end with a polar head group. these unique membrane adaptations help archaea thrive in harsh environments.

aside from certain Mycoplasma bacteria

cholesterol is rarely seen in prokaryotic cells while it is stranded in eukaryotic cell membranes.

the cell wall

confers a rigid structure to prokaryotes and also as an extra layer of protection from the environment. prokaryotic cells live in diverse environments so it makes sense that they would develop diverse cell walls that would help them to live under specialized condtions such as allowing them to more easily adhere to a host cell or to resist a host's immune defense.

the chemical linkages in bacterial plasma membranes

differs from those in archaea. this is actually one of the ways we distinguish bacteria from archaea.

in bacterial membranes

fatty acids are joined by an ester bond while archaea use an ether bond. archaeal membranes can be monolayer or bilayer formats; bacterial membranes are strictly bilayers.

because the plasma membrane is selectively permeable

gases, water, and other small noncharged substances can diffuse in and out of the cell without assistance. however ions and larger polar substances like sugars often require specific protein transporters found in the membrane to enter or exit a cell. many of these membrane proteins also detect changes in the environment and help coordinate cellular responses to these changes.

gram negative bacteria

have a cell wall with a thin peptidoglycan layer surrounded by an outer membrane.

gram negative bacteria

have an outer membrane along with a thin peptidoglycan layer in their cell walls.

microbes that thrive in extremely cold climates

have been shown to incorporate a higher proportion of short unsaturated fatty acids into their phospolipids to preserve membrane fluidity. similarly organisms in high temperature environments lean toward longer and more saturated fatty acids in their phospholipids to keep membranes from becoming too fluid. integrating steriod based lipids like cholesterol in membranes can also improve fluidity.

unsaturated fatty acids

have double bonds in their hydrocarbon backbone, which limits how tightly they pack together, thereby keeping them fluid at colder temperatures.

the plasma membrane

is a thin flexible phospholipid bilayer that serves as a selective barrier separating the cell's cytoplasm from the external environment. it is the cell's main platform for interacting with the environment. the plasma membrane is also a site for metabolic reactions that prokaryotes rely on to make ATP.

peptidoglycan

is built by alternating N-actylglucosamine and N-acetylmuramic acid residues. the long strands of these alternating residues are then cross linked by short peptides to create a meshlike structure much like a chain link fence.

the lipid bilayer

is not a static structure, but is a fluid-mosaic model where the membrane lipids and proteins mover around within the bilayer. this fluidity enables proteins to relocate to areas of the membrane where they are most needed. maintaining a certain level of membrane fluidity is essential to the cell and can impact its physiological functions.

the outer membrane

is structurally similar to the plasma membrane in that it is a lipid bilayer rich in phospholipids.

the plasma membrane is built like a sandwhich

it includes a layer of bread on both the top and bottom that consists of water loving hydrophilic phosphates that directly touch the water based cytoplasm within the cell and the watery environment beyond it. the filling part of the membrane sandwich lies between the bread layers. it contains hydrophilic fatty acids that dislike interacting with water and are therefore chemically content to be sheltered within this water free environment.

saturated fatty acids

lack double bonds in their hydrocarbon backbone.

gram positive bacteria

lack outer membranes and have a thick peptidoglycan layer in their cell walls. have a cell wall with a thick peptidoglycan layer and they lack an outer membrane. the gram positive cell wall stains purple in the gram stain procedure. note that gram positive cells with damaged cell walls may appear to be gram negative. in a given sample of cells its very likely that some may have cell wall damage. thus its typical that a culture of gram positive cells will contain some cells that appear gram negative.

unlike the plasma membrane the outer membrane is enriched with a glycolipid called

lipoplysaccharide (LPS) contains a lipid portion (lipid A or endotoxin) that is poisonous to animals

between the peptidoglycan layer and the outer membrane is a gap called the

periplasmic space. this space is filled with a gel like fluid that is enriched with various factors that have important roles in helping the bacterium obtain nutrients and nueutralize toxic substances.

all cells whether they are prokaryotic or eukaryotic are surrounded by an outer boundary called

plasma membrane (also called cytoplasmic membrane or cell membrane)

most prokaryotic cells also have a cell wall

that lies just outside of the plasma membrane. while most cells are able to repair minor damage to their plasma membrane and cell wall drastic damage can be fatal. a number of antimicrobal drugs, detergents, and disenfectants target these cellular structures to limit bacterial growth.

membrane proteins

serve as transporters, anchors, receptors, and enzymes. the types of proteins found in plasma membranes are tremendously diverse and confer specific capabilities to a cell. in many cases these membrane proteins are linked to sugar groups (glycoproteins) or lipid groups (lipoproteins). these added groups contribute important structural and functional properties to the membrane.

the types of lipids present in the plasma membrane differ considerably by

species, and many prokaryotes can alter their membrane lipid profiles in response to certain stresses or environmental changes. additionally in many cases proteins constitute at least half of the plasma membrane mass.

Gram negative cell walls

stain red/pink in the Gram staining procedure.

phospholipids

the main components of plasma membranes are built from glycerol, phosphate, and 2 fatty acids. the phosphate containing region of a phospholipid is polar while fatty acids are nonpolar.