Download Cell walls of eubacteria

Microm 410 Fall 2008: Cell membrane and Transport
Cell Membrane
Fluid mosaic model of Singer and Nickelson (1972)
Chemistry of Phospholipid
Fatty acids
isoprene
Establishes a compartment different from environment.
Mediates the exchange of matter and information between
the cell and its environment.
See Fig. 4.5
Cell walls of eubacteria
Fig. 4.27
hyperthermophiles
Porins exclude molecules larger than 500 Da.
Amino acids about 115 Da; Glucose, 180 Da
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Microm 410 Fall 2008: Cell membrane and Transport
Membrane Transport
Semi-premeable membrane
Selective permeability achieved by:
• Use of substrate-specific carrier proteins
(permeases, transporters, carriers)
• use of dedicated nutrient-binding proteins in
periplasmic space
• use of membrane-spanning protein channels,
pores, that discriminate between substrates
Concentration dependence of
diffusion vs transport rates
via permeases/transport
proteins
Carrier-mediated transport shows saturation at
low solute concentrations
Fig. 4.10
Simple and Facilitated Diffusion
• no energy required
• movement of solute depends on have a
gradient across membrane (solutes always
moves down gradient)
• can never have more solute on one side of
membrane than exist on other side
• solute not modified as crosses membrane
Few molecule can cross membrane by simple
diffusion (small, non-polar or fat-soluble
molecules, weak acid/bases, O2, CO2, NH3,
aquaporins
and H2O)
Weak acids: HA <-----> H+ + AWeak base: B + H2O <-----> BH+ + OH-
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Microm 410 Fall 2008: Cell membrane and Transport
Facilitated Diffusion
Glucose transport in animal cells uses Glut-1protein
Energy-consuming transport
systems in bacteria
• Require a
membrane
protein(s)
• Three classes of
transporters
consume energy
in different ways
• Two can
concentrate a
transported solute
in cytoplasm
(active transport)
• One results in the
modification of the
solute during
transport
Fig. 4.11
Overview of PMF-dependent transporters
Mediate movement of ions, sugars, etc. (1 or 2 molecules/transporter)
Active transport via PMF
• Transport requires
PMF to move solute
• Solutes are not
modified during
transport
• Common transport
mechanism in both
bacteria and
eukaryotes
Fig. 4.23
Fig. 4.13
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Microm 410 Fall 2008: Cell membrane and Transport
ABC Transporters
Conformational
change in
membrane
protein is
energized by
ATP hydrolysis.
Examples: Mal
or His permease
Fig. 4.15
Group translocation
Transport mechanisms common to
Bacteria (especially in facultative
anaerobes), but absent in Eukarya and
Archaea.
Group Translocation:
PhosphoTransferase System (PTS)
Commonly used for transport of sugars
Sugar is modified during transport by
addition of Pi group.
Energy is consumed in the form of PEP,
which is the source of the Pi group.
No concentration gradient is exploited
because of the solute modification.
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Microm 410 Fall 2008: Cell membrane and Transport
PTS transport of sugars
Fig. 4.25
Pi transfer: Enzyme I (= Enz I or EI) cleaves Pi from PEP and
attaches Pi to itself. Pi transfer proceeds from subunit to subunit,
until the Pi attached to the sugar as it comes through the IIC complex.
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