文档介绍：Nutrient Transport across Membranes
Except for H2O, most polar molecules
do NOT move across the lipid bilayers
Membranes as barriers
Relative speed of nutrient movement across bacterial membranes
substance
permeability
water
glycerol
Tryptophan (色氨酸)
glucose
Cl-
K+
Na+
100
10-6
10-7
10-8
Note that many nutrients are polar
Membrane transport systems
are the systems to move nutrients and
waste products across membranes
Uniporter transport
Passive
Active
Antiport
Passive diffusion
Facilitated diffusion
Channel proteins
Symport
ABC system
group translocation
Molecules move along a concentration gradient (from region of higher concentration to lower concentration)
Movement is driven by random thermal action - no energy output anism
Concentration reaches equilibrium
Water, gasses, lipids, small uncharged polar molecules
Not primary mode for hydrophilic ions
What is diffusion?
Examples of gases that cross membranes by passive diffusion include N2, O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea.
Passive 1 Passive diffusion
Channel proteins蛋白通道 transport water or certain ions down either a concentration gradient, in the case of water, or an electric potential gradient, in the case of certain ions from an area of higher concentration to lower concentration.
While water molecules can directly cross the membrane by simple diffusion, as mentioned above, their transport can be enhanced by channel proteins called aquaporins(运水蛋白).
Passive 2. Channel proteins
Diffusion aided by a carrier protein - permease - in cell membrane
Carrier provides specificity and increases rate to equilibrium
is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy.
Not highly important in prokaryotes
What is facilitated diffusion?
Passive 3. facilitated diffusion
Passive 3 Facilitated diffusion
Diffusion facilitator
protein
Nutrient molec in high conc
Specific b