2nd
Law of Thermodynamics
•All things tend toward entropy
(randomness).
•Molecules move (diffuse) from an
area of high concentration to areas of
low concentration.
•This is a driving force, like gravity. It
happens spontaneously. To go against
it, for example, to gather molecules
together where there are already
many, takes the expenditure of energy.
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Type of molecule affects transport
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• The major function of a cell membrane is to prevent
substances from entering or escaping the cell.
• Small molecules can pass through a lipid bilayer
– Water; otherwise, no osmosis
– Gases such as O2 and CO2
• Lipid molecules can
– Dissolve in lipid bilayer, pass through membrane
– Many antibiotics, drugs are lipid soluble
• Larger, hydrophilic molecules cannot
– Ions, sugars, amino acids cannot pass through lipids
– Needed to provide raw materials & energy for cell.
How things get in (and out) of cells
• Eukaryotic cells
– Have transport proteins in membrane
– Have a cytoskeleton made of microtubules
• Allows for receptor mediated endocytosis,
phagotcytosis, etc.
• Cell membrane pinches in, creates vesicle
• Prokaryotic cells
– Have a stiff cell wall
– Can NOT carry out endocytosis
– Entry of materials into cell by diffusion or transport
processes ONLY.
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Illustrations: entry into cells
Only eukaryotes.
Both prokaryotes and
eukaryotes.
http://bio.winona.msus.edu/bates/genbio/images/endocytosis.gif
http://www.gla.ac.uk/~jmb17n/Teaching/JHteaching/Endocytosis/figures/howdo.jpg
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Transport through membranes
• Simple diffusion
– Molecules travel down concentration gradient
– Membrane is not a barrier to their passage
• Facilitated diffusion
– Molecules travel down concentration gradient
– Cannot pass through lipid bilayer; their passage is
facilitated by protein transporters
• Active transport
– Molecules travel against concentration gradient
– Requires input of metabolic energy (ATP), transporter
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How molecules get through the membrane
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Membranes/bauerp/diff.gif
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ABC transport systems
• Include a periplasmic binding
protein, a transmembrane channel,
and an ATP-hydrolyzing enzyme.
• High affinity binding system.
• Family of related proteins.
– Eubacteria, Archaea, Eukaryotes
• Example of Active Transport
– Requires transport protein
– Requires metabolic energy
http://www.ugr.es/~eianez/Microbiologia/images/06memb3.jpg
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Group Translocation
• “Straddles” facilitated diffusion and active transport.
• Used by bacteria to transport various sugars. As molecule
passes through the membrane, it is chemically changed.
• Requires energy in the form of PEP.
• Requires series of proteins
• Energy brings sugar in
AND activates it for
metabolism.
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Permeases
• Transport proteins are often called permeases (-ase
= enzyme) because they have the same properties
–
–
–
–
Instead of changing a chemical, they change its location
Permeases have an “active site”
Permeases are specific
Permeases are saturable
http://cwx.prenhall.com/horton/media
lib/media_portfolio/text_images/FG0
9_32.JPG
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ATP is not always used directly in
active transport
• An electrochemical gradient exists across the cell
membrane (membrane potential)
– Positive just outside the membrane, negative within
– Gradient in the form of H+ ions
Maintained by the hydrolysis of
ATP or by the same metabolic
reactions that make ATP
Powers uniports, symports and
antiports
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Uniport
• Transport of a single substance
• Example: transport of K+ into the cell
– Against its chemical gradient, but down its electrical
gradient.
– (red ball = K+)
– Doesn’t require energy
DIRECTLY, but making the
electrical gradient DOES
require energy.
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Antiport and Symport
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www.cat.cc.md.us/.../ prostruct/u1fig6e1.html
Molecules (red balls) transported against a gradient.
Coupling to flow of H+ into the cell powers this.