2nd Law of Thermodynamics

1
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.

2
Type of molecule affects transport

• 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.

3
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.

4
Illustrations entry into cells
Both prokaryotes and eukaryotes.
Only eukaryotes.
http//bio.winona.msus.edu/bates/genbio/images/end
ocytosis.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/M
embranes/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/06m
emb3.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
  formetabolism.

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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/medialib/media_port
folio/text_images/FG09_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)
• Doesnt require energy DIRECTLY, but making
  theelectrical gradient DOESrequire energy.

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Antiport and Symport
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.