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.

5. Transport through membranes
Simple diffusion
Molecules travel down concentration gradient
Membrane is not a barrier to their passage
Facilitated diffusion
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

6. How molecules get through the membrane

7. 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

8. 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.

9. 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

10. 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

11. 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.

12. Antiport and Symport
prostruct/u1fig6e1.html
Molecules (red balls) transported against a gradient. Coupling to flow of H+ into the cell powers this.