1 2 nd 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 O 2 and CO 2 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 –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

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

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. 8

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 lib/media_portfolio/text_images/FG0 9_32.JPG

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