CHEM 2301 Lecture 5: Ch12-Membrane Transport-F14
Document Summary
Cells must exchange molecules with their environment to live and grow. The hydrophobic lipid bilayer blocks the passage of most water soluble molecules. A few solutes (c02, o2) can diffuse across the bilayer, but others require membrane transport proteins (figures 12- 1, 12- 2). There are two general classes of membrane transport proteins, transporters and channels (figure 12- 3). Tranporters move small molecules by changing their shape. Channels form small hydrophilic pores that allow solutes to pass by diffusion down a concentration or electrochemical gradient. Cells maintain an internal ion composition very different from the ion composition of the fluid around them (table 12- 1) K+ and h+ are higher inside the cell. To maintain electrical neutrality, the cell has many other negatively charged molecules (carbonate, phosphate, nucleic acids, etc. ) Membrane transport proteins are multipass transmembrane proteins (figure 11- 23). Movement of a molecule down its concentration gradient is passive transport (figure 12- 4).
