Cell Transport Mechanisms: Diffusion, Osmosis, and Active Transport

CHAPTER 5 The Working Cell 5.3-passive transport is diffusion across a membrane with no energy cost I. Diffusion A. Intro 1. In addition to chemical signals, cells transport H2O, food, and ions across membrane a. Ion charged particle B. Diffusion 1. The mixing of 2 substances by the random motion of molecules 2. Molecules will always diffuse from and area of high [ ] to an area of low [ ] 5.4-Osmosis is diffusion of water across a membrane C. Osmosis: Diffusion of H2O 1. The diffusion of H2O across a selectively permeable membrane 2. Again from an area of high [ ] to an area of low [ ] 3. Requires no energy by the cell a. Water moves freely across membrane

5.5-Water balance between cells and their surroundings is crucial to organisms I. Controlling osmosis A. [ ] gradient 1. The difference in the [ ] of particles that occurs across a selectively permeable membrane 2. Remember that water moves from high [ ] to low [ ] 3. Moving with or down the [ ] gradient a. Particles moving from high [ ] to low [ ] 4. Moving against or up the [ ] gradient a. Particles moving from low [ ] to high [ ] B. Isotonic solution 1. [ ] of particles is equal on both sides of the membrane 2. Cells in an isotonic solution do not experience osmosis

C. Hypotonic solution 1. [ ] of particles outside the cell is lower than inside the cell a. More water outside the cell than inside 2. Water moves inside the cell causing the cell to swell a. If solution is to hypotonic cell will burst

D. Hypertonic solution 1. [ ] of particles outside the cell is higher than inside the cell a. More water inside the cell than outside 2. Water leaves the cell causing it to shrink E. Osmotic pressure 1. The pressure that results from the force of water pushing against the plasma membrane 2. Also called turgor pressure

5.6-Transport proteins may facilitate diffusion across membranes I. Selective transport A. Review 1. Not all substances can pass freely across plasma membrane like water B. Intro 1. Protein channels and pumps a. Channels 1. Passageways across plasma membrane 2. Allows only certain molecules to cross membrane depending on shape of molecule b. Pumps 1. Passageways across plasma membrane that requires a certain amount of energy 2. Used to pump a molecules against or up the [ ] gradient

C. Facilitated diffusion 1. “free of charge” a. Costs the cell no energy to perform 2. If molecule can fit through the protein channel it may enter or leave the cell 3. Movement occurs due to diffusion, hence the name a. high [ ] to low[ ] 4. Called facilitated because channels assist, or facilitate the diffusion of molecule

5.8-Cells expend energy in the active transport of a solute against its [ ] gradient I. Active transport A. “comes at a price” 1. Energy must be used to perform B. Allows molecules to move in only one direction 1. Like a turnstile in a stadium C. Allows cells to stockpile certain substances at higher [ ] inside the cell than outside

D. Sodium-potassium pump (Na, K)

1. Moves Na out of cell and K into the cell 2. Na fits into protein receptors inside the cell 3. Once energy is provided Na is pumped out of the cell a. once released outside Na cannot reenter through this channel 4. Once Na is released K attaches to same protein receptor outside the cell 5. Na reenters the cell by facilitated diffusion once [ ] is low enough inside the cell a. K does the same in the opposite direction

6. So what is the point of all this? a. One mechanism used by cells to maintain homeostasis b. The flow of Na and K is vital for restoring the balance that is needed for nerve cells to function 5.9-Exocytosis & endocytosis transport large molecules across membranes I. Moving large molecules A. Some particles are to large to fit through any protein channels B. Endocytosis 1. The process of bringing molecules into a cell using extensions of the cell membrane C. Exocytosis 1. Molecules w/in a sac inside the cell are discharged from the cell in an opposite manner to endocytosis D. Both endocytosis and exocytosis require energy

Cell Transport Mechanisms: Diffusion, Osmosis, and Active Transport