CHAPTER 5 The Working Cell

5.3-passive transport is diffusion across a membrane with no energy costI. Diffusion

A. Intro1. In addition to chemical signals, cells transport H2O, food, and ions

across membranea. Ion charged particle

B. Diffusion1. The mixing of 2 substances by the random motion of molecules2. Molecules will always diffuse from and area of high [ ] to an area of

low [ ] 5.4-Osmosis is diffusion of water across a membraneC. Osmosis: Diffusion of H2O

1. The diffusion of H2O across a selectively permeable membrane2. 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. [ ] gradient1. The difference in the [ ] of particles that occurs across a selectively

permeable membrane2. 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 membrane2. Cells in an isotonic solution do not experience osmosis

C. Hypotonic solution1. [ ] of particles outside the cell is lower than inside the cell

a. More water outside the cell than inside2. Water moves inside the cell causing the cell to swell

a. If solution is to hypotonic cell will burst

E. Osmotic pressure1. The pressure that results from the force of water pushing against the plasma membrane2. Also called turgor pressure

D. Hypertonic solution1. [ ] of particles outside the cell is higher than inside the cell

a. More water inside the cell than outside2. Water leaves the cell causing it to shrink

5.6-Transport proteins may facilitate diffusion across membranesI. Selective transport

A. Review1. Not all substances can pass freely across plasma membrane like

waterB. Intro

1. Protein channels and pumpsa. Channels

1. Passageways across plasma membrane2. Allows only certain molecules to cross membrane depending

on shape of molecule

b. Pumps1. Passageways across plasma membrane that requires a certain

amount of energy2. Used to pump a molecules against or up the [ ] gradient

C. Facilitated diffusion1. “free of charge”

a. Costs the cell no energy to perform2. If molecule can fit through the protein channel it may enter or leave

the cell3. 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 [ ] gradientI. Active transport

A. “comes at a price”1. Energy must be used to perform

B. Allows molecules to move in only one direction1. 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 cell2. Na fits into protein receptors inside the cell3. Once energy is provided Na is pumped out of the cella. once released outside Na cannot reenter through this channel4. Once Na is released K attaches to same protein receptor outside

the cell5. Na reenters the cell by facilitated diffusion once [ ] is low enough

inside the cella. 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 homeostasisb. 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 membranesI. Moving large molecules

A. Some particles are to large to fit through any protein channels

B. Endocytosis1. The process of bringing molecules into a cell

using extensions of the cell membraneC. 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