SECTION 3-4 How Things Get Into and Out of Cells

Permeability The ease with which substances can cross the cell membrane Nothing passes through an impermeable barrier Anything can pass through a freely permeable barrier Cell membranes are selectively permeable

Diffusion Movement of a substance from an area of high concentration to low Continues until concentration gradient is eliminated

Figure 3.18 Diffusion Figure 3.18

Figure 3.19 Diffusion across the Cell Membrane

Osmosis Diffusion of water across a semipermeable membrane in response to solute differences Osmotic pressure = force of water movement into a solution Hydrostatic pressure opposes osmotic pressure Water molecules undergo bulk flow

Figure 3.20 Osmosis Figure 3.20

Tonicity The effects of osmotic solutions on cells Isotonic = no net gain or loss of water Hypotonic = net gain of water into cell Hemolysis Hypertonic = net water flow out of cell Crenation

Figure 3.21 Osmotic flow across a cell membrane