Chapter 5 Homeostasis and Transport Section 5.1

Passive Transport The movement of substances across a cell membrane without any input of energy from the cell

Diffusion Simplest type of passive transport Molecules move from an area of higher concentration to an area of lower concentration Driven by kinetic energy

Equilibrium When the concentration of the molecules of a substance are the same throughout a space

Diffusion Across Membranes Cell membranes allow some molecules to pass through, but not others Depends on size and type of molecule Also depends on the chemical nature of the membrane

Osmosis Water molecules diffuse across a cell membrane from an area of higher concentration of H2O to an area of lower concentration

Direction of Osmosis Hypotonic- the concentration of solute molecules outside the cell is LOWER than the inside of the cell Water moves INTO the cell

Hypertonic- the concentration of solute molecules outside the cell is HIGHER than the inside of the cell Water moves OUT OF the cell

Isotonic- the concentrations of solutes outside and inside the cell are equal No movement of water occurs

How Cells Deal with Osmosis Contractile vacuole- organelles to remove water Turgor pressure- pressure of water against the cell wall Plasmolysis- cells shrink away from cell walls

Facilitated Diffusion The movement of molecules across a cell membrane through the use of carrier proteins First, a carrier protein binds to a molecule on one side of the cell membrane

Second, the carrier protein changes shape shielding the molecule from the interior of the membrane Finally, the molecule is released on the other side of the membrane

Diffusion Through Ion Channels Ion channels provide small passages for ions to diffuse across the cell membrane Channels may have “gates” that respond to stretching of the cell membrane, electrical signals, or chemicals in the cytosol

Ions such as Na+, K+, Ca2+, and Cl- are important for a variety of cell functions