1. Ch. 4 Cells and Their Environment

2. The purpose of this chapter is to learn how substances move into and out of cells. Two ways in which this is done: 1) passive transport 2) active

3. Passive transport: does not require energy Active transport: requires energy

4. Diffusion Your body responds to external conditions to maintain a stable internal environment. When organisms adjust internally to changing external conditions, they are maintaining homeostasis. One way cells maintain homeostasis is by controlling the movement of substances across their cell membrane.

5. Cells must use energy to transport some substances across the cell membrane. Other substances move across the cell membrane without any use of energy by the cell. Passive transport:

6. Concentration gradient: Equilibrium: fig. 1, p 75. Diffusion:

7. To diffuse “down” its concentration gradient-from an area of high concentration to an area of lower concentration- a substance must be able to pass through the cell membrane. The cell membrane is selectively permeable to substances. The nonpolar interior of the lipid bilayer repels ions and most polar molecules. Refer back to p. 32, Polarity section.

8. These substances are prevented from diffusing across the cell membrane. Molecules that are very small or nonpolar can diffuse across the cell membrane down their concentration gradient. The diffusion of such molecules across the cell membrane is the simplest type of passive transport.

9. Osmosis Osmosis: Table 1, p. 77. Hypertonic solution:

10. Hypotonic solution: Isotonic solution:

11. Crossing the Cell Membrane Ions and most polar molecules cannot diffuse across the cell membrane because they cannot pass through the nonpolar interior of the lipid bilayer. Such substances can cross the cell membrane when they are aided by transport proteins. Transport proteins called channels provide polar passageways through which ions and polar molecules can move across the cell membrane.

12. Each channel allows only a specific substance to pass through the cell membrane. Ex.: Some channels allow only one type of ion to cross the cell embrane, while others transport a particular kind of sugar or amino acid.

13. Diffusion through ion channels: Electrical charge and ion transport:

14. Facilitated Diffusion: What are the 3 steps? Carrier proteins: