1. Chapter Three: Section 2
3.2 The Cell in It’s Environment

2. Learning Targets Describe how small molecules cross the cell membrane
Explain why osmosis is important to cells
Compare/Contrast passive vs. active transport

3. Key Vocabulary Words Diffusion Osmosis Passive Transport
Active Transport
Selectively permeable

4. Diffusion and Osmosis How does water move into and out of a cell?
The diffusion and egg lab……….. (Thurs 11/6)

5. The Cell Membrane The cell membrane has many functions: protection
obtaining food
ridding wastes
communication
interaction with other cells
A soap bubble is like a cell membrane.

6. A closer look at the cell membrane
The cell membrane is made of several types of molecules.

8. Diffusion
Molecules move across the cell membrane by a process called diffusion.
Diffusion is the movement of molecules from areas of higher concentration to areas of lower concentration.

9. Diffusion
Molecules move into or out of the cell until there is an equal number on both sides of the cell membrane.

10. Diffusion (Passive Transport)
Water molecules are small enough to pass through the cell membrane by diffusion.
Osmosis is the diffusion of water across the cell membrane.
This is passive transport… no energy is required.

12. Osmosis Isotonic Solutions Hypertonic Solutions Hypotonic Solutions
Diffusion of water from an area of greater concentration of water to an area of lesser concentration of water -- across a selectively permeable membrane.
Selectively Permeable: Only some molecules can pass through (small ones: water, salt)
Isotonic Solutions
Hypertonic Solutions
Hypotonic Solutions

13. Isotonic Solution
Concentration of solutes in the solution is the same as inside the cell.
Cell is in dynamic equilibrium, i.e., no net gain or loss of water.
H20
H20

14. Hypotonic Solution
Concentration of H20 is higher outside the cell than inside cell.
Net movement of water will be INTO the cell.
H20
H20
H20

15. Hypotonic Solutions

16. Hypertonic Solution
Concentration of H20 is higher inside the cell than outside cell.
Net movement of water will be OUT OF the cell.
H20
H20
H20

17. Other types of transport
Larger molecules like sugars, starches, and proteins sometimes diffuse through protein channels.

18. Active transport
Active transport is a process that allows molecules to move across the cell membrane from lower to higher concentrations.
To do this requires energy.

19. Other types of transport
A cell can take in large particles of food by “engulfing” them.
This is also a form a active transport…. It takes energy.

20. Why are cells so small?
To understand why the volume of a cell increases faster than its surface area, let’s imagine a perfectly square cell.

21. Why are cells so small?
Which cell type has a higher surface area to volume ratio?
Surface area =
2(16x4)+2(.125x4)+2(.125x16) = 133