1. Cells and their Environment

2. Maintaining Homeostasis
One way that cells maintain homeostasis is by controlling the movement of substances across the cell membrane
The cell membrane is fluid, not rigid

3. Cell Membrane
The cell membrane is made up of a fluid “sea” of phospholipids
A phospholipid is a special lipid made up of a phosphate “head” (polar – attracted to water) and two fatty acid “tails” (nonpolar – repelled by water)

4. Cell Membrane
The phospholipids form a bilayer (two layers) in which the polar heads face the exterior and interior of the cell and the nonpolar tails face each other
Only small, nonpolar substances can pass directly through this bilayer
Why?

5. Membrane Proteins
Different proteins float within the fluid “sea” of phospholipids and perform many functions
Cell surface markers act as name tags to identify each type of cell
Receptor proteins enable a cell to sense its surroundings (sense in what type of environment it’s in)
Enzymes help with important biochemical reactions inside the cell
Transport proteins help substances that cannot easily cross the cell membrane to get into and out of the cell

6. Membrane Proteins

7. Cell Transport Substances can enter and leave the cell in many ways
Sometimes, energy is needed, and other times, it is not…

8. Diffusion
Diffusion occurs when substances move from an area of high concentration to an area of low concentration (when a concentration gradient exists)
This does not require energy

9. Passive Transport
Occurs when substances cross the cell membrane down their concentration gradient (they “diffuse” across the membrane)
This does NOT require energy

10. Passive Transport
Simple diffusion allows small nonpolar molecules to pass directly through lipid bilayer (ex: oxygen)
Facilitated diffusion occurs when transport proteins help substances that can’t pass directly through lipid bilayer (ions and polar molecules) cross the membrane.
Even though transport proteins are involved, energy is NOT required!

11. Passive Transport
Simple Diffusion
Facilitated Diffusion

12. Osmosis
Osmosis is the diffusion of water molecules across a cell membrane
Osmosis allows cells to maintain water balance as their environment changes
Since water molecules are polar, they must diffuse through special transport proteins (a form of facilitated diffusion)

13. Predicting Water Movement
If in biology, there is a constant need to achieve equilibrium, where will the water move?
Water will move into the cell
The cell will expand
This solution is HYPOTONIC
80% water
20% sugar
20% water
80% sugar

14. Predicting Water Movement
If in biology, there is a constant need to achieve equilibrium, where will the water move?
Water will move out of the cell
The cell will shrink
This solution is HYPERTONIC
20% water
80% sugar
80% water
20% sugar

15. Predicting Water Movement
If in biology, there is a constant need to achieve equilibrium, where will the water move?
Water will move into and out of the cell evenly
The cell will remain the same
This solution is ISOTONIC
50% water
50% sugar
50% water
50% sugar

16. Predicting Water Movement
A solution is HYPERTONIC when it has a higher concentration of solutes and lower concentration of water than the cell – water will move out of the cell
A solution is HYPOTONIC when it has a lower concentration of solutes and higher concentration of water then the cell – water will move into the cell
A solution is ISOTONIC when it has an equal concentration of solutes and solvents than the cell – water will move into and out of the cell evenly with no net gain or loss

17. Predicting Water Movement
The solutions in which these red blood cells are in are…
Hypertonic
Hypotonic
Isotonic

18. Active Transport Energy is needed when
Cells transport substances against their concentration gradient
Large molecules need to get into/out of a cell

19. Active Transport
Proteins can be used as pumps to pump substances against their concentration gradient
These are the same kinds of proteins used in facilitated diffusion, except ENERGY is used

20. Active Transport
Large substances cross the cell membrane in vesicles (membrane bound sacs)
Endocytosis is when substances move INTO the cell
Exocytosis is when substances move OUT OF the cell