1. All living cells must maintain a balance regardless of internal and external conditions. Survival depends on the cell’s ability to maintain the-

2. This process of maintaining the cell’s environment is called-
Selective permeability is a process used to maintain homeostasis in which the plasma membrane allows some molecules into the cell-

3. Cell (plasma) membrane
Cells need an inside & an outside…
separate cell from-
cell membrane - IN
food
- sugars
- proteins
- fats
salts O2
H2O
OUT
waste
- ammonia
- salts
- CO2
- H2O
products
- proteins
cell needs materials in & products or waste out

4. Building a membrane
How do you build a barrier that keeps the watery contents of the cell separate from the watery environment?
 FATS 
 LIPIDS 
Remember: oil & water don’t mix!!
What substance do you know that doesn’t mix with water?

5. Lipids of cell membrane
Membrane is made of special kind of lipid -
“split personality”
Membrane is a double layer -
“attracted to water”
phosphate
inside cell
outside cell
lipid
“repelled by water”

6. Makeup of the phospholipid bilayer
The describes the plasma membrane as a flexible boundary of a cell. The phospholipids move within the membrane.

7. Semi-permeable membrane
Cell membrane controls what gets in or out
Need to allow some materials — but not all — to pass through the membrane
semi-permeable -
So what needs to get across the membrane?
sugar
lipids aa O2
H2O
salt
waste

8. How do you build a semi-permeable cell membrane?
Channels -
proteins both “like” water & “like” lipids
bi-lipid
membrane
protein channels in bi-lipid membrane