1. 8.1 Cell Membrane I. Homeostasis
A. The maintenance of stable internal conditions in a changing environment
1. ex. – body temp
B. Cells stay stable by controlling what comes in and out of the cell
II. Lipid bilayer
A. Phospholipid
1. specialized lipid made of phosphate “head” & acid “tails”

2. 2. “heads” = polar, attracted to water
3. “tails” = nonpolar, repel water
B. Tails point inward on both sides of the bilayer and heads make up the outside
C. Only allows certain substances in the cell
III. Membrane Proteins
A. Cell-surface markers- identifies each cell, like a name tag
B. Receptor proteins- allows cell to sense its surroundings by binding to substances
C. Enzymes- help with reactions in the cell

3. D. Transport Proteins- aide in the. movement of substances across the
D. Transport Proteins- aide in the movement of substances across the membrane

4. 8.2 Cell Transport I. Passive Transport
A. Cell does not use energy in this process
B. Equilibrium is reached when space in a cell is filled evenly
C. Concentration gradient- areas of high & low concentration of substances
D. Diffusion
1. Movement down the concentration gradient

5. 2. Simple diffusion- small, nonpolar
2. Simple diffusion- small, nonpolar molecules pass directly through bilayer
a. ex.- oxygen, CO2, natural steroids
3. Facilitated diffusion- transport proteins help substances diffuse through bilayer

6. E. Osmosis
1. Allows cells to keep their water balance in a changing environment
2. Moves from high to low concentration

7. II. Active Transport
A. Uses energy
B. Pulls substances against concentration gradient
C. Sodium-Potassium pump

8. 8.3 Cell Communication D. Vesicles 1. Membrane bound sacs
2. Endocytosis- movement of large substances into a cell
3. Excytosis- movement of materials out of a cell
8.3 Cell Communication
I. Sending Signal
A. Cells produce signals, often a molecule, that are detected by the target cell

9. B.Targets
1. Neighboring cells communicate through direct contact
2. Long-distance signals are carried by hormones and nerves
C. Environmental Signals
1. ex- the length of sunlight
II. Receiving Signals
A. Target cells are hit by many signals but only recognize and respond to a few of them

10. 1. Receptor proteins are responsible for
1. Receptor proteins are responsible for identifying which signals to accept
a. Signals only bind if they are the correct shape of the binding site, works like a lock & key
III. Responding to Signals
A. The cell may respond to a signal by changing its membrane permeability, by activating enzymes, or by forming a second messenger