1. Cells & Their Environment

2. Cell Membrane
**Every cell is surrounded by a cell membrane! - protects cell - helps move substances in & out of the cell

3. I. Homeostasis
Definition – maintenance of stable internal conditions in a changing environment
Cell Membrane & Homeostasis
Cell Membrane controls the movement of substances in & out of the cell
Provides structural support
Recognizes foreign material
Communicates with other cells

4. II. Lipid Bilayer
Phospholipid – cell membrane is a “sea” of phospholipids in which proteins “float”
Phosphate head is polar, hydrophilic, water-loving
Two fatty acid tails are nonpolar, hydrophobic, water-fearing

5. Membrane Structure
Lipid Bilayer – hydrophobic tails are inside, protected by hydrophilic heads on outside
Barrier – phospholipids form a barrier through which only small, nonpolar substances can pass

6. Cell Membranes are Fluid
Phospholipids are the most abundant lipid in the plasma membrane
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

7. Phospholipid bilayer Hydrophobic regions of protein
Figure 7.3
Phospholipid bilayer
Figure 7.3 The original fluid mosaic model for membranes.
Hydrophobic regions of protein
Hydrophilic regions of protein 7

8. Inside of extracellular layer Inside of cytoplasmic layer
Figure 7.4
TECHNIQUE
Extracellular layer
Proteins
Knife
Plasma membrane
Cytoplasmic layer
RESULTS
Figure 7.4 Research Method: Freeze-fracture
Inside of extracellular layer
Inside of cytoplasmic layer 8

9. Phospholipids in the plasma membrane can move within the bilayer
Most of the lipids, and some proteins, drift laterally
Rarely does a molecule flip-flop transversely across the membrane

10. Lateral movement occurs 107 times per second.
Figure 7.6
Lateral movement occurs 107 times per second.
Flip-flopping across the membrane is rare ( once per month).
Figure 7.6 The movement of phospholipids. 10

11. III. Membrane Proteins Proteins in Lipids
Integral Proteins – proteins stretch across the bilayer & face both inside and outside
Peripheral Proteins – proteins that are only on one side (face inside or outside) of the membrane

13. Types of Proteins
Receptor Proteins – sense the cell’s surroundings by binding to substances outside the cell
Enzymes – proteins that help biochemical reactions in a cell
Transport Proteins – aid the movement of certain substances in & out of the cell

14. Types of Proteins in the Membrane