Cells & Their Environment
Cell Membrane
**Every cell is surrounded by a cell membrane!
- protects cell
- helps move substances in & out of the cell
I. Homeostasis
A. Definition – maintenance of stable internal
conditions in a changing environment
B. Cell Membrane & Homeostasis
1. Cell Membrane controls the movement of
substances in & out of the cell
2. Provides structural support
3. Recognizes foreign material
4. Communicates with other cells
II. Lipid Bilayer
A. Phospholipid – cell membrane is a “sea” of
phospholipids in which proteins “float”
1. Phosphate head is polar, hydrophilic, waterloving
2. Two fatty acid tails are nonpolar, hydrophobic,
water-fearing
B. Membrane Structure
1. Lipid Bilayer – hydrophobic tails are inside,
protected by hydrophilic heads on outside
2. Barrier – phospholipids form a barrier through which
only small, nonpolar substances can pass
C. Cell Membranes are Fluid
1. Phospholipids are the most abundant lipid in the
plasma membrane
2. The fluid mosaic model states that a membrane
is a fluid structure with a “mosaic” of various
proteins embedded in it
Figure 7.3
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Figure 7.4
TECHNIQUE
Extracellular
layer
Knife
Plasma membrane
Proteins
Cytoplasmic layer
RESULTS
Inside of extracellular layer
Inside of cytoplasmic layer
3. Phospholipids in the plasma membrane can
move within the bilayer
4. Most of the lipids, and some proteins, drift
laterally
• Rarely does a molecule flip-flop transversely
across the membrane
Figure 7.6
Lateral movement occurs
107 times per second.
Flip-flopping across the membrane
is rare ( once per month).
III. Membrane Proteins
A. Proteins in Lipids
1. Integral Proteins – proteins stretch across the
bilayer & face both inside and outside
2. Peripheral Proteins – proteins that are only on
one side (face inside or outside) of the
membrane
B. Types of Proteins
1. Receptor Proteins – sense the cell’s surroundings
by binding to substances outside the cell
2. Enzymes – proteins that help biochemical
reactions in a cell
3. Transport Proteins – aid the movement of certain
substances in & out of the cell
Types of Proteins in the Membrane