Plasma Membrane and Cell Junctions

The plasma membrane is a flexible, transparent barrier that contains cell contents and separates them from the surrounding environment.
Terms like "cell membrane" or "cytoplasmic membrane" are sometimes used, but this text consistently refers to the outer limiting membrane as the plasma membrane.
The plasma membrane defines the limits of the cell and plays a dynamic role in cellular activities.

Describe the chemical composition of the plasma membrane and relate it to membrane functions.
Compare the structure and function of tight junctions, desmosomes, and gap junctions.

The plasma membrane structure consists of two phospholipid layers arranged "tail to tail."
Cholesterol and floating proteins are scattered among the phospholipid layers.
Some phospholipids have sugar groups attached, forming glycolipids.
Proteins can move freely and form a constantly changing pattern or mosaic.

Some proteins float freely in the lipid bilayer, whereas others are anchored in specific locations. What structure(s) could anchor the stationary proteins?

The phospholipid bilayer forms the basic "fabric" of the membrane.
The polar "heads" of phospholipids are hydrophilic (water-loving) and attracted to water, positioning themselves on the inner and outer surfaces of the membrane.
Nonpolar fatty acid "tails" are hydrophobic (water-fearing) and line up in the center (interior) of the membrane.
The self-orienting property of phospholipids allows biological membranes to reseal quickly when torn.
The hydrophobic interior makes the membrane relatively impermeable to most water-soluble molecules.
Cholesterol stabilizes the membrane and keeps it flexible.

Proteins scattered in the lipid bilayer are responsible for most specialized functions.
- Some proteins are enzymes.
- Many protruding proteins act as receptors for hormones or chemical messengers.
- Proteins can be binding sites for anchoring the cell to fibers or other structures.
- Most transmembrane proteins are involved in transport, forming protein channels or acting as carriers.

Branching sugar groups are attached to most proteins abutting the extracellular space, forming glycoproteins.
The cell surface is a fuzzy, sticky, sugar-rich area called the glycocalyx.
Glycoproteins determine blood type and act as receptors for bacteria, viruses, or toxins.
They play a role in cell-to-cell recognition and interactions.
Changes in glycoproteins occur in cells transforming into cancer cells.

Phospholipids are polar molecules; their charged end interacts with water, and the fatty acid chains do not.
This polarity makes phospholipids a good foundation for cell membranes.

Blood cells, sperm cells, and phagocytic cells are "footloose," but many other cell types, especially epithelial cells, are knit into tight communities.
Cells are bound together in three ways:
- Glycoproteins in the glycocalyx act as an adhesive or cellular glue.
- Wavy contours of adjacent cell membranes fit together in a tongue-and-groove fashion.
- Special cell membrane junctions are formed.

The main types of junctions include tight junctions, desmosomes, and gap junctions.

Impermeable junctions encircle cells and bind them together into leakproof sheets.
Adjacent plasma membranes fuse tightly, preventing substances from passing through the extracellular space.
Example: In the small intestine, tight junctions prevent digestive enzymes from seeping into the bloodstream.

Anchoring junctions scattered like rivets along the sides of adjacent cells.
They prevent cells subjected to mechanical stress from being pulled apart.
Found in heart muscle cells and skin cells.
Structurally, they are buttonlike thickenings of adjacent plasma membranes (plaques) connected by fine protein filaments.
Thicker protein filaments extend from plaques inside the cells to plaques on opposite sides, forming an internal system of strong "guy wires."

Communicating junctions that function mainly to allow communication.
Commonly found in the heart and between embryonic cells.
Neighboring cells are connected by hollow cylinders composed of proteins (connexons) that span the entire width of the abutting membranes (transmembrane proteins).
Chemical molecules, such as nutrients or ions, can pass directly through the water-filled connexon channels from one cell to another.