The Cell Membrane, Cytoskeleton, and Cell-Cell Interactions
4.1 The Cell Membrane Controls Cell Function
1. The cell surface is a selective interface between the cell and the outside environment. It receives and transmits incoming messages, controls which substances enter and leave the cell, and mediates attachments to and interactions with other cells and extracellular material.
2. The features of a cell's surface identify it as belonging to a particular species, individual, and tissue. The surface consists of molecules embedded in and extending from the cell membrane.
3. A biological membrane consists of a phospholipid bilayer embedded with movable proteins, glycoproteins, and glycolipids. The percentage and distribution of membrane proteins varies in different cell types. Membrane proteins carry out a variety of functions.
4. Substances cross cell membranes in several ways. In diffusion, a molecule passes through openings in a membrane following its concentration gradient. Osmosis is the simple diffusion of water across a semipermeable membrane. Terms describing tonicity (isotonic, hypotonic, hypertonic) predict whether cells will swell or shrink when the surroundings change. Cells are adapted to maintain shape when solute concentration changes.
5. Transport proteins move molecules across membranes. In facilitated diffusion, which is a form of passive transport, a channel protein admits, or a carrier protein carries, a substance across the membrane. In active transport, a carrier protein plus energy move the substance. In cotransport, a protein carries one substance down its concentration gradient, as well as a second substance.
6. In exocytosis, vesicles inside the cell carry substances to the cell membrane, where they fuse with the membrane and release the cargo outside. In endocytosis, molecules are brought into the cell by a vesicle in the cell membrane. Endosomes are vesicles that shuttle substances within cells. Receptor-mediated endocytosis is more specific. Proteins guide vesicles to particular organelles. Substances cross cells, entering by endocytosis and exiting by exocytosis, in transcytosis.
7. The cytoskeleton is a network of rods and tubules that provides cells with form, support, and the ability to move.
8. Microtubules self-assemble from hollow tubulin subunits to form cilia, flagella, and the spindle fibers that separate chromosomes during cell division. Some microtubules have a characteristic 9 + 2 configuration. Dynein causes adjacent microtubules to slide, which moves the overall structure.
9. Microfilaments are solid and smaller than microtubules. They are composed of the protein actin and provide contractile motion when they interact with myosin.
10. Intermediate filaments are intermediate in diameter between microtubules and microfilaments. They consist of entwined dimers of various proteins. They strengthen the cytoskeleton.
4.3 Cells Interact and Respond to Signals
11. Junctions connecting animal cells include tight junctions, desmosomes, and gap junctions. Tight junctions create a seal between adjacent cells. Desmosomes anchor cells in place. Gap junctions allow adjacent cells to exchange cytoplasmic material.
12. Most organisms other than animals have cell walls, which provide shape and mediate signals. Plant cell walls consist of cellulose fibrils connected by hemicellulose, plus pectin and various proteins. Plasmodesmata are continuations of cell membranes between cells through thinned parts of the cell wall.
13. Cellular adhesion molecules enable cells to contact each other in precise steps that carry out a particular function.
14. In signal transduction, receptors in the cell membrane receive input from first messengers and transmit the messages through a series of membrane proteins. Eventually this signaling activates a second messenger, which stimulates the cell to carry out a specific function.