Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 5

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.
The features of a cell's surface identify it as belonging to a particular species, to a particular individual, and to a particular tissue within that individual. The surface consists of molecules embedded in and extending from the cell membrane. Cell surfaces guide embryonic cells to form tissues and organs. The human leukocyte antigen cell surface markers establish self and are associated with increased risk of developing certain disorders.
A biological membrane consists of a phospholipid bilayer embedded with movable proteins, glycoproteins, and glycolipids, like a fluid mosaic. The percentage and distribution of membrane proteins varies in different cell types. Membrane proteins include receptors, carriers, adhesion molecules, and cell surface proteins establishing self.
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 the cell membrane or other semipermeable membrane. Terms describing tonicity (isotonic, hypotonic, hypertonic) predict whether cells will swell or shrink when the surroundings change. Several adaptations maintain cell shape in the face of solute concentration changes.
In facilitated diffusion, molecules cross a membrane following the concentration gradient with the aid of a carrier protein. In active transport, molecules cross a membrane against the concentration gradient and require a carrier protein and energy from ATP.
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. In some processes, vesicles shuttle between exocytosis and endocytosis, including intermediate endosomes. Endocytosis is more specific when a receptor in the vesicle binds a specific molecule. Within cells, proteins guide vesicles to particular organelles.
Intercellular junctions join cells. In animals they include tight junctions, desmosomes, and gap junctions. In plants they include a pectin layer and plasmodesmata.
The cytoskeleton is a network of rods and tubes that provides cells with form, support, and the ability to move. Microtubules self-assemble from hollow tubulin subunits to become cilia, flagella, and the spindle fibers that separate one cell into two during cell division. Microtubules have a characteristic 9+ 2 configuration. Dynein causes adjacent microtubules to slide, which moves the overall structure. Microfilaments are solid and smaller than microtubules. They are composed of the protein actin and provide contractile motion. Intermediate filaments provide scaffolding.
Red blood cells and muscle cells derive their strength from cell membrane proteins that connect cytoskeletal elements to extracellular materials.
In signal transduction, receptors in the cell membrane receive input from first messengers (hormones and growth factors) 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.
Cellular adhesion molecules enable cells to contact each other in precise steps that carry out a particular function.