Biology Chapter: The Plasma Membrane

Two layers of phospholipids arranged "tail to tail" with cholesterol and proteins scattered among them
Sugar groups may be attached to the phospholipids
Hydrophilic ("water loving") polar "heads" on the inner and outer surfaces of the membrane
Hydrophobic ("water fearing") nonpolar "tails" form the center of the membrane
This interior makes the plasma membrane relatively impermeable
Responsible for specialized membrane functions such as enzymes, receptors for hormones or other chemical messengers, and transport as channels or carriers
Glycoproteins are branched sugars attached to proteins that abut the extracellular space
Glycocalyx is the fuzzy, sticky, sugar-rich area on the cell’s surface

Cells are bound together in 3 ways:
- Glycoproteins in the glycocalyx act as an adhesive or cellular glue
- Wavy contours of the membranes of adjacent cells fit together in a tongue-and-groove fashion
- Special cell membrane junctions are formed

Tight junctions:
- Impermeable junctions
- Plasma membranes fuse like a zipper to prevent substances from passing through the extracellular space between cells
Desmosomes:
- Anchoring junctions, like rivets, that prevent cells from being pulled apart as a result of mechanical stress
Gap junctions (communicating junctions):
- Allow communication between cells
- Hollow cylinders of proteins (connexons) span the width of the abutting membranes
- Molecules can travel directly from one cell to the next through these junctions

Nuclear envelope (membrane):
- Consists of a double membrane that bounds the nucleus
- Contains nuclear pores that allow for exchange of material with the rest of the cell
- Encloses the jellylike fluid called the nucleoplasm
Nucleolus:
- Nucleus contains one or more dark-staining nucleoli
- Sites of ribosome assembly
- Ribosomes migrate into the cytoplasm through nuclear pores to serve as the site of protein synthesis
Chromatin:
- Composed of D N A wound around histones (proteins)
- Scattered throughout the nucleus and present when the cell is not dividing
- Condenses to form dense, rodlike bodies called chromosomes when the cell is dividing

Cytosol: Fluid that suspends other elements and contains nutrients and electrolytes
Inclusions: Chemical substances, such as stored nutrients or cell products, that float in the cytosol
Organelles: Metabolic machinery of the cell that perform functions for the cell. Many are membrane-bound, allowing for compartmentalization of the cell

“Powerhouses” of the cell
Mitochondrial wall consists of a double membrane with cristae on the inner membrane
Carry out reactions in which oxygen is used to break down food into A T P molecules

Made of protein and ribosomal R N A
Sites of protein synthesis in the cell
Found at two locations:
- Free in the cytoplasm
- Attached to the rough endoplasmic reticulum

Fluid-filled tunnels (or canals) that carry substances within the cell
Continuous with the nuclear membrane
Two types:
- Rough E R: Studded with ribosomes; synthesizes proteins; transport vesicles move proteins within cell; abundant in cells that make and export proteins
- Smooth E R: Lacks ribosomes; functions in lipid metabolism, detoxification of drugs and pesticides

Appears as a stack of flattened membranes associated with tiny vesicles
Modifies and packages proteins arriving from the rough E R via transport vesicles
Produces different types of packages:
- Secretory vesicles: Pathway for proteins to leave the cell via exocytosis

Network of protein structures that extend throughout the cytoplasm
Provides the cell with an internal framework that determines cell shape, supports organelles, and provides the machinery for intracellular transport
Three different types of elements:
- Microfilaments (largest)
- Intermediate filaments
- Microtubules (smallest)

Surface extensions found in some cells
Cilia: Move materials across the cell surface (located in the respiratory system to move mucus)
Flagella: Propel the cell (the only flagellated cell in the human body is sperm)
Microvilli: Tiny, fingerlike extensions of the plasma membrane that increase surface area for absorption

Epithelial cell: Packs together in sheets; intermediate fibers resist tearing during rubbing or pulling

Skeletal muscle and smooth muscle cells: Contractile filaments allow cells to shorten forcefully

White blood cells, such as the macrophage (a phagocytic cell): Digests infectious microorganisms

Oocyte (female): Largest cell in the body; divides to become an embryo upon fertilization
Sperm (male): Built for swimming to the egg for fertilization; flagellum acts as a motile whip