Biology Chapter: The Plasma Membrane

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

Cell Membrane Junctions

• 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

Main Types of Cell Junctions

• 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

The Nucleus

• Control center of the cell
• Contains genetic material known as deoxyribonucleic acid, or D N A
• D N A is needed for building proteins
• D N A is necessary for cell reproduction

Anatomy of the Nucleus

• 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

The Cytoplasm

• The cellular material outside the nucleus and inside the plasma membrane
• Site of most cellular activities
• Includes cytosol, inclusions, and organelles

Three Major Components of the Cytoplasm

• 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

Mitochondria

• “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

Ribosomes

• 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

Endoplasmic Reticulum (E R)

• 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

Golgi Apparatus

• 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

Lysosomes

• Membranous “bags” that contain digestive enzymes
• Enzymes can digest worn-out or nonusable cell structures
• House phagocytes that dispose of bacteria and cell debris

Peroxisomes

• Membranous sacs of oxidase enzymes
• Detoxify harmful substances such as alcohol and formaldehyde
• Break down free radicals (highly reactive chemicals)
• Free radicals are converted to hydrogen peroxide and then to water

Cytoskeleton

• 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)

Centrioles

• Rod-shaped bodies made of nine triplets of microtubules
• Generate microtubules
• Direct the formation of the mitotic spindle during cell division

Cell Extensions

• 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

Cell Diversity

• The human body houses over 200 different cell types
• Cells vary in size, shape, and function
• Cell shape reflects its specialized function

Cells that Connect Body Parts

• Fibroblast: Secretes cable-like fibers
• Erythrocyte (red blood cell): Carries oxygen in the bloodstream

Cells that Cover and Line Body Organs

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

Cells that Move Organs and Body Parts

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

Cell that Stores Nutrients

• Fat cells: Lipid droplets stored in cytoplasm

Cell that Fights Disease

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

Cell that Gathers Information and Controls Body Functions

• Nerve cell (neuron): Receives and transmits messages to other body structures

Cells of Reproduction

• 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