The generalized cell

Proteasome
Hollow, drumlike cylinder made up of protein subunits 
Found throughout cytoplasm
Breaks apart abnormal / misfolded proteins or proteins that are no longer needed

• Small proteins called ubiquitins tag proteins for destruction by proteasomes
• Breaks proteins into small segments, which are later broken apart to individual amino acids (which in turn are recycled)
• Failure to dispose of misfolded proteins could result in buildup of harmful plaques

Proteasome
A protein (green) is shown moving from top to bottom through the hollow proteasome. Middle part is cut away to see where active enzymes cut the protein into small segments, which then move out ofthe bottom end.
(click image to enlarge)

Cytoskeleton
Made up of

• Microfilaments
• Intermediate filaments
• Microtubules
• Adsorption of water on proteins and cross-linking of proteins gives cytoplasm a gel consistency 

Functions include

• Support
• Movements of a cell
• Movement within a cell
• Forming/supporting a cell’s shape
• Anchoring/forming connections with other cells (see below)

Centrosome

• Also called microtubule-organizing center (MTOC) 
• Guides formation and elongation of microtubules, as in mitosis 
• Includes two cylindrical centrioles

Molecular motors

• Help to move materials or organelles within cell
• Provide power to move the cytoskeleton (and thus move or change the shape of cells
• Examples: dynein, myosin, kinesin

Cell extensions

• Microvilli (sing. microvillus) 
  • Increase surface area for absorption
• Cilia (sing. cilium) 
  • Groups of moving cilia move fluids along the surface of a sheet of cells 
  • Primary cilium
    • Most cells have one or more (blood cells do not)
    • Some have a sensory function (taste, smell, movement, etc)
    • Involved in replication of centrioles
• Flagella (sing. flagellum)
  • Only the sperm cell has one
  • Molecular motors at base allow flagellem to move, thus enabling swimming of sperm   

Extracellular matrix (ECM)
Material outside of cells 
The ECM is a complex arrangement of fibers and other molecules that interact with cells to perform body functions

See the discussion at the beginning of Chapter 5 of Anatomy & Physiology textbook

Cell connections
Cells must be held together in a multicellular organism, or the tissues would simply fall apart
In some tissues, cells are held together by fibrous “nets” that are not part of the cells themselves
In some tissues, cells form junctions with each other

Desmosome

• Spot desmosomes: small patches of filaments from adjoining cells “tangle” together like Velcro patches, holding cells together (example: skin cells)

• Belt desmosomes: connecting band (rather than small patch) encircling the cell and connecting it to nearby cells 

Tight junction 

• Bands of protein units in adjoining cells “snap together” to form a tight seal all the way around one “end” of a cell, forming a sort of “collar” that sticks to the collars of nearby cells and thus forms a seal to prevent molecules from passing by a membrane made of cells held together by tight junctions (example: lining of intestines)

Tight junction
click to enlarge

Group junction 

• Protein units form channels that link together to form “tunnels” that lead from one cell to the next
• This arrangement not only joins cells structurally but also functionally, because molecules can move back and forth through gaps and the plasma membrane of each cell is now a continuous sheet—as if it’s now one giant cell (example: heart muscle cells)

Gap junction
click to enlarge

Cell Life Cycle
Life Cycles
All organisms have “life cycles” of development and reproduction—so do cells

• Parent (mother) cell divides to produce two genetically identical daughter cells
  • Daughter cells may not be the same size or have exactly equal number of organelles

Phases
Cell cycle includes many phases of growth and development
Interphase (in-between phase)

• Phase during which cell is not actively dividing
• G1 phase (first growth [gap] phase) – new daughter cell is growing
• S phase (synthesis phase) – cell prepares for eventual cell division by replicating nuclear DNA
  • So that there are now two identical sets (so that each daughter cell will receive one complete, identical set)
  • Each DNA molecule splits (unzips) and new, complementary nucleotides fill in the missing side
  • Each daughter DNA molecule (chromatid) is held together at the centromere
• G2 phase (second growth [gap] phase) – cell continues to grow, often “stockpiling” extra cytoplasm for an eventual split
• Centrioles (in the centrosome) are replicated during interphase

Mitosis

• Coordinated division of the nuclear DNA and equal distribution to each daughter cell
  • Provides genetic integrity
  • Also called nuclear division or M phase
• Phases of mitosis
  • Prophase (preliminary phase)
    • Nuclear envelope dissolves
    • Chromatin (DNA) strands shorten into compact chromosomes (each chromosome is made up of two chromatids held together by a centromere)
    • Each centriole moves toward an opposite pole of the cell, constructing a spindle of fibers (microtubules) across the cell
    
    Prophase

• Metaphase (positioning phase) – chromosomes align at equator of cell and attach to spindle fibers

Metaphase

• Anaphase (separation phase) – spindle fibers pull toward poles, separating the chromatids to form individual chromosomes moving away from their sisters

Anaphase

• Telophase (end phase) – everything goes back to the way it “should be”—the chromosomes unwind to chromatin, the nuclear envelope reforms, the spindle is dismantled

Telephase

• Cytokinesis – the “pinching in” of the membrane and eventual separation of the two daughter cells; not as precise as nuclear division (mitosis); overlaps the end of mitosis

Cytoknesis

Daughter cells are now separate and in G1 of interphase

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