Cells and the cytoskeleton

• Organelles are membrane-bound

  Like “mini cells” with a big cell
• Each organelle contains specific molecules for specific chemical reactions and cellular processes
• Why organelles are so great

  • Compartmentalization! Incompatible chemical reactions can be separated things the cell wants to keep intact
  • For example, digestive enzymes can be kept away from
• Why organelles are so great

  • Specialization! Increased efficiency of chemical rxs and cellular processes
  • Substrates required for chemical reactions can be clustered in one compartment and/or embedded in the organelle membrane for easy access, thus speedier rxs
• Why organelles are so great

  • Complexity
  • With the help of a supportive cytoskeleton, cells can become bigger, perform more diverse functions, give rise to multicellularity
• The nucleus houses the DNA and is the site of RNA synthesis
• Some ribosomes float freely in the cytosol, where they make proteins that function in the cytosol
• Other ribosomes are attached to the rough endoplasmic reticulum, where they synthesize proteins that will be:

  1. embedded in a cell membrane
  2. secreted from the cell
  3. put inside of an organelle
• The rough endoplasmic reticulum gets its name from the rough appearance of the ribosomes on its surface
• The attached ribosomes grab RNA strand from nucleus and start injecting protein product directly into the lumen of the rough ER as its being translated
• The smooth ER (which is “smooth” because it’s not associated with ribosomes) is where lipids such as phospholipids and fatty acids are synthesized
• The Golgi apparatus receives proteins and lipids from the rough and smooth ER and:

  • further modifies them (e.g., by attaching a sugar molecule) and
  • sorts and packages them for delivery to their final locations in the membrane or outside of the cell
• The Golgi apparatus is also where most of the cell’s carbohydrates are synthesized
• The nucleus, ER and Golgi make up a continuous “endomembrane system” (or “secretory pathway”)– connected by vesicles –

  • to streamline synthesis and delivery of proteins and lipids to their final destinations
• Lysosomes break down damaged or unneeded macromolecules (large molecules) into smaller components
• Lysosomes form from the fusion of Golgi vescicle containing enzymes and the vesicle containing macromolecules to be broken down
• Mitochondria synthesize most of the ATP (“energy currency”) used by the cell
• Chloroplasts (in plant cells) capture energy from sunlight and synthesize simple sugars
• The biconcave shape of a red blood cell maximizes its surface area for gas exchange and allows it to deform as it passes through the circulatory system
• Long, slender extensions of the plasma membrane allow neurons to communicate with other cells
• Thin, comb-like projections, called microvilli, on intestinal cells increase their absorptive surface area
• Long flagellum facilities movements, packed with a lot of mitochondria to support this energy-intensive swimming function
• Peroxisomes break down specific organic molecules, such as fatty acids, and synthesize other organic molecules such as cholesterol and some types of phospholipids
• Cytoskeleton (the "bones of the Cell)
• The cytoskeleton provides internal structural support and enables movement of substances within the cell

  • 3 types

  • Each type is formed from long chains of protein subunits joined together

  •The cytoskeleton is DYNAMIC! (unlike bones)
• Microtubules

  • Subunits (“Building Blocks”):
  • • Tubulin dimers
• Microtubules

  • Structure:
  • • Hollow tube
  • • Directional: plus end and minus end
• Microtubules

  • Major Microtubule Functions:
  • • Cell shape and support
  • • Vesicle transport and organelle arrangement
  • Cell movement (by cilia, flagella)
  • • Cell division (chromosome segregation)
• Microtubules: vesicle transport

  • Motor proteins kinesin and dynein “walk” on microtubule tracks to carry cargo-filled vesicles through the cell.
• Microtubules: Cell movement

  • Microtubules are a key part of the structures of eukaryotic cilia and flagella.
  • Cilia and flagella beat to either propel a cell or allow it to move fluidly across the cell surface
• Microtubules are dynamic

  • Microtubules lengthen and shorten at their ends
• Microfilaments

  Subunits (“Building Blocks”):• Actin monomers
• Microfilaments

  Structure:

  • Thin double helix of actin protein monomers

  • Directional: plus end and minus end
• Microfilaments

  Major Microfilament Functions:

  • Cell shape and support

  • Cell movement (by crawling)

  • Vesicle transport

  • Muscle contraction

  • Cell division (cytokinesis)
• Microfilaments: Cell shape and support

  • Actin microfilaments help maintain cell shape
  • Lots of actin microfilaments at plasma membrane!
• Microfilaments: Cell movement (by crawling)

  • Elongating actin filaments at the edge of the cell membrane allow crawling motion of cells
• Microfilaments: Vesicle transport

  • Actin microfilaments work together with myosin motor proteins to carry out important functions.
  • Myosin motor proteins can “walk” along microfilaments carrying cargo-filled vesicles (similar to kinesin and dynein on microtubules)
• Microfilaments: Muscle contraction

  • Interactions between actin microfilaments and myosin motor proteins are responsible for muscle contractions
• Microfilaments: Cell division (cytokinesis)

  • Actin microfilaments play a key role in cell division.
  • A band of actin contracts, causing the two new cells to “pinch” apart
• Intermediate Filaments

  Subunits (“Building Blocks”):

  • Diverse, cell - type - specific protein subunits

  Major Intermediate Filament Function:

  • Provide mechanical strength to cell