P1: CELLS AND ORGANELLES

• Cells


  • Organizational complexity
  • Molecular components
  • Sizes and shapes
  • Specialization
• Types of organisms


  • Bacteria
  • Archaea
  • Eukaryotes

  
• Prokaryotes


  Simpler type of cells characteristic of bacteria
• Eukaryotes


  More complex type of cells characteristic of plants, animals, fungi, algae and protozoa

  
• Distinction between prokaryotic and eukaryotic cells


  Eukaryotic cells have a membrane-bounded nucleus

  
• The term 'prokaryote' is unsatisfactory in describing the non-nucleated cells
• Sharing of a gross structural feature is not necessarily evidence of relatedness
• Prokaryotic cells can be divided into the widely divergent bacteria and archaea based on rRNA sequence analysis

  
• Bacteria and archaea are as divergent from one another as humans and bacteria are

  
• Three domains


  • Archaea
  • Bacteria
  • Eukarya (eukaryotes)

  
• Bacteria


  • Escherichia coli
  • Pseudomonas
  • Streptococcus
• Archaea


  Originally called archebacteria before they were discovered to be so different from bacteria
• Types of archaea


  • Methanogens
  • Halophiles
  • Thermacidophiles
• Archaea and Eukarya have a common ancestor which means that they are much more related as compared to archaea and bacteria

  
• Cell size limitations


  • Need for adequate surface area relative to volume
  • Rates at which molecules can diffuse
  • Need to maintain adequate local concentrations of substances required for necessary cellular functions
• As cell size increases

  Volume increases with the cube of its length, but surface area increases with the square of its length, so larger cells have proportionately smaller surface areas

  
• Cells specialized for absorption


  • Have characteristics to maximize surface area/volume ratio, e.g. microvilli in cells lining the small intestine

  
• Diffusion


  The unassisted movement of a substance from a region of high concentration to a region of low concentration
• Eukaryotic cells


  Can avoid the problem of slow diffusion rates by using carrier proteins to actively transport materials through the cytoplasm, using cytoplasmic streaming, or moving molecules through the cell in vesicles transported along microtubules
• For a reaction to occur, the appropriate reactants must collide with and bind to a particular enzyme, and the frequency of such collisions is greatly increased by higher concentrations of enzymes and reactants
• Compartmentalization of activities within specific regions of the cell


  A solution to the concentration problem in eukaryotic cells
• Most eukaryotic cells have a variety of organelles, membrane-bounded compartments that are specialized for specific functions

  
• Presence of a membrane-bounded nucleus


  • Eukaryotic cells have a true, membrane-bounded nucleus with a nuclear envelope consisting of two membranes and a nucleolus, while the genetic information of bacterial or archaeal cells is folded into a compact structure called the nucleoid and is attached to the cell membrane

  
• Eukaryote organelles


  • Endoplasmic reticulum
  • Golgi complex
  • Mitochondria
  • Chloroplasts
  • Lysosomes
  • Peroxisomes
  • Various types of vacuoles and vesicles

  
• Cytoskeleton


  • Nonmembranous, proteinaceous structures for cellular contraction, motility and support, including microtubules, microfilaments, and intermediate filaments
• Exocytosis and endocytosis


  Processes involving membrane fusion events unique to eukaryotic cells that allow exchange of materials between compartments within the cell and the exterior
• Differences in DNA organization

  • Bacterial DNA: circular molecule associated with a few proteins
  • Eukaryotic DNA: organized into linear molecules complexed with large amounts of proteins (histones)
  • Archaeal DNA: circular molecule complexed with proteins similar to eukaryotic histone proteins
• The circular DNA of bacteria or archaea is much longer than the cell itself and must be folded and packed tightly
• Most eukaryotic cells have more than 1000 times more DNA than prokaryotes and encode only 5-10 times more proteins
• Junk DNA


  Excess noncoding DNA in eukaryotes that may have important functions in gene regulation and evolution
• Chromosomes


  The solution to the problem of DNA packaging in eukaryotes, containing equal amounts of histones and DNA
• Segregation of genetic information


  • Prokaryotes and archaea: Replicate DNA and divide by binary fission
  • Eukaryotes: Replicate DNA and then distribute chromosomes into daughter cells by mitosis and meiosis, followed by cytokinesis
• Eukaryotic cells have more than 1000 times more DNA than prokaryotes and encode only 5-10 times more proteins
• JUNK DNA


  The excess noncoding DNA but it may have important functions in gene regulation and evolution
• Chromosomes


  • They contain equal amounts of histones and DNA
  • The DNA is folded and compressed into chromosomes
• Prokaryotes and eukaryotes differ in


  How genetic information is allocated to daughter cells upon division
• Bacterial and archaeal cell division


  Replicate DNA and then distribute their chromosomes into daughter cells by binary fission
• Eukaryotic cell division


  Replicate DNA and then distribute their chromosomes into daughter cells by mitosis and meiosis, followed by cytokinesis
• Eukaryotic cells transcribe genetic information in the nucleus into large RNA molecules which are processed and transported into the cytoplasm for protein synthesis
• Bacteria transcribe genetic information into RNA and the RNA molecules produced may contain information for several polypeptides