There are other membrane bound organelles that are NOT part of the endomembrane system:

• Mmitchondria

• Peroxisomes

• Chloroplasts

Mitochondria and chloroplasts are evolutionary remnants of bacteria that became symbiotic with early single celled eukaryotes

• Endosymbiotic theory

• Mitochondria and chloroplasts still have a small amount of DNA in them, and ribosomes 

  • Mitochondrial and chloroplast genes are more similar to bacterial gene than eukaryotic genes, key piece of evolutionary evidence

Mitochondria

• Generates nearly all of the ATP for eukaryotic cells

  • Kreb’s cycle and electron transport chain of cellular respiration

  • In prokaryotes, processes are done in plasma membrane

• In mitochondria, (inner) membrane became bigger and extensively folded

  • Provides much more SA for generation ATP

• Infoldings (christae) produce the intermembrane space and the mitochondrial matrix

  • Electron transport chain sees H cation pass between these two species, generates ATP

• Many mitochondria per cell (even more in cells that require lots of energy)

Chloroplasts

• Are also products of endosymbiosis

  • Blue-green algae (cyanobacteria) invaded an early plant cell

  • Also have DNA and ribosomes

  • Two outer membranes, and third membrane system (thylakoids)

    • Convoluted membrane forms flattened sacs, and stacked (grana)

    • Surrounded by a fluid (stroma)

• Is one of the plastids, others are:

  • Amyloplasts ; stores starch in roots and tubers

  • Chromoplasts ; storage of pigments

Peroxisomes

•  Beta-oxidation (catabolism) of very long fatty acid chains

• Contians enzymes (oxiadases) that strip hydrogens from various compounds and transer them to O2, making H2O2

  • E.g detoxification of alcohol in the liver

  • E.g uric acid oxidase

    • significantly , this is missing in humans and other primates

      • Another example of synapomorphy, indicating evolutionary relatedness

      • Buildup of uric acid lead to gout

  • Peroxide is also toxic, but peroxisomes contain enzymes (catalases) that convert it to water

• Peroixsomes the site of synthesis of plasmalogen, a phospholipid critical to the myelin sheath of neurons

• Peroxisomes are bounded by a single membrane

• Formed by pinching-off from pre-existing peroxisomes after they have grown larges

Cytoskeleton

• A network of fibres that give the ccell some structural rigidity and regularity, and allow for various types of cellular movement

  • Microtubules

    • Straight, hallow rods of repeating tubulin molecules

    • 25nm in diamete

    • 200 - 25,000nm in length

    • Alpha tubulin and beta tubulin subunits

    • Assembled in centrosome

      • Act as rails for organelles which are pulled along by “motor” proteins (e.g kinesin), require energy

    • Make up the centrioles of centrosomes

      • Nine triplets of microtubules arranged in a ring

      • Often have role in organizing microtubules of spinde in mitosis and meiosis

      • Base of flagella and cilia of eukaryotic cells

  • intermediate filaments

    • Composed of diverse keratin proteins 

    • More permanent than other two types

      • Basic framework for entire cytoskeleton

    • Used to fix organelle positions

    • Compose for nuclear lamina

  • Microfilaments

    • Composed of two long chains of actin monomers, wound into a helix

    • 7nm in diamete

    • Resist tensional stress on cell shape

      • Form a network just insed the plasma membrane

    • Support cells specialized for membrane transport

      • E.g microvilli of intestinal epithelial cells

    • Muscle contraction

      • Strands of myosin (also a motor molecule) “walk” along actin filaments, causing shortening the muscle cell, thus contraction works

      • Powered by ATP

    • Actin-myosin complesxes are also responsible for:

      • Pinching an animal cell in two during division (at cleavage furrow)

      • Action of pseudopodia during movement in amoebae

      • Cytoplasmic streaming in plant cells

Cilia and flagella - responsible for locomotion of eurkaryotic cells

• Cilia - 2-20nm in length, numerous on cell surface, work like an oar

• Flagella - 10-220nm in length, one or a few at most per cell, undulating motion

• Both have similar ultrastructure

  • Outer surface is an extension of plasma membrane 

  • Inside is a “9+2” arrangement of microtubules

  • These are connected to center by radial spokes

  • Each set of outer doublets connected to the next dynein sidearms

• Deynein is a motor molecule

  • Motion of flagella and cilia occurs by dynein “walking” along the next doublet, powered by ATP

• Flagela and cilia are anchored to the cell by a basal body

  • Structurally identical to a centriole

Cell Surfaces

• Plant cells have a thick cell wall

• Cellulose in a matrix of other polysaccharides and proteins

• Primary wall is laid down first 

• Middle lamella (pectins) found between adjacent cells

• Secondary walls (maybe several) laid down as the cell stops growing; comprise wood

• Animal cells have an extracellular matrix (ECM)

  • ECM is involved in the support, adhesion, movement and regulation of cellular and gene function

  • ECM contains a great deal of collagen (half of total body proteins)

    • Connected to cell by fibronecetin 

    • Enmeshed in a series of proteoglycan complexes

    • Mechanical stimuli on the ECM communicated to the cytoskeleton via integrin proteins

• Adjacent cells can communicate with each other directly

• Plasmodesmata between plant cells 

  • Water and small solutes can pass 

• Animal cells have:

  • Gap junctions: equivalent to plasmodesmata

  • Desmosomes: fastens cells tightly together

    • Anchors of intermediate filaments

  • Tight junctions: sealants, prevents leakage of fluid from top of a cell layer to bottom