Another group of organelles bounded by an envelope:
Plastids.
 THE ‘type’organelle of plants!!
A general category of oval to lens shaped organelles.
 Surrounded by a double membrane envelope.
 The most prominent plastid is called the CHLOROPLAST.
            (Fig. 3.11)
  Main function is photosynthesis.
  Up to 100 chlorplasts per cell in leaves.
  Generally, around 4-6 um in length.
  Internally, chloroplasts are a 2 phase organelle.
   Water soluble phase, or STROMA.
   Internal membranous sacs called THYLAKOIDS.
 Other kinds of plastids:
  leucoplasts, chromoplasts (Fig. 3.12), proplastids.
 
All of the plastids in a plant derive from proplastids in the egg.
   Plastids DIVIDE.
 Plastids evolved from a primitive ENDOSYMBIONT,
   probably around 1.5-2 billion years ago.
  The evidence for this is overwhelming.

Mitochondria   (Fig. 3.13).
 Surrounded by a double membrane.
 Approximately 0.5-1.0 um in length.
 Like plastids, a 2 phase internal organization.
  Water soluble phase, or MATRIX.
  Internal, sac-like membranes called CRISTAE.
 These are the respiratory organelles of the cell. They generate energy
                  in the form of ATP needed to drive metabolism.
Mitochondria also originated evolutionarily by endosymbiosis of a primitive bacterium by a host cell.

There are lots of other organelles, vesicles and other membranous  compartments in plant cells:
 E.g. Endoplasmic reticulum, Golgi apparatus, ribosomes

The CYTOSKELETON.
 Fibrous structures only found in Eukaryotes
  Plants have two broad kinds, each composed of specific proteins.
They are VERY important, and serve several functions.
 E.g. motility and locomotion
  Cilia and flagella.
   E.g. in algae, sperm of certain land plants.
  Mitosis and meiosis.
   Chromosome separation.
  Cell shape determination.
   The cell wall connection.

1. Microtubules
 Long flexuous tubes, 25 nm in diameter, indeterminate length.
 Composed of the protein TUBULIN.
  Tubulin is a globular protein.
   Many tubulin subunits associate to make a microtubule.
     (Fig. 3.15)

 In cilia and flagella.
  Wave-like motion moves algal cells, plant sperm cells.
 Spindle fibers of the MITOTIC APPARATUS.
 Cortical microtubules.  (Fig. 3.15)
  Determine location of wall synthesis and orientation of new cellulose microfibrils.
  That in turn determines cell shape.   Why? How?

Microtubules are said to occur in groups or assemblies called ARRAYS.
 E.g.The whole mitotic spindle, with individual spindle fibers composed of many microtubules, is an array.
    The cortical microtubules of a cell comprise another array.
  Each array performs certain functions.
Microtubule arrays undergo a generalized cycle during the life of a plant cell depending on what the cell is doing.

Microtubules are just skeletal structures, like bones.
  To carry the analogy further, what actually causes movement  in our bodies?  Bones and muscles.
  Microtubules are like bones, and require something analogous to muscles to perform movement.
        These are specific protein machines, called 'motors', that run on energy.

Composition
 Lipid bilayer and associated proteins.
  Phospholipids.
Phospholipids are the principle components of  biological membranes, and are responsible for
 membrane properties.
  A class of lipids with a highly charged, or POLAR, phosphate group attached to one end.
    This adds a specific property to the molecule. It is NEGATIVELY charged.
Because it is charged, this end interacts well with water and is said to be HYDROPHILIC.
The other end consists of two 'tails' each composed of nonpolar or uncharged carbon chains. This end hates or fears water, so it's
  HYDROPHOBIC.

In other words, the phosphate group and tails have very different chemical properties. Phospholipids have a SPLIT PERSONALITY.

This property of phospholipids is quite special, and important.
Membranes consist of two layers of phospholipids ‘cemented’ together. The fatty acid chains, being hydrophobic, interact with each other rather than water on the outside of the cell and in the cytoplasm, and so face the middle of the membrane. The  hydrophilic phosphate groups face the aqueous, or water filled medium to either side of  the membrane.
This arrangement is called a lipid BILAYER. (Fig. 3.7)
 The bilayer has fluid properties, like oil. The phospholipids are mobile -- they move around.