13/11/2015 · Plant Cell Structure
Cells are living entities made up of a central nuclear region which contains the hereditary material, surrounded on all sides by cytoplasm, which, encompassed by a delimiting membrane, contains all the structures required for biological processes, such as making protein and extracting utilizable energy from food. These events may occur in separate compartments, the organelles, which include the endoplasmic reticulum, the Golgi apparatus, the mitochondria, the chloroplasts and the lysosomes; there is also an internal cytoskeleton.
In higher plants, chloroplast ..
Mitochondria have been obtained from a large number of plant sources including roots, storage tissue, stems and photosynthetic tissues. The usual problems of isolation, regardless of the source, are (a) the rupture of a rather rigid cell wall and (b) the prevention of damage to organelles through the release of intracellular, particularly vacuolar, contents. Ikuma (1970) listed a number of conditions for successful isolation of tightly-coupled mitochondria. These include (a) gentle tissue disruption, (b) rigorous exclusion of contaminating particles and (c) the use of a buffered grinding medium isotonic with mitochondria and containing a variety of protective reagents. Most investigators employ some device to reduce quickly the tissue to a coarse slurry, which is passed through a cloth filter to remove large debris. The fraction which sediments between 1,000 and 10,000 is collected as the mitochondrial fraction. This fraction will oxidize all the intermediates of the Krebs tricarboxylic acid cycle, exhibit respiratory control and yield ADP to O ratios approaching the theoretical value for the substrate used. The mitochondrial fraction can be further purified by density gradient centrifugation. This may be done in discontinuous sucrose gradients (Baker 1968; Douce 1972a) or Dextran-40 gradients (Solomos 1973). Mitochondria form a band at the interface between 1.2 and 1.5 sucrose (Douce 1972a). This is recovered and diluted slowly to 0.3 sucrose. This procedure yields mitochondria with intact outer and inner membranes as shown by electron microscopy. The integrity of the outer membrane is also shown enzymatically by the inability to reduce exogenous cytochrome with NADH or succinate as substrates, unless the mitochondria have been subjected to mild osmotic shock which renders the outer membrane permeable to high molecular weight solutes.
Plastids are organelles which are bounded by double membranes and which occur, as far as is known, in all cells of eukaryotic green plants at some stage, usually becoming modified according to their function. In their undifferentiated form they may remain as proplastids, which are characteristic of epidermal and meristematic cells, for example. In the green parts of plants the proplastids normally develop into chloroplasts, which are the site of photosynthesis, while in starchstoring organs they form amyloplasts which produce the starch grains. However these two functions are not mutually exclusive as most chloroplasts will form starch under appropriate physiological conditions and the exposure of starch-storing organs to illumination results in the amyloplasts forming some thylakoids and chlorophyll. In certain plant parts, such as flowers, fruits and some leaves, the thylakoids of the chloroplasts become degraded, forming chromoplasts, which contain large amounts of carotenoids, the pigments responsible for 'autumn colouration' and the characteristic colours of certain flowers and fruits.