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Plastids

Plant cells possess a variety of membrane-bound organelles that have specific functions. Although most organelles are small and difficult to see with light microscopy, plastids are large and fairly obvious. They function as synthetic and storage structures.

In identifying plant cells, plastids of various types provide insight into cell activities. For example, leaf cells are photosynthetic and therefore contain an abundance of light-gathering plastids called chloroplasts. Chloroplasts are also found in cells forming the actively growing "green" shoots and young stems of plants. Root cells on the other hand, are engaged in nutrient acquisition and storage. It would be a waste for these cells to possess photosynthetic chloroplasts underground. Root cells therefore, commonly contain storage plastids or amyloplasts.

The chloroplast can be used as a model to highlight features of a typical plastid. Note these are double membrane-bound structures. The inner membranes typically possess the important enzymes that determine what plastids do. In chloroplasts, thylakoid membranes of stacked, flattened grana possess the chlorophylls and enzymes used for photosynthesis.

Plastids are self-replicating structures with circular DNA. In young, embryonic cells all plastids began as proplastids. Proplastids replicate to produce all types of plastids. Roll your mouse over the top granum on the far right stack of grana to see some examples of plastids.

 

 

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Chloroplasts in a Corn Stem

Visualize a young, actively growing corn plant and you will agree the stems are green and therefore photosynthetic. In this microscopic view of a corn stem, note the presence of green chloroplasts in most cells. These are also pressed to the periphery by central vacuoles. Most of the cells shown here possess thin cell walls and are therefore parenchyma cell types. Parenchyma cells with chloroplasts are more specifically named chlorenchyma. We will learn more about these later.

The circular structure to the right is a vascular bundle that transports water and sugars through the stem. Larger, green cells around the perimeter of this bundle are bundle sheath cells. These specialized cells play a key role in the photosynthetic efficiency of the corn plant.

Now, roll your mouse through the most central parenchyma cell along the top border of the image.

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Amyloplasts in Ranunculus Root Cells

A plastid commonly found in root cells is the amyloplast, a storage site for starch molecules. Depending on the stain, starch granules in an amyloplast can appear in various colors. Here these appear as dark purple or black granules within cells.

Now, roll over the largest amyloplast you can find in the image.