Delphinidin

Most living plant cells have several kinds of plastids, with the chloroplasts being the most conspicuous. They occur in a variety of shapes and sizes, such as the beautiful corkscrew-like ribbons found in cells of the green alga Spirogyra  and the bracelet-shaped chloroplasts of other green algae, such as Ulothrix. The chloroplasts of higher plants, however, tend to be shaped somewhat like two Frisbees glued together along their edges, and when they are sliced in median section, they resemble the outline of a rugby football.

Although several algae and a few other plants have only one or two chloroplasts per cell, the number of chloroplasts is usually much greater in a green cell of higher plants. Seventy-five to 125 is quite common, with the green cells of a few plants having up to several hundred. The chloroplasts may be from 2 to 10 micrometers in diameter, and each is bounded by an envelope consisting of two delicate membranes. The outer membrane apparently is derived from endoplasmic reticulum, while the inner membrane is believed to have originated from the cell membrane of a cyanobacterium.

Within the chloroplast are numerous grana (singular: granum), which are formed from membranes and have the appearance of stacks of coins with double membranes. There are usually about 40 to 60 grana linked together by arms in each chloroplast, and each granum may contain from two or three to more than 100 stacked thylakoids. In reality, thylakoids are part of an overlapping and continuous membrane system suspended in the liquid portion of the chloroplast.

The thylakoid membranes contain green chlorophyll and other pigments. These “coin stacks” of grana are vital to life as we know it, for it is within the thylakoids that the first steps of the important process of photosynthesis. In photosynthesis, green plants convert water and carbon dioxide (from the air) to simple food substances, harnessing energy from the sun in the process. The existence of human and all other animal life depends on the activities of the chloroplasts.

The liquid portion of the chloroplast is a colorless fluid matrix called stroma, which contains enzymes involved in photosynthesis. Genes in the nucleus dictate most of the activities of chloroplasts, but each chloroplast contains a small circular DNA molecule that encodes for production of certain proteins related to photosynthesis and other activities in the chloroplast and cell. The chloroplast also contains RNA and ribosomes, which facilitate some protein synthesis.

Some plastids (e.g., those of tobacco) store proteins. There are usually four or five starch grains in the stroma, as well as oil droplets and enzymes. Chromoplasts are another type of plastid found in some
cells of more complex plants. Although chromoplasts are similar to chloroplasts in size, they vary considerably in shape, often being somewhat angular. They sometimes develop from chloroplasts through internal changes that include the disappearance of chlorophyll.

Chromoplasts are yellow, orange, or red in color due to the presence of carotenoid pigments, which they synthesize and accumulate. They are most abundant in the yellow, orange, or some red parts of plants, such as ripe tomatoes, carrots, or red peppers. These carotenoid pigments, which are lipid soluble, are not, however, the predominant pigments in most red flower petals. The anthocyanin pigments of most red flower petals are water soluble.

Leucoplasts are yet another type of plastid common to cells of higher plants. They are essentially colorless and include amyloplasts, which synthesize starches, and elaioplasts, which synthesize oils. If exposed to light, some leucoplasts will develop into chloroplasts, and vice versa.

Plastids of all types develop from proplastids, which are small, pale green or colorless organelles having roughly the size and form of mitochondria. They are simpler in internal structure than plastids and have fewer thylakoids, the thylakoids not being arranged in grana stacks. Proplastids frequently divide and become distributed throughout the cell. After a cell itself divides, each daughter cell has a proportionate share. Plastids also arise through the division of existing mature plastids.