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Chloroplasts and Other Plastids

Plastid is the collective name given to a series of organelles found exclusively in plant (and some protozoan) cells. At one level or another these organelles are responsible for harvesting light energy, converting it to chemical energy, processing this chemical energy into storage molecules and then packaging this stored energy in a form that can be kept for long periods of time.

The central organelle in this complex energy harvesting process is the chloroplast . Like the mitochondrion, a chloroplast has two membranes; an outer membrane which defines the shape of the organelle and a series of inner membranes that form flattened disk-shaped sacs called thylakoids. At times of high activity, thylakoids stack up one on top of another to create structures called grana. Embedded in the membranes of the grana are photosynthetic pigments which begin the energy trapping process by harvesting light.

In a complex series of energy transfer reactions, light energy is converted to electron energy, and as the electron is passed from one type of pigment to another, this energy is used to pump hydrogen ions across the grana membrane. As the hydrogen ions return across the membrane ATP is formed. This form of short term energy storage is only the beginning. The liquid stroma between the membranes contains many dissolved enzymes that use the ATP energy to create sugars and eventually starch molecules. These are the long term energy storage molecules.

Chromoplasts are other plastids which are common in plant cells and are used to store red, yellow and orange pigments. In large numbers these chromoplasts give fruits their characteristic colors. Leucoplasts have no color, but are created in storage organs, such as potatoes and used for starch storage.

Science@a Distance
© 2002, Professor John Blamire