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Cell Biology
The World of Cells
Plastids Plant cells can contain several different types of plastids. These are organelles such as the amyloplast, which is involved in starch storage, and chromoplasts which contain colored pigments.

However the most easily recognizable plastid is the chloroplast, a green organelle that harvests light, then uses the trapped energy to synthesize sugar molecules, which are then stored as starch.

Chloroplasts have an outer membrane enclosing a fluid matrix known as the stroma. Packed inside the stroma are membranous disks (thylakoids) which are assembled into piles called grana.

Linking the grana are tubules called stroma lammellae.

Sunlight striking a chloroplast is absorbed and the energy the sunlight contains is converted to electron energy in the grana membranes. This electron energy is eventually converted to chemical energy by molecules and enzymes in the stroma matrix of the chloroplast. The final product is usually the sugar glucose which is then polymerized into starch.

Use of sunlight to make sugar is called photosynthesis.

Figure legend: Chloroplast. The double-membraned organelles that carry out the process of photosynthesis.

© 2001, Professor John Blamire