Chloroplast | Definition, Function, Structure, Location, & Diagram

Structural features

Chloroplasts are roughly 1–2 μm (1 μm = 0.001 mm) thick and 5–7 μm in diameter. When thin sections of a chloroplast are examined under the electron microscope, several features are apparent. Chief among these are the intricate internal membranes (i.e., the lamellae) and the stroma, a colorless matrix in which the lamellae are embedded. Also visible are starch granules, which appear as dense bodies.

Chloroplasts are enclosed in a chloroplast envelope, which consists of a double membrane with outer and inner layers, between which is a gap called the intermembrane space. A third, internal membrane, extensively folded and characterized by the presence of closed disks (or thylakoids), is known as the thylakoid membrane. In most higher plants, the thylakoids are arranged in tight stacks called grana (singular granum). Grana are connected by stromal lamellae, extensions that run from one granum, through the stroma, into a neighboring granum. The thylakoid membrane envelops a central aqueous region known as the thylakoid lumen. The space between the inner membrane and the thylakoid membrane is filled with stroma, a matrix containing dissolved enzymes, starch granules, and copies of the chloroplast genome.

A single lamella, which contains all the photosynthetic pigments, is approximately 10–15 nanometers thick. The lamellae exist in more-or-less flat sheets, a few of which extend through much of the length of the chloroplast. Examination of cross sections of lamellae under the electron microscope shows that their edges are joined to form closed hollow disks that are called thylakoids (“saclike”). The chloroplasts of most higher plants have regions, called grana, in which the thylakoids are very tightly stacked. When viewed by electron microscopy at an oblique angle, the grana appear as stacks of disks. When viewed in cross section, it is apparent that some thylakoids extend from one grana through the stroma into other grana. The thin aqueous spaces inside the thylakoids are believed to be connected with each other via these stroma thylakoids. These thylakoid spaces are isolated from the stroma spaces by the relatively impermeable lamellae.

Lamellae consist of about equal amounts of lipids and proteins. Most of the lipids are polar compounds, such as phospholipids and galactolipids, that form the lipid bilayer of the membrane. Approximately one-fifth of the lamellar lipids are chlorophyll molecules; one type, chlorophyll a, is more abundant than the second type, chlorophyll b. These chlorophyll molecules are specifically bound to small protein molecules and act as “light-harvesting” pigments. These absorb light and pass its energy on to special chlorophyll a molecules that are directly involved in the conversion of light energy to chemical energy. In addition to the chlorophyll-protein complexes, other lamellar proteins include enzymes and protein-containing coenzymes, which serve as organic catalysts for specific reactions within the lamellae. Finally, small molecules called plastoquinones are also found in substantial numbers in the lamellae. They are lipid soluble, meaning they can diffuse through the membrane, and help carry electrons between the components of the light reactions.

Explore Britannica Premium!

The trusted destination for professionals, college students, and lifelong learners.

SUBSCRIBE