How big is Chloroplast? | Scale of the Universe

A chloroplast is the part of a plant cell that captures the energy of sunlight and uses it to turn carbon dioxide into sugar, the process called photosynthesis. In plant leaves, chloroplasts are shaped like lenses or American footballs, typically about 5 micrometers long. A micrometer is a millionth of a meter, or a thousandth of a millimeter.

Size

Because a chloroplast is flattened, its size is given as its length or diameter, not its thickness. In vascular plants, which include ferns, conifers and flowering plants, chloroplasts have a typical diameter of about 4–6 micrometers. Leaf chloroplasts are usually described as about 5 micrometers long and 1–2 micrometers thick.

Size also varies between species. Chloroplasts in corn seedlings have an average volume of about 20 cubic micrometers, where a cubic micrometer is a cube 1 micrometer on each side. In the leaves of Arabidopsis, a plant widely used in research, the average is 93 cubic micrometers, more than four times as much. In algae, chloroplasts take more varied forms, including tubes and branching networks.

Size comparisons

A chloroplast 5 micrometers long is about two-thirds the width of a red blood cell, which is 7–8 micrometers across.
About 16–20 chloroplasts, lined up end to end, would span the width of a human hair, which is 80–100 micrometers wide.
A chloroplast in a corn seedling has about 20 times the volume of an E. coli bacterium, which is about 1 cubic micrometer.
A single photosynthetic cell in the leaf of a plant such as Arabidopsis or wheat holds about 100 chloroplasts.
If a chloroplast were enlarged to the width of a basketball, about 24 centimeters (9.4 inches), each of the 40–60 stacks of membranes inside it would be 1.4–2.9 centimeters (0.6–1.1 inches) across.

How a chloroplast works

A chloroplast is wrapped in two membranes, an outer one and an inner one. Inside is a third system of membranes, the thylakoids, piled in stacks called grana. The thylakoid membranes are packed with the equipment that captures light, including chlorophyll, the green pigment. Chlorophyll is the most abundant pigment in living things.

The energy captured by the thylakoids drives reactions in the watery space around them. There an enzyme called Rubisco captures carbon dioxide for the reactions that build sugar. Rubisco fills almost all of that space, and it is the most abundant soluble protein in living things. The sugar is stored as grains of starch.

Descended from bacteria

Chloroplasts are thought to descend from cyanobacteria, bacteria that carry out photosynthesis. Somewhere between about 1 and 1.5 billion years ago, another cell engulfed one of them, and over time the two came to work closely together. The chloroplasts of green algae, red algae and all plants trace back to that event.

Chloroplasts still carry some DNA of their own, about 100,000 base pairs long, in tens to hundreds of copies each. Their ancestors probably had more than 3,000 genes. Most of those genes moved into the cell's nucleus, and only about 130 remain in the chloroplasts of plants today.