Chloroplast Diagram: Label the Parts and Learn What Each Does

What a chloroplast diagram shows

A chloroplast is an organelle, a tiny working part inside a cell. It is where plants and algae turn light energy into chemical energy stored in sugar. A chloroplast diagram cuts one open so you can see its layers, the way a drawing of an orange might show the peel, the white pith and the segments.

Chloroplasts are small: a few micrometers long, which is a few thousandths of a millimeter, so many of them fit inside a single leaf cell. You need a microscope to see them at all, and an electron microscope to see the folded membranes inside. That is why a clear diagram is so helpful.

The diagram above shows four parts you can tap, plus the five steps of photosynthesis that happen inside them:

Parts of a chloroplast, one at a time

The envelope

The envelope is a double layer: an outer membrane and an inner membrane, with a thin space between them. The outer membrane lets many small molecules pass. The inner membrane is choosier, and it uses special transport proteins to move sugars and other molecules in and out. Think of it as a building with an outer fence and a front door with a guard.

The stroma

The stroma is the thick, protein-rich fluid that fills the chloroplast around the thylakoids. It is the chloroplast's kitchen. The enzymes of the Calvin cycle work here, including RuBisCO, the enzyme that grabs carbon dioxide from the air. RuBisCO is often called the most abundant protein on Earth because every plant leaf contains so much of it.

The stroma also holds things that surprise most students: the chloroplast's own circular DNA, its own ribosomes for making some of its proteins, and grains of starch, which is how the chloroplast stores extra sugar for later.

Thylakoids and grana

Thylakoids are flattened sacs of membrane, shaped like coins. Each one surrounds a small inner space called the lumen. Thylakoids stack on top of each other like a roll of coins, and each stack is a granum. Thin sheets of membrane, sometimes called stroma lamellae, connect one granum to the next, so the whole thylakoid system is really one connected network.

Why stack them? The light reactions happen in the thylakoid membrane itself, so more membrane means more room for chlorophyll and the proteins that use light. Stacking packs a large area of membrane into a tiny space, the same way folding a long sheet of paper fits it into a small drawer.

Built into the thylakoid membranes are the main machines of the light reactions: photosystem II, an electron transport chain, photosystem I and ATP synthase, a protein that spins like a tiny turbine to make ATP.

What happens where: thylakoids versus stroma

Photosynthesis has two stages, and the chloroplast keeps them in different places. This is the single most important idea a chloroplast diagram teaches.

Here are the five steps you can put in order in the Order it activity:

1. Light absorbed. Chlorophyll in the thylakoid membranes absorbs light energy.
2. Water split, oxygen released. That energy is used to split water molecules. The oxygen leaves the leaf as a gas.
3. ATP and NADPH made. Electrons from water travel along the membrane, and the energy they carry is stored in two "rechargeable battery" molecules, ATP and NADPH.
4. Calvin cycle. In the stroma, RuBisCO attaches carbon dioxide to a five-carbon molecule, and the cycle uses ATP and NADPH to rebuild it into sugar.
5. Sugar made. The sugar is used for energy, turned into starch for storage, or built into cellulose for cell walls.

The two stages depend on each other. If the light goes out, the thylakoids stop making ATP and NADPH, and the Calvin cycle soon stops too. That is why scientists now prefer "light-independent reactions" to the old name "dark reactions": the Calvin cycle doesn't need darkness, it just doesn't use light directly.

Why chloroplasts are green, and why leaves change color

Chlorophyll absorbs red and blue light strongly and reflects green, so leaves look green. Chloroplasts also hold yellow and orange pigments called carotenoids, which help collect light and protect the chlorophyll.

In autumn, many trees stop making new chlorophyll and break down what they have. As the green fades, the yellow and orange carotenoids that were there all along show through. The USDA Forest Service explains that the reds and purples in some trees come from a different set of pigments, anthocyanins, which many leaves make in the fall.

Chloroplast vs mitochondria

Plant cells have both chloroplasts and mitochondria, and the two are easy to mix up. One stores energy in sugar; the other releases it.

Notice that the inputs of one are the outputs of the other. A plant cell does both at once during the day, and only respiration at night.

Where chloroplasts came from

Chloroplasts have their own DNA, their own ribosomes and a double membrane, and they divide by splitting in two, much like bacteria. These clues support the endosymbiotic theory: long ago, an early cell swallowed a photosynthetic bacterium, related to today's cyanobacteria, and instead of digesting it, kept it. Over time the bacterium became the chloroplast. OpenStax notes that mitochondria likely arose the same way from a different kind of bacterium.

Most of the genes a chloroplast needs have since moved to the cell's nucleus, so a chloroplast can no longer live on its own. It still keeps a small circle of DNA with a little over a hundred genes in most land plants.

Where chloroplasts are in a plant

Most chloroplasts sit in the leaf's mesophyll cells, especially the tall, tightly packed palisade cells just under the top surface, where the light is strongest. Green stems have some too. Roots, which grow in the dark, usually have none. Guard cells around the stomata have chloroplasts, but the cells of the leaf's clear outer skin, the epidermis, mostly don't.

To see where a chloroplast fits in the whole picture, open the interactive plant cell diagram, then the leaf view of the photosynthesis diagram, which follows light, water and carbon dioxide into the leaf. Comparing a plant cell with the animal cell diagram shows the three parts animal cells lack: the cell wall, the large central vacuole and chloroplasts.

How to teach the chloroplast

• Grades 5 and 6: Stick to the big idea and three parts: the envelope keeps things in, the thylakoid stacks catch light, and the stroma makes sugar. Use the Kid depth and the Explore mode.
• Middle school: Add the two stages and where each happens. Have students fill in the thylakoid-versus-stroma table from memory, then check it here.
• High school: Use the High school depth for photosystems, the electron transport chain, ATP synthase and RuBisCO, and connect the Calvin cycle to cellular respiration.

A simple model helps at every age. Stack coins or pancakes for grana, set them in a zip-top bag of clear gel for the stroma, and put that bag inside a second bag for the double envelope. Then print the labeled chloroplast diagram and finish with the photosynthesis worksheet. If you are choosing a full course, our guide to the best homeschool science curriculum compares programs that cover cells and photosynthesis in depth.