Mitochondria and plastids

Two organelles with their own DNA

Most organelles of a eukaryotic cell belong to the endomembrane system of the last course. Mitochondria and plastids do not. Each is wrapped in two membranes of its own. Each carries its own DNA and its own ribosomes, and each can divide.

Mitochondria

Mitochondria (singular, mitochondrion) are hard to see in an ordinary light microscope. They show up only after special staining.

• How many. The number varies with the work a cell does. A busy cell has many, a quiet one few.
• Shape and size. They are usually sausage-shaped or cylindrical. They are $0.2$ to $1.0$ µm across, about $0.5$ µm on average, and $1.0$ to $4.1$ µm long.

Each mitochondrion is a double-membrane-bound structure.

• The outer membrane is smooth. It forms the boundary.
• The inner membrane folds inwards again and again. The folds are called cristae (singular, crista).
• The two membranes divide the inside into two watery compartments. The outer compartment lies between them. The inner compartment is filled with a dense, even material, the matrix.
• Each membrane carries its own enzymes, suited to its own work.

Mitochondria are the sites of aerobic respiration. They release energy from food and store it as ATP. That is why they are called the power houses of the cell.

The matrix holds a single circular DNA molecule, a few RNA molecules and 70S ribosomes. It also holds what is needed to make proteins. Mitochondria multiply by fission, splitting in two.

Why fold the inner membrane?

The enzymes that make ATP sit on the inner membrane. More membrane means room for more of them. Folding packs a long membrane into a small space, without making the mitochondrion any bigger.

The simulation shows this in a cross-section of one mitochondrion. The outer outline stays at $5.6$ µm. The inner outline, flat, would be $5.2$ µm. Every crista adds two walls.

Plastids

Plastids are found in all plant cells and in euglenoids. They are large, so they are easy to see under a microscope. Many carry pigments, which give a plant part its colour. By what they hold, plastids are of three kinds.

• Chloroplasts hold chlorophyll and carotenoid pigments. They trap light energy for photosynthesis.
• Chromoplasts hold fat-soluble carotenoid pigments, such as carotene and xanthophylls. These give a part its yellow, orange or red colour, as in the skin of a ripe tomato.
• Leucoplasts have no colour. They store food, and they come in three kinds.
  • Amyloplasts store carbohydrates, mainly starch, as in a potato tuber.
  • Elaioplasts store oils and fats.
  • Aleuroplasts store proteins.

The chloroplast

Most chloroplasts of green plants lie in the mesophyll cells of the leaves. They may be lens-shaped, oval, spherical, discoid or even ribbon-like. They are $5$ to $10$ µm long and $2$ to $4$ µm wide. Chlamydomonas, a green alga, has just one in each cell. A mesophyll cell may have 20 to 40.

• Like a mitochondrion, a chloroplast has two membranes. The inner one lets fewer things through.
• The space inside the inner membrane is the stroma.
• In the stroma lie flat, membranous sacs, the thylakoids. They are piled like coins in stacks. Each stack is a granum (plural, grana).
• Flat membranous tubules, the stroma lamellae, join the thylakoids of different grana.
• The membrane of each thylakoid encloses a space, the lumen.
• The chlorophyll pigments are in the thylakoids.
• The stroma holds the enzymes that make carbohydrates and proteins. It also holds small, double-stranded circular DNA molecules and 70S ribosomes.

The two compared

• Membranes. Each has two.
• The fluid inside. A mitochondrion has a matrix, and a chloroplast has a stroma.
• Inner membranes. A mitochondrion has cristae. A chloroplast has thylakoids, stacked in grana.
• DNA and ribosomes. Each has circular DNA and 70S ribosomes.
• Work. A mitochondrion carries out aerobic respiration. A chloroplast carries out photosynthesis.

Because each has its own DNA and ribosomes, it can make some of its own proteins. So these two are called semi-autonomous organelles.

Try it in Python

The program names the plastid that does each job in three plant parts.

Practise with new numbers

Common mistakes

• "Mitochondria are found only in animal cells." Plant cells respire too, so they have mitochondria as well as plastids.
• "The cristae are folds of the outer membrane." They are folds of the inner membrane.
• "All plastids are green." Only chloroplasts are green. Chromoplasts are yellow, orange or red, and leucoplasts have no colour.
• "Grana and stroma are the same thing." Grana are stacks of thylakoids. The stroma is the fluid around them.
• "A potato stores its starch in chloroplasts." It stores it in amyloplasts, a kind of leucoplast.

Remember these

• A mitochondrion: two membranes, cristae from the inner one, outer compartment and matrix.
• Size: $0.2$ to $1.0$ µm across and $1.0$ to $4.1$ µm long.
• Mitochondria are the sites of aerobic respiration and make ATP, the power houses of the cell.
• Plastids: chloroplasts (chlorophyll), chromoplasts (carotene, xanthophylls) and leucoplasts (stores).
• Leucoplasts: amyloplasts (starch), elaioplasts (oils and fats), aleuroplasts (proteins).
• A chloroplast: two membranes, stroma, thylakoids stacked in grana, stroma lamellae between grana.
• Both organelles have circular DNA and 70S ribosomes, and both divide.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the two compartments of a mitochondrion. Which one holds its DNA?

2. Why are mitochondria called the power houses of the cell?

3. A plant part is bright orange. Which plastid gives it the colour, and which pigments does it hold?

4. What is a granum, and what joins one granum to the next?

5. Give two features that mitochondria and chloroplasts share and other organelles lack.