A single liver cell can hold a thousand of them or more. Tiny grains, yet together they fill about 20% of the cell's volume. What do they do with all that room? They make the cell's energy, and every part of their structure is arranged for that one job.
1. The name and the history
There is a cytoplasmic organelle wrapped in a double membrane, where the Krebs cycle (also called the TCA or citric acid cycle) and electron transport take place and energy is produced. That is the mitochondrion. It is called the power house of the cell.
Von Kolliker discovered it with a light microscope in 1850. The naming story is a long one: Fleming called it fila in 1882, Altman called it bioplast in 1890, and in 1897 Carl Benda named it mitochondria. Years and names are easy to swap, so go through them one at a time on the line below.
historyput them in order
Who and when?
1850. Kolliker discovers mitochondria with a light microscope.
1882. Fleming names it fila.
1890. Altman names it bioplast.
1897. Carl Benda names it mitochondria. That is the name we use today.
The name comes from Greek: Mitos means thread and chondrion means grain. The singular is mitochondrion, the plural mitochondria.
2. How many, and what shape
Mitochondria make up about 20% of the cell's volume. The usual number is 300-400 per cell, and 1000 or more in a liver cell. They increase in number by dividing.
The shape may be round, rod-like, filamentous, star-like or coiled. The size follows the shape: a round one is 0.2-2.0 micron across, a rod type is up to 3 micron long and 0.5 micron wide, and a filamentous type is 80-90 micron long.
3. What is inside
The outer membrane is smooth and plain, and its job is protection. It holds no electron transport chain (ETC), no ATP synthase and no ATP-making enzymes. The gap between the two membranes is 6-8 nm.
The space between the two membranes is the outer chamber, or intermembrane space. The central region inside the inner membrane is the inner chamber, filled with a jelly-like matrix.
The inner membrane folds inward like fingers, and these outgrowths are the cristae. The space between cristae is the intracristal space, and it is joined to the outer chamber.
Tiny granules cling to the inside of the inner membrane: the oxisomes. A stalked oxisome has three parts, a head, a stalk and a base. The cristae carry ATP-synthase, where ATP is made, and many electron transport chains (ETC) run along them.
The matrix holds the mitochondrion's own circular DNA and 70S ribosomes. Both are prokaryotic features. Now look at the whole structure drawn after Figure 1.11, and tap each part.
Figure 1.11tap a point
What is inside
The outer membrane. Protective and smooth. It holds no ETC and no ATP synthase.
The inner membrane. It folds to form cristae, and oxisomes cling to its inner face.
The outer chamber. The gap between the two membranes, 6-8 nm wide. It is joined to the intracristal space.
The inner chamber. Enclosed by the inner membrane and filled with jelly-like matrix.
The cristae. Finger-like folds of the inner membrane. The space between them is the intracristal space.
ATP-synthase. It sits on the cristae, and ATP is made here.
The ribosome. 70S, in the matrix. A prokaryotic feature.
DNA. Circular, in the matrix. The mitochondrion's own.
All eight seen. The ETC and ATP-synthase are not on the outer membrane; they are on the cristae of the inner one.
Drawn by hand after the book's figure, not to scale.
4. Chemistry, and the numbers
By dry weight a mitochondrion is about 65% protein, 29% glycerides, 4% lecithin and cephalin, and 2% cholesterol. Of the lipid, 90% is phospholipid.
It holds about 100 kinds of enzymes and coenzymes, and 0.5% RNA.
The numbers of this topic come up in exams again and again. Tap each cell of the grid and match the value to its description.
number matchtap a cell
Numbers and sizes
description
value
share of cell volume
20%
protein by dry weight
65%
number in a liver cell
1000+
gap between the membranes
6-8 nm
matrix ribosome
70S
20%. Mitochondria make up about 20% of the cell's volume.
65%. About 65% of the dry weight is protein.
1000+. A liver cell holds 1000 or more; the usual cell holds 300-400.
6-8 nm. The gap between the two membranes, the width of the outer chamber.
70S. The ribosome of the matrix, a prokaryotic feature.
5. The work: shield outside, factory inside
The jobs of the mitochondrion: produce and control energy; run the Krebs cycle, electron transport and oxidative phosphorylation; make its own DNA and RNA; actively transport Ca and K; and control apoptosis.
Run the power house yourself. Guess where ATP is made, then switch it on.
the ATP factoryguess first
Where is ATP made?
Match these jobs to the structure. The outer membrane protects and controls exchange. Energy is made on the cristae of the inner membrane. So the outer structure is protective and the inner structure does the work. Choose which membrane each of the six features belongs to.
sort themtap a chip, then pick a bin
Which membrane?
Pick up a chip, then drop it in a bin. Ask: does it guard, or does it make energy?
Right. Next one.
Not yet. Folds, the ETC and oxisomes belong to the inside; guarding and staying smooth belong to the outside.
All correct. Outer: smooth and straight, no ATP synthase, protective, guards the inside. Inner: folds into cristae, the ETC lies on the cristae, oxisomes sit on its inner face.
One last idea. Chloroplasts and mitochondria are regarded as endosymbionts. They are thought to have evolved from bacteria taken in by endophagocytosis. The circular DNA and 70S ribosomes of the matrix fit that idea, since both are prokaryotic features.
6. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
Look at the figure: [Figure: a longitudinal section of an oval organelle; a double membrane, and the inner membrane folded like fingers (cristae); the line marked ‘P’ points to the organelle.] The figure:
i. Is known as the powerhouse of the cell ii. Is found in bacteria iii. Contains a little DNA
i and ii: ii is wrong, since bacteria have no mitochondria.
ii and iii: ii is wrong.
Right. The mitochondrion is the powerhouse and holds a little DNA of its own in the matrix.
Not all three, because ii is wrong.
In the second, recall which structure does what.
check yourselfpick one answer
Question 2
What percentage of the dry weight of mitochondria is protein?
4% is the share of lecithin and cephalin.
29% is the share of glycerides.
2% is the share of cholesterol.
Right. About 65% of the dry weight of mitochondria is protein.
The last one is about numbers and names again.
check yourselfpick one answer
Question 3
Which scientist gave the name “mitochondria” to this organelle?
Kolliker first saw mitochondria in 1850 but did not name them.
Fleming saw thread-like mitochondria in 1882 and called them fila, not mitochondria.
Right. In 1897 Carl Benda gave this organelle the name mitochondria.
Altman called them bioplasts in 1890, not mitochondria.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionWhat is a mitochondrion? What is it called?
AnswerThe double-membrane organelle in which the Krebs cycle and electron transport occur and energy is made. It is the power house of the cell.
QuestionWhat share of cell volume is mitochondria? How many in a liver cell?
AnswerAbout 20%; 1000 or more in a liver cell (usual average 300-400).
QuestionDoes the outer membrane hold the ETC and ATP synthase? What does it do?
AnswerNo. It is protective and smooth; the gap between the two membranes is 6-8 nm.
QuestionWhat are cristae? What is the intracristal space?
AnswerFinger-like folds of the inner membrane. The gap between cristae is the intracristal space, joined to the outer chamber.
QuestionWhich prokaryote-like features lie in the matrix?
AnswerIts own circular DNA and 70S ribosomes.
QuestionState four functions of mitochondria.
AnswerEnergy production; Krebs cycle, electron transport and oxidative phosphorylation; making own DNA and RNA; controlling apoptosis.
QuestionHow do the roles of the outer and inner membranes differ?
AnswerThe outer protects and controls exchange; energy is made on the cristae of the inner.
QuestionWhy are mitochondria and chloroplasts called endosymbionts?
AnswerThey are thought to have evolved from bacteria taken in by endophagocytosis.
Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.3.