Plant and Animal Cells
Senior 1 · Biology
Plant and Animal Cells
Lesson 4 of 5
20 min
Run up one of Kigali's steep hills and three things happen: your leg muscles burn, you breathe faster and deeper, and you grow hot. All three are clues to the same process. Your muscle cells are releasing energy from the food you ate, using the oxygen you breathe in, and some of that energy escapes as heat. The release happens inside tiny organelles found in almost every plant and animal cell: the mitochondria.
A mitochondrion is only about 1 to 2 µm long. A school light microscope cannot separate details smaller than about 0.2 µm, so even at ×400 mitochondria show up at best as faint dots, and only with special stains. Their inner structure was first seen in the 1950s with the electron microscope, which uses a beam of electrons instead of light and can magnify more than 100 000 times.
Definition
Mitochondrion (plural mitochondria): an organelle, surrounded by two membranes, in which aerobic respiration releases energy from food. Found in plant and animal cells.
Aerobic respiration: the chemical reactions in cells that use oxygen to break down glucose, releasing energy and producing carbon dioxide and water.
Cristae (singular crista): the folds of the inner membrane. Matrix: the fluid inside the inner membrane.
Figure 1: Inside a mitochondrion, and what it takes in and gives out.
The outer membrane is smooth. The inner membrane is folded again and again into cristae, which gives it a very large surface area; many of the reactions of respiration happen on it, so more folds mean more energy released. The reactions are summed up as:
The energy is not "made": it was stored in the glucose, and respiration releases it in a form the cell can use, packed into a small carrier molecule called ATP. Cells use this energy to:
So the number of mitochondria in a cell tells you how hard the cell works. A liver cell can contain more than a thousand of them; in heart muscle they fill about a third of each cell; a mature red blood cell has none at all.
Warning
Respiration is not breathing. Breathing is the movement of air into and out of the lungs (unit 8). Respiration is a chemical reaction that happens in the mitochondria of every living cell, including plant cells, day and night. Breathing only supplies the oxygen for it and removes the carbon dioxide.
Worked example 1: the real size of a mitochondrion
In an electron micrograph taken at ×30 000, a mitochondrion is 45 mm long. Find its actual length in micrometres.
Answer: 1.5 µm, which is why a light microscope cannot show its cristae.
Worked example 2: is the equation balanced?
Check that no atoms are created or lost in .
| Atom | Left side | Right side |
|---|---|---|
| carbon (C) | 6 | |
| hydrogen (H) | 12 | |
| oxygen (O) |
Answer: every type of atom has the same number on both sides, so the equation is balanced. One glucose molecule needs six oxygen molecules.
Riders preparing for the Tour du Rwanda train for months on the hills around Musanze and Rubavu. Regular endurance training makes muscle cells build more mitochondria, so the muscles can release more energy aerobically and tire less quickly. The same happens, more slowly, to anyone who walks or runs to school every day. It is one reason why fitness improves with practice.
Mitochondria: The Powerhouse of the Cell, Bozeman Science. Watch the first part, on the double membrane and the cristae; the later detail on the stages of respiration goes beyond S1.
Tip
Memory hook: "More work, more mitochondria." Busy cells (muscle, liver, sperm, root hairs) are packed with them.
Practice: the mitochondrion
Key points
Sources and credits
Figure 1, the worked examples and the practice questions are written by Ganzaa.
