Remember “metabolism” from our last instruction? As you have learned, metabolism is the sum total of all the chemical reactions that must go on for an organism to survive.

You have also read about photosynthesis and cellular respiration. Photosynthesis is the process by which plants make carbohydrates (sugar).
Cellular respiration is the process by which cells change these sugars into energy. The energy that it turns into is a chemical called ATP (adenosine triphosphate). This ATP is the basic food for cells.

Organisms must eat, digest and absorb the ingredients that the plants produced. This is known as digestion. You will read about digestion in a later lesson. For now, imagine that your cells have already absorbed the glucose (sugar/carbohydrate). As soon as it enters the cells, it undergoes glycolysis. Glycolysis is just a fancy word for “sugar splitting.” One molecule of glucose gets split into two totally different molecules.

Glycolysis
http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html

You can see in this animation that as soon as glucose enters the cell, some chemical changes happen to it. It is not so important that you memorize the names of these steps, just understand that the glucose gets changed. These changes trap the glucose in the cell. After several steps, the glucose is turned into two molecules of pyruvic acid. Remember you don’t have to try to memorize each step. Just remember three things as far as glycolysis is concerned. Glucose enters the cell, gets trapped there and is ultimately changed into pyruvic acid. So now we have taken one molecule and split it into two molecules. This all took place in the cytoplasm of the cell. These two new molecules will enter the mitochondria.

Mitochondria are the powerhouses of cells. It gets this distinction because most of the cell's ATP production happens here. Remember the two molecules that were the final product of glycolysis? Well those molecules undergo some changes, and once changed, cellular respiration occurs and a big amount of energy is the result. This cellular respiration takes place inside the mitochondria helping it to earn its name as powerhouse. It is important to note that this takes place in the presence of oxygen. When reactions take place in the presence of oxygen, they are considered to be aerobic. Anaerobic reactions take place without the presence of oxygen. Glycolysis takes place in the cytoplasm and requires anaerobic conditions. The cellular respiration that takes place in the mitochondria of the cell requires oxygen and is an aerobic reaction.

It all can sound intimidating, but, in a nutshell:

• plants make food sugar (a.k.a. carbohydrates) and other organic food molecules
• we eat these sugars
• our body splits the sugar by something called glycolysis (takes place in cytoplasm)
• The end product of this sugar splitting gets converted to ATP through respiration (takes place in the mitochondria)

Mitochondria are the powerhouses of cells. Their whole purpose is to break down the molecules in ATP (cell food) so the cell can have the energy it needs to live.

Let’s do a little comparison.

You eat and your intestines break down the food for you to use. A cell eats and its mitochondria break down cell food for the cell to use.  So mitochondria are kind of like a cell’s intestines.

Mitochondria / Cristae / Matrix
http://www.cytochemistry.net/Cell-biology/mitochondria_1.htm

Let’s take a look at what mitochondria look like:

Mitochondria have two membranes. The outer membrane covers the mitochondria. 

As you learned in our Cell Membrane instruction, the outer membranes of cells are “semi permeable,” which means that material (small molecules and ions) can enter and leave the cell.

The inner membranes of mitochondria fold many, many times. These folds are called cristae. These cristae are important because they make more surface area where chemical reactions can take place. The molecules and some of the enzymes responsible for making ATP are located in and on the folds of these inner membranes (cristae).

There is a space between the outer and inner membranes called, not surprisingly, intermembrane space. It doesn’t do much, but you need to know it’s there.

On the other hand, what’s inside the inner membrane does a lot. It’s called the matrix. The matrix is a fluid that has water and proteins (enzymes) all mixed together. Here’s where the rest of the enzymes that make ATP come from.

The enzymes in and on the inner membranes and the enzymes in the matrix work closely together with their substrates (remember substrates from our Instruction on enzymes?) to synthesize (make) the final product – ATP.

Mitochondria and Energy
http://www.johnkyrk.com/mitochondrion.html 

There are many many mitochondria in the eukaryotic cells that make up animal tissues.

The metabolic activity (energy level) of a cell is related to the number of mitochondria and cristae it contains. Cells with a lot of metabolic activity, such as those in the heart muscle, contain lots and lots of mitochondria.

In other words – the more energy an organ needs, the more mitochondria it's got.

Where Mitochondria Came From

The double membrane of mitochondria has led biologists to an interesting theory.

Scientists think that mitochondria may be the descendents of bacteria (prokaryotic cells) that lived “free” billions and billions of years ago.

These bacteria, so the theory goes, invaded larger cells and lived inside them without being digested (that’s called symbiosis).

This may have led to the development of eukaryotic cells (cells with a nucleus inside) from their prokaryotic ancestors.

This theory is supported by the fact that mitochondria contain DNA and ribosomes that are more like those in prokaryotic bacteria than they are like those in other eukaryotic cells (as you know, mitochondria are only found in eukaryotic cells).

Research on this is still going on – another example of the fascinating changes that make science science.

Experiments for Home and Classroom

You couldn't live without ATP (adenosine triphosphate) -- cell food. Actually, ATP has been described as a rechargeable battery for your cells. In this simple "Biology in Motion" activity, students are invited to recreate the process of cellular metabolism online. This activity requires Flash plug-in. Click:
http://www.biologyinmotion.com/atp/index.html 

Play The Cellular Respiration Game to see how much you know about the process. Start with the URL shown below -- then click "Cellular Respiration," then "10 Steps of Glycolysis" and finally "Play the Game." Begin at:
http://biology.about.com/od/cellularprocesses/a/aa102204a.htm 

Many of the organelles in cells have scientific names that are difficult to pronounce. To hear how these names should sound, go to this URL and click on the picture of an organelle -- like mitochondria -- that you want to learn to pronounce. When you have finished with the pronunciation activity, click the "back" button to take an interactive cell tour. Click:
http://personal.tmlp.com/Jimr57/tour/cell/sounds/cellmap.html