Biology for Kids: Respiration and Enzymes
Why Respiration Matters
Every cell, in every single living organism on the planet, needs a continual supply of energy if it is to remain alive. All the activities of life—growing, moving, thinking, and all the rest—require energy. Without energy, cells and organisms stop and die.
The energy needed is released in the process called respiration. Respiration is absolutely crucial to our survival. If respiration stops, life stops.
So what is this process and how does it work?
What Is the Definition of Respiration?
Respiration is a set of chemical reactions going on inside cells which releases energy for use by the cell during the breakdown of food.
Fine. So, what does that actually mean?
- Respiration is a set of chemical reactions, it is not the same as breathing.
- Respiration happens inside cells. Every cell in an organism needs energy to live, and every cell releases energy by respiration. To emphasise this point, biologists sometimes refer to "cell respiration".
- Respiration happens when food is broken down. The process involves chemical reactions which break down larger molecules into smaller molecules, which releases the energy stored in the larger ones. The most important of these larger molecules found in food is glucose.
Respiration is a chemical process taking place in cells which releases energy stored in food. It doesn't "make" energy. Energy can't be created or destroyed, only changed from one form to another.
What's the Difference Between Aerobic and Anaerobic Respiration?
Respiration happens in two different ways. They both start with glucose.
- In aerobic respiration glucose is broken down using oxygen. In this case, it's broken down completely into carbon dioxide and water and most of the chemical energy from the glucose is released
- In anaerobic respiration the glucose molecule is only partly broken down, without the help of oxygen, and only about 1/40th of its chemical energy is released
Of these two types of respiration, aerobic respiration is the most efficient and is always done by cells if they have sufficient oxygen available. Anaerobic respiration only happens when cells run short of oxygen.
Let's examine each of these types of respiration in a bit more detail.
Aerobic respiration can be described by the following word equation:
glucose + oxygen gives carbon dioxide + water (+ energy)
This means that glucose and oxygen get used up while carbon dioxide and water are made. The chemical energy stored in the glucose molecule is released in this process. Some of this energy is captured and used by the cell.
The above word equation is only a simple summary of a much longer and more complicated chemical process. The large glucose molecule is really dismantled in a series of much smaller steps, a few of which happen in the cytoplasm and the later ones (the steps which make use of the oxygen) happen in the mitochondria. Still, the word equation correctly gives the starting point, the carbon dioxide and water, of the whole process.
The Symbol Equation for Aerobic Respiration
In addition to the word equation, it's helpful to any budding biologist to understand how to write the balanced chemical symbol equation for aerobic respiration.
You'll need to know a bit of chemistry to get this. But much of biology comes down to chemistry in the end!
In case you're not sure about this aspect of things, let's take a quick look at chemical formulae, what the symbols mean, and how to write them.
How to Write Chemical Formulae
In chemical formulae, each element is given a symbol of one or two letters. In biology, the symbols and elements you'll come across most often are shown in the table below.
Table of Chemical Elements and Symbols
Molecules contain two or more atoms joined together. In the formula for a molecule, each atom is represented by its symbol.
- A carbon dioxide molecule has the formula CO2. This means it contains one carbon atom joined to two oxygen atoms
- A water molecule has the formula H2O. This means it contains two hydrogen atoms joined to one oxygen atom
- A glucose molecule has the formula C6H12O6. This means it contains six carbon atoms joined to twelve hydrogen atoms and six oxygen atoms
- An oxygen molecule has the formula O2. This means it contains two oxygen molecules joined together
What Is a Chemical Compound?
A compound is a substance whose molecules contain more than one kind of atom. So, carbon dioxide (CO2), water (H2O), and glucose (C6H12O6) are all compounds, but oxygen (O2) isn't.
Easy, really, isn't it?
How to Write the Symbol Equation for Aerobic Respiration
Now we've got that straightened out, the rest should make sense. This, then, is how you write the symbol equation for aerobic respiration:
C6H12O6 + 6O2 => 6CO2 + 6H2O (+ energy)
Get it? The equation means that each glucose molecule is broken down with the help of 6 oxygen molecules to produce six carbon dioxide molecules and six water molecules, which releases energy.
While aerobic respiration is much the same in all organisms, anaerobic respiration can happen in a number of different ways. But the following three factors are always the same:
- Oxygen is not used
- Glucose isn't fully broken down into water and carbon dioxide
- Only a small amount of chemical energy is released
There are three important types of anaerobic respiration it's useful to know about. In each case, the cells involved are capable of aerobic respiration and only turn to anaerobic respiration when they run short of oxygen.
All cells can perform aerobic respiration and prefer it as a way of releasing energy. They only turn to anaerobic respiration when there isn't enough oxygen available.
Respiration in Yeasts
Yeasts break down glucose into ethanol (alcohol) and carbon dioxide. That's why we use yeasts to make bread and beer. The chemical formula for ethanol is C2H5OH, and the word equation for the reaction is:
glucose => ethanol + carbon dioxide (+ some energy)
Respiration in Bacteria and Protozoa
Bacteria, protozoa, and some plants, break down glucose to methane. This happens in the digestive system of cows, in garbage dumps, in marshes, and rice fields, for example. Methane released like this contributes to global warming. The chemical formula for methane is CH4
Anaerobic Respiration in Human Muscle
When the blood can't get enough oxygen to the muscles (maybe during prolonged or intense exercise) human muscles break down glucose into lactic acid. Afterwards, the lactic acid is broken down into carbon dioxide and water using oxygen, although it won't release useful energy at that stage. This process is sometimes referred to as "paying back the oxygen debt".
The chemical formula for lactic acid is C3H6O3
The word equation for the reaction is:
glucose => lactic acid (+ some energy)
Every cell is kept working by a huge number of different chemical reactions taking place in the cytoplasm and the nucleus. These are called metabolic reactions and the sum total of all these reactions is called metabolism. Respiration is just one of these important chemical reactions.
But these reactions must be controlled, to make sure they don't go too fast or too slow, or the cell will malfunction and may die.
So, each metabolic reaction is controlled by a special protein molecule called an enzyme. There's a different kind of enzyme specialized for each type of reaction.
The key roles of an enzyme in controlling metabolic reactions are:
- to speed reactions up. Most reactions would happen too slowly to sustain life at normal temperatures, so enzymes help make them work fast enough. This means that enzymes are biological catalysts. A catalyst is something that speeds up a chemical reaction without being used up or changed during the reaction
- once an enzyme has catalyzed a reaction, it works to control the rate at which the reaction takes place, to make sure it doesn't go too fast or too slow
As with all other metabolic reactions, enzymes also catalyze and control the rate of respiration.
How Do Enzymes Work?
Each enzyme is a large protein molecule with a particular shape. One part of its surface is called the active site. During the chemical reaction, the molecules which are going to be changed, called the substrate molecules, bind onto the active site.
Binding onto the active site helps the substrate molecules change into their products more easily. These then drop off the active site, and the next set of substrate molecules bind.
The active site is exactly the right shape to fit its substrate molecules, much in the same way that a lock is just the right shape to fit its key. This means that each enzyme can control only one chemical reaction, just like each lock can only be opened by one key. Biologists say that an enzyme is specific to its reaction. This means that each enzyme can only act on its particular reaction.
What Effect Does Temperature Have on Enzymes?
Chemical reactions controlled by enzymes go faster if you warm them up. There are two reason for this:
- a reaction can only occur when the substrate molecules have reached the enzyme's active site. The higher the temperature the faster the particles move and the less time an enzyme molecule has to wait for the next set of substrate molecules to reach its active site
- the higher the temperature, the more energy, on average, each substrate particle has. Having more energy makes the substrate molecule more likely to react once it is bound onto the active site
But if you keep increasing temperature above about 40 degrees Celsius, the reaction slows down and eventually stops. This is because, at higher temperatures, the enzyme molecule vibrates more and more. The shape of its active site changes, and although the substrate molecules get there faster they can't bind so well once they arrive. Eventually, at a high enough temperature, the shape of the active site is completely lost, and the reaction stops. Biologists then say that the enzyme has become denatured.
The temperature at which the reaction occurs fastest and most efficiently is called the optimum temperature. For most enzymes this is close to, or just above, human body temperature (about 37 degrees Celsius).
What Effect Does pH Have on Enzymes?
Changing the acidity (pH) of a solution also changes the shape of an enzyme molecule and therefore the shape of its active site. In the same way that there's an optimum temperature at which enzymes can function, there's an optimum pH also, at which an enzyme's active site is exactly the right shape to do its job.
The cytoplasm of cells is maintained at a pH of about 7, which is neutral, so enzymes which work inside cells have an optimum pH of about 7. But the enzymes which break down food in the digestive system are different. As they work outside the cells, they are adapted to the particular conditions in which they operate. For example, the enzyme pepsin, which digests protein in the acidic environment of the stomach has an optimum pH of about 2; while the enzyme trypsin, which works in the alkaline conditions of the small intestine has a much higher optimum pH.
Enzymes and Respiration
As respiration is a kind of metabolic reaction (or, more accurately, a series of metabolic reactions) its various stages are catalyzed and controlled by specific enzymes every step of the way. Without enzymes, neither aerobic or anaerobic respiration would occur and life would not be possible.
© 2019 Amanda Littlejohn