One of the criteria for life was that...
All life forms have a method by which they take energy from the surroundings and convert it into energy that helps them live.
This criteria is the focus of this post.
Life's Energy Cycle
You can divide creatures into three different categories: producers, consumers and decomposers. Producers are organisms that produce their own food. They take the energy from the sun, carbon dioxide and water and make a sugar called glucose and then the organism uses this glucose as food for energy.
Consumers are organisms that eat living producers and/or other living consumers for food.
Lastly, although you could classify decomposers as consumers since they do not produce their own food, they are classified separately because they recycle dead organisms and this is a uniquely vital component in life's energy cycle.
Consumers can be further classified into herbivores (those who eat producers exclusively), carnivores (those who eat only other consumers) and omnivores (those that eat both producers and other consumers).
How Do Organisms Get Energy From Food?
Organisms get energy from burning their food. One can see the relationship between consuming food and the organism's need for respiration if you think about what combustion needs. In order for our food to be burned for energy, we need two things, something to burn, or food and oxygen because without oxygen, combustion cannot occur.
|What Combustion Needs|
Combustion produces energy, carbon dioxide and water. We inhale oxygen because combustion requires oxygen. We exhale carbon dioxide and water as products of combustion as can be seen from our breath outside on a cold day.
Combustion in Living Organisms
The combustion of a monosaccharide takes place in three steps in each cell of the body, two of which occur in an organelle called the mitochondrion. The first step is called glycolysis. In this step, glucose is broken into two parts. This results in a small release of energy and a little bit of hydrogen. They are then sent to a particular organelle in the cell called the mitochondrion. In the mitochondrion, the second step, called the Krebs cycle, occurs. In this step, the two pieces of glucose are broken down int their two main elements, carbon and hydrogen. Oxygen combines with the carbon to make carbon dioxide. That results in a small release of energy as well. The hydrogen from glycolysis and the hydrogen released in the Krebs cycle then go through the third step, called the electron transport system. In the electron transport system, oxygen combines with the hydrogen to make water. This results in a large release of energy. Since the majority of the energy release occurs during this step and this step takes place in the mitochondrion of the cell, the mitochondrion is often referred to as the powerhouse of the cell. This process of combustion is amazingly complex in order to provide energy in a gentle enough fashion.
What Actually Gets Burned For Energy?
Our bodies burn three macronutrients: carbohydrates, fats and proteins, in that order. There are many types of carbohydrates. There are simple carbohydrates, called monosaccharides. Glucose, the chemical that plants produce for food is an example. Two monosaccharides can link together to become a disaccharide, of which table sugar is a good example. If many monosaccharides link together, they form a polysaccharide. Starch, such as found in potatoes, is a good example of a polysaccharide.
Starches or carbohydrates can be detected easily with an iodine solution. Most drugstores carry "tincture of iodine" which contains iodine and alcohol, which can be used for this demonstration. If the iodine turns black or blue, there is starch present. If it remains reddish brown, there is no a significant amount of starch. Obviously once the food has had iodine on it, it must be discarded and cannot be eaten. Bread, pasta, potatoes and fruit such as bananas and apples all contain starches, and therefore are good things to test, but make sure you also have things that do not have starch so they can compare them.
Sam (age 10) tests several food items. Most of the results turned out as he expected, but he was surprised that peanut butter tested positive for the presence of starch.
There are also many different types of fats in the food we eat. Unsaturated fats tend to be liquid at room temperature while saturated fats are solid at room temperature. Your body produces fats from excess carbohydrates and proteins as a way of storing them for future use.
Proteins are a third macronutrient. Your body burns these molecules only if you have too many of them, because they are essential to many other chemical processes which occur in your body. Like polysaccharides, proteins are formed when smaller substances, called amino acids, are linked together in long chains. Next to DNA, proteins are the most important chemicals to life because nearly every chemical reaction that occurs in the body is affected by proteins. Your cells manufacture 12 of the 20 amino acids that you need but there are 8 that your cells cannot manufacture. These are called essential amino acids and we get them from the proteins we eat. The main reason, then, that we eat proteins is not as a source of energy but as a source of the amino acids that are bodies cannot produce themselves.
Energy Use in the Body
Our bodies use energy to control all of our voluntary muscles as well as the involuntary muscles which keep our organs and internal processes functioning. They also use energy to keep our internal temperatures at a constant rate.
Calories and Food
The total energy that our bodies use each day is called our metabolic rate. The basal metabolic rate is the minimum amount of energy required by the body every day. The amount of activity we engage in affects our metabolic rate, so the total metabolic rate is the sum of the basal metabolic rate and our activity level. Calories can be used to measure the macronutrient intake, and so, in order to maintain current weight, one should roughly take in the same number of calories that are used every day. A healthy life requires more than just eating the proper number of calories, however. It also requires us to get a certain distribution of macronutrients. Although there is no real consensus among nutritionists because there are still many things we do not understand about the human body, many say that 21% of our calories should come from fat, 70% from carbohydrates and the rest (9%) should come from protein.
Metabolic Rates in Mammals
Another factor of caloric intake is metabolic rate. Generally, the smaller the mammal, the larger the metabolic rate, because the smaller mammal has a larger percentage of their total body exposed to the outside air, losing more heat. Since a mammal has to keep his internal temperature the same, the smaller mammal must expend more energy for that purpose than the larger mammal.
Sources and Resources:
- Exploring Creation with General Science, Jay Wile, chapter 13
- Exploring Creation with Human Anatomy and Physiology, chapter 5