Heterotrophs get both their energy and building blocks from what they eat. Here we will use humans as an example and examine how energy is transferred from the food we take in to a form that can be used by our cells to drive life processes, such as growth, muscle contraction, and so on.
Although all the organic molecules we eat contain free energy, three types of molecules are commonly used to supply our energy demands:
| Carbohydrates (sugars, starches) | 4 Cal/gm |
| Fat | 9 Cal/gm |
| Protein | 4 Cal/gm |
A calorie is a unit of energy. Nutritionists use a different calorie unit than biologists and biochemists. The calorie you see on food labels (which can be written with capital C) is equal to 1,000 of the calories used by biologists (i.e., 1 Cal = 1,000 cal). Here we will use the nutritionist's calorie. As you can see, fat contains more calories per gram than does carbohydrate or protein. This makes fat a good storage molecule and accounts for our bodies tendency to convert what we eat into fat. For our bodies to use these foods, the molecules must be broken down into their subunits so they can get into our cells. For example, starch that we eat is broken down into simple sugars, which are transported into the cell.
Energy Reserves of a 70 kg male
| Fat | 15 kg | 135,000 Cal |
| Protein | 6 kg | 24,000 Cal |
| Carbohydrate | 0.225 kg | 900 Cal Cal |
Glucose Oxidation
How does a cell use the free energy contained in a simple sugar molecule like glucose? The complete process is called oxidative respiration. It is important to keep in mind that all of the processes described below happen inside of cells. What happens is that the energy contained in the glucose molecule is transferred to a smaller molecule, ATP, that can be use for a variety of tasks within the cell. ATP can be thought of as the energy currency of the cell. It is small, so it can move around easily, and the free energy stored in it is easy for the cell to use. Almost every cellular process that requires energy uses ATP. Overall, in oxidative respiration one molecule of glucose provides enough energy to produce about 36 molecules of ATP. The electrons that are removed from the glucose are transferred to oxygen molecules which end up in water. The carbon in the glucose is released as carbon dioxide. The efficiency of oxidative respiration is about 35%. This means that about 35% of the free energy originally contained in a glucose molecule ends up stored in ATP.
Oxidation/Reduction Reactions
Oxidation is the removal of an electron or hydrogen atom from a molecule or molecular group. The free energy stored in a glucose molecule is stored in bond energy. Within the cell electrons are removed from glucose in a manner that transfers the free energy to other molecules. Electron transfer reactions are known as oxidation and reduction reactions and are a key element of energy conversion within cells.
Oxidation - The removal of an electron or H from a molecule
Three stages of respiration:
In glycolysis the 6-C glucose molecule is split into two 3-C compounds. The reaction progresses by a linear series of enzymatic reactions and yields two molecules of NADH and two molecules of ATP. Most of the free energy is remains in the 3-C compounds. In the presence of oxygen, the 3-C compounds are transported into the mitochondrion, where the next two stages of oxidative respiration occur. ATP is made during glycolysis by a process known as substrate-level phosphorylation. In this process a phosphate group on a carbon compound is transferred to ADP by an enzyme (Fig. 7.4).
In mitochondria, the 3-C compound is modified and attached to a carrier molecule (CoA). The Krebs Cycle removes hydrogen atoms from the carbon skeleton, transferring electrons and hydrogen to NAD+. The result is that the carbon originally contained in the glucose is released as carbon dioxide. Most of the energy that was originally stored in glucose is now in NADH. Some ATP is produced, but the lion's share of ATP is made in the next stage.
In the mitochondrion electrons are transferred from NADH to oxygen by the respiratory electron transport chain. The respiratory electron transport chain is a series of proteins that form a linear pathway for electrons to travel from NADH to oxygen. The electron carriers are arranged in the inner mitochondrial membrane (Fig. 7.8). The electron transport reactions drive proton transfer across the inner mitochondria membrane, creating a proton gradient. The energy that was in the NADH molecules is then stored in the proton gradient across the membrane. These protons move through the ATP synthase enzyme, which combines ADP and phosphate, yielding ATP. This process is known as oxidative phosphorylation and is how most of the ATP within our cells.
Under anaerobic conditions (low oxygen) many cells can produce ATP by adding to glycolysis a few additional reactions. This is known as fermentation. During exercise our muscle cells can run out of oxygen, in which case they can convert glucose to lactic acid. This allows ATP to be made, but the lactic acid accumulates, causing a problem for the muscles cell. The ache you feel in your legs after brief, vigorous exercise is due to the build up of lactic acid. In yeast fermentation yields the product ethanol (drinking alcohol). Fermentation has the advantage of producing ATP in the absence of oxygen, but has the disadvantage of only using a small fraction of the bond energy available in the glucose molecule.
Energy Use During Exercise
We are very efficient at taking in the calories we eat. For a healthy person, over 90% of the calories eaten end up in our body. Efficiency of calorie uptake by our bodies
| Sugar | 97% |
| Fat | 95% |
| Protein | 92% |
Studies of humans doing physical work show that the efficiency is about 25%. This means that when we oxidize glucose to do work, that 25% of the energy goes into doing work and 75% of the energy goes into heat. Imagine you drink a coke that contains 140 Cal in the form of sugar. This is enough energy to lift a 60 kg person 1,000 m into air if the energy is used at 100% efficiency. However, if you want to climb stairs and guarantee that you burn up all the calories taken in from the coke, you don't need to climb so high. You need to climb up only 250 m because our bodies are only about 25% efficiency. (Most of the wasted energy occurs in the cells during oxidative respiration. Once ATP is formed, the efficiency of doing muscular work is quite high - 80%.) This is equivalent to climbing to the top of a five story building about 10 times. If you do this you can be sure you used the 140 Cal you got from the coke, but the actual number of calories you burn could be more.
The number of calories you use during exercise depends on how fast you move. The number of calories used by a 70 kg runner during a 30 minute period is shown below. Note that running twice as fast for the same period of time burns twice a many calories.
Calories used during a 30 minute run (70 kg runner)
| 12 min. miles | 280 Cal |
| 8 min. miles | 420 Cal |
| 6 min. miles | 560 Cal |