14 February 2005
Reading, Chapter 24
D. Benefits and costs of aerobic respiration
Anaerobic cell respiration (glycolysis + fermentation) produces 2 ATP/glucose consumed. Aerobic cell respiration (glycolysis + the Krebs cycle + respiratory electron transport) produces 36 ATP/glucose consumed. Aerobic cell respiration is roughly 18 times more efficient than anaerobic cell respiration. Your cells require a lot of energy and are dependent on the high efficiency of aerobic respiration. They quickly die if deprived of oxygen.
Overall, aerobic respiration converts about 40% of the available energy of glucose into ATP. The remaining 60% is lost as heat and helps to generate your relatively high body temperature. 40% efficiency may seem poor but it is several times more efficient than the best automobile engines.
Aerobic cell respiration is so much better than anaerobic respiration that the overwhelming majority of life on Earth employs this pathway. Using oxygen entails some costs, however. Oxygen can be converted by cell activities into toxic forms that damage biological molecules. For this reason, if you were forced to breathe 100% oxygen rather than the 21% of our present atmosphere, you would die in a few days.
A toxic form of oxygen that is generated by respiratory electron transport (and other cell activities) can damage the mitochondrial chromosome, causing mutations to its genes. Accumulation of such mutations over a lifetime has been proposed to contribute to human aging. In some cases, loss of muscular strength (including that of the heart muscle) and loss of nerve function with age have been proposed to result from populations of mitochondria that have been crippled by such mutations in nerve and muscle tissue, which perform high rates of aerobic respiration.
When generated outside mitochondria, toxic forms of oxygen can cause mutations in genes of the nucleus that lead to uncontrolled cell division (cancer). Toxic oxygen can be made from water by ionizing radiation and it is thought that the carcinogenic effect of ionizing radiation is mediated by the toxic oxygen it forms.
To limit the harm that can be caused by toxic forms of oxygen, your cells have "antioxidants" that convert them back into harmless O2 or water. Some of these antioxidants are enzymes and others are not. Plant cells have high levels of antioxidants, since photosynthesis generates toxic forms of oxygen. You acquire some of these antioxidants when you eat plant cells. In some cases, they act as antioxidants in your cells too. Vitamin C, Vitamin E, and the precursor to Vitamin A are all antioxidant compounds that you need in your diet. Vitamin C is mostly needed to maintain connective tissues while Vitamin A is mostly needed by the light-sensitive cells in the back of your eye. Both Vitamin A and C may also act as antioxidants in your cells, however. Vitamin E is an effective antioxidant in your cells and those of plants.
You can gain whatever antioxidant benefits these compounds provide by including plenty of fresh fruits and vegetables in your diet. You can also get them from supplements (vitamin pills) but take care. Both Vitamin A and E are toxic at high dosages.
E. Energy flow at the organism scale
Now that you understand how cells get energy from the sun (photosynthesis by green plant cells) or from food (non-photosynthetic cells, including yours), let's discuss energy flow through a multi-celled organism, which is an assemblage of many cells working together. Let's use your body as an example.
1. The human digestive system
In a multi-celled organism, cells are organized into tissues. Tissue are combined into organs and organs cooperate in "organ systems". One of these is the digestive system of your body.
The digestive system includes the digestive tract, a pathway through your body that is taken by the food you eat. The digestive tract includes mouth, esophagus, stomach, and intestines (small and large). The liver and pancreas are part of the digestive system also. The liver stores blood sugar (glucose) as glycogen (animal starch) and produces bile salts for the digestion of fats in the small intestine. The pancreas secretes digestive enzymes and bicarbonate buffer (to neutralize stomach acid) into the small intestine. The pancreas also releases insulin and glucagon into the blood. These are two hormones that act to maintain a stable concentration of glucose in the blood. Adipose tissue can also be considered part of the digestive system. It stores fat for later conversion to glucose, if necessary.
2. After a meal
All of your cells need glucose and oxygen to perform aerobic respiration. The different cells and organs of your body coordinate to provide glucose and oxygen to all while taking into account the constraints of gathering and eating the food that provides the glucose.
After eating, the different components of your food contribute to cell respiration in different ways:
a. Starches and sugars are readily converted to glucose by enzymes in your mouth and stomach. The glucose is taken up by your blood in your intestine. All of your body's cells can use the glucose to make ATP but some do other things with it.
Nerve cells use only glucose for aerobic respiration. Unlike other cells, they cannot take up fatty acids from the blood as an alternative. Your brain contains many nerve cells that need lots of ATP. It consumes about 2/3 of the glucose in your blood, on average.
Your liver is the largest organ in your body. Its cells take up glucose from your blood and convert it to glycogen for short-term storage (recall that glycogen is a form of starch made by animals).
Muscle cells also take up blood glucose. Some is used to make ATP for muscle contraction. The excess is converted to glycogen for storage.
Adipose tissue is composed of fat cells. They can take up excess blood glucose and convert it into long fatty acid molecules. Two or three of these combine to produce a fat molecule. Accumulated fats look like large oily droplets in fat cells.
b. Fats and oils in your food are converted to fatty acids by enzymes in your intestine and taken up by your blood there. Since there is already glucose in the blood after a meal, the fatty acids are taken up by the fat cells in adipose tissue and converted to fats for storage.
c. Proteins in your food are broken up into their component amino acids by enzymes and hydrochloric acid in your stomach. The amino acids are taken up by your blood in your intestine.
All cells can take up amino acids from the blood to make new proteins. Most of your body's protein is in muscle cells, which contain large amounts of cytoskeleton and motor proteins for contraction. Muscles consume lots of amino acids if they are growing.
Amino acids in the blood are also taken up by the liver and converted into glycogen. This entails a little extra processing. Glycogen is a carbohydrate. To convert amino acids to carbohydrates, the amino groups must be removed. Free amino groups are NH3 (ammonia), which is toxic to cells. Cells in your liver convert NH3 to urea, which is less toxic than ammonia, and load it into your blood. Your kidneys remove the urea from your blood and release it into your urine.