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Plants use the energy stored by photosynthesis for their own growth, to acquire nutrients, to fight off insects and to create the oxygen required for respiration. Respiration is the reverse of photosynthesis; carbohydrate and oxygen are converted to carbon dioxide and water. In addition, a relatively large amount of energy is released and captured throughout the process (Figure 1). One major difference between the two processes is that while photosynthesis is possible only in the presence of sunlight (daytime), respiration occurs continuously, regardless of whether or not there is light (day and night). Humans continuously utilize respiration as the source of energy for daily activities.

C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6 O<sub>2</sub> --> 6 CO<sub>2</sub> + 6 H<sub>2</sub>O + Energy (1)

[[Image:Photosynthesis and respiration.jpg|thumb|500 px|alt=Figure 1 Photosynthesis and respiration. Carbon dioxide and water, through the process of photosynthesis, are turned into organic carbon. During respiration, the reverse happens and plants’ and animals’ organic carbon is broken down into carbon dioxide and water. |Figure 1 Photosynthesis and respiration. Carbon dioxide and water, through the process of photosynthesis, are turned into organic carbon. During respiration, the reverse happens and plants’ and animals’ organic carbon is broken down into carbon dioxide and water.]]

[[Image:The flow of nutrients and energy.jpg|thumb|500 px|alt=Figure 2 The flow of nutrients and energy through biological systems. Note that while nutrients are recycled over and over again, the sun must continuously supply energy. |Figure 2 The flow of nutrients and energy through biological systems. Note that while nutrients are recycled over and over again, the sun must continuously supply energy.]]

This is the energy that ultimately, through the food chain, passes on to other living organisms to sustain life. The simple food chain starts from plants (producers) and passes on to other animals (consumers) through three or more stages called trophic (nutritional) levels. At the base of the food pyramid are autotrophs, which are responsible for the primary food production. Autotrophs (self-feeders or producers) are those organisms, such as green plants, algae, and certain bacteria that convert inorganic compounds into energy-rich organic compounds, mostly by photosynthesis ([[#Additional Comments|a]]). Heterotrophs (other-feeders) are those organisms that cannot produce their own food, but rather feed on other organisms. Trees and most other plants are autotrophs. All animals, including humans, are heterotrophs, as are many microscopic organisms. Heterotrophs can be divided into primary consumers or grazers (herbivores), colloquially
referred to as vegetarians, and secondary consumers (carnivores), or
meat-eaters. A simple three-level food chain is grassgsheepgwolf.
Plantginsectg froggman is an example of a simple four-level food
chain. Things are not always that simple in nature, however, as there are
multiple types of prey for a single predator. Each link in the food chain
can itself be linked with many other food chains, making a complex set of
feeding relations which is more accurately called a food web.

Although nutrients undergo cyclic processes, energy is not cyclical (See
Figure 2). Organisms use energy as fuel to help their movements and
growth. In this fueling process, some energy is discarded as waste heat. To
keep the system operating, as energy degrades, the sun must replenish it.

==Energy Flow through the Biosphere==

As was discussed above, photosynthesis is not only responsible for
providing energy for a plant’s own growth, but must also provide food
for other consumers as they are passed through one trophic level to the
next. The photosynthetic capacity, i.e., the rate at which solar energy
is fixed by plant photosynthesis varies with temperature, rainfall and
the biochemistry of the plant. The rate of gross production is therefore
dependent not only on the amount of sunlight, the nutrient supply, and
the stability of the surface water in an area, but also to geography. Overall,
it is estimated that a mere 1% of the incident solar energy is responsible
for the production of all plants, and only a portion of this energy is used
for a plant’s own respiration. The remainder is used by consumers and
decomposers and is referred to as the net production. Additional losses
cause efficiency to drop at each level of consumption beginning with
plants; as a result, only ten percent of the energy is converted from one
trophic level to the next (See box “Rule of 10”). For example, only 10%
of the energy contained in plants is used to warm an herbivore and make
it grow. The rest is excreted as waste. Similarly, our body uses only 10% of
the energy we take in as food (plants or meat) for our various needs.

[[Image:The flow of nutrients and energy.jpg|thumb|500 px|alt=Figure 3 Energy flow in vegetarian and non-vegetarian diets. |Figure 3 Energy flow in vegetarian and non-vegetarian diets.]]

Question: Plenty of sunshine is often treated as a necessary condition
for growing plants. Does plant production increase with light
intensity?

Answer: Yes, to a certain extent. As light intensity increases, the
chlorophyll traps more light, but as traps are filled, additional light
will not be useful and may actually harm the plant.

The demand for food production increases because of two factors: the
increase in population and change to a more affluent diet. At the present
rate of growth, it is estimated that by mid-century, the world population
will increase to between 8 and 11 billion before gradually stabilizing.
Affluent diets enriched with animal fat, meat, dairy, and eggs use up as
much as three times more biomass than vegetarian diets (Figure 3).

Raising domestic animals and livestock such as cows, sheep, goats, pigs, horses, and camels was a major factor that led to the development of ancient societies. These animals not only provided meat, milk, and milk products such as butter, cheese, and yogurt, but also fertilizer and plowing power needed to grow various plants. In fact, these mammals yielded several times more energy over their lifetime than if they were slaughtered and consumed as meat ([[#References|1]]).

Question: In India, cattle are considered sacred and cannot
be slaughtered for food. How does this practice contribute to the food shortage in India?

Answer: Although on the surface this practice may appear unjustifiable, it is actually an effective way to combat hunger. Cattle are fed mostly with grass and crop wastes, but provide
milk and other dairy products, which are sources of high-
quality protein. Besides producing food, cow dung is used as fuel and a source of high-quality fertilizer.

Question: What are the consequences of vegetarian and non-vegetarian diets on total energy consumption?

Answer: Raising livestock for meat is a very inefficient way of generating food. For example, it takes far more resources (fuel, water, etc.) to produce a kilogram of meat than a kilogram of corn. Adding in all the energy used for transportation, feedlot, and storage, it takes the equivalent of one gallon of gasoline to produce one pound of grain-fed beef. In other words, to provide the yearly average beef consumption in an American family, we require over 260 gallons of fossil fuel ([[#References|2]]).

The rate of food production varies widely among developed and developing countries. Modern agricultural machinery, better fertilizers, and more complex irrigation techniques have allowed developed countries to produce food at a rate exceeding the rate of their population increase. Unfortunately, the same cannot be said for developing and underdeveloped countries. In these countries, although yearly food production has remained relatively constant, the population has been steadily increasing. As a result, the amount of food per capita has been decreasing dramatically, and many countries have faced severe food shortages and even famine. Furthermore, the food surplus in the richer countries that was traditionally exported to the poorer countries is now being used either to feed the increasing appetite of the local population or is exported to other developing countries. At the same time, much of the high-quality food produced in less-developed countries is being exported to industrial countries. For example, for the last few years, the United States has been the world’s largest importer of beef and fish while Latin America has been the major exporter of these same products ([[#References|3]]).

===Units===

Food energy is usually expressed in Calories. One Calorie (with a capital C) is equal to the heat energy that is required to raise the temperature of one kilogram of water by one degree centigrade. A smaller unit of thermal energy is a calorie (with a small c), which is 1/1000th of the food calorie:

:1 Calorie = 1 kilocalorie = 1,000 calories = 4,184 joules

Unfortunately, the calorie notation can cause some confusion. What should be remembered is that when we talk about a food calorie, we are talking about kilocalories, whether capitalized or not. The same is true when we talk about weight; we usually mean mass, especially when we are using terms such as “weight loss.”

==Energy of Food Production==

Food production has become less and less efficient as we have replaced the traditional pre-industrial non-mechanized practices with modern agricultural technologies and farming practices. Food production efficiency defined as the ratio of energy output (energy content of foods) to the energy input (energy used to produce, process, package, and transport food) has decreased from roughly 100 in the pre-industrial era to less than 1 today ([[#References|4]]).

The reason for this huge loss in efficiency is that much of the commercial production and delivery of our food on all stages (planting, irrigation, feeding and harvesting, processing, packaging and distribution) depends heavily on oil as the primary source of energy. Fossil fuel is important in cultivating the land, planting seeds, manufacturing fertilizers and pesticides, irrigating, and harvesting. Fossil fuel is also needed in the construction of roads and the transportation of farm workers and food. As a result, only one in every 7-11 calories of energy is available through food. Efficiencies of around 15% in Europe and 9% in the United States have been reported ([[#References|5]]).

==Metabolism==

[[Image:Energy Expenditure.jpg|thumb|500 px|alt=Table 1 Energy Expenditure. |Table 1 Energy Expenditure.]]

It takes energy to stay alive. Whether we are eating or participating in rigorous exercise, we metabolize food as a source of energy. Even when we are not doing any work or are sleeping, we consume food energy. The minimum energy needed for survival and to maintain equilibrium of all vital functions (nervous, cardiovascular, respiratory, and digestive systems) when a body is at rest (sedentary) is called the basal metabolic rate (BMR). The brain and the liver, two organs which jointly make up only 4% of body weight, are responsible for half of all metabolic activity. As chemical energy in food converts to heat, body temperature tends to rise. To maintain a constant temperature, the body reacts by transferring the energy to the circulatory system and to the skin, where it eventually dissipates into the environment. BMR varies with sex, body size, general health, and age, and is generally higher in males and for heavier, healthier, and younger animals. BMR is about 1.45 watts for a rat, 80 watts for an average sized person, and 266 watts for a medium size cow ([[#References|6]]).

BMR does not account for any physical activity, so additional energy is needed to carry out daily tasks (See Table 1). Physical activities can be divided into aerobic and non-aerobic activities. During aerobic activities, oxygen breaks down carbohydrates, fat, and protein and converts them to energy. Examples of aerobic exercises are dancing, jogging, swimming, and biking. Anaerobic activities burn carbohydrates without oxygen with maximum bursts of energy of short duration. Examples are weight lifting, pushups, chin ups, and sprinting. The amount of daily energy needed is different for different people and can vary with gender, weight (mass), and levels of physical and mental activity. As a rule of thumb, it is usually assumed that an average man requires 2,500 Calories, whereas an average woman needs only 1,800 Calories per day in food intake.

Example: What is the minimum number of calories that an average person needs to barely stay alive?

Solution: Assuming BMR of 80 W, the daily caloric intake must be at least

:<math>80W = 80 \frac{J}{s}x\frac{1 kcal}{4.1868 kJ}x\frac{1 kJ}{1000 J} x \frac{24 x 3600 s}{day} = 1650 kcal/day</math>

Question: How much weight does a starving male (zero food intake) lose in one week?

Answer: Assuming an average person requires 2,500 kilo-calories of food to meet his daily energy needs, and that each gram of fat metabolizes 9 Calories (37.6 MJ/kg), he burns 2,500/9=277 grams of his body fat each day (1.9 kilograms in one week). It is no surprise that people are known to survive many weeks without food ([[#Additional Comments|b]]).

Question: Which one has a higher BMR, a tall and thin person or a short and fat person?

Answer: For the same mass, tall thin persons have a greater skin surface area and thus lose heat to the environment faster. They should have a higher BMR to maintain the equilibrium body temperature.

Question: Which organism has a higher basal metabolic rate, a hummingbird or an elephant?

Answer: Hummingbirds are very small birds, weighing approximately 2.5-4.5 grams. Because small birds have proportionately larger surfaces in relation to their body mass, they can lose heat faster and therefore have higher metabolic rates. Gram by gram, hummingbirds have the highest metabolic rate of any animal, roughly 12 times that of a human being and 100 times that of an elephant. Their BMR is around 29 W, which means they need to consume 600 Calories of food every day. No wonder each day they have to visit hundreds of flowers to gather enough nectar to survive.

==The Human Heat Engine==

[[Image:Human body is a heat engine.jpg|thumb|500 px|alt=Figure 4 Human body is a heat engine, taking input energy in the form of food, converting a part of it into muscle work, and rejecting the waste heat as sweat and other excrements. |Figure 4 Human body is a heat engine, taking input energy in the form of food, converting a part of it into muscle work, and rejecting the waste heat as sweat and other excrements.]]

The human body functions like an engine, converting part of the food’s chemical energy into useful mechanical energy when muscle cells carry out physical work, and dumping the rest into the environment (Figure 4). The conversion efficiency is about 17%, that is, only about one in every six units of energy in the food we eat is converted to muscle work. What remains is used as internal energy in our muscles, essential to maintaining our body temperature (through sweating and other forms of heat losses collectively called waste heat) or stored in body in form of fat . Metabolic efficiency is not the same for all people, and can vary between 15-25% depending on gender, weight, condition of health, athletic abilities, and age. Although maximum power can be quite high, average power output is not and is limited to around 20 W of mechanical energy per kilogram of body mass. By combining the food production efficiency (~10-15%) and metabolic efficiency (~15-25%), a net figure for the human power efficiency is calculated at around 1.5-3.75%. In other words, on average, each unit of mechanical (muscle) energy comes at the expense of 17 to 66 units of primary (fossil) energy.

Example: Can we harness waste energy from the human body to perform work? What is the maximum power that can be utilized using this energy?
Solution: Assuming ambient temperature of 25oC, the Carnot efficiency is

η= 1-298/310 = 3.87%

where waste is taken to be at the body temperature of 37oC. While sitting, an average person can expend 116 W, of which 4.5 W is available in the form of useful power.

Example: Compare the cost of power generated by human muscles to that from nuclear or coal power plants.

Solution: An average human consumes 2,500 Calories of food every day. Each Calorie is equal to 1,000 calories or 4,180 joules, and each kilowatt-hour is 3.6x106 joule, so 2,500 kcal = 10,450 kJ = 2.9 kWh

This amount of energy can be supplied by about 2 pounds of steak and 10 slices of bread for an average cost of $10 (or $300 a month for food cost). The energy cost is therefore estimated at around $3.45/kWh. The cost of electricity being charged to American consumers varies between 8-15 cents per kilowatt-hour, which is an order of magnitude cheaper than the cost of energy delivered by humans. No wonder machines are rapidly replacing people whenever possible.

===Food, exercise, and dieting===

[[Image:Food Calories.jpg|thumb|500 px|alt=Table 2 Food Calories. |Table 2 Food Calories.]]

The energy required by our body is provided by the food we eat. The major food categories are carbohydrates (mainly sugar, bread, and rice), proteins (primarily meat, milk, and eggs), and fats. In addition, most plant-based foods contain other nutrients like vitamins, minerals, and water which are drawn up from their roots during growth. Although they are small in quantity, these are essential ingredients in metabolizing the calories stored in food. The human body metabolizes these foods differently. While each gram of fat yields 9.5 kilocalories when burned, carbohydrates and proteins yield only 4.3 and 5.3 kcal of energy, respectively. Carbohydrates and proteins take some time to digest into simple sugars, whereas sugar is instantly metabolized. The energy contents (calorific values) of some common foods are given in Table 2.

Physiologists and physicians have studied, but differ in opinion on the root causes of weight gains by people. Some attribute the propensity to gain weight to a special gene and hormonal imbalance. Others believe bodies develop fat cells during childhood that become active as the body ages. Still others attribute obesity solely to overeating. No matter what the actual medical reason for weight gain, to maintain our weight, the total energy intake by food must balance the energy expenditure of normal basal metabolic rate and that expended by work and other physical activities. If the energy intake exceeds the energy expenditure, the excess energy is stored as fat. As a rule of thumb, we need to burn roughly 3,500 extra Calories to lose one pound of fat.

Example: According to one study ([[#References|7]]), since the 1960s, the average individual living in the United States has increased caloric intake by about 250 Calories each day. How much weight would one gain by staying on the new diet?

Solution: The total excess intake of energy over one year is:
250*365 = 91,250 kcal. The added weight is 91,250/3,500 = 26 lbs (11.8 kg).

Example: A 50-kg woman is jumping rope at a rate of 50 times a minute for 15 minutes. Each jump on average raises the center of mass 0.5 m. How much energy does this woman consume?

Solution: The work performed per jump is W = mgh = 50x9.8x0.5 = 245 J. Total work performed is (15 minutes)x(50 jumps/minute)x(245 J/jump) = 183,750 J = 44 kilocalories.

Example: A 110-lb (50 kg) woman pedals a stationary bike for 20 minutes at an average speed of 20 km/h (12.5 mph). Calculate power consumption rate, total Calories burned and the metabolic equivalent (METs). 1 MET is defined as the energy expenditure rate in kcal per hr per kilogram of body mass.

Solution: Referring to Table 1, energy expenditure rate (power) is given as 450 kcal/hr or 520 W. Total calories burned is E = P.t = 450 kcal/hr x 20 min = 150 kcal. The metabolic equivalent is calculated as the power (450 kcal/hr) divided by her weight (50 kg), or 9 METs.

Example: It is experimentally found that in the photosynthetic reaction given by Equation 1, for each mole of carbohydrate (CH2O), 112 kcal of light energy is needed. What is the amount of energy supplied to our body through the consumption of one 12-oz cola containing 36 grams of sugar (glucose)?

Solution: The respiration reaction is the inverse of equation 5-1; for each mole of glucose (180 g) we consume 6x112 = 672 kcal (Calories) of energy is taken in. The soda contains 36x672/180 = 134 Calories.

Example: How many stairs should a 60-kg woman climb to burn off a chocolate chip cookie that has 100 Calories?

Solution: To maintain her weight, the woman needs to burn off 100 Calories. Assuming there are no other losses (friction from shoes and air currents, for example), this energy must be compensated for by the work that she does against gravity (i.e. gain in the potential energy), or 100 kcalx4.18 kJ/kcal = 418 kJ. The height she needs to climb is h = E/mg = 418,000/(60x9.81) = 710 m. Assuming each step is 20 cm in height, she must climb up 710/0.20 = 3,550 steps! In reality, the human body is only about 20% efficient at converting chemical energy into gravitational potential energy. In other words, 80% of the metabolic energy generated goes into heat. Consequently, the number of steps is really smaller by a factor of 5, but it still is a pretty big number, something to think about before you take another cookie.

Example: A 90-kg (200-lb) man has a cheeseburger, regular fries, and a glass of beer for lunch and a milk shake for dessert. How long would it take him to burn off the calories, if (a) he returns to continue his routine office work or (b) he goes to a gym and bicycles at a moderate speed?

Solution: The total caloric intake can be calculated using data in Table 2, as 350+150+250+420 = 1,170 Calories. An average person (70-kg) can burn off calories at the rate of 100 Calories/hr by sitting in his office or 450 Calories/hr by bicycling. Since the rate of energy consumption is higher for a heavier person, the 90-kg man burns calories faster at the rate of 100x90/70 = 129 Calories/hr doing office work, and 450*90/70 = 579 Calories/hr bicycling. The time needed is 1170/129 = 9 hours of office work, or 1170/579 = 2 hours of bicycling. Clearly, the best way to lose the weight is by eating less. Exercising has many health benefits, but not necessarily the most effective for losing weight.

Example: In order to maintain constant weight, we must consume about 2,000 kilocalories of food energy per day of inactivity. How long could you live off your fat?

Solution: In two days of starvation and inactivity one would be able to lose about 4,000 kcal or about one pound of fat. On the average, 15% of an adult male’s weight is in the form of body fat, and 22% for the adult female. Therefore, assuming about 30 pounds per person, we could, in principle, live for about two months (30x4100/2000 = 61.5 days) simply by burning off our own fat.

Example: Suppose a 65-kg person spends 8 hours sleeping, 1 hour performing moderate physical labor, 4 hours engaging in light activity, and 11 hours working at a desk or relaxing. Is this person likely to gain weight if he maintains a 3500-Cal daily diet?

Solution: Referring to table, we calculate the energy expenditure to be (8x70+1x460+4x230+11x115) x 3600 = 11.5 MJ = 2800 Cal/day. Since this person is taking in 3500 Cal and burning only 2,800 Cal, he will gain weight.

Among health care professionals, perhaps the best known method for assessing body size is the Body Mass Index (See box “Body Mass Index”). Other factors, such as the waist-to-hip ratio, may also be important in determining the ideal weight for a healthy person.

==References==

(1) Diamond, J., Guns, Germs, and Steel: The Fate of Human Societies, W. W. Norton and Company, 1999. p. 88

(2) Rifkin, J., Beyond Beef: The Rise and Fall of the Cattle Culture, Penguin Press, N.Y., 1992.

(3) United Nations Food and Agricultural Organization (http://www.hsus.org/ace/352).

(4) Green, B. M., Eating Oil – Energy Use in Food Production, Westview Press, Boulder, Colorado, 1978.

(5) Günther, F., “Fossil Energy and Food Security,” Energy & Environment, Volume 12, Number 4, July 2001, pp. 253-272.

(6) Faughn, J., Life Science Applications for Physics, Harcourt, 1998.

(7) Thompson, D. J., et al., “Lifetime health and economic consequences of obesity,” Archives of Internal Medicine, 159:2177-83 (1999).

(8) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

==Additional Comments==

(a) Biomass may also be produced without sunlight through the utilization of certain inorganic compounds called chemo-autotrophs.

(b) They still need to drink water to prevent dehydration that may occur in a few days.

==Further Reading==

Sims, R., Bioenergy Options for a Cleaner Environment in Developed and Developing Countries, Elsevier, 2003.

Tillman, D., Combustion of Solid Fuels & Wastes, Academic Press, 1991.

Biofuels for Transport: Global Potential and Implications for Energy and Agriculture, The Worldwatch Institute, 2007.

Biomass and Bioenergy, Science Direct Elsevier Science Publishing Company.

==External Links==

National Renewable Energy Laboratory: Biomass Research (http://www.nrel.gov/biomass).

US Department of Energy (http://www1.eere.energy.gov/biomass).

Biomass Energy Research Association (http://www.bera1.org).

American Bioenergy Association (http://www.biomass.org).

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