Energy Units

Biologists are concerned with energy flows over a wide range of scales, from a single bacterium to the whole earth. Here are some useful conversion factors and some tables of data illustrating biological energy problems.

Units of Force

Units of Pressure

Units of Energy & Work

Units of Power

Energy for Bacterial Growth

 

 % Dry
Weight

 Approx.
MW

 Molecules
per Cell

 Molecules
Synthesized
per Second

 ATPs used
per Second

 %Total
Energy for
Synthesis
 DNA

 5

 2 billion

 1

 .00083

 60,000

 2.5

 RNA

 10

 1,000,000

 15,000

 12.5

 75,000

 3.1

 Protein

 70

 60,000

 1,700,000

 1,400

 2,120,000

 88.0

 Lipid

 10

 1,000

 15,000,000

 12,500

 87,500

 3.7

 Sugars

 5

 200,000

 39,000

 32.5

 65,000

 2.7

These figures were calculated by Albert L. Lehninger for the E. coli bacterium, which has a division time of 20 minutes. Every 20 minutes during growth the cell must make all of the components needed for a new cell.The biosynthesis requires 2,400,000 molecules of ATP and 400,000 molecules of oxygen every second. Note that almost 90% of the ATP energy goes into making new proteins. From: Albert Lehninger. Biochemistry: the Molecular Basis of Cell Structure and Function. NY: Worth Publishing, 1975.

Energy for Hummingbird Migration

Some ruby-throated hummingbirds do non-stop migration flight across the Gulf of Mexico for 800 kilometers (about 500 miles). The flight takes 10 hours, a speed of 80 km/hr (50 mph). It is amazing because the bird weighs only 3 to 4 grams (a little more than a penny). This weight must include both the flying machine and the fuel.

During flight the birds use about 250 ml of O2 per hour. The energy required can be calculated from the oxygen consumption:

Power = (0.25 liters O2/hr)(4.82 kcal/liter O2) = 1.2 kcal/hr

Total energy required = (1.2 kcal/hr)(10 hrs) = 12 kcal

The amount of fuel required can be calculated by assuming that the hummingbird stores the energy as fat at 9 kcal/gm:

Fat required = (12 kcal)/(9 kcal/gm) = 1.3 gm

If the energy were stored as carbohydrates (glycogen) 3.0 gm would be needed, plus some extra water that glycogen carries with it. About 3 times more fuel weight would be required. This is why migrating animals store energy as fat instead of carbohydrates. Hummingbird data is from: Oliver Pearson. The metabolism of hummingbirds. Scientific American, January, 1953, p. 69-72.

Energy Stores of the Human Body

 Storage Form

 Amount
kg

 Energy Stored
kcal

 Time of Use
min at 3 mph

 Miles at
3 mph

ATP & Creatine
Phosphate

 ATP = 0.1
CP = 0.15

 10

 3

 0.15

 Glycogen

 0.425

 1700

 510

 25

 Fat
(triglycerides)

 15

135,000

40300

2015

In addition to these molecules the body can burn protein for energy. There is 10-15 kg of protein in the body, giving a potential of another 40,000 to 60,000 calories. Much of the protein is required for cell structure and function, however, so it is not clear how much protein is available for energy. Data from:

Peter Hochachka & George Somero. Biochemical Adaptation. Princeton University Press, 1984, chapter 4.
George Brooks & Thomas Fahey. Fundamentals of Human Performance. NY: Macmillan, 1987, chapter 1.

Power Output for Human Walking and Running

 Velocity
mph

 Velocity
meters/min

 Oxygen
Consumption
mL/min

 Power
kcal/min

 Power
watts

 0

 0

 210

 1.01

 70

 2

 54

 530

 2.57

 179

 3

 81

 700

 3.35

 233

 4

 107

 850

 4.11

 286

 5

 134

 1820

 8.76

 610

 6

 161

 2140

 10.3

 719

 7.5

 201

 2630

 12.7

 883

 10

 268

 3430

 16.5

 1150

 15

 403

 5050

 24.3

 1690

Values from 0 to 4 mph are for walking. For 5 mph and above figures are for running. Calculations are for a 60 kg person.This person will use 1440 kcal each day at rest. If she walks 2 miles/day this will add 94 kcal to her energy use. If she were to run the 2 miles she would add 188 kcal (running costs twice as much as walking in energy). Figures are calculated from equations in: American College of Sports Medicine. Guidelines for Exercise Testing and Prescription, 4th edition. Philadelphia: Lea & Febiger, 1991, p. 285-300.

Power Costs of Human Activities

This table gives a picture of relative energy costs of common activities. A MET is 0.0169 kcal per min per kilogram. To calculate your energy output multiply the MET figures by 0.169 and your weight in kilograms. Suppose y
 Activity

 Energy Cost
METs

 Sleep, watching TV while lying

 0.9

 Reclining or sitting while talking, writing, reading, kissing

 1 to 2

 Standing quietly

 1.2

 Light home activities: cooking, dish washing, watering lawn

 1.5 to 2.5

 Office work

 1.5 to 2.5

 Driving car

2

 Playing music

  2 to 3

 Light carpentry, plumbing, electrical work

 3.5

 Walking 3 mph

 3.5

 Bicycling, leisurely

 4 to 6

 Painting, remodeling

 4.5 to 5

 Playing baseball

 5

 Dancing

 5 to7

 Carrying groceries, boxes, furniture

 6 to 8

 Backpacking, cross country skiing

 7 to 9

 Playing basketball, football

 8 to 9

 Digging ditches, carrying bricks

 8 to 9

 Running 6 mph

 10

 Fast rope jumping

 12

 Running, 8 mph

 13.5

 Running, 10 mph

 16

ou are backpacking (8 METs) and weight 70 kg:

Power = (0.0169 kcal/min-kg)(8)(70 kg) = 9.46 kcal/min = 660 watts

These figures are from: Barbara Ainsworth, William Haskell, Arthur Leon, David Jacobs, Jr., Henry Montoye, James Sallis & Ralph Paffenbarger, Jr. Compendium of physical activities: classification of energy costs of human physical activities. Medicine and Science in Sports and Exercise 25: 71-80, 1993.

I question 2 of the figures from the Ainsworth table. Sexual activity ("active, vigorous effort") is given only a 1.5 MET rating, while showering is given a value of 4.0 METs! I wonder if they mixed up the 2 values.

Human Power Limits

 Activity

 Duration

 Power
watts

 Power
kcal/min

 Energy Used
Power X Time
kcal

 Single vigorous jump or lift

 < 1 sec

 4500

 64

 1

 Sprint

 20 sec

 2200

 32

 10

 Mile run

 5 min

 1100

 16

 80

 Marathon

 2 hr

 400

 5

 600

 Manual labor

 10 hr

 150

 2

 1200

 Rest

 24 hr

 75

 1

 1440

The maximum power the body can produce is seen in jumping or in rapidly lifting heavy weights. We can sustain this level of activity for a second or less ("the harder one works the sooner one must stop" Bent). If we cut down the power output we can go for a longer period of time. Power output of the body is determined by the amounts of different energy stores and by the enzymes that release the energy. ATP and creatine phosphate can be broken down very rapidly (high power), but the total amount is small. Burning of fats for energy is very slow (low power), but there are huge amounts.

The table comes from: Henry A. Bent. Energy and exercise. I: How much work can a person do? Journal of Chemical Education 55: 456-458, 1978.

Use of Energy in Human Society

In addition to metabolism of food we use energy, mostly in the form of fossil fuels derived from living creatures, for industry and transportation. Use of energy varies widely from country to country. In the US we use about 42 barrels of oil equivalent per capita every year, while in India, Nigeria and the Phillipines the amount is 2 to 3 barrels per capita per year.

Using the conversion factors from above (a barrel of oil is equivalent to 6.1 million joules) it is easy to convert the 42 barrels/year into watts or calories/day:

(42 barrels/yr)(6.1 billion joules/barrel)/(365 days/yr) = 702 million joules/day

(702 million j/day)/(4.18 j/cal)(1000 cal/kcal) = 168,000 kcal/day = 8140 watts

A fairly good estimate for the average power output of a human is 100 watts. Comparing this with the figure for industrial energy use we see that industrial user is about 80 X the metabolic energy use. In a sense each of us has 80 energy servants working for him continuously. For further information on this subject see the September 1990 special issue of Scientific American (Energy for Planet Earth).

Energy Budget of the Earth

 

 

Return to Review Index / Return to Homepage