270
7 Light in Biology and Medicine
7.30 Solar Radiation and Biology
The Sun supplies about 10 24 kcal of energy over the surface of the Earth in 1
year. Above the atmosphere, this solar power amounts to about 1370 W per square
meter. (Your toaster uses about 1000 W of power while in operation.) With the Sun
overhead on a clear day, plant leaves would receive about 1000 W per square meter.
Including the solar ray angle variations and day/night cycles, the average plant on
Earth gets only about 200 W/m 2 . Using photosynthesis, a green plant 49 can convert
about 1–2% of the arriving solar energy into stored chemical energy in organic
molecules, such as glucose. 50
This could explain why you are not green. Assuming you could expose a quarter
of your green skin to sunlight, you would produce less than about 0.03×400 W/m 2 ×
0.5 m 2 = 6 W of power. But your metabolism will require an average of 160 W. A
person is supported in his/her energy needs by at least 27 m 2 of productive leaf
surfaces. That’s about 17,000 healthy tree leaves. A medium size maple tree can
have that many, but lots of the leaves are in partial shadows, so we should look for
a large maple tree to sustain just one of us.
Currently, photovoltaic cells can convert about 10% of the incoming solar energy
into chemical energy in the disassociation of water into hydrogen and oxygen. Your
metabolic energy requirement could be fulfilled by a solar panel aimed at the Sun
and covering about 8 m 2 .
Plants fix about 150 trillion kilograms of carbon each year; i.e. about 10 18 kcal of
equivalent glucose energy per year. Largely to support industry and food production
and distribution, man is using about 4 × 10 17 kcal per year in fossil fuel, the result
of plants geologically converted over hundreds of millions of years.
The chemical energy used by organisms is stored in carbohydrates, lipids, waxes,
and proteins. The energy stored in these molecules can be later converted into the
energy in a phosphate bond of adenosine triphosphate (ATP), which is the ready fuel
for the activity of cells.
The analysis of energy exchanges in organisms follows the principles of nonequilibrium thermodynamics. (See Sect. 9.23.)
49 Chlorophyll (a and b) absorb in the red and blue part of the spectrum, while green light tends to
be reflected. See Fig. 7.13.
50 The pigment-protein complex which absorbs photons through electron excitation acts as a highly
efficient antenna. However, It is possible that quantum coherence plays a role in this transfer
of energy, as described by Engel et al., Evidence for wavelike energy transfer through quantum
coherence in photosynthetic systems, Nature 446, 782–786 (2007).
7 Light in Biology and Medicine
7.30 Solar Radiation and Biology
The Sun supplies about 10 24 kcal of energy over the surface of the Earth in 1
year. Above the atmosphere, this solar power amounts to about 1370 W per square
meter. (Your toaster uses about 1000 W of power while in operation.) With the Sun
overhead on a clear day, plant leaves would receive about 1000 W per square meter.
Including the solar ray angle variations and day/night cycles, the average plant on
Earth gets only about 200 W/m 2 . Using photosynthesis, a green plant 49 can convert
about 1–2% of the arriving solar energy into stored chemical energy in organic
molecules, such as glucose. 50
This could explain why you are not green. Assuming you could expose a quarter
of your green skin to sunlight, you would produce less than about 0.03×400 W/m 2 ×
0.5 m 2 = 6 W of power. But your metabolism will require an average of 160 W. A
person is supported in his/her energy needs by at least 27 m 2 of productive leaf
surfaces. That’s about 17,000 healthy tree leaves. A medium size maple tree can
have that many, but lots of the leaves are in partial shadows, so we should look for
a large maple tree to sustain just one of us.
Currently, photovoltaic cells can convert about 10% of the incoming solar energy
into chemical energy in the disassociation of water into hydrogen and oxygen. Your
metabolic energy requirement could be fulfilled by a solar panel aimed at the Sun
and covering about 8 m 2 .
Plants fix about 150 trillion kilograms of carbon each year; i.e. about 10 18 kcal of
equivalent glucose energy per year. Largely to support industry and food production
and distribution, man is using about 4 × 10 17 kcal per year in fossil fuel, the result
of plants geologically converted over hundreds of millions of years.
The chemical energy used by organisms is stored in carbohydrates, lipids, waxes,
and proteins. The energy stored in these molecules can be later converted into the
energy in a phosphate bond of adenosine triphosphate (ATP), which is the ready fuel
for the activity of cells.
The analysis of energy exchanges in organisms follows the principles of nonequilibrium thermodynamics. (See Sect. 9.23.)
49 Chlorophyll (a and b) absorb in the red and blue part of the spectrum, while green light tends to
be reflected. See Fig. 7.13.
50 The pigment-protein complex which absorbs photons through electron excitation acts as a highly
efficient antenna. However, It is possible that quantum coherence plays a role in this transfer
of energy, as described by Engel et al., Evidence for wavelike energy transfer through quantum
coherence in photosynthetic systems, Nature 446, 782–786 (2007).