at the outset that the ability of certain molecules to capture and use
small amounts of energy efficiently is a central aspect of life. The continuous
chemical reactions and changes that go on in an organism is called metabolism.
Accumulation and use of energy is the basis of metabolism.
In the long
run, almost all the energy for life on Earth comes from sunlight, through
photosynthesis by plants. Energy is stored as potential energy in chemical
bonds of carbon compounds. This is our "free reserve."
X: Cycle of photosynthesis and oxidation
-- energy capture and release.
shows the cycle of energy transformation overall.
and Energy Storage
In most of photosynthesis, electromagnetic energy from sunlight is used
to store chemical potential energy through oxidation of water,
using CO2 for the carbon.
+ H2O + sunlight energy
rich carbon compounds + O2 + heat
may be written in general as:
+ H2O + sunlight (CH2O)n
is the generic name for the energy-rich compounds--sugars and starches
such as C6H12O6 [(CH2O)6]
or C11H22O11 [(CH2O)11].
the energy captured is lost
as heat. Photosynthesis is less than 1% efficient--only 1% or so of the
total energy falling on leaves is used for photosynthesis--mainly the
blue and ultraviolet region. Because photosynthesis only uses blue and
ultraviolet wavelengths of energy, it only actually captures energy from
a small fraction of the solar spectrum.
Then again only 1% of this captured energy is converted into food.
appear green because they reflect most of the sunlight in the visible
region. The main molecules responsible for photosynthesis is the family
of pigments called chlorophyll (NOTE: Pigments are light absorbing molecules
that by absorbing light preferentially of one or a set of wavelengths
gives the color to the material that contains them.) Chlorophylls are
formed in cell bodies called chloroplasts. Chlorophyll and other similar
energy-transforming molecules (phycocyanin, fucoxanthin, phycoerythrin)
are characteristic of autotrophic organisms. Autotrophs are organisms
that are able to manufacture their own basic supply of energy-rich carbon
compounds from CO2. Heterotrophs (like us) on the other hand,
have to be supplied with energy-rich carbon compounds from outside sources.
All autotrophs reduce CO2 to carbohydrates (written in general
as (CH2O)n) or related organic compounds.
may also occur through oxidation of compounds other than water. There
are a few bacteria called chemoautotrophic bacteria that get their energy
for storage not from light (photosynthesis) but from other inorganic chemicals
(chemosynthesis). Many of these bacteria can also do without pure oxygen
as long as they have CO2 and energy from chemical bonds. Thus
some use H2S found in volcanic ash and make (CH2O)
+ 2H2S + sunlight or chemical bond energy
+ H2O + 2S
can use bond energy from H2S instead of sunlight. Recall how
we said in the previous section that sulfur could have been a "contender"
for the position oxygen holds in making life possible.
species oxidize H2:
+ 2H2 [CH2O]
some complicated molecule like isopropanol:
+ 2CH3CHOHCH3 [CH2O]
+ H2O + 2CH3COCH3
photosynthesis may be written as:
+ 2H2A [CH2O]
+ H2O + 2A
(e.g. H2O, H2S) is the compound that gets oxidized
to A. The role of H2A is to donate hydrogen to make the energy-rich
carbohydrates (CH2O)n from CO2. <<NOTE
oxidation is the process--often a series of processes--by which the energy
in (CH2O)n is eventually used by the organism with the final
products being CO2 and H2O which are excreted. Glucose
is a type of sugar made through photosynthesis. The oxidation of glucose
can be used as an example to show the oxidation reaction:
+ 6CO2 + 688 kcal/mole
that comes into the body through respiration enables the biological oxidation
(or "combustion) of carbohydrates with the release of energy. Note
that this energy is of a different type than the original light energy
which helped form the sugar.
similarity of this reaction to the burning
of coal described in the Energy System. We had to have high temperatures
for the combustion of coal. Compared to that, our combustion takes place
at low (body) temperatures. Glucose is a complex
and it breaks down in steps. Actually, this breakdown happens very very
slowly at normal temperatures. Certain catalysts--enzymes and coenzymes--speed
these up as needed, in the absence of heat.
used for energy are not directly derivable from glucose. The vertebrate
animals use carbohydrates, fats, and proteins to store energy. These are
broken down into smaller units before oxidation begins. Adenosine triphosphate
(ATP) and adenosine diphosphate (ADP) play a unique role in mobilizing
and transforming energy in living systems. The phosphate bonds in ATP
and ADP are used to transfer energy. It takes a long chain of processes
to eventually complete the breakdown of sugars and starches to H2O
and CO2, material being constructed and deposited along the
Y: Adenosine monophosphate.
shows the structure of adenosine monophosphate, made of the protein
adenine and the sugar ribose with a phosphate group hanging at the side.
One or two more phosphate groups can be linked on to the phosphate in
AMP to give ADP and ATP respectively. The ADP phosphate bond is about
10 kcal/mole and breaking and building this (the ADP-ATP cycle) transfers
energy from glucose to where it is needed. The high energy phosphate bond
is denoted by ~ph. This reaction, called phosphorylation, is represented
in the equation:
+ ATP glucopyromose-6-phosphate
+ ~ph GP6P
higher energy than glucose and we say the glucose has been phosphorylated.
This compound can take part in reactions that glucose cannot, and use
the energy in these reactions. One example is muscular contraction, where
the ~ph bonds transfer the energy needed.
the extraction of energy and sugar breakdown occurs through fermentation
of sugars to alcohol and acids (alcohols have the -OH group, acids have
the -COOH group).