Capturing Energy for Life

We said 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."

Figure ?: Cycle of photosynthesis and oxidation -- energy capture and release.

Figure ? shows the cycle of energy transformation overall.

Photosynthesis 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.

CO2 + H2O + sunlight energy rich carbon compounds + O2 + heat
	(e.g. C12H22O11)

This equation may be written in general as:

CO2 + H2O + sunlight (CH2O)n + O2

where (CH₂O)_n is the generic name for the energy-rich compounds--sugars and starches such as C₆H₁₂O₆ [(CH₂O)₆] or C₁₁H₂₂O₁₁ [(CH₂O)₁₁].

Part of the energy captured is lost as heat. (link to Energy--but where in Energy?) 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 energy is converted into food. (Indira- i'm confused....is it 1% of the total energy falling on plants that is converted into food, or is it 1% of the energy in the "proper" wavelengths?)

Leaves appear green because they reflect most of the sunlight in 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 **** 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.

Photosynthesis 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 oxygen as long as they have CO₂ and electromagnetic energy. Thus some use H₂S found in volcanic ash and make (CH₂O) according to:

CO2 + 2H2S + sunlight [CH2O] + H2O + 2S

Recall how we said in the previous section that sulfur could have been a "contender" for the position oxygen holds in making life possible. Some species oxidize H₂:

CO2 + 2H2 [CH2O] + H2O

or even some complicated molecule like isopropanol:

CO2 + 2CH3CHOHCH3 [CH2O] + H2O + 2CH3COCH3

In general, photosynthesis may be written as:

CO2 + 2H2A [CH2O] + H2O + 2A

where H₂A (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 TO TEACHER>>

Biological 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:

C6H12O6 +6O2 6H2O + 6CO2 + 688 kcal/mole

The oxygen 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.

Note the 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 compound and it breaks down in steps. Actually, this breakdown to lead eventually to CO₂ and H₂O happens very very slowly at low temperatures. Certain catalysts--enzymes and coenzymes-- are essential to speed these up as needed.

All compounds 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. (Note.) 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 way.

Note from above:

The ATP-ADP System

Figure ?: Adenosine monophosphate.

Figure? 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:

glucose + ATP glucopyromose-6-phosphate + ADP
	(known as GP6P)

or

glucose + ~ph GP6P

GP6P has 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.

Another example is muscular contraction where the ~ph bonds transfer the energy needed.

Part of 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).

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