ENERGY TRANSFORMATIONS

We are going to learn about photosynthesis and respiration.
First, we need to review our understanding of energy  transformations in cells.

Energy is the capacity to do work.

Energy comes in different forms that can be interconverted.
  E.g. light, electricity, heat.

What we need to know about energy is bound up in the  laws of thermodynamics.

First Law of Thermodynamics: In a closed system, energy can be changed  from one form to another but cannot be created or destroyed.
  (We’re not counting energy at the atomic level, where mass and energy are interconvertable).

We need to introduce the idea of Potential Energy.
 The energy in a system that could be used to do work.
  I.e. USABLE ENERGY.
   Analogy of water behind a dam.

Second Law of Thermodynamics: In energy exchanges and conversions, if no energy leaves or enters a system (i.e. a closed system),    
    the potential energy of the final state will always be less than the potential energy of the initial state.

Different ways to say the same thing:

Energy goes from forms that are more useful for work to a form that’s less useful (heat).
Closed systems tend to go from greater order  (complexity) to less order (greater entropy). Closed systems tend to become
    random, simpler.
Closed systems go from disequilibrium and instability,  to equilibrium and stability.   Fig. 2.8. Read p. 20 of text.
    What happens to stacked books on a shelf, if left to themselves over time?

Biological systems are very organized and complex.
  I.e. Non-random.
     Organelles, cells, large organisms, ecosystems.
They become increasingly more complex with time.
That takes an input of energy to fight the tendency   towards disorder.
    Where does that energy come from?

Energy transformations are very important to cells,  organisms and ecosystems.

An ecosystem is “a fountain of energy flowing through a  circuit of soils, plants and animals”.  Aldo Leopold  

Glucose is more complex than the ingredients from  which it’s synthesized, carbon dioxide and water.
It takes energy to make glucose during photosynthesis.
 The plant gets that energy from the SUN.

When glucose is synthesized, much of the energy used to  make it is stored in the molecule as
  CHEMICAL ENERGY, a form of potential energy.

Think of rolling a boulder uphill.
    The processes of photosynthesis and respiration are analogous.
        In photosynthesis, the plant in effect rolls the ball uphill to make glucose from carbon dioxide and water, only instead the
            light energy stored in glucose is chemical potential energy.
        During respiration, the ball rolls downhill in effect, releasing the chemical potential energy in a form the cell uses to run its
            many reactions and create even more complexity, i.e. ATP or adenosine triphosphate.

Oxidation/Reduction
 These are also chemical terms.
  Oxidation - loss of electrons (or H atoms)
  Reduction - gain of electrons (or H atoms)
For something to be oxidized, something else must be    reduced.
 Redox reactions.

Let’s look at the oxidation states of carbon.
 Carbon with a lot of H’s, is highly reduced.
 Remove H’s, add oxygens, carbon is oxidized.
  Electrons go to lower energy levels.
   less energetic.
  Lower caloric value.
 Reduced carbon molecules have more potential energy.

Methane                    211 kcal/mole liberated by combustion
methanol                   171
formaldehyde           134
formic acid                 63
carbon dioxide            ?
                                                          (What’s a calorie? a kcal?   A mole?)

Can you burn carbon dioxide?

Review: Fats have more energy than carbohydrates. Why?

Compounds involved in energy transformations.
    ATP -- adenosine triphosphate.
        A ribonucleotide.

The bonds between the first and second, and second and  third phosphates, ‘store’ energy. They are said to   be high energy
    bonds.
 Due to the repulsive forces of the negative charges   on the phosphates.
 When the bonds are broken, or hydrolyzed, 7.3 kcal/mole are released. 
This is called an  EXERGONIC REACTION.
  It releases energy.

ATP is the energy currency of the cell. Cells use ATP as  an immediate source of energy to do work.

 1. The energy is in an immediately available form.   Cells can use it quickly, for many reactions.
  Approx. 2 million ATPs used/sec in an average cell.
 2. The energy of hydrolysis of the terminal   phosphate is just the  right amount for many different reactions.

ATP is the cell’s ENERGY CURRENCY. Like Money!

 Starch, fats etc are like real estate, stocks and bonds.
 Glucose, fats and starch have a lot more energy per mole, but it’s not immediately usable.
    The molecules have to be  broken down.

How is ATP used?
 Cells couple the hydrolysis of ATP to reactions, those that require an input of energy, e.g. in creating
        complexity.

Cells COUPLE REACTIONS to each other.

Kinase enzyme takes phosphate from ATP and adds it to another molecule.
 PHOSPHORYLATION
That molecule is now ‘energized’ to react with a second  molecule to  create a product. In the process, the  phosphate is released.

E.g. the synthesis of sucrose.

ATP + glucose --> glucose phosphate + ADP
ATP + fructose --> fructose phosphate + ADP

glucose phosphate + fructose phosphate --> sucrose + 2 inorganic phosphate

Reaction uses 2 X 7.3 kcal/mole sucrose = 14.6 kcal/mole -- released by hydrolysis of ATP.
    But only 5.5 kcal/mole of energy needed to actually join glucose and fructose.

Sooooooo, 14.6 - 5.5 = 9.1 kcal/mole excess energy drives the reaction. What form does this 'excess' energy take?
  In other words, there’s a net decrease in usable energy, even though complexity is increasing.

That also satisifies the second law of thermodynamics
 

Preview: PHOTOSYNTHESIS

First, how important is it?

“This process -- photosynthesis -- is the route by which virtually all energy enters our biosphere. Each year, more than 250
billion tons of sugar are produced worldwide by photosynthetic organisms. The importance of photosynthesis, however, extends
far beyond the sheer weight of this product. Without this flow of energy from the sun, channeled largely through the chloroplasts
of eukaryotic cells, the pace of life on this planet would swiftly dininish and then would virtually cease altogether, as dictated by
the inexorable second law of thermodynamics.”   Raven et al., 6th ed.

This is what photosynthesis is about:

        3 CO2 +6H2O ---->  C3H6O3 + 3O2 + 3 H2O

The 3 carbon compound is a simple sugar called a triose.
 Trioses are then used to make more complex sugars, including glucose, fructose and starch.

Can express this more simply as:

CO2 + H2O  ------> (CH2O) + O2
    (...) -- just a symbolic term for carbohydrate
            It takes the energy of light to accomplish this process

Carbon dioxide is reduced to produce carbohydrate (CH2O).
 That requires a source of energy and reductant, or high energy electrons.
 The energy ends up as chemical potential energy stored in carbohydrate and other complex molecules.