Bioenergetics

Bioenergetics is the study of energy and energy transformations in biological systems. While this may appear to be a very dry (and boring) topic, a basic understanding of energy and its use in living organisms is invaluable. Living organisms must perform work to stay alive, grow and reproduce. All living organisms must possess the ability to obtain energy and to be able to transform that energy into a form that can be used by its cells. Practically speaking , knowing the fundamentals of bioenergetics aids in the understanding of cell function and allows us to understand why and how the cell is able to harness energy.


Laws of Thermodynamics

A quick review of the laws of thermodynamics is in order. While these laws should be familiar to you, you should make sure that you have a good understanding of what these laws mean.

The First Law: The conservation of energy

Energy cannot be created or destroyed

This law is telling you that energy cannot come from nowhere. Therefore, a reaction which requires the input of energy cannot occur unless there is a source for the needed energy. This is not foreign to you as you have experience with this concept. Imagine that there is a pen on the table next to the computer. You would never expect the pen to suddenly rise off the table and move towards the ceiling. That is because such a movement requires energy to move the pen against the force of gravity. Unless someone (or thing) gives the pen this energy, it will not move. Understand that a reaction that requires energy is not impossible. You could pick up the pen and throw it so it hits the ceiling. In this case, you are giving energy to the pen. This energy ultimately comes from the nutrients that you eat and convert into the muscle and fuel for the muscle that you used to move the pen.

The Second Law: Nothing is Perfect

Every time energy is changed, there is an increase in the entropy of the universe

The second law implies that whenever energy is changed, some of the energy will be wasted which will lead to an increase in the disorder of the universe. When dealing with biological systems, this wasted energy is usually in the form of heat. Thus, whenever any reaction occurs, some of the energy in the reaction will be released as heat.


Gibbs Free Energy

Now, these two laws are relatively easy to repeat and understand - but how can we use them to answer questions dealing with biological reactions. Through our experience, you have a feeling for how these laws apply to macro events that surround us (such as the pen moving toward the ceiling). You do not, however, have an understanding about how these laws apply to biochemical reactions that not only you can't see but you didn't even know were occurring. For example, the following is a reaction that every cell performs, the adding of a phosphate group (PO42-) to the sixth carbon of the monosaccharide, glucose.

PO42- + glucose <----------> glucose-6-phosphate

Is this reaction possible? Most likely, you have no idea as you have little practical experience dealing with this reaction. You might assume that since every cell can phosphorylate glucose, this reaction is thermodynamically permissible. But in actuality, this reaction, as written, is (usually) impossible. This is because this reaction requires energy to occur and there is no source for this energy (thus this reaction is analogous to the pen moving to the ceiling). How were you to know this? You couldn't! At some time, a scientist measured the change in the energy between the reactants (PO42- + glucose) and the product (glucose-6-phosphate) and found that under certain conditions, the product contained more energy (I bet that got the scientist tenure!). Note: the scientist's measurements took into account both actual energy changes as well as entropy (disorder) changes. These measurements are reported using a value known as Gibbs Free Energy, abbreviated delta G. For a reaction in which the products have more energy than the reactants (thus a gain in energy), the delta G for that reaction is positive. That means that a reaction with a positive delta G cannot occur. For a reaction in which the products have less energy than the reactants (thus a loss in energy), the delta G for that reaction is negative and the reaction is thermodynamically permissible. You should remember this!

This should leave you wondering - the phosphorylation of glucose (as shown above) has a positive delta G and thus cannot occur, but every cell phosphorylates glucose! How can this be? In order for the reaction to occur, an source of energy is needed ( like the pen to the ceiling). A common source of energy that cells use is a compound call ATP (adenosine triphosphate). ATP can undergo the hydrolysis (breaking off) of one of the phosphate groups to produce adenosine diphosphate, ADP and a free phosphate. This is written as:

ATP ----------> ADP + PO42-

This reaction has a negative delta G. Thus if the cell breaks up ATP at the same time that it is adding the phosphate to glucose, the cell can use the energy released from ATP to fuel the addition of the phosphate to glucose. This requires that the amount of energy released from the hydrolysis of ATP is greater than the amount of energy needed to add the phosphate. If this occurs, then these reactions are said to be coupled. If so, then the reaction would be written as:

ATP + PO42- + glucose ----------> glucose-6-phosphate +ADP + PO42-

This reaction can be simplified (just like in Algebra) by noticing that there is PO42- on each side of the reaction and can be cancelled.

ATP + glucose ----------> glucose-6-phosphate +ADP

Now, where do cells get the ATP? Obviously, if the breakdown of ATP has a negative delta G (gives off energy), the formation of ATP must have a positive delta G.

ADP + PO42- ----------> ATP

Where do cells get the energy to make ATP (remember, most of the cellular reactions that require energy use the hydrolysis of ATP as the source - ATP is like money to the cell). The ultimate source of this energy is from the nutrients eaten by organisms. The breakdown of nutrients such as sugars and fats into carbon dioxide and water has a negative delta G. The cells are capable of taking these nutrients, breaking them down and using the energy in them to make ATP. You will know more about these processes later.


Terms to Remember

Metabolism
the sum of all chemical reactions that occur in a cell or organisms
Anabolism
that part of metabolism that synthesizes complicated biomolecules with the use of energy
Catabolism
that part of metabolism that degrades biomolecules to release energy

You must remember that reactions with a positive delta G are not permissible and those with a negative delta G are permissible. Also, the delta G for a given reaction is not a constant but is dependent on several factors. The most important factor that affects the value of delta G is the relative concentration of the products and the reactants.

 

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