MCAT Biochemistry Review
Chapter 12: Bioenergetics and Regulation of Metabolism
You got up this morning with a really ambitious plan: study for the MCAT! The day started with a big breakfast, and then you dove into MCAT Biochemistry Review. A few chapters in you noticed your stomach growling, but you were having so much fun that you ignored it. A little while later, your body realized it wasn't getting any more food for a while, but it still needed energy. Where does it come from?
The human body is an incredible system. When we skip lunch on a study day, we produce hormones that help raise the level of certain energy molecules in the bloodstream, mainly glucose. This is a good thing because the brain relies solely on glucose for most of its metabolism, and we always want to be thinking at our peak. Glucose in the blood comes from either our diet, such as when we eat a big breakfast, or from our fuel stores, through the processes of gluconeogenesis and glycogenolysis. These processes, just like the formation and consumption of ATP, are highly regulated.
In this chapter, we'll highlight the basic principles of bioenergetics, including thermodynamics: the sources of energy and the reactions that play a key role in moving that energy around. Then we'll examine the different energy states of the body before taking a look at the intimate relationship of hormones with metabolism. We'll spend some time examining the regulation of metabolism, regulatory enzymes for some common pathways, and how specific tissues react to regulatory pathways. By the end of this chapter, you'll be able to tell where and how your food is being used, and you probably won't choose to skip lunch again—no matter how much fun you're having!
12.1 Thermodynamics and Bioenergetics
If we take a look back at what we've learned about thermodynamics in Chapter 3 of MCAT Physics and Math Review and Chapter 7 of MCAT General Chemistry Review, it becomes evident that we already know quite a bit. However, most of the data that we've seen so far has been obtained under standard-state conditions (25°C, 1 atm pressure, and 1 M concentrations). These assumptions work in a chemistry lab, but must be adjusted for application in the human body.
Biological systems are often considered open systems because they can exchange both energy and matter with the environment. Energy is exchanged in the form of mechanical work when something is moved over a distance, or as heat energy. Matter is exchanged through food consumption and elimination, as well as respiration. Most biochemical studies are performed on the cellular or subcellular level rather than in an entire organism. These systems can be considered closed because there is no exchange of energy with the environment. In such a system, we can make useful simplifications about the internal energy, U. Internal energy is the sum of all of the different interactions between and within atoms in a system; vibration, rotation, linear motion, and stored chemical energies all contribute.
The energy of chemical reactions are described as part of general chemistry, while work is generally associated with physics. Be aware that on Test Day, you may see crossover that allows you to draw on knowledge of the other subjects and to use that background information to your advantage.
Because the system is closed, the change in internal energy can come only in the form of work or heat. This can be represented mathematically through the First Law of Thermodynamics, ΔU = Q – W. Work in thermodynamics refers to changes in pressure and volume. These are constant in most living systems, so the only quantity of interest in determining internal energy is heat.
ENTHALPY, ENTROPY, AND FREE ENERGY
Bioenergetics is used to describe energy states in biological systems. Changes in free energy (ΔG) provide information about chemical reactions and can predict whether a chemical reaction is favorable and will occur. In biological systems, ATP plays a crucial role in transferring energy from energy-releasing catabolic processes to energy-requiring anabolic processes.
Whether a chemical reaction proceeds is determined by the degree to which enthalpy and entropy change during a chemical reaction. Enthalpy measures the overall change in heat of a system during a reaction. At constant pressure and volume, enthalpy (ΔH) and thermodynamic heat exchange (Q) are equal. Changes in entropy (ΔS) measure the degree of disorder or energy dispersion in a system. While the MCAT will not test on the level of statistical thermodynamics, this conceptual understanding of entropy (ΔS) will be helpful. Entropy carries the units
When combined together mathematically, along with temperature (T), these quantities can be related through the Gibbs free energy equation:
ΔG = ΔH − TΔS
which predicts the direction in which a chemical reaction proceeds spontaneously. Spontaneous reactions proceed in the forward direction, exhibit a net loss of free energy, and therefore have a negative ΔG. In contrast, nonspontaneous reactions, which would be spontaneous in the reverse direction, exhibit a net gain of energy and have a positive ΔG. Free energy approaches zero as the reaction proceeds to equilibrium and there is no net change in concentration of reactants or products.
Enthalpy, entropy, and free energy are discussed more thoroughly in Chapter 7 of MCAT General Chemistry Review.
The change in free energy (ΔG) that we have been discussing up to this point predicts changes occurring at any concentration of products and reactants and at any temperature. In contrast, standard free energy (ΔG°) is the energy change that occurs at standard concentrations of 1 M, pressure of 1 atm, and temperature of 25°C. These can be related by the equation:
ΔG = ΔG° + RT ln (Q)
where R is the universal gas constant, T is the temperature, and Q is the reaction quotient. Biochemical analysis works well under all standard conditions except one: pH. A 1 M concentration of protons would correspond to a pH of 0, which is far too acidic for most biochemical reactions. Therefore, in the modified standard state, [H+] = 10−7 M, and the pH is 7. With this additional condition, ΔG° is given the special symbol ΔG°′, indicating that it is standardized to the neutral buffers used in biochemistry. Note that if the concentrations of reactants and products differ from 1 M, these must still be adjusted for in the equation above.
The shift in ΔG as a result of changing concentration is not universally toward or away from spontaneity. There is a general trend that reactions with more products than reactants have a more negative ΔG, while reactions with more reactants than products have a more positive ΔG. While this trend is useful for making quick assessments, always double check with numbers on Test Day.
MCAT Concept Check 12.1:
Before you move on, assess your understanding of the material with these questions.
1. What conditions does ΔG°′ adjust for that are not considered with ΔG°?
2. Why can heat be used as a measure internal energy in living systems?
3. Complete the following table relating the change in entropy and enthalpy of a reaction with whether the reaction is spontaneous.