Chapter 4 Exercise Metabolism and Bioenergetics:
Be Familiar with all definitions throughout the chapter
Bioenergetics and Metabolism
- Bioenergetics – Study of energy in the human body.
- Metabolism – All of chemical reactions that occur in the body to maintain itself. Metabolism is process in which nutrients are acquired, transported, used, and disposed of by the body.
- Exercise Metabolism – Examination of bioenergetics as it relates to unique physiologic changes and demands placed on the body during exercise.
Fuel for Energy Metabolism
- Substrates – Material or substance on which enzyme acts. Proteins, carbs, fats are main substrates used to transfer metabolic energy to be used for all types of cellular activity and life.
- Carbohydrates – Organic compounds of carbon, hydrogen, oxygen which include starches, cellulose, and sugars. Important source of energy. All carbs broken down into glucose(simple sugar).
- Glucose – Absorbed and transported in the blood. Simple sugar manufactured from carbs, fat, and lesser extent protein.
- Glycogen – Complex carb molecule used to store carbs in liver and muscle cells. When carb energy is needed, glycogen is converted into glucose for use by muscle cells.
- Fat – Helps the body use vitamins and keep skin healthy, serve as energy stores for the body. Two types of fats in food saturated and unsaturated.
- Triglycerides – Chemical substrate form in which most fat exists in food as well as in the body.
- Protein – Amino acids linked by peptide bonds, which consist of carbon, hydrogen, nitrogen, oxygen, and usually sulfur. Protein rarely supplies much energy during exercise, ignored as significant fuel for energy metabolism. Protein becomes significant source of fuel during starvation.
- Gluconeogensis – Formation of glucose from noncarbohydrate sources, such as amino acids.
Energy and Muscle Contraction
- Adenosine Triphosphate – Energy storage and transfer unit within the cells of the body. When chemical bonds holding ATP are broken, energy is released for cellular work(such as muscle contraction), breaking the bond leaves behind molecule called adenosine diphosphate (ADP).
- Adenosine Diphosphate – High-energy compound occuring in all cells from which ATP is formed. Free energy is harnessed, used to attach phosphate group to an ADP and retore ATP levels back to normal to perform more work.
- Energy is used to form myosin-actin cross-bridges that faciliate muscle contraction. Cross-bridges is an enzyme that separates phosphate from ATP, releasing energy. Energy is needed to allow cross-bridge to ratchet thin actin filament toward center of sarcomere. Once that process is complete another ATP is needed. For one cycle of a cross-bridge two ATPs are needed.
Energy and Mechanical Work
- Any form of exercise can be defined by intensity and duration.
- 40% of energy released from ATP is actually used for cellular work, remainder is released as heat.
- ATP = ADP + Pi + Energy release
- Phosphorylation – process of adding phosphate group onto ADP to create ATP
- Three metabolic pathways cells can use to generate ATP – 1. ATP-PC system, 2. Glycolytic system(glycolysis), 3. Oxidative system(oxidative phosphorylation)
- ATP-PC System – Transfers phosphate group from another high energy molecule called phosphocreatine(PC or CP) to ADP molecule enough energy can be produced to faciliate one cross-bridge cycle. ATP and PC are called phosphagens. Creating new ATP from phosphocreatine molecule(ATP-PC system) is simplest and fastest way. Occurs without presence of oxygen(anaerobic). Only supplies energy for 10-15 secs before exhausted, this system is activated at onset of activity regardless of intensity.
- Glycolysis – The other anaerobic means of producing ATP. Chemical breakdown of glucose. Anaerobic glycolysis. Glucose or glycogen must be converted to glucose-6-phosphate before it can generate energy. Conversion of glucose to glucose-6-phosphate takes 1 ATP molecule, with glycogen it does not. Glucose and glycogen are broken down into pyruvic acid(aerobic glycolysis) or lactic acid(anaerobic glycolysis). 2 ATP for each mole or unit of glucose and 3ATP for each unit of glycogen. This system can produce significantly greater amount of energy than ATP-PC system, it too is limited to approximately 30 to 50 seconds of duration.
- Oxidative System – Most complex of three energy systems. Uses substrates with aid of oxygen to generate ATP. Three oxidative systems include aerobic glycolysis, krebs cycle, elecron transport chain(ETC).
- B-oxidation – Breakdown of triglycerides into smaller subunits called free fatty acids(FFAs) to convert FFAs into acyl-CoA molecules, which then are available to enter the Krebs cycle and ultimately lead to the production of additional ATP.
Energy During Exercise
- Most important factor regulating energy utilization during exercise is the intensity and duration of exercise.
- After 90 mins of exercise majority of muscle glycogen stores are depleted.
Metabolism during Steady-State Exercise
- Excess Postexercise Oxygen Consumption(EPOC) – State in which the body’s metabolism is elevated after exercise. Energy demands fall back to baseline after exercise but oxygen consumption remains elevated for short period to keep generating ATP aerobically, this is the EPOC. ATP Above and beyond what is needed for recovery is produced to help reestablish baseline levels of ATP and PC and assist in clearing metabolic end products(like lactic acid).
Metabolism during Intermittent Work
- Most of energy comes from anaerobic metabolism. When intensity is decreased there is a continued period of high, but briefly elevated oxygen consumption. If high intensity work is short, fueld by ATP-PC, then recovery period if brief. Recovery of ATP-PC cycle is complete in 90 seconds. If period of high intensity work is longer recovery period will take longer.
The Myth of the Fat Burning Zone
- Even though a RQ of .8 results in 67% of energy coming from fat and 33% from carbs, you’re only expending 4.8 cals per minute equaling 3.2 from fat. If you double intensity to RQ of .86, 54% of energy comes from fat. But 9.75 cals are expended per minute, thus more cals from fat are still expended even though fat burning % is lower.