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CH.4 Nutrient Role in Bioenergetics

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Bioenergetics
Bioenergetics refers to the flow of energy within a living system.

Energy is the capacity to do work.

Aerobic reactions require oxygen.

Anaerobic reactions do not require oxygen.
Anaerobic and aerobic breakdown of ingested food nutrients provides
the energy source for synthesizing the chemical fuel that powers all forms of biologic work.
First law
Energy is neither created nor destroyed, but instead, transforms from one state to another without being used up.
There are six forms of interchangeable energy states:
Chemical
Light
Electric
Mechanical
Heat
Nuclear
During photosynthesis
chlorophyll absorbs radiant energy to synthesize glucose from carbon dioxide and water and releases oxygen.
Solar energy and photosynthesis power
the animal world with food and oxygen.
Respiration is
the reverse of photosynthesis.
During respiration
the chemical energy stored in glucose, lipid, or protein molecules is extracted in the presence of oxygen.
Takes one of three forms:
Mechanical work
Chemical work
Transport work
Mechanical work
of muscle contraction
muscles, cells, cilia.
Chemical work
for synthesizing cellular molecules
continuous synthesis of cellular components occurs as other components break down.
Transport work
that concentrates diverse substances in body fluids
passive and active transport.
Potential energy
refers to energy associated with a substance's structure or position.
Kinetic energy
refers to energy of motion.
Potential energy and kinetic energy
constitute the total energy of any system.
Releasing potential energy
transforms it into kinetic energy of motion.
Oxidation
a substance loses electrons
Reduction
a substance gains electrons
Redox reactions
power the body's energy transfer processes.
Potential energy is
extracted from food and conserved within the bonds of ATP.
Chemical energy is
extracted and transferred in ATP to power biologic work.
Powers all forms of biologic work.
The energy liberated during ATP breakdown transfers directly to other energy-requiring molecules.
In the degradation of 1 mole of ATP to adenosine diphosphate (ADP), the outermost phosphate bond splits and liberates approximately 7.3 kCal of free energy.
In addition to ATP, PCr is another high-energy phosphate compound.
Releases large amounts of energy when bonds between creatine and phosphate are broken.
Cells store 4-6 times more PCr than ATP.
Provide a reservoir of high-energy phosphate bonds
The onset of intense exercise triggers PCr hydrolysis for energy;
it does not require oxygen and reaches a maximum in about 10 seconds.
ATP and PCr provide anaerobic sources of phosphate-bond energy.
The energy liberated from the hydrolysis of PCr rebonds ATP and P to form ATP
Phosphorylation
Refers to energy transfer through phosphate bonds
Most of the energy for ATP phosphorylation comes from
oxidation of carbohydrates, lipids, and proteins.
Oxidative phosphorylation synthesizes ATP by
transferring electrons from NADH and FADH2 to oxygen.
Cellular Oxidation
Constitutes the mechanism for energy metabolism
Involve the transfer of hydrogen atoms
Loss of hydrogen: oxidation
Gain of hydrogen: reduction
Mitochondria contain
carrier molecules that remove electrons from hydrogen and pass them to oxygen
Sources for ATP formation include:
Glucose derived from liver glycogen
Triacylglycerol and glycogen molecules stored within muscle cells
Free fatty acids derived from triacylglycerol (in liver and adipocytes) that enter the bloodstream for delivery to active muscle
Intramuscular and liver-derived carbon skeletons of amino acids
The primary function of carbohydrates is to
supply energy for cellular work.
The complete breakdown of 1 mol of glucose liberates
686 kcal of energy.
Of this, ATP bonds conserve about
233 kcal (34%), with the remainder dissipated as heat.
Glucose Degradation
Occurs in two stages:
Anaerobic: Glucose breaks down relatively rapidly to 2 molecules of pyruvate.
Aerobic: Pyruvate degrades further to carbon dioxide and water.
Glycolysis
Glycogen catabolism
Substrate-level phosphorylation in glycolysis
Hydrogen release in glycolysis
Lactate formation
Glycogenolysis describes the cleavage of glucose from stored glycogen.
Energy transfers directly via phosphate bonds in the anaerobic reactions called substrate-level phosphorylation.
During glycolysis, two pairs of hydrogen atoms are stripped from the substrate (glucose), and their electrons are passed to NAD+ to form NADH .
Lactate provides a valuable source of chemical energy that accumulates in the body during heavy exercise.
The second stage of carbohydrate breakdown is known as the
citric acid cycle (Krebs cycle).
Degrades acetyl-CoA substrate to carbon dioxide and hydrogen atoms within the mitochondria
The acetyl portion of acetyl-CoA joins with oxaloacetate to form citrate (citric acid).
Citric Acid Cycle Results
Pyruvate prepares to enter the citric acid cycle by joining with the vitamin B (pantothenic acid)-derivative coenzyme A to form the 2-carbon compound acetyl-CoA.
Each acetyl-CoA molecule entering the citric acid cycle releases two carbon dioxide molecules and four pairs of hydrogen atoms.
Stored fat represents
the body's most plentiful source of potential energy.
Energy sources for fat catabolism include:
Triacylglycerol stored directly within the muscle fiber
Circulating triacylglycerol in lipoprotein complexes
Circulating free fatty acids
Prior to energy release from fat
hydrolysis (lipolysis or fat breakdown) splits the triacylglycerol molecule into glycerol and three water-insoluble fatty acid molecules.
Adipose tissue
serves as an active and major supplier of fatty acid molecules.
Triacylglycerol fat droplets occupy up to
95% of the adipocyte cell's volume.
Free fatty acids either form
intracellular triacylglycerols or bind with intramuscular proteins and enter the mitochondria for energy metabolism.
Epinephrine, norepinephrine, glucagon, and growth hormone
augment lipase activation.
Fat breakdown or synthesis depends on
the availability of fatty acid molecules.
Hormonal release triggered by exercise stimulates
adipose tissue lipolysis.
Plasma concentrations of these lipogenic hormones
increase during exercise to continually supply active muscles with energy-rich substrate.
Glycerol
Provides carbon skeletons for glucose synthesis
Fatty acids
Beta (ß)-oxidation converts a free fatty acid to multiple acetyl-CoA molecules.
Hydrogens released during fatty acid catabolism oxidize through the respiratory chain.
Lipogenesis
begins with carbons from glucose and the carbon skeletons from amino acid molecules that metabolize to acetyl-CoA.
The formation of fat, mostly in the cytoplasm of liver cells
Occurs when excess glucose or protein is not used immediately to sustain metabolism, so it converts into stored triacylglycerol
The lipogenic process
requires ATP energy and the B vitamins biotin, niacin, and pantothenic acid.
Protein plays a role as an
energy substrate during endurance activities and heavy trainings.
Deamination:
Nitrogen is removed from the amino acid molecule.
Transamination:
when an amino acid is passed to another compound
The remaining carbon skeletons
enter metabolic pathways to produce ATP.
Protein catabolism
facilitates water loss.
The amine group and other solutes from protein breakdown must be eliminated.
must be eliminated.
This requires excretion of "obligatory" water as the
waste products of protein catabolism leave the body dissolved in fluid (urine).
Excessive protein catabolism
increases the body's water needs.
The citric acid cycle is
a vital link between food energy and the chemical energy of ATP.
The citric acid cycle also provides
intermediates that cross the mitochondrial membrane into the cytosol to synthesize bionutrients.
Food energy from macronutrients.
Bionutrients are necessary for maintenance and growth.
Example: Excess carbohydrates provide glycerol and acetyl fragments to synthesize triacylglycerol; acetyl-CoA also functions as the branch point for synthesizing cholesterol, bile, and many hormones as well as ketone bodies and fatty acids.