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Anatomy
and
Physiology
Biology 2402
Chapter-25
Metabolism and Energetics
Metabolic Reactions
• Metabolism is the sum of all chemical reactions that occur in
the body. It can be subdivided into two groups of reactions:
– Anabolism:
Anabolic reactions require energy: here monomers (small molecules) join
to form polymers (macromolecules).
– Catabolism:
catabolic reactions release energy: macromolecules are broken down to
monomers.
• Metabolism balances anabolism and catabolism: the energy
released by catabolic reactions is transferred to anabolic
reactions.
• The energy generated by the metabolic break down of
carbohydrates, lipids, proteins is used to produce ATP
(Adenosine TriPhosphate) through oxidation-reduction reactions
Anabolic and Catabolic Reactions
Oxidation-Reduction Reactions
• Oxidation-Reduction Reactions are also known as Redox
reactions.
• These reactions are coupled: they occur simultaneously
between two molecules: one molecule become oxidized, the
other becomes reduced.
• Oxidation is the removal of electrons from a molecule.
• Reduction is the gain of electrons by a molecule.
• Redox reactions are transfer reactions: electrons are
transferred from one molecule to the other.
Coenzymes
• Are molecules that can accept, carry and transfer electrons.
• Two of the most common electron carriers also known as
coenzymes are NAD and FAD:
– NAD (nicotinamide adenine dinucleotide)
The reduced form of NAD is NADH + H+
The oxydized form of NAD is NAD+
– FAD (flavin adenine dinucleotide)
The reduce form of FAD is FADH2
The oxidized form is FAD
ATP
• ATP is the energy currency of the cell.
• It is generated by phosphorylation (addition of a phosphate) of
ADP (Adenosine DiPhosphate):
ADP + Pi  ATP
• ATP can be generated by 3 mechanisms:
– Substrate level phosphorylation: a high energy phosphate group is
transferred from a phosphorylated compound (substrate) to ADP.
This occurs in the cytosol of cells.
– Oxidative phosphorylation: ATP synthase uses the proton motive
force generated by an electron transport chain to transfer one
phosphate group to ADP. This occurs in the innermitochondrial
membrane.
– Photophosphorylation: this occurs during photosynthesis in plants,
or in photosynthetic bacteria.
• Functions of Organic Compounds
1.
Perform structural maintenance and repairs
2.
Support growth
3.
Produce secretions
4.
Store nutrient reserves
Types of Organic Compounds
• Glycogen- storage form of carbohydrates.
• Triglycerides (fat) – energy source
• Proteins – most abundant organic compounds in our
body.
Figure 25-2 Nutrient Use in Cellular Metabolism
Structural, functional, and storage components
Organic
compounds that
can be absorbed
by cells are
distributed to
cells throughout
the body by the
bloodstream.
Nutrient
pool
Triglycerides
Glycogen
Proteins
Fatty acids
Glucose
Amino acids
Three-carbon chains
Two-carbon chains
MITOCHONDRIA
KEY
 Catabolic
pathway
 Anabolic
pathway
Citric
acid
cycle
Coenzymes
Electron
transport
system
Carbohydrate Metabolism:
Glucose
• Largely based on glucose metabolism although other
carbohydrates are involved.
• Glucose is a monosaccharide, a 6 carbon sugar (hexose) from
the aldose family.
• In the body it is mainly generated by the digestion of starch
and glycogen.
• After absorption by secondary active transport in the small
intestine, glucose enters the blood and it is routed with
other nutrients to the liver.
• In the liver (and in skeletal muscle) it is stored as glycogen, a
polysaccharide.
• When cells need glucose, glycogen is broken down.
Glucose Catabolism
Aerobic Respiration
• Cells generate ATP by cellular respiration.
• Aerobic respiration is the complete oxidation of glucose by
glycolysis, the Krebs cycle, and the electron transport chain
(ETC) and oxidative phosphorylation.
• The complete degradation of glucose by cellular respiration can
be written:
C6H12O6 + 6O2  6CO2 + 6H2O + Energy
Glycolysis
• Break down of glucose into pyruvic acid in ten enzymatically
catalyzed steps.
• Occurs in the cytosol with or without oxygen
• One can roughly breakdown glycolysis into 2 phases:
– The energy investment phase (first five reactions)
– The energy payoff phase (last five reactions)
• In the end it produces 2 pyruvates, 2 ATP, and 2 NADH + 2H+.
Pyruvic Acid
• Pyruvic acid is a 3 carbon molecule produced by
catabolism of glucose.
• During metabolism it can be used in two ways:
– When oxygen concentration is high pyruvic acid is
transformed into AcetylCoA in the matrix of mitochondria
and enters the Krebs cycle (aerobic pathway)
– When oxygen concentration is low, pyruvic acid is reduced to
lactic acid by NADH + H+. This is called lactic acid
fermentation. This process oxidizes NAD into NAD+ which
allows glycolysis to continue, and lactic acid diffuses into the
blood. Lactic acid is converted back into pyruvic acid in the
liver.
Anaerobic Respiration
• Anaerobic respiration does not require oxygen.
• Glycolysis occurs, and may be followed by lactic
acid fermentation, which is a way to continue
glycolysis without the Krebs Cycle or the ETC.
Krebs Cycle
• The Krebs cycle which is also called the citric acid cycle,
or the tricarboxylic acid (TCA) cycle is a group of 8
metabolic reactions that occur in the matrix of
mitochondria.
Electron Transport Chain
• The ETC is a group of membrane bound electron carriers
arranged in a redox cascade in the inner mitochondrial
membrane.
• These membrane bound carriers receive electrons from NADH +
H+ and FADH2.
Chemiosmosis
ATP synthesis is linked to redox reactions and H+ ions pumping
Summary of Cellular Respiration
C6H12O6 + 6O2  + 6CO2 + 6H2O + 36 or 38ATP
• During aerobic respiration, 36 ATPs can be generated from one molecule of
glucose.
– Two ATPs come from glycolysis.
– Two come from substrate-level phosphorylation in the Krebs cycle.
Glycogenesis & Glycogenolysis
• Glycogenesis is the synthesis of glycogen from glucose in the
liver and skeletal muscle.
– It is regulated by insulin, and occurs when the blood glucose levels
are high.
– Insulin stimulate liver and mucle cells to absorb glucose and store it
as glycogen.
• Glycogenolysis is the break down of glycogen to release
glucose.
– It is regulated by glucagon and epinephrine, and occurs in response
to low glucose blood levels.
– Liver and muscle cells are stimulated by these two hormones to
break down glycogen to release glucose.
– Only liver cells are able to release glucose to the blood.
Gluconeogenesis
• Gluconeogenesis is the generation of glucose from non
carbohydrate molecules like pyruvic acid, lactic acid, glycerol,
and glucogenic amino acids.
• It is stimulated by cortisol and glucagon and occurs primarily in
the liver. It can also occur in the kidney.
Lipid Metabolism
• Lipids are transported under 4 different forms of
lipoproteins:
– Chylomicrons: transport triglycerides from intestinal
epithelial cells to lacteals (special lymph capillaries in villi).
Once in circulation they travel to adipose tissue where
triglycerides are stored. (2% protein)
– Very low-density lipoproteins (VLDLs): carry triglycerides
synthesized in the liver to adipose tissue. They are converted
to LDLs. (10% protein)
– Low-density lipoproteins (LDLs): transport cholesterol to
body cells via blood circulation. They are also called “bad
cholesterol” because in high amounts they may deposit
cholesterol around blood vessels. (25% protein)
– High-density lipoproteins (HDLs): they remove excess
cholesterol from cells and send to the liver for elimination.
“Good cholesterol” (40% protein)
Lipolysis and Other Lipids Functions
• Adipose triglycerides are broken down and released as
free fatty acids.
• Free fatty acids are taken up by cells and broken down
by beta-oxidation into acetylCoA which can enter the
citric acid cycle or can be converted to ketone bodies.
• Other lipids are used as structural molecules or to
synthesize essential molecules. Examples include
–
–
–
–
–
phospholipids of plasma membranes
lipoproteins that transport cholesterol
thromboplastin for blood clotting
myelin sheaths to speed up nerve conduction
cholesterol used to synthesize bile salts and steroid hormones.
Lipid Metabolism
• Lipid Synthesis (Lipogenesis)
– Can use almost any organic substrate
• Because lipids, amino acids, and carbohydrates can be converted to acetylCoA
– Essential fatty acids
• Cannot be produced by the body, must be consumed
• Unsaturated 18-carbon fatty acid from plants
• Linoleic acid
• Linolenic acid
Lipolysis and Lipogenesis
Protein Metabolism
• During digestion proteins are broken down into amino acids
that are quickly asorbed.
• Like monosaccharides they are absorbed by villi capillaries and
sent to the liver via the hepatic portal system.
• From the liver they enter general circulation to reach body cells
by active transport.
• Amino acids are used to synthesize proteins or as energy
source.
• Amino acids are not stored in the body.
Amino Acids Catabolism
• Amino acids must be converted
to vaious substances to enter
the Krebs cycle for oxidation.
• Amino acid conversion occurs in
the liver.
• Amino acids can be:
– Deaminated
– Decarboxylated
– Dehydrogenated
• Deamination removes the
amino group and creates
nitrogenous wastes such as
urea.
Essential Amino Acids
• Essential amino acids are molecules that we
cannot synthesize, or that we synthesize in low
amount.
• Humans cannot synthesize leucine, isoleucine,
lysine,methionine, phenylalanine, threonine,
tryptophan, and valine.
• Arginine and histidine are not synthesized in
sufficient amounts.
Metabolic Adaptations
• Absorptive state
– The period immediately after eating when nutrients are absorbed
through intestinal wall into circulatory and lymphatic systems
– about 4 hours after each meal
Metabolic Adaptations
• Postabsorptive state
– Occurs late in morning, afternoon, night after absorptive state
concluded
– Blood glucose levels are maintained by conversion of other
molecules to glucose
Metabolic Pathways during the Absorptive State
Metabolic Adaptations
Metabolic Pathways during the
Postabsorptive State
Fasting and Starvation
• Fasting means going without food for many hours or a
few days.
• Starvation implies weeks or months of food deprivation
or inadequate food intake.
• Catabolism of stored triglycerides and structural proteins
can provide energy for several weeks.
• The amount of adipose tissue determines the lifespan
possible without food.
• During fasting and starvation, nervous tissue and red
blood cells continue to use glucose for ATP production.
Metabolic Rate
• Total amount of energy produced and used by body
per unit of time
– Estimated by amount of oxygen used per minute
• Components
– Basal metabolic rate
• Energy used at rest, 60% of metabolic rate
– Thermic effect of food
• Energy used to digest and absorb food, 10%
– Muscular activity
• Energy used for muscle contraction, 30%
Body Temperature Regulation
• A balance between heat gain and loss
– Heat is produced through metabolism
– Heat is exchanged through radiation, conduction,
convection, evaporation
• The greater the temperature difference between
body and environment, the greater the rate of heat
exchange
• Regulated by a “set point” in the hypothalamus
calle hypothalamic thermostat.
Hypothalmic Thermostat
• Several negative feedback loops work to raise body temperature when it is too
low or it raises too high.
•
Heat conservation mechanisms
– Vasoconstriction
– sympathetic stimulation
– skeletal muscle contraction (shivering)
– thyroid hormone production
Nutrition
• Nutrition
– Is the absorption of nutrients from food
– The body’s requirement for each nutrient varies
• Balanced Diet
– Contains all nutrients needed for homeostasis
• Malnutrition
– An unhealthy diet
• Kilocalories
– Measure of energy supplied by food and released
through metabolism
My Pyramid
Table 25-2-1 Minerals and Mineral Reserves
Table 25-3 The Fat-Soluble Vitamins
Table 25-4 The Water-Soluble Vitamins