Download Chapter 24 - Metabolism

Chapter 24 – Nutrition,
Metabolism and Thermoregulation
Use the video clip,
CH 24 Nutrition
for a review of
general nutrition
G.R. Pitts, J.R. Schiller, and
James F. Thompson, Ph.D.
Nutrition
 We eat, we digest, we absorb, then
what?
 3 fates for food = nutrients
1) Most are used to supply energy for life
2) Some are used to synthesize structural
or functional molecules
3) The rest are stored for future use – love
handles!
Nutrition for College Students
Four
Groups:
Grease
Salt
Sugar
Alcohol
Weight Management
If energy consumption (food intake)
equals energy utilized (activity), then
body weight will remain constant
Activity and consumption levels vary
day to day, but individuals keep
relatively constant weight for long
periods of time
Many individuals in affluent nations
have an imbalance between intake and
use  obesity
Regulation of Food Intake
 Hypothalamus - complex
integrating center receiving
sensory information from all
parts of the body
 Two hypothalamic centers
control eating
• Feeding (hunger) center
 located in lateral hypothalamus
 when stimulated, it initiates
feeding, even if one is “full”
• Satiety center
 ventromedial nuclei
 when stimulated, they cause
cessation of eating, even if one
has been starved for days
 Controls for their set points
are unknown!
Regulation of Food Intake (cont.)
Feeding center is always active; but
the satiety center can inhibit it
• May be driven by changes in blood
composition
glucostatic theory – blood glucose levels vary
lipostatic theory - blood lipid levels vary; fat
released from adipose tissue between meals
• Other influences
blood amino acid levels
temperature - high temp decreases appetite
GI tract distension (a slow and variable reflex)
social and psychological factors
hormones (CCK), neurotransmitters, ions (Zn+)
Metabolism
All biochemical reactions in the body
Balance between synthesis (anabolic) and
breakdown (catabolic) reactions
• Anabolism
chemical reactions that combine simple, smaller
molecules into more complex molecules
uses energy
protein formation from amino acids
carbohydrate formation from simple sugars
etc.
• Catabolism
chemical reactions that break down complex organic
molecules into simpler ones
releases energy
proteins are broken down by various proteases
etc.
Relationship of
Catabolism to Anabolism
Adenosine Tri-Phosphate
 ATP is the link between anabolism and catabolism
3 Phosphates
 ATP energy is the “currency” used in most cellular
energy exchanges
 catabolic reactions provide the ATP energy that
most anabolic reactions require
 only about 10-30% of the energy released by
catabolic reactions can be used
• most chemical energy is lost as “waste heat”
• “waste heat” is not wasted; it is essential in maintaining a
constant body temperature
ATP Metabolism
Allows for transfer of small but useful
amounts of energy from one molecule to
another
Cell's entire amount of ATP is recycled
approximately every minute
ATP is NOT for long term energy storage
• too reactive in the cell
• other molecules available for energy storage
(neutral fats, glycogen, creatine phosphate, etc.)
About 8kg (17 lb) of ATP is produced every
hour in an average male
Total amount of ATP present in the body at
any time is only about 50g
ATP Metabolism (cont.)
 energy is released by
breaking the third
phosphate group’s bond
• ATP  ADP + Pi
 a reversible reaction
 the energy released is
enough to drive anabolic
reactions
• ATP  ADP + CrP
 creatine provides energy
storage in skeletal muscle
 allows for more ATP to be
formed when O2 is less
readily available during
skeletal muscle contraction
Energy Production
 Energy is stored in chemical bonds
 Oxidation-Reduction (Redox) reactions:
• Oxidation component:
 also known as dehydrogenation reactions
 remove electrons from molecules
o decreases the energy remaining in the oxidized molecule
o generally, 2e- (and 2H+) are removed simultaneously
 Can also be the gain of oxygen
Energy Production (cont.)
OxidationReduction
reactions (cont.)
• Reduction
component:
addition of electrons
to a molecule
increases the energy
of the reduced
molecule
These 2 component
reactions are
always coupled:
oxidation-reduction
reactions
Energy Production (cont.)
 ATP Generation
• Addition of phosphate to
a chemical compound is
phosphorylation
• 3 mechanisms for this:
 (1) substrate-level
phosphorylation – a
high-energy phosphate
group is transferred
directly from a molecule
to ADP to make ATP
 For example, when the
energy stored on a highenergy phosphate group
on creatine phosphate
is transferred to ADP to
make ATP in skeletal
muscles
CK transfers
its high
energy
phosphate
to ADP
Energy Production (cont.)
 ATP Generation
• Adding a phosphate ion
to a molecule is
phosphorylation
• 3 mechanisms:
 (2) oxidative
phosphorylation
o electrons (H+)
removed from
molecules
o enzymes combine
H+ with O2
releasing enough
energy for ATP
formation
 (3) photophosphorylation photosynthesis
Carbohydrate Metabolism
General
• 80% of carbohydrates ingested contain
glucose; remainder: fructose, galactose
• glucose is the body's preferred
carbohydrate energy source
Fate of carbohydrates -- depends on
needs of body cells
•
•
•
•
•
ATP production
Amino acid synthesis
Glycogenesis
Lipogenesis
Excretion in urine (minimal)
Carbohydrate Metabolism (cont.)
 Glucose anabolism
• Glucose storage:
glycogenesis
 glycogen formation is
stimulated by insulin
 glucose not needed
immediately is stored in
the liver (25%) and in
skeletal muscle (75%)
• Glucose release:
glycogenolysis
 converts glycogen to
glucose
 occurs between meals,
stimulated by glucagon
and epinephrine
Carbohydrate Metabolism (cont.)
Glucose anabolism (cont.)
• Formation of glucose from proteins, fats:
gluconeogenesis
when blood glucose level is low, you eat; if glucose
remains low, body catabolizes some proteins and fats
stimulated by cortisol and thyroid hormone
o cortisol (glucocorticoids) mobilizes proteins, making
AA's available
o thyroid hormone mobilizes proteins (AA's) and may
mobilize lipids
epinephrine, glucagon, hGH also stimulate
These five hormones are often referred to as the
“insulin antagonists.”
Glucose Metabolism
Glucose Catabolism
• glucose oxidation is known as cellular
respiration
complete catabolism of each molecule of
glucose to CO2, H2O
maximum yield of 36 ATP molecules/glucose
o 38% of the energy present in a glucose
o excellent efficiency for a biological system
o the rest of the energy is “waste heat”
2 linked enzymatic pathways are involved in
glucose catabolism
o glycolysis
o Kreb’s cycle
Glucose Metabolism (cont.)
Glycolysis - Overview
Occurs in cytosol
1 glucose 2 pyruvates
(pyruvic acid)
net gain 2 ATP’s
• 2 ATP’s used
• 4 ATP’s made
net gain 2 NADH + 2H+
(aerobic conditions)
Glucose Metabolism (cont.)
Fate of pyruvate (pyruvic acid) - depends
on availability of O2
• without O2: NADH + H+ + pyruvate  lactic acid
• with O2 available to the cell
pyruvate converted to acetyl coenzyme A (acetyl CoA)
this reaction couples glycolysis to the Krebs cycle
Glucose Metabolism (cont.)
 Pyruvic acid - formation of acetyl coenzyme A
(Acetyl CoA) + CO2
• lose one carbon from pyruvate to form CO2 (waste)
• the remaining two carbons, the acetyl group, join with
CoA, to generate NADH + H+ (1 from each pyruvate =
2 NADH + 2H+ total from one glucose)
Glucose Metabolism (cont.)
Krebs cycle (Citric Acid
Cycle or Tricarboxylic
Acid Cycle (TCA)
• oxidation of acetyl
Coenzyme A
• reduction of coenzymes
(NAD+, FAD+)
Oxidative
phosphorylation
• uses NADH2‘s and
FADH2‘s to make
additional ATPs
Glucose Metabolism (cont.)
Glycolysis and
Krebs Cycle combined
total:
6 CO2 (waste) + 6 H2O
10 NADH2 + 2 FADH2 +
4 ATP (energy harvest)
Electron Transport
 Electrons Source: NADH2/FADH2 from glycolysis
and Krebs cycle
 High-energy electrons enter the system, and
low-energy electrons leave
Animal Physiology, Hill et al., 2004
Electron Transport
System
• Oxidative
phosphorylation
O2 is the final
electron acceptor for
low-energy electrons
from last of the
carrier molecules
NADH2  3 ATP
FADH2  2 ATP
• Enzyme cytochrome
oxidase splits apart
O2 molecules
Combines each O
atom with 2 H+ ions
to makes H2O = water
Animal Physiology, Hill et al., 2004
Glucose
Metabolism
Overview
C6H12O6 + 6 O2

6 CO2 (waste)
+ 12 H2O
+ 36 ATP
(useful energy)
Lipids
Beta oxidation breaks
down fatty acids to form
acetyl Coenzyme A.
Lipids are more reduced
(have fewer oxygens);
therefore, they have
more potential chemical
energy and can be more
fully oxidized as an
energy fuel.
Therefore, we gain
more energy, gram for
gram, from fats than
from carbohydrates.
Protein Metabolism
Amino acids may be
deaminated and the
resulting “carbon
skeletons” of whatever
composition, can be
entered into the
glycolytic or Krebs
cycle pathways to
yield an energy
harvest of ATPS. The
amino groups will be
joined with CO2
molecules to form the
nitrogenous waste
urea.
Nutrient Catabolism Pathways
Are All Interconnected
Ketone bodies result from
excessive lipolysis and fat
catabolism; a symptom of
diabetes mellitus
The Daily Metabolic Cycle
The body shifts back and forth
physiologically between the absorptive
state and the postabsorptive state.
The absorptive state occurs for
approximately 4 hours after each regular
meal.
The postabsorptive state takes over until
the next meal can be absorbed.
The Absorptive State
Insulin
Dominates
the
Absorptive
State
All Cells
Rely on
Glucose
from the
Meal for
Energy
The Post-Absorptive State
Glucagon dominates the post-absorptive state and is
assisted by the “insulin antagonists (glucocorticoids,
thyroid hormones, epinephrine, and hGH).
Only Brain and Spinal Cord Cells Rely on Glucose for Energy;
Most Other Body Cells Rely on Fatty Acids for Energy.
The Post-Absorptive State
Glycogenolysis provides glucose fuel for
skeletal muscle.
Glycogenolysis and gluconeogenesis in the
liver provide plasma glucose for nervous tissue.
Lipolysis supplies lipids to fuel all other cells.
Lipid Transport by Lipoproteins
Lipoproteins transport hydrophobic lipids in a droplet
which is emulsified by an external layer of phospholipids
and proteins which make the surface water soluble.
Lipid Transport by Lipoproteins
 Chylomicrons carry
absorbed fat from the meal
to adipose tissue via lymph
 VLDL and LDL carry
cholesterol synthesized in
the liver and fat stored in
the liver to body tissues
 HDL carries excess
cholesterol back to the
liver for catabolism and
excretion
 Increased total cholesterol
and increased LDL are
linked to vessel and heart
disease
Thermogenesis
Heat Loss to the Environment
radiation away of infrared radiation
convection and conduction of heat to air
or water surrounding the body
evaporation from sweating and from
ventilating respiratory membranes
vasodilation of cutaneous capillary beds
decreased hormonal activity leading to
decreased basal metabolic rate (BMR)
behavioral: stop exercising; move to the
shade, take off clothes, turn on a/c, etc.
Heat Production/Conservation
increased hormone activity (thyroxine,
epinephrine) leading to increased (BMR)
increased sympathetic ANS activity
leading to increased (BMR)
shivering of skeletal muscles
vasoconstriction of dermal capillary beds
behavioral: start exercising, huddle
together, use clothing and shelter, use
fire or other means of heating the
surroundings
Thermogenesis (cont.)
 A complex regulation involving negative feedback
control through endocrine and autonomic pathways:
Pathology of Temperature Control
fever
heat cramps, heat exhaustion, heat stroke
heat-induced dehydration
burns
hypothermia
alcohol-induced hypothermia
End Chapter 24