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Three Stages of Metabolism
3 Stages of Metabolism
•First stage:
• It is the breakdown & degradation of
polysaccharides, proteins & lipids into
their simplest units either through
intracellular catabolism or during
digestion
Carbohydrates Proteins Lipids
Glucose Amino acids Free Fatty acids
1st Stage of Metabolism
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2
3
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Digestive juices
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Disaccharidase
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DipeptidaseDipeptide
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CoA.CoA
of Kreb’s cycle.
& glycerol.
Carbohydrates Proteins Lipids
Glucose Amino acids Free Fatty acids
Pyruvate Acetyl CoA
ATP
(substrate level phosphorylation)
1st
Stage
2nd
Stage
1st and 2nd Stages of Metabolism
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H2O ++
Carbohydrates Proteins Lipids
Glucose Amino acids Free Fatty acids
Pyruvate Acetyl CoA
ATP
(Substrate level phosphorylation)
NADH/FADH2
Oxidative phosphorylation (CO2 + H2O) + ATP
1st
Stage
2nd
Stage
Stages of Metabolism
3rd
Stage
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The Sun is Energy for Life
• Phototrophs use light to drive synthesis
of organic molecules (C6H12O6)
• Heterotrophs use these organic
molecules as building blocks
• CO2, O2, and H2O are recycled
Metabolism
Metabolism includes:
1. Digestion
2. Absorption
3. Catabolism
4. Anabolism
5. Excretion
Metabolism
• Catabolism: degradative
pathways, Usually energy-
yielding!
• Anabolism: biosynthetic
pathways, energy-requiring!
Different Types of Metabolic Pathways
a) Linear metabolic pathway:
The product of each step is the substrate
for the next step
b) Cyclic pathway forms a closed loop:
In the citric acid cycle, an acetyl group is
metabolized (oxidized) via reactions that
regenerate the intermediates of the cycle
c) Spiral pathway:
The same set of enzymes catalyze a
progressive lengthening of the acyl chain
Different Types of Metabolic Pathways
Linear pathway Cyclic pathway Spiral pathway
Kreb’sycle Fatty acid synthesis
Conversion of 3-phospho-
glycerate into serine
Enz 1
Enz 2
Enz 3
Comparing Pathways
• Anabolic & catabolic pathways involving the
same product are not the same
• Some steps may be common to both
• Metabolic pathways are multi-step pathways
• This also allows regulation mechanisms to turn
on pathway and other turned off
Single-step versus Multi-
step Pathways
(capturing chemical energy)
• The uncontrolled combustion
of glucose releases a large
amount of energy all at once
A
C
E
F
• A multi-step enzyme –
catalyzed pathway – releases
the same amount of energy
but conserves much of it in a
manageable form
1. Multiple points where
metabolites can enter & leave
2. Multiple points for regulation
3. Substrates & products can be
involved in more than one
metabolic reaction
A
C
E
F
Single-step versus Multi-
step Pathways
(capturing chemical energy)
Starting Substrate
Final End ProductStarting Substrate
Final End Product
Regulation of Metabolic Pathways
Irreversible Reactions
Regulation of Metabolic Pathways
Feedback Inhibition
• Feedback inhibition occurs when a product (usual the
final product) of a pathway controls the rate of its own
synthesis through inhibition of an early step, usually the
first step.
Regulation of Metabolic Pathways
Feed-forward Activation
• Feed-forward activation occurs when a metabolite
produced early in a pathway activates an enzyme that
catalyzes a reaction further down the pathway.
Regulation of Metabolic Pathways
Coordinated Regulation
+
• Can serve to deplete intermediates &
accelerate their re-supply
• The existence of multiple control points is to be expected
Regulation of Metabolic Pathways
• It is the breakdown &
degradation of polysaccharides &
oligosaccharides into their
simplest units (Monosaccharides)
Digestion of Carbohydrates
1. Digestion
Digestible
Monosaccharides
Do not need
digestion
Starch
Hexoses
Carbohydrates
Glycogen
Lactose
Sucrose
Pentoses
Non-Digestible
Cellulose
The Digestive System
PH 6.4 – 6.9. Activated
by Cl-, digests starch to
dextrins, maltose &
isomaltose
Acidic PH of stomach (1-2) is
unsuitable for salivary
amylase
3. Brush border (Intestinal juice,
Succus Entericus) containing:
Maltase, Lactase & Sucrase
(Activated by Cl-).
4.
Carbohydrate Digestion in Small Intestine
• Salivary amylase stops working in acidic
stomach (if pH 4.5)
• 50% of dietary starch digested before it
reaches small intestine
• Brush border enzymes act upon
oligosaccharides, maltose, sucrose & lactose
• lactose indigestible after age 4 in most
humans (due to lack of lactase)
Carbohydrate Digestion
in Small Intestine
& Isomaltose
Monosaccharides Absorption
• Sodium-glucose transport proteins (SGLT)
in membrane help absorption of glucose &
galactose
• Fructose absorbed by facilitated diffusion
then converted to glucose inside the cell
2. Absorption
Absorption of Pentoses & Hexoses
Pentoses absorbed
by Passive Diffusion
Hexoses absorbed
by Active Transport
According
concentration
gradient
Against
concentration
gradient
Types of Transport Carriers
Monosaccharides Absorption
Liver
Digestion of Lactose
(Lactase)
Lactose Intolerance• Some individuals [90% of certain races (adult blacks &
Orientals)] have a defect in lactase enzyme
• Undigested maltose (osmotically active compound)
passes to the bowel (large intestine), acted upon by
bacteria of large intestine producing short chain fatty
acids & CO2 gas, leading to:
1. Abdominal distention (cramping)
2. Abdominal pain
3. Nausea
4. Bloating
5. Watery diarrhea
Lactase deficiency
Fate of Absorbed Glucose
• Glucose enter Liver & Brain by Passive
diffusion
• Glucose enter other tissues (skeletal muscle,
adipose tissue, etc…) by Active transport
• Active transport is enhanced by Insulin
Utilization of Glucose
Anabolic
Reactions Catabolic
Reactions
1) Storage in the form of
Glycogen (Glycogenesis)
2) Storage in the form of TAG
(Lipogenesis)
3) Synthesis of sugar alcohols
and amino sugars
4) Interconversion between
monosaccharides
1) Glycolysis
2) Kreb’s Cycle
3) Pentose Shunt
4) Formation of
Uronic acids