3. 3
Digestion is the biochemical process
by which food molecules, through
hydrolysis, are broken down into
simpler chemical units that can be
used by cells for their metabolic
needs.
DIGESTION AND ABSORPTION OF CARBOHYDRATES
4. 4
DIGESTION AND ABSORPTION OF CARBOHYDRATES
The digestion of carbohydrates begins
in the mouth, where the enzyme
salivary a-amylase catalyzes the
hydrolysis of a-glycosidic linkages in
starch from plants and glycogen from
meats to produce smaller
polysaccharides and the disaccharide
maltose.
5. 5
DIGESTION AND ABSORPTION OF CARBOHYDRATES
The primary site for carbohydrate
digestion is within the small intestine,
where a-amylase, this time secreted by
the pancreas, again begins to function. The
pancreatic a-amylase breaks down
polysaccharide chains into shorter and
shorter segments until the disaccharide
maltose (two glucose units; and glucose
itself are the dominant species.
6. 6
DIGESTION AND ABSORPTION OF CARBOHYDRATES
The final step in carbohydrate digestion occurs
on the outer membranes of intestinal mucosal
cells, where the enzymes that convert
disaccharides to monosaccharides are located.
The important disaccharidase enzymes are
maltase, sucrase, and lactase. These enzymes
convert, respectively, maltose to two glucose
units, sucrose to one glucose and one fructose
unit, and lactose to one glucose and one
galactose unit.
7. 7
DIGESTION AND ABSORPTION OF CARBOHYDRATES
The three major breakdown products
from carbohydrate digestion are thus
glucose, galactose, and fructose.
These monosaccharides are absorbed
into the bloodstream through the
intestinal wall.
9. 9
DIGESTION AND ABSORPTION OF CARBOHYDRATES
After their absorption into the
bloodstream, monosaccharides are
transported to the liver, where fructose
and galactose are rapidly converted into
compounds that are metabolized by the
same pathway as glucose.
10. 10
Glycolysis
Glycolysis is the metabolic pathway by
which glucose (a C6 molecule) is converted
into two molecules of pyruvate (a C3
molecule), chemical energy in the form of
ATP is produced, and NADH-reduced
coenzymes are produced.
11. 11
Glycolysis
The conversion of glucose to pyruvate is an
oxidation process in which no molecular oxygen
is utilized. The oxidizing agent is the coenzyme
NAD. Metabolic pathways in which molecular
oxygen is not a participant are called anaerobic
pathways. Pathways that require molecular
oxygen are called aerobic pathways. Glycolysis is
an anaerobic pathway.
12. 12
Six-Carbon Stage of Glycolysis (Steps 1–3)
Step 1: Phosphorylation: Formation of
Glucose 6-Phosphate.
13. 13
Six-Carbon Stage of Glycolysis (Steps 1–3)
Step 2: Isomerization: Formation of
Fructose 6-Phosphate.
14. 14
Six-Carbon Stage of Glycolysis (Steps 1–3)
Step 3: Phosphorylation: Formation of
Fructose 1,6-Bisphosphate.
28. 28
Regulation of Glycolysis
Glycolysis, like all metabolic
pathways, must have control
mechanisms associated with it. In
glycolysis, the control points are
Steps 1, 3, and 10
29. 29
FATES OF PYRUVATE
Three common fates for
pyruvate are of prime
importance: conversion into
acetyl CoA, into lactate, and
into ethanol
31. 31
FATES OF PYRUVATE
A key concept in considering these
fates of pyruvate is the need for a
continuous supply of NAD for
glycolysis. As glucose is oxidized to
pyruvate in glycolysis, NAD is reduced
to NADH.
34. 34
FATES OF PYRUVATE
Fermentation Processes
Fermentation processes solve this
problem. Fermentation is a
biochemical process by which NADH is
oxidized to NAD without the need for
oxygen.
35. 35
FATES OF PYRUVATE
Lactate Fermentation
Lactate fermentation is the enzymatic
anaerobic reduction of pyruvate to
lactate. The sole purpose of this process is
the conversion of NADH to NAD. The
lactate so formed is converted back to
pyruvate when aerobic conditions are
again established in a cell
38. 38
FATES OF PYRUVATE
Ethanol Fermentation
Ethanol fermentation is the enzymatic anaerobic
conversion of pyruvate to ethanol and carbon
dioxide. Ethanol fermentation involving yeast
causes bread and related products to rise as a
result of CO2 bubbles being released during
baking. Beer, wine, and other alcoholic drinks are
produced by ethanol fermentation of the sugars
in grain and fruit products.
44. 44
GLYCOGEN SYNTHESIS AND DEGRADATION
Glycogen, a branched polymeric form of
glucose, is the storage form of
carbohydrates in humans and animals. It is
found primarily in muscle and liver tissue.
In muscles it is the source of glucose
needed for glycolysis. In the liver, it is the
source of glucose needed to maintain
normal glucose levels in the blood.
45. 45
GLYCOGEN SYNTHESIS AND DEGRADATION
Glycogenesis
Glycogenesis is the metabolic
pathway by which glycogen is
synthesized from glucose 6-phosphate.
Glycogenesis involves three reactions
(steps).
50. 50
GLYCOGEN SYNTHESIS AND DEGRADATION
Glycogenolysis
Glycogenolysis is the metabolic pathway
by which glucose 6-phosphate is produced
from glycogen. This process is not simply
the reverse of glycogen synthesis
(glycogenesis), because it does not require
UTP or UDP molecules. Glycogenolysis is a
two-step process rather than a three-step
process.
55. 55
GLUCONEOGENESIS
Gluconeogenesis is the metabolic
pathway by which glucose is
synthesized from noncarbohydrate
materials. Glycogen stores in muscle
and liver tissue are depleted within
12–18 hours from fasting or in even
less time from heavy work or
strenuous exercise.
56. 56
GLUCONEOGENESIS
The processes of gluconeogenesis
(pyruvate to glucose) and glycolysis
(glucose to pyruvate) are not exact
opposites. The most obvious
difference between these two
processes is that 12 compounds are
involved in gluconeogenesis and only
11 in glycolysis.
61. 61
The CORI CYCLE
The Cori cycle is a cyclic biochemical
process in which glucose is converted
to lactate in muscle tissue, the lactate
is reconverted to glucose in the liver,
and the glucose is returned to the
muscle tissue.
65. 65
THE PENTOSE PHOSPHATE PATHWAY
The pentose phosphate pathway
is the metabolic pathway by which
glucose is used to produce NADPH,
ribose 5-phosphate (a pentose
phosphate), and numerous other
sugar phosphates.
70. 70
HORMONAL CONTROL OF CARBOHYDRATE METABOLISM
A second major method for regulating
carbohydrate metabolism, besides
enzyme inhibition by metabolites, is
hormonal control. Among others,
three hormones—insulin, glucagon,
and epinephrine—affect carbohydrate
metabolism.
71. 71
HORMONAL CONTROL OF CARBOHYDRATE METABOLISM
Insulin
Insulin, a 51-amino-acid protein, is a
hormone produced by the beta cells of
the pancreas. Insulin promotes the
uptake and utilization of glucose by
cells. Thus its function is to lower
blood glucose levels. It is also involved
in lipid metabolism.
72. 72
HORMONAL CONTROL OF CARBOHYDRATE METABOLISM
The release of insulin is triggered by high
blood-glucose levels. The mechanism for
insulin action involves insulin binding to
protein receptors on the outer surfaces of
cells, which facilitates entry of glucose into
the cells. Insulin also produces an increase
in the rates of glycogen synthesis,
glycolysis, and fatty acid synthesis.
73. 73
HORMONAL CONTROL OF CARBOHYDRATE METABOLISM
Glucagon
Glucagon is a polypeptide hormone (29 amino
acids) produced in the pancreas by alpha cells. It
is released when blood-glucose levels are low. Its
principal function is to increase blood-glucose
concentrations by speeding up the conversion of
glycogen to glucose (glycogenolysis) and
gluconeogenesis in the liver. Thus glucagon’s
effects are opposite to those of insulin.
74. 74
HORMONAL CONTROL OF CARBOHYDRATE METABOLISM
Epinephrine
Epinephrine, also called adrenaline, is
released by the adrenal glands in response
to anger, fear, or excitement. Its function is
similar to that of glucagon—stimulation of
glycogenolysis, the release of glucose from
glycogen. Its primary target is muscle cells,
where energy is needed for quick action. It
also functions in lipid metabolism.
