Nutrition
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Nutrient – a substance that promotes normal
growth, maintenance, and repair
Major nutrients – carbohydrates, lipids, and
proteins
Other nutrients – vitamins and minerals (and
technically speaking, water)
USDA Food Guide Pyramid
Figure 24.1a
Nutrition
Figure 24.1b
Carbohydrates


Complex carbohydrates (starches) are found in
bread, cereal, flour, pasta, nuts, and potatoes
Simple carbohydrates (sugars) are found in
soft drinks, candy, fruit, and ice cream
Carbohydrates



Glucose is the molecule ultimately used by
body cells to make ATP
Neurons and RBCs rely almost entirely upon
glucose to supply their energy needs
Excess glucose is converted to glycogen or fat
and stored
Carbohydrates



The minimum amount of carbohydrates
needed to maintain adequate blood glucose
levels is 100 grams per day
Starchy foods and milk have nutrients such as
vitamins and minerals in addition to complex
carbohydrates
Refined carbohydrate foods (candy and soft
drinks) provide energy sources only and are
referred to as “empty calories”
Lipids

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The most abundant dietary lipids, triglycerides,
are found in both animal and plant foods
Essential fatty acids – linoleic and linolenic
acid, found in most vegetables, must be
ingested
Dietary fats:


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Help the body to absorb vitamins
Are a major energy fuel of hepatocytes and skeletal
muscle
Are a component of myelin sheaths and all cell
membranes
Lipids

Fatty deposits in adipose tissue provide:

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A protective cushion around body organs
An insulating layer beneath the skin
An easy-to-store concentrated source of energy
Lipids

Prostaglandins function in:

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Smooth muscle contraction
Control of blood pressure
Inflammation
Cholesterol stabilizes membranes and is a
precursor of bile salts and steroid hormones
Lipids: Dietary Requirements


Higher for infants and children than for adults
The American Heart Association suggests that:

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Fats should represent less than 30% of one’s total
caloric intake
Saturated fats should be limited to 10% or less of
one’s total fat intake
Daily cholesterol intake should not exceed 200 mg
Proteins


Complete proteins that meet all the body’s
amino acid needs are found in eggs, milk, milk
products, meat, and fish
Incomplete proteins are found in legumes,
nuts, seeds, grains, and vegetables
Proteins

Proteins supply:



Essential amino acids, the building blocks for
nonessential amino acids
Nitrogen for nonprotein nitrogen-containing
substances
Daily intake should be approximately 0.8g/kg
of body weight
Proteins: Synthesis and
Hydrolysis

All-or-none rule


All amino acids needed must be present at the
same time for protein synthesis to occur
Adequacy of caloric intake

Protein will be used as fuel if there is insufficient
carbohydrate or fat available
Proteins: Synthesis and
Hydrolysis

Nitrogen balance




The rate of protein synthesis equals the rate of
breakdown and loss
Positive – synthesis exceeds breakdown (normal in
children and tissue repair)
Negative – breakdown exceeds synthesis (e.g.,
stress, burns, infection, or injury)
Hormonal control

Anabolic hormones accelerate protein synthesis
Essential Amino Acids
Figure 24.2
Vitamins


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Organic compounds needed for growth and
good health
They are crucial in helping the body use
nutrients and often function as coenzymes
Only vitamins D, K, and B are synthesized in
the body; all others must be ingested
Water-soluble vitamins (B-complex and C) are
absorbed in the gastrointestinal tract

B12 additionally requires gastric intrinsic factor to
be absorbed
Vitamins


Fat-soluble vitamins (A, D, E, and K) bind to
ingested lipids and are absorbed with their
digestion products
Vitamins A, C, and E also act in an antioxidant
cascade
Minerals

Seven minerals are required in moderate
amounts

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
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Calcium, phosphorus, potassium, sulfur, sodium,
chloride, and magnesium
Dozens are required in trace amounts
Minerals work with nutrients to ensure proper
body functioning
Calcium, phosphorus, and magnesium salts
harden bone
Minerals


Sodium and chloride help maintain normal
osmolarity, water balance, and are essential in
nerve and muscle function
Uptake and excretion must be balanced to
prevent toxic overload
Metabolism


Metabolism – all chemical reactions necessary
to maintain life
Cellular respiration – food fuels are broken
down within cells and some of the energy is
captured to produce ATP


Anabolic reactions – synthesis of larger molecules
from smaller ones
Catabolic reactions – hydrolysis of complex
structures into simpler ones
Metabolism


Enzymes shift the high-energy phosphate
groups of ATP to other molecules
These phosphorylated molecules are activated
to perform cellular functions
Stages of Metabolism

Energy-containing nutrients are processed in
three major stages


Digestion – breakdown of food; nutrients are
transported to tissues
Anabolism and formation of catabolic
intermediates where nutrients are:



Built into lipids, proteins, and glycogen
Broken down by catabolic pathways to pyruvic acid and
acetyl CoA
Oxidative breakdown – nutrients are catabolized to
carbon dioxide, water, and ATP
Oxidation-Reduction (Redox)
Reactions

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Oxidation occurs via the gain of oxygen or the
loss of hydrogen
Whenever one substance is oxidized, another
substance is reduced
Oxidized substances lose energy
Reduced substances gain energy
Coenzymes act as hydrogen (or electron)
acceptors
Two important coenzymes are nicotinamide
adenine dinucleotide (NAD+) and flavin
adenine dinucleotide (FAD)
Carbohydrate Metabolism


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Since all carbohydrates are transformed into
glucose, it is essentially glucose metabolism
Oxidation of glucose is shown by the overall
reaction:
C6H12O6 + 6O2  6H2O + 6CO2 + 36 ATP +
heat
Glucose is catabolized in three pathways

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Glycolysis
Krebs cycle
The electron transport chain and oxidative
phosphorylation
Glycolysis

A three-phase pathway in which:

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Glucose is oxidized into pyruvic acid
NAD+ is reduced to NADH + H+
ATP is synthesized by substrate-level
phosphorylation
Pyruvic acid:


Moves on to the Krebs cycle in an aerobic pathway
or
Is reduced to lactic acid in an anaerobic
environment
Glycolysis
ATP
Glycolysis
Krebs
cycle
ATP
Electron transport chain
and oxidative
phosphorylation
ATP
Glucose
Phase 1
Sugar
activation
Key:
= Carbon
atom
Pi = Inorganic
phosphate
2 ATP
2 ADP
Fructose-1,6bisphosphate
P
P
Phase 2
Sugar
Dihydroxyacetone
cleavage
phosphate
P
Pi
Glyceraldehyde
phosphate
P
2 NAD+
4 ADP
2 NADH+H+
Phase 3
Sugar
oxidation
and formation
of ATP
4 ATP
2 Pyruvic acid
2 NADH+H+
O2
To Krebs
cycle
(aerobic
pathway)
O2
2 NAD+
2 Lactic acid
Figure 24.6
Krebs Cycle: Preparatory Step

Occurs in the mitochondrial matrix and is
fueled by pyruvic acid and fatty acids
Krebs Cycle: Preparatory Step

Pyruvic acid is converted to acetyl CoA in
three main steps:

Decarboxylation


Carbon is removed from pyruvic acid
Carbon dioxide is released
Krebs Cycle: Preparatory Step

Oxidation

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Hydrogen atoms are removed from pyruvic acid
NAD+ is reduced to NADH + H+
Formation of acetyl CoA – the resulting acetic acid
is combined with coenzyme A, a sulfur-containing
coenzyme, to form acetyl CoA
Krebs Cycle

An eight-step cycle in which each acetic acid
is decarboxylated and oxidized, generating:

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Three molecules of NADH + H+
One molecule of FADH2
Two molecules of CO2
One molecule of ATP
For each molecule of glucose entering
glycolysis, two molecules of acetyl CoA enter
the Krebs cycle
Cytosol
Pyruvic acid from glycolysis
Glycolysis
ATP
Krebs
cycle
Electron
transport chain
and oxidative
phosphorylation
NAD+
CO2
CoA
Acetyl CoA
ATP
Mitochondrion
(fluid matrix)
NADH+H+
ATP
Oxaloacetic acid
(pickup molecule)
NADH+H+
Citric acid
CoA (initial reactant)
NAD+
Isocitric acid
Malic acid
NAD+
Krebs cycle
CO2
NADH+H+
a-Ketoglutaric acid
Fumaric acid
CO2
FADH2
FAD
Key:
Succinic acid
Succinyl-CoA
CoA
NAD+
NADH+H+
CoA
= Carbon atom
GTP
GDP + Pi
ADP
ATP
Pi = Inorganic phosphate
CoA = Coenzyme A
Figure 24.7
Electron Transport Chain

Food (glucose) is oxidized and the released
hydrogens:

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Are transported by coenzymes NADH and FADH2
Enter a chain of proteins bound to metal atoms
(cofactors)
Combine with molecular oxygen to form water
Release energy
The energy released is harnessed to attach
inorganic phosphate groups (Pi) to ADP,
making ATP by oxidative phosphorylation
Glycolysis
Krebs
cycle
Electron
transport chain
and oxidative
phosphorylation
ATP
ATP
ATP
H+
H+
H+
H+
Intermembrane
space
Core
Cyt c
e-
eQ
1
3
2
Inner
mitochondrial
membrane
2 H+ +
FADH2
NADH +
Mitochondrial
matrix
O2
H2O
FAD
ATP
ADP + Pi
H+
(carrying efrom food)
1
2
NAD +
H+
Electron Transport Chain
ATP Synthase
Figure 24.8
Electronic Energy Gradient

The electrochemical proton gradient across the
inner membrane:



Creates a pH gradient
Generates a voltage gradient
These gradients cause H+ to flow back into the
matrix via ATP synthase
ATP Synthase



The enzyme consists of three parts: a rotor, a
knob, and a rod
Current created by H+ causes the rotor and rod
to rotate
This rotation activates catalytic sites in the
knob where ADP and Pi are combined to make
ATP
Structure of ATP Synthase
Figure 24.10
Summary of ATP Production
Figure 24.11
Glycogenesis and Glycogenolysis


Glycogenesis –
formation of glycogen
when glucose supplies
exceed cellular need
for ATP synthesis
Glycogenolysis –
breakdown of
glycogen in response
to low blood glucose
Figure 24.12
Gluconeogenesis


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The process of forming sugar from
noncarbohydrate molecules
Takes place mainly in the liver
Protects the body, especially the brain, from
the damaging effects of hypoglycemia by
ensuring ATP synthesis can continue
Lipid Metabolism


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Most products of fat metabolism are
transported in lymph as chylomicrons: a large
plasma lipoprotein particle, occurring as a
droplet consisting primarily of triglycerides
and functioning in the transport of neutral
lipids from the intestine to the tissues by way
of the lymph.
Lipids in chylomicrons are hydrolyzed by
plasma enzymes and absorbed by cells
Only neutral fats are routinely oxidized for
energy
Lipid Metabolism

Catabolism of fats involves two separate
pathways


Glycerol pathway
Fatty acids pathway
Lipid Metabolism- Glycerol
Pathway

Glycerol is converted to glyceraldehyde
phosphate


Glyceraldehyde is ultimately converted into acetyl
CoA
Acetyl CoA enters the Krebs cycle
Lipid Metabolism- Fatty Acids
Pathway

Fatty acids undergo beta oxidation which
produces:


Two-carbon acetic acid fragments, which enter the
Krebs cycle
Reduced coenzymes, which enter the electron
transport chain
Lipid Metabolism
Figure 24.13
Lipogenesis and Lipolysis


Excess dietary glycerol and fatty acids undergo
lipogenesis to form triglycerides
Glucose is easily converted into fat since
acetyl CoA is:


An intermediate in glucose catabolism
The starting molecule for the synthesis of fatty
acids
Lipogenesis and Lipolysis


Lipolysis, the breakdown of stored fat, is
essentially lipogenesis in reverse
Oxaloacetic acid is necessary for the complete
oxidation of fat

Without it, acetyl CoA is converted into ketones
(ketogenesis)
Lipid Metabolism:
Synthesis of Structural Materials

Phospholipids are important components of
myelin and cell membranes
Lipid Metabolism:
Synthesis of Structural Materials

The liver:

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Synthesizes lipoproteins for transport of
cholesterol and fats
Makes tissue factor, a clotting factor
Synthesizes cholesterol for acetyl CoA
Uses cholesterol to form bile salts
Certain endocrine organs use cholesterol to
synthesize steroid hormones
Protein Metabolism

Excess dietary protein results in amino acids
being:



Oxidized for energy
Converted into fat for storage
Amino acids must be deaminated prior to
oxidation for energy
Protein Metabolism

Deaminated amino acids are converted into:



Pyruvic acid
One of the keto acid intermediates of the Krebs
cycle
These events occur as transamination,
oxidative deamination, and keto acid
modification
Amino Acid Oxidation
Figure 24.15