METABOLISM
Vocabulary
Catabolism:
Anabolism:
Glycolysis:
Citric Acid Cycle:
Electron Transport Chain:
Acetyl CoA:
Pyruvate:
Ketones:
Glucogenic Amino Acids:
Ketogenic Amino Acids:
Carbon Skeleton:
Deanimation:
Definition of Metabolism: Chemical reactions involved in releasing energy, breaking down
compounds, making new compounds, and transporting compounds from place to place.
The Principal Organs:
 Digestive Organs
 Liver
 Pancreas
 Heart and Blood Vessels
 Kidneys
Energy Metabolism Centers on 4 Basic Units:
 From Carbohydrate – glucose
 From Lipids – glycerol and fatty acids
 From Protein – amino acids
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Metabolic pathways break down compounds (
compounds (
)
Metabolic pathways are never completely inactive
) or build more complex
BREAKDOWN and RELEASE of ENERGY - CARBOHYDRATE
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Glycolysis: the metabolic pathway that breaks down glucose
Glucose  Pyruvate  Acetyl CoA  Citric Acid Cycle → Electron Transport Chain
→ ATP
If oxygen is unavailable, these reactions cannot occur and lactate is formed
Lactate is an alternative fuel that muscle cells can use, or liver cells can convert to
glucose
BREAKDOWN and RELEASE of ENERGY – FATS
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To begin breaking down fat, the body breaks triglycerides into glycerol and fatty acids
Most of the energy is stored in the fatty acids (glycerol can be converted to glucose or
pyruvate)
Fatty acids cannot be converted to glucose
Fatty acids usually produce substantially more energy than glucose
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BREAKDOWN and RELEASE of ENERGY – PROTEIN
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Protein is only used for energy in the absence of carbohydrate or fat
Carbon skeletons: are formed by the deamination of amino acids and can enter the
metabolic pathways at several points depending on their structure (# carbons)
Glucogenic Amino Acids: become pyruvate or a citric acid cycle intermediate
Ketogenic Amino Acids: become acetyl CoA
FEASTING
 Excess fat?
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Excess protein?
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Excess carbohydrate?
FASTING
 The body’s top priority is to meet energy needs for on-going cellular activity. Normally,
we eat or “refuel” to supply this energy. If we are starving or fasting, the body must use
fuel reserves from its own tissues
 Glycogen is used first, along with some fat breakdown. Glycogen is exhausted within
several hours.
 Low blood glucose serves as a signal to promote further fat breakdown
 A few hours into the fast, most of the cells are depending on fatty acids, except brain,
nerve, and red blood cells. Body proteins must always breakdown to some extent during
fasting to provide glucose for these cells. However, the body cannot afford to breakdown
its own protein at a high rate.
 Eventually, the body adapts, and ketone bodies are made from fatty acids to provide fuel
for some brain cells. After several weeks of fasting, ketones are providing much of the
energy needs of these cells, although many areas of the brain still rely on glucose.
 The average person has ~3 weeks of fat stores. As the later stages of starvation exhaust
fat stores, the body again turns to protein, its sole remaining fuel source.
 In the final stages of protein depletion, the body deteriorates rapidly.
 Death usually occurs when ~half of the body’s protein has been depleted.
How long can a person survive total starvation? Depends on:
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