Metabolism
Reactions and Metabolic Pathways
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A progression of metabolic reactions from beginning to end is called a pathway
Intermediates of reactions
Anabolic pathways
Catabolic pathways
Energy for the cell
 Energy used in cells come from the chemical bonds found between atoms in
carbohydrate, fat, protein, and alcohol
 Most energy is from the sun and involved in reactions converting carbon dioxide
and water into glucose – (photosynthesis)
 Glucose used in cell respiration to produce ATP used by all reactions in all cells
 Types of energy: chemical, mechanical, electrical, osmotic
ATP
 ATP --> ADP + P + energy (liberated)
[catabolic]
 Energy + ADP + P --> ATP
[anabolic]
Oxidation-Reduction Reactions
 A substance is oxidized when it loses one or more electrons
 A substance is reduced when it gains one or more electrons
 Oxidation-reduction reactions are controlled by enzymes
 Antioxidants – compounds that donate electrons to oxidized compounds, putting
them into a more reduced (stable) state
 Oxidized compounds tend to be highly reactive
 Vitamins E and C are antioxidants
 Remember phytochemicals!
Glycolysis, Citric Acid Cycle (also called Krebs Cycle), and
Electron Transport Chain (ETC)
Glycolysis
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Occurs in the cytosol of the cell
Begin process with glucose
2 ATP used 4 ATP produced = 2 ATP net
Water molecule is removed
Hydrogen atoms removed from intermediates by NAD molecules
2 pyruvate molecules produced at end of the pathway
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If no oxygen present in the cell then pyruvates are converted into lactate –
this process is called anaerobic respiration
Intermediate step: Pyruvate to Acetyl CoA (occurs in the mitochodria)
Citric Acid Cycle
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Occurs in the mitochondria
Acetyl CoA added to compound in the cycle
Hydrogens are removed by NAD molecules and FAD molecules
Carbon dioxide is removed from intermediates
GTP produced (a usable energy source like ATP)
Electron Transport Chain
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Hydrogenated NAD and FAD molecules from Citric Acid Cycle are oxidized and
electrons and hydrogen ions are removed
Electrons are passed down a chain of proteins until accepted by oxygen
Hydrogen ions
 Some are used with oxygen to form water
 Most are used to form a concentration gradient
Hydrogen ion gradient used to synthesize many ATPs (Chemiosmosis)
Lipolysis - Breakdown of triglycerides to glycerol and fatty acids
 Takes place in peroxisomes and mitochondria of cells
 Fatty acids
 Liberated from lipid storage in adipose cells by an enzyme (hormonesensitive lipase)
 Are taken up from the bloodstream by cells
 Are converted to acetyl CoA which enter the Citric Acid Cycle (CAC)
 Ketogenesis and ketone bodies
 Ketogenesis is the process by which ketone bodies are produced as a
result of fatty acid breakdown
 Process is dependent upon availability of carbohydrates (glucose):
 If sufficient glucose present - acetyl CoA is recycled (in CAC)
 If excess glucose present - acetyl CoA is used to form fatty acids
 If no glucose present - fats are broken down to produce acetyl CoA
- HOWEVER acetyl CoA is not recycled and to get more CoA
available to be used in CAC, the used acetyl CoA molecules react
with one another to remove CoA yet produce ketone bodies (such
as acetoacetate, beta-hydroxybutyrate and acetone)
 Ketone bodies can be acidic and if they accumulate in the blood,
the pH drops and causes ketoacidosis
Lipogenesis - Building of fatty acids using derivatives of acetyl CoA
Protein Metabolism
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Begins after proteins are degraded into amino acids
Amino group removed from amino acids
Amino acid metabolism mostly takes place in the liver
Gluconeogenesis – carbon skeletons of amino acids enter directly into the Citric Acid
Cycle or become pyruvate
 Glucogenic amino acids
 Ketogenic amino acids
Excess amino acids are converted to urea
Chapter Objectives
After reading chapter four - A student should be able to…
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Define: anabolic pathway, catabolic pathway, and intermediates of reactions
Discuss the function of ATP Describe what oxidation-reduction reactions are
Define antioxidant and discuss how they relate to oxidation-reduction reactions
List and discuss in detail the process of Glycolysis, Krebs Cycle, and Electron Transport
Chain List the products of the metabolisms
Discuss the role of oxygen in metabolism
Describe what happens when oxygen is not available for cell respiration
Discuss lipolysis, lipogenesis, and ketogenesis
Discuss gluconeogenesis and protein metabolism