23. Cellular Metabolism
Catabolism: substances are broken down into
molecules
“destructive” process
Large molecules broken down into smaller
molecules
Usually by hydrolysis
“splitting with water”
Adds H2O back into molecule
Breaks covalent bonds
24. Cellular Metabolism
Catabolism
Energy is released when bonds break
Reverse of dehydration synthesis
(condensation)
Hydrolysis = chemical digestion
Occurs simultaneously (and
continuously) with anabolism
Processes controlled by enzymes
25. Cellular Energy
Cellular energy is chemical energy
Derived from breaking chemical
bonds
~ ½ Energy is stored as ATP
~ ½ Energy is released as heat
Helps maintain body temperature
Enzymes control in the process
26. Cellular Energy
• All nutrient molecules are ultimately
degraded or converted to glucose
• Only glucose can be used to make
ATP
• Oxidation: cellular process of
chemically breaking apart a glucose
molecule to release energy
27. Cellular Energy
• Glucose oxidation occurs in 2 phases
• Anerobic metabolism
• Occurs in cytoplasm
• Without oxygen
• AKA glycolysis
• Splits glucose into two 3-Carbon
molecules: pyruvate
28. Cellular Energy
• Glycolysis
• Process also produces 2 ATPs
• In yeast, plant cells:
• Pyruvate can undergo alcoholic
fermentation
• In bacteria, animal cells:
• Pyruvate can produce lactic acid
29. Cellular Energy
• Aerobic metabolism
• Uses oxygen
• AKA Kreb’s Cycle or Citric Acid
cycle or Tricarboxylic Acid (TCA)
Cycle
• Occurs in mitochondria
• Makes more ATP than anerobic
processes
30. Cellular Energy
• Aerobic metabolism
• CO2 and H2O are waste products
• CO2:
• Diffuses out of cells
• Dissolves in plasma
• Produces HCO3
- in blood
• Exhaled from lungs
31. Cellular Energy
• Aerobic metabolism
• H2O:
• “metabolic” water
• Exhaled from lungs
• Final products of glucose oxidation:
• CO2, H2O, ATP
32. Cellular Energy
• Aerobic metabolism
• For each molecule of glucose:
• 2 ATP formed in glycolysis
• 36 ATP formed in TCA cycle
• Energy stored in phosphate
bonds
• A reversible reaction
33. Metabolic Pathways
• “A particular sequence of enzymatic
reactions”
• Such as glycolysis, TCA cycle
• Carbohydrate pathways
• Carbos should comprise most of
our diet (~ 50% complex carbs)
• Used as a primary energy source
• Produce 4kcal/gm
34. Metabolic Pathways
• Carbohydrate pathways
• Excess carbs converted to energy
storage forms
• Glycogen (muscle, liver)
• Adipose tissue (hips)
• Process is anabolism
35. Metabolic Pathways
• Lipid pathways
• Metabolism controlled by liver
• Should comprise <30% of calories
in diet
• Get 9 kcal/gm (more ATP!)
• Must be degraded into glycerol,
fatty acids, then pyruvate
• A reversible catabolic process
36. Metabolic Pathways
• Protein pathways
• Proteins should comprise ~30% of
diet
• Get 4 kcal/gm
• Catabolism is more complex
• Proteins contain nitrogen
37. Metabolic Pathways
• Protein pathways
• Deamination: removal of nitrogen
from amino acids
• Occurs in liver
• Nitrogen is converted to urea
» A nitrogenous waste product
• Sent to kidneys for excretion
38. Metabolic Pathways
• Protein pathways
• After deamination:
• amino acid “skeleton” is
processed
• in TCA cycle
• May produce CO2, H2O, ATP
• May form glucose or fat
39. Metabolic Pathways
• Protein pathways
• Glucose formed from amino acid
skeletons may be re-converted to
amino acids
• “Essential” amino acids:
• Body cannot make these
• Must obtain in the diet
40. Regulation of Metabolic
Pathways
• Enzyme “saturation”
• Too much substrate for number of
enzyme molecules
• Reaction rate cannot increase
• A single enzyme can control an
entire metabolic pathway
• “rate limiting” enzyme