metabolism is the sum of all reactions in the body
metabolic reactions are of two types
In anabolic reactions energy is used to synthesize large
molecules from smaller molecules.
Anabolic reactions create materials for growth and repair.
In catabolic reactions large molecules are decomposed into
Catabolic reactions release energy for cellular use
Dehydration synthesis is a type of anabolic reaction.
triglycerides, polysaccharides, and proteins are synthesized through dehydration
A molecule of water is released from dehydration synthesis.
Amino acids are joined by
polysaccharides from monosaccharides
proteins from amino acids
nucleic acids from nucleotides
fats by joining fatty acids to glycerol
hydrolysis is the reverse of dehydration synthesis
water is used to break apart molecules
hydrolysis releases energy from chemical bonds
Decomposes Polysaccharides into monosaccharides & disaccharides
Decomposes proteins into amino acids
Decomposes Fats into fatty acids & glycerol
Decomposes Nucleic Acids into nucleotides
The critical amount of energy required for a
reaction to occur is called the activation energy.
Enzymes are biological catalysts
They greatly reduce the activation
energy required to start a reaction.
Characteristics of enzymes
Most enzymes are Proteins
Enzymes lower the activation energy of a reaction
Enzymes catalyze reactions
– they increase the rate of reactions, but are not consumed by the reaction
Enzymes are specific to one substrate.
A substrate is the target molecule of an enzyme
Enzymes are named for substrate they act upon and
their name usually ends with _____ase.
Examples of enzymes:
Lipase: decomposes lipids
Protease: decomposes proteins
Nuclease: decomposes nucleic acids
ATP Synthase: synthesizes ATP molecules
The Active site of an enzyme is the region that binds to the substrate
The enzyme temporarily binds to the substrate forming an Enzyme-Substrate Complex
The Enzyme releases the product and enzyme is reused for a new reaction.
Rate of enzyme-catalyzed reactions
The rate of a reaction is limited by:
1. The concentration of substrate
2. The concentration of enzyme
3. The efficiency of enzymes
Some enzymes handle 2-3 molecules per second
Other enzymes handle thousands per second
A metabolic pathway is a complex series of reactions leading to a product
Metabolic Pathways are controlled by several enzymes
Example: The catabolic pathway
for the breakdown of glucose is
The product of each reaction becomes the substrate of next reaction.
Each step requires its own enzyme
The least efficient enzyme is the “Rate-Limiting Enzyme”
Rate-limiting enzyme is usually first in sequence
• Enzyme A = Rate-limiting Enzyme
Negative Feedback in Metabolic Pathway
Negative feedback prevents too much product from being produced.
The product of the metabolic pathway often inhibits the rate-limiting enzyme.
substance that increases the efficiency of an enzyme
Cofactors include ions (zinc, iron, copper) and coenzymes
Coenzymes are organic cofactors
Coenzymes include Vitamins (Vitamin A, B, D)
Reusable – required in small amounts
Vitamins are essential organic molecules that humans cannot synthesize, so they
must come from diet
Many vitamins are coenzymes
Vitamins can function repeatedly, so can be used in small amounts.
Example: Coenzyme A
Energy for Metabolic Reactions
Energy: is the capacity to change something, or ability to do work.
Common forms of energy:
Energy cannot be created or destroyed, but it may
be transferred from one form to another.
example of energy transfer: combustion engine
The combustion of fuel converts chemical energy in
the gasoline into kinetic energy, heat, sound. Water
and CO2 are produced as waste.
Fuel (chemical energy)
= Kinetic Energy + CO2 + H2O
Cell Respiration is the transfer of energy from food molecules into a
form the cells can use
Energy from foods such as glucose is used to make ATP for the cell.
Reaction of Cell Respiration
Initial fuel or
End of Section 1, Chapter 4
ATP = Energy
currency for cells
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