Download Spotlight on Metabolism

Chapter
7
Metabolism
Metabolism
• Metabolism: All chemical reactions within
organisms that enable them to sustain life.
The two main categories are catabolism and
anabolism.
Metabolism
• Catabolism: Any metabolic process whereby
cells break down complex substances into
simpler, smaller ones.
• Anabolism: Any metabolic process whereby
cells convert simple substances into more
complex ones.
Metabolism
• Thousands of chemical reactions occur every
moment in cells throughout the body.
• The most active metabolic sites are the liver,
muscle, and brain cells.
Energy: Fuel for Work
• Energy source
– Chemical energy (stored in molecular bonds) in
carbohydrates, fat, protein
• Food energy to cellular energy
– Stage 1: digestion, absorption, transport
– Stage 2: breakdown of molecules to a few key
metabolites
– Stage 3: transfer of energy to a form cells can use
What Is Metabolism?
• Catabolism
– Reactions that
breakdown
compounds into
small units
• Anabolism
– Reactions that build
complex molecules
from smaller ones
What Is Metabolism?
• Cell is the metabolic
processing center
– Nucleus
– Cytoplasm
• Cytosol + organelles
• ATP is the body’s energy currency
– ATP = adenosine triphosphate
– Form of energy cells use
• NAD and FAD: transport shuttles
– Accept high-energy electrons for use in
ATP production
Breakdown and Release of Energy
• Extracting energy from carbohydrate
– Glycolysis
• Pathway splits glucose into 2
pyruvates
• Transfers electrons to NAD
• Produces 2 ATP
• anaerobic
– Pyruvate to acetyl CoA
• Releases CO2
• Transfers electrons to NAD
Breakdown and Release of Energy
• Extracting energy from carbohydrate
– Citric acid cycle
• Releases CO2
• Produces GTP (like ATP)
• Transfers electrons to NAD
and FAD
– Electron transport chain
• Accepts electrons from NAD
and FAD
• Produces large amounts of ATP
• Produces water
– End products of glucose
breakdown
• ATP, H2O, CO2
Breakdown and Release of Energy
• Extracting energy from fat
– Split triglycerides into glycerol and fatty acids
– Beta-oxidation
• Breaks apart fatty acids into acetyl CoA
• Transfers electrons to NAD and FAD
– Citric acid cycle
• Acetyl CoA from beta-oxidation enters cycle
– Electron transport chain
– End products of fat breakdown
• ATP, H2O, CO2
Breakdown and Release of
Energy
• Extracting energy from protein
– Split protein into amino acids
– Split off amino group
• Converted to urea for excretion
– Carbon skeleton enters breakdown pathways
– End products
• ATP, H2O, CO2, urea
Breakdown
and
Release of
Energy
Cellular Respiration
• Cellular respiration is the controlled release of
chemical-bond energy from large, organic
molecules.
• This energy is utilized for many activities to
sustain life.
• Both autotrophs and heterotrophs carry out
cellular respiration.
Aerobic Vs. Anaerobic
• Aerobic respiration requires oxygen.
• Anaerobic respiration does not require
oxygen.
Aerobic Respiration
• Aerobic cellular respiration is a specific series
of enzyme controlled chemical reactions in
which oxygen is involved in the breakdown of
glucose into carbon-dioxide and water.
• The chemical-bond energy is released in the
form of ATP.
• Sugar + Oxygen  carbon dioxide + water +
energy (ATP)
Aerobic Respiration
• Simplified Reaction:
• C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) ΔHc 2880 kJ
• Covalent bonds in glucose contain large
amounts of chemical potential energy.
• The potential energy is released and utilized
to create ATP.
Glycolysis
• Glycolysis is a series of enzyme controlled
anaerobic reactions that result in the
breakdown of glucose and the formation of
ATP.
• A 6-carbon sugar glucose molecule is split into
two smaller 3-carbon molecules which are
further broken down into pyruvic acid or
pyruvate.
Glycolysis
• 2 ATP molecules are created during glycolysis
and electrons are released during the process.
Krebs Cycle
• The Krebs cycle is a series of enzymecontrolled reactions that take place inside the
mitochondrion.
• Pyruvic acid formed during glycolysis is broken
down further.
• Carbon dioxide, electrons, and 2 molecules of
ATP are produced in this reaction.
Electron Transport System
• The electrons released from glycolysis and the
Krebs cycle are carried to the electrontransport system (ETS) by NADH and FADH2.
• The electrons are transferred through a series
of oxidation-reduction reactions until they are
ultimately accepted by oxygen atoms forming
oxygen ions.
• 32 molecules of ATP are produced.
Aerobic Respiration Summary
• Glucose enters glycolysis.
– Broken down into pyruvic acid.
• Pyruvic acid enters the Krebs cycle.
– Pyruvic acid is further broken down and carbon-dioxide is
released.
• Electrons and hydrogen ions from glycolysis and the
Krebs cycle are transferred by NADH and FADH2 to the
ETS.
– Electrons are transferred to oxygen to form oxygen ions.
– Hydrogen ions and oxygen ions combine to form water.
Anaerobic Cellular Respiration
• Anaerobic respiration does not require oxygen
as the final electron acceptor.
• Some organisms do not have the necessary
enzymes to carry out the Krebs cycle and ETS.
• Many prokaryotic organisms fall into this
category.
• Yeast is a eukaryotic organism that performs
anaerobic respiration.
Fat Respiration
• A triglyceride (neutral fat) consists of a
glycerol molecule with 3 fatty acids attached
to it.
• A molecule of fat stores several times the
amount of energy as a molecule of glucose.
• Fat is an excellent long-term energy storage
material.
• Other molecules such as glucose can be
converted to fat for storage.
Protein Respiration
• Protein molecules must first be broken down
into amino acids.
• The amino acids must then have their amino
group (-NH2) removed (deamination).
• The amino group is then converted to
ammonia. In the human body ammonia is
converted to urea or uric acid which can then
be excreted.
Biosynthesis and Storage
• Making carbohydrate (glucose)
– Gluconeogenesis
• Uses pyruvate, lactate, glycerol, certain amino acids
• Storing carbohydrate (glycogen)
– Liver, muscle make glycogen from glucose
• Making fat (fatty acids)
– Lipogenesis
• Uses acetyl CoA from fat, amino acids, glucose
• Storing fat (triglyceride)
– Stored in adipose tissue
Biosynthesis and Storage
• Making ketone bodies (ketogenesis)
– Made from acetyl CoA
• Inadequate glucose in cells
• Making protein (amino acids)
– Amino acid pool supplied from
• Diet, protein breakdown, cell synthesis
Regulation of Metabolism
• May favor either anabolic or catabolic
functions
• Regulating hormones
– Insulin
– Glucagon
– Cortisol
– Epinephrine
Special States
• Feasting
– Excess energy
intake from
carbohydrate, fat,
protein
• Promotes storage
Special States
• Fasting
– Inadequate
energy intake
• Promotes
breakdown
– Prolonged
fasting
• Protects body
protein as
long as
possible
The ADP–ATP Cycle
• When extracting energy
from nutrients, the
formation of ATP from
ADP + P captures energy.
• Breaking a phosphate
bond in ATP to ADP + P,
releases energy for
biosynthesis and work.
When Glycolysis Goes Awry
• Red blood cells do not have mitochondria, so they rely on
glycolysis as their only source of ATP.
• They use ATP to maintain the integrity and shape of their cell
membranes.
• A defect in red blood cell glycolysis can cause a shortage of
ATP, which leads to deformed red blood cells.
• Destruction of these cells by the spleen leads to a type of
anemia called hemolytic anemia.
Electron Transport Chain
• This pathway produces most of the ATP available from
glucose. NADH molecules deliver pairs of high-energy
electrons to the beginning of the chain.
• The pairs of high-energy electrons carried by FADH2 enter this
pathway farther along and produce fewer ATP than electron
pairs carried by NADH.
• Water is the final product of the electron transport chain.
Carnitine
• Without assistance,
activated fatty acid cannot
get inside the mitochondria
where fatty acid oxidation
and the citric acid cycle
operate.
• This entry problem is solved
by carnitine, a compound
formed from the amino acid
lysine.
• Carnitine has the unique
task of ferrying activated
fatty acids across the
mitochondrial membrane,
from the cytosol to the
interior of the
mitochondrion.
Deamination
• A deamination
reaction strips the
amino group from an
amino acid.
Fuel for Distance Walking
• A recent study sought to explore
whether or not humans naturally
select a preferred walking speed
(PWS), and that the body’s fuel
selection can be critical to the
total distance traveled. The
hypothesis maintained that
humans select a preferred
walking speed that primarily uses
fat as fuel and does not deplete
carbohydrate (CHO) stores.
• The major finding of this study
was that able-bodied subjects
naturally selected a walking
speed just below the speed
preceding an abrupt rise in CHO
oxidation that would deplete the
body’s small stores of CHO
quickly.
Ketones
• Organic compounds that contain a chemical group
consisting of C=O (a carbon–oxygen double bond)
bound to two hydrocarbons.
• Pyruvate and fructose are two examples of ketones.
• Acetone and acetoacetate are both ketones and
tetone bodies.
• While betahydroxybutyrate is not a ketone, it is a
ketone body.
Cholesterol
• Your body can make cholesterol from acetyl CoA by way of
ketones. In fact, all 27 carbons in synthesized cholesterol
come from acetyl CoA.
• The rate of cholesterol formation is highly responsive to
cholesterol levels in cells. If levels are low, the liver makes
more. If levels are high, synthesis decreases.
• That is why dietary cholesterol in the absence of dietary fat
often has little effect on blood cholesterol levels.
Transamination
• A transamination
reaction transfers the
amino group from one
amino acid to form a
different amino acid.
Indispensable and Dispensable
Amino Acids
• Proteins are made from combinations of
indispensable and dispensable amino acids.
• The body synthesizes dispensable amino acids from
pyruvate, other glycolytic intermediates, and
compounds from the citric acid cycle.
• To form amino acids, transamination reactions
transfer amino groups to carbon skeletons.