Chapter 6 Metabolism: Energy and
Enzymes
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Cells and the Flow of Energy
• Energy is the ability to do work.
• Living things need to acquire energy; this
is a characteristic of life.
• Cells use acquired energy to:
• Maintain their organization
• Carry out reactions that allow cells to
develop, grow, and reproduce
Flow of energy
Cells and Entropy
• The term entropy is used to indicate the relative
state of disorganization.
• Cells need a constant supply of energy to
maintain their internal organization.
• Complex molecules like glucose tend to break
apart into their building blocks, in this case
carbon dioxide and water.
• This is because glucose is more organized, and
thus less stable, than its breakdown products.
• The result is a loss of potential energy and an
increase in entropy.
Metabolic Reactions and Energy
Transformations
• Metabolism is the sum of all the chemical
reactions that occur in a cell.
• Reactants are substances that participate
in a reaction; products are substances that
form as a result of a reaction.
• A reaction will occur spontaneously if it
increases entropy.
• Biologists use the term “free energy”
instead of entropy for cells.
ATP: Energy for Cells
• ATP (adenosine triphosphate) is the energy
currency of cells.
• ATP is constantly regenerated from ADP
(adenosine diphosphate) after energy is
expended by the cell.
• Use of ATP by the cell has advantages:
• 1) It can be used in many types of reactions.
• 2) When ATP → ADP + P, energy released
is sufficient for cellular needs and little
energy is wasted.
The ATP cycle
Photosynthesis
• The overall reaction for photosynthesis
can be written:
• 6CO2 + 6H2O + energy → C6H12O6 + 6O2
• During photosynthesis, hydrogen atoms
are transferred from water to carbon
dioxide, and glucose is formed.
• Water has been oxidized; carbon dioxide
has been reduced.
• Energy to form glucose comes from the
sun.
Cellular Respiration
• The overall equation for cellular respiration
is opposite that of photosynthesis:
• C6H12O6 + 6O2 → 6CO2 + 6H2O +
Energy
• In this reaction, glucose is oxidized and
oxygen is reduced to become water.
• The complete oxidation of a mol of
glucose releases 686 kcal of energy that is
used to synthesize ATP.
Organelles and the Flow of Energy
• During photosynthesis, chloroplasts capture
solar energy and use it to convert water and
carbon dioxide into carbohydrates that provide
food for other living things.
• Cellular respiration, the breakdown of glucose
into carbon dioxide and water, occurs in
mitochondria.
• It is the cycling of molecules between
chloroplasts and mitochondria that allows a flow
of energy from the sun through all living things.
Relationship of chloroplasts to
mitochondria
Chapter 7:
Cellular Respiration
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cellular respiration
Cellular Respiration takes place in 4 phases
Lets follow ONE molecule of glucose through its complete metabolism
Cellular Respiration takes place in 4 phases
1. Glycolysis is the breakdown of glucose into
pyruvate, 2 ATP molecules are made
Step 1:
2
Cellular Respiration takes place in 4 phases
2. In the transition reaction, pyruvate is broken down
into acetyl CoA, no ATP is made
Step 2:
2
Cellular Respiration takes place in 4 phases
3. The Citric Acid Cycle also called the “Krebs” cycle, and
breaksdown Acetyl-CoA into CO2….2 ATP are made
Step 3:
2
2
MITOCHONDRIA
Cellular Respiration takes place in 4 phases
4. The Electron Transport System uses the ELECTRONS
removed from glucose molecules to provide
engergy to make TONS of ATP…..about 32-34 ATPs!!!
Step 4:
2
2
MITOCHONDRIA
32
Cellular Respiration takes place in 4 phases
Therefore, ONE molecule of glucose generates
2+2+32/34 ATP molecules…….a total of 36-38!
Step 2:
Step 1:
2
Step 3:
Step 4:
2
32
Where is all this occuring?!?!
Cell
Outside of the
Mitochondria!
ATP is not only produced….but also NADH and FADH!
ATP is the currency to run machinery within the cell…..
But NADH and FADH2 run the electron transport system
*which then makes the ATP!*
NAD+ and FAD
• Each step of cellular respiration requires a
separate enzyme.
• Some enzymes use the oxidationreduction coenzyme NAD+ (nicotinamide
adenine dinucleotide).
• FAD (flavin adenine dinucleotide) is
sometimes used instead of NAD+.
The function of NADH and FADH2 is to carry and then
donate electrons to the electron transport system…..
The function of NADH and FADH2 is to carry and then
donate electrons to the electron transport system…..
*remember electron transport is occuring across the mitochondria’s cristae!*
What happens when breakdown of glucose is incomplete?
FERMENTATION!!!!
• When oxygen is available, pyruvate enters the
mitochondria, where it undergoes further breakdown,
through the citric acid and electron transport cycles.
• If oxygen is not available, fermentation occurs and
pyruvate undergoes reduction.
• Fermentation is an anaeorbic process and does not
require oxygen. (“an-aeorobic" means, without oxygen)
• In humans, pyruvate is reduced to lactic acid during
fermentation.
In humans…..
In bacteria or yeast…..
The Fermentation Process…… remember, it’s ANAEOROBIC
this situation only occurs when oxygen levels are LOW!
In humans…..
In bacteria or yeast…..
Notice, that in low oxygen you only make 2 ATP, compared to 36!
this is why, when you have an oxygen debt, you get a
lactic acid buildup in your muscles! AND, you pant to try to
Bring more oxygen into your body to complete cellular respiration
Efficiency of Fermentation
• Two ATP produced during fermentation are
equivalent to 14.6 kcal; complete oxidation
of glucose to CO2 and H2O represents a
yield of 686 kcal per molecule of glucose.
• Thus, fermentation is only 2.1% efficient
compared to cellular respiration.
• (14.6/686) x 100 = 2.1%
Advantages and Disadvantages of
Fermentation
• Fermentation can provide a rapid burst of
ATP in muscle cells, even when oxygen is
in limited supply.
• Lactate, however, is toxic to cells.
• Initially, blood carries away lactate as it
forms; eventually lactate builds up,
lowering cell pH, and causing muscles to
fatigue.
• Oxygen debt occurs, and the liver must
reconvert lactate to pyruvate.
But we don’t just eat carbohydrates…..so
what happens with proteins and lipids?
?
?
?? ?
?
? ? ?
?
?
The metabolic pool concept
Catabolism
catabolic reactions, break it down
• Molecules aside from glucose can enter
the catabolic reactions of cellular
respiration.
• When a fat is used for energy, it breaks
down into glycerol and three fatty acids;
glycerol is converted to PGAL, and the
fatty acids are converted to acetyl-CoA,
thus both types of molecules can enter the
citric acid cycle.
Fat breaks down into glycerol and
three fatty acids; glycerol is converted
to PGAL, and the fatty acids are
converted to acetyl-CoA, and both
molecules can then enter the
citric acid cycle.
Catabolism
catabolic reactions, break it down
• The carbon backbones of amino acids can
also enter the reactions of cellular
respiration to provide energy.
• The amino acid first undergoes
deamination, or the removal of the amino
group in the liver; the amino group
becomes ammonia (NH3) and is excreted
as urea.
• Where the carbon portion of the amino
acid enters the reactions of respiration
depends on its number of carbons.
Anabolism
Anabolic reactions build things up
• The substrates of the pathways of cellular
respiration can also be used as starting
materials for synthetic reactions.
• This is the cell’s metabolic pool, in which one
type of molecule can be converted into another.
• In this way, dietary carbohydrates can be
converted to stored fat, and come substrates of
the citric acid cycle can be transaminated into
amino acids.
Phases of Complete Glucose
Breakdown
• The oxidation of glucose by removal of
hydrogen atoms involves four phases:
• Glycolysis – the breakdown of glucose to
two molecules of pyruvate in the
cytoplasm with no oxygen needed; yields
2 ATP
• Transition reaction – pyruvate is oxidized
to a 2-carbon acetyl group carried by CoA,
and CO2 is removed; occurs twice per
glucose molecule
• Citric acid cycle – a cyclical series of oxidation
reactions that give off CO2 and produce one ATP
per cycle; occurs twice per glucose molecule
• Electron transport system – a series of carriers
that accept electrons removed from glucose and
pass them from one carrier to the next until the
final receptor, O2 is reached; water is produced;
energy is released and used to synthesize 32 to
34 ATP
• If oxygen is not available, fermentation occurs in
the cytoplasm instead of proceeding to cellular
respiration.
Citric acid cycle
Overview of the electron transport
system