Cellular Metabolism
Chapter 4
All the chemical processes that take place in
a cell are called metabolism.
The building up of molecules is called
anabolism.
The breaking down of molecules, usually
with the release of energy, is catabolism.
The process of breaking down organic
molecules in the presence of oxygen to
get energy to do work is called cellular
respiration.
When oxygen is required, the process is
said to be aerobic.
Often many steps are needed to accomplish
what the cell needs to do. These steps
taken together are a metabolic pathway.
Energy is stored in the form of chemical
bonds.
ATP is the energy currency of the cell.
Cellular respiration in a nutshell:
C6H12O6 + 6O2 → 6CO2 + 6 H2O
Glucose + oxygen →carbon + water
dioxide
Steps of Cellular Respiration
1.
2.
3.
4.
Glycolysis
Formation of acetyl-CoA
Krebs or Citric Acid Cycle
Oxidative Phosphorylation or The
Electron Transport Chain
Remember:
Glycolysis takes place in the cytosol
(cytoplasm).
All the other steps of cellular respiration
take place in the mitochondrion.
Glycolysis
•
•
•
•
•
We start with a six-carbon molecule of
glucose
2 ATP are added to start the reactions
4 ATP are released by reactions
Net gain of 2 ATP per molecule of
glucose
The end products of glycolysis are two
three-carbon molecules called pyruvic
acid.
Redox Reactions
• Oxidation and reduction reactions are
always coupled inside a cell.
• Oxidation is the removal of electrons from
a molecule and results in a decrease in
the energy content of the molecule.
• Reduction is the addition of electrons to a
molecule and results in an increase in the
energy content of the molecule.
• Two molecules participate in the redox
reactions of cellular respiration: NAD+ and
FAD. These two molecules act as
coenzymes, and accept electrons given off
by other reactions. NAD+ is reduced to
NADH and FAD is reduced to FADH2 . They
each accept 2 electrons (and some
hydrogen ions) and take them to the
electron transport chain.
When there is not enough oxygen in a
cell we can run glycolysis alone.
• We need to regenerate our electron carrier.
• NADH gives its electrons to the end product
of glycolysis, pyruvic acid, and forms lactic
acid.
• The NAD+ can now accept more electrons.
• This process is rapid, but inefficient – gives
only 2 ATP for each molecule of glucose.
• Doesn’t require oxygen.
Oxygen Debt
• When we make lactic acid, we need to take in
more oxygen than usual to convert the lactic
acid back to pyruvic acid and run it through the
mitochondrion
• We also need oxygen because our temperature
is higher after exercising and our cellular
reactions run faster, needing ATP.
• We also need oxygen for ATP for tissue repair.
From here on, all steps take place
inside the mitochondrion
• Pyruvic acid is broken down into a twocarbon acetyl group and combined with
Coenzyme A to make acetyl-Coenzyme A
• Acetyl-CoA enters the Krebs cycle, where
it is combined with Oxaloacetic acid to
make Citric acid (hence the citric acid
cycle)
This is what the Krebs Cycle
actually looks like:
This is what I want you to
remember:
• Many reactions take place in the Krebs
cycle.
• NADH, FADH2, 2CO2 and 2ATP are
produced
• Oxaloacetic acid is formed again, ready to
repeat the cycle.
The Electron Transport Chain
(oxidative phosphorylation)
• Composed of several proteins called
cytochromes, which act like a “bucketbrigade” for electrons.
• Electrons that enter the chain have a lot of
energy. Every time an electron is passed
by a cytochrome, it takes a little of that
energy and uses it to push a hydrogen ion
(H+) up a concentration gradient across
the inner mitochondrial membrane.
Cytochromes of the electron
transport chain
Chemiosmosis
• Now we have a great deal of stored
(potential) energy in the gradient we have
made.
• When we let the hydrogen ions fall down
that concentration gradient, we can catch
that energy using a special molecule in the
membrane called ATP synthase. It adds a
phosphate group to ADP to make ATP.
• This process is called chemiosmosis.
• Cellular Respiration takes more time and
more steps, but it is very efficient. We can
get 32- 36 molecules of ATP from one
molecule of glucose, instead of just 2.
• We can also use these same processes to
get energy from fats and other
carbohydrates, they just enter the
pathways in different places.
• We can even use proteins and nucleic
acids for energy, but first our liver has to
remove the nitrogen. (deamination)
• So, we can break substances down for
energy, or use them to build other
molecules.
Protein Synthesis
How DNA works – continued
separately