The human cell - 101
Cellular Respiration
What is cellular respiration?
The purpose of cellular respiration is to harvest
high-energy ELECTRONS from
glucose ,
with which to produce
ATP Energy. This is accomplished by a 4-step
process, which oxidizes the Carbons of
glucose
to Carbon Dioxide and Water.
• Step
1. GLYCOLYSIS
•
Step 2. PYRUVIC ACID CYCLE
• Step
3. KREBS CYCLE
•
Step 4. OXIDATIVE PHOSPORYLATION/
ELECTRON TRANSPORT CHAIN
• In Summary
Step 1 - DOES NOT REQUIRE OXYGEN and Takes place in the CYTOSOL (Cell's Internal
Fluid)
Anaerobic cells can use this method to
produce a small amount of ATP energy.
Steps 2, 3 and 4 - REQUIRE OXYGEN and Take place in the
MITOCHONDRION
Mitochondria (Cells'"Power
Plants") are large organelles with double
membranes. The outer membrane is smooth, while the
inner membrane has long folds called cristae.
Initial removal of electrons
from
glucose
to make two
3-carbon Pyruvate molecules
The word "Glycolysis" means to break
down sugar: glyco = sugar, and lysis = to break.
Glycolysis occurs in the cytoplasm of eukaryote cells. Glycolysis has 8 steps each
catalyzed by a specific enzyme which nets 2 ATP molecules
and 2 NADH from each molecule of
glucose broken down.
Glucose →
2 Pyruvate + 2 NADH (electron
carriers) + 2 ATP
After glycolysis, the pyruvate molecules
enter step 2.
The next step(PYRUVIC ACID CYCLE) can also be fueled by other sources of
pyruvate:
- Fatscan be oxidized to make pyruvate
- Fructose
can be metabolized to
pyruvate in the fructolytic
pathway
Formation of Acetyl CoEnzyme A
from
Pyruvate
Pyruvate is shuttled
into the mitochondrion where it is decarboxylated (a carbon
group is removed as carbon dioxide) - and the now 2-carbon compound is attached
to Co-enzyme A (CoA ,
a derivative of vitamin B5) forming a molecule called
Acetyl CoA, stripping off another
2 electrons , which are carried by
NADH .
2 Pyruvate +
CoA + 2 NAD+
(electron acceptor) → 2 Acetyl CoA +
2 NADH (electron carrier) +
H+ + CO2
Insufficient Oxygen
produces
Lactic Acid -
Pyruvate is turned into lactic acid
instead of forming
Acetyl-CoA .
Inefficient Fat Metabolism causes Brain fogging.
ß-oxidation of fats
supplies the best source of
Acetyl-CoA ,
however the brain can only generate Acetyl
CoA from glucose , not from
fat .
Aging causes fat metabolism inefficiencglucose instead
of fat (glucose
that would otherwise have been available for the brain), to form
Acetyl CoA . This explains why older people complain
of brain fatigue. Acetyl CoA only lasts 2 hours
in the system.
Amino acids can also be converted to
Acetyl CoA
for entry into the Kreb's Cycle.
Changes
Acetyl CoA
into Energy.
Electrons
are removed from
Acetyl CoA
forming carbon dioxide. This cycle occurs twice per
glucose molecule.
The Citric Acid Cycle has 8 steps each mediated by a specific enzyme. As each
acetyl CoA
goes around the cycle
2 carbon dioxide molecules are given off,
3 NADH, 1 FADH2 ,
and 1 ATP .
Net energy gain from Krebs per molecule of glucose
is 2 ATP.
2 Acetyl CoA →
4 CO2 +
6 NADH + 2 FADH2
(coenzymes carrying electrons as hydride ions + protons) +
2 ATP
(Coenzymes
NAD + &
FAD *
are reduced (negatively charged hydride ions added) to become
electron-carrying coenzymes
NADH& FADH2 . A hydride ion H-
is a hydrogen atom which has gained an
electron.
By adding this to the NAD+ , the group containing nitrogen becomes
neutral, forming NADH)
Electrons from the Kreb's Cycle are used
to make a maximum 32 ATP molecules
The goal is to create a strong potential difference
across the mitochondrial membrane, which can be used to create
ATP energy - Specialized proteins and enzymes located
on the inner mitochondrial membrane form a molecular "wire" (an electron
transport chain). By a process called oxidative phosphorylation (the coupling of
oxidation with the addition of a phosphate molecule),
NADH & FADH2
donate their electrons via this "wire"(through a series of intermediate
compounds)to molecular
oxygen , which
becomes reduced to water, producing
ATP.
6 NADH &2 FADH2
→. . .. . .. . . →2
H+ and O→ H2 O +32
ATP
Chemiosmosis - Hydrogen ions
(protons) are used to maintain an Electrochemical
Gradient that turns the electron energy
into
ATP.
Involves pumping protons across mitochondrial membranes to establish
proton gradient, which passes protons down the gradient via the enzyme ATP synthase,
whereby the energy of the protons is used to generate
ATP.
- When
NADH & FADH2
release their electrons,
hydrogen ions (H+) are also released.
These positively-charged hydrogen ions are pumped out of the mitochondrial
matrix, using ATP energy , across the
inner mitochondrial membrane into the intermembrane space creating an electrochemical
gradient (this process is called the cytochrome oxidase system, which uses an enzyme
proton pump called cytochrome oxidase acting as a step-down transformer)
-
At the last stage of the respiratory chain these hydrogen ions flow back across
the inner mitochondrial membrane through ion-channels - where
they drive a molecular enzyme "motor"called
ATP synthase
in the creation of ATP from adenosine diphosphate (ADP) and phosphoric acid (ADP is phosphorylated
into ATP ), somewhat like water drives a water wheel.
Electron Transport Chain
(ETC)
ATP Energy is
made using the electrons
that were passed down the line from
glucose:
C6 H12 6 (Glucose) +
6O2 (Oxygen) →
6 CO2 (Carbon Dioxide) +6 H2 O + ~36 ATP + heat
The energy released from ATP through hydrolysis (a chemical reaction with
water) can then be used for biological work.
