Transcript Cellular respiration - College of the Atlantic

cellular respiration
biology 1
• Cellular respiration and fermentation are catabolic
(energy yielding) pathways
• Redox reactions release energy when electrons
move closer to electronegative atoms
• Electrons ‘fall’ from organic molecules to oxygen,
stepwise, via NAD+ and an electron transport chain
• Cellular respiration consists of
– Glycolysis
– Krebs Cycle
– Electron transport chain
• Fermentation - an anaerobic alternative
Fermentation and Respiration
• Fermentation is an anaerobic ATP producing
catabolic pathway
• Cellular respiration is an aerobic catabolic
pathway, where O2 acts as the final electron
acceptor
– Summarized as:
C6H12O6 + 6O2
6H2O + 6CO2 + energy
– Energy from respiration is used to recycle ADP to ATP
Respiration as a redox reaction
• Oxidation = partial or complete loss of
electrons
• Reduction = partial or complete gain of
electrons
• Redox reaction = shunt of electrons from one
reactant to another. e.g., in respiration,
– O2 (oxidizing agent) receives electrons from sugar
(oxidized)
– Sugar (reducing agent) donates electrons to O2
(reduced)
– Movement of electrons to more electronegative
state causes loss of potential energy, and
therefore release of energy
• In respiration, hydrogen is transferred to oxygen,
and carbon is oxidized
oxidation
C6H12O6 + 6O2
6H2O + 6CO2 + energy
reduction
– Carbohydrates and fats are excellent energy stores
because they are rich in C-H bonds
– Respiration does not occur in one explosion - its done
stepwise so that energy can be harnessed at each step
– 1 mole of glucose = 2870 kJ of energy
– Catabolic pathway of respiration is aided by enzymes
that lower the activation energies of the reactions
How energy is harnessed in respiration
• Remember that energy from respiration comes from
electrons falling from a high potential energy to a lower
potential energy
• This fall is performed stepwise
• Electrons are not passed directly to O2, but are picked up
by an electron acceptor, NAD+ (nicotinamide adenine
dinucleotide), which acts as an interim oxidizing agent
– NAD+ is aided by dehydrogenases that remove a pair of hydrogen
atoms
– 2 electrons and one proton go to NAD+ (becomes NADH)
– Remaining proton ‘floats’
• Purpose of first two stages of respiration is to produce
NADH, which goes to an Electron Transport Chain, which is
the main source of ATP production in cellular respiration
Respiration Stage 1: Glycolysis
• Converts 1 molecule of glucose (hexose
sugar) to 2 molecules of pyruvate (triose
sugar) in 10 steps
• Requires initial investment of 2 ATP(energy
investment phase)
• Yields 4ATP (net gain = 2 ATP), and 2 NADH
(energy yield phase)
• Conversion is through series of substratelevel phosphorylations and enzymes
• Occurs in the cytoplasm
A summary of Glycolysis
C6H12O6
+ 2 NAD+
+ 2 ADP + 2 Pi
2 C3H4O3 (pyruvate)
+ 2 NADH + 2 H+
+ 2 ATP
+ 2 H 2O
Respiration Stage 2: The Krebs Cycle
• Completes the energy yielding oxidation
of pyruvate
• Occurs in mitochondrion
– Translocation across mitochondrial
membrane by multienzyme complex. This
results in (per molecule of glucose)
• Release of 1 CO2
• Reduction of 1 NAD+ to NADH
• Attachment of coenzyme A
– Forms Acetyl Coenzyme A
• Acetyl Coenzyme A enters into Krebs
cycle (in mitochondrial matrix), where
remaining acetyl groups are oxidized
• The Krebs cycle is an energy mill that
produces (per molecule of pyruvate)
– 2 CO2
– 3 NADH
– 1 FADH
– 1 ATP
• Regenerates CoA
• Two turns of Krebs cycle required to
oxidize 1 molecule of glucose
Head count so far...
• Per molecule of glucose:
Molecule Glycolysis Krebs
ATP
2
2
NADH
2
6
FADH2
-
2
The electron transport chain (ETC)
• All ATP produce so far by substrate-level
phosporylation (not much!)
• A majority of ATP production is via oxidative
phosphorylation in the ETC
• Analogy: the ETC is like a salmon ladder
operating in reverse. Each step represents a
level of potential energy. Electrons ‘fall’ down
the ladder to reach their lowest potential state
(ie bound to O2. Each ‘fall’, releasing some
potential energy, is used to convert ADP TO
ATP
• The ladder starts with NADH donating
its electrons to the first ‘rung’ - an
electron carrier
• Each successive rung is an electron
carrier of increasing electronegative
potential. Electron carriers include:
– Flavoproteins
– Iron-sulfur proteins
– Cytochromes
• FADH donates its electrons further
down the ladder
How does the ETC harness energy chemiosmosis
• The ETC generates a proton gradient:
released potential energy is used to
pump a proton (H+) across the inner
membrane of a mitochondrion into its
intermembrane space
• H+ can’t leak back across membrane - it
has to pass through a specific gate - a
protein (enzyme) called ATP-synthase
• ATP-synthase uses proton gradient to
convert ADP to ATP
The final count (per molecule of glucose)
ATP produce
directly by
substrate-level
Reduced
ATP produced by
oxidative
process
Glycolysis
phosphorylation
Net 2 ATP
coenzymes
2 NADH
phosphorylation
4-6 ATP
Total
6-8
Oxidation of
pyruvate
-
2 NADH
6 ATP
6
Krebs cycle
2 ATP
6 NADH
2FADH2
18 ATP
4 ATP
24
Total
36-38
Fermentation
• Glycolysis oxidizes glucose to pyruvate
using NAD+, not oxygen
• Alcohol fermentation: glucose is
reduced to ethanol
• Lactic Acid fermentation: glucose is
reduced to lactate
• Organisms may be obligate aerobes,
obligate anaerobes, or facultative
aerobes
The control of respiration
• Catabolic pathways are controlled by
regulating enzymes at key points
• Key step is 3rd stage of glycolysis,
catalyzed by phosphofructokinase
– Sensitive to ratio of ATP:ADP
– Citrate (produced in Krebs) and ATP are
allosteric inhibitors of Phosphofructokinase
– Other allosteric enzymes that control the
rate of cellular respiration