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

C6H12O6 + 6O2 6H2O + 6CO2 + energy

– 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

oxidation

reduction

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 substrate-level phosphorylations and enzymes

• Occurs in the cytoplasm

A summary of Glycolysis

C6H12O6 2 C3H4O3 (pyruvate)

+ 2 NAD+ + 2 NADH + 2 H+

+ 2 ADP + 2 Pi + 2 ATP

+ 2 H2O

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)

process

ATP produce

directly by

substrate-level

phosphorylation

Reduced

coenzymes

ATP produced by

oxidative

phosphorylation Total

Glycolysis Net 2 ATP 2 NADH 4-6 ATP 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