CELLULAR RESPIRATION

and FERMENTATION

Energy flow and chemical

recycling in ecosystems

Two major catabolic pathways

• Fermentation: partial degradation of organic molecules in the absence of oxygen

• Cellular respiration: complete breakdown of organic molecules, more efficient, uses oxygen

– Most processes occur in mitochondria

– General reaction: organic compound + O2 → CO2 + H2O + energy

– Example glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP + heat)

How is energy harvested?

• Redox reactions yield energy in catabolic pathways

• General definition: Redox reactions = reactions that result in the transfer of one or more electrons from one reactant to another

• Oxidation: loss of electrons

• Reduction: addition of electrons

• General reaction: Xe- + Y → X + Ye-

– X (electron donor) is the reducing agent and reduces Y

– Y (electron recipient) is the oxidizing agent and oxidizes X

• Oxygen is one of the most potent oxidizing agents (most electronegative atom)

As electrons “fall” from a less electronegative atom to a more electronegative atom, they lose free potential energy

• Example: methane combustion

• Good fuels:carbs and fats

How is energy harvested?

• Fuels are broken down gradually in a series of steps, each catalyzed by a specific enzyme

Principles of cellular respiration• Transfer of electrons to oxygen is not direct, but via a

coenzyme (NAD+ = nicotinamide adenine dinucleotide) NAD+: oxidizing agent in many redox steps

• The stepwise “fall” of electrons during cellular respiration:

– Energy is tapped to synthesize ATP as electrons “fall” from NADH to oxygen

– Reaction steps are called electron transport chain

Three stages during cellular respiration1. Glycolysis2. Krebs cycle (Citric acid cycle, tricarboxylic acid cycle)3. Electron transport chain and oxidative phosphorylation

Glucose: the example

Glycolysis (“Splitting of sugar”)

• Glucose is split into two three-carbon sugars, which are oxidized and rearranged to pyruvate

• Ten steps in two phases

• Net yield: 2 ATP and 2 NADH per glucose

• In cytosol

Energy investment phase

Energy payoff phase

The Krebs cycle • Completes the energy-yielding oxidation of organic molecules• Pyruvate is channeled into mitochondrion and modified into

Acetyl CoA

3 H2O are needed to provide 3O for 2CO2. These

3H2O give 6H+ and 6e-. There are 3H to start, one

gets to CoA. So, totally 8 e- and 8H+ produced.

O

CH3 —C —S —CoA 2CO2 + HS—CoA + e- + H+

2 CO2 and 8e- are produced in the Krebs cycle from each acetyl group.

• Each molecule pyrovate is broken down into 3 CO2

• Each cycle yields

– 4 NADH

– 1 FADH2

– 1 GTP

• Per glucose molecule two cycles!

• Within mitochondrial matrix

Krebs cycle summary

• Within inner membrane of mitochondrion (cristae)

• Electrons carried by NADH are transferred along the ETC to oxygen

The electron transport chain

(ETC)

The electron transport chain (ETC)

• Produces a proton (H+) gradient between the intermembrane space and the matrix

The electron transport chain (ETC)

• H+ concentration is a proton-motive force: H+ diffuse back to the matrix through “molecular mill” → generation of ATP

• Chemiosmosis: The coupling of the redox reactions of the electron transport chain to ATP synthesis

Cellular respiration – summary• Most energy flows from glucose → NADH → ETC → proton-

motive force → ATP • Efficiency of respiration = 40% of initial energy of glucose

Fermentation

• Anaerobic catabolism of sugars through a different “finish” of glycolysis

• NAD+ is recycled by transferring electrons from NADH to pyruvate or derivatives of pyruvate in various fermentation pathways

2

In anaerobic condition:

2

In organisms that can grow anaerobically, such as yeasts:

Alcohol fermentation• Pyruvate is converted to ethanol in two steps

• Many bacteria and some fungi can carry out alcohol fermentation

Alcohol fermentation - applications

When inadequate oxygen is present, for example, in a muscle cell undergoing vigorous contraction:

Lactic acid fermentation• Pyruvate is reduced directly by NADH to form lactate (ionized

form of lactic acid).

• Muscle cells switch to lactic acid fermentation when O2 is scarce

• Some fungi and bacteria can carry out lactic acid fermentation

Lactic acid fermentation - applications

• Both use glycolysis to oxidize sugars to pyruvate with a net production of 2 ATP by substrate-level phosphorylation

• Both use NAD+ as an electron acceptor

Fermentation and cellular respiration: common features

Fermentation and cellular respiration: differences

• Fermentation:

– NAD+ is recycled by passing the electrons of NADH to an organic molecule

– One molecule glucose yields 2 ATP (energy stored in pyruvate is unavailable to the cell)

• Respiration:

– Electrons of NADH are ultimately passed to O2,

generating ATP by oxidative phosphorylation

– Additional ATP is generated from the oxidation of pyruvate in the Krebs cycle

– One molecule glucose yields ~38 ATP

Other carbohydrates as well as fats and proteins can also be broken down

• Intermediate product enter at different steps of glycolysis and Krebs cycle

Metabolic pathways of respiration also important for the synthesis of molecules

Feedback mechanisms control cellular respiration

• E.g. phosphofructokinase (catalyzes third step of glycolysis):– inhibited by ATP and citrate

(first product of the Krebs cycle)

– stimulated by AMP– allosteric regulation

Rate of glycolysis and Krebs cycle are synchronized

Facultative anaerobes

• Some yeast and many bacteria that can survive using either fermentation or respiration

• At a cellular level, human muscle cells can behave as facultative anaerobes

Saccharomyces Yeast Cells

Evolutionary perspective

• Glycolysis

– the most widespread metabolic pathway and occurs in the cytosol without membrane-enclosed organelles

– probably evolved early in the history of life

• Oldest bacterial fossils

– are over 3.5 billion years old

– appeared long before appreciable quantities of O2

accumulated in the atmosphere (green algae and plants hadn’t evolved yet)

First prokaryotes may have generated ATP exclusively from glycolysis

Other carbohydrates as well as fats and proteins can also be broken down

Carbohydrates:

• Polysaccharides (e.g. starch) can be hydrolyzed to glucose monomers that enter glycolysis

• Hexose sugars other than glucose (e.g. fructose) are modified to enter glycolysis

Other carbohydrates as well as fats and proteins can also be broken down

Proteins:

• Must first be digested to individual amino acids

• Amino acids must have their amino groups removed (excreted as ammonia, urea, etc.)

• Carbon skeletons are then modified by enzymes and enter as intermediaries into glycolysis or the Krebs cycle depending on their structure

Other carbohydrates as well as fats and proteins can also be broken down

Fats • Must be digested to glycerol and

fatty acids• Glycerol can be converted to an

intermediate of glycolysis• Fatty acids are split into two-

carbon fragments via beta oxidation, then enter the Krebs cycle as acetyl CoA

• A gram of fat will generate twice as much ATP as a gram of carbohydrate via aerobic respiration

Metabolic pathways of respiration also important for the synthesis of molecules

• Not all organic molecules of food are completely oxidized to make ATP

• Intermediaries in glycolysis and the Krebs cycle can be diverted to anabolic pathways:– E.g. a human cell can synthesize about half the 20

different amino acids by modifying compounds from the Krebs cycle

– Glucose can be synthesized from pyruvate and fatty acids from acetyl CoA

• Cells can convert one kind of molecule to another using glycolysis and Krebs cycle:– E.g. excess carbohydrates and proteins can be

converted to fats

Feedback mechanisms control cellular respiration

• As ATP levels drop, catabolism speeds up to produce more ATP

• Excess ATP leads to inhibition of catabolism

• Control is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway

• E.g. phosphofructokinase (catalyzes third step of glycolysis):– inhibited by ATP and citrate (first

product of the Krebs cycle)– stimulated by AMP

Rate of glycolysis and Krebs cycle are synchronized

Metabolic pathways are complex…

Review of key processes

• During glycolysis and Krebscycle ATP is produceddirectly through substrate-basedphosphorylation

• In the electron transport chain ATP is produced indirectly through oxidative phosphorylation → much greater yield

Substrate-based phosphorylation

• Directly catalyzed by an enzyme

• Remember: enzymes

– lower the activation energy needed for a reaction to proceed

– Speed up reactions

– Are substrate-specific

– Make reactions controllable through feedback mechanisms

Oxidative phosphorylation

• Ultimately driven by the loss of electrons from food molecules

• Electrons are shuttled via NADH (and FADH2)

1. Acetyl CoA adds its two-carbon fragment

2. Citrate is converted to isocitrate by removal and subsequent addition of a water molecule

3. CO2 is lost, resulting

compound is oxidized, reducing NAD+ to NADH

4. CO2 is lost, resulting

compound is oxidized, reducing NAD+ to NADH, remaining molecule

is attached to CoA

5. CoA is displaced by a phosphate group, which is eventually transferred to ADP → ATP

6. Two hydrogens are transferred to FAD → FADH2

7. Addition of water rearranges bonds

8. Compound is oxidized, reducing NAD+ → NADH

Cycle starts again