Respiration

Respiration is the process by which the chemical energy of "food" molecules is released and captured in the form of high energy molecules such as ATP. Carbohydrates, fats, and proteins can all be used as fuels for respiration.

C6H12O6 + 6O2 → 6H2O + 6 CO2

The energy released is trapped for use by all the energy-consuming activities of the cell.

Respiration is also called cellular respiration

Adenosine TriphosphateAdenosine Triphosphate (ATP) is the universal energy carrier

(currency) of the cell.

It is made from the nucleotide Adenosine monophosphate (AMP). This nucleotide consists of a ribose (5C) sugar, a nitrogenous base (adenine) and a phosphate group.

AMP condenses with a phosphate group to produce ADP, which subsequently condenses with a phosphate group to produce ATP.

AMP + 2Pi ADP + Pi ATPThe energy used to add Pi to ADP comes from oxidation of

sugar

Adenosine Triphosphate Cont’dAdenosine monophosphate (AMP)

Respiration Cont’d

Respiration occurs in two phases: • glycolysis, which is the breakdown of glucose to pyruvic

acid • Kreb cycle, a process in which there is the complete

oxidation of pyruvic acid to carbon dioxide and water

In eukaryotes, glycolysis occurs in the cytosol. The Kreb cycle takes place in mitochondria.

Glycolysis

Glycolysis is a metabolic pathway, which consists of about 10 enzymatic steps.

It involves the oxidation of glucose to pyruvate (a 3C compound), and this is accompanied by the net generation of 2 ATP molecules.

It takes place in the cytoplasm and does not require the presence of oxygen.

Glycolysis is divided into three stages: 1. Phosphorylation of glucose; 2. Lysis to produce two 3C sugar phosphates;3. Oxidation by dehydrogenation

Glycolysis

Glycolysis

Glycolysis Cont’dOverall, two ATP molecules are used for phosphorylation

reactions, while four ATP molecules are produced.This results in a net gain of two ATP molecules.

The fate of pyruvate depends on the availability of oxygen in the cell.

If oxygen is present, pyruvate will enter a mitochondrion and get completely oxidized to CO2 and water.

If oxygen is unavailable, pyruvate will be converted to ethanol (yeast) or lactate (muscle).

Aerobic RespirationIn aerobic respiration, the pyruvate from glycolysis is

completely oxidized to carbon dioxide and water using oxygen (oxygen dependent stage).

In the first stage, pyruvate is broken down to carbon dioxide and hydrogen. This takes place in the matrix of the mitochondria via a series of reaction called the Krebs Cycle.

In the second stage, hydrogen is oxidized by oxygen to form water. This takes place by a series of reaction in the mitochondria cristae (electron transport chain).

Mitochondrion

Mitochondrion

Aerobic Respiration Cont’d

Each pyruvate molecule (3C) is converted to acetylcoenzyme A (acetylcoA – 2C), which allows it to enter the mitochondrion. CO2 molecule is released in the process.

The mitochondria matrix is the site of the Kreb Cycle and the formation of acetylcoA allows the product of glycolysis to enter the Kreb cycle.

The conversion of pyruvate to AcetylcoA includes a dehydrogenation and the hydrogen released results in the reduction to NAD to produce NADH.

Aerobic Respiration Cont’dNADH oxidation to NAD occurs on the inside of the

inner membrane is catalyzed by a dehydrogenase enzyme. The cristae give a large surface area to take these enzymes so NAD is recycled quickly.

NAD+ + 2H NADH + H+

NADH is the hydrogen carrier, which releases the hydrogen to oxygen to form water in the electron transport chain.

Kreb Cycle

Kreb Cycle Cont’d

For every turn of the cycle for the oxidation of the acetyl group from glycolysis

• Two carbon atoms are lost as carbon dioxide in two decarboxylation reactions.

• Four dehydrogenation reactions causes hydrogen to be added to the hydrogen carriers, resulting in the formation of 3 NADH and 1 FADH

• One high energy molecule is formed (ATP or GTP).• Three molecules of water is used, as they are needed as a

source of oxygen in decarboxylation reactions.

Electron Transport Chain

The energy released as electrons pass down the gradient from NADH to oxygen is harnessed by three enzyme complexes of the respiratory chain (I, III, and IV).

They pump protons (H+) against their concentration gradient from the matrix of the mitochondrion into the intermembrane space (an example of active transport).

Electron Transport Chain

Electron Transport Chain

As the H+ ion concentration increases (which is the same as saying that the pH decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex.

As in chloroplasts, the energy released as these protons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion.

Amount of ATP Produced

Most of the ATP is generated by the proton gradient that develops across the inner mitochondrial membrane.

The number of protons pumped out as electrons drop in energy level from NADH, through the respiratory chain to oxygen, produces enough energy that is theoretically large enough to generate 3 ATPs per electron pair (but only 2 ATPs for each pair donated by FADH2) as they return through ATP synthase, .

Amount of ATP Produced Cont’dWith 12 pairs of electrons removed from each glucose

molecule, 10 by NAD+ (so 10x3=30); and 2 by FADH2 (so 2x2=4), this could generate 34 ATPs.

Add to this the 4 ATPs that are generated by the 3 exceptions and one arrives at 38. But the energy stored in the proton gradient is also used for the active transport of several molecules and ions through the inner mitochondrial membrane into the matrix.

NADH is also used as reducing agent for many cellular reactions.

Overall Output for Aerobic Respiration

CO2 ATP NADH + H+ FADH + H+

Glycolysis - 2 2 -

Pyruvate Acetyl coA

2 - 2 -

Krebs cycle 4 2 6 2

TOTAL 6CO2 4ATP 10(NADH + H+) 2(FADH + H+)

Electron Transport (Respiratory) Chain

ATP ATP

NAD+ FADH+H+ Fe3+ Cu+ 1/2O2

NADH Dehydrogenase Cytochrome Cytochrome oxidase

NADH+H+ FAD+ Fe2+ Cu2+ H2O

Oxidized Reduced oxidized

ATPFe is a part of a haem group in the cytochrome protein complex;

while Cu is a part of a group of proteins called cytochrome oxidase.

Overall Activity of the Electron Transport Chain

Each NADH results in production of 3 ATP and H is released to be combined with O2 to form H2O.

Each FADH results in the production of 2 ATP.

Entering Produced Used

12H2 in the form of 10NADH + H+ 30ATP + 10H2O 5O2

2FADH + H+ 4ATP + 2H2O O2

TOTAL 34 ATP + 12H2O 6O2

Anaerobic Respiration

Many microorganisms use anaerobic respiration as their major source of ATP.

Some bacteria have to live where there is low levels of oxygen. They are called obligate anaerobes.

Organisms such as yeast or tapeworm can respire

aerobically or anaerobically. They are called facultative anaerobes.

Anaerobic Respiration Cont’dThe first part of anaerobic respiration is glycolysis. A net of

2 ATP and 2 NADH are produced.In aerobic respiration, H from NADH is reacted to form H2O.

However, in the absence of oxygen (anaerobicity), Hydrogen is added to pyruvate.

In Fungi such as yeastPyruvate acetaldehyde (ethanal) + CO2

The reaction is catalysed by pyruvate decarboxylase

Ethanal (CH3-CHO) + NADH + H+ ethanol + NAD+

Catalysed by alcohol dehydrogenase

Anaerobic Respiration in Muscle Tissue

Pyruvate + NADH + H+ Lactate + NAD+Catalysed by lactate dehydrogenase.

No CO2 is produced

Only a net of 2 ATP is produced in the glycolysis stage of glucose oxidation

Oxygen Debt

Oxygen debt (deficit) is the amount of oxygen that was needed, but not supplied from outside the body by breathing.