Module 2-Cellular Respiration

8/2/2019 Module 2-Cellular Respiration

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Objectives:

In the end of this Module, the student must be able to:

y Enumerate the 10 enzymes that react on glycolysisyMemorize the process of the citric acid cycley Discuss the electron transport chainy Differentiate the mitochondrial membranes

Cellular Respiration

All living things in the world require energy in order tofunction. This energy is obtained from the food we eat. The cells

break down the energy stored in the food through a unique system known

as cellular respiration. In plain language, cellular respiration means

the procedure through which the food is broken down by the cells of

living things in order to produce the energy which is in the form of

ATP molecules (also known as the Adenosine Tri Phosphate molecules).

In order to function, living things must acquire energy from what

we eat. The term cellular respiration refers to the biochemical

pathway by which cells release energy from the chemical bonds of food

molecules and provide that energy for the essential processes of life.All living cells must carry out cellular respiration in order to

survive. Without cellular respiration, our body cannot produce ATP.

Without ATP, our body will not function because the energy that is

used to drive the processes in our body is absent.

Mitochondrion

The Mitochondrion is a membrane-enclosed organelle distributed

through the cytosol of most eukaryotic cells. Their numberwithin the cell ranges from a few hundred to, in very activecells, thousands. Their main function is the conversion glucoseinto ATP.

Mitochondria have:


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y an outer membrane that encloses the entire structurey an inner membrane that encloses a fluid-filledmatrixy between the two is the intermembrane spacey the inner membrane is elaborately folded with shelflike cristae

projecting into thematrix.

y a small number (some 510) circular molecules of DNA

The Outer Membrane

The outer membrane contains many complexes of integral membrane

proteins that form channels through which a variety of molecules and

ions move in and out of the mitochondrion.

The Inner Membrane

The inner membrane contains 5 complexes of integral membrane proteins:

y NADH dehydrogenase (Complex I)y succinate dehydrogenase (Complex II)y cytochrome c reductase (Complex III; also known as the cytochrome

b-c1 complex)

y cytochrome c oxidase (Complex IV)y ATP synthase (Complex V)

The Matrix

The matrix contains a complex mixture of soluble enzymes thatcatalyze the respiration of pyruvic acid and other small organicmolecules.


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Here pyruvic acid is

y oxidized by NAD+ producing NADH + H+y decarboxylated producing a molecule of

o carbon dioxide (CO2) ando a 2-carbon fragment of acetate bound to coenzyme A forming

acetyl-CoA

Phases of cellular respiration

y GlycolysisGlycolysis

(a splitting process) is a pathway for the catabolismof carbohydrates in which the six-carbon sugars are split to three-

carbon compounds with the release of energy used to transform ADP to

ATP. Glycolysis can proceed to anaerobic (without oxygen) and aerobic

reactions. Glucose will be the first reactant in glycolisis. The

final product of glycolisis is pyruvate.

y Preparatory Reaction

The produced Pyruvate in glycolysis, which is a 3-carbon sugar,

will enter the mitochondrion to transform this pyruvate into a 2-

carbon acetyl group called Acetyl CoA.

y Citric Acid Cycle or Krebs CycleThe Citric Acid Cycle or Krebs Cycle Acetyl CoA. Through a series

of steps, several compounds capable of storing "high energy" electrons

are produced along with two ATP molecules. These compounds, known as

nicotinamide adenine dinucleotide (NAD) and flavin adenine

dinucleotide (FAD), are reduced in the process. These reduced forms

carry the "high energy" electrons to the next stage. The Citric Acid

Cycle occurs only when oxygen is present but it doesn't use oxygen

directly.


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y Electron Transport ChainDuring the process of glycolysis and Krebs cycle, very little

energy is produced. The energy that remains inside the original

molecule of glucose gets released through the electron transport

chain. This chain is actually a widespread network of electroncarrying proteins, which are found inside the inner membrane of the

mitochondrion. The work of these proteins is to transfer the electrons

from one to another and finally adds itself with the protons to the

oxygen, which is known as the final electron acceptor. Though water is

produced during this procedure, no ATP is produced. ATP is produced

later through a proton. Thus the work of the electron transport chain

is only to produce an ingredient from which ATP can be produced.

GLYCOLYSIS

Glycolysis is a series of steps that converts glucose to

pyruvate. The energy released during these oxidation reactions is

used to form ATP, the energy currency of the cell. The initial steps

in glycolysis are the additions of two phosphates to the glucose

molecule, with the use of two molecules of ATP. The result is a six

carbon sugar diphosphate molecule and 2 ADP molecules. This six carbon

sugar diphosphate molecule is then split into two three carbon

molecules. Each of the three carbon molecules is converted through a

series of steps, to pyruvate. During these reactions, electrons are

transferred to the coenzyme NAD+ to form NADH and ATP is formed. Under

aerobic conditions, the pyruvate is further oxidized to yield more ATP

and under anaerobic conditions, fermentation follows.

Energy-Investment Steps

The first five steps of glycolysis is the energy-investmentsteps.

Glycolysis step 1:

Glucose phosphorylation catalysed by Hexokinase:


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The enzyme hexokinase phosphorylates (adds a phosphate group to)

glucose. In the process, a phosphate group from ATP is transferred to

glucose producing glucose 6-phosphate.

Glucose Glucose 6-phosphate

Glycolysis step 2:

Isomerization of glucose-6-phosphate catalysed by

Phosphoglucoisomerase:

The enzyme phosphoglucoisomerase converts glucose 6-phosphate into its

isomer fructose 6-phosphate. Isomers have the same molecular formula,

but the atoms of each molecule are arranged differently.

Glucose 6-phosphate Fructose 6-phosphate

Glycolysis step 3:

Second phosphorylation catalysed by Phosphofructokinase:

The enzyme phosphofructokinase uses another ATP molecule to

transfer a phosphate group to fructose 6-phosphate to form fructose 1,

6-bisphosphate.


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Fructose 6-phosphate Fructose 1, 6-bisphosphate

Glycolysis step 4:

Cleavage to two Triose phosphates catalysed by Aldolase:

The enzyme aldolase splits fructose 1, 6-bisphosphate into two sugars

that are isomers of each other. These two sugars are dihydroxyacetone

phosphate and glyceraldehyde phosphate.

Fructose 1, 6-bisphosphate Dihydroxyacetone phosphate +

Glyceraldehyde phosphate

Glycolysis step 5:

Isomerization of dihydroxyacetone phosphate catalysed by Triose

phosphate isomerase:

The enzyme triose phosphate isomerase rapidly inter-converts the

molecules dihydroxyacetone phosphate and glyceraldehyde phosphate.

Glyceraldehyde phosphate is removed as soon as it is formed to be used

in the next step of glycolysis.


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Energy-Harvesting Steps

Glycolysis step 6:

Generation of 1,3-Bisphosphoglyceratecatalysed by Glyceraldehyde-3-

phosphate:

The enzyme triose phosphate isomerase

serves two functions in this step. First the enzyme transfers a

hydrogen (H-) from glyceraldehyde phosphate to the oxidizing agent

nicotinamide adenine dinucleotide (NAD+) to form NADH. Next, triose

phosphate dehydrogenase adds a phosphate (P) from the cytosol to the

oxidized glyceraldehyde phosphate to form 1, 3-bisphosphoglycerate.

This occurs for both molecules of glyceraldehyde phosphate produced in

step 5.

Glycolysis step 7:

Substrate-level phosphorylation, 3-

Phosphoglycerate catalysed by

Phosphoglycerokinase:

The enzyme phosphoglycerokinase

transfers a P from 1,3-bisphosphoglycerate to a molecule of ADP toform ATP. This happens for each molecule of 1,3-bisphosphoglycerate.

The process yields two 3-phosphoglycerate molecules and two ATP

molecules.

Glycolysis step 8:

Phosphate transfer to 2-

Phosphoglycerate catalysed by

Phosphoglyceromutase:

The enzyme phosphoglyceromutase relocates the P from 3-

phosphoglycerate from the third carbon to the second carbon to form 2-

phosphoglycerate.

3-Bisphosphoglycerate

3-Phosphoglycerate


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Glycolysis step 9:

Synthesis of Phosphoenolpyruvate

catalysed by Enolase:

The enzyme enolase removes a molecule of water from 2-

phosphoglycerate to form phosphoenolpyruvic acid (PEP). This happens

for each molecule of 2-phosphoglycerate.

Glycolysis step 10:

Substrate-level phosphorylation. Pyruvate

synthesis catalysed by Pyruvate kinase:

The enzyme pyruvate kinase transfers a P from PEP to ADP to form

pyruvic acid and ATP. This happens for each molecule of PEP. This

reaction yields 2 molecules of pyruvic acid and 2 ATP molecules.

Enzymes (arranged in order):

1. Hexokinase2. Phosphoglucoisomerase

3. Phosphofructokinase

4. Aldolase

5. Triose phosphate isomerase

6. Glyceraldehyde-3-phosphate

7. Phosphoglycerokinase

8. Phosphoglyceromutase

9. Enolase

10. Pyruvate kinase

PREPARATORY REACTION

The preparatory reaction is so called because it prepares the

end product of glycolysis which is the pyruvate to enter to the


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mitochondrion. Thepyruvate will be transformed to a two carbon acetyl

group called Acetyl CoA. In this process, two molecules of NAD+ are

used because of the splitting of the fifth molecule in glycolysis.

KREBS CYCLE/ CITRIC ACID CYCLE

The krebs cycle occurs in the matrix of the mitochondrion.

Krebs cycle is named after Hans Krebs, the chemist who worked

out the fundamental s of the process in the 1930s. The Krebs

cycle starts with the two carbon acetyl group, which is produced

in glycolysis, called Acetyl CoA. Acetyl CoA binds with a four

carbon molecule and results to a six carbon molecule called

citrate.The CoA carrier is released. Carbon dioxide is the

released from the six carbon molecule, forming a five carbon

compound. In this step, hydrogen is removed and transferred to

NAD+ to form NADH. This compound is called Ketoglutarate. Next,

a second oxidation and decarboxylation occurs. Again, NADH and

carbon dioxide are produced. In addition, a molecule of ATP is

produced As a result of the reactions, a four carbon molecule is

formed in the Kerbs cycle. This four carbon compound is called

succinate. Succunate then rearranges to form fumarate. Finally,the four carbon molecule is further oxidized and the hydrogens

that are removed are used to form NADH and FADH2. These

reactions regenerate the four carbon molecule that initially

reacts with the acetyle CoA, Oxaloacetate.Each glucose molecule

is broken down into two pyruvate molecules during glycolysis.

Then each pyruvate is converted to Acetyl CoA, which enters the


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Krebs Cycle. Thus, for each glucose molecule,, the Krebs cycle

must complete two circuits to completely break down the pyruvate

molecules.

ELECTRONTRANSPORT CHAIN

In the mitochondrion, the energy stored in NADH is udes to

generate a proton gradient across the mitochondrial membrane and the

energy of the proton gradient is used to make ATP. When glucose is

oxidized during glycolysis and the Krebs Cycle, the co-enzymes NAD and

FAD are reduced to NADH and FADH2. Inside the mitochondrial matrix,

the electrons from NADH are transferred to the electron carrier

Coenzyme Q by NADH dehydrogenase, and the protons are transferred

across the membrane to the intermembrane space. Coenzyme Q carries the

electrons to the cytochrome bc1 complex As the electrons moves from

the bc1 to cytochrome c, more protons are carried from the inside to

the outside of the membrane. Electrons are also transferred from FADH2

to coenzyme Q, with the protons being transferred across the membrane.

Cytochrome c oxidase complex. Protons are also transferred to the


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outside of the membrane by the cytochrome c oxidase complex. The

cytochrome oxidase complex then transfers electrons from cytochrome c

to oxygen, the terminal electron acceptor and water is formed as the

product. The transfer of protons to the intermembrane space generates

a proton motive force across the inner membrane of the mitochondrion.

Since membranes are impermeable to ions, the protons that reenter the

matrix pass through special proton channel proteins called ATP

synthase. The energy derived from the movement of these protons is

used to synthesize ATP from ADP and phosphate. Formation of ATP by

this mechanism is referred to as oxidative phosphorylation.

FERMENTATION

Fermentation is the conversion of a carbohydrate such as sugar

into an acid or an alcohol. More specifically, fermentation can refer


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to the use of yeast to change sugar into alcohol or the use of

bacteria to create lactic acid in certain foods. Fermentation occurs

naturally in many different foods given the right conditions, and

humans have intentionally made use of it for many thousands of years.

TESTYOURSELF

A.

1.Which of the following statements correctly describes glycolysis?a.Glycolysis occurs in the mitochondrionb.Glycolysis requires Oxygenc.Glycolysis produces two molecules of pyruvated.All of the above

2. The first phase of glucose metabolism is __________.a.The Citric acid cycleb.Glycolysisc.The electron transport chaind.The preparatory reaction

3.Which of the following is associated with cellular respiration?a. NAD+b. O2c. FADd. Both A and C

4.Glycolysis occurs in thea.

Cytosol

b.Matrixc.Cristaed.Nucleus

5.The reactants of glycolysis include __________.a.Glucoseb.NAD+c.ATPd.All of the above

6.There is/are ____pyruvate molecule(s) produced per glucose moleculeduring glycolysis.

a.Oneb.Twoc.Sixd.Ten

7.If oxygen is available _____ follows glycolysis.a.Fermentationb.Prep reactionc.Citric Acid Cycle


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d.Electron transport Chain8.When oxygen is available, pyruvate enters the _________.

a.Nucleusb.Chloroplastc.Plasma membraned.

Mitochondria

9.The NET result of a single glycolysis run is the formation of______.a.1 NADH and 1 ATPb.2 NADH and 2 ATPc.2 NADH and 4 ATPd.4 NADH and 2 ATPe.4 NADH and 4 ATP

10. The electron transport system is located in the _________.a.Stromab.Matrixc.Cytosold.Cristae

B. Enumerate the 10 enzymes that react in Glycolysis.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.


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Module 2-Cellular Respiration

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