RESPIRATION PROCESS- GLYCOLYSIS AND CREBS CYCLE

  CELLULAR RESPIRATION  
 Cellular respiration is a physiological process in which food substance such as carbohydrates, lipids and proteins are broken down to release energy, CO2, and H2O. The process may require oxygen or not, If it requires oxygen it is described as aerobic respiration and if it takes place in absence of oxygen is described as Anaerobic respiration.
CONT…
The purpose of respiration is to produce energy that was interlocked in the bonds (organic bonds) of organic food synthesized during photosynthesis and make it available for use. The energy produced/released is in form of ATP (Adenosine triphosphate).
Adenosine triphosphate released from the process of cellular respiration is made up of sugar, base and three phosphate.
IMPORTANCE OF ATP
ATP is the source of energy for the different body activities, as a metabolically active cell may require up to two million ATP molecules every second.The following are some of the importances of ATP.

Anabolic processes
Synthesis of materials in living cells ( macromolecules) requires energy e.g synthesis of proteins from amino acids, DNA replication and polysaccharides from monosaccharide units.

Movement
Respiration provides energy for many forms of cellular movements including muscle movements,(contractions), ciliary actions, spindle (fibres) active in cell division.

Active transport
This involves movement of materials against a concentration gradient.
Secretion of different chemicals
Secretion of different chemicals and hormones in the body cells require energy from respiration in form of ATP.

Activation of chemicals
Respiration provides energy which makes chemicals more reactive enabling them to react readily example in phosphorylation

Bioluminescence
Some organisms (animals) have ability to produce light during the night. This process requires energy from respiration.

Nerve impulse transmittion

Warming the body (production of heat)


COMBUSTION  AND  CELLULAR RESPIRATION
Combustion is the rapid chemical combination of a substance with oxygen involving the production of heat and light. Combustion requires oxygen as a factor for it to occur as it is in aerobic respiration. Hence these two process have some similarities and some differences.

 SIMILARITIES BETWEEN CELLULAR RESPIRATION AND COMBUSTION
Both involve breaking down of substances to release energy.
They both take place in the presence of oxygen gas
In both processes CO2 is released as a bi-product from the system

DIFFERENCES BETWEEN COMBUSTION AND CELLULAR RESPIRATION
OCCURANCE
Cellular respiration takes place inside the cell, while combustion takes place outside the cell. Hence combustion does not need cellular environment for it to occur but respiration requires cell environment hence the name cellular respiration.

• ENERGY RELEASED

In cellular respiration energy is released in form of ATP, While in combustion energy is produced in form of heat thus no ATP formation.

• TEMPERATURE

In cellular respiration temperature remain constant(there is no remarkable raise in temperature) while in combustion temperature rises very high.

• STAGES INVOLVED

Cellular respiration involves mainly three stages which are Glycolysis, crebs cycle and electron transport chain. While combustion doesnot involve these stages.

• CHEMICAL REACTIONS

Chemical reactions in cellular respiration are enzyme controlled. while chemical reactions in combustion are not.

• TIME

Cellular respiration occurs slowly,  in stages and different intermediates are formed during the process. While  combustion occurs rapidly and no intermediate products are fomed during the process.

CELLULAR RESPIRATION AND PHOTOSYNTHESIS
Photosynthesis is the process by which organisms( autotrophs) capture sunlight energy from the sun and use it to manufacture food from carbondioxide and water in the environment and release oxygen as a waste product. Cellular respiration and photosynthesis cannot be separated i.e  depends on one another. For heterotrophic organisms to survive they require presence of autotrophs (organisms capable to manufacture their own food).

SIMILARITIES BETWEEN PHOTOSYNTHESIS AND RESPIRATION

• Both Photosynthesis and cellular respiration both involve production of chemical energy (ATP)
• In both processes the production of ATP is involved in the electron transport chain and chemo-synthesis
• Both processes takes place within the living cells.
• Both reactions involve chemical intermediate reactions.
• Both Photosynthesis and respiration are essential process in the living organisms.

DIFFERENCE BETWEEN PHOTOSYNTHESIS AND RESPIRATION

• ENERGY

Photosynthesis is an endothermic reaction as it absorbs sunlight energy while Respiration is an exothermic process as it releases energy.

• SITE OF OCCURANCE

Photosynthesis takes place in the chloroplast which contains the appropriate enzymes for the process to take place. While cellular respiration occurs in the mitochondria of the cell which contains enzymes specific for the process.

• TIME OF OCCURANCE

Photosynthesis takes place during the day time in presence of sunlight only while respiration takes place during the day and night i.e all the time.

• PRODUCTS

Photosynthesis process results into the production of oxygen and glucose as its end product, while respiration produces ATP, carbondioxide and water as the end product of catabolism of organic substances

• MATERIALS USED

Photosynthesis requires carbondioxide and water as the raw materials for the production of organic substance, while respiration requires glucose and oxygen as raw materials for the production of energy.

• SIGNIFICANCE

Plants can live for many days without photosynthesis taking place.While organism cannot remain alive without respiration.

• TYPE OF CELL

Photosynthesis occurs in green plant cells and in some photosynthetic organisms the green algae and some bacteria. While respiration occurs in all type of cells i.e in plant cell and animal cells.

RELATIONSHIP BETWEEN CELLULAR RESPIRATION AND PHOTOSYNTHESIS
Photosynthesis is the process by which organism capture energy from sunlight and use it to manufacture sugars from CO2 and H2O in the environment and release oxygen in the process, respiration is the process by which chemical energy in organic molecules is released by oxidation.

THE SITE OF RESPIRATION
Respiration occurs in the cell in which glycolysis occur in the cytoplasm, where as Krebs cycle and electron transport system occurs in mitochondria.
Its structure
Mitochondria have double membrane.The outer membrane which contain many proteins channels called Pornis which allow passage of small molecules through. Inner membrane is impermeable to most materials. It is folded into folders called cristae, giving a large surface area for reactions to take place.

The space inside the inner membrane is called matrix and it is where the Krebs cycle takes place.
The matrix contain some materials like DNA, tRNA and Ribosomes

ADAPTATIONS OF MITOCHONDRIA
Have a small size which gives large surface area to volume ratio for rapid uptake/release of materials for chemical reactions to take place.
The presence of enzymes of Krebs cycle in the matrix

Presence of Double membranes which provide compartmentalization of mitrochondria (separate all enzymatic reactions/ processes occurring in the mitrochondrion from processes in the cytoplasm.
Matrix is in aqueous form allowing enzymes to work best.

Presence of enzymes involved in generation of ATP by the oxidation of NADH2 and FADH2(electron transport chain) are tightly bound to the inner and outer membrane.
Presence of DNA which act as genetic materials to control mitochondrion replication and codes for synthesis of its own proteins and enzymes.

Narrow gap between inner and outer membrane allow proton concentration gradient to be rapidly established.
Presence of finger like projections called cristae to increase the surface area for electron transport chain(Terminal oxidation)

RESPIRATORY SUBSTRATES
Refers to the substance required for cellular respiration to derive energy through oxidation. Respiration involves oxidation of organic compound, the respiratory substrate which are carbohydrates, fats and proteins in which all may be used as required by the body.

• CARBOHYDRATES

When these are available they are usually used first by most cells. Polysacharides such as starch(in plants) and glycogen (in animals and fungi) are hydrolysed to monosacharides before they enter the respiratory pathway.

• LIPIDS (fats and oils)

Lipids are used mainly when carbohydrates reserves have been exhausted. They are first converted to glycerol and fatty acids. Fatty acids are energy rich and some cells such as skeletal muscle cells, in particular gain some of their energy from hydrolysis of lipids during normal activity.

• PROTEINS

Since proteins have other essential functions they are only used when all carbohydrates and lipid reserves have been used up, as during prolonged starvation. Proteins are first hydrolysed to amino acids and then deaminated (their amino groups are removed). The remaining acids can enter the krebs cycle or be converted first to a fatty acid for oxidation.

STAGES OF RESPIRATION

• Glycolysis
• Fermentation
• Krebs cycle/Tricarboxylic acid cycle(TAC)
• Electron Transport chain(Terminal oxidation)

Before studying the details of cell respiration it is useful to have an overview of the process. There is one aerobic pathway (krebs cycle) and two anaerobic pathway (glycolysis and fermentation). The first stage in all these pathways is glycolysis.


GLYCOLYSIS
is the oxidation of glucose to pyruvate. One glucose molecule( six carbon atoms) is broken into two molecules pyravate(3C).It occurs in cytoplasm of cells, not in the mitochondria and does not require the presence of oxygen. Glycolysis reaction can classified into two way, firstly according to type of reaction and secondly according to number of carbon.

According to type of reactions;
Phosphorylation of the sugar
This activates the sugar making it more reactive. The process uses some ATP. Bearing in mind that the whole point of respiration is to make ATP, This may seem unfortunate but it can be regarded as an investment which allows ATP producing reactions to occur later.

LYSIS
The phosphorylated 6C sugar is split into two 3C sugar phosphate hence the name glycosis, which means sugar splitting. The sugar phosphates are isomers of each other before continuing giving two identical 3C sugar phosphates.

OXIDATION BY DEHYDROGENATION
Each 3C sugar phosphate is converted to pyruvate. This involves a  dehydrogenation, making a reduced NAD molecule (Nicotinamide adenine dinucleotide) hydrogen carrier and production of two ATP molecules. The process happen twice, once for each 3C sugar phosphate molecule, so two reduced NAD and four ATP molecules are made.

Another way to classify the steps of glycolysis is through the number of carbons as follows;

The hexose stage
This is the stage which involves the conversion of glucose into dehydroxyacetone phosphate and phosphoglyceraldehyde( Glycerate-3- phosphate)
The Triose stage
This involves the conversion of two molecules of phosphoglyceraldehyde into pyruvate molecule.

STAGES OF GLYCOLYSIS

HEXOSE STAGE ( STAGE ONE)
First Reaction
Conversion of glucose to glucose-6- phosphate by phosphohexokinase enzyme.
    Glucose (6C)                           Glucose-6-phosphate (6C)

Second Reaction
Conversion of Glucose-6-Phosphate to fructose 6-phosphate by enzyme phosphohexoisomerase (phosphoglycoisomerase)
    Glucose-6-phosphate                              Fructose-6-phosphate (6C)

Third Reaction
Fructose-6-Phosphate is coverted to fructose-1,6-diphosphokinase
     Fructose-6-phosphate                         Fructose-1,6-diphosphate(6C)

Forth Reaction
Conversion of fructose-1,6-diphosphate to Dihydroxyacetone phosphate and phosphoglyceraldehyde (PGAL) by aldose enzyme.
    Fructose-1,6-diphosphate                           Dihydroxyacetone+ PGA(3C)

TRIOSE STAGE
Fifth Reaction
Dihydroxyacetone phosphate is converted to phosphoglyceraldehyde phosphate (PGAL) by triose phosphate Isomerase
  Dihydroxyacetonephosphate                            Phosphoglyceraldehyde phosphate (3C)


Sixth Reaction
Phosphoglyceraldehyde is converted to 1,3-diphosphoglycerate by enzyme Glyceraldehyde-3-phosphate dehydrogenase.
   Phosphoglyceraldehyde phosphate                    1,3-diphosphoglycerate(1,3 PGA)

Seventh Reaction
Conversion of 1,3-diphospoglycerate to 3-phosphoglycerate by ana enzyme phosphoglyceratekinase.
     1,3-diphosphoglycerate                   3-phosphoglycerate(3-PGA)

Eighth Reaction
3-Phosphoglycerate is converted into two 2-Phosphoglycerate by enzyme Phosphoglycerate
   3-Phosphoglycerate                   2-Phosphoglycerate (2-PGA)

Ninth Reaction
Conversion of 2-phosphaglycerate into phosphoenolpyruvate by enalase enzyme
      2-phosphoglycerate                         Phosphoenol pyruvate

Tenth Reaction
Phosphoenil pyruvate is converted into pyruvic acid by the enzyme pyruvate-kinase
     Phosphoenol pyruvate                       Pyruvate (3C)

PRODUCTS OF GLYCOLYSIS
2 ATP MOLECULES
2 NADH2 MOLECULES
2 PYRUVIC ACID MOLECULES
In glycolysis oxygen is not required and also carbondioxide is not produced.

FATES OF PYRUVIC ACIDS
KREBS CYCLE
FERMENTATION

FERMENTATION
This refers to the formation of alcohol by micro organisms and plants as well as lactic acid in animal cell. This involves the formation of incomplete oxidized product without the use of atmospheric oxygen.
TYPES OF FEMENTATION
There are two types of fermentation
i) Alcohol Fermentation
ii) Lactic acid Fermentation

ALCOHOL FERMENTATION
This is the kind of fermentation formed by microorganisms and plants. It involves the following processes.
Pyruvic  Acid from Glycolysis is converted into ethanal (Acetylaldehyde) through the loss of carbondioxide
                 Pyruvic Acid                   ethanol
                                            CO2
The ethanal then combines with hydrogen atom under the enzyme called ethanal dehydrogenase to form ethanol
                   Ethanal                  Ethanol

In summery

 Glucose                  2-ethanol+ 2ATP+2CO2    

The total energy released by conversion of Glucose to Ethanol is 210kJ/mol

LACTIC ACID FERMENTATION
This is the kind of fermentation which occurs in animal cells (muscle cells).In lactic Acid fermentation the pyruvic acid accepts the hydrogen atoms of NADH directly and becomes lactic acid.The reaction is catalysed by lactate dehydrogenase(enzyme).
         Pyruvic Acid                    lactic Acid
In this process the molecules of ATP are formed from Glucose molecule
          Glucose                 2ATP + 2 lactic Acid

IMPORTANCE OF FERMENTATION
Fermentation processes are important in the household and industries these importance include;

• In souring of milk or formation of curd
• Formation of alcohols in industries (brewing industries) where various alcohols such as beer and wines are made.
• Baking industries for making breads and biscuits
• Production of vinegar by Acetic Acid Bacteria
• In the presence of oxygen the lactic acid is broken down to yield energy, it can also be used to synthesize carbohydrate also it can be excreted. Lactic acid is poisonous and causes pain to the muscles if left to accumulate which in turn causes muscle cramps.

DIFFERENCES BETWEEN ALCOHOL FERMENTATION AND LACTIC ACID FERMENTATION

• PRODUCTS

Alcohol is produced in the process of alcohol fermentation while in lactic acid fermentation no alcohol is produced and instead lactate(lactic acid) is produced.

• CO2 LIBERATION

Alcohol fermentation liberate carbondioxide during conversion of pyruvic acid from glycolysis  to form ethanol. While in lactic acid fermentation do not liberate CO2.

• ENERGY PRODUCED

The total energy liberated in alcohol fermentation is 210KJ/mol ,While only 150KJ  is liberated as energy in lactic acid fermentation hence alcohol fermentation produce more energy compared to lactic acid fermentation.

• OXYGEN EFFECT

In alcohol fermentation energy locked in ethanol cannot be rectrieved later even when there is presence of oxygen. While in lactic acid fermentation energy locked in lactate is avaible in later stage when oxygen is present

• ANAEROBIC RESPIRATION

This is the type of respiration whereby oxygen is not required for the breakdown of respiratory substrates. In this process there is no gaseous exchange between the respiratory sites and the environment which results to the incomplete breakdown of the respiratory substrates

DIFFERENCES BETWEEN AEROBIC AND ANAEROBIC RESPIRATION

• OXYGEN DEMAND

Aerobic respiration requires oxygen for breaking down of respiratory substrates ie  Carbohydrates, lipids, and proteins. While Anaerobic respiration do not require oxygen for it to perform the respiratory process of  breaking down of respiratory substrates.

• REACTION PROCESS

In Aerobic respiration respiratory materials are completely oxidized ie it involves complete breakdown of respiratory substrates to yield energy  and  carbondioxide and water as waste products. while Anaerobic respiration involves incomplete breakdown of respiratory substrates.

• GASEOUS EXCHANGE

In aerobic respiration there is gaseous exchange between between the organism and the environment to bring in oxygen for the process. While in anaerobic respiration there is no gaseous exchange between the organism and the environment since oxygen is not required for the process.

• END PRODUCTS

End products of aerobic respiration are CO2,H2O and energy. While in anaerobic respiration the end products are CO2,energy and alcohol and also there is no formation of water at all in anaerobic respiration.

• STAGES INVOLVED

In aerobic respiration all three stages are involved ie Glycolysis, krebs cycle and terminal oxidation while in anaerobic respiration it involves only two stages which are Glycolysis and incomplete breakdown of pyruvic acid.

TRANSITION STAGE BETWEEN GLYCOLYSIS AND KREBS CYCLE
Each pyruvate molecule enters the matrix of a mitochondria where it is converted to an acetyl group (CH3COO).These are carried by coenzyme A as Acetylcoenzyme A. Acetyl groups have two carbon atoms (2C), so the conversion of pyruvate (3C) to acetyl involves loss of carbon. This is lost as carbondioxide in decarboxylation reaction. A dehydrogenation also occurs so that reduced NAD is made from NAD.


KREBS CYCLE
It was named after the British biochemist sir Hans Krebs who discovered it. Glycolysis yields a little amount of energy from the glucose molecule and the majority of the energy remain locked in pyruvic acid. In presence of oxygen it is broken down to carbondioxide and hydrogen atoms in a series of reactions called the krebs cycle.
CONT…
Krebs cycle is also known as citric acid cycle due to the acid it contains. Another name of krebs cycle is tricarboxylic acid  cycle because citrate has three carboxyl group. It takes place in the matrix of the mitochondrion in the eukaryotic organism.
The pyruvic molecule produced by the glycolysis are incoperated in the tricarboxylic acid cycle (TCA) where it combines with acetyl co enzyme forming 2 acetyl attached to co enzyme A. Under the influence of enzyme pyruvic dehydrogenase where pyruvic molecule is oxidized and decarboxylated.

Pyruvic acid                       Acetyl group +Coenzymes
                              CO2


Pyruvic acid                       Acetyl group +Co enzyme+ acetylCo A
                             CO2
CONT…
It mainly involves three intermediate stages
i) Removal of CO2 from Pyruvate
ii) Reduction of NAD+ to NADH which is the significant source of metabolic energy.
iii) Oxidation of pyruvate, the acetyl group removed from pyruvate combined with co factor called Co enzyme A to form a compound known as Acetyl-co A.
CONT..
Acetyl Co A is the starting point for Krebs cycle. This reaction is known as transition reaction because it connects glycolysis to the krebs cycle.


KREBS CYCLE
In this stage Acetyl-Co A is oxidized in series of reactions in the cycle the two carbon acetyl group or acetyl-Co A combine with four carbon molecules called oxaloacetate.
1st Reaction
The Acetyl Co A of two carbon groups join with four carbon molecules oxaloacetate to form six molecule citrate.
        Oxaloacetate +  AcetylCo A                      6 citrate(6C)
However the reaction is inhibited when the cells ATP concentration is high and stimulated when it is low.

2nd Reaction and 3rd Reaction
Before oxidation the hydroxyl group(OH) of citrate is repositioned whereby water molecules are removed from carbon then water is added to a different carbon as the result of interchange of H+  and OH- the product is an isomer of citrate called Isocitrate.

4th Reaction
The citrate is oxidatively decarboxylated to α- ketogluterate in the process of reduced NAD+ to NADH2 and it yields 5 carbon molecules called α- ketogluterate.
       Citrate                  α-ketogluterate (5C)

5th  Reaction
In this stage oxidation occurs in presence of succinate Co A synthesize enzymes α- ketoglutarate is oxidized and decarboxylated to succinate.
In this process molecules of ATP is synthesized from ADP and Pi and the central carbon atom is slit off to form CO2.
α-Ketogluterate                         Succinate (4C)

6th Reaction
The bond between four carbon succinyl group of Co A is cleaved and the energy is released which drives the phosphorylation of guanosine diphosphate(GDP).Forming guanosine triphosphate GTP which is readly converted to ATP. And the remaining 4 carbon molecule is called succinate.

7th Reaction
Oxidation occurs where succinate is oxidized to fumerate by succinic dehydrogenase enzyme and the hydrogen acceptor in this case is FAD. Unlike NAD+, FAD is not free to diffuse within the mitochondria membrane it reduces to form FADH2.
           Succinate                       Fumarate.

8th and 9th Reaction
In this stage fumerate is hydrated to form Malate by fumarate enzymes.
          Fumarate                    Malate
 Malate is then oxidized to yield a four carbon molecules of oxaloacetate and two electrons that reduce molecules of NAD+ to NADH. The enzymes involved is malicdehydrogenase and NAD. Oxaloacetate molecules are now free to combine with another two carbon acetyl Co A.

NOTE
To keep the cycle operational supply of oxaloacetate must be maintained
Since the cycle involves only one molecule of pyruvic acid each molecule of glucose has got two complete cycle.

DIAGRAM OF KREBS CYCLE

Overal budget of krebs cycle

        CO2
        ATP
 NADH+ H+
FADH+H+

Pyruvate Acetyl Co A
        2
        -
       2
      -

Krebs Cycle
         4
       2
      6
      2

Total
       6
       2
       8
      2


IMPORTANCE OF KREBS CYCLE
During Krebs cycle carbon skeleton are obtained for use in growth.

Many intermediate compounds are formed which are used to synthesize other biomolecules such as amino acids, Nucleotides, Chlorophyll, cytochromes and fats.

During this pathway amino acids are synthesized from ketoglutaric pyruvic acid and oxaloacetic acid.

Here succinyl Co A acts as starting molecule for synthesis of chlorophyll.

Krebs cycle is a major pathway for generation of ATP molecules which are the energy currency of the cell.

It brings about degeneration of macromolecules the 3-carbon pyruvate is broken down to carbondioxide.

It produce and reduce NAD and FAD which pass through electron transport system to release energy.

ELECTRON TRANSPORT CHAIN
Refers to a sequence of electron carrier molecules (membrane proteins) that shuttle electrons during the redox reactions that release energy used to make ATP. Most components of the chain are proteins which exist in multiprotein complexes numbered I through IV. Tightly bound to those proteins are prosthetic groups, non-protein components essential for the catalytic functions of certain enzymes.

DIAGRAM OF ELECTRON TRANSPORT CHAIN

The diagram above shows the sequence of electron carriers in the electron transport chain and the drop in free energy as electron travel down the chain.During electron transport along the chain electron carriers alternate between reduction and oxidation as they accept and donate electrons.
CONT..
Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor which has a lower affinity for electrons (ie less electronegative). It then returns to its oxidized form as it passes electrons to its “ downhill” more electronegative neighbor.
Now lets take a closer look at the electron transport chain in a diagram.Electron removed from food by NAD+, during glycolysis and the citric acid cycle, are transferred from NADH to the first molecule of the electron transport chain. This molecule is a flavoprotein, so named because it has a prosthetic group called flavin mononucleotide (FMN is complex). In the next redox reaction, the flavoproteins returns to its oxidized form as it passes electrons to an iron-sulphur protein(Fe.s in complex one). One of a family of proteins with both iron and sulphur tightly bound the iron sulphur protein then passes the electrons to a compound called ubiquinone. This electron carrier is the small hydrophobic molecule, the only member of the electron transport chain that is not a protein. Ubiquinone is mobile within the membrane rather than residing in a particular complex.
CONT..
Most of the remaining electron carriers between ubiquinone and oxygen are proteins called cytochrome. Their prosthetic group, called a heme group has an iron atom that accept and donate electrons.(It is similar to the heme group in hemoglobin, the proteins of red blood cells, except iron in haemoglobin carries oxygen not electrons) the electron transport chain has several types of cytochromes, each a different protein with slightly different electron- carrying heme group. The last cytochrome of the chain, cyt a,passes its electrons to oxygen, which is very electronegative. Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution forming water.
 CONT…
Another source of electrons for the transport chain is FADH2 ,the other reduced product of the citric acid cycle. Notice in the diagram that FADH2, adds its electrons to the electron transport chain at complex II, at a lower energy level than NADH does, the electron transport chain provide about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH.
The electron transport chain makes no ATP directly. Its function is to ease the fall of electrons from food to oxygen, breaking a larger free-energy drop into a series of smaller steps that release energy in manageable amount. How does the mitochondria couple this electron transport and energy release to ATP synthesis? This is through the process called chemiosmosis.

ATP SUMMATION
The number of ATP molecules produced during respiration of one molecule of glucose depends on the nature of glucose metabolism.

ANAEROBIC RESPIRATION
When the glucose molecule is completely oxidized anaerobically the only molecule of ATP formed are those formed during glycolysis only 2 ATP molecules.

AEROBIC RESPIRATION
When glucose molecules is completely oxidized aerobically the source of ATP are as follows.

GLYCOLYSIS
In this process a total of four ATP are synthesized directly. However 2ATP molecules are used to pay back for the initial energy investment used to initiate the process of respiration hence the net yield is only 2ATP.

KREBS CYCLE
Yield only 2ATP molecules

RESPIRATORY CHAIN
The number of ATP molecules depend on the number of NADH2 and FADH2  available thus
Glycolysis yield 2 NADH2
Conversion of pyruvate to acetyle yield 2NADH2
Krebs cycle yield 6NADH2
CONT…
Therefore there are 10NADH2  molecules in which each NADH2  yield a total of  3ATP. In the respiratory chain. This is because it shunts its hydrogen atoms at the NAD on the beginning of the chain.
CONT..
The FADH2 on the other hand yield 2ATP molecules this is because it shunts its hydrogen atom at FAD. Therefore the total number of ATP in the respiratory chain is 3ATP×10NADH2=30ATP Molecules and 2ATP×FADH2=4ATP. The respiratory chain thus yield 34ATP.


SUMMATION
Glycolysis              2ATP
TAC                         2ATP
Respiration chain 34ATP

Thus molecules of glucose when completely oxidized aerobically yield 38ATP Molecules.

Overal  budget for aerobic respiration of one glucose molecule.

           CO2
         ATP
      NADPH2
       FADH2

Glycolysis
        _
         2
       2
         _

Pyruvate-AcetylCO-A

        2

        _

       2

         _

KREBS CYCLE
        4
         2
      6
         2

TOTAL
         6
        4
      10
         2


The overall equation
C6H12O6  +6H2O ----------> (6CO2+4ATP+12H2………………….(1)
The overall equation for respiratory chain
 12H2+6O2(12H2O +34 ATP……………………………….(2)
Combining the two equations we get…
C2H2O6  +  6O2 (6O2 +6 H2O + 38ATP
Thus 38 molecules of ATP are produced for every glucose molecule oxidized in aerobic respiration.
ATP  BOOK KEEPING
      PROCESS
       DIRECTLY
      INDIRECTLY
       TOTAL

 GLYCOLYSIS
       2 ATP
      6 ATP
     8 ATP

 PYRUVATE
     _
       6 ATP
     6 ATP

 TAC
       2 ATP
      22 ATP
    24 ATP

 TOTAL
      4 ATP
      34 ATP
    38 ATP



ALTERNATIVE RESPIRATORY SUSTRATE

RESPIRATION OF FATS
Initially fats are hydrolysed by lipase enzyme into fatty acids and glycerol.
FATS(Fatty Acids + Glycerol
Glycerol is first phosphorylated by ATP to form glycerol phosphate. Then it is later dehydrogenated by NAD into dihydroxyacetone which is then converted into its isomers 3-PGAL
CONT..
 The NADH2 is passed through the respiratory chain with yield of 3 ATP Molecules. The product of 3-PGAL enter the cycle where it yields 17ATP. Thus a complete oxidation of glycerol yield 19ATP molecules. The one ATP is consumed by the process in the initial stage.

Fatty acids
In the matrix of the mitochondria fatty acids are oxidized by the process called β-oxidation. β-oxidation means fragmentation of the fatty acid molecules into 2- carbon fragments. The later are then converted into acetylco A which is then fed into the Krebs cycle at the point where acetylcoA occurs.
Fatty acid-----------> (2-carbon+ CoA------------> ( acetylcoA-----------> (Krebs cycle
The advantage of respiring fatty acids is that it provides large number of hydrogen atom to which when passed through the respiratory chain yields large number of ATP.

METABOLISM OF PROTEINS
Proteins are first hydrolysed into amino acids. The amino group is removed and is attached to a non amino acid residue (organic acid) to form a used substance. This process is known as transamination. Deamination takes place where amino acids are converted into ammonia and α- keto acid. The α-keto acid can either be glucogenic which becomes converted into glucose then carbohydrate metabolism. The formation of glucose from non carbohydrate source is termed as glucogenesis.



SUMMERY OF THE RESPIRATORY PATHWAY

RESPIRATORY QUOTIENT
Respiratory quotient is the measure of the ratio of CO2 evolved to the oxygen consumed by an organism.
                          RQ= volume of CO2 evolved

                                    ---------------------------------
                                   Volume of O2  consumed

SIGNIFICANCE OF RQ
Helps to tell the type of substrate being respired. For example the respiration quotient of glucose is one since the volume of CO2 evolved is 6 and volume of oxygen consumed is 6.

When glucose is completely oxidized the RQ is 1. When protein is completely oxidized the RQ is 0.9. When fatty acid is completely oxidized RQ is less than that of protein.
For example; 2C51H98O6 +145O2 -------> ( 102CO2 +98H2O   since RQ=CO2/O2=102/145=0.7
CONT…
It is not easy to have the above calculations because substrates are rarely completely oxidized.

The value of respiratory quotient is used to tell the sort of metabolism that is going on, if the RQ value is greater than 1 this means that the respiring cells or tissue is in short supply of oxygen ie respiring anaerobically. If the RQ is lower than 1 it indicates that the mixture of substrate is respired or some of the CO2 evolved is put into some use by the organism e.g photosynthesis in plants.

BASAL METABOLIC RATE
Basal metabolic rate is the minimum amount of energy needed to keep the body alive at absolute rest. Before the BMR of the human subject is measured they undergo standardized rest period of 12-18 hours of physical and mental relaxation. No meal is eaten to ensure that the alimentary canal is empty before measurements are taken.

The following are the factors affecting BMR
AGE
TEMPERATURE-  during cold respiration increase to maintain body temperature
PHYSICAL ACTIVITY- a man who is involved in heavy activities needs more oxygen for the release of energy.
BODY SIZE- Rate of respiration is correlated with body surface area to volume ratio
Physiological condition- pregnant women need more oxygen than non-pregnant women because much oxygen is needed to fulfill the needs of the maternal and fetus.

REFERENCE
G  ILENN and  SUSAN  Toole (2014) Understanding  Biology for Advanced level forth edition.
Oxford University press United Kingdom Ermes Kira (2009) Advanced level Biology Text book for form 5 & 6. Nyambari Nyangwine publishers East Africa
D.J TAYLOR, N.P.O GREEN and G.W STOUT (2012). Biological science 1&2 Third Edition Britain Cambridge University PRESS.