Respiratory Electron transport Chain

The last step in aerobic respiration is the oxidation of reduced co enzymes NADH and FADH, produced in glycolysis and Krebs cycle by molecular oxygen. The pairs of hydrogen atoms released from glucose during glycolysis and Krebs cycle of aerobic respiration are not received directly by oxygen but pass along a series of electron carriers called coenzymes and cytochromes. This series of electron carriers constitute respiratory electron transport chain. The final electron acceptor at the end of the electron transport chain is oxygen forming water. Various molecules involved in the electron transport are NADH,,FADH.coenzyme Q. cytochrome b (Cyt.b), cytochrome c(Cyt.c). cytochrome a (Cyt.a) and Cytochrome a, (Cyt.a, ). The coenzyme Q and cytochromes are alternately reduced and oxidized. Electrons are passed along a series of carriers as they lose energy at each transfer. Some of this energy is used in the formation of ATP from ADP and inorganic phosphate. In the electron transport chain each next molecule is at lower energy level than the previous one. At the end oxygen accepts electrons and hydrogen to form water

Electron transport Chain

Chemiosmotic ATP Synthesis

The synthesis of ATP from ADP and inorganic phosphate in the electron transport system through the joint event of chemical and osmotic

processes is called chemiosmotic ATP synthesis.

Chemiosmotic theory of ATP synthesis suggests how ATP formatio is coupled with the energy release in the electron transport system. Mitochondria are surrounded by double membrane. The outer membrane is

smooth while the inner membrane forms enfolding which are shelf-like projections or protuberances called cristae. The cristae are present in the inner chamber or mitochondrial matrix that is filled with a gel-like substance. The carriers of electron transport system are present on the

cristae. A space is present between the outer and inner membrane called intermembrane space.

During the passage of electrons through the electron transport system certain carriers of the system pump hydrogen ions from the mitochondrial matrix into the intermembrane space. As result hydrogen ions accumulate on the outside of the inner membrane in the intermembrane space. Difference of hydrogen ion concentration increases across the membrane which develops a gradient of hydrogen ions between the matrix and the intermembrane space i.e. across the inner membrane. Hydrogen ions diffuse down the inner membrane through electrochemical gradient

from the intermembrane space into the matrix. The passage of hydrogen ions through the membrane is coupled to ATP synthesis from ADP and inorganic phosphate through ATP synthase complex. This process of ATP synthesis is called chemiosmosis because electrochemical and osmotic

events are involved.

Cellular Respiration of Proteins and Fats.

Cells degrade mostly glucose to release energy. However cells can

oxidize and degrade other food molecules such as proteins and fats to release energy.

Fats, Protein and Glucose Metabolism.

Proteins are broken down to amino acids. Amino group is removed from amino acids forming ammonia and the remaining molecule enters the Krebs cycle. Entry point to the Krebs cycle depends on the number of

carbon atoms of the entering molecule.

When fat is used as energy source it is hydrolyzed into glycerol and three fatty acids. Glycerol ( a 3-C compound) is converted to PGAL which enters the process of respiration into the glycolytic pathway. Each fatty acid is degraded into two carbon fragments acetyl groups which enter into

Krebs cycle. For example oleic acid is a fatty acid with 18 carbon atoms. It breaks down into nine acetyl groups. It is estimated that these nine acetyl groups would generate 108 ATP molecules. Because of very large number of production of ATP molecules fats are regarded as very efficient form of

stored energy

B. Anaerobic Respiration

The incomplete breakdown of glucose without the utilization of oxygen is

called anaerobic respiration. Anaerobic respiration (Fermentation) occurs

in the absence of oxygen. It involves incomplete breakdown of organic

food molecule and only a small amount of energy is released. Pyruvate

formed in glycolysis has two pathways. In human cells it depends on the

availability of oxygen. If oxygen is available then pyruvic acids is completely degraded into CO and water in mitochondria i.e. aerobic respiration. If oxygen is not available then anaerobic respiration continues and fermentation occurs. The process of fermentation consists of two steps

i.e. glycolysis and the reduction of pyruvate into alcohol or to lactate

Anaerobic respiration is of two types.

I. Lactic Acid Fermentation

This form of fermentation occurs ib muscles cells of human and in many microorganisms. It completes in two steps. In the first step glucose is broken down into pyruvic acid which is basically glycolysis. In the next step pyruvic acid is reduced by NADH into lactic acid. Compared to

aerobic respiration which yields 36 ATP molecules from the breakdown of

one glucose , anaerobic respiration

yields only 2 ATP molecules. Despite its

low yield of ATP, anaerobic respiration has its importance because of rapid

production of ATP(energy) when demanded.

A) Glucose + 2ATP→ Pyruvic acid +4ATP

B) 2NADH 2NAD2(C,H,0)Pyruvic Acid2 (C,H,O)Lactic Acid

ii. lcoholic Fermentation:

Alcoholic Fermentation is brought about by microorganisms. This type of anaerobic respiration also completes in two steps.

First step is the same glycolysis during which glucose molecule is broken down into two molecules of pyruvic acid and NAD is reduced to NADH . In the next step NADH, gives hydrogen to pyruvic acid, which is converted into ethyl alcohol and CO,. In alcoholic fermentation also 2 ATP molecules

are produced from one glucose molecule.

This is called alcoholic fermentation because alcohol is produced at the end.

2CO, 2NADH 2NAD (CH,0) Pyruvic Acid

2(C,H,O) Acetaldehyde 2 (C,H,OH) + ATP

Ethyl Alcohol

Photorespiration and its affects

Photorespiration is defined as the process in which oxygen combines with ribulose biphosphate (RuBP) in the presence of sunlight and CO, is produced. The process is called photorespiration because in the presence of light (photo), oxygen is taken up and CO2 is evolved (respiration). Photosynthesis needs optimum concentration of the requirements for normal functioning. If however one of the requirement is present in less concentration than the optimum, the process of photosynthesis is affected and slows down.

In the Dark reaction of photosynthesis, normally CO combines with RuBP (carboxylation) forming PGA molecules. The process occurs in the presence of an enzyme called ribulose biphosphate carboxylase (rubisco). This enzyme can act both as carboxylase and oxygenase. The reaction

depends on the concentration of CO, and O. If the concentration of CO, is more, then rubisco combines with co, and photosynthesis proceeds normally. On the other hand if the concentration of O, is more, then rubisco combines with , and photorespiration occurs. (Rubisco can act both as carboxylase as well as oxygenase).

stro Plants have stomata for the exchange of gases. Diffusion of water vapours from leaf to the external environment also occurs through the stomata. In dry and hot weather plants close up stomata so as to conserve

water. In such condition CO, cannot enter the leaf and cannot leave it. Dry and hot conditions are usually accompanied by intense sunlight therefore light reaction occurs with maximum rate which results in

maximum use of CO. Since concentration of CO, lowers down in the leaf and photorespiration proceeds.The following steps are involved in photorespiration:

1. Oxygen combines with RuBP (present

of chloroplast) and a compound called Glycolate is produced. RuBP Chcolate

2. Glycolate is converted into glycine (simplest amino acid) in the peroxisome

Chcolate → olisine

3. Glycine is transported to mitochondria where it is converted into serine and a molecule of CO is produced. Ghisine

Serine +Co2

Disadvantages of Photorespiration (Consequences)

1. Photorespiration is just the reverse of photosynthesis hampering the fixation of CO photosynthesis.

2. The process wastes energy and does nothing to serve the needs of the plant

Photosynthesis

In normal process of photosynthesis a 3C carbon compound called PGA is formed as the first detected product of photosynthesis (carbon dioxide fixation) and therefore these plants are called C plants. There are

some plants growing in dry and hot conditions produce a 4C carbon compound called oxaloacetate as the first product of carbon dioxide fixation in dark reactions of photosynthesis. These plants are called C, plants and this type of photosynthesis is called C photosynthesis. C, plants use rubisco to react Co, with RuBP. On the other hand plants use another enzyme called pepco (phosphoenolpyruvate carboxylase) to fix Co to a compound called phosphoenolpyruvate (PEP). This molecule is reduced to another called bundle sheath cells where Calvin cycle proceeds. molecule called malate. The malate carries CO, to the special type of cells. In C, plants chloroplasts are present only in mesophyll cells of leaf. However in a C.plant chloroplasts are present both in mesophyll cells and in bundle sheath cell. In a C, plant all the mesophyll cells carry out Calvin

cycle by fixing CO and producing elucose. In a plant the mesophyll cells only fix co, by using pepco while the bundle sheath cells carry out. Calvin cycle producing glucose,

This is a special condition evolved by C. plants so as to prevent photorespiration even in dry and hot environment as pepco does not bind to O, irrespective of the concentration of Co. Examples of plants are sugar cane, maize etc.

In C. plants the rate of photosynthesis remains high even when the stomata are closed and temperature is high. The rate of CO, fixation is also high as

compared to C, plants. C, cycle is basically an adoptability of C. plants to carry out CO, fixation in dry and hot condition and to reduce the rate of

photorespiration

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