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The electron transport chain ( [link] a ) is the last component of aerobic respiration and is the only part of metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plants for this purpose. In animals, oxygen enters the body through the respiratory system. Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where oxygen is the final electron acceptor and water is produced. There are four complexes composed of proteins, labeled I through IV in [link] c , and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain    . The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of prokaryotes. In each transfer of an electron through the electron transport chain, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions across the inner mitochondrial membrane into the intermembrane space, creating an electrochemical gradient.

Art connection

Part a: This illustration shows the electron transport chain embedded in the inner mitochondrial membrane. The electron transport chain consists of four electron complexes. Complex I oxidizes NADH to NAD+ and simultaneously pumps a proton across the membrane into the intermembrane space. The two electrons released from NADH are shuttled to coenzyme Q, then to complex III, to cytochrome c, to complex IV, then to molecular oxygen. In the process, two more protons are pumped across the membrane into the intermembrane space, and molecular oxygen is reduced to form water. Complex II removes two electrons from FADH2, thereby forming FAD. The electrons are shuttled to coenzyme Q, then to complex III, cytochrome c, complex I, and molecular oxygen as in the case of NADH oxidation. Part b: This illustration shows an ATP synthase enzyme embedded in the inner mitochondrial membrane. ATP synthase allows protons to move from an area of high concentration in the intermembrane space to an area of low concentration in the mitochondrial matrix. The energy derived from this exergonic process is used to synthesize ATP from ADP and inorganic phosphate. Part c: This illustration shows the electron transport chain and ATP synthase enzyme embedded in the inner mitochondrial membrane, and the citric acid cycle in the mitochondrial matrix. The citric acid cycle feeds NADH and FADH2 into the electron transport chain. The electron transport chain oxidizes these substrates and, in the process, pumps protons into the intermembrane space. ATP synthase allows protons to leak back into the matrix and synthesizes ATP.
(a) The electron transport chain is a set of molecules that supports a series of oxidation-reduction reactions. (b) ATP synthase is a complex, molecular machine that uses an H + gradient to regenerate ATP from ADP. (c) An overview of the entire process.

Electrons from NADH and FADH 2 are passed to protein complexes in the electron transport chain. As they are passed from one complex to another (there are a total of four), the electrons lose energy, and some of that energy is used to pump hydrogen ions from the mitochondrial matrix into the intermembrane space. In the fourth protein complex, the electrons are accepted by oxygen, the terminal acceptor. The oxygen with its extra electrons then combines with two hydrogen ions, further enhancing the electrochemical gradient, to form water. If there were no oxygen present in the mitochondrion, the electrons could not be removed from the system, and the entire electron transport chain would back up and stop. The mitochondria would be unable to generate new ATP in this way, and the cell would ultimately die from lack of energy. This is the reason we must breathe to draw in new oxygen.

In the electron transport chain, the free energy from the series of reactions just described is used to pump hydrogen ions (via active transport) across the membrane. The uneven distribution of H + ions across the membrane establishes an electrochemical gradient, owing to the H + ions’ positive charge and their higher concentration on one side of the membrane.

Hydrogen ions diffuse through the inner membrane through a membrane protein called ATP synthase    ( [link] b ). This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient from the intermembrane space, where there are many mutually repelling hydrogen ions to the matrix, where there are few. The turning of the parts of this molecular machine regenerate ATP from ADP and phosphate.

Chemiosmosis ( [link] c ) is used to generate 90 percent of the ATP made during aerobic glucose catabolism. The result of the reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the electron transport system, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen ions attract hydrogen ions (protons) from the surrounding medium, and water is formed.

Atp yield

The number of ATP molecules generated from the catabolism of glucose varies. In general, processing of each NADH yields approximately 3 ATP and each FADH 2 yields approximately 2 ATP. Overall, a total of 10 NADH and 2 FADH 2 were produced in glycolysis, transition reaction, and the citric acid cycle per glucose molecule. This results in the production of approximately 34 ATP. Remember, that two additional ATP were produced directly in both glycolysis and the citric acid cycle, resulting in a total yield of 38 ATP per glucose. This represents an efficiency of approximately 35%, with the remaining energy potential lost as heat or other products.

Careers in action

Mitochondrial disease physician

What happens when the critical reactions of cellular respiration do not proceed correctly? Mitochondrial diseases are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial disease, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disease.

Section summary

The citric acid cycle is a series of chemical reactions that removes high-energy electrons and uses them in the electron transport chain to generate ATP. One molecule of ATP (or an equivalent) is produced per each turn of the cycle.

The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor for electrons removed from the intermediate compounds in glucose catabolism. The electrons are passed through a series of chemical reactions, with a small amount of free energy used at three points to transport hydrogen ions across the membrane. This contributes to the gradient used in chemiosmosis. As the electrons are passed from NADH or FADH 2 down the electron transport chain, they lose energy. The products of the electron transport chain are water and ATP. A number of intermediate compounds can be diverted into the anabolism of other biochemical molecules, such as nucleic acids, non-essential amino acids, sugars, and lipids. These same molecules, except nucleic acids, can serve as energy sources for the glucose pathway.

Questions & Answers

In the digestive system,what are nucleotides and what is their use
Leo Reply
what is the difference between kidney and liver
Raphael
what's an isomer
marybertiny Reply
is a chemical species with the same number and types of atoms as another chemical species but with distinct properties because rhe atoms are arranged into different chemical structures.
yaad
How are they important to anmals
Leo
immune system protect body from harmful foreign cells
Hira Reply
that's right some time when bacteria attached the body the immune system product
abdala
what is function of the body
Sarat Reply
Body funtion. Our bodies consist of a number of biological systems that carry out specific functions necessary for everyday living. The job of the circulatory system is to move blood, nutrients, oxygen, carbon dioxide, and hormones, around the body. It consists of the heart, blood, blood vessels,art
Sayed
what's the function of the lymphatic system
Samuel
how long does it take for food process to reach the deudenum
Braveson
how does it takes for food to digested
Kelliann
how long does it takes for food to digested
Kelliann
what are tissue
Nkeng Reply
what are the difference type of connective tissue
Nkeng
Types of connective tissue 1. Loose connective tissue: a.Areolar connective tissue b.Adipose connective tissue 2. Dense (fibrous) connective tissue: I. Compact tissue a.White fibrous tissue ( tendon and sheath) b.Yellow elastic tissue (Ligament) II. Supporting connective tissue: types- 1. Cartilag
iti
2. Bone III. Fluid or liquid connective tissue: Blood lymph
iti
What is cytoplasm
Uyi Reply
what is cytoplasm
Morgan
a cytoplasm is a fluid found inside the cell membrane which contains dissolved minerals e.t.c.
Fortune
ok
Morgan
why is the the mitochondria known as the power house?
Fortune
The mitochondria is involved in releasing energy from food..this process is known as cellular respiration.
Sherice
ok
Fortune
what is Deoxyribonucleic Acid (DNA)?
Jackson
is a molecule composed of two chains.
Fortune
is a molecule composed of two chains that coil around each other to form a double helix carrying the genetics instructions used in growth,development e.t.c.
Fortune
Thanks
Jackson
structur of the heart
Munyah Reply
hi
Unimke
Hello
Mariam
miss Mariam u are welcome
Emmanuel
hello
wana
hii
Dein-aboh
hi
Wani
how
Renzo
hello every one
BOBO
hello
iti
hello
Hilmi
hello
Eshiemoghie
hi
Vaishnavi
what is mutation?
Oliver
Hi to everybody
Hussein
essay for photosynthesis
NiCho Reply
essay of the human body
NiCho
how do female menstrual uterus prepare for fertilisation?
Jackson
what is blood pressure
Sid Reply
to my understanding blood pressure is where u are shock with something emergency I think that can also course blood pressure
Badu
what is the meaning of extended family
Shellyann Reply
extended family is two are more nuclear family living together under one roof
Nordalee
what is genetic engineering
Ebot Reply
good day please what is a cell?
Colota Reply
cell is the basic unit of all living things/organism
Hassan
differenciate b/w cilia and flagellum
Abshira
Cilia: 1. They are smaller in number. 2. Cilia usually occur throughout or major part of the surface of a cell. 3. They beat oar like and in a co-ordinated rhythm 4. Cilia help in locomotion, feeding, circulation, etc.. 5. Example: Paramecium
iti
Flagella: 1. They are larger in size. ADVERTISEMENTS: 2. Flagella are commonly found at one end of the cell. 3. The flagella beat whip-like and independently. 4. Flagella help in locomotion. 5. Example: Euglena
iti
smallest structural and functional unit of an organism!
zidane
cell is the basic unit of all living things
Lukman
A cell is a basic unit of life
Badu
please I want to know the meaning of Tissues
Badu
hellooo here
Ebot
h2
Oliver
meaning of dispesing
Oliver
thnk u iti
Abshira
tissue is a group of identical cells having common origin
iti
what is the word? dispensing or dispersing...check the spelling of the word.
iti
dispersing
Oliver
Smaller in size not in number
Md
They are small in size and more in number
Md
Not identical but similar
Md
 cell is the smallest unit of life. Cellsare often called the "building blocks of life". The study of cells is called cellbiology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.
Harun
hi
Vaishnavi
how are u doing
Emmanuel
this thing I don't know whether is an instruments use to measure pressure Sphygmomanometer is it true or not because in my formal school that's my master told me
Badu
dispersing means distribution or spreading
iti
sphygmomanometer is used for measuring blood pressure
iti
thanks
Oliver
mention the functions of mitochondria?
Oliver
mitochondria is the powerhouse of cell. it generates and stores energy in the form of ATP molecules in its oxysomes. it has its own DNA so its capable of self replicating and hence also called as semi autonomous organelle
iti
welcome
iti
explain briefly anatomy
Oliver
anatomy is thr study of internal structures and their organization as well as functions of living organisms
iti
OK thanky
Oliver
Yes
Hilmi
Good
Moses
welcome
iti
differenciate b/w red blood cell and whight blood cell
BOBO
d
Wani
what is homeostasis
alex Reply
the ability of an organism to maintain stable internal environment, even when the external environment changes
Bee
what's the function of the hypothalamus
Agyekum
The hypothalamus is apart of the lower middle brain that tells the pituitary glands when to release hormones
taehyunggg
what is homeostasis
Edward Reply
constant maintenance of internal body temperature
Agyekum
What is the function of the Pituitary Gland?
WeNdlovu
how can homeostasis be maintained?
alex
pituitary gland also known as the master gland secretes hormones which triggers other gland to secretes their hormones
Agyekum
what's the full meaning of DNA and RNA
Deborah Reply
DNA is Deoxyribonucleic acid and RNA means Ribonucleic acid
Christian

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Source:  OpenStax, Human biology. OpenStax CNX. Dec 01, 2015 Download for free at http://legacy.cnx.org/content/col11903/1.3
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