<< Chapter < Page Chapter >> Page >

Q and complex ii

Complex II directly receives FADH 2 , which does not pass through complex I. The compound connecting the first and second complexes to the third is ubiquinone    (Q). The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced, (QH 2 ), ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from complex I and the electrons derived from FADH 2 from complex II, including succinate dehydrogenase. This enzyme and FADH 2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH 2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.

Complex iii

The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe ++ (reduced) and Fe +++ (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time).

Complex iv

The fourth complex is composed of cytochrome proteins c, a, and a 3 . This complex contains two heme groups (one in each of the two cytochromes, a, and a 3 ) and three copper ions (a pair of Cu A and one Cu B in cytochrome a 3 ). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water (H 2 O). The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis.

Chemiosmosis

In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the membrane. The uneven distribution of H + ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane.

If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient. Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase ( [link] ). This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.

Questions & Answers

what are products of the light reactions of photosynthesis that are utilised in the Calvin cycle?
Nellie Reply
light stage reaction of photosynthesis is represented in the chemical equation ; 4H+ >>>>>> ²H20 + O2 (Hydroxyl). (water) (oxygen)
Emmanuel
a man with blood group A married a woman with blood group B,their child has blood group O,what are the genotype of the parents
Adesewa Reply
your question is not clear
Emmanuel
Ai woman Bi man
INGIPHILE
Is this a science student asking question
Sando
how are living organisms grouped?
Jimmy Reply
what are the basic concepts of biology
Dolapo Reply
what features differentiate metaphase and anaphase stage
Paul Reply
please I need full biology textbook
Ugwuanyi Reply
what are the vital component of biochemistry
Isaiah Reply
what are the vital components of biochemistry
Isaiah
what is the importance of cells
Samuel Reply
epiphytes grow on branches of large trees and competing for light in the canopy by climbing up those large trees while using them for support at same time. what type of ecological interaction is seen here?
Kenneth Reply
how can we practically eliminate or reduce climate change?
Thomas Reply
Fine question
Sando
According to science hepatitis is a disease cause by hepatitis virus which I know, and also do not have a fixed definition or treatment, can the discover better explain
Sando
hello
eunice
Welcome
Sando
by modernizing man's activity
Jimmy
what will the under secretion of somatotropin result
Jovita Reply
The under secretion of Somatotropin growth hormone results in DWARFISM, a condition of retarded growth....
Emmanuel
what are nutrients needed by plants
Mabeni Reply
calcium, phosphorus, potassium, nitrogen ,magnesium
Shukurat
why is the cell made up of celullose wall
Mungudit Reply
what are prokaryotic and eukàryotic
Desire Reply
what is ribose
Quadri Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask