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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.


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

state any one specialized animal cell and it's function
Pascal Reply
sperm cell: Fertilization
Can DNA and RNA be praticalized
Oyewale Reply
you can make models of them and show how they replicate
what is dentition
Tisa Reply
Study of teeths and their functions
what are the function
it's not teeths. teeth is plural as well as singular noun. the function of the teeth is to cut and grind food for easier swallowing and for increasing the surface area if the fish as the substrate in which digestive enzymes work
dentition is not the study of teeth but the arrangement of teeth in the mouth of species or individuals
what are the two types of teeth
Function of probosis
Adeola Reply
why do we study biology?
where biology began
What are chromesome
Talkmore Reply
condensed DNA wrapped with histone
what is chromosome?
Royd Reply
How can a person who has been in the vascular disease repairs his or her DNA
Rabson Reply
i need its explanation sir..pls provid na...🙁
Shaf Reply
Of wat
explanation for wat
Of war na so dat I can provide
Is d process which food is converted into substance dat can be utilize by d body
the meaning of disgestion
Charity Reply
because they are considered to be proteins ..lipids,fats, and nucleic acids
Geeta Reply
what is osmosis
Festus Reply
Is the movement of solvent molecules from the region of a higher concentration to the region of lower concentration
It is the movement of solvent from the region of lower concentration to the region of higher concentration through a semi-permeable membrane.
and diffusion
it is the movement of water molecules fro region of higher conc. to the region of lower conc.thro a semi-permeable membrane.
Diffusion is the movement of solute molecules from a region of higher concentration to a region of lower concentration.
osmosis is the movement of water molecule from the region higher water potential to a region of lower water potential
what is the function of Nerve cell?
Hello Everyone!! Any good study app recommendations?
For which subject
Hello everyone!! I want to be sure does exoskeleton consist of endoskeleton? plz help right nw
Try this one and also biology Olevels
Hey, I'm completely new to biology. I'm computer science graduate but I want to learn Neuroscience from scratch. So I have decided to study this book as my entry point to the field. is this book good for a complete beginner in the field?
Adeetya Reply
Yes it can help since u ar a beginner
Thanks for the clarification.
WhAt are the differences between prokaryotic cell and eukaryotic cell
oluwapelumi Reply
Prokaryotic cell is a single celled organism that do not have membrane -bound nuclei.
prokaryotes also lack other cell organelles whereas eukaryotes possess an organized nucleus with nuclear envelope and also they have complex locomotory and cytoskeletal structures
genetic material is found in cytoplasm in prokaryotic cell while in eukaryotes genetic material is found in nucleus
What cells are the basic unit of life?

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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