AP Biology
Notes:  Electron Transport

 The pathway of electron transport:

The electron transport chain is made of electron carrier molecules embedded in the inner mitochondrial membrane.

• Each successive carrier in the chain has a higher electronegativity than the carrier before it, so the
  electrons are pulled downhill towards oxygen, the final electron acceptor and the molecule with
  the highest electronegativity.
• Except for ubiquinone (Q), most of the carrier molecules are proteins and are tightly bound to
  prosthetic groups (nonprotein cofactors).
• Prosthetic groups alternate between reduced and oxidized states as they accept and donate electrons.

Heme group = Prosthetic group composed of four organic rings surrounding a single iron atom

Cyytochrome = type of protein molecule that contains a heme prosthetic group and that functions as an
electron carrier in the electron transport chains of mitochondria and chloroplasts.

• There are several cytochromes, each a slightly different protein with a heme group.
• It is the iron of cytochromes that transfers electrons.

As molecular oxygen is reduced it also picks up two protons from the medium to form water. 
For every two NADHs, one O₂ is reduced to two H₂O molecules.

• FADH₂ also donates electrons to the electron transport chain, but those electrons are added
  at a lower energy level than NADH
• The electron transport chain does not make ATP directly.  It generates a proton gradient
  across the inner mitochondrial membrane, which stores potential energy that can be used
  to phosphorylate ADP.
  

  Chemiosmosis:  the Energy-Coupling Mechanism
The mechanism for coupling exergonic electron flow from the oxidation of food to the endergonic
process of oxidativephosphorylation is chemiosmosis.

Chemiosmosis = The coupling of exergonic electron flow down an electron transport chain to
endergonic ATP production by
     the creation o a proton gradient across a membrane.  The proton gradient drives ATP synthesis as protons 
     diffuse back across the membrane.

• Proposed by British biochemist, peter Mitchell (1961)
• The term chemiosmosis emphasizes a coupling between (1) chemical reactions (phosphorylation) and (2) transport process (proton transport)
• Process involved in oxidative phosphorylation and photophosphorylation.

The site of oxidative phosphorylation is the inner mitochondrial membrane, which has many copies of a
protein complex, ATP synthase. 
 This complex:

• Is an enzyme that makes ATP
• Uses an existing proton gradient across the inner mitochondrial membrane to power ATP synthesis

Cristae, or infoldings of the inner mitochondrial membrane, increase the surface area available for chemiosmosis to occur

Membrane structure correlates with the prominent functional role membranes play in chemiosmosis:.

• Using energy from exergonic electron flow, the electron transport chain creates the proton gradient by pumping H⁺s from the mitochondrial matrix, across the innermembrane to the intermembrane space.
• This proton gradient is maintained,, because the membrane's phospholipid bilayer is impermeable to H⁺s and prevents them from leaking back across the membrane by diffusion.
• ATP synthases use the potential energy stored ina proton gradient to make ATP by allowing H⁺ to diffuse down the gradient, back across the membrane.  protons diffuse through the ATP synthase complex, which causes the phosphorylation of ADP

Mobile carriers transfer electrons between complexes.

When the transport chain is operating:

• The pH in the intermembrane space is one or two pH units lower than in the matrix
• The pH in the intermembrane space is the same as the pH of the cytosol because the outer mitochondrial membrane in permeable to protons.

Proton motive force = Potential energy stored in the proton gradient created across biological membranes that are involve in chemiosmosis.

• This force is an electrochemical gradient with two components:
  1. Concentration radient of protons (chemical gradient)
  2.  Voltage across the membrane because of a higher concentration of positively charged protons on one side (electrical gradient)

Cellular respiration s remarkably efficient in the transfer of chemical energy from glucose to ATP.

• Estimated efficiency in eukaryotic cells is about 40%.
• Energy lost in the process is released as heat.