What is the source of the energy for the phosphorylation of ADP into ATP by the mitochondria?

Understanding:

•  Phosphorylation of molecules makes them less stable

   
Adenosine triphosphate (ATP) is a high energy molecule that functions as an immediate power source for cells

• One molecule of ATP contains three covalently bonded phosphate groups – which store potential energy in their bonds
• Phosphorylation makes molecules less stable and hence ATP is a readily reactive molecule that contains high energy bonds
• When ATP is hydrolysed (to form ADP + Pi), the energy stored in the terminal phosphate bond is released for use by the cell

Relationship between ATP and ADP

ATP has two key functions within the cell:

• It functions as the energy currency of the cell by releasing energy when hydrolysed to ADP (powers cell metabolism) 
• It may transfer the released phosphate group to other organic molecules, rendering them less stable and more reactive

ATP is synthesised from ADP using energy derived from one of two sources:

• Solar energy – photosynthesis converts light energy into chemical energy that is stored as ATP
• Oxidative processes – cell respiration breaks down organic molecules to release chemical energy that is stored as ATP

Understanding:

•  Cell respiration involves the oxidation and reduction of electron carriers

 
Cell respiration is the controlled release of energy from organic compounds to produce ATP

• Anaerobic respiration involves the incomplete breakdown of organic molecules for a small yield of ATP (no oxygen required)
• Aerobic respiration involves the complete breakdown of organic molecules for a larger yield of ATP (oxygen is required)

The breakdown of organic molecules occurs via a number of linked processes that involve a number of discrete steps

• By staggering the breakdown, the energy requirements are reduced (activation energy can be divided across several steps)
• The released energy is not lost – it is transferred to activated carrier molecules via redox reactions (oxidation / reduction)

Energy Conversions in Sugar Breakdown (Direct Combustion vs Cell Respiration)

Redox Reactions

When organic molecules are broken down by cell respiration, the chemical energy is transferred by means of redox reactions

• Redox reactions involved the reduction of one chemical species and the oxidation of another (redox = reduction / oxidation)

Most redox reactions typically involve the transfer of electrons, hydrogen or oxygen 

• Reduction is the gain of electrons / hydrogen or the loss of oxygen
• Oxidation is the loss of electrons / hydrogen or the gain of oxygen

Redox reactions can be summarised according to the following table:

Redox Mnemonics

Redox reactions involving electrons can be remembered using any of the following mnemonics:

• OIL RIG  –  Oxidation Is Loss (of electrons) ; Reduction Is Gain (of electrons)
• LEO goes GER  –  Loss of Electrons is Oxidation ; Gain of Electrons is Reduction
• ELMO  –  Electron Loss Means Oxidation

Understanding:

•  Energy released by oxidation reactions is carried to the cristae of the mitochondria by reduced NAD and FAD

  
Cell respiration breaks down organic molecules and transfers hydrogen atoms and electrons to carrier molecules

• As the organic molecule is losing hydrogen atoms and electrons, this is an oxidation reaction
• Energy stored in the organic molecule is transferred with the protons and electrons to the carrier molecules

The carrier molecules are called hydrogen carriers or electron carriers, as they gain electrons and protons (H+ ions)

• The most common hydrogen carrier is NAD+ which is reduced to form NADH  (NAD+ + 2H+ + 2e–  →  NADH + H+)
• A less common hydrogen carrier is FAD which is reduced to form FADH2  (FAD + 2H+ + 2e–  →  FADH2)

The hydrogen carriers function like taxis, transporting the electrons (and hydrogen ions) to the cristae of the mitochondria

• The cristae is the site of the electron transport chain, which uses the energy transferred by the carriers to synthesize ATP
• This process requires oxygen to function, and hence only aerobic respiration can generate ATP from hydrogen carriers
• This is why aerobic respiration unlocks more of the energy stored in the organic molecules and produces more ATP

Energy Transfer via Hydrogen Carriers

⇒  Click on the diagram to display taxi representation

Mitochondria are unusual organelles. They act as the power plants of the cell, are surrounded by two membranes, and have their own genome. They also divide independently of the cell in which they reside, meaning mitochondrial replication is not coupled to cell division. Some of these features are holdovers from the ancient ancestors of mitochondria, which were likely free-living prokaryotes.

What Is the Origin of Mitochondria?

Mitochondria are thought to have originated from an ancient symbiosis that resulted when a nucleated cell engulfed an aerobic prokaryote. The engulfed cell came to rely on the protective environment of the host cell, and, conversely, the host cell came to rely on the engulfed prokaryote for energy production. Over time, the descendants of the engulfed prokaryote developed into mitochondria, and the work of these organelles — using oxygen to create energy — became critical to eukaryotic evolution (Figure 1).

Modern mitochondria have striking similarities to some modern prokaryotes, even though they have diverged significantly since the ancient symbiotic event. For example, the inner mitochondrial membrane contains electron transport proteins like the plasma membrane of prokaryotes, and mitochondria also have their own prokaryote-like circular genome. One difference is that these organelles are thought to have lost most of the genes once carried by their prokaryotic ancestor. Although present-day mitochondria do synthesize a few of their own proteins, the vast majority of the proteins they require are now encoded in the nuclear genome.

What Is the Purpose of a Mitochondrial Membranes?

As previously mentioned, mitochondria contain two major membranes. The outer mitochondrial membrane fully surrounds the inner membrane, with a small intermembrane space in between. The outer membrane has many protein-based pores that are big enough to allow the passage of ions and molecules as large as a small protein. In contrast, the inner membrane has much more restricted permeability, much like the plasma membrane of a cell. The inner membrane is also loaded with proteins involved in electron transport and ATP synthesis. This membrane surrounds the mitochondrial matrix, where the citric acid cycle produces the electrons that travel from one protein complex to the next in the inner membrane. At the end of this electron transport chain, the final electron acceptor is oxygen, and this ultimately forms water (H20). At the same time, the electron transport chain produces ATP. (This is why the the process is called oxidative phosphorylation.)

During electron transport, the participating protein complexes push protons from the matrix out to the intermembrane space. This creates a concentration gradient of protons that another protein complex, called ATP synthase, uses to power synthesis of the energy carrier molecule ATP (Figure 2).

Is the Mitochondrial Genome Still Functional?

Mitochondrial genomes are very small and show a great deal of variation as a result of divergent evolution. Mitochondrial genes that have been conserved across evolution include rRNA genes, tRNA genes, and a small number of genes that encode proteins involved in electron transport and ATP synthesis. The mitochondrial genome retains similarity to its prokaryotic ancestor, as does some of the machinery mitochondria use to synthesize proteins. In fact, mitochondrial rRNAs more closely resemble bacterial rRNAs than the eukaryotic rRNAs found in cell cytoplasm. In addition, some of the codons that mitochondria use to specify amino acids differ from the standard eukaryotic codons.

Still, the vast majority of mitochondrial proteins are synthesized from nuclear genes and transported into the mitochondria. These include the enzymes required for the citric acid cycle, the proteins involved in DNA replication and transcription, and ribosomal proteins. The protein complexes of the respiratory chain are a mixture of proteins encoded by mitochondrial genes and proteins encoded by nuclear genes. Proteins in both the outer and inner mitochondrial membranes help transport newly synthesized, unfolded proteins from the cytoplasm into the matrix, where folding ensues (Figure 3).

How Many Mitochondria Do Cells Have?

Mitochondria cannot be made "from scratch" because they need both mitochondrial and nuclear gene products. These organelles replicate by dividing in two, using a process similar to the simple, asexual form of cell division employed by bacteria. Video microscopy shows that mitochondria are incredibly dynamic. They are constantly dividing, fusing, and changing shape. Indeed, a single mitochondrion may contain multiple copies of its genome at any given time.

Conclusion

Mitochondria, the so-called "powerhouses" of cells, are unusual organelles in that they are surrounded by a double membrane and retain their own small genome. They also divide independently of the cell cycle by simple fission. Mitochondrial division is stimulated by energy demand, so cells with an increased need for energy contain greater numbers of these organelles than cells with lower energy needs.