The mitochondrial shuttles are systems used to transport reducing agents across the inner mitochondrial membrane

Inner mitochondrial membrane

The mitochondrial inner membrane forms internal compartments known as cristae, which allow greater space for the proteins such as cytochromes to function properly and efficiently. The electron transport chain is located on the inner membrane of the mitochondria...

. NADH cannot cross the membrane, but it can reduce another molecule that can cross the membrane, so that its electrons can reach the electron transport chain

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

.

The two main systems in humans are:

Name

In, to mitochondrion

To ETC Electron transport chain An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

Out, to cytosol

>- | Glycerol phosphate shuttle Glycerol phosphate shuttle The glycerol-3-phosphate shuttle is a mechanism that regenerates NAD+ from NADH, a by-product of glycolysis. Its importance in transporting reducing equivalents is secondary to the malate-aspartate shuttle.-Reaction:...

glycerol 3-phosphate Glycerol 3-phosphate Glycerol 3-phosphate is an organophosphate derived from the reaction catalysed by glycerol kinase where ATP + glycerol ADP + sn-glycerol 3-phosphate. It is a component of glycerophospholipids. It should not be confused with the similarly named glycerate 3-phosphate or glyceraldehyde 3-phosphate...

QH2 Coenzyme Q Coenzyme Q10, also known as ubiquinone, ubidecarenone, coenzyme Q, and abbreviated at times to CoQ10 , CoQ, Q10, or Q, is a 1,4-benzoquinone, where Q refers to the quinone chemical group, and 10 refers to the number of isoprenyl chemical subunits in its tail.This oil-soluble, vitamin-like substance...  (~2 ATP)

>- | Malate-aspartate shuttle Malate-aspartate shuttle The malate-aspartate shuttle is a biochemical system for translocating electrons produced during glycolysis across the semipermeable inner membrane of the mitochondrion for oxidative phosphorylation in eukaryotes. These electrons enter the electron transport chain of the mitochondria via reduction...

malate Malate Malate is the ionized form of malic acid. It is an important chemical compound in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle....

NADH Nicotinamide adenine dinucleotide Nicotinamide adenine dinucleotide, abbreviated NAD, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base and the other nicotinamide.In metabolism, NAD is involved...  (~3 ATP)

oxaloacetate/aspartate

In humans, the glycerol phosphate shuttle

Glycerol phosphate shuttle

The glycerol-3-phosphate shuttle is a mechanism that regenerates NAD+ from NADH, a by-product of glycolysis. Its importance in transporting reducing equivalents is secondary to the malate-aspartate shuttle.-Reaction:...

 is primarily found in brown adipose tissue

Brown adipose tissue

Brown adipose tissue or brown fat is one of two types of fat or adipose tissue found in mammals....

, as the conversion is less efficient, thus generating heat, which is one of the main purposes of brown fat. It is primarily found in babies, though it is present in small amounts in adults around the kidneys and on the back of our necks. The malate-aspartate shuttle

Malate-aspartate shuttle

The malate-aspartate shuttle is a biochemical system for translocating electrons produced during glycolysis across the semipermeable inner membrane of the mitochondrion for oxidative phosphorylation in eukaryotes. These electrons enter the electron transport chain of the mitochondria via reduction...

is found in much of the rest of the body.