What is the function of the glycerol phosphate shuttle?

the malate aspartate shuttle is used predominantly in heart and liver, whereas the glycerol is used in skeletal muscle and brain. the m shuttle is bidirectional and makes 3 atps per nadh. the glycerol shuttle is unidirectional and makes less atp. the malate shuttle is mediated by 2 transport proteins whereas the glycerol has none involved.