The ABCs of Cellular Membrane Transport

This link and the website of which it is a part contain much useful information about ABC genes; an important family of genes involved in cell membrane transport functions. Quoting from the linked source (in red):

The ATP-binding cassette (ABC) genes represent the largest family of transmembrane (TM) proteins. These proteins bind ATP and use the energy to drive the transport of various molecules across all cell membranes (1–3) (Figure 1). Proteins are classified as ABC transporters based on the sequence and organization of their ATP-binding domain(s), also known as nucleotide-binding folds (NBFs).

The article goes on to explain that the NBFs, which are located on the cytoplasmic side of the cell membrane, enable a transfer of energy needed to transport certain substrates across cellular membranes. ABC proteins can be viewed as molecular pumps which primariliy pump in one direction- that direction being determined by the nature of the cell. For example, in prokaryotic organisms nutrients are pumped from the exterior of a cell to inside of it. This includes sugars, vitamins and metal ions.

There is greater complexity to flow in eukaryotic cells. ABC proteins transport compounds from inside to ouside cells but also move compounds from place to place within cells. In the cytoplasm compounds are transported to within organelles like endoplasmic reticulum or mitochondria.

References:

1. The Human ATP-Binding Cassette (ABC) Transporter Superfamily by Michael Dean; Introduction to ABC Protein and Gene Organization; http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mono_001.section.139