The substrate is transported through the membrane without changes. The carrier changes its affinity for the substrate during its translocation through the membrane. The affinity is high at the outside of the membrane and low at its inside. Active sugar transport by means of substrate modification. The substrate is chemically modified during the transport.
Examples are the phosphorylation of sugars and the glycosylation of adenine.
The process is also called vectorial phosphorylation. The transport is initiated by a chemical reaction during which the substrate is 'phosphorylated into the cell'.