It is easy enough to assert that active transport across membranes occurs, but much harder to explain how.

Because energy is required for this process, it is known as 'active' transport.

Several models for possible active transport through plasmodesmata exist.

This protein is one of many which is involved in active transport.

This situation is common in active transport and is described thus:

Ascorbic acid is absorbed in the body by both active transport and simple diffusion.

Some plants, however, appear not to load phloem by active transport.

Water as well as salts can also be moved through the skin by active transport.

Therefore, these co-transporters are an example of secondary active transport.

Endocytosis requires energy and is thus a form of active transport.