Answer:
b. Proteins
Step-by-step explanation:
Several transmembrane proteins serve as transport proteins and move charged, polar and large substances such as ions, amino acids, sugars through membranes. These are the carrier proteins and channel proteins. Each type of transport protein transports a specific type of substance. Carrier proteins bind the ion or molecule and undergo changes in shape. This change moves the molecule across the membrane. Channel proteins form pores through the membrane to allow transport of substances.
For example, aquaporins are transmembrane proteins that allow water to pass through membranes.