<prompt> Glucose absorption from intestine occurs byFacilitated diffusionPassive diffusionPrimary active transportSecondary active transport  <response>: Ans. d Secondary active transport. Ref. Guyton, Physiology, 10th ed., 773 In GI membrane and in renal tubules, glucose is transported by the mechanism of active sodium-glucose co-transport while at all other tissue cells glucose is transported by facilitated diffusion. Glucose as such cannot diffuse through the pores of cell membrane, as max molecular wt. of glucose is 180.TRANSFER OF MATERIAL AND INFORMATION ACROSS MEMBRANES Cross-membrane movement of small molecules- Diffusion Passive AND Facilitated- Active transport Primary active transport AND Secondary active transport Cross-membrane movement of large molecules- Endocytosis Fluid-phase endocytosis random and nondirected Receptor-mediated endocytosis selective and occurs in coated pits lined with clathrin- Exocytosis Signal transmission across membranes- Cell surface receptors Signal transduction eg, glucagon -cAMP Signal internalization coupled with endocytosis, eg, the LDL receptor- Movement to intracellular receptors steroid hormones; a form of diffusion Intercellular contact and communicationSome properties of ion channels They are composed of transmembrane protein subunits. Most are highly selective for one ion; a few are nonselective. They allow impermeable ions to cross membranes at rates approaching diffusion limits. They can permit ion fluxes of 106-107s. Their activities are regulated. The two main types are voltage-gated and ligand-gated. They are usually highly conserved across species. Most cells have a variety of Na, K, Ca2, and CI- channels. Mutations in genes encoding them can cause specific diseases. Their activities are affected by certain drugs. 