Glucose absorption from intestine occurs by
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 106-107/s.# 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.
Facilitated diffusion
Passive diffusion
Primary active transport
Secondary active transport
G.I.T.