Items 13-15

The following set of items pertain to membrane transport phenomena.

13. All of the following substances move across the plasma membrane by simple diffusion EXCEPT:

(A) testosterone
(B) estrogen
(C) aldosterone
(D) Na+
(E) palmitic acid

14. All of the following substances move across the plasma membrane by ediated transport EXCEPT:

(A) testosterone
(B) glucose
(C) aspartate
(D) glutamate
(E) galactose

15. Which statement best characterizes the difference between active mediated transport (AMT) and passive mediated transport (PMT)?

(A) Only PMT requires energy input.
(B) Only AMT can occur against a concentration gradient.
(C) Only AMT shows saturation kinetics.
(D) Only PMT shows specificity for solute transport.
(E) Only AMT can be inhibited.

ANSWERS AND TUTORIAL ON ITEMS 13-15

The answers are: 13-D; 14-A; 15-B. The plasma membrane is a selective barrier between the inside and the outside of individual cells. Solutes can across the membrane by diffusion or by mediated transport. Diffusion is relatively slow for molecules such as glucose because of their low lipid solubility. In contrast, steroids and fatty acids have relatively high lipid solubility and are able to cross the plasma membrane by diffusion.

For simple diffusion, there is a linear relationship between rate of transport and the ratio of external concentration of solute/internal concentration of solute, i.e., the higher the ratio, the greater the rate of transport. Mediated transport shows saturation kinetics: as solute concentration increases, the rate will reach a maximal velocity when the carrier for mediated transport becomes saturated with solute. Ions such as Na+, K+, Ca²- (cations), HPO 4²-, CI- and HCO3- (anions) as well as amino acids sugars are transported by mediated transport professional-high-power microscope.

Mediated transport is facilitated by transporter membrane proteins (translocases) and can be either passive o active.Both passive mediated transport and active mediated transport show saturation kinetics, show specificity fro solute transport and can be inhibited.

In passive mediated transports, solute moves from a high external concentration to a low internal concentration without the expenditure of energy. In active mediate transport, solute can move against a concentration gradient because energy (the energy of hydrolysis of ATP) is generated by the Na+-K+ ATPase to which the transport is coupled. Video Eyepiece Cameras Video Zoom Inspection microscopes.