Chapter 7, Problem 7.1PS

Functions of Membranes. For each of the following statements, specify which one of the five general membrane functions (permeability barrier, localization of function, regulation of transport, detection of signals, or intercellular communication) the statement illustrates.

(a) When cells are disrupted and fractionated into subcellular components, the enzyme cytochrome P-450 is recovered with the endoplasmic reticulum fraction.

(b) On their outer surface, cells of multicellular organisms carry specific glycoproteins that are responsible for cell-cell adhesion.

(c) The interior of a membrane consists primarily of the hydrophobic portions of phospholipids and amphipathic proteins.

(d) Photosystems I and II are embedded in the thylakoid membrane of the chloroplast.

(e) All of the acid phosphatase in a mammalian cell is found within the lysosomes.

(f) The membrane of a plant root cell has an ion pump that exchanges phosphate inward for bicarbonate outward.

(g) Ions and large polar molecules cannot cross the membrane without the aid of a transport protein.

(h) Insulin does not enter a target cell but instead binds to a specific membrane receptor on the external surface of the membrane, thereby activating the enzyme adenylyl cyclase on the inner membrane surface.

(i) Adjacent plant cells frequently exchange cytoplasmic components through membrane-lined channels called plasmodesmata.