PERMEABILITY OF CYTOPLASMIC MEMBRANES 119 



can also be demonstrated by the manipulation of living protoplasm with fine 

 glass needles (Chambers and Hofler, 1931). It has been found possible to 

 withdraw a plasmolyzed protoplast (Chap, XI) from its cell wall and to 

 strip off the protoplasm from the vacuole leaving the latter as a free-floating 

 sac full of cell sap enclosed by a delicate membrane — the tonoplast (Fig. 25). 

 Most evidence indicates that the 

 tonoplast and probably also the 

 plasmalemma are thin liquid films 

 which are immiscible with water. 



A complete picture of the ^„ ,^, ^, ^ ' TZI^T^ T^ 



, _ ^ ^ . NUCLEUS CYTOPLASM VACUOLE 



permeability of a cell to different 



, , , , . Fig. 25. Removal of the outer cytoplasm 



compounds can be drawn only m r^„^ <.!,<.*„„ 1 .. j i r 1 



' ■' irom the tonoplast and vacuole of a plasmo- 



terms of the permeability of both lyzed cell. Redrawn from Seifriz (1928). 

 the cell wall and the cytoplasmic 



membranes. In entering a cell a substance must first pass through the various 

 layers of the cell wall, thence in turn through the plasmalemma, the interior 

 cytoplasm, and the tonoplast before reaching the vacuole. Although some 

 investigators prefer to refer all permeability phenomena of plant cells to the 

 cytoplasm as a whole, the evidence that the plasmalemma and the tonoplast 

 exist as distinct membranes with the property of differential permeability must 

 be regarded as very substantial. 



It is also customary to speak of certain plant structures composed of one 

 or more layers of cells as membranes. Examples are the seed coats of many 

 seeds and fruits (especially grains), epidermal layers such as those peeled 

 from the scales of an onion bulb, the "skin" of potatoes, etc. Such mem- 

 branes may be termed for convenience 7nulticellular membranes. They may 

 be either living, as for example onion epidermis, or non-living, as for example 

 many seed coats. 



The Permeability of the Cytoplasmic Membranes. — The cytoplasmic 

 membranes are differentially permeable membranes par excellence. The term 

 "cytoplasmic membranes" will be employed in a loose sense to refer to the 

 entire cytoplasmic system of membranes : plasmalemma plus interior cyto- 

 plasm plus tonoplast. Actually many substances entering plant cells are inter- 

 cepted and utilized in one way or another in the cytoplasm, and never diffuse 

 through the tonoplast. Similarly com.pounds synthesized in the cytoplasm 

 may pass out of the cell without crossing the tonoplast. Hence the plasma- 

 lemma is often regarded as the most important unit in the cytoplasmic sj'Stem 

 of membranes. 



A pertinent concept as a background for a discussion of the permeability 

 of the cytoplasmic membranes is the distinction between "polar" and "non- 



