The Self-Made Tapestry Phần 2 pptx

Soap bubbles and foams do not last for ever, and I suppose that is part of their appeal: fragile beauty,

gone in a moment. The collapse of foams is brought about partly by the drainage of the films, under the

influence of gravity and capillary forces, until they become too thin to resist the slightest disturbancea

vibration or a breath of air. But in their passing, soap films can treat us to a wonderful display. Held

vertically on a wire frame, a thinning soap film becomes striated with bands of rainbow colours that

pass from top to bottom (Plate 2). Finally the top becomes silvery and then black; and the blackness,

like a premonition of the film's demise,

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moves over the entire surface. Once it is black, the film is doomed to burst at the merest perturbation.

Fig. 2.17

Cell membranes are made from double layers of surfactants called

phospholipids. These bilayers are studded with other membrane

components, such as protein molecules, and are sometimes strengthened with

a protein web called the cytoskeleton.

These colours are the result of interference between light reflected from the front and the back of the

film. Interference takes place when the distance between the front and back becomes comparable to the

wavelength of light (a few hundred millionths of a millimetre), and as this distance changes, so too does

the wavelength (that is, the colour) of the light affected by interference. When the film is black, all

reflected visible light cancels itself out by interference. The film is by that stage only about four-and-a￾half millionths of a millimetre thickabout the same thickness as a double layer of soap molecules. The

two films at the surfaces have almost met back to back.

This back-to-back arrangement of surfactant molecules has some similarities to the wall of a living cell.

Cell membranes (Fig. 2.17) are composed of amphiphilic moleculesbiological surfactants, if you

likecalled phospholipids, or just lipids. A double layer of lipids, called a bilayer, is one of the

fundamental architectural features of living organisms, providing the housing in which nature's

chemistry takes place. Lipid bilayers also divide up cells of multicelled organisms (like ourselves) into

several compartments, each of which acts as the location for specific biological processes. One critical

difference between a black soap film and a lipid bilayer, however, is that in the former the surfactants

meet head to head and in the latter they meet tail to tail. Thus lipid bilayers present a horde of water￾soluble head groups at their surface, and the water-insoluble tails are buried within, where they are

shielded from water. In a loose sense, cell membranes can be considered to be microscopic, inside-out

bubbles, afloat in a watery sea. Of course, real cells are anything but 'hollow'their insides are filled with

biological hardware, including the DNA that allows the cells to generate copies of themselves. But in

the 1960s, researchers at Cambridge University found that phospholipids would come together

spontaneously in solution to form empty cell-like structures called vesicles, when the solution was

jiggled by sound waves. This self-assembly of vesicles is driven by the tendency of lipids to form

bilayers_in order to bury their insoluble tails.

A lot of work has been devoted to studying the shapes that lipid bilayer vesicles can adopt, because

these can provide clues about the factors that control the shapes of real cells. The range of shapes is far

more varied and interesting than those of soap bubbles: vesicles can be spherical, but they can also take

on other stable shapes too. In the broadest sense, these shapes are determined by the same driving force

that dictates the shapes of soap films: the tendency to minimize the total (free) energy. The principal

contribution to the energy of a soap film comes from the surface tension, so the film adopts a shape that

minimizes this by finding the smallest surface area. But for a vesicle, the surface area is essentially

fixed: once a vesicle is formed, the number of surfactant molecules in its wall stays pretty much the

same, and each molecule occupies a fixed area on the bilayer surface. This means that another factor is

able to exercise a dominant influence on the energy: the surface curvature. The way that shape affects

the curvature energy is rather subtle, and it may turn out that the lowest-energy shape is not that with

constant mean curvaturea spherebut some other, more complex shape. This balance can be shifted by

changing the nature of the vesicle's environmentfor example, by warming it upand so the vesicle may

undergo changes in shape as the temperature is changed.

The German biophysicist Erich Sackmann and co-workers have shown that under certain conditions,

the most stable shape of a vesicle is that of a disk with dim-

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ples in the top and bottom (Fig. 2.18a), which is precisely the shape that a red blood cell adopts. They

saw these vesicles change shape to become bowl-like entities as the temperature was increased (Fig.

2.18a), and were able to show theoretically that these shape changes are to be expected because of the

changing balance in energies. The bowl-like shape, called a stomatocyte, may eventually curl up on

itself to generate a small, spherical vesicle inside a larger one, connected via a narrow neck which

eventually became pinched off. Under different conditions, a vesicle can become elongated from an

egglike shape into a pear shape, ultimately pinching off a little bud at the thin end (Fig. 2.18b). Both of

these processesthe budding and expulsion of a small vesicle from the outside of a cell membrane and

the engulfing and budding off of a small interior vesiclehappen in real cells, where they are called

exocytosis and endocytosis. The former process allows cells or interior sub-compartments of cells

called organelles to send out little chemical messagesa package of protein molecules, perhapsin soft

wrappers, while the latter enables a cell to ingest material. In cells these processes are controlled by

protein molecules embedded in the cell membranes, but we can see that they can also come about

through nothing more than the 'blind' physical forces that determine a membrane's geometry.

Fig. 2.18

Vesicles are closed, cell-like bilayer membranes.

They adopt a range of different shapes at different temperatures,

which are determined by the subtle influences of elastic and

curvature energy. In (a) a flattened vesicle with a shape like

a red blood cell develops a concavity which becomes a

separate internal vesicle. In (b) an elongated vesicle

develops a bud, which eventually separates from the main

body. Both of these sequences, seen experimentally under

a microscope (top frames), can be reproduced by calculations

of the equilibrium shape that minimizes the total

energy (lower frames). (Images: Reinhard Lipowsky, Max

Planck Institute for Colloid Science, Teltow-Seehof, Germany.)

Fig. 2.19

Starfish vesicles (a), and the corresponding shapes

calculated with an energy-minimization model (b).

(Images: Udo Seifert, Max Planck Institute for Colloid Science,

Teltow-Seehof, Germany.)

Udo Seifert and co-workers at the Max-Planck Institute for Colloid Science in Teltow-Seehof,

Germany, have found that under some conditions the driving force to minimize curvature energy can

push vesicles through extremely bizarre contortions. Under the microscope they saw multi-armed

vesicles that looked like starfish or ink blots (Fig. 2.19a). If these were living amoeba dragging

themselves around by extending pseudopodia, we might not consider the shapes surprising; but they are

merely empty sacs whose shapes are the product of a mathematically well-defined minimization

principle! Seifert and colleagues showed that they could reproduce the shapes theoretically by

minimizing the curvature energy of the bilayers subject to the constraints of fixed surface area and

enclosed volume (Fig. 2.19b). This 'mathematics of blobs' appears to hold some symmetry principles:

the researchers could