4 DESCRIBE THE PRESENT PAPER
Introduction — Writing Task 25 1.3.4 Testing the Model Th e next step is to look at the way this model works in a real Introduction. Here are some full-length Introductions from real research articles. Read them through, and mark the model components (1, 2, 3 or 4) wherever you think you see them. For example, if you think the fi rst sentence of the Introduction corresponds to number 1 in our model, write 1 aft er it, etc. Th e height of biomolecules measured with the atomic force microscope depends on electrostatic interactions INTRODUCTION Because the atomic force microscope (AFM) (Binnig et al., 1986) makes it possible to image surfaces in liquids, it has become an important tool for studying biological samples (Drake et al., 1989). Recent reports document the observation of protein assemblies under physiological conditions at nanometer resolution (Butt et al., 1990; Hoh et al., 1991; Karrasch et al., 1993, 1994; Yang et al., 1993, Schabert and Engel, 1994; Mou et al., 1995b; Muller et al., 1995b, 1996b). As demonstrated on solids under vacuum conditions (Sugawara et al., 1995) and in liquid (Ohnesorge and Binnig, 1993), the AFM also makes it possible to measure sample heights with subangstrom accuracy. However, the heights of native biological samples measured with the AFM in aqueous solution vary signifi cantly, and may diff er from values estimated with other methods (Butt et al., 1991; Apell et al., 1993; Muller et al., 1995b, 1996a; Schabert and Rabe, 1996). For example, the height reported for single purple membranes ranges from 5.1 ± 0 nm to 11.0 ± 3.4 nm (see Table 1). Height measurements on actin fi laments (Fritz et al., 1995b), bacteriophage ø29 connectors (Muller et al., 1997c), cholera toxin (Yang et al., 1994; Mou et al., 1995b), DNA (Hansma et al., 1995; Mou et al., 1995a; Wyman et al., 1995), gap junctions (Hoh et al., 1993), GroEL (Mou et al., 1996), hexagonally packed intermediate layer (HPI) (Karrasch et al., 1993; Muller et al., 1996a; Schabert and Rabe, 1996), lipid bilayers (Mou et al., 1994, 1995b; Radler et al., 1994), and microtubules (Fritz et al., 1995a) exhibit a similar variability. Height anomalies