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4 Low-density gases obey a universal law The founders of chemistry arrived at the idea that atoms combine in definite proportions by noticing that gases combine in simple, fixed ratios of volume. Eventually it became clear that this observation reflects the fact that the number of gas molecules in a box at atmospheric pressure is just proportional to its volume. More precisely, one finds experimentally that the pressure p, volume V , number of molecules N , and temperature T of any gas (at low enough density) are related in a simple way called the ideal gas law: pV = N kB T.

Digital image kindly supplied by J. ] obtain high-quality energy by capturing sunlight. 6). 3). The part of the cytoplasm not contained in any membrane-bounded organelle is collectively called the cell’s cytosol. In addition, cells create a variety of vesicles (small bags). Vesicles can form by “endocytosis,” as a part of the cell’s outer membrane engulfs some exterior object or fluid, then pinches off to form an internal compartment. The resulting vesicle then fuses with internal vesicles containing digestive enzymes, which break down its contents.

Cytol. ) Gross anatomy of a white blood cell. A single plasma membrane surrounds the cell. Other organelles are visible, most of them small compartments defined by membranes. 9 and discussed in Chapter 12. Chapters 10 and 11 will discuss the osmotic regulation mechanism that keeps the cell’s interior full of fluid. ] internal energy-carrying molecule ATP. Instead of metabolizing food, however, they 38 Chapter 2. ) Fusion of synaptic vesicles with the nerve cell membrane (upper solid line) at the junction, or synapse, between a neuron (above) and a muscle fiber (below).