consisting of the charged heads, the inner surface of uncharged tails. Cell membranes are composed of a lipid bilayer, two sheets of lipid molecules sandwiched together.
Figure 2.1 Lipid bilayer spherical shell (cross-section) with a protein pore (hemolysin) on right showing how molecules can cross the otherwise impermeable bilayer.
Collectively, these three cellular elementsânucleic acids, proteins, and the lipid bilayer membraneâexist for the purpose of maintaining the cellas a living system, but for this it needs a fourth class of materials, sugars (saccharides, typically glucose or sucrose). Sugars are the material sources of a cellâs energy. But in order to be actually utilized as energy, they must first be converted into a biochemically accessible form. This conversion occurs in a process called glycolysis (âsugar breakingâ), a complex, ten-step progression of events that is the bane of biochemistry students (although other metabolic pathways take even more twists and turns). The final result of glycolysis is the creation of ATP (adenosine triphosphate), the molecules that directly power the cell. Those same ATP molecules power our bodily cells.
To be a cell, then, is to be a deterministic system governed by DNA, composed largely of proteins and lipids, and energized by ATP. Some cells aremore like us than you may imagine â E. coli , for example, the standard organism of genetic engineering.
This bacterium was discovered in 1885 by the German pediatrician Theodor Escherich, after whom it was named. It has since become the most biochemically well-defined organism known to science. Like ourselves, it is mobile and self-propelled, although the medium through which it travels and its propulsion system are quite different from our own. Individual E. coli cells are small, rod-shaped objects about four micrometers in length, easily visible in a light microscope. Extending from the cellâs surface are a number of long, corkscrew-shaped flagella. They propel the cell through a watery medium that, to them, is as viscous as molasses. At this scale, where gravity has little effect, there is no up or down. Since the average E. coli cell lives inside the human intestinal tract, the cell has no vision, and since it has no brain or nervous system, it has no conscious experience. But believe it or not, the bacterium has a primitive sense of perception.
An E. coli cell can be observed swimming through its aqueous medium in a straight line, propelled by its flagella motors. Inside the cell, floating in the cytoplasm, certain kinds of protein molecules react to the density of nutrients in the surrounding medium. If the nutrient levels remain the same or are increasing, the bacterium will continue straight ahead in its travels. But if the nutrient level declines, these same proteins will react with the flagellar motor in such a way that causes the organism to reverse direction. It will then flail about and wander all over, as if exploring the universe, until the nutrient gradient increases, whereupon it will again swim in the direction of the nutrient. In its simple and rudimentary way, this is how the bacterium senses its surroundings.
There is, finally, one other commonality between ourselves and E. coli , and that is the ability to reproduce. An E. coli cellâs capacity for reproduction is unmatched in speed by few other organisms. The basic processes take place in the cytoplasm, which has no precise analog on the human scale. Itâs a scene of apparent pandemonium where every possible space is filled, all molecules are in a state of constant thermal agitation, and objects of multiple shapes and sizes are bumping into each other, fusing together, breaking apart, and popping into and out of existence seemingly at random.
Biochemist David Goodsell, in his book The Machinery of Life , likens the conditions inside the cytoplasm to an airport terminal crowded with passengers who are pushing,
Figure 2.1 Lipid bilayer spherical shell (cross-section) with a protein pore (hemolysin) on right showing how molecules can cross the otherwise impermeable bilayer.
Collectively, these three cellular elementsânucleic acids, proteins, and the lipid bilayer membraneâexist for the purpose of maintaining the cellas a living system, but for this it needs a fourth class of materials, sugars (saccharides, typically glucose or sucrose). Sugars are the material sources of a cellâs energy. But in order to be actually utilized as energy, they must first be converted into a biochemically accessible form. This conversion occurs in a process called glycolysis (âsugar breakingâ), a complex, ten-step progression of events that is the bane of biochemistry students (although other metabolic pathways take even more twists and turns). The final result of glycolysis is the creation of ATP (adenosine triphosphate), the molecules that directly power the cell. Those same ATP molecules power our bodily cells.
To be a cell, then, is to be a deterministic system governed by DNA, composed largely of proteins and lipids, and energized by ATP. Some cells aremore like us than you may imagine â E. coli , for example, the standard organism of genetic engineering.
This bacterium was discovered in 1885 by the German pediatrician Theodor Escherich, after whom it was named. It has since become the most biochemically well-defined organism known to science. Like ourselves, it is mobile and self-propelled, although the medium through which it travels and its propulsion system are quite different from our own. Individual E. coli cells are small, rod-shaped objects about four micrometers in length, easily visible in a light microscope. Extending from the cellâs surface are a number of long, corkscrew-shaped flagella. They propel the cell through a watery medium that, to them, is as viscous as molasses. At this scale, where gravity has little effect, there is no up or down. Since the average E. coli cell lives inside the human intestinal tract, the cell has no vision, and since it has no brain or nervous system, it has no conscious experience. But believe it or not, the bacterium has a primitive sense of perception.
An E. coli cell can be observed swimming through its aqueous medium in a straight line, propelled by its flagella motors. Inside the cell, floating in the cytoplasm, certain kinds of protein molecules react to the density of nutrients in the surrounding medium. If the nutrient levels remain the same or are increasing, the bacterium will continue straight ahead in its travels. But if the nutrient level declines, these same proteins will react with the flagellar motor in such a way that causes the organism to reverse direction. It will then flail about and wander all over, as if exploring the universe, until the nutrient gradient increases, whereupon it will again swim in the direction of the nutrient. In its simple and rudimentary way, this is how the bacterium senses its surroundings.
There is, finally, one other commonality between ourselves and E. coli , and that is the ability to reproduce. An E. coli cellâs capacity for reproduction is unmatched in speed by few other organisms. The basic processes take place in the cytoplasm, which has no precise analog on the human scale. Itâs a scene of apparent pandemonium where every possible space is filled, all molecules are in a state of constant thermal agitation, and objects of multiple shapes and sizes are bumping into each other, fusing together, breaking apart, and popping into and out of existence seemingly at random.
Biochemist David Goodsell, in his book The Machinery of Life , likens the conditions inside the cytoplasm to an airport terminal crowded with passengers who are pushing,
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