H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003), страница 4

Each embryo will be genetically identical to theadult from which the nucleus was obtained. Low percentages ofembryos survive these procedures to give healthy animals, andthe full impact of the techniques on the animals is not yet known.[Geoff Tompkinson/Science Photo Library/Photo Researchers, Inc.]merous diseases in which particular cell types are damagedor missing, and of repairing wounds more completely.1.2 The Molecules of a CellMolecular cell biologists explore how all the remarkableproperties of the cell arise from underlying molecular events:the assembly of large molecules, binding of large moleculesto each other, catalytic effects that promote particular chemical reactions, and the deployment of information carried bygiant molecules.

Here we review the most important kinds ofmolecules that form the chemical foundations of cell structure and function.Small Molecules Carry Energy, Transmit Signals,and Are Linked into MacromoleculesMuch of the cell’s contents is a watery soup flavored withsmall molecules (e.g., simple sugars, amino acids, vitamins)and ions (e.g., sodium, chloride, calcium ions). The locationsand concentrations of small molecules and ions within thecell are controlled by numerous proteins inserted in cellularmembranes. These pumps, transporters, and ion channelsmove nearly all small molecules and ions into or out of thecell and its organelles (Chapter 7).1.2 • The Molecules of a CellOne of the best-known small molecules is adenosinetriphosphate (ATP), which stores readily available chemicalenergy in two of its chemical bonds (see Figure 2-24).

Whencells split apart these energy-rich bonds in ATP, the releasedenergy can be harnessed to power an energy-requiringprocess like muscle contraction or protein biosynthesis. Toobtain energy for making ATP, cells break down food molecules. For instance, when sugar is degraded to carbon dioxide and water, the energy stored in the original chemicalbonds is released and much of it can be “captured” in ATP(Chapter 8). Bacterial, plant, and animal cells can all makeATP by this process. In addition, plants and a few other organisms can harvest energy from sunlight to form ATP inphotosynthesis.Other small molecules act as signals both within and between cells; such signals direct numerous cellular activities(Chapters 13–15).

The powerful effect on our bodies of afrightening event comes from the instantaneous flooding ofthe body with epinephrine, a small-molecule hormone thatmobilizes the “fight or flight” response. The movementsneeded to fight or flee are triggered by nerve impulses thatflow from the brain to our muscles with the aid of neurotransmitters, another type of small-molecule signal that wediscuss in Chapter 7.Certain small molecules (monomers) in the cellular soupcan be joined to form polymers through repetition of a singletype of chemical-linkage reaction (see Figure 2-11).

Cellsproduce three types of large polymers, commonly calledmacromolecules: polysaccharides, proteins, and nucleicacids. Sugars, for example, are the monomers used to formpolysaccharides. These macromolecules are critical structuralcomponents of plant cell walls and insect skeletons. A typicalpolysaccharide is a linear or branched chain of repeatingidentical sugar units.

Such a chain carries information: thenumber of units. However if the units are not identical, thenthe order and type of units carry additional information. Aswe see in Chapter 6, some polysaccharides exhibit the greaterinformational complexity associated with a linear code madeup of different units assembled in a particular order.

Thisproperty, however, is most typical of the two other types ofbiological macromolecules—proteins and nucleic acids.Proteins Give Cells Structure and Perform MostCellular TasksThe varied, intricate structures of proteins enable them tocarry out numerous functions. Cells string together 20 different amino acids in a linear chain to form a protein (seeFigure 2-13). Proteins commonly range in length from 100 to1000 amino acids, but some are much shorter and otherslonger. We obtain amino acids either by synthesizing themfrom other molecules or by breaking down proteins that weeat. The “essential” amino acids, from a dietary standpoint,are the eight that we cannot synthesize and must obtain fromfood. Beans and corn together have all eight, making theircombination particularly nutritious.

Once a chain of aminoacids is formed, it folds into a complex shape, conferring adistinctive three-dimensional structure and function on eachprotein (Figure 1-9).InsulinGlutamine synthetaseHemoglobin▲ FIGURE 1-9 Proteins vary greatly in size, shape, andfunction. These models of the water-accessible surface of somerepresentative proteins are drawn to a common scale and revealthe numerous projections and crevices on the surface.

Eachprotein has a defined three-dimensional shape (conformation)that is stabilized by numerous chemical interactions discussed inChapters 2 and 3. The illustrated proteins include enzymes9DNA moleculeImmunoglobulinAdenylatekinaseLipid bilayer(glutamine synthetase and adenylate kinase), an antibody(immunoglobulin), a hormone (insulin), and the blood’s oxygencarrier (hemoglobin). Models of a segment of the nucleic acidDNA and a small region of the lipid bilayer that forms cellularmembranes (see Section 1.3) demonstrate the relative width ofthese structures compared with typical proteins.

[Courtesy ofGareth White.]10CHAPTER 1 • Life Begins with CellsSome proteins are similar to one another and thereforecan be considered members of a protein family. A few hundred such families have been identified. Most proteins are designed to work in particular places within a cell or to bereleased into the extracellular (extra, “outside”) space. Elaborate cellular pathways ensure that proteins are transportedto their proper intracellular (intra, within) locations or secreted (Chapters 16 and 17).Proteins can serve as structural components of a cell, forexample, by forming an internal skeleton (Chapters 5, 19, and20). They can be sensors that change shape as temperature, ionconcentrations, or other properties of the cell change.

Theycan import and export substances across the plasma membrane (Chapter 7). They can be enzymes, causing chemical reactions to occur much more rapidly than they would withoutthe aid of these protein catalysts (Chapter 3). They can bind toa specific gene, turning it on or off (Chapter 11). They can beextracellular signals, released from one cell to communicatewith other cells, or intracellular signals, carrying informationwithin the cell (Chapters 13–15). They can be motors thatmove other molecules around, burning chemical energy (ATP)to do so (Chapters 19 and 20).How can 20 amino acids form all the different proteinsneeded to perform these varied tasks? Seems impossible atfirst glance. But if a “typical” protein is about 400 aminoacids long, there are 20400 possible different protein sequences.

Even assuming that many of these would be functionally equivalent, unstable, or otherwise discountable, thenumber of possible proteins is well along toward infinity.Next we might ask how many protein molecules a cellneeds to operate and maintain itself. To estimate this number, let’s take a typical eukaryotic cell, such as a hepatocyte(liver cell).

This cell, roughly a cube 15 m (0.0015 cm) ona side, has a volume of 3.4 109 cm3 (or milliliters). Assuming a cell density of 1.03 g/ml, the cell would weigh3.5 109 g. Since protein accounts for approximately 20percent of a cell’s weight, the total weight of cellular protein is 7 10 10 g. The average yeast protein has a mo-lecular weight of 52,700 (g/mol). Assuming this value istypical of eukaryotic proteins, we can calculate the totalnumber of protein molecules per liver cell as about 7.9 109 from the total protein weight and Avogadro’s number,the number of molecules per mole of any chemical compound (6.02 10 23).

To carry this calculation one stepfurther, consider that a liver cell contains about 10,000different proteins; thus, a cell contains close to a millionmolecules of each type of protein on average. In actualitythe abundance of different proteins varies widely, from thequite rare insulin-binding receptor protein (20,000 molecules) to the abundant structural protein actin (5 10 8molecules).Nucleic Acids Carry Coded Informationfor Making Proteins at the Right Time and PlaceThe information about how, when, and where to produce eachkind of protein is carried in the genetic material, a polymercalled deoxyribonucleic acid (DNA). The three-dimensionalstructure of DNA consists of two long helical strands that arecoiled around a common axis, forming a double helix. DNAstrands are composed of monomers called nucleotides; theseoften are referred to as bases because their structures containcyclic organic bases (Chapter 4).Four different nucleotides, abbreviated A, T, C, and G,are joined end to end in a DNA strand, with the base partsprojecting out from the helical backbone of the strand.

EachDNA double helix has a simple construction: wherever thereis an A in one strand there is a T in the other, and each C ismatched with a G (Figure 1-10). This complementary matching of the two strands is so strong that if complementarystrands are separated, they will spontaneously zip back together in the right salt and temperature conditions. Suchhybridization is extremely useful for detecting one strand usingthe other. For example, if one strand is purified and attachedto a piece of paper, soaking the paper in a solution containing the other complementary strand will lead to zippering,ParentalstrandsDaughterstrandsA G T C▲ FIGURE 1-10 DNA consists of two complementarystrands wound around each other to form a double helix.(Left) The double helix is stabilized by weak hydrogen bondsbetween the A and T bases and between the C and G bases.(Right) During replication, the two strands are unwound and usedas templates to produce complementary strands.

The outcome istwo copies of the original double helix, each containing one ofthe original strands and one new daughter (complementary)strand.1.2 • The Molecules of a Celleven if the solution also contains many other DNA strandsthat do not match.The genetic information carried by DNA resides in its sequence, the linear order of nucleotides along a strand. Theinformation-bearing portion of DNA is divided into discretefunctional units, the genes, which typically are 5000 to100,000 nucleotides long. Most bacteria have a few thousand genes; humans, about 40,000.