난감형을 위한 토론자료 Overview of the Cellular Basis of Life

[극강 심검]

1.   All living organisms are composed of cells—the basic structural and functional units of life. Cells vary widely in both shape and size.

2.  The principle of complementarity states that the biochemical activity of cells reflects the operation of organelles.

3.  The generalized cell is a concept that typifies all cells. The generalized cell has three major regions—the nucleus, cytoplasm, and plasma membrane.

The Plasma Membrane: Structure  (pp. 59-63)

1.  The plasma membrane encloses cell contents, mediates exchanges with the extracellular environment, and plays a role in cellular communication.

The Fluid Mosaic Model  (pp. 60-62)

2.  The fluid mosaic model depicts the plasma membrane as a fluid bilayer of lipids (phospholipids, cholesterol, and glycolipids) within which proteins are inserted.

3.  The lipids have both hydrophilic and hydrophobic regions that organize their aggregation and self-repair. The lipids form the structural part of the plasma membrane.

4.  Most proteins are integral transmembrane proteins that extend entirely through the membrane. Some, appended to the integral proteins, are peripheral proteins.

5.  Proteins are responsible for most specialized membrane functions: some are enzymes, some are receptors, and others mediate membrane transport functions. Externally facing glycoproteins contribute to the glycocalyx.

Specializations of the Plasma Membrane  (pp. 62-63)

6.  Microvilli are extensions of the plasma membrane that increase its surface area for absorption.

7.  Membrane junctions join cells together and may aid or inhibit movement of molecules between or past cells. Tight junctions are impermeable junctions; desmosomes mechanically couple cells into a functional community; gap junctions allow joined cells to communicate.

The Plasma Membrane: Functions  (pp. 63-79)

Membrane Transport  (pp. 63-76)

1.  The plasma membrane acts as a selectively permeable barrier. Substances move across the plasma membrane by passive processes, which depend on the kinetic energy of molecules or on pressure gradients, and by active processes, which depend on the use of cellular energy (ATP).

2.  Diffusion is the movement of molecules (driven by kinetic energy) down a concentration gradient. Fat-soluble solutes can diffuse directly through the membrane by dissolving in the lipid.

3.  Facilitated diffusion is the passive movement of certain solutes across the membrane either by their binding with a membrane carrier protein or by their moving through a membrane channel. As with other diffusion processes, it is driven by kinetic energy, but the carriers and channels are selective.

4.  Osmosis is the diffusion of a solvent, such as water, through a selectively permeable membrane. Water diffuses through membrane pores (aquaporins) or directly through the lipid portion of the membrane from a solution of lesser osmolarity to a solution of greater osmolarity.

5.  The presence of impermeable solutes leads to changes in cell tone that may cause the cell to swell or shrink. Net osmosis ceases when the solute concentration on both sides of the plasma membrane reaches equilibrium.

6.  Solutions that cause a net loss of water from cells are hypertonic; those causing net water gain are hypotonic; those causing neither gain nor loss of water are isotonic.

7.  Filtration occurs when a filtrate is forced across a membrane by hydrostatic pressure. It is n[안내]태그제한으로등록되지않습니다-xxonselective and limited only by pore size. The pressure gradient is the driving force.

8.  Active transport (solute pumping) depends on a carrier protein and ATP. Substances transported move against concentration or electrical gradients. In primary active transport, such as that provided by the Na1-K1 pump, ATP directly provides the energy. In secondary active transport, the energy of an ion gradient (produced by a primary active transport process) is used to transport a substance passively. Many active transport systems are coupled, that is, cotransported substances move in the same (symport) or opposite (antiport) directions across the membrane.

9.  Vesicular transport also requires that ATP be provided. Exocytosis, which uses SNARES to anchor the vesicles to the plasma membrane, ejects substances (hormones, wastes, secretions) from the cell. Endocytosis brings substances into the cell in protein-coated vesicles. If the substance is particulate, the process is called phagocytosis; if the substance is dissolved molecules, the process is pinocytosis. Receptor-mediated endocytosis is selective; engulfed particles attach to receptors on the membrane before endocytosis occurs.

      Most endocytosis (and transcytosis) is mediated by clathrin-coated vesicles. Caveolin-coated vesicles engulf some substances but appear to be more important as sites that accumulate receptors involved in cell signaling.

      Coatomer-coated vesicles mediate substance (vesicular) trafficking within the cell.

Generating and Maintaining a Resting Membrane Potential  (pp. 76-77)

10. All cells in the resting stage exhibit a voltage across their membrane, called the resting membrane potential. Because of the membrane potential, both concentration and electrical gradients determine the ease of an ion’s diffusion.

11. The membrane potential is generated by concentration gradients of and differential permeability of the plasma membrane to sodium and potassium ions. Sodium is in high extracellular–low intracellular concentration, and the membrane is poorly permeable to it. Potassium is in high concentration in the cell and low concentration in the extracellular fluid. The membrane is more permeable to potassium than to sodium. Protein anions in the cell are too large to cross the membrane and Cl2, the main ECF anion, is repelled by the negative charge on the inner membrane face.

12. The greater outward diffusion of potassium (than inward diffusion of sodium) leads to a charge separation at the membrane (inside negative). This charge separation is maintained by the operation of the sodium-potassium pump.

Cell-Environment Interactions  (pp. 77-79)

13. Cells interact directly and indirectly with other cells. Indirect interactions involve extracellular chemicals carried in body fluids or forming part of the extracellular matrix.

14. Molecules of the glycocalyx are intimately involved in cell-environment interactions. Most are cell adhesion molecules or membrane receptors.

15. Activated membrane receptors act as catalysts, regulate channels, or, like G protein-linked receptors, act through second messengers such as cyclic AMP and Ca21. Ligand binding results in changes in protein structure or function within the targeted cell.

The Cytoplasm  (pp. 79-94)

1.  The cytoplasm, the cellular region between the nuclear and plasma membranes, consists of the cytosol (fluid cytoplasmic environment), inclusions (nonliving nutrient stores [lipid droplets, glycosomes], pigment granules, crystals, etc.), and cytoplasmic organelles.

Cytoplasmic Organelles  (pp. 79-86)

2.  The cytoplasm is the major functional area of the cell. These functions are mediated by cytoplasmic organelles.

3.  Mitochondria, organelles limited by a double membrane, are sites of ATP formation. Their internal enzymes carry out the oxidative reactions of cellular respiration.

4.  Ribosomes, composed of two subunits containing ribosomal RNA and proteins, are the sites of protein synthesis. They may be free or attached to membranes.

5.  The rough endoplasmic reticulum is a ribosome-studded membrane system. Its cisternae act as sites for protein modification. Its external face acts in phospholipid and cholesterol synthesis. Vesicles pinched off from the ER transport the proteins to other cell sites.

6.  The smooth endoplasmic reticulum synthesizes lipid and steroid molecules. It also acts in fat metabolism and in drug detoxification. In muscle cells, it is a calcium ion depot.

7.  The Golgi apparatus is a membranous system close to the nucleus that packages protein secretions for export, packages enzymes into lysosomes for cellular use, and modifies proteins destined to become part of cellular membranes.

8.  Lysosomes are membranous sacs of acid hydrolases packaged by the Golgi apparatus. Sites of intracellular digestion, they degrade worn-out organelles, and tissues that are no longer useful, and release ionic calcium from bone.

9.  Peroxisomes are membranous sacs containing oxidase enzymes that protect the cell from the destructive effects of free radicals and other toxic substances by converting them first to hydrogen peroxide and then water.

10. The cytoskeleton includes microfilaments, intermediate filaments, and microtubules. Microfilaments, formed of contractile proteins, are important in cell motility or movement of cell parts. Microtubules organize the cytoskeleton and are important in intracellular transport. Motility functions involve motor proteins. Intermediate filaments help cells resist mechanical stress and connect other elements.

Cellular Extensions  (pp. 86–88)

11. Centrioles form the mitotic spindle and are the basis of cilia and flagella.

The Nucleus  (pp. 88-91)

1.  The nucleus is the control center of the cell. Most cells have a single nucleus. Without a nucleus, a cell cannot divide or synthesize more proteins; thus, it is destined to die.

2.  The nucleus is surrounded by the nuclear envelope, a double membrane penetrated by fairly large pores.

3.  Nucleoli are nuclear sites of ribosome subunit synthesis.

4.  Chromatin is a complex network of slender threads containing histone proteins and DNA. The chromatin units are called nucleosomes. When a cell begins to divide, the chromatin coils and condenses, forming chromosomes.

Cell Growth and Reproduction  (pp. 91-103)

The Cell Life Cycle  (pp. 91-96)

1.  The cell life cycle is the series of changes that a cell goes through from the time it is formed until it divides.

2.  Interphase is the nondividing phase of the cell life cycle. Interphase consists of G1, S, and G2 subphases. During G1, the cell grows and centriole replication begins; during the S phase, DNA replicates; and during G2, the final preparations for division are made.

3.  DNA replication occurs before cell division; it ensures that all daughter cells have identical genes. The DNA helix uncoils, and each DNA nucleotide strand acts as a template for the formation of a complementary strand. Base pairing provides the guide for the proper positioning of nucleotides.

4.  The products of the semiconservative replication of a DNA molecule are two DNA molecules identical to the parent molecule, each formed of one “old” and one “new” strand.

5.  Cell division, essential for body growth and repair, occurs during the M phase. Cell division is stimulated by certain chemicals (including growth factors and some hormones) and increasing cell size. Lack of space and inhibitory chemicals deter cell division. Cell division is regulated by cyclin-Cdk complexes, of which one example is MPF. Cell division consists of two distinct phases: mitosis and cytokinesis.

6.  Mitosis, consisting of prophase, metaphase, anaphase, and telophase, results in the parceling out of the replicated chromosomes to two daughter nuclei, each genetically identical to the mother nucleus. Cytokinesis, which begins late in mitosis, divides the cytoplasmic mass into two parts.

Protein Synthesis  (pp. 96-103)

7.  A gene is defined as a DNA segment that provides the instructions for the synthesis of one polypeptide chain. Since the major structural materials of the body are proteins, and all enzymes are proteins, this amply covers the synthesis of all biological molecules.

8.  The base sequence of DNA provides the information for protein structure. Each three-base sequence (triplet) calls for a particular amino acid to be built into a polypeptide chain.

9.  The three varieties of RNA are synthesized on single strands of the DNA template. RNA nucleotides are joined following base-pairing rules.

10. Ribosomal RNA forms part of the protein synthesis sites; messenger RNA carries instructions for making a polypeptide chain from the DNA to the ribosomes; transfer RNA ferries amino acids to the ribosomes and recognizes codons on the mRNA strand specifying its amino acid.

11. Protein synthesis involves (1) transcription, synthesis of a complementary mRNA, and (2) translation, “reading” of the mRNA by tRNA and peptide bonding of the amino acids into the polypeptide chain. Ribosomes coordinate translation.

12. Soluble proteins that are damaged or no longer needed are targeted for destruction by attachment of ubiquitin. Such proteins are degraded by cytosolic enzymes or proteasomes.

Extracellular Materials  (pp. 103)

1. Extracellular materials are substances found outside the cells. These include body fluids, cellular secretions, and extracellular matrix. Extracellular matrix is particularly abundant in connective tissues.

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