CHAPTER TWO SUMMARY
INTRODUCTION
• The complex organization and interaction of the chemicals within a cell confer the unique
characteristics of life. The key chemicals of life are water, certain small inorganic solutes such as
salts, and four categories of organic molecules: lipids, carbohydrates, amino acids and proteins,
and nucleotides and nucleic acids.
•Large organic complexes and macromolecules—membranes, polysaccharides, proteins, and
nucleic acids—are responsible for the main structures and functions of cells. Functions often
require that these complexes and macromolecules be flexible and dynamic, but in turn, they are
susceptible to disruption by some environmental factors such as temperature. Thus, homeostasis
of the interior environment in organisms in part evolved to protect the macromolecules, especially
proteins.
•Cells are the living building blocks of multicellular bodies. Body cells, which are too small to be
seen by the unaided eye, have been shown by microscopic techniques to consist of three major
subdivisions: (1) the plasma membrane, which encloses the cell and separates the intracellular
and extracellular fluid; (2) the nucleus, which contains deoxyribonucleic acid (DNA), the cell’s
genetic material; and (3) the cytoplasm, the portion of the cell’s interior not occupied by the
nucleus.
•The cytoplasm consists of cytosol, a complex gelatin-like mass, and organelles, which are highly
organized, membrane-enclosed structures dispersed within the cytosol.
NUCLEUS, CHROMOSOMES AND GENES
•The eukaryotic nucleus contains deoxyribonucleic acid, DNA, in linear strings called
chromosomes. DNA consists of a varying 4 nucleotides designated A, G, T, and C. The sequence
of a particular stretch of DNA, called a gene, contains a code for making proteins, and a
regulatory sequence for controlling the coding region of the gene.
•A gene is transcribed (copied) as an mRNA (messenger RNA) (ribonucleic acid, made of 4
nucleotides designate A, G, U, and C). mRNA then binds to a ribosome, where it is translated (by
synthesis of a molecule made of amino acids) into a protein.
•Different genes are expressed in different tissues and organs. Special proteins, transcription
factors, which often differ among tissues, recognize the regulatory sequence of a gene. Following
recognition by these factors, transcription of the nearby coding section of the gene ensues.
•Telomeres protect the ends of chromosomes, and are lost at each cell division. When telomeres
are gone, cell division ceases, a stage associated with lifespans of species.
•Genomics analyzes the genetic codes of organisms and seeks to understand the functions and
regulations of all genes. New technologies associated with genomics include: 1)
immunofluorescence, using fluorescent antibodies that bind to proteins in tissues, revealing their
locations visually; 2) “knocking out” (removing) a gene to see its effects; 3) DNA microarrays
(“gene chips”), which can detect active and inactive genes in different tissues and under different
conditions; 4) transgenic organisms, in which genes from one species are inserted into another,
for the purposes of researching how genes work, production of useful products, and gene therapy
(adding functional genes to an individual with a malfunctioning gene); 5) cloning of an
individual, which involves taking a nucleus from an advanced cell (e.g., adult skin cell) and
inserting it into an activated egg which has had its own nucleus removed;
ORGANELLES
•Compartmentalization of specific sets of chemicals within the organelles permits chemical
activities that would not be compatible with each other to occur simultaneously within separate
organelle compartments. Six types of organelles are found in most cells: endoplasmic reticulum,
Golgi complex, lysosomes, peroxisomes, mitochondria, and vaults.
•The endoplasmic reticulum (ER) is a single, complex membranous network that encloses a fluidfilled lumen. The primary function of the ER is to serve as a factory for synthesizing proteins and
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lipids to be used for (1) the production of new cellular components, particularly cell membranes,
and (2) the secretion of special products such as enzymes and hormones to the exterior of the cell.
There are two types of endoplasmic reticulum: rough endoplasmic reticulum, which is studded
with ribosomes, and smooth endoplasmic reticulum, which lacks ribosomes.
•The rough endoplasmic reticular ribosomes synthesize proteins, which are released into the ER
lumen so that they are separated from the cytosol. Also entering the lumen are lipids produced
within the membranous walls of the ER. Synthesized products move from the rough ER to the
smooth ER, where they are packaged and discharged as transport vesicles. Transport vesicles are
formed as a portion of the smooth ER “buds off,” containing a collection of newly synthesized
proteins and lipids wrapped in smooth ER membrane.
•The Golgi complex, which consists of stacks of flattened, membrane- enclosed sacs, serves a
twofold function: (1) to act as a refining plant for modifying into a finished product the newly
synthesized molecules delivered to it in crude form from the endoplasmic reticular factory; and
(2) to sort, package, and direct molecular traffic to appropriate intracellular and extracellular
destinations.
•Each cell contains several hundred lysosomes, which are membrane- enclosed sacs that contain
powerful hydrolytic (digestive) enzymes. Serving as the intracellular digestive system, lysosomes
destroy phagocytized foreign material such as bacteria, demolish worn-out cell parts to make way
for new replacement parts, and eliminate the entire cell if it is severely damaged or dead.
•Peroxisomes, small membrane-enclosed sacs containing powerful oxidative enzymes, are
specialized for carrying out particular oxidative reactions, including certain detoxification
activities.
•The rod-shaped mitochondria are the energy organelles of the cell. They house the enzymes of
the citric acid cycle and electron transport chain, which efficiently convert the energy in food
molecules to the usable energy stored in ATP molecules. During this process, which is known as
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oxidative phosphorylation, the mitochondria utilize molecular oxygen and produce carbon
dioxide and water as by-products.
•In the absence of oxygen, animal cells rely on glycolysis, and (in some animals) other reactions
which yield considerably less ATP than oxidative phosphorylation. These anaerobic processes
yield byproducts such as lactic acid.
•The body’s cells use ATP as an energy source for synthesis of new chemical compounds, for
membrane transport, and for mechanical work.
•Vaults, which are shaped like octagonal barrels, are the same shape and size as the nuclear pores.
It is speculated that vaults may transport messenger RNA and/or the ribosomal subunits from the
nucleus to the cytoplasmic sites of protein synthesis.
CYTOSOL AND CYTOSKELETON
•The cytosol contains the enzymes involved in intermediary metabolism and the ribosomal
machinery essential for synthesis of these enzymes as well as other cytosolic proteins.
Furthermore, many cells store unused nutrients within the cytosol in the form of glycogen
granules or fat droplets.
•Also present in the cytosol are various secretory, transport, and endocytotic vesicles containing
materials that are being moved between various organelles and between these organelles and the
plasma membrane.
•Pervading the cytosol is the cytoskeleton, which serves as the “bone and muscle” of the cell. The
three types of cytoskeletal elements—microtubules, microfilaments, and intermediate filaments—
are each composed of different proteins and perform various roles. Collectively, the cytoskeletal
elements give the cell shape and support, enable it to organize and move its internal structures as
needed, and, in some cells, allow movement between the cell and its environment.
•The eukaryotic cell is a very crowded environment, particularly because of the cytoskeleton.
Crowding affects diffusion and reaction rates that are still being investigated.
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CELL-TO-CELL ADHESIONS
• Special cells locally secrete a complex extracellular matrix, which serves as a biological “glue”
between the cells of a tissue.
• The extracellular matrix consists of a watery, gel-like substance interspersed with three major
types of protein fibers: collagen, elastin, and fibronectin.
• Many cells are further joined by specialized cell junctions, of which there are three types:
desmosomes, tight junctions, and gap junctions.
• Desmosomes serve as adhering junctions to hold cells together mechanically and are especially
important in tissues subject to a great deal of stretching.
• Tight junctions actually fuse cells together to seal off passage between cells, thereby permitting
only regulated passage of materials through the cells. These impermeable junctions are found in
the epithelial sheets that separate compartments with very different chemical compositions.
• Gap junctions are communicating junctions between two adjacent but not touching cells. Cells
joined by gap junctions are connected by small tunnels that permit exchange of ions and small
molecules between the cells. Such movement of ions plays a key role in the spread of electrical
activity to synchronize contraction in heart and smooth muscle.
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