Biology Chapter 1

Embed or link this publication

Description

Biology 8th Edition Chapter 1 provided by Julian M.

Popular Pages


p. 1

a-pdf split demo purchase from www.a-pdf.com to remove the watermark introductio themes in the study of life key concepts figure 1.1 what properties of life are demonstrated by this flower 1.1 1.2 themes connect the concepts of biology the core theme evolution accounts for the 1.3 unity and diversity of life scientists use two main forms of inquiry in their study of nature r about the world of life he flower featured on the cover of this book and in figure 1.1 is from a magnolia a tree ofancient lineage that is native to asian and american forests the magnolia blossom is a sign of the plant s status as a living organism for flowers contain organs of sexual reproduction and reproduction is a key property ochfe as you will learn later like all organisms the magnolia tree in figure 1.2 is living in close association with other organisms though it is a lone specimen far from its ancestral forest for example it depends on beetles to carry pollen from one flower to another and the beetles in turn eat from its flowers the flowers are adapted to the beetles in several ways their bowl shape allows easy access and their multiple reproductive organs and tough petals see figure 1.1 help ensure that some survive the voracious beetles such adaptations are the result of evolution the process of change that has transformed life on earth from its earliest beginnings to the diversity oforganisms living today as discussed later in this chapter evolution is the fundamental organizing principle of biology and the main theme of this book although biologists know a great deal about magnolias and other plants many mysteries remain for instance what exactly led to the origin of flowering plants posing questions about the living world and seeking science-based answersscientific inquiry-are the central activities ofbiology the sci t entific study of life biologists questions can be ambitious they may ask how a single tiny cen becomes a tree or a dog how the human mind works or how the different forms of life in a forest interact can you think ofsome questions about living organisms that interest you when you do you are already starting to think like a biologist more than anything else biology is a quest an ongoing inquiry about the nature of life perhaps some of your questions relate to health or to societal or environmental issues biology is woven into the fabric of our culture more than ever before and can help answer many questions that affect our lives research breakthroughs in genetics and cell biology are transforming medicine and agriculture neuroscience and evolutionary biology are reshaping psychology and sociology new models in ecology are helping societies evaluate environmental issues such as global warming there has never been a more important time to em· bark on a study of life figure 1.2 a magnolia tree in early spring.

[close]

p. 2

order this close-up of a sunflower illl,lstrates the highly qrdered strl,lctl.lre that characterizes life regulation the regulation of blood flow through the blood vessels of this jackrabbit s ears helps maintain a constant body temperature by adjusting heat exchange with the surrounding air energy processing this hummingbird obtains fuel in the form of nectar from flowers the hummingbird will use chemical energy stored in its food to power flight and other work reproduction organisms living things reproduce their own kind here an emperor penguin protects its baby · ·· 1.3 ii s a gasoline-powered lawn mower life which of these properties does it have which properties does it lack alive figure some properties of but what is life even a small child realizes that a dog or a plant is alive while a rock is not yet the phenomenon we call life defies a simple one-sentence definition we recognize life by what living things do figure 1.3 highlights some of the properties and processes we associate with life while limited to a handful of images figure 1.3 reminds us that the living world is wondrously varied how do biologists make sense of this diversity and complexity this opening 2 cilapte one chapter sets up a framework for answering this question the first part of the chapter provides a panoramic view of the biological landscape organized around some unifying themes we then focus on biojogy soverarching theme evolution with an introduction to the reasoning that led charles darwin to his explanatory theory finally we look at scientific inquiryhow scientists raise and attempt to answer questions about the natural world introduction themes in the study ofufe

[close]

p. 3

r c ect the concepts of biology biology is a subject of enormous scope and anyone who follows the news knows that biological knowledge is expanding at an ever-increasing rate simply memorizing the factual details ofthis huge subject is nota reasonable option how then can you as a student go beyond the facts to develop a coherent view of life one approach is to fit the many things you learn into a set of themes that pervade all ofbiology-ways of thinking about life that will still apply decades from now focusing on a few big ideas will help you organize and make sense of all the information you ll encounter as you study biology to help you we have selected seven unifying themes to serve as touchstones as you proceed through this book emergent properties ifwe now zoom back out from the molecular level in figure 1.4 we can see that novel properties emerge at each step properties that are not present at the preceding level these emergent properties are due to the arrangement and interactions of parts as complexity increases for example if you make a testtube mixture of chlorophyll and all the other kinds of molecules found in a chloroplast photosynthesis will not occur photosynthesis can take place only when the molecules are arranged in a specific way in an intact chloroplast to take another example if a serious head injury disrupts the intricate architecture ofa human brain the mind may cease to function properly even though all of the brain parts are still present our thoughts and memories are emergent properties of a complex network of nerve cells at a much higher level ofbiological organization-at the ecosystem level-the recycling of chemical elements essential to life such as carbon depends on a network ofdiverse organisms interacting with each other and with the soil water and air emergent properties are not unique to life we can see the importance of arrangement in the distinction between a box of bicycle parts and a working bicycle and while graphite and diamonds are both pure carbon they have very different properties because their carbon atoms are arranged differently but compared to such nonliving examples the unrivaled complexity ofbiological systems makes the emergent properties of life especially challenging to study evolution the overarching theme of biology evolution is biology s core theme-the one idea that makes sense of everything we know about living organisms life has been evolving on earth for billions of years resulting in a vast diversity of past and present organisms but along with the diversity we find many shared features for example while the sea horse jackrabbit hummingbird crocodile and penguins in figure 1.3 look very different their skeletons are basically similar the scientific explanation for this unity and diversity-and for the suitability of organisms to their environments-is evolution the idea that the organisms living on earth today are the modified descendants of common ancestors in other words we can explain traits shared by two organisms with the idea that they have descended from a common ancestor and we can account for differences with the idea that heritable changes have occurred along the way many kinds of evidence support the occurrence of evolution and the theory that describes how it takes place we ll return to evolution later in the chapter after surveying some other themes and painting a fuller picture of the scope of biology the power and limitations of reductionism because the properties of life emerge from complex organization scientists seeking to understand biological systems confront a dilemma on the one hand we cannot fully explain a higher level of order by breaking it down into its parts a dissected animal no longer functions a cell reduced to its chemical ingredients is no longer a cell disrupting a living system interferes with its functioning on the other hand something as complex as an organism or a cell cannot be analyzed without taking it apart reductionism-the reduction of complex systems to simpler components that are more manageable to study-is a powerful strategy in biology for example by studying the molecular structure of dna that had been extracted from cells james watson and francis crick inferred in 1953 how this molecule could serve as the chemical basis of inheritance the central role of dna in cells and organisms became better understood however when scientists were able to study the interactions of dna with other molecules biologists must balance the reductionist strategy with the larger-scale holistic objective of understanding emergent properties-how the parts of cells organisms and higher levels of order such as ecosystems work together at the cutting edge of research today is the approach called systems biology chapte one theme new properties emerge at each level in the biological hierarchy the study of life extends from the microscopic scale of the molecules and cells that make up organisms to the global scale ofthe entire living planet we can divide this enormous range into different levels of biological organization imagine zooming in from space to take a closer and closer look at life on earth it is spring and our destination is a forest in ontario canada where we will eventually explore a maple leaf right down to the molecular level figure 1.4 on the next two pages narrates this journey into life with the circled numbers leading you through the levels of biological organization illustrated by the photographs introduction themes in the study of life 3

[close]

p. 4

· figun 1 exploring levels of biological organization 1 the biosphere as soon as we are near enough to earth to make out its continents and oceans we begin to see signs oflife-in the green mosaic of the planet s forests for example this is our first view of the biosphere which consists of all the environments on earth that are inhabited by life the biosphere includes most regions ofland most bodies of water and the atmosphere to an altitude of several kilometers 2 ecosystems as we approadl earth s surfare fof an imaginary landing in ontario we can be to make out a forest with an abunc1arxr of deciduous trei s trei s that lose their leaves in one season and grow new ones in another such a deciduous forest is an exam of an eals stem grasslands deserts and the crean s roral red5 are other types of erosrstems an ecosystem consists of au the imng thing in a particular area along with au the nonliving romponents ofthe environment with whidllife interacts such as soil water atmospheric gases and ighl all of earth s eros stems combined make up the biosphere 3 communities the entire array of organisms inhabiting a particular ecosystem is called a biological community the community in our forest ecosystem includes many kinds of trees and other plants a diversity of animals various mushrooms and other fungi and enormous numbers of diverse microorganisms which are living forms such as bacteria that are too small to see without a microscope each of these forms of life is called a species 4 populations a population consists of all the individuals of a species living within the bounds ofa specified area for example our ontario forest includes a population of sugar maple trees and a population of white-tailed deer we can now refine our definition ofa community as the set of populations that inhabit a particular area 5 organisms individual living thing are called organisms each of the maple trees and other plants in the forest is an organism and so is each forest animal such as a frog squirrel deer and beede 1be soil teems with microorganisms such as bacteria 4 hahuionf introduction themes in the study of life

[close]

p. 5

· 8 cells 6 organs and organ systems the structural hierarchy oflife continues to unfold as we explore the architecture ofthe more complex organisms amaple leaf is an example ofan organ a body part consisting of two or more tissues which we11 see upon our next scale change an organ carries out a particular function in the body stems and roots are the other major organs of plants examples of human organs are the brain heart and kidney the organs of humans other complex animals and plants are organized into organ systems each a team oforgans that cooperate in a specific function for example the human digestive system includes such organs as the tongue stomach and intestines the cell is life s fundamental unit of structure and function some organisms such as amoebas and most bacteria are single cells other organisms including plants and animals are multicellular instead of a single cell performing all the functions ofhfe a multicellular organism has a division of labor among specialized cells a human body consists of trillions of microscopic cells of many different kinds such as muscle cells and nerve cells which are organized into the various specialized tissues for example muscle tissue consists of bundles of muscle cells in the photo below we see a more highly magnified view of some of the cells in a leaf tissue each of the cells is only about 25 micrometers 11m i l across it would take morethan 7000f 10 /1m these cells to reach across a penny as s~l a v vv that each contains numerous green structures called chloroplasts which are responsible for photosynthesis 9 organelles cbloroplasts are examples of organelles the various functional components that make up cells in this image a very power· ful tool called an electron microscope brings a single chloroplast into sharp focus 7 tissues our next scale changeto see a leaf s tissuesrequires a microscope the leaf shown here has been cut on an angle the honeycombed tissue in the interior of the leaf left portion of photo is the main location of photosynthesis the process that converts light energy to the chemical energy of sugar and other food we are viewing the sliced leaf from a perspective that also enables us to see the jigsaw puzzle-like tissue called epidermis the skin on the surface of the leaf right part of photo the pores through the epidermis allow the gas carbon dioxide a raw material for sugar production to reach the photosynthetic tissue inside the leaf at this scale we can also see that each tissue has a cellular structure in fact each kind of tissue is a group of similar cells · 10 molecules 50/lm our last scale change vaults us into a chloroplast for a view ofhfe at the molecular level amolecule isa chemical structure consisting oftwo or more small chemical units called atoms which are represented as balls in this computer graphic ofa chlorophyll molecule chlorophyll is the pigment molecule that makes a maple leaf green one of the most important molecules on earth chlorophyll absorbs sunlight during the first step of photosynthesis within each chloroplast millions of chlorophylls and other molecules are organized into the equipment that converts light energy to the chemical energy offood chapte one introduction themes in the study of life 5

[close]

p. 6

systems biology a system is simply a combination ofcomponents that function together a biologist can srudy a system at any level of organization a single leaf cell can be considered a system as can a frog an ant colony or a desert ecosytem to understand how such systems work it is not enough to have a parts list,n even a complete one realizing this many researchers are now complementing the reductionist approach with new strategies for studying whole systems this changing perspective is analogous to moving from ground level on a street corner to a helicopter high above a city from which you can see how variables such as time of day construction projects accidents and traffic-signal malfunctions affect traffic throughout the city the goal of systems biology is to construct models for the dynamic behavior of whole biological systems successful models enable biologists to predict how a change in one or more variables will affect other components and the whole system thus the systems approach enables us to pose new kinds of questions how might a drug that lowers blood pressure affect the functions of organs throughout the human body how might increasing a crop s water supply affect processes in the plants such as the storage of molecules essential for human nutrition how might a gradual increase in atmospheric carbon dioxide alter ecosystems and the entire biosphere the ultimate aim of systems biology is to answer big questions like the last one systems biology is relevant to the study of life at all levels during the early years of the 20th century biologists studying animal physiology functioning began integrating data on how multiple organs coordinate processes such as the regulation of sugar concentration in the blood and in the 1960s scientists investigating ecosystems pioneered a more mathematically sophisticated systems approach with elaborate models diagramming the network of interactions between organisms and nonliving components of ecosystems such as salt marshes such models have already been useful for predicting the responses of these systems to changing variables more recently systems biology has taken hold at the cellular and molecular levels as we ll describe later when we discuss dna affected by the interactions between them the tree also interacts with other organisms such as soil microorganisms associated with its roots and animals that eat its leaves and fruit ecosystem dynamics the operation ofany ecosystem involves two major processes one process is the cycling of nutrients for example minerals acquired by a tree will eventually be returned to the soil by organisms that decompose leaf litter dead roots and other organic debris the second major process in an ecosystem is the one-way flow of energy from sunlight to producers to consumers producers are plants and other photosynthetic organisms which use light energy to make sugar consumers are organisms such as animals that feed on producers and other consumers the diagram in figure 1.5 outlines the h o processes acting in an african ecosystem energy conversion moving growing reproducing and the other activities of life are work and work requires energy the exchange of energy bety een an organism and its surroundings often involves the transformation of one form ofenergy to another for example the leaves ofa plant absorb light energy and convert it to chemical energy stored in sugar molecules vhen an animal s muscle cells use sugar as fuel to power movements they convert chemical energy to kinetic energy the energy of motion and ecosystem cycling of chemical heal nutrients theme organisms interact with their environments exchanging matter and energy turn back again to figure 1.4 this time focusing on the forest in this or any other ecosystem each organism interacts continuously with its environment which includes both nonliving factors and other organisms a tree for example absorbs water and minerals from the soil through its roots at the same time its leaves take in carbon dioxide from the air and use sunlight absorbed by chlorophyll to drive photosynthesis converting water and carbon dioxide to sugar and oxygen the tree releases oxygen to the air and its roots help form soil by breaking up rocks both organism and environment are 6 ilapte one heat figure 1.5 nutrient cycling and energy flow in an ecosystem introduction themes in the study ofufe

[close]

p. 7

a a bird s wings have an aerodynamically efficient shape b wing bones have a honeycombed internal structure that is strong but lightweight infoldings of membrane mitochondrion 0.5 i1m c the flight muscles are controlled by neurons nerve cells which transmit signals with long extensions neurons are especially well structured for communication within the body figure 1.6 form fits function in a gull s wing a bird s build and the structures of its components make flight possible how does form fit function in a human hand d the flight muscles obtain energy in a usable form from organelles called mitochondria a mitochondrion has an inner membrane with many infoldings molecules embedded in the inner membrane carry out many of the steps in energy produdion and the illfoldings pack a large amount of this membrane into a small container ii in all these energy conversions some ofthe energy is converted to thermal energy which dissipates to the surroundings as heat in contrast to chemical nutrients which recycle within an ecosystem energy flows through an ecosystem usually entering as light and exiting as heat see figure 1.5 theme cells are an organism s basic units of structure and function in life s structural hierarchy the cell has a special place as the lowest level of organization that can perform all activities required for life moreover the activities of organisms are all based on the activities ofcells for instance the division ofcells to form new cells is the basis for all reproduction and for the growth and repair of multicellular organisms figure 1.7 to theme structure and function are correlated at all levels of biological organization another theme evident in figure 1.4 is the idea that form fits function which you ll recognize from everyday life for example a screwdriver is suited to tighten or loosen screws a hammer to pound nails how a device works is correlated with its structure appjied to biology this theme is a guide to the anatomy ofjife at all its structural levels an example from figure 1.4 is seen in the leaf its thin flat shape maximizes the amount of sunlight that can be captured by its chloroplasts analyzing a biological structure gives us dues about what it does and how it works conversely knowing the function of something provides insight into its construction an example from the animal kingdom the wing of a bird provides additional instances ofthe structure-function theme figure 1.6 in exploring life on its different structural levels we discover functional beauty at every turn figure 1.7 a lung cell from a newt divides into two smaller cells that will grow and divide again c~apte one introduction themes in the study of life 7

[close]

p. 8

cite another example the movement of your eyes as you read this line is based on activities of muscle and nerve cells even a global process such as the recycling ofcarbon is the cumulative product ofcellular activities including the photosynthesis that occurs in the chloroplasts ofleafcells understanding how cells work is a major focus of biological research all cells share certain characteristics for example every cell is enclosed by a membrane that regulates the passage of materials between the cell and its surroundings and every cell uses dna as its genetic information however we can distinguish between two main forms ofcells prokaryotic cells and eukaryotic cells the cells of two groups of microorganisms called bacteria and archaea are prokaryotic all other forms of life including plants and animals are composed of eukaryotic cells a eukaryotic cell is subdivided by internal membranes into various membrane-enclosed organelles such as the ones you see in figure 1.8 and the chloroplast you saw in figure 1.4 in most eukaryotic cells the largest organelle is the nucleus which contains the cell s dna the other organelles are located in the cytoplasm the entire region between the nucleus and outer membrane of the cell as figure 1.8 also shows prokaryotic cells are much simpler and generally smaller than eukaryotic cells in a prokaryotic cell the dna is not separated from the rest of the cell by enclosure in a membrane-bounded nucleus prokaryotic cells also lack the other kinds of membrane-enclosed organelles that characterize eukaryotic cells but whether an organism has prokaryotic or eukaryotic cells its structure and function depend on cells theme the continuity of life is based on heritable information in the form of dna inside the dividing cell in figure 1.7 on the previous page you can see structures called chromosomes which are stained with a blue-glowing dye the chromosomes have almost all of the cell s genetic material its dna short for deoxyribonucleic acid dna is the substance of genes the units of inheritance that transmit information from parents to offspring your blood group a b ab or 0 for example is the result of certain genes that you inherited from your parents dna structure and function each chromosome has one very long dna molecule with hundreds or thousands of genes arranged along its length the dna of chromosomes replicates as a cell prepares to divide and each ofthe two cellular offspring inherits a complete set of genes each of us began life as a single cell stocked with dna inherited from our parents replication of that dna with each round of cell division transmitted copies of it to our trillions of cells in each cell the genes along the length of the dna molecules encode the information for building the cell s other molecules in this way dna controls the development and maintenance ofthe entire organism and indirectly everything it does figure 1.9 the dna serves as a central database the molecular structure of dna accounts for its ability to store information each dna molecule is made up of two long chains arranged in a double helix each chain link is one of four kinds of chemical building blocks called nucleotides figure 1.10 the way dna encodes information is analogous to the waywe arrange the letters ofthe alphabet into precise sequences with specific meanings the word rat for example evokes a rodent the words tar and art which contain the same letters mean very different things libraries are filled with books containing information encoded in varying sequences of only 26 letters we can think of nucleotides as the alphabet of inheritance specific sequential arrangements of these four chemical letters encode the precise information in genes which are typically hundreds or thousands of nucleotides long one gene in a bacterial cell may be translated as build a certain component of the cell membrane a particular human gene may mean make growth hormone more generally genes like those just mentioned program the cell s production oflarge molecules called proteins other human proteins include a muscle cell s contraction proteins and the defensive proteins called antibodies a class of proteins crucial to all cells are enzymes which catalyze speed up specific chemical reactions thus dna provides the blueprints and proteins serve as the tools that actually build and maintain the cell and carry out its activities the dna of genes controls protein production indirectly using a related kind ofmolecule called rna as an intermediary h prokaryotic cell eukaryotk cell dna no nucleus liik membrane cytoplasm organelles nucleus contains dna figure 1.8 contrasting eukaryotic and prokaryotic cells in size and complexity cilapte one 8 introduction themes in the study ofufe

[close]

p. 9

sperm cell nuclei contammg dna · ·1 ·· n j h · · y 7 · fertilized egg with dna from both parents egg cell figure 1.9 inherited dna directs development of an organism embyro s cells with copies of inherited dna offspring with traits inherited from both parents nucleus dna cell nucleotide the sequence of nucleotides along a gene is transcribed into rna which is then translated into a specific protein with a unique shape and function in the translation process all forms of life employ essentially the same genetic code a particular sequence of nucleotides says the same thing to one organism as it does to another differences between organisms reflect differences between their nucleotide sequences not all rna in the cell is translated into protein we have known for decades that some types of rna molecules are actually components of the cellular machinery that manufactures proteins recently scientists have discovered whole new classes of rna that play other roles in the cell such as regulating the functioning of protein-coding genes the entire library of genetic instructions that an organism inherits is called its genome a typical human cell has two similar sets of chromosomes and each set has dna totaling about 3 billion nucleotides if the one-letter symbols for these nucleotides were written in letters the size of those you are now reading the genetic text would fill about 600 books the size of this one vithin this genomic library of nucleotide sequences are genes for about 75,000 kinds of proteins and an as yet unknown number of rna molecules systems biology at the leyels of cells and molecules a dna double helix this mcxlel shows each atom in a segment of dna made up of two long chains of building blocks called nucleotides a dna molecule takes the three-dimensional form of a double helix b single strand of dna these geometric shapes and letters are simple symbols for the nucleotides in a small section of one chain of a dna molecule genetic information is encoded in specific sequences of the four types of nucleotides their names are abbreviated here as a t c and g figure 1.10 dna the genetic material the entire sequence of nucleotides in the human genome is now known along with the genome sequences of many other organisms including bacteria archaea fungi plants and animals these accomplishments have been made possible by the development of new methods and dna-sequencing machines such as those shown in figure 1.11 on the next page the sequencing of the human genome is a scientific and te

[close]

p. 10

figure 1.11 modern biology as an information science automatic dna· sequencing machines and abundant computing power made the sequencing of the human genome possible this facility in walnut creek california was one of many labs that collaborated in the international human genome project an even bigger research endeavor an effort to learn how the activities of the myriad proteins encoded by the dna are co· ordinated in cells and whole organisms the best way to make sense of the deluge of data from genome-sequencing projeds and the growing catalog of known protein functions is to apply a systems approach at the cellular and molecular levels figure 1.12 illustrates the results of a large study that mapped a network of protein interactions within a cell ofa fruit fly a popular research organism the model is based on a database of thousands of known proteins and their known interactions with other proteins for example protein a may attach to and alter the activities of proteins b c and d which then go on to interact with stit other proteins the figure maps these protein partnerships to their cellular locales the basics of the systems strategy are straightforward first it is necessary to inventory as many parts of the system as possible such as all the known genes and proteins in a cell an application of reductionism then it is necessary to investigate how each part behaves in relation to others in the working system-all the protein-protein interactions in the case of our fly cell example finally with the help of computers and specialized software it is possible to pool all the data into the kind of system network pictured in figure 1.12 though the basic idea ofsystems biology is simple the practice is not as you would expect from the complexity ofbiological systems it has taken three key research developments to bring systems biology within reach one is high-throughput technology tools that can analyze biological materials very rapidly and produce enormous amounts of data the automatic dna-sequencing machines that made the sequencing of 10 chapte one figure 1.12 a systems map of interactions among proteins in a cell this diagram maps 2,346 proteins dots and their network of interadions lines connecting the proteins in a fruit fly cell systems biologists develop such models from huge databases of information about molecules and their interadions in the cell a major goal of this systems approach is to use the models to predict how one change such as an increase in the activity of a particular protein can ripple through the cell s molecular circuitry to cause other changes the tolal number of proteins in this type of cell is probably in the range of 4,000 10 7,000 introduction themes in the study of life

[close]

p. 11

the human genome possible are examples of high-throughput devices see figure 1.11 the second is bioinformatics which is the use ofcomputational tools to store organize and analyze the huge volume of data that result from high-throughput methods the third key development is the formation of interdisciplinary research teams-melting pots of diverse specialists that may include computer scientists mathematicians engineers chemists physicists and of course biologists from a variety of fields negative feedback lllllll i rl!i enzyme 1 1 it 11111111i t excess d blocks a step enzyme 2 theme feedback mechanisms regulate biological systems a kind of supply-and-demand economy applies to many biological systems consider your muscles for instance when your muscle cells require more energy during exercise they increase their consumption of the sugar molecules that provide fuel in contrast when you rest a different set of chemical reactions converts surplus sugar to storage molecules like most of the cell s chemical processes those that decompose or store sugar are accelerated or catalyzed by the specialized proteins called enzymes each type of enzyme catalyzes a specific chemical reaction in many cases these reactions are linked into chemical pathways each step with its own enzyme how does the cell coordinate its various chemical pathways in our example ofsugar management how does the cell match fuel supply to demand regulating its opposing pathways of sugar consumption and storage the key is the ability of many biological processes to self-regulate by a mechanism called feedback in feedback regulation the output or product of a process regulates that very process in life the mostcommon form of regulation is negative feedback in which accumulation of an end product of a process slows that process for example the cell s breakdown of sugar generates chemical energy in the form of a substance called atp vhen a cell makes more atp than it can use the excess atp feeds back and inhibits an enzyme near the beginning of the pathway figure 1.13a though less common than processes regulated by negative feedback there are also many biological processes regulated by positive feedback in which an end product speeds up its production figure 1.13b the clotting of your blood in response to injury is an example when a blood vessel is damaged structures in the blood called platelets begin to aggregate at the site positive feedback occurs as chemicals released by the platelets attract more platelets the platelet pile then initiates a complex process that seals the wound with a clot feedback is a regulatory motif common to life at all levels from the molecular level to ecosystems and the biosphere such regulation is an example of the integration that makes living systems much greater than the sum of their parts enzyme 3 a negative feedback this three-step chemical pathway converts substance a to substance d a specific enzyme catalyzes each chemical reaction accumulation of the final product oj inhibits the first enzyme in the sequence thus slowing down production of mored w enzyme 4 positive feedback x 0 llllllllll iii enzyme 5 11 i i 1111 iii nt excess z stimulates a step enzyme 6 b positive feedback in a biochemical pathway regulated by positive feedback a product stimulates an enzyme in the reaction sequence incteasing the tate of production of the product ii what would happen if enzyme 2 were missing concept check figure 1.13 regulation by feedback mechanisms 1.1 1 for each biological level in figure 1.4 write a sentence that includes the next lower level example a community consists of populations of the various spedes inhabiting a specific area 2 what theme or themes are exemplified by a the sharp spines of a porcupine b the cloning of a plant from a single cell and c a hummingbird using sugar to power its flight 3 _1,i tuia for each theme discussed in this section give an example not mentioned in the book for suggested answers see appendik a hapte one introduction themes in the study of life ii

[close]

p. 12

r 4 e~h~me evolution accounts for the unity and diversity of life the list of biological themes discussed under concept 1.1 is not absolute some people might find a shorter or longer list more useful there is consensus among biologists however as to the core theme of biology it is evolution to quote one of the founders of modern evolutionary theory theodosius dobzhansky nothing in biology makes sense except in the stand dobzhansky s statement we need to discuss how biolo gists think about this vast diversity organizing the diversity of life diversity is a hallmark oflife biologists have so far identified and named about 1.8 million species to date this diversity of life is known to include at least 6,300 species of prokaryotes organisms with prokaryotic cells 100,000 fungi 290,000 plants 52,000 vertebrates animals with backbones and 1 million insects more than half of all known forms of life researchers identify thousands of additional species each year estimates of the total number of species range from about 10 million to over 100 million whatever the actual number the enormous variety of life gives biology a very broad scope biologists face a major challenge in attempting to make sense of this variety figure 1.14 light of evolution in addition to encompassing a hierarchy of size scales from molecules to the biosphere biology extends across the great diversity of species that have ever lived on earth to under genus order class kingdom domain ursus amerkanus american black bear m #if ii e · · figure 1.14 classifying life to help organize the diversity of life biologists classify species into groups that are then combined into even broader groups in the traditional linnaean system species that are very closely related such as polar bears and brown bears are placed in the same genus genera plural are grouped into families and so on this example classifies the species ursus americanus the american black bear alternative classification schemes will be discussed in detail in chapter 26 12 chapte one introduction themes in the study of life

[close]

p. 13

a domain bacteria bacteria are the most diverse and widespread prokaryotes and are now divided among multiple kingdoms each of the rod-shaped structures in this photo is a bacterial cell grouping species the basic idea there is a human tendency to group diverse items according to similarities for instance perhaps you organize your music collection by artist and then maybe you group the various artists into broader categories such as rock jazz and classical in the same way grouping species that are similar is natural for us we may speak of squirrels and butterflies though we recognize that many different species belong to each group we may even sort groups into broader categories such as rodents which include squirrels and insects which include butterflies taxonomy the branch of biology that names and classifies species formalizes this ordering of species into groups of increasing breadth see figure 1.14 you will learn more about this taxonomic scheme in chapter 26 for now we wm focus on kingdoms and domains the broadest units of classification b domain archaea 2 j.im many of the prokaryotes known as archaea live in earth s extreme environments such as salty lakes and boiling hot springs domain archaea includes multiple kingdoms the photo shows a colony composed of many cells the three domains of life until a few decades ago most biologists adopted a taxonomic scheme that divided the diversity of life into five kingdoms plants animals fungi single-celled eukaryotic organisms and prokaryotes since then new methods such as comparisons of on a sequences from different species have led to an ongoing reevaluation of the number and boundaries of kingdoms researchers have proposed anywhere from six kingdoms to dozens of kingdoms but as debate continues at the kingdom level there is a consensus that the kingdoms of life can now be grouped into three even higher levels of classification called domains the three domains are named bacteria archaea and eukarya figure 1.15 the organisms making up domain bacteria and domain archaea are all prokaryotic most prokaryotes are singlecelled and microscopic in the five-kingdom system bacteria i i c domain eukarya o.5l m protists multiple kingdoms are unicellular eukaryotes and their relatively simple multicellular relatives pictured here is an assortment of protists inhabiting pond water scientists are currently debating how to assign the protists to kingdoms that accurately reflect their evolutionary relationships kingdom fungi is defined in part by the nutritional mode of its members such as this mushroom which absorb nutrients from their surroundings kingdom animalia consists of multicellular eukaryotes that ingest other organisms kingdom plantae consists of multicellular eukaryotes that carry out photosynthesis the conversion of light energy to the chemical energy in lood figure 1.15 the three domains of life chapte one introduction themes in the study of life 13

[close]

p. 14

and archaea were combined in a single kingdom because they shared the prokaryotic form of cell structure but much evidence now supports the view that bacteria and archaea represent two very distinct branches of prokaryotic life different in key ways that you ll learn about in chapter 27 there is also evidence that archaea are at least as closely related to eukaryotic organisms as they are to bacteria all the eukaryotes organisms with eukaryotic cells are now grouped in domain eukarya in the era of the five-kingdom scheme most single-celled eukaryotes such as the microorganisms known as protozoans were placed in a single kingdom protista many biologists extended the boundaries ofkingdom protista to include some multicellular forms such as seaweeds that are closely related to certain unicellular protists the recent taxonomic trend has been to split the protists into several groups at the kingdom level in addition to these protistan groups domain eukarya includes three kingdoms of multicellular eukaryotes kingdoms plantae fungi and animalia these three kingdoms are distinguished partly by their modes of nutrition plants produce their own sugars and other foods byphotosynthesis fungi absorb dissolved nutrients from their surroundings many decompose dead organisms and organic wastes such as leaflitter and animal feces and absorb nutrients from these sources animals obtain food by ingestion which is the eating and digesting of other organisms animalia is of course the kingdom to which we belong unity in the diversity of life as diverse as life is it also displays remarkable unity earlier we mentioned the similar skeletons of different vertebrate animals but similarities are even more striking at the molecular and cellular levels for example the universal genetic language of dna is common to organisms as different as bacteria and animals unity is also evident in many features of cell structure figure 1.16 how can we account for life s dual nature of unity and diversity the process of evolution explained next illuminates both the similarities and differences in the world ofhfe and introduces another dimension of biology historical time charles darwin and the theory of natural selection the history of life as documented by fossils and other evidence is a saga of a changing earth billions of years old inhabited by an evolving cast of living forms figure 1.17 this evolutionary view of life came into sharp focus in november 1859 when charles robert darwin published one of the most important and influential books ever written entitled on the origin of species by means of natural selection darwin s book was an immediate bestseller and soon made ~darwin ism almost synonymous with the concept of evolution figure 1.18 cilia of paramecium the alia of the single-celled paramecium propel the organism through pond water 0,1 j.lm cilia of windpipe cells the cells that line the human windpipe are equipped with cilia that help keep the lungs clean by sweeping a film of debris-trapping mucus upward cross section of a cilium as viewed with an electron microscope figure 1.16 an example of unity underlying the diversity of life the architecture of cilia in eukaryotes cilia singular cilium are extensions of cells that fundion in locomotion they occur in eukaryotes as diverse as paramecia and humans even organisms so different share a common architecture for their cilia which have an elaborate system of tubules that is striking in cross-sectional views 14 chapte one introduction themes in the study of life

[close]

p. 15

figure 1.17 digging into the past paleontologist paul sereno of the university of chicago gingerly excavates the leg bones of a dinosaur fossil in niger the origin of species articulated two main points first darwin presented evidence to support his view that contemporary species arose from a succession ofancestors we will discuss the evidence for evolution in detail in chapter 22 darwin called this evolutionary history of species descent with modification it was an insightful phrase as it captured the duality of life s unity and diversity-unity in the kinship among species that descended from common ancestors diversity in the modifications that evolved as species branched from their com· mon ancestors figure 1.19 darwin s second main point was to proposea mechanism for descent v.ith modification he called this evolutionary mechanism natural selection darwin synthesized his theory of natural selection from observations that by themselves were neither new nor profound others had the pieces of the puzzle but darwin saw how they fit to figure 1.18 charles darwin as a young man gether he started with the following observations from nature individuals in a population vary in their traits many of which seem to be heritable passed on from parents to offspring also a population can produce far more offspring than can survive to produce offspring of their own with more individuals than the environment can support competition is inevitable lastly species generally suit their environments for instance birds living where tough seeds are a good food source may have especially strong beaks darwin made inferences from these observations to arrive at his theory of evolution he reasoned that individuals with inherited traits that are best suited to the local environment are more likely to survive and reproduce than less fit individuals over many generations a higher and higher proportion figure 1.19 unity and diversity in the orchid family these three rain forest orchids are variations on a common floral theme for example each of these flowers has a liplike petal that helps attrad pollinating inseds and provides a landing platform for the pollinators chapte one introduction themes in the study of life 15

[close]

Comments

no comments yet