Plant Biology - Systematics - Quick Guides | Biology

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Study GuidePlant Biology–Systematics1. Plants Among the Diversity of OrganismsLiving organisms on Earth show an amazing variety. To make sense of this diversity, scientists classifyorganisms into groups. However, classification isnot fixed. It keeps changing as new scientific toolsand information become available—especially data fromDNA and RNA sequencing.Because scientists often interpret this data differently,disagreements are common. Some biologistsbelieve organisms that look and behave very differently should not be grouped together just becausetheir DNA is similar. On the other hand,cladistssupport grouping organisms based mainly on sharedancestry shown by molecular data, even if they look very different. This debate is a normal andhealthy part of science.1.1Major Groups and Modern Ways of Classifying OrganismsIn the18th century, classification took a major step forward due to the work ofCarl Linnaeus. Heintroduced a systematic way of naming and grouping organisms.Later, in the19th century,Charles Darwintransformed biology by explaining how organisms evolveover time and share common ancestors.By theend of the 20th century, another revolution occurred. Advances inmolecular sequencingreshaped our understanding of evolutionary relationships. Scientists realized that life on Earth can betraced back tothree major evolutionary lineages, all originating from acommon ancestor.

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Study GuideThese three major groups are now calledDomains, a level above kingdoms.The Three Domains of Life1.BacteriaoUnicellular organismsoNo true nucleus (prokaryotic)oIncludes groups like cyanobacteria and gram-positive bacteria2.Archaea(formerly called Archaebacteria)oAlso unicellular and prokaryoticoGenetically very different from bacteriaoOften live in extreme environments, such as hot springs or salty lakes3.EukaryaoOrganisms with a true nucleus (eukaryotic)oIncludesplants, animals, fungi, and protistsThe traditionalLinnaean hierarchywas modified to include this new top level called theDomain,making classification more accurate based on modern evidence.1.2Changes Within the Domain EukaryaRearranging groups withinEukaryahas caused even more debate. This is not because the data areunreliable, but because scientists differ in how to organizenew molecular informationalongsideolder, traditional knowledge.In the past, organisms were grouped intofive kingdoms. Today, those organisms are redistributedinto:•Four kingdoms within Eukarya•Two prokaryotic domains: Bacteria and Archaea1.3A Challenge for BotanistsThese changes raise an interesting question for plant scientists. What happens when organismstraditionally studied by botanists areremoved from the plant kingdom?Are botanists still plant scientists if they study organisms that are no longer classified as plants?Many textbooks still include these groups. The reason is simple:Even if some organisms are no longer officially “plants,” theyshare many plant-like features.Studying them still helps scientists understand plant structure, function, and evolution.

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Study Guide1.4Why Classification Keeps ChangingBiological classification is based oncurrent knowledge, and science is always progressing. As aresult:•Classifications are updated•Groups are rearranged•Differences of opinion remain commonRarely do all scientists agree completely. However, someolder group namesare still useful forinformal discussion and learning, even if they are no longer used in formal classification systems.2. Classifying Groups of OrganismsBiologists classify living organisms to better understand their similarities, differences, and evolutionaryrelationships. To do this, they focus onkey structural and functional features. These features helpscientists decide how organisms should be grouped in modern classification systems.2.1Features Used for ClassificationBiologists use the following characteristics to classify organisms:•Presence or absence of anucleus•Whether the organism isunicellular or multicellular•Mode of nutrition•Presence or absence of acell wall•Compositionof the cell wall•Ability to move (motility)•Type of reproduction•Kind of life cycleEach of these features gives important clues about how organisms are related.

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Study Guide2.2Presence or Absence of a NucleusThe most basic division of living organisms is based on whether their cells contain atrue nucleus.•Anucleusis a membrane-bound organelle that holds most of a cell’s DNA.•Prokaryotesdonothave a membrane-bound nucleus. Their DNA lies freely in the cell.•Eukaryotesdohave a well-defined nucleus.This single feature separates all life forms into two large groups.2.3Cellularity: One Cell or Many?Organisms can differ greatly in their body structure.•Unicellular organismsare made of just one cell.•Multicellular organismsconsist of many cells with specialized functions.Some unicellular organisms form:•Filaments(chains of cells),•Sheetsheld together by pectins, or•Coloniesthat look multicellular but are not true tissues.True multicellular organisms form:•Tissues–similar cells working together•Organs–groups of tissues performing specific functionsSome organisms even switch between unicellular and multicellular stages during their life cycle.All eukaryotes containorganelles, which are membrane-bound structures that perform specific jobsinside the cell.

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Study Guide2.4Nutrition: How Organisms Get EnergyAll living organisms need energy to carry outmetabolism, the chemical reactions that keep themalive.There are two main nutritional strategies:Autotrophs•Make their own organic food from inorganic substances.•Photoautotrophsuse sunlight throughphotosynthesis.•Chemoautotrophsuse chemical energy throughchemosynthesis.•Chloroplasts are found only in photoautotrophs.Heterotrophs•Cannot make their own food.•Obtain nutrients from organic matter made by autotrophs.Different heterotrophs feed in different ways:•Animalsingest food and digest it internally.•Fungirelease enzymes outside their bodies and absorb nutrients.•Manyprotistsusephagotrophy, engulfing food particles.•Saprophagesfeed on dead and decaying organic matter.2.5Cell Wall: Present or Absent?•Animals and animal-like protistsdo not have cell walls.•Most other organisms do, but thecomposition differs:oBacteria: cell walls containpeptidoglycanoArchaea: lack peptidoglycanoFungi: walls made ofchitinoPlants and many algae: walls made ofcellulosePlant cell walls may also contain extra substances such as lignin, waxes, and suberin.

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Study Guide2.6Motility: Ability to MoveMany organisms can move, but movement alone does not define a group.•Sessile organismsstay attached to a surface.•Even sessile organisms may havemotile cells.Movement usually occurs using:•Flagella–long, whip-like structures•Cilia–shorter, hair-like structuresImportant differences:•Prokaryotic flagellarotate and are made of a single fiber.•Eukaryotic flagella and ciliahave a9 + 2 microtubule structureand move by slidingaction.2.7Type of ReproductionReproduction creates new individuals and can occur in two main ways.Asexual Reproduction•No fusion of sex cells (gametes).•Occurs in all kingdoms.•Common types include:oFission–cell splits into twooBudding–new individual grows from the parentoSpore formation–spores form in structures calledsporangiaSexual Reproduction•Involves fusion ofhaploid gametes.•Fusion (fertilization) forms adiploid zygote.•Found in all groups except Archaea.Some organisms producesexual spores, which are formed through meiosis.

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