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Plant Biology - Systematics

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    Study Guide Plant Biology – Systematics 1. Plants Among the Diversity of Organisms Living organisms on Earth show an amazing variety. To make sense of this diversity, scientists classify organisms into groups. However, classification is not fixed . It keeps changing as new scientific tools and information become available — especially data from DNA and RNA sequencing . Because scientists often interpret this data differently, disagreements are common . Some biologists believe organisms that look and behave very differently should not be grouped together just because their DNA is similar. On the other hand, cladists support grouping organisms based mainly on shared ancestry shown by molecular data, even if they look very different. This debate is a normal and healthy part of science. 1.1 Major Groups and Modern Ways of Classifying Organisms In the 18th century , classification took a major step forward due to the work of Carl Linnaeus . He introduced a systematic way of naming and grouping organisms. Later, in the 19th century , Charles Darwin transformed biology by explaining how organisms evolve over time and share common ancestors. By the end of the 20th century , another revolution occurred. Advances in molecular sequencing reshaped our understanding of evolutionary relationships. Scientists realized that life on Earth can be traced back to three major evolutionary lineages , all originating from a common ancestor .

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    Study Guide These three major groups are now called Domains , a level above kingdoms. The Three Domains of Life 1. Bacteria o Unicellular organisms o No true nucleus (prokaryotic) o Includes groups like cyanobacteria and gram - positive bacteria 2. Archaea (formerly called Archaebacteria) o Also unicellular and prokaryotic o Genetically very different from bacteria o Often live in extreme environments, such as hot springs or salty lakes 3. Eukarya o Organisms with a true nucleus (eukaryotic) o Includes plants, animals, fungi, and protists The traditional Linnaean hierarchy was modified to include this new top level called the Domain , making classification more accurate based on modern evidence. 1.2 Changes Within the Domain Eukarya Rearranging groups within Eukarya has caused even more debate. This is not because the data are unreliable, but because scientists differ in how to organize new molecular information alongside older, traditional knowledge . In the past, organisms were grouped into five kingdoms . Today, those organisms are redistributed into: • Four kingdoms within Eukarya • Two prokaryotic domains : Bacteria and Archaea 1.3 A Challenge for Botanists These changes raise an interesting question for plant scientists. What happens when organisms traditionally studied by botanists are removed 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,” they share many plant - like features . Studying them still helps scientists understand plant structure, function, and evolution.

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    Study Guide 1.4 Why Classification Keeps Changing Biological classification is based on current knowledge , and science is always progressing. As a result: • Classifications are updated • Groups are rearranged • Differences of opinion remain common Rarely do all scientists agree completely. However, some older group names are still useful for informal discussion and learning, even if they are no longer used in formal classification systems. 2. Classifying Groups of Organisms Biologists classify living organisms to better understand their similarities, differences, and evolutionary relationships. To do this, they focus on key structural and functional features . These features help scientists decide how organisms should be grouped in modern classification systems. 2.1 Features Used for Classification Biologists use the following characteristics to classify organisms: • Presence or absence of a nucleus • Whether the organism is unicellular or multicellular • Mode of nutrition • Presence or absence of a cell wall • Composition of the cell wall • Ability to move ( motility ) • Type of reproduction • Kind of life cycle Each of these features gives important clues about how organisms are related.

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    Study Guide 2.2 Presence or Absence of a Nucleus The most basic division of living organisms is based on whether their cells contain a true nucleus . • A nucleus is a membrane - bound organelle that holds most of a cell’s DNA. • Prokaryotes do not have a membrane - bound nucleus. Their DNA lies freely in the cell. • Eukaryotes do have a well - defined nucleus. This single feature separates all life forms into two large groups. 2.3 Cellularity: One Cell or Many? Organisms can differ greatly in their body structure. • Unicellular organisms are made of just one cell. • Multicellular organisms consist of many cells with specialized functions. Some unicellular organisms form: • Filaments (chains of cells), • Sheets held together by pectins, or • Colonies that look multicellular but are not true tissues. True multicellular organisms form: • Tissues – similar cells working together • Organs – groups of tissues performing specific functions Some organisms even switch between unicellular and multicellular stages during their life cycle. All eukaryotes contain organelles , which are membrane - bound structures that perform specific jobs inside the cell.

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    Study Guide 2.4 Nutrition: How Organisms Get Energy All living organisms need energy to carry out metabolism , the chemical reactions that keep them alive. There are two main nutritional strategies: Autotrophs • Make their own organic food from inorganic substances. • Photoautotrophs use sunlight through photosynthesis . • Chemoautotrophs use chemical energy through chemosynthesis . • 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: • Animals ingest food and digest it internally. • Fungi release enzymes outside their bodies and absorb nutrients. • Many protists use phagotrophy , engulfing food particles. • Saprophages feed on dead and decaying organic matter. 2.5 Cell Wall: Present or Absent? • Animals and animal - like protists do not have cell walls. • Most other organisms do, but the composition differs : o Bacteria : cell walls contain peptidoglycan o Archaea : lack peptidoglycan o Fungi : walls made of chitin o Plants and many algae : walls made of cellulose Plant cell walls may also contain extra substances such as lignin, waxes, and suberin.

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    Study Guide 2.6 Motility: Ability to Move Many organisms can move, but movement alone does not define a group. • Sessile organisms stay attached to a surface. • Even sessile organisms may have motile cells . Movement usually occurs using: • Flagella – long, whip - like structures • Cilia – shorter, hair - like structures Important differences: • Prokaryotic flagella rotate and are made of a single fiber. • Eukaryotic flagella and cilia have a 9 + 2 microtubule structure and move by sliding action. 2.7 Type of Reproduction Reproduction 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: o Fission – cell splits into two o Budding – new individual grows from the parent o Spore formation – spores form in structures called sporangia Sexual Reproduction • Involves fusion of haploid gametes . • Fusion (fertilization) forms a diploid zygote . • Found in all groups except Archaea. Some organisms produce sexual spores , which are formed through meiosis.

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