Plant Biology - Protista - Quick Guides | Biology

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Study GuidePlant Biology–Protista1. AlgaeAlgae are a diverse group of organisms, and they can look very different from one another. Somemarine algae, calledkelps, even resemble large plants. They havestem-like stalks,leaf-likeblades, and aholdfastthat anchors them to rocks on the sea floor. These kelps grow in cold northernoceans and can reach an incredible height of50–60 meters, making them thelargest protistsonEarth.However, most algae are much simpler and much smaller. Many areunicellular(made of just onecell) andmicroscopic. Others formcolonies,filaments,net-like structures, or longtubularbodies. Some move freely using flagella, while others remain fixed in place. This wide variety offorms shows how adaptable algae are to different environments.1.1How Do Scientists Classify Algae?Because algae come in so many shapes and sizes,body structure alone is not very usefulforclassifying them. Their reproductive structures also don’t help much. Instead, scientists focus onspecific features that are more reliable.Thefive main characteristicsused to classify algae are:1.Photosynthetic pigmentsthey contain2.Type of food they store3.Material of the cell wall4.Number, type, and position of flagella5.Details of cell structure, such as chloroplast shape or the presence of special structures likeeye spots or pyrenoidsA sixth and very important featureis now used:•DNA and RNA molecular sequencesModern genetic studies have greatly improved our understanding of algal relationships. In fact, thisnew data hasreshaped the algal family tree, changing many ideas scientists once believed werecorrect.

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Study Guide1.2Algae and the Origin of PlantsEven with these new discoveries, one idea remains strong: certain green algae are closely related toland plants. In particular, scientists agree that members of the ordersColeochaetalesandCharales,which belong to the classCharophyceae, are theclosest living relatives of land plants.1.3Why Are Green Algae “Green”?Green algae get their color fromchlorophyll a and b, along withcarotenoids, in the sameproportions found in green plants. This similarity goes beyond color:•Theystore starch inside plastids, just like plants•Most havecellulose cell walls•Some produce reproductive cells withtwo whiplash flagella, similar to plant sperm cellsBecause of these shared features, scientists long believed thatgreen algae were the ancestors ofland plants(also calledembryophytes).1.4Are Green Algae Plants?Today, scientists still debate how to classify green algae. Some biologists suggest that algae aresimplyaquatic green plantsand propose a broad group calledChlorobiota, which would includeboth aquatic algae and land plants.Others prefer to keep algae and land plants separate. At present,there is no clear agreement, andboth views are actively discussed in biology.SummaryAlgae may look simple, but they are incredibly diverse and important. They help scientists understandphotosynthesis,evolution, and even theorigin of plants on land. Studying algae gives us aclearer picture of how life adapted from water to land—and that makes them a fascinating topic inbiology!

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Study Guide2. A Mixture of Life Forms2.1Understanding the Kingdom ProtistaTheKingdom Protistais a very diverse group of organisms. In fact, it is so diverse that scientistsoften describe it as amixed collectionof life forms rather than a single, closely related group.Most protists share some features withplants, animals, or fungi, but they do not completely fit intoany one of these kingdoms. Because of this, scientists place them in a separate kingdom calledProtista (also called Protoctista).2.2Why Is Protista So Hard to Classify?Protists do not all come from one clear evolutionary line. Some are photosynthetic like plants, somebehave like animals, and others resemble fungi.Because of this confusion,cladists—scientists who classify organisms based on evolutionaryrelationships—have suggestedreorganizing the tree of life. Using modern tools likegenesequencing, they propose:•Creatingthree new kingdoms•Expanding theplant and animal kingdoms•Redistributing many protists into these new groupsThis idea is still debated among scientists and has not been universally accepted.2.3Why Protists Are Extremely ImportantEven though their classification is debated, protists areessential to life on Earth:•Many protists aresingle-celled, photosynthetic organisms•They are theprimary producersin marine and freshwater ecosystems•They form thebase of aquatic food chains•They have existed forover a billion yearsProtists also help scientists understandhow early life evolved. They show many different waysorganisms manage energy and survive. In many ways, protists representearly experiments in life,some of which eventually led to plants, animals, and fungi.

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