BIO 202 Chapter 28, Spring 2018

© 2017 Pearson Education, Ltd. Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge 28 Green Algae and Land Plants Chapter 28 Opening Roadmap. © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Introduction • The Viridiplantae, or green plants, consist of the green algae and land plants • Green algae are important photosynthetic organisms in freshwater habitats • Land plants are the key photosynthesizers in terrestrial environments © 2017 Pearson Education, Ltd. Introduction • Green algae have traditionally been considered protists, but we study them along with land plants for two reasons: 1. They are the closest living relatives to land plants 2. The transition from aquatic to terrestrial life occurred when land plants evolved from green algae © 2017 Pearson Education, Ltd. Introduction • Land plants were the first organisms that could thrive with their tissues completely exposed to the air • Before land plants evolved, terrestrial life was limited to bacteria, archaea, and single-celled protists • Plants transformed the nature of life on Earth © 2017 Pearson Education, Ltd. Why Do Biologists Study Green Plants? • Biologists study plants not only because they are fascinating organisms but also because we could not live without them • People rely on plants for food, fuel, the fibers used to make clothing and other products, building materials, and pharmaceuticals • Agriculture, forestry, and horticulture are among the most important industries based on plants © 2017 Pearson Education, Ltd. Why Do Biologists Study Green Plants? • Biologists also study plants that cause problems for humans: • Weeds that decrease crop productivity • Newly introduced species that invade and then degrade natural areas © 2017 Pearson Education, Ltd. Plants Provide Ecosystem Services • An ecosystem consists of • All of the organisms in a particular area • Nonliving physical components of the environment, such as • The atmosphere • Precipitation • Surface water • Sunlight • Soil • Nutrients © 2017 Pearson Education, Ltd. Plants Provide Ecosystem Services • Ecosystem services provided by green algae and land plants include • Producing oxygen via oxygenic photosynthesis • Building soil by providing food for decomposers • Holding soil and preventing nutrients from being lost to wind or water erosion • Holding water in the soil • Moderating the local climate by providing shade and reducing the impact of wind on landscapes Figure 28.1 © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Plants Are Primary Producers • Land plants are the dominant primary producers in terrestrial ecosystems • They convert energy in sunlight into chemical energy • The sugars produced by land plants support virtually all other organisms in terrestrial habitats © 2017 Pearson Education, Ltd. Plants Are Primary Producers • Land plants are the key to the carbon cycle • Take CO2 from the atmosphere and reduce it to make sugars • Fix much more CO2 than they release • The loss of plant-rich prairies has contributed to increased concentrations of CO2 in the atmosphere • Partly responsible for rising temperatures associated with global climate change © 2017 Pearson Education, Ltd. Plants Provide Humans with Food, Fuel, Fiber, Building Materials, and Medicines • Food • Agricultural research began with the initial domestication of crop plants • Artificial selection for plants with certain properties has led to dramatic changes in plant characteristics Figure 28.2 (a) Plants were domesticated at an array of locations. Wheat Sunflower Maize Potato Millet Barley Rice Soybeans (b) Artificial selection changes the traits of domesticated species. Domestic maize Wild maize (teosinte) © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Plants Provide Humans with Food, Fuel, Fiber, Building Materials, and Medicines • Fuel • Historically, humans relied on wood burning for energy • In industrialized countries, fossil fuels including coal and petroleum or natural gas have replaced wood • Fossil fuels are derived from plants—living or long-deceased © 2017 Pearson Education, Ltd. Plants Provide Humans with Food, Fuel, Fiber, Building Materials, and Medicines • Fiber and Building Materials • Plants provide us with important sources of raw material for clothing, rope, and household articles such as towels and linens • Woody plants provide • Lumber for houses and furniture • Fibers used to make paper © 2017 Pearson Education, Ltd. Plants Provide Humans with Food, Fuel, Fiber, Building Materials, and Medicines • Medicines • About 25% of the prescriptions written in the United States each year include at least one molecule derived from plants • Most of these compounds are synthesized by plants to repel herbivores Table 28.1 © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. How Do Biologists Study Green Algae and Land Plants ? • To understand diversification: 1. Compare morphological traits 2. Analyze the fossil record 3. Estimate phylogenetic trees © 2017 Pearson Education, Ltd. Analyzing Morphological Traits • The green algae include species that • Are unicellular, colonial, or multicellular • Live in marine, freshwater, or moist terrestrial habitats • The vast majority of green algae are aquatic • The vast majority of land plants are terrestrial © 2017 Pearson Education, Ltd. Similarities between Green Algae and Land Plants • Biologists have long hypothesized that green algae are closely related to plants on the basis of several key traits: • Their chloroplast structure is the same • Their thylakoid arrangements are similar • Their cell walls, sperm, and peroxisomes are similar in structure and composition • Their chloroplasts synthesize starch as a storage product © 2017 Pearson Education, Ltd. Major Morphological Differences among Land Plants 1. Nonvascular plants • Lack vascular tissue • Specialized groups of cells that conduct water or dissolved nutrients throughout the plant body • Include mosses • Use spores, not seeds, for reproduction and dispersal © 2017 Pearson Education, Ltd. Major Morphological Differences among Land Plants 2. Seedless vascular plants • Have well-developed vascular tissue • Do not make seeds; use spores for reproduction • Includes ferns © 2017 Pearson Education, Ltd. Major Morphological Differences among Land Plants 3. Seed plants • Have vascular tissue • Make seeds • Seeds consist of an embryo and a store of nutritive tissue, surrounded by a tough protective layer • Include angiosperms (“encased-seeds”), or flowering plants and gymnosperms (“naked seeds’) © 2017 Pearson Education, Ltd. Preventing Water Loss: Cuticle and Stomata • Cuticle is a watertight sealant that covers the aboveground parts of the plant and gives them the ability to survive in dry environments • However, cuticle also keeps CO2 out of plant • Most plants solve this problem with stoma (“mouth”; plural: stomata) that consists of an opening surrounded by specialized guard cells • Pore opens and closes as guard cells change shape • Gas exchange accomplished through pore Figure 28.7 (a) Cuticle is a waxy layer that prevents water loss from stems and leaves. Leaf cross section Cuticle Moist photosynthetic cells (b) Stomata have pores that allow gas exchange in photosynthetic tissues. Pore 25 µm Guard cell Cuticle Stoma © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Providing Protection from UV Irradiation • Plants out of water exposed to harmful UV rays of the sun • UV light damages DNA by causing thymine dimers • Water absorbs UV light, so algae did not face this problem to the same extent © 2017 Pearson Education, Ltd. Providing Protection from UV Irradiation • Many species of algae probably colonized wet soil near home ponds, but only some were able to survive the harsh sunlight • The plants that survived were those that by chance made compounds that absorb UV light • Most plants today accumulate UV-absorbing compounds (flavonoids) that protect DNA from damage © 2017 Pearson Education, Ltd. The Importance of Upright Growth • First land plants were small or had a low, sprawling growth habit • Had to grow in a way that kept many or most of their tissues in direct contact with moist soil • Competition for space and light would have become intense soon after the first plants began growing on land • In a terrestrial environment, individuals that can grow upright have much better access to sunlight than individuals that cannot © 2017 Pearson Education, Ltd. Elaboration of Vascular Tissue: Tracheids and Vessels • Simple water-conducting tissues evolved by natural selection into more complex, efficient supportive and water-conducting tissues • Long, thin, tapering, water-conducting cells called tracheids evolved about 380 mya © 2017 Pearson Education, Ltd. Elaboration of Vascular Tissue: Tracheids and Vessels • Tracheids have • A thickened, lignin-containing secondary cell wall in addition to a cellulose-based primary cell wall • Pits in the sides and ends of the cell, which allow water to flow efficiently between tracheids • The secondary cell wall increased structural support, but water could still move easily through the cells because of the pits • Today, all vascular plants contain tracheids © 2017 Pearson Education, Ltd. Elaboration of Vascular Tissue: Tracheids and Vessels • Vessel elements, the most specialized type of water- conducting cell, appeared about 250 to 270 mya • Vessel elements • Are shorter and wider than tracheids • Have gaps on both ends where both cell walls are missing • In stems and branches of some vascular plants, tracheids or a combination of tracheids and vessels form wood • Strong support material © 2017 Pearson Education, Ltd. Mapping Evolutionary Changes on the Phylogenetic Tree • Cuticle, stomata, and vascular tissue were key adaptations that allowed early plants to colonize land • Fundamental adaptations to dry conditions—such as cuticle, pores, stomata, vascular tissue, and tracheids—evolved just once • Convergent evolution also occurred, as vessels evolved independently in angiosperms, gnetophytes, and several species of seedless vascular plants © 2017 Pearson Education, Ltd. How Do Plants Reproduce in Dry Conditions? • Life cycles of sexually reproducing eukaryotes serve several functions: • Increasing genetic variability due to meiosis and fertilization • Increasing the number of individuals • Dispersing individuals to new habitats © 2017 Pearson Education, Ltd. How Do Plants Reproduce in Dry Conditions? • Three innovations were instrumental for efficient plant reproduction in a dry environment: 1. Spores that resist drying because they are encased in a tough coat of sporopollenin 2. Gametes that were produced in complex, multicellular structures (gametangia: archegonie and antheridia) 3. Embryos that were retained on and nourished by the parent plant © 2017 Pearson Education, Ltd. Desiccation-Resistant Spores • Spores resist drying because they are encased in a tough coat of sporopollenin • Tough outer coating of spores helps them survive for fairly long periods of time • Can be dispersed hundreds of miles by wind currents • Resistant, lightweight nature of spores helps increase their chances of being dispersed to a suitable environment for growth © 2017 Pearson Education, Ltd. Protective, Complex Reproductive Organs • Individuals produce distinctive male and female gametangia • Antheridium (plural: antheridia)—sperm-producing • Archegonium (plural: archegonia)—egg-producing • Analogous to the testes and ovaries of animals Figure 28.10 Male Antheridia (containing sperm) Female Eggs Archegonia Moss gametophytes are either male or female 0.2 mm 0.2 mm © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Embryos Nourished by Parental Tissues • Instead of shedding their eggs into the water or soil, as did their ancestors, land plants retain them • Eggs form inside archegonia • In contrast to most green algae, zygotes of land plants • Begin development on the parent plant • Form multicellular embryos that remain attached to and can be nourished by the parent plant © 2017 Pearson Education, Ltd. Embryos Nourished by Parental Tissues • The retention of the embryo was a key event in land plant evolution • The group’s formal name is Embryophyta, literally, the “embryo-plants” • The retention of the fertilized egg in embryophytes is analogous to pregnancy in mammals © 2017 Pearson Education, Ltd. Alternation of Generations • All land plants undergo alternation of generations, in which individuals have • A multicellular haploid phase called the gametophyte • A multicellular diploid phase known as the sporophyte • The two phases of the life cycle are connected by distinct types of reproductive cells—gametes and spores © 2017 Pearson Education, Ltd. Alternation of Generations • Alternation of generations does not occur in the algal groups most closely related to land plants Figure 28.11 Haploid (n) Diploid (2n) Spores (n) Multicellular adult (n) Zygote (2n) (retained on parent) Sperm (n) Gametes are produced in gametangia Egg (n) © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Alternation of Generations • Alternation of generations always involves the same sequence of events: 1. The sporophyte produces haploid spores by meiosis 2. Spores germinate and divide by mitosis and develop into multicellular, haploid gametophytes 3. Gametophytes produce unicellular haploid gametes by mitosis 4. Two gametes unite during fertilization to form a diploid zygote 5. The zygote divides by mitosis and develops into a multicellular, diploid sporophyte Figure 28.12 Haploid (n) Diploid (2n) Spores (n) (2n) Sporophyte (multicellular, diploid) Gametophyte (multicellular, haploid) (n) Zygote (2n) Gametes (n) 2 3 3 4 1 5 MITO SIS © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Alternation of Generations • Compare and contrast zygotes, spores, and gametes: • Zygotes and spores are both single cells that divide by mitosis to form a multicellular individual. Zygotes develop into sporophytes; spores develop into gametophytes • Zygotes are diploid and spores and gametes are haploid • Zygotes result from the fusion of two haploid cells, such as a sperm and an egg, but spores are not formed by the fusion of gametes • Spores are produced by meiosis inside structures called sporangia; gametes are produced by mitosis inside gametangia © 2017 Pearson Education, Ltd. From Gametophyte-Dominant to Sporophyte- Dominant • In nonvascular plants, sporophyte is small and short lived and is largely dependent on gametophyte for nutrition • Gametophyte-dominant life cycle • In ferns and other vascular plants, sporophyte is much larger and longer lived than gametophyte • Sporophyte-dominant life cycle • Gametophytes of gymnosperms and angiosperms are microscopic Figure 28.13 Haploid (n) Mature sporophyte (2n) Diploid (2n) Spores dispersed by wind (n) Developing gametophytes (n) Sperm swim to egg Developing sporophyte (2n) Mature female gametophyte (n) Zygote (2n) Egg (n) Sperm develop in antheridia 2 µm Mature female gametophyte (n) Mature male gametophyte (n) FERTILIZATION Archegonium 2 µm Eggs develop in archegonia © 2017 Pearson Education, Ltd. Figure 28.14 Developing gametophyte (n) 1 mm Haploid (n) Spores are produced in sporangia Spores dispersed by wind (n) Diploid (2n) Mature gametophyte (n) Sporophyte (2n) Sperm (n) develop in antheridia Zygote (2n) Sperm swim to egg Egg (n) Gametophyte (n; side view) Mature sporophyte (2n) Archegonium Eggs develop in archegonia © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. From Gametophyte-Dominant to Sporophyte- Dominant • The transition from gametophyte-dominated life cycles to sporophyte-dominated life cycles is striking • Sporophyte-dominated life cycles were advantageous • Diploid cells can respond to varying environmental conditions more efficiently than haploid cells can • This is especially true if the individual is heterozygous at many genes © 2017 Pearson Education, Ltd. Heterospory • Heterospory—production of two distinct types of spores by different structures • All of the nonvascular plants and most of the seedless vascular plants are homosporous • Produce a single type of spore • Spores develop into bisexual gametophyte that produces both eggs and sperm • These gametophytes can self-fertilize and produce offspring Figure 28.16a (a) Nonvascular plants and most seedless vascular plants are homosporous. © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Heterospory • The two types of spore-producing structures in heterosporous species are microsporangia and megasporangia • Microsporangia produce microspores that develop into male gametophytes, which produce small gametes called sperm • Megasporangia produce megaspores that develop into female gametophytes, which produce large gametes called eggs • The gametophytes of seed plants are either male or female, but never both Figure 28.16b (b) Seed plants are heterosporous. © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Pollen • Sperm of the nonvascular and seedless vascular plants have to swim to the egg to fertilize it • Water must be available for fertilization to occur • Pollen grain in land plants allowed plants living in dry habitats to reproduce efficiently • Tiny male gametophytes surrounded by a tough coat of sporopollenin • Can be exposed to air for long periods of time without drying out • Carried to female gametophyte by wind or animals © 2017 Pearson Education, Ltd. Seeds • The evolution of embryo retention had a downside: • In ferns and horsetails, sporophytes have to live in the same place as their parent gametophyte • Seed plants overcome this limitation • Embryos of seed plants are portable and can disperse to new locations © 2017 Pearson Education, Ltd. Seeds • A seed is a structure that includes an embryo and a store of nutrients provided by the mother and surrounded by a tough, protective coat • Spores are an effective dispersal stage for nonvascular plants and seedless vascular plants, but they lack the stored nutrients found in seeds Figure 28.17 Embryo Nutritive tissue Protective coat 1 mm © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Seeds • Life cycle of a pine tree—typical heterospory in gymnosperms: 1. Starting with the sporophyte on the left, note separate structures, called cones, where microsporangia and megasporangia develop 2. Microsporangia contain microspore mother cells that divide by meiosis to form microspores, which then divide by mitosis to form pollen grains 3. Megasporangia are found inside protective structures called ovules. Each megasporangium contains a megaspore mother cell that divides by meiosis to form a megaspore © 2017 Pearson Education, Ltd. Seeds • Life cycle of a pine tree—typical heterospory in gymnosperms: 4. Megaspore undergoes mitosis to form the female gametophyte, which then produces egg cells through further rounds of mitosis 5. The female gametophyte stays attached to the sporophyte as pollen grains arrive and produce sperm that fertilize the eggs 6. Seeds mature as the embryo develops. Inside the seed, cells derived from the female gametophyte become packed with nutrients provided by the sporophyte