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Lecture 9

BIO153 Lecture 9.pdf

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Christoph Richter

2009 BIO153: Lecture 9 Plants (I) February 9, 2009 Plants are a clade composed of the green algae or Chlorophyta (which are classified in cotemporary taxonomies as protists) and the land plants. Our current thinking is that green algae themselves are a grade (the Charales are the sister taxon of the land plants); the land plants (the non-vascular plants; the seedless vascular plants; the gymnosperms and angiosperms) are a clade, and the land plants + the green algae are a clade. Green algae vary a great deal in their structure (some are unicellular; some colonial, some multicellular etc.), but in many ways they are similar to land plants: they have: ▯ chloroplasts with double membranes ▯ chlorophyll a & b ▯ cellulose in the cell wall The first fossils of green algae are from ~700 million years ago, and a corresponding rise in O2levels is observed in the geological record. (My favourite green alga: Chlamydomonas nivalis, which causes watermelon snow!) 1 Green algae are ecologically important today because of their contribution to global photosynthesis. Also: ▯ green algae in symbiosis with fungi form lichen ▯ some live as endosymbionts in protists Charales (stoneworts) are green algae that are the sister taxon of the land plants ▯ features of growth & reproduction more complex than most green algae ▯ multicellular individuals are haploid, but gametes can fuse and form a diploid zygote ▯ diploid zygote undergoes immediate meiosis to restore haploidy; divides to form a haploid gametophyte (TYPE I life cycle; see below). Land Plants: ▯ 1 truly terrestrial organisms (land plants originated about 470 million years ago) ▯ are a clade (suggests that the transition to land happened once) ▯ form the foundation of terrestrial food webs (major terrestrial autotroph) ▯ allowed for the terrestrial invasion (i.e. colonization of land) by fungi and animals Features of land plants ▯ in general, all are autotrophs (with a few exceptions -- a few species have developed a purely parasitic lifestyle and no longer make their own food) ▯ they exhibit an enormous range of size:~ 1mm ▯ ~100m ▯ most are sessile (a few aquatic plants such as Lemma (duckweed) are planktonic- float or drift in water) ▯ Plants may be extremely long-lived: some individuals of the creosote bush (Larrea spp.) may be greater than 14,000 years old (longer than recorded human history)! 2 The story of land plants is the story of adaptation to life out of water What are the benefits of life on land? ▯ unimpeded access to CO , 2unlight ▯ compared to living in the water: no absorption of light by the medium; no reflection at surface; no turbidity ▯ thus, much higher rates of photosynthesis are possible! However, there are problems with moving to land from water: ▯ air provides no support (no buoyancy as in water) ▯ air provides no nutrients (water has many dissolved nutrients) ▯ water loss and access becomes an issue ▯ terrestrial environments experience a greater temperature range ▯ completing the life-cycle becomes problematic Modern plant structure = evolutionary response to these factors 1. Physiological/structural adaptations to life in air: ▯ structural support: a woody skeleton fortified with lignin ▯ nutrient & water transport: roots; a vascular system ▯ access to water: roots & root hairs (root hairs increase surface area for absorption), mycorrhizae (symbiotic relationship with fungi that increases absorption of nutrients & water – we’ll discuss these further in the lectures on Fungi) ▯ control of loss: waxy cuticle on leaves and stems; stomata (pores) on leaves control movement of water and gases The transpiration stream: a necessary evil – keep in mind that plants, unlike animals, do not have muscles to pump nutrients and water around 3 the body and thus must rely on solute gradients and transpiration to accomplish these tasks ▯ to transport water, water must be lost! ▯ capillary action moves H 2O 2-3m at most (and slowly!) ▯ most water movement is driven by evaporation at the leaves, which creates a water pressure deficit, causing water to move down the pressure gradient (i.e., from the roots to the leaves) ▯ in order to move water upwards from the roots, a large tree such as a redwood may lose 160 gal./d through evaporation at the leaf surface Reproduction out of water: Green algae in water complete their life cycle entirely in the water: swimming gametes (haploid cells) can meet and fuse to form a diploid zygote in an aquatic environment. Gametes are unprotected cells that can quickly dry out, so reproduction is harder to complete in the terrestrial environment. 2. Adaptations to the life cycle in response to life out of water: ▯ reduction of reliance on swimming gametes ▯ alternation of generations** ▯ land plants are called embryophytes (eggs, zygotes & embryos are retained on parent plant as an adaptation to prevent desiccation of these delicate structures) Thus, along with many structural/physiological adaptations to life on land, the evolution of land plants involves changes in the life cycle: In order to make sense of the evolutionary changes in the plant life cycle (and the diversity of life cycles in plants), it’s necessary to step back and talk about the possible life cycles in sexual organisms. 4 Sexually reproducing organisms all have haploid and diploid phases to the life cycle. (See the webcast from Friday Feb 6 tin the lab folder for a discussion of the three types of life cycle.) There are 3 general types of life cycles in all sexually reproducing organisms (fungi have a weird variation on one of these cycles that we will discuss in an upcoming lecture). Recall that the first eukaryotic organisms were haploid & asexual. The beginning of sexual reproduction (the creation of genetically distinct offspring): fusion of 2 haploid individuals to form a 2N zygote. Immediate meiosis restores haploidy. By immediate, I mean that there is no proliferation (mitosis) of the zygote. The zygote does not type I: zygotic meiosis become an embryo – it is a single diploid cell that undergoes meiosis to produce haploid cells genetically distinct from the haploid individuals that fused to produce the zygote. This is the Type I mitosis gametes life cycle (also known as zygotic meiosis). The Type I life cycle: the ancestral form of the sexual life cycle haploid ▯ most of the life cycle is haploid meiosis diploid zygote ▯ If there is a multicellular phase, the only multicellular phase is the haploid phase ▯ no multicellular diploid phase; only diploid part is the zygote ▯ This type of life cycle is found in green algae: e.g. Chlamydomonas, which stays unicellular, or Charales, which have multicellular haploid gametophyte (more about this term later) type II: gametic meiosis
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