Sea Star Life Cycle

Animals are multicellular, heterotrophic eukaryotes that obtain nutrients by ingestion. Ingestion means eating food. Most animals are diploid and reproduce sexually. Male and female adult animals make haploid gametes by meiosis, and an egg and a sperm fuse, producing a zygote. The zygotes divides by mitosis, forming an early embryonic stage called a blastula, which is usually a hollow ball of cells. In the sea star and most other animals, one side of the blastula folds inward, forming a stage called a gastrula. The internal sac formed by gastrulation becomes the digestive tract, lined by a cell layer called the endoderm. The embryo also has an ectoderm, an outer cell layer that gives rise to the outer covering of the animal and, in some phyla, to the central nervous system. Most animals have a third embryonic layer, known as the mesoderm, which forms the muscles and most internal organs. After the gastrula stage, many animals develop directly into adults. Others, such as the sea star, develop into one or more larval stages first. A larva is an immature individual that looks different from the adult mammal. The larva undergoes a major change of body form, called metamorphosis, in becoming an adult capable of reproducing sexually. Biology book pg 366

 

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Sym Gene Testing

The hypothesized importance of sym genes to plants also predicts that their function should have changed little over time. That is, the sym genes of liverworts should work similarly to those of flowering plants. Scientists tested this prediction by using Agrobacterium to introduce a functional liverwort sym gene into an angiosperm possessing a nonfunctional version of the gene that could not form mycorrhizae. The investigators then applied fungal spores to the roots of transgenic plants and a control group of mutant plants. After several weeks, mycorrhizae were present in the transgenic plants--the result of the liverwort sym gene functioning normally. The control plants had no mycorrhizae. -Biology book pg 360

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Fungi Life Cycle

Fungal reproduction typically involves the release of vast numbers of haploid spores, which are transported easily over great distances by wind or water. When the hyphae meet, their cytoplasms fuse. But this fusion of cytoplasm is often not followed immediately by the fusion of "parental" nuclei. Thus, many fungi have what is called a heterokaryotic stage, in which cells contain two genetically distinct haploid nuclei. Hours, days, or even centuries may pass before the parental nuclei fuse, forming the usually short-lived diploid phase. Zygotes undergo meiosis producing haploid spores. In asexual reproduction, spore-producing structures arise from haploid mycelia have undergo neither a heterokaryotic stage nor meiosis. Many fungi that reproduce sexually can also produce spores asexually. In addition, asexual reproduction is the only known means of spore production in some fungi, informally known as imperfect fungi. Mold and yeast is also imperfect fungi. The term mold refers to any rapidly growing fungi that reproduces asexually by producing spores, often at the tips of specialized hyphae. The term yeast refers to any single-celled fungus. -Biology book pg 356

 

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Angiosperm Life Cycle

In angiosperms, the sporophyte generation is dominant and produces the gametophyte generation within its body. Meiosis in the anthers of the flower produces haploid spores that undergo mitosis and form the male gametophytes or pollen grains. Meiosis in the ovule produces a haploid spore that undergoes mitosis and forms the few cells of the female gametophyte, one of which becomes an egg. Pollination occurs when a pollen grain, carried by the wind or an animal, lands on the stigma. As in gymnosperms, a tube grows from the pollen grain to the ovule, and a sperm fertilizes the egg, forming a zygote. A seed develops from each ovule. While the seeds develop, the ovary's wall thickens, forming the fruit that encloses the seeds. When conditions are favorable, a seed germinates, which means it began to grow. Biology book pg 351

 

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Plant Life Cycles

Plants have cycles that are very different from ours. Humans are diploid individuals--that is, each of us have two sets of chromosomes, one from each parent. Gametes are the only haploid stage in the human life cycle. Plants have an alternation of generations: the diploid and haploid stages are distinct, multicellular bodies. The haploid generation of a plant produces gametes and is called the gametophyte. The diploid generation produces spores and is called the sporophyte. In a plant's life cycle, these two generations alternate in producing each other. In mosses, as in all nonvascular plants, the gametophytes is the larger, more obvious stage of the life cycle. Ferns, like most plants, have a life cycle dominated by the sporophyte, Today about 95% of all plants, including all seeds plants, have a dominant sporophyte in their life cycle. The life cycles of all plants follow a pattern. -Biology book pg. 346

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Plant Evolution

After plants originated from an algal ancestor approximately 470 million years ago, early diversification gave rise to seedless, non-vascular plants, including mosses, liverworts, and hornworts. These plants are formally called bryophytes, resemble other plants in having apical meristems and embryos that are retained on the parent plant, but they lack true roots and leaves. Without lignified cell walls, bryophytes with an upright growth habit lack support. The origin of vascular plants occurred about 425 million years ago. Their lignin-hardened vascular tissues provide strong support, enabling stems to stand upright and grow tall on land. Two clades of vascular plants are informally called seedless vascular plants: lycophytes and widespread monilophytes. The first vascular plants with seeds evolved about 360 million years ago. Seeds and pollen are key adaptions that improved the ability of plants to diversify in terrestrial habitats. A seed consists of an embryo packaged with a food supply within a protective covering. -Biology book pg. 344

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