WEB TUTORIAL 21.2 Alternation of Generations

Text Sections Section 21.5 Plants: The Foundation for Much of Life, p. 358 Introduction All plants go through an alternation of generations: a life cycle in which successive plant generations alternate between the diploid sporophyte condition and the hap- loid gametophyte condition. Learning Objectives • Understand what is meant by the phrase "alternation of generations." • Know the forms that a plant may take during its life cycle. Narration

Human Reproduction Almost all cells in human beings are diploid or 2n, meaning they contain paired sets of chromosomes. An exception to this is human gametes (eggs and sperm), produced through meiosis, which are haploid or 1n, meaning they have a single set of chromosomes. In the moment of conception, a haploid sperm fuses with a hap- loid egg to produce a diploid zygote that grows into a complete human being through mitosis.

Alternation of Generations A typical plant in its diploid phase will, like human beings, produce a specialized set of haploid reproductive cells. Instead of these cells being egg or sperm, howev- er, what's produced is a spore: a reproductive cell that can develop into a new organism without fusing with another reproductive cell. This organism represents a separate adult generation of the plant. These adults are called gametophytes because, through mitosis, they produce haploid gametes. These haploid gametes then fuse to form the diploid zygote that begins the new diploid generation.

In most higher plants the diploid generation is dominant to the much smaller game- tophyte, for example the Pine tree is a sporophyte and the gametophytes form is the pine cone.

You should now be able to… • Compare and contrast reproduction in humans and plants. • Explain the relationship between the sporophyte and gametophyte in a plant's life cycle. • Describe the products of mitosis and meiosis in a plant.