Plant Life Cycle – Alternation of Generations

Today, I will start posting about plants. Over the next two weeks, leading up the Praxis II Biology Exam on June 9th, I will spend some time writing posts about areas of biology that I think might be important for the exam. The first major concept that I would like to review is the plant life cycle, in particular, alternation of generations.

Plants spend part of their life as diploid individuals and other parts as haploid. This is in stark contrast to animals where we pretty much spend our entire lives as a diploid. Diploid means that we have a double set of chromosomes. Humans have 23 unique chromosomes, but double that up and each of our cells (aside from our gametes) have 46 chromosomes. Haploid is the term used to define cells with just one unique set of chromosomes. Sperm and eggs only have 23 chromosomes, they are haploid.

Land plants can form two different types of multicellular organisms: a haploid individual called a gametophyte and a diploid individual called a sporophyte. Gametophytes produces (you guessed it!) haploid gametes by mitosis. These gametes will join together (egg and a sperm) in a process called fertilization to form a diploid zygote. The zygote grows into a multicellular organism called a sporophyte (again diploid). Sporophytes produce spores via meiosis and therefore are haploid. Spores grow into gametophytes and the cycle continues again.

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