Learning Objectives:
 

What does it mean to be haploid?

What does it mean to be diploid?

What is a sporophyte?

What is a spore?

What is a gametophyte?

What is a gamete?

The terms haploid and diploid refer to the DNA present in the cells of an organism. Nearly all of our cells has 2 sets of genes (2 sets of chromosomes). We received one set from our mother and one set from our father. A diploid cell is a cell with 2 sets of chromosomes, 2 sets of genes. Haploid refers to cells that have only one set of genes, 1 set of chromosomes. We produce cells called GAMETES, sperms and ova (eggs)  that have only one set of genes. When a sperm fuses with an egg, a single diploid cell is produced, and that cell has the potential to divide the numerous millions of times required to produce a new individual. In our life history, the only haploid cells produced are gametes.

To grow and reproduce, we need two kinds of cell division. In one kind, MITOSIS, one cell divides to produce two identical cells. A diploid cells divides by mitosis to produce 2 dipliod cells. A haploid cell divides by mitosis to produce 2 haploid cells. In the other kind of cell division, MEIOSIS, a diploid cell divides to produce haploid cells. Cells with two sets of chromosomes divide to produce cells with one set of chromosomes. In humans, the products of meiosis are gametes. Thus humans are said to have a GAMIC life history. In plants, the products of meiosis are SPORES, and thus plants are said to have a SPORIC life history.

We will start with spores. Spores will be haploid. The haploid spores divide by mitosis (germinate) to produce a multicellular organisms called a GAMETOPHYTE. Gametes develop within structures on the gametophyte. In gametophytes, gametes are produced by mitosis of haploid gametophyte cells that then develop into gametes.

Gametes fuse to produce a diploid cell (a sperm and egg will fuse to produce a diploid cell) that is called a ZYGOTE in all sexually reproducing organisms (including humans).

In plants, the zygote divides by mitosis, diploid cells divide to produce diploid cells, to produce a multicellular organism called a SPOROPHYTE.

So in the plant life history we have two distinct multicellular phases, the gametophyte and the sporophyte.

In sporophytes, structures called SPORANGIA will develop. In sporangia, some cells divide by meiosis to produce spores.