Alternation of generations

Definition

Alternation of generations is a biological life cycle that occurs in plants and some algae, involving a switch between a haploid gametophyte stage and a diploid sporophyte stage. This cycle allows for genetic diversity and adaptation to different environments, with each generation playing distinct roles in reproduction and survival.

5 Must Know Facts For Your Next Test

1. In the alternation of generations, the sporophyte is typically larger and more complex than the gametophyte in seedless vascular plants, while in bryophytes, the gametophyte is more dominant.
2. Green algae exhibit this life cycle, which illustrates the evolutionary link between these aquatic organisms and terrestrial plants.
3. The evolution of seed plants introduced a reduced gametophyte stage that remains within the sporophyte, allowing for better protection and nutrition of the developing gametes.
4. In angiosperms, the gametophyte is reduced to just a few cells, such as pollen grains and embryo sacs, enhancing reproductive efficiency.
5. The alternation of generations contributes to genetic variation through sexual reproduction, allowing plants to adapt to changing environments.

Review Questions

• How does the alternation of generations enhance genetic diversity in plant life cycles?
  • The alternation of generations promotes genetic diversity through sexual reproduction during the gametophyte stage, where gametes are formed. When these gametes fuse during fertilization, they create a new diploid sporophyte. This cyclical process allows for recombination of genetic material, enabling plants to adapt better to their environments and increasing their chances of survival in varying conditions.
• Compare the roles of the gametophyte and sporophyte stages in various plant groups and their adaptations to terrestrial life.
  • In bryophytes, the gametophyte is dominant and photosynthetic, relying on water for fertilization, while in seedless vascular plants, the sporophyte becomes dominant with structures that facilitate spore dispersal. Seed plants have evolved further by reducing the gametophyte stage into microscopic forms retained within the sporophyte. This adaptation protects the gametes from environmental stresses and provides them with nutrition, showcasing a significant shift toward more complex terrestrial life.
• Evaluate the significance of reducing the size of the gametophyte generation in angiosperms compared to earlier plant forms.
  • The reduction of the gametophyte generation in angiosperms marks a critical evolutionary advancement. By minimizing this phase to just a few cells, such as pollen grains and embryo sacs, angiosperms enhance reproductive efficiency and protection of developing structures. This adaptation allows for more effective pollination strategies and ensures greater survival rates for offspring. The compact nature of the gametophyte also contributes to successful fertilization in various environments, demonstrating how evolutionary pressures shaped plant reproduction.

Related terms

Gametophyte: The haploid stage in the plant life cycle that produces gametes (sperm and eggs) through mitosis.

Sporophyte: The diploid stage in the plant life cycle that produces spores through meiosis, which can develop into gametophytes.

Meiosis: A type of cell division that reduces the chromosome number by half, resulting in the formation of haploid spores from a diploid sporophyte.