Mycorrhizae, the mutualistic relationship between fungus and plant roots, benefits both the fungus and the plant. The fungus that grows on the plant’s roots increases the surface area for nutrient absorption of the roots. While plants provide the fungus with organic nutrients, the fungus passes minerals that that the fungus does not absorb to the plant roots. Mycorrhizae are very common - 95% of plants have mycorrhizae and all three phyla of fungi have mycorrhizae-forming members.
Fungi of the phylum Zygomycota are almost entirely terrestrial and live on dead plant and animal material. Zygotes are able to reproduce both asexually and sexually. In the zygote asexual reproductive cycle, spores germinate into hyphae and and produce haploid spores from sporangium.
Sexual reproduction of zygotes requires the conjugation of plus and minus hyphal strands. The temporary uniting of the plus and minus strands during which the strands exchange genetic material causes each strand to produce a gametangium. The nuclei of the plus and minus strands then fuse in fertilization, creating a diploid zygospore. The zygospore undergoes meiosis to produce four haploid nuclei, out of which only one is functional and the other three degenerate. The zygospore is able to withstand harsh conditions and germinates into hyphae, thus beginning the asexual stage.
Ascomycota, also known as sac fungi, have both sexual and asexual reproductive stages. In the asexual stage, conidia form on the fungi’s conidiophores, special aerial hyphae. Spores then form on the conidia and are dispersed away from the parent by the wind.
Ascomycota sexually reproduce by first fusing the hyphae of different mating strains. Mitotic nuclear division leads to multiple nuclei in the ascogonium and anthoridium (male and female gametangia, respectively), then the nuclei are transferred to the ascogonium. The haploid nuclei from two different strains are now in the ascogonium, which divides cytokinetically with walls between the nuclei, thus creating dikaryotic cells. These dikaryotic cells divide via mitosis and the dikaryotic and sterile hyphae form cuplike structures called ascocarps containing special cells called asci, or sacs. The fusing of two nuclei in the ascus forms a zygote, which undergoes meiosis to form four haploid ascospores. Each ascospore undergoes mitosis to form two ascospores, thus forming eight ascospores in total. The spores are then released and germinate into new hyphae.
Basidiomycota form mushrooms as their fruiting bodies, called basidiocarps, to reproduce. Fungi in phylum Basidiomycota reproduce sexually. Different mating strains meet underground and the cells fuse, however, the nuclei join together without fusing. Dikaryotic hyphae then form and grow into basidiocarps. A basidiocarp is a structure consisting of a stalk, which supports a cap with gills on the underside. The basidiocarp gills give rise to stalks called basidia. Haploid nuclei fuse at the tips of the basidium. each basidium then undergoes meiosis and forms four basidiospores, which are released into the environment and germinate into new hyphae.
Characteristics of Ancestral Cells that Allowed Evolution of Eukaryotic Cells
Many factors are part of the combined circumstances that allowed for the evolution of eukaryotic cells. Flexibility of cell membranes was necessary for the enfolding of those membranes, a trait which eventually led to the possibility of engulfing other cells. The rise of cytoskeleton supported cells and gave them structure to allow organized growth. Endosymbiosis by the engulfing of prokaryotes, including mitochondria and chloroplasts, that eventually led to a symbiotic relationship also contributed to the evolution of eukaryotic cells.
Endosymbiosis is the engulfing of a cell by a larger cell, in which the smaller cell evolves into an organelle of the larger cell. The theory of endosymbiosis proposes that organelles such as mitochondria and chloroplasts in the eukaryotic cell were once free-living cells that had been engulfed by larger cells and evolved to become organelles. Evidence for endosymbiosis includes the facts that
- organelles (such as mitochondria & chloroplasts) have their own DNA, which is similar to that of bacteria and suggests that the organelles were once separate from the host cell
- chloroplasts and mitochondria reproduce via binary fission (like bacteria) instead of mitosis
Mitochondria: By engulfing energy-producing bacteria whose respiration produced H2 as a byproduct, anaerobic (living/active in the absence of free oxygen) host cells whose metabolic pathways depended on hydrogen then developed a dependence for the H2 producing bacteria. This internal supply of hydrogen allowed the host cell to adapt to an atmosphere with high amounts of O2.
Chloroplasts: Engulfing of photosynthetic bacteria; secondary endosymbiosis (engulfing a cell that has an endosymbiotic relationship with another engulfed cell) of red algae by brown algae to obtain chloroplasts.
Note: the cells are not called “mitochondria” or “chloroplasts” until they have been engulfed and evolved into organelles.
Due to the evolution of flexible cell membranes, bacterial cell membranes were able to invaginate and form cavities within the cell. The endoplasmic reticulum (ER) arose as a result of the infolding of the cell’s pasma membrane. The nucleus is proposed to have evolved from the infolding of cell membrane, which eventually led to the encasing of the cell’s DNA within a cavity formed from the invagination of the cell membrane.
Fungi are heterotrophic and feed through absorption, meaning that a fungi grows on its food source, releases digestive chemicals, and absorbs the food that is digested outside of its body. The digestive chemicals that fungi secrete are hydrolytic enzymes that break down large molecules, after which the fungi absorb the simple molecules. The formation of mycellium in thin layers increases surface area, thus allowing fungi to absorb nutrients more efficiently.
There are three main methods by which Fungi feed: saprophytic, parasitic, and symbiotic.
- Saprophytic feeding is the decomposition of dead remains or waste materials of other organisms. These decomposers are called saprobes.
- Parasitic fungi feed on living creatures, often causing harm to their hosts. These fungi that take nutrients from their host cells are called parasites.
- Symbiotic fungi live with other living organisms in mutually benefitting relationships, providing the other organism with nutrients while gaining nutrients from that organism. These fungi that absorb nutrients from another organism’s cells while benefitting the other organism in some way are called mutualists. (An example is mycorrhizal fungi, which grows on plant roots and helps the roots absorb nutrients more efficiently as well as boosting overall plant health)
Some fungi are predators - the fungi trap protists or small nematodes and absorb N compounds from the prey. Fungi can also be lichens and have mutualistic relationships with algae in which the alga provide nutrients and nitrogen for the fungus, and the fungus provides a suitable, shady, environment that retains moisture and minerals while facilitating gas exchange for the alga.
Pollination in a flower occurs when pollen is transferred from the anther of a flower to the same flower (self-pollination) or another flower’s (cross-pollination) stigma. One of the strategies for pollination of an angiosperm flower is the transport of pollen grains from one flower to the next on the body of a pollinator. When the organism lands on the next flower, the pollen grains that were picked up by the organism sticks to the stigma of the flower. Flowers can also be pollinated by the wind, which carries pollen from one flower to the next. Wind pollination requires many flowers in the same general vicinity to have a good chance of pollen being successfully carried to another flower’s stigma by the wind.
Life on land poses various challenges to plants. Previously adapted to aquatic environments, plants would struggle with being at risk for dehydration because the land atmosphere is much dryer in comparison to that of aquatic environments. Plants on land are at higher risk of damage to their structures and DNA from UV light because air is less dense than water, thus UV light from the sun is less obstructed from objects on land. Air’s low density also provides no structural support, therefore plant structures would collapse. This minimizes efficiency because leaves that are not upright have less surface area on which photosynthesis and gas exchange can occur. In a non-aquatic environment, plants are also posed with the challenges of reproducing - waterproof gametes are necessary to fertilize land plants, and protection of embryos is necessary to prevent them from drying out.
Along with the challenges to living on land, plants also receive a large amount of benefits from their new environment. Photosynthesis occurs more efficiently with the abundance of CO2 in the surrounding environment. Respiration is also improved because O2 is more abundant on land. Light from the sun is more plentiful on land, thus there are more resources for the production of energy. Life on land offers fewer predators and less competition than life in marine environments.
Over time, plants adapted to life on land. The rise of vascular tissue (xylem and phloem) allowed transportation of water, nutrients, and ions throughout the plant body. Tracheids, specialized transport cells, in particular allow for more efficient transport of materials throughout the cell by facilitating water and mineral transport. Evolved higher efficiency in the transportation of nutrients from the leaves down to the roots, and water from the roots up to the leaves allowed plants to grow larger. Lignin arose in the walls of xylem cells, an adaptation which helped support the plant’s upright structure as well as waterproof the cells. Cuticles, waxy coverings on the outside of plants, helps plants retain water as well as keeping air out. Because of the cuticles, gas exchange (which is essential to photosynthesis and respiration) is limited. The adaptation of stomata, tiny openings in the plant epidermis, allowed for gas diffusion in and out of the cell. The evolution of protection for reproductive structures keeps them from drying out. Different ways of fertilization allow eggs and sperm to unite without water, and to protect the embryo from drying out. The development of a root system allows the plant to absorb water and nutrients from its environment for use in plant growth.