Mycorrhizae & Their Mutualistic Relationship with Plants
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.
Fruits arise from fertilized flowers. The ovary wall, called the pericarp, of a flower is made of three layers: the exocarp, mesocarp, and endocarp; these layers develop to become the exterior skin, fleshy area beneath the skin, and internal area in which seeds can be found, respectively, of a fruit. The endocarp is the inner area of the fruit in which the seeds are contained; ovule chambers of flowers from the endocarp in fruits. The seeds of a fruit are typically toward the center of the fruit and are mature ovules from the flower’s ovary. The remains of the sepals and stem can be seen on some fruits.
Plants have multiple benefits from fruits. Having fruits allows embryo dispersal far enough away from the parent as to reduce competition for resources, thus also allowing angiosperms to colonize large areas.
Fruits also allow plants various ways of dispersing their seeds. These dispersal methods include fleshy fruits, which are dispersed by animals, that have seeds with adapted hard coats to protect the embryo from stomach acids and digestive enzymes. Certain types of fruits have hooked spines and stick to mammals’ fur or clothing, thus being dispersed wherever the mammal travels when the fruits lose grip of the fur/clothing and fall off. Some fruits have adapted wing-like structures that aid in distribution by wind when those fruits are blown away from the parent plant. Some fruits are dispersed via floating in water.
There are five main types of simple fruits. Drupes form their skin, fleshy area, and protective seed coating from the ovary wall of a flower with only one ovary and one chamber (example: peach). Pods form from flowers with one ovary and one carpel chamber; the flowers from which pods develop have many ovaries in a single row in the carpel chamber and its seeds are attached to the placenta on one side of the ovary (example: string bean). Achenes are hard protective fruits that develop from flowers with a single-chambered ovary, whose wall becomes hard and protective (example: sunflower seed). Pomes develop from flowers with one multi-chambered ovary. The seeds of pomes are separated in carpel chambers and are attached to the placenta at the center of the fruit, and the fleshy part of pomes develop from the petals, sepals, etc. (example: apple). Berries form from flowers with a singe multi-chambered carpels and a single ovary. Carpel chambers form the segments in which seeds are housed; the seeds are attached to the placenta at the center of the berry (example: tomato).
Aggregate fruits develop from a multi-ovary flower, in which each ovary has one ovule, matures into a section of the fruit, and is attached to the central stem (example: strawberry).
Double fertilization in an angiosperm plant is the process by which a zygote and triploid endosperm are created. One sperm nucleus fuses with the eg nucleus, thus producing a zygote. The other sperm nucleus fuses with the polar nuclei, thus producing a triploid endosperm that serves as nutrients and feeds the developing embryo.
There are two main slime mold phyla in Kingdom Protista, each with its own distinct characteristics.
Phylum Acrasiomycota (cellular slime molds)
Cellular slime molds closely resemble amoebas in structure. These molds live independently until food runs out. A starving amoeba secretes the hormone cyclic AMP into the environment. Other amoebas detect the cyclic AMP as a food source and aggregate from great distances to follow the concentration gradient to the dying amoeba. The amoebas then attach to one another and become what seems to be a functioning multicellular organism. The moving slug finds a suitable habitat before forming itself into a diploid fruiting body called a sorocarp, which releases encysted amoebas or diploid macrocysts. The released amoebas live independently until food resources are depleted, then the cycle is repeated.
Phylum Myxomycota (plasmodial slime molds)
A plasmodial slime mold is essentially one large cell, which is possible due to the mold’s multiple nuclei that create many RNA. This type of mold lives as a non-walled multi-nucleated mass of slime in its feeding phase. The nuclei of the mold, which can be either a diploid or haploid species, undergo mitosis. Fruiting bodies are formed under harsh conditions. The spores produced by the sporangium are either diploid or haploid, depending on the ploidy of the plasmodium as a whole. The spores undergo meiosis to produce gametes. After the spores are released, the gametes fuse when they come in contact with one another and undergo repeated mitosis to form a multinucleate plasmodium.
The molds of these phylums are similar in that those of phylum Acrasiomycota converge to form a seemingly individual organism from many amoebas, while a mold of the phylum Myxomycota is essentially one large cell that functions as individual cells because the large cell is multi-nucleated. The reproductive cycles of the two phylum differ in that cellular slime mold gives rise to fruiting bodies and release amoebas, while plasmodial slime molds produce gametes that must fuse to and undergo mitosis to form the new plasmodial slime mold.
Organisms in the kingdom Fungi have certain general characteristics. The cell walls of fungi are made of chitin. Fungi have filamentous bodies made of hyphal strands, which are long strings of cells connected end to end by septa. Because the septa only partially separates the cells, cytoplasm flows freely throughout the body of a fungus. A mass of hyphae is called a mycelium.
Reproductive Cycles of 3 Main Fungi Phylums
Zygomycota
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
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
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.
The dicotyledonous root contains many structures. In a longitudinal section, the root cap is at the tip of the root and serves to protect the growing root and the layer of meristematic tissue under it. Underneath the root cap is the apical meristem, which gives rise to undifferentiated cells. The zone of cell division is after the apical meristem and includes procambium tissue. The zone of elongation is the area where cells mature and lengthen, thus pushing the root into the soil. The zone of maturation is after that of cell division and elongation; the maturation zone has vascular tissue in its center and root hairs extending from individual epidermis cells, which increases surface area for nutrient absorption. From the zone of cell division up to the zone of maturation there is ground tissue between the epidermis and endodermis.
The cross section of a dicot root has xylem for water transport arranged in star shape in the middle, surrounded by phloem, which transports sugars, with a ring of pericycle and endodermis surrounding the vascular tissue. A layer of cortex lies between the endodermis and the exodermis.
In a monocot root cross section, ground tissue forms a pith in the center of the section, with xylem and phloem in respective rings around the pith. Just as in the dicot root cross section, that of the monocot root also has a ring of pericycle and endodermis, respectively, encompassing the vascular tissue, and a layer of cortex between the endodermis and exodermis.
Evolution of the Eukaryotic Cell
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
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.
Nucleus
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.
Monocot and dicot plants differ mainly in plant structure; however, both have three types of plant tissue – vascular, ground, and dermal – and indeterminate growth due to the presence of apical meristems. External differences include leaf venation – monocots have parallel veins while dicots’ veins are netted. The roots of monocots contain a larger area of ground tissue in the cortex, with the vascular tissue composed of xylem and phloem centered in the middle of the cross-section. Dicot roots have a ring of cambium tissue between the layers of xylem and phloem. Dicot leaves have a layer of parenchyma cells under the upper epidermis, with small vascular bundles in the spongy mesophyll layer between the layer of parenchyma cells and the lower epidermis. Monocot leaves are more condensed to preserve water and have larger vascular bundles. The vascular bundles in monocot stems are scattered throughout the cortex while those in the dicot stems are organized in rings, thus allowing secondary growth. In dicot stems, layers of secondary xylem and phloem are added to either side of the layer of vascular cambium tissue between those of the xylem and phloem. Both monocots and dicots have cotyledons, however, monocots have one while dicots have two inside of their seeds.
