Liverwort and Photosynthesis
Photosynthesis uses energy from light to combine carbon dioxide from air and hydrogen from water to make nutrient sugars. Liverworts do not have true roots, stems or leaves. Tallose liverwort is ribbon-shaped and flattened; leafy liverwort has a vascular center with leaf-like lobes on each side. How liverwort evolved to conduct photosynthesis is best understood by comparing it to the method of vascular plants.
Leaves of vascular plants have tiny holes called stomata that can open to let in the air necessary to obtain carbon dioxide for photosynthesis. Stomata can open to release oxygen and close to prevent the loss of water.
Liverwort has no comparable leaves; the surface layer of the plant contains permanently-open pores. These single-celled, open pores must bring in carbon dioxide and exhale oxygen.
Both vascular plants and liverworts need to get water to the chlorophyll in order for photosynthesis to take place.
Vascular plants have roots that have no cuticle or hard covering and so can absorb water. They transport water from the soil through xylem, which are hollow tubes, to the phloem, live cells that distribute sugars around the plant.
Water moves through the flat, tallose liverwort by osmosis in the space between cells. Some tallose liverworts float on still water where they absorb the moisture they need.
Liverwort does not have roots, but leafy liverwort has a kind of foot that functions somewhat like a root. Water moves through a thickened center that contains water-bearing hydroid cells, the function of which can be likened to xylem, and elongated leptoid cells that function somewhat like phloem.
In vascular plants, the middle of the leaf contains mesophyll, tissue containing chloroplasts. Chloroplasts are subunits of a cell that capture light energy. They are necessary for photosynthesis.
The necessary chloroplasts of liverwort form a thin layer in a special chamber below the permanently-open surface pores. Liverwort stores the nutrient sugars in a final, colorless layer of cells beneath the chloroplast cells. Chloroplasts of liverwort contain multiple copies of circular DNA molecules that contain some, but not all, of the genes needed for photosynthesis and their own replications.
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