The protists belong to the kingdom Protista that comes under the domain Eukarya. Protists are a very broad group, comprising of various eukaryotes that do not fit into the other kingdoms, like animals, plants, and fungi. The protists were the initial eukaryotes.
They would go on to become the precursors for each of the other eukaryotic kingdoms. By examining protists one can witness the transformation between the single-cell prokaryotes and modern multicellular eukaryotes.
Red blood cell is shaped as a biconcave discs, this shape allows them to squeeze through small capillaries.
The biconcave shape of the cell allows oxygen exchange at a constant rate over the largest possible area. The biconcave shape increases the cell's surface area compared to a flat disk of the same size. The greater surface area makes it easier for gases to move into and out of the red blood cell.
The biconcave shape provides a large surface area compared to the volume of the red blood cell, allowing diffusion to happen efficiently. This shape optimizes the ratio of surface area to volume, facilitating gas exchange. It also enables them to fold up as they move through narrow blood vessels.
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