In our impression, single-celled organisms are mostly very tiny microorganisms that cannot be seen by the naked eye, is this really the case?
Many friends may have heard that egg cells are the largest cells, such as human eggs to the extent visible to the naked eye, and their diameter can reach 0.1 to 0.2 mm, which is also the largest cell in the human body. However, the size of human egg cells is far from the size of the egg cells of birds or reptiles.
Some birds and reptiles lay eggs, which are mainly a single cell, that is, the yolk in the egg, we can think of the yolk in the bird egg as a single cell, such as the yolk of the egg, as long as it is not fertilized, then it is a single cell, if it is fertilized, it becomes a fertilized egg, the fertilized egg will have cell division, it is not a cell.
From the structure of the yolk, the surface of the yolk has a very thin membrane, called the yolk membrane, it is the cell membrane in the cell structure, but also the innermost protective structure in the internal structure of the egg, the yolk inside is composed of the yolk and the white yolk in concentric circles, the middle is the yolk heart, there is a small white disk on it, it is composed of the nucleus and part of the cytoplasm, that is, the placenta of the egg cell. After the placenta is fertilized, the egg becomes a fertilized egg and hatches under the right conditions. The egg whites, ties, eggshells, and shell membranes on the outside of the yolk are all non-cellular structures, which mainly provide nutrition and protection for the basic egg yolk.
Bird eggs also have life characteristics, so if the bird eggs see an organism, the eggs laid by the largest ostrich in birds are a huge single cell, of course, there have been dinosaurs in the history of life on the earth, and the single cells in the eggs they lay are larger.
If the bird egg is not an organism, we can also explore the single-celled life with self-activity characteristics, first of all, the trumpet worm and ciliate, they are common small life forms in natural waters, the body length is between 1 and 5 mm, their body structure is basically a cell, relative to the animal's naked eye invisible tissue cells, the size of these single-celled life forms is undoubtedly very huge.
However, biologists have found that some single-celled structures of amoeba are larger, some can reach 5-20 cm in length, amoeba is already a very complex single-celled organism, but it already has many multicellular organisms.
However, the larger single-celled organisms are some algae and fungi, some of which can grow up to tens of centimeters, is not the big one a bit unexpected? Cognition has been completely subverted? However, the champion of single-celled organisms should belong to certain ferns, such as Caulerpa taxifolia, which is a species of green algae phylum, fern family, fern algae genus, some aquarium stores common staghorn seaweed, green grape algae, large feather algae belong to the genus fern algae, some single-celled fern algae can be more than one meter long.
Many friends may not understand, why is a single-cell organism so big? It's beyond imagination! To know that ferns are all leafy, how can it be a single-celled organism? In fact, the leaves of ferns are fundamentally different from the leaves of grass, they can shrink in volume, have many folds, and look like leaves when they are unfolded. It can collect nutrient molecules with the help of photosynthesis, and the growth process of this organism alone is also similar to the growth changes of single-celled organisms. However, it must be pointed out that some biologists believe that this type of plant is multinucleated, but there is no cell membrane between cells, so it is more like a multicellular organism.
In any case, some single-celled organisms are indeed very large, why can these single-celled organisms grow so large? There is a physical phenomenon here, that is, when the volume of the object increases, the surface area is not very large compared to the increase, and the larger surface area is conducive to the cells to obtain nutrients from the external environment, so some single-celled organisms have evolved relatively large. But the larger volume also means that more nutrients need to be consumed and excreted, and the functions that can be extended by a single cell tend to be extreme, which is not conducive to the survival of single-cell organisms, so most single-celled organisms are still very small. Moreover, when the cell evolved to a higher end, it chose the comprehensive functional development direction of the multicellular functional differentiation combination, which led to more and more advanced multicellular organisms in the biological world.