Question

Which of the following organisms can be seen only under the microscope?
A. Unicellular
B. Multicellular
C. Vertebrates
D. All of the above

Answer 1

Hint: Unicellular organisms are composed of only one cell which is responsible for carrying out all of the required functions of an organism, while multicellular organisms are made up of many different cells which perform diverse functions.

Complete answer: Bacteria, yeast and protists are the unicellular organisms. For example, a paramecium is a unicellular organism which is found in pond water. It obtains its food from the water. The nutrient from the food reaches organelles surrounding the cell through the cytoplasm. This nourishes the cell thus keeping it functional.
However, multicellular organisms are composed of more than one cell. Groups of cells differentiate to perform specialized functions. In human beings, during early developmental stages, cells differentiate and specialize to become muscle cells, nerve cells, blood cells, skin cells, and various other types of cells. These differences can be observed under a microscope easily. Their function determines their structure. Cells take up particular forms for performing diverse functions in an effective way.
The majority of unicellular organisms have a microscopic size therefore they are also classified as microorganisms. For example, the size of protozoans and phytoplankton range in between 0.001 mm to about 0.25 mm. On the other hand, the majority of the vertebrates and other multicellular organisms are visible to the naked eye so there is no need to use a microscope.
 So, the correct answer is option “A” that unicellular organisms can be seen only under the microscope.

Note: Function of cells differs in both unicellular and multicellular organisms. However, in all organisms, specialized cell structures, or organelles are present which are responsible for a number of cellular activities or functions, such as energy production, achieving nutrients, and synthesizing proteins.