Humans require organ systems to carry out life processes. single-celled organisms do not have organ systems and yet they are able to carry out life processes. this is because
human organ systems lack the organelles found in single-celled organisms

a human cell is more efficient than the cell of a single-celled organism

it is not necessary for single-celled organisms to maintain homeostasis

organelles present in single-celled organisms act in a manner similar to organ systems

Organelles present in single-celled organisms act very similar to organ systems in multicellular organisms.

Organelles:

They are the small bio-molecular system that carry out the vital processes inside a cell.

For example-

Therefore, organelles present in single-celled organisms act very similar to organ systems in multicellular organisms.

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Multicellular organisms

organ system play a critical role for the smooth functioning of whole body. Millions of cells combined to make an organism. The role of organ system in whole body is to provides oxygen and nutrients to the cells and eliminates the toxic material such as carbon dioxide. Organ systems like, respiratory system, digestion system, and cardiovascular play an important role in the human body to perform various functions for its survival.  

Unlike multicellular organisms the unicellular only consists of single cell e.g. prokaryotic organisms and eukaryotic organisms namely, Amoeba, Euglena, salmonella, Fungi etc. Single celled organisms are able to carry out all processes mandatory for the sustenance of life with the help of single cell only e.g. respiration and digestion.They can survive with the help of single cell. Unlike multicellular organisms unicellular do not require multiple number of cell for its survival

is Organelles present in single-celled organisms act in a manner similar to organ systems

Explanation:  

Multicellular organisms organ system play a critical role for the smooth functioning of whole body. Millions of cells combined to make an organism. The role of organ system in whole body is to provides oxygen and nutrients to the cells and eliminates the toxic material such as carbon dioxide. Organ systems like, respiratory system, digestion system, and cardiovascular play an important role in the human body to perform various functions for its survival.  

Unlike multicellular organisms the unicellular only consists of single cell e.g. prokaryotic organisms and eukaryotic organisms namely, Amoeba, Euglena, salmonella, Fungi etc. Single celled organisms are able to carry out all processes mandatory for the sustenance of life with the help of single cell only e.g. respiration and digestion.They can survive with the help of single cell. Unlike multicellular organisms unicellular do not require multiple number of cell for its survival.