An organism that operates at the cellular level of organization, carries out its physiological activities by using its

Analyze the question The question asks about an organism at the cellular level of organization. This means a single-celled (unicellular) organism such as Amoeba, Paramecium, or Chlamydomonas. It asks what structures such an organism uses to carry out its physiological activities, such as respiration, excretion, and reproduction.

Explain the concept Levels of organization in biology run from cell, to tissue, to organ, to system, to organism.

• Unicellular organisms operate at the cellular level. One cell does everything.
• Multicellular organisms divide the work among tissues, organs, and systems.
• Since a single cell has no organs or systems, it relies on its organelles. These are the "mini-organs" inside the cell: • Mitochondria for respiration • Contractile vacuole for osmoregulation and excretion • Ribosomes for protein synthesis • Nucleus for control and reproduction • Chloroplasts for photosynthesis, in some unicellular organisms

Evaluate the options

• A. Cell membrane: It controls what enters and leaves the cell, but it is only one structure and cannot carry out all the physiological activities.
• B. Organelles: Correct. These are specialized structures that perform specific functions inside the cell.
• C. Small size: This is a feature of the organism, not a structure that does work.
• D. Cytoplasm: It is the medium in which reactions take place and the organelles sit, but the specialized functions are done by the organelles.

Answer: B. organelles

Exam tip: Match each level to its working parts:

• Cellular level → organelles
• Tissue level → cells
• Organ level → tissues
• System level → organs

If the question mentions a unicellular organism, think "organelles do the jobs."