Characteristics and Classification of Living Organisms: Question 1
Syllabus 1.1
A student places a single-celled organism called an Amoeba under a microscope while it lives in a drop of pond water. The student notices a small, bubble-like structure inside the cell called a contractile vacuole.
Water constantly diffuses into the Amoeba's cytoplasm from the surrounding pond water. Over a few seconds, the contractile vacuole slowly fills with this water. It then suddenly contracts, squeezing the water out of the cell and back into the pond.
Which life process is the Amoeba carrying out when it forces this water out of its cell?
Show worked solution Hide worked solution
Worked solution
Step 1: Recall the definition of excretion
The syllabus defines excretion as the removal of the waste products of metabolism, and of substances that are in excess of an organism’s requirements, from the body of that organism.
Step 2: Apply the definition to the Amoeba
The water diffusing into the Amoeba’s cytoplasm is not itself a waste product of metabolism, but if it were allowed to build up it would dilute the cytoplasm and eventually cause the cell to burst. This water is therefore a substance that is in excess of the Amoeba’s requirements. The contractile vacuole collects this excess water and forces it back out of the cell, so this is an example of excretion.
Step 3: Rule out the other life processes
- Growth would mean the Amoeba permanently increasing in size and dry mass. Removing water does not add to the organism’s mass, so this is not growth.
- Nutrition is the taking in of materials for energy, growth and development. This scenario describes water leaving the cell, the opposite direction, so it is not nutrition.
- Reproduction would involve the Amoeba producing more amoebae. No new organisms are described here, so this is not reproduction.
Final answer
- Forcing excess water out of the cell using the contractile vacuole is an example of excretion, option A.