Chapter 19, Problem 28A

Summary Introduction

Introduction :

The given figure is of Euglena gracilis . They are single-celled algae of genus Euglena. They usually found in freshwater and have characteristics of both animals and plants. They have secondary chloroplasts for the process of photosynthesis which is a characteristic of plants, but they lack a cell wall which is a characteristic of animals. They are mixotrophs that means they are able to feed themselves by the process of photosynthesis or by phagocytosis.

Answer to Problem 28A

Correct answer :

The correct answer is option (B) eyespot.

Explanation of Solution

Explanation/justification for the correct answer :

Option (B) eyespot. In euglena, the eyespot is found in the gullet near the base of flagellum. It is also known as stigma. Eyespot is a highly pigmented area that functions in the light reception. It is a light-sensitive receptor that directs the euglena towards the light for the process of photosynthesis. It means euglena use eyespot to sense the light. So, the correct answer is option (B).

Explanation for incorrect answer :

Option (A) chloroplast. Chloroplasts are the cell organelles that perform photosynthesis. They have the ability to absorb sunlight. Euglena contain photosynthesizing chloroplasts that enable them to sustain by the autotrophy. So, this is an incorrect option.

Option (C) nucleus. The nucleus is found in the center of euglena’s cell body. The nucleus contains the genetic material of the cell and regulates the different types of cell activities. It is not used as a light-sensing structure. So, this is an incorrect answer.

Option (D) pellicle. Euglena don’t have cell walls but instead of cell wall they have a pellicle which is composed of a protein layer with a substructure of microtubules. They are arranged in strips around the cell. The strips of pellicle slide over one another and permit the euglenoid to crawl in the mud when the level of water is very low to swim. So, option (D) is also incorrect.