In certain motile protists such as Euglena, which light sensitive organelle, also called a stigma, helps the cell detect the direction of light?

Introduction / Context:
Many protists live in aquatic environments where light is an important cue for finding optimal conditions for photosynthesis or avoiding harmful radiation. Some motile protists have specialized structures that help them sense and respond to light direction, allowing them to move toward or away from light sources. This question asks you to identify the organelle that performs this light detecting function in organisms such as Euglena.

Given Data / Assumptions:

• The focus is on motile protists, for example Euglena.
• These protists often perform photosynthesis and move using flagella.
• They have an organelle also known as a stigma.
• The options include eyespot, contractile vacuole, food vacuole, and cilia.

Concept / Approach:
An eyespot, also called a stigma, is a light sensitive organelle found in some photosynthetic protists. It is typically located near the base of the flagellum and contains pigments that can detect light intensity and direction. When combined with flagellar movement, the eyespot enables phototaxis, movement toward or away from light. A contractile vacuole regulates water balance, a food vacuole is involved in digestion, and cilia are hair like structures used for locomotion or moving fluid but do not themselves detect light. The correct answer must therefore be the eyespot.

Step-by-Step Solution:
Step 1: Recall that Euglena and similar protists live in water and carry out photosynthesis using chloroplasts. Step 2: Recognize that to optimize photosynthesis, they benefit from moving toward moderate light intensity, which requires sensing light direction. Step 3: Identify the eyespot, or stigma, as a pigmented region associated with light detection, usually near the flagellum. Step 4: Evaluate the contractile vacuole. This organelle collects excess water and expels it to maintain osmotic balance, not to detect light. Step 5: Evaluate the food vacuole. It encloses ingested particles and participates in digestion; it is not light sensitive. Step 6: Evaluate cilia. These structures, where present, help with movement or fluid flow but do not act as light detecting organelles by themselves. Step 7: Conclude that the eyespot is the correct light sensitive organelle that enables these protists to detect light direction.

Verification / Alternative check:
Microscopic images and diagrams of Euglena commonly label the eyespot as a reddish or orange structure near the flagellum. Textbooks describe it as a photoreceptive organelle that works together with the flagellum to achieve phototaxis. Experimental observations show that when light direction changes, Euglena adjusts its swimming behavior in response, which would be impossible without some form of light sensing structure. Contractile and food vacuoles are usually labeled separately and have distinct roles in water regulation and digestion.

Why Other Options Are Wrong:
Option B: The contractile vacuole is involved in osmoregulation, not in sensing light.

Option C: The food vacuole helps in digesting ingested food particles but does not respond to light.

Option D: Cilia enable movement and feeding in many protists and animal cells, but light sensitivity is not their primary function and they are not called stigmas.

Common Pitfalls:
Some students confuse different small organelles in protists and may misidentify vacuoles as general sensory structures. Another pitfall is to associate any external appendage like cilia or flagella with sensory roles, when in fact these structures mainly provide movement. Remember that the eyespot or stigma is specifically named and described as a light sensitive area that helps guide movement relative to light sources.

Final Answer:
The light sensitive organelle that helps certain protists detect the direction of light is the eyespot, also known as the stigma.