Problem 48 - Objective Biology for NEET

Question

Euglena posses all except (a) Pellicle (b) Two equal flagella (c) Two unequal flagella (d) Mixotrophic nutrition

Step-by-Step Solution

Answer

The correct answer is (b) 'Two equal flagella'. This is not a feature of Euglena as they possess two unequal flagella.

Step 1: Identifying the features of Euglena

Euglena posses a flexible outer layer called Pellicle which allows it to change shape. Hence, option (a) is incorrect because Euglena does indeed possess a pellicle.

Step 2: Evaluating the nature of flagella

Euglena has two flagella that are unequal in length, one is short and the other one is long. Thus, option (c) is also incorrect because Euglena does have two unequal flagella.

Step 3: Evaluating the type of nutrition

Euglena uses mixotrophic nutrition for energy, that is, they can make their own food by photosynthesis and they can also ingest food from their environment. Therefore, option (d) is incorrect since Euglena does practice mixotrophic nutrition.

Step 4: Evaluating the remaining option

After eliminating options (a), (c), and (d) that are true for Euglena, we are left with option (b), 'Two equal flagella', which is not a feature of Euglena.

Key Concepts

Pellicle

Euglena is distinctive for its flexible outer layer known as the pellicle. This structure is crucial for several reasons. Firstly, the pellicle offers protection while still allowing Euglena to maintain flexibility, which is unusual compared to the rigid cell walls found in many other microorganisms.

Composed of a series of protein strips lying beneath the cell membrane, the pellicle enables Euglena to change its shape, mostly seen during locomotion or when reacting to environmental pressures. This elasticity lets Euglena squeeze through tiny spaces and evade predators, by altering its morphology.

Importance of the Pellicle

Additionally, the pellicle plays a role in cell division, ensuring that the structural integrity of the cell is maintained throughout the process. This feature is so central to Euglena's survival and functionality that it's often highlighted in scientific descriptions of the organism.

Flagella in Euglena

Movement in Euglena is facilitated by flagella, which are whip-like appendages. A typical Euglena has two flagella protruding from its cell, but there's an intriguing detail: these flagella are of unequal length.

While one flagellum is quite short and usually not visible without advanced microscopy, the other is considerably longer and can often be seen beating rhythmically to propel the organism through its aquatic habitat. This juxtaposition in flagellar length is both a defining characteristic and a tool for movement and navigation.

Functionality of Flagella

The long flagellum is actively involved in steering and moving Euglena towards light sources, a behavior known as phototaxis, which is crucial for photosynthesis. Moreover, the flagella can detect chemical changes in the environment, helping Euglena respond to different stimuli.

Mixotrophic Nutrition

One of the most fascinating aspects of Euglena is its mixotrophic nutrition, combining autotrophic and heterotrophic modes of sustenance. When light is available, Euglena can photosynthesize, thanks to its chloroplasts which contain the pigment chlorophyll. This means that it can produce its own food by converting sunlight into energy-rich molecules, a process known as autotrophy.

However, in the absence of light or when photosynthesis is not possible, Euglena can switch to a heterotrophic lifestyle. It ingests food particles from its environment by phagocytosis, enveloping the food with its cell membrane and creating a food vacuole where digestion occurs.

Versatility in Nutrition

This nutritional flexibility allows Euglena to thrive in varying conditions where other organisms might struggle to survive. The term 'mixotrophic' perfectly encapsulates the dual capability of Euglena to alternate between capturing energy from the sun and consuming organic matter as needed.