Q. 98 The primary function of polysaccharides attached to glycoproteins in the animal cell membrane is
to
(A) facilitate diffusion of molecules down their concentration gradients.
(B) maintain membrane fluidity at low temperatures.
(C) maintain the integrity of a fluid mosaic membrane.
(D) mediate cell-to-cell recognition

This article breaks down a key biology multiple-choice question on cell membrane structure, focusing on glycoproteins. It’s tailored for students preparing for exams like NEET, CSIR NET, or undergrad biology courses, providing the correct answer, detailed explanations, and why other options fall short.

Glycoproteins play a crucial role in the plasma membrane of animal cells, where proteins attach to carbohydrate chains called polysaccharides. These structures, known as the glycocalyx, contribute to various cellular processes. But what is their primary function? Let’s dive into this common MCQ from biology exams.

Correct Answer: (D) Mediate Cell-to-Cell Recognition

Polysaccharides on glycoproteins act as identification tags, enabling cells to recognize each other. This is essential for processes like immune responses, tissue formation, and preventing attacks from the body’s defenses. For instance, blood types (A, B, AB, O) rely on specific polysaccharide antigens on red blood cell glycoproteins—mismatched transfusions trigger immune rejection. In development, these markers guide cell sorting and adhesion.

This function aligns with the fluid mosaic model, where the glycocalyx provides unique “signatures” for cell communication, far outweighing other roles.

Why Not the Other Options?

Here’s a clear breakdown of each incorrect choice, grounded in membrane biology:

• (A) Facilitate diffusion of molecules down their concentration gradients
  Diffusion relies on membrane phospholipids and proteins like channels or carriers (e.g., aquaporins). Glycoprotein polysaccharides don’t form pores or transporters; they project outward into the extracellular space. This option confuses glycoproteins with transport proteins.

• (B) Maintain membrane fluidity at low temperatures
  Fluidity comes from phospholipid fatty acid saturation—unsaturated chains prevent packing at low temperatures, aided by cholesterol. Polysaccharides are hydrophilic sugars on the external surface; they don’t insert into the lipid bilayer to influence packing or phase transitions.

• (C) Maintain the integrity of a fluid mosaic membrane
  The fluid mosaic model describes proteins floating in a phospholipid bilayer, stabilized by hydrophobic interactions and cytoskeletal links. While glycoproteins contribute to structure, their polysaccharides aren’t the primary stabilizers—integrity stems from lipids and proteins, not carbs. This option is vague and doesn’t pinpoint recognition.

Key Takeaways for Exam Prep

• Focus on context: In animal cells, glycoprotein carbs are asymmetric (extracellular only), optimized for recognition.

• Real-world links: Think lectins (proteins binding specific sugars) in cell adhesion or pathogen evasion.

• Related concepts: Compare with plant cells, where cell walls handle rigidity, not membrane glycoproteins.

Mastering this reinforces your understanding of cell signaling and membrane dynamics. For more MCQs on primary function of polysaccharides in glycoproteins.