Problem 51 - Objective Biology for NEET

Question

What is external to cell wall in a prokaryotic cell? (a) Glycocalyx layer (b) Plasma membrane (c) Both (a) and (b) (d) None of these

Step-by-Step Solution

Answer

The structure that is external to cell wall in a prokaryotic cell is the Glycocalyx layer. Hence, the correct answer is (a) Glycocalyx layer.

Step 1: Understand the structure of a prokaryotic cell

First, one should recall or revise that prokaryotic cells are simple and smaller as compared to eukaryotic cells. They consist of various layers which include the plasma membrane, cell wall, and an outermost layer known as the glycocalyx.

Step 2: Determine the external layer of a prokaryotic cell

According to the structure of a prokaryotic cell, from inside to outside, it generally includes plasma membrane, cell wall, and glycocalyx. The question asks for the part that is external to the cell wall. Observing the structure, it is clear that the glycocalyx is located outside the cell wall.

Key Concepts

Glycocalyx

The glycocalyx is an outermost layer found in prokaryotic cells like bacteria. It is a significant aspect because it serves multiple functions essential for the cell's survival and adaptability. Glycocalyx can be further classified into two forms: the capsule and the slime layer. The capsule is dense and difficult to remove, whereas the slime layer is more loosely attached.
One of the primary roles of the glycocalyx is protection. It shields the cell from environmental stress, such as desiccation (drying out) and phagocytosis by other cells or organisms. It also aids in adherence to surfaces, allowing bacteria to colonize and form biofilms effectively.

• Protects against dehydration and engulfment by other cells
• Helps in attachment to surfaces, facilitating biofilm formation
• Can act as a barrier against certain antibiotics

Overall, the glycocalyx enhances the survivability of prokaryotic cells in varied environments.

Cell Wall

The cell wall is a critical structure for most prokaryotic cells, providing them with shape and strength. It surrounds the plasma membrane and determines the form and rigidity of the cell.
In bacteria, the cell wall is primarily composed of a unique molecule known as peptidoglycan. This compound consists of sugars and amino acids, forming a mesh-like structure that retains the cell's shape and protects it from osmotic pressures.
Functions of the cell wall include:

• Maintaining the shape of the cell
• Preventing the cell from bursting in hypotonic environments
• Serving as a protective barrier against external insults

Notably, the cell wall's specific composition can vary, leading to different staining properties like Gram-positive and Gram-negative, which are crucial for bacterial classification.

Plasma Membrane

The plasma membrane is an essential component of prokaryotic cells, as well as all living cells. It is the innermost layer of the cell's external boundaries, located just beneath the cell wall.
This membrane is primarily composed of phospholipids and proteins forming a bilayer. Its primary function is to create a semi-permeable barrier that regulates the movement of substances in and out of the cell.
Key functions of the plasma membrane include:

• Controlling the entry and exit of nutrients, waste products, and gases
• Facilitating communication between the cell and its environment
• Supporting processes like energy production and metabolism

Within prokaryotic cells, the plasma membrane also takes on additional roles, such as helping to anchor the DNA during cell division and playing a part in other essential cellular functions.