Problem 5 - Biology Today and Tomorrow Without Physiology

Question

Unlike eukaryotic cells, prokaryotic cells ________________ . a. have no plasma membrane b. have RNA but not DNA c. have no nucleus d. a and \(c\)

Step-by-Step Solution

Answer

Option c: Prokaryotic cells have no nucleus.

Step 1: Understand the Characteristics of Prokaryotic Cells

Prokaryotic cells are a fundamental type of cell that lacks a membrane-bound nucleus. Their genetic material, unlike eukaryotic cells, is not enclosed in a nuclear membrane.

Step 2: Evaluate Option A

Consider whether prokaryotic cells have a plasma membrane. All cells, including prokaryotic cells, have a plasma membrane that protects and encloses their contents.

Step 3: Evaluate Option B

Check if prokaryotic cells have RNA but not DNA. Prokaryotic cells have both DNA and RNA, just like eukaryotic cells, although their DNA is organized differently.

Step 4: Evaluate Option C

Consider the existence of a nucleus in prokaryotic cells. Prokaryotic cells do not have a true nucleus; their genetic material is not surrounded by a nuclear membrane.

Step 5: Evaluate Option D

Option D combines statement a and c. Since option a is incorrect (prokaryotes do have a plasma membrane) and option c is correct, option D is not correct because one of its components is false.

Step 6: Determine the Correct Answer

Since option c is the only correct statement regarding prokaryotic cells, which lack a nucleus, it stands as the accurate choice.

Key Concepts

Cell Structure

Prokaryotic cells have a simple yet efficient structure that allows them to thrive in a variety of environments. Unlike eukaryotic cells, prokaryotic cells do not have membrane-bound organelles. Their internal structure is more rudimentary compared to the complexity observed in eukaryotic cells. This simplicity is a key feature:

• Lack of Membrane-Bound Nucleus: Genetic material is found freely in the cytoplasm, organized in a region called the nucleoid.
• Small Size: Generally range from 0.1 to 5.0 micrometers, facilitating rapid growth and reproduction.
• Shapes: Can be spherical (cocci), rod-shaped (bacilli), or spiral (spirilla).

The simplicity and efficiency of the prokaryotic cell structure enable it to adapt and rapidly proliferate in diverse habitats.

Nucleus

A defining characteristic of prokaryotic cells is the absence of a true nucleus. In contrast to eukaryotic cells, which enclose their DNA within a nuclear membrane, prokaryotic DNA is not separated from the rest of the cell by a membrane. This results in several distinct features:

• Nucleoid: The region in the cytoplasm where DNA is concentrated. It lacks a surrounding membrane, setting it apart from the nucleus in eukaryotic cells.
• Circular DNA: Prokaryotic cells typically have a single, circular chromosome as opposed to the multiple linear chromosomes found in eukaryotes.
• Plasmids: Small, circular DNA molecules that may be present and carry additional genes beneficial for survival, such as antibiotic resistance.

The lack of a membrane-bound nucleus allows for more straightforward and rapid transcription and translation processes.

Plasma Membrane

Prokaryotic cells are surrounded by a plasma membrane, a crucial structure that maintains the integrity and homeostasis of the cell. This semi-permeable membrane plays several key roles:

• Barrier Function: Separates the interior of the cell from the external environment, protecting the cell from unwanted substances.
• Transport: The plasma membrane controls the entry and exit of nutrients, ions, and waste, essential for sustaining cell functions.
• Sensory Receptors: Contains proteins that sense environmental changes, enabling the cell to respond accordingly.

Comprised of a lipid bilayer, the plasma membrane’s structure is similar to that of eukaryotic cells, although typically simpler in prokaryotes.

Genetic Material

In prokaryotic cells, genetic material is present in a unique and efficient form that serves the organism's simple structure and rapid reproductive needs. Here are the main characteristics:

• DNA Structure: Prokaryotic cells typically contain a single circular DNA strand located in the nucleoid region.
• Genome Size: Generally smaller and more compact than eukaryotic genomes due to fewer non-coding regions.
• Binary Fission: Prokaryotic cells reproduce through a process called binary fission, where the DNA is duplicated and the cell divides into two identical cells.

Additionally, the simplicity of prokaryotic genetic material allows for quick replication and adaptation, contributing to the prolific nature of these organisms.