College Biology on 06 March 2025 at 13.38.09 PM

Prokaryotic Cells

Ribosomes

• Definition: Non-membrane bound organelles found in prokaryotic cells.

• Function: Synthesize proteins using instructions from DNA, playing a crucial role in translating genetic information into functional proteins essential for cellular activities.

• Composition: Made up of ribosomal RNA (rRNA) and proteins, these structures vary in size between prokaryotes and eukaryotes, serving as the site of protein synthesis.

DNA Location

• Nucleoid: Found in the nucleoid, an irregularly shaped region within the cell, which is not membrane-bound. The nucleoid contains the cell's genetic material and facilitates the rapid transcription and replication processes crucial in prokaryotic cells.

• Plasmids: Contains plasmids, which are small, circular DNA molecules separate from chromosomal DNA that often carry genes beneficial for survival, such as antibiotic resistance. Plasmids can be transferred between bacteria through horizontal gene transfer, contributing to genetic diversity.

Chromosome Structure

• Shape: Prokaryotic chromosomes are circular in shape, in contrast to eukaryotic chromosomes, which are linear. This circular structure allows for efficient replication and compact storage within the nucleoid region.

• Supercoiling: The DNA is often supercoiled, allowing it to fit within the limited cell volume while maintaining integrity and accessibility for transcription.

Cell Wall Composition

• Material: Most bacteria have cell walls primarily composed of peptidoglycan, a polymer consisting of sugars and amino acids, which provides structural support and protection against environmental stress, contributing to the overall shape and rigidity of the bacterial cell.

• Types of Cell Walls: Cell walls may vary between gram-positive and gram-negative bacteria, influencing antibiotic susceptibility and the effectiveness of immune responses.

Eukaryotic Cells

Nucleus

• Membrane-bound Organelles: Eukaryotes possess a membrane-bound nucleus, which is the largest organelle and the most discernible feature.

• Location of Genetic Material: Houses the majority of the cell's genetic material (DNA), typically linear in structure, with organized chromatin that plays a vital role in gene expression and regulation.

• Nuclear Envelope: Composed of double membranes with nuclear pores that control the exchange of materials between the nucleus and the cytoplasm, regulating gene expression and cellular activity.

Size Comparison

• Comparison with Prokaryotic Cells: Eukaryotic cells are significantly larger than prokaryotic cells, typically ranging from 10 to 100 micrometers in diameter, allowing for more complex functions and compartmentalization of cellular processes.

Membrane-bound Organelles

• Variety of Organelles: Eukaryotic cells have various organelles with specific functions, such as mitochondria for energy production, the endoplasmic reticulum for protein and lipid synthesis, and the Golgi apparatus for modifying and packaging proteins.

• Specialized Functions: The presence of these organelles enhances cellular processes by allowing compartmentalization and decreasing the likelihood of conflicting chemical reactions.

Phospholipid Bilayer

• Structure: The primary component of eukaryotic cell membranes is phospholipids, forming a semi-permeable bilayer that regulates the entry and exit of substances while providing a barrier against harmful agents.

• Fluid Mosaic Model: This model describes the cell membrane’s structure, highlighting the dynamic nature of protein and lipid movement, which is essential for cell signaling and transport mechanisms.

Cytosol and Cytoskeletal Elements

Cytosol

• Definition: The liquid component of the cytoplasm, primarily composed of water (40-60%), with dissolved ions, small molecules, and large water-soluble molecules such as proteins.

• Function: Acts as a medium for biochemical reactions and provides volume to the cell, facilitating the movement of molecules and organelles within the cytoplasm.

Cytoskeleton

• Overview: Composed of three main types of protein filaments that play roles in providing structural support, shaping the cell, facilitating movement, and transporting materials:

  • Microfilaments (Actin Filaments): Dynamic structures involved in cell shape and movement, contributing to processes such as phagocytosis in white blood cells and muscle contraction.

  • Intermediate Filaments: Maintain structural integrity and stability, providing resistance to mechanical stress and anchoring organelles in place.

  • Microtubules: The largest filaments composed of tubulin dimers, playing crucial roles in transport within cells (e.g., vesicle movement) and chromosome movement during mitosis (cell division).

Cell Wall Structures in Different Organisms

• Plant Cells: Composed mainly of cellulose, providing rigidity and maintaining structure crucial for support, water retention, and protection against pathogens.

• Fungi: Cell walls made of chitin, which is structurally different from plant cell walls, provide support and protect against environmental stress.

• Bacterial Cells: Generally have a peptidoglycan layer for structural strength, the composition affecting the integrity and shape of bacteria.

• Animal Cells: Lack a cell wall, resulting in greater variability in shape compared to plant and fungal cells, allowing for complex tissue formation and functionality.

Summary of Key Differences between Prokaryotic and Eukaryotic Cells

• Nucleus: Present in eukaryotes; absent in prokaryotes, influencing cellular organization and gene regulation.

• Size: Eukaryotic cells are generally larger than prokaryotic cells, accommodating more complex cellular machinery and functions.

• Organelles: Varied organelles in eukaryotes provide specialized functions, unlike prokaryotes which have ribosomes and a nucleoid region, demonstrating the contrast in cellular complexity and specialization.