Cell Structure, Microscopy, and Prokaryotic vs. Eukaryotic Cells

The Cell

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. They can be studied using various types of microscopes, each providing different levels of detail and magnification.

• Prokaryotic cells are simpler, lacking a nucleus and most organelles.

• Eukaryotic cells are more complex, containing a nucleus and membrane-bound organelles.

Microscopy and Cell Visualization

Types of Microscopes

Microscopes are essential tools for studying cells and their structures. The choice of microscope depends on the size of the object and the level of detail required.

• Light Microscopes (LMs): Use visible light to magnify specimens up to about 1000x. Suitable for viewing most plant and animal cells, but not detailed internal structures.

• Electron Microscopes (EMs): Use beams of electrons for much higher resolution and magnification.

  • Scanning Electron Microscopes (SEMs): Focus a beam of electrons onto the surface of a specimen, producing detailed 3D images of cell surfaces.

  • Transmission Electron Microscopes (TEMs): Transmit electrons through a thin specimen, allowing detailed study of internal cell structures.

Example: The Klebsiella pneumoniae bacterium can be visualized at high magnification using a TEM, revealing its internal and surface structures.

Scale of Biological Structures

Biological structures vary greatly in size, from atoms and small molecules to entire cells and organisms. Understanding scale is crucial for selecting the appropriate microscopy technique.

• Most plant and animal cells: 10–100 μm

• Bacteria: 1–10 μm

• Viruses: 20–300 nm

• Proteins: ~5–10 nm

• Atoms: ~0.1 nm

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

Prokaryotic cells are found in organisms of the domains Bacteria and Archaea. They are generally smaller and simpler than eukaryotic cells.

• No nucleus: DNA is located in a region called the nucleoid.

• No membrane-bound organelles.

• Plasma membrane: Composed of a phospholipid bilayer with embedded proteins.

• Cell wall: Provides structural support; in bacteria, often contains peptidoglycan.

• Ribosomes: Sites of protein synthesis, smaller than those in eukaryotes.

• Cytoplasm: The interior of the cell, filled with cytosol.

Example: Klebsiella pneumoniae is a prokaryotic bacterium that can be visualized using electron microscopy.

Gram-Positive vs. Gram-Negative Bacteria

Bacterial cell walls differ in structure, affecting their staining properties and susceptibility to antibiotics.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
Staining	Retains crystal violet (purple)	Does not retain crystal violet (pink/red)

Eukaryotic Cells

Eukaryotic cells are found in animals, plants, fungi, and protists. They are characterized by internal compartmentalization and specialized organelles.

• Nucleus: Contains the cell's genetic material (DNA) and is surrounded by a nuclear envelope.

• Membrane-bound organelles: Include mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and (in plants) chloroplasts.

• Plasma membrane: Similar in structure to prokaryotes, but with additional complexity.

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

Endosymbiont Theory

The endosymbiont theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. It proposes that these organelles originated as free-living prokaryotes that were engulfed by an ancestral eukaryotic cell.

• Evidence: Both mitochondria and chloroplasts have double membranes, their own DNA, and ribosomes similar to those of prokaryotes. They can also grow and reproduce independently within the cell.

Cellular Organelles and Their Functions

Membranous Organelles

• Nucleus: Stores genetic information and coordinates cell activities.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery to different destinations.

• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.

• Mitochondria: Sites of aerobic respiration and ATP production.

• Chloroplasts (in plants): Sites of photosynthesis, converting light energy into chemical energy.

Nonmembranous Organelles

• Ribosomes: Sites of protein synthesis; can be free in the cytoplasm or bound to the rough ER.

• Cytoskeleton: Provides structural support, maintains cell shape, and assists in cell movement.

• Centrioles: Involved in organizing microtubules during cell division (mainly in animal cells).

Chromatin, Chromosomes, and Histones

Definitions and Functions

• Chromatin: Complex of DNA and proteins (mainly histones) found in the nucleus; condenses to form chromosomes during cell division.

• Chromosomes: Threadlike structures composed of DNA and proteins; carry genetic information.

• Histones: Proteins that DNA wraps around to form nucleosomes, aiding in DNA packaging and gene regulation.

Example: Nucleosomes are the basic units of chromatin, consisting of DNA wrapped around histone proteins.

Cell Compartmentalization

Importance and Mechanisms

Compartmentalization in eukaryotic cells allows for specialized environments and efficient metabolic processes. Membranes divide the cell into distinct organelles, each with unique functions.

• Enables incompatible processes to occur simultaneously.

• Facilitates regulation and coordination of cellular activities.

Summary Table: Major Organelles and Their Functions

Organelle	Main Function	Membranous?	Part of Endomembrane System?
Nucleus	Stores genetic material, controls cell activities	Yes	No
Ribosome	Protein synthesis	No	No
Rough ER	Protein synthesis and processing	Yes	Yes
Smooth ER	Lipid synthesis, detoxification	Yes	Yes
Golgi Apparatus	Modification, sorting, packaging of proteins/lipids	Yes	Yes
Lysosome	Digestion of macromolecules	Yes	Yes
Mitochondrion	ATP production (cellular respiration)	Yes	No
Chloroplast	Photosynthesis	Yes	No

Key Equations and Calculations

• Magnification Calculation: To determine the actual size of a specimen from a micrograph:

  • $\text{Actual size} = \frac{\text{Measured size on image}}{\text{Magnification}}$

Additional info:

• Some context and explanations were inferred from standard biology textbooks to clarify fragmented notes and images.

• Scientific names such as Klebsiella pneumoniae are italicized according to convention.