Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Introduction

This chapter explores the structural and functional differences between prokaryotic and eukaryotic cells, which are fundamental to understanding microbiology. The content covers cell types, their distinguishing features, and the diversity of bacterial shapes and structures.

Two Kinds of Cells

Classification of Cells

All living organisms are composed of either prokaryotic or eukaryotic cells. These two cell types differ in their structural organization and complexity.

• Prokaryotes: Organisms whose cells lack a true nucleus and membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: Organisms whose cells have a true nucleus enclosed by a nuclear membrane and possess membrane-bound organelles. Includes Animals, Plants, Fungi, Algae, and Protozoa.

Cell Structure Overview

Prokaryotic and eukaryotic cells can be visually distinguished by their internal complexity and the presence or absence of a nucleus.

• Prokaryote comes from the Greek for "prenucleus." Their DNA is not enclosed within a membrane.

• Eukaryote comes from the Greek for "true nucleus." Their DNA is enclosed within a nuclear membrane.

Differences Between Prokaryotic and Eukaryotic Cells

Genetic Material and Internal Structure

• Prokaryotes:

  • DNA is not enclosed in a nuclear membrane; usually a single circular chromosome.

  • DNA is not associated with histone proteins.

  • No membrane-enclosed organelles.

  • Cell wall, if present, is complex.

• Eukaryotes:

  • DNA is enclosed in a nuclear membrane; multiple linear chromosomes.

  • DNA is associated with histones and non-histone proteins.

  • Contains membrane-bound organelles (e.g., Golgi complex, mitochondria, lysosomes).

  • Cell wall, if present, is simple.

Cell Division and Other Features

• Prokaryotes:

  • Divide by binary fission.

  • Size: 0.2–20 μm in diameter.

  • Plasma membrane lacks carbohydrates and generally sterols.

  • No cytoskeleton or cytoplasmic streaming.

  • Ribosomes are smaller (70S).

  • Asexual reproduction (may transfer DNA fragments).

• Eukaryotes:

  • Divide by mitosis; sexual reproduction involves meiosis.

  • Size: 10–100 μm in diameter.

  • Plasma membrane contains carbohydrates and sterols (serve as receptors).

  • Cytoskeleton present.

  • Ribosomes are larger (80S).

The Prokaryotes

General Characteristics

Prokaryotes include Bacteria and Archaea. Some bacteria, such as cyanobacteria, are photosynthetic. Bacterial species are differentiated by:

• Morphology (shape)

• Chemical composition (detected by staining)

• Nutritional requirements

• Biochemical activities

• Source of energy (sunlight or chemicals)

Shapes of Bacterial Cells

Basic Shapes

Bacteria exhibit a variety of shapes, which are important for identification and classification.

• Bacillus: Rod-shaped

• Coccus: Spherical

• Spiral: Includes Spirillum (rigid, spiral), Vibrio (curved rods), and Spirochete (flexible, helical)

Arrangements of Cocci

• Single

• Diplococci: Remain in pairs

• Tetrad: Groups of four

• Sarcinae: Cube-like groups of eight

• Streptococci: Chain-like pattern

• Staphylococci: Grape-like clusters

Arrangements of Bacilli and Spiral Forms

• Bacillus: Single, diplobacilli (pairs), streptobacilli (chains), coccobacilli (short rods)

• Spiral: Vibrios (curved rods), spirilla (helical, rigid), spirochetes (helical, flexible)

Unusual Shapes

• Star-shaped Stella

• Square Haloarcula

Monomorphic and Pleomorphic Bacteria

• Monomorphic: Most bacteria maintain a single shape, determined by heredity.

• Pleomorphic: Some bacteria can have more than one genetically controlled shape (e.g., Rhizobium, Corynebacterium).

Prokaryotic Cell Structure

Diagram of a Prokaryotic Cell

The prokaryotic cell contains several key structures, both internal and external to the cell wall.

Structures External to the Cell Wall

Glycocalyx

• A sugar coat on the surface of many cells.

• Bacterial glycocalyx is viscous (sticky) and is a gelatinous polymer of polysaccharide and/or polypeptide.

• Made inside the cell and secreted to the surface.

• If organized and firmly attached, called a capsule.

• If unorganized and loose, called a slime layer.

Capsule

• Important virulence factor (e.g., Streptococcus pneumoniae is virulent if it has a capsule).

• Provides protection from phagocytosis (e.g., Bacillus anthracis).

• Allows pathogenic bacteria to attach to surfaces (e.g., Klebsiella in the respiratory tract).

• Can serve as a source of nutrition when energy stores are low.

• Protects against dehydration.

• Capsules are antigenic (contain antigens).

Flagella

• Long, filamentous appendages that propel bacteria (motility).

• Flagellar protein (H-antigen) is used to distinguish among serovars.

• Four arrangements:

  • Monotrichous: Single polar flagellum

  • Amphitrichous: Tuft of flagella at both ends

  • Lophotrichous: Two or more flagella at one pole

  • Peritrichous: Flagella distributed over the entire cell

Structure of a Prokaryotic Flagellum

• Three basic parts:

  1. Filament: Outermost region, constant diameter, composed of flagellin protein arranged in intertwining chains forming a helix around a hollow core.

  2. Hook: Attaches the filament to the cell, wider than the filament, composed of a different protein.

  3. Basal body: Anchors the flagellum to the cell wall and plasma membrane (not shown in the provided images, but important for context).

Additional info: The basal body is a complex structure that differs between Gram-positive and Gram-negative bacteria, anchoring the flagellum and enabling its rotation for motility.