Cell Structure and Function

Chapter 3: Cell Structure and Function

Overview of Bacterial Predator

Bdellovibrio bacteriovorus: A Gram-negative predatory bacterium that targets Escherichia coli and is being investigated for biological therapy to combat infections caused by pathogenic strains of E. coli. Notable methods of its study include cryo-electron tomography, which created detailed images of its cell structure.
Reference: Oikonomou, C., Chang, YW., & Jensen, G. (2016). A new view into prokaryotic cell biology from electron cryotomography, Nat Rev Microbiol 14, 205–220. https://doi.org/10.1038/nrmicro.2016.7

Learning Objectives

3.1 The Bacterial Cell: An Overview

  • Describe the overall structure and function of a bacterial cell, including its major compartments.

  • Explain laboratory techniques for studying bacterial cell structures.

3.2 Membrane Molecules and Transport

  • Identify key components of cell membranes, including phospholipids and proteins.

  • Explain the interaction of the bacterial cell membrane with small molecules.

3.3 The Envelope and Cytoskeleton

  • Outline different types of cell envelopes (cell wall, Gram-negative outer membrane) and explain the role of the cytoskeleton in maintaining cell shape.

3.4 Bacterial Cell Division

  • Discuss coordination of DNA replication, cell expansion, and septation during bacterial cell division.

3.5 Cell Asymmetry, Membrane Vesicles, and Extensions

  • Explain how asymmetry and polarity influence bacterial reproduction and the use of vesicles and extensions.

3.6 Specialized Structures

  • Describe the functions of various subcellular structures in different bacterial taxa, such as carboxysomes and attachment pili.

Introduction to Bacterial Cells

Bacteria and archaea are prokaryotic domains of life characterized by their lack of a nucleus. Eukaryotes, in contrast, possess distinct membranous organelles. Bacterial cells have permeable phospholipid bilayers, compact genomes, and a high degree of functional coordination.

Bacterial Cell Characteristics

  1. Cell Envelope: Comprising a complex outer layer that protects the cell and mediates environmental exchange.

  2. Compact Genome: Prokaryotic genomes are streamlined to optimize resource use.

  3. Function Coordination: Interdependent cell functions allow rapid reproduction.

The Bacterial Cell Structure

Model Bacterial Cell

The model organism for discussion is Escherichia coli, characterized by its Gram-negative structure featuring an inner membrane and an outer membrane.

  1. Cytoplasm: Contains enzymes, mRNA, ribosomes, and chromosomal DNA within a gel-like matrix.

  2. Inner Membrane: Barriers for cytoplasmic contents, crucial for maintaining ion gradients.

  3. Cell Wall & Periplasm: Peptidoglycan forms a rigid structure contributing to turgor pressure.

  4. Outer Membrane: Composed of phospholipids and lipopolysaccharides (LPS), offers additional protection and can act as an endotoxin.

  5. Capsule: A polysaccharide layer that enhances virulence by inhibiting phagocytosis.

Key Definitions

  • Chemotaxis: Movement towards nutrients via chemoreceptors responding to environmental signals.

  • Turgor Pressure: The pressure within a cell that maintains rigidity.

Membrane Structure

The bacterial cell membrane is a crucial physiological structure that embodies a phospholipid bilayer consisting predominantly of phospholipids, which are amphipathic molecules featuring both hydrophobic (water-repelling) and hydrophilic (water-attracting) properties. This unique architecture allows the membrane to self-assemble into a bilayer, where the hydrophilic head groups face the aqueous environment, and the hydrophobic tails are shielded from water, creating a hydrophobic core. This bilayer not only provides structural integrity but also serves as a dynamic barrier that controls the movement of substances in and out of the cell, thereby maintaining cellular homeostasis.

The fluid nature of the bacterial cell membrane is significant. It is not a static structure; rather, it is fluid and flexible due to the presence of unsaturated fatty acids in the phospholipids. This fluidity enables proteins to move laterally within the membrane, facilitating efficient interaction between proteins and other molecules. Membrane proteins serve various roles, including forming channels and transporters that regulate the passage of ions, nutrients, and waste products.

ATP synthase, a critical protein embedded in the membrane, plays a vital role in cellular energy production. It utilizes the proton motive force generated across the membrane to synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate. This process occurs during cellular respiration and photosynthesis, highlighting the membrane's importance in energy metabolism.

Additionally, the cell membrane houses various receptors that detect environmental signals, allowing the bacterium to respond dynamically to changes in its surroundings. These receptors initiate signaling cascades that can affect cellular behavior, such as motility, virulence, and nutrient acquisition.

Another important aspect of the bacterial cell membrane is its role in maintaining osmotic balance. The membrane's selective permeability ensures that only certain substances can pass through, preventing the loss of essential ions while allowing waste products to exit. This selective transport is critical in protecting the cell from osmotic shock and ensuring viability in fluctuating environmental conditions.

Biochemical Composition of Bacteria

The major biochemical components include water, proteins, nucleic acids, lipids, and small organic molecules. A balance of these components ensures cellular function.

Table of E. coli Composition

  • Water: 70%

  • Proteins: 16%

  • RNA: 6% (including rRNA, tRNA)

  • DNA: 1%

  • Lipids: 4% (e.g., phospholipids, LPS)

Cell Fractionation Techniques

Cell fractionation separates components like membranes and ribosomes for detailed studies, despite losing interactions between components.

Ultracentrifugation

This method employs high speeds to separate cell components based on their density, and techniques like lysozyme treatment facilitate the lysis of cell walls for further analysis.

Summary of Bacterial Cells

Bacterial cells feature protective envelopes, composed of various layers depending on the type. They are metabolically efficient, rich in essential biomolecules, and maintain intricate structures facilitating survival and reproduction under diverse conditions.

Glossary

  • Cell membrane: The phospholipid bilayer that encloses the cytoplasm.

  • Peptidoglycan: A polymer that forms the bacterial cell wall.

  • Spheroplast: A cell with modified integrity due to disrupted cell wall.

  • Lipopolysaccharides (LPS): Molecules that play a critical role in the outer membrane structure of Gram-negative bacteria.

  • Porin: A membrane protein that allows ions and small molecules to pass.

Chapter Addendum

This chapter also explores specialized structures that contribute to survival, such as thylakoids for photosynthesis, carboxysomes for CO2 fixation, and various organelles to store energy and manage cellular processes.

Bacterial Cell Division

Coordination of Factors

Bacterial division occurs through a highly regulated process known as binary fission, characterized by cell elongation followed by septation. During this process, the bacterium prepares for division by first elongating its cell envelope and synthesizing new cellular components to ensure a balanced division of materials. This elongation phase is crucial as it increases the surface area of the cell and ensures that vital cellular components are present in sufficient quantities for both daughter cells.

The septation phase involves the coordinated assembly and action of a range of proteins, the most critical of which is FtsZ. FtsZ is a tubulin-like protein that polymerizes to form a contractile structure known as the Z-ring at the future division site of the cell. The Z-ring serves as a scaffold for other proteins that are recruited to assemble the divisome, a complex molecular machine responsible for cell division.

As DNA replication commences, the FtsZ ring constricts, pulling the cytoplasmic membrane inwards and forming a septum that ultimately divides the cell into two distinct daughter cells. This process not only ensures that each daughter cell receives a copy of the genetic material but also that essential cellular machinery is evenly distributed. Other proteins involved in this process include Min proteins, which help localize the Z-ring to the center of the cell, and proteins like ZipA and FtsA that anchor the Z-ring to the membrane and link it to the cell wall synthesis machinery.

Additionally, cellular signaling systems and checkpoints are in place to ensure that division occurs only when conditions are favorable, preventing premature or inappropriate division. Understanding bacterial cell division is crucial for potential antibiotic developments targeting peptidoglycan synthesis, as disrupting this process can lead to cell death and offer a pathway for treating bacterial infections effectively.

Summary of Division Dynamics

Bacterial cells continuously synthesize RNA and proteins throughout their growth cycle, ensuring an efficient and coordinated division process. This synthesis is crucial as it provides the necessary components for cellular function and allows for the rapid adaptation to changing environments. The growth cycle includes several phases: lag, exponential (log), stationary, and death phases. During the exponential phase, the rate of cell division increases significantly, leading to a substantial increase in cell numbers.

Understanding bacterial cell division is crucial for developing potential antibiotics that target peptidoglycan synthesis, which is vital for maintaining cell wall integrity. Disruption of this process can lead to cell lysis and death, thus representing a promising avenue for treating bacterial infections. Additionally, studying bacterial division helps uncover alternative mechanisms unique to different bacterial taxa, which could be exploited for novel antimicrobial strategies.

Furthermore, the regulation of the division process is influenced by a variety of environmental factors such as nutrient availability, temperature, and the accumulation of metabolites. Bacteria can sense these changes through signaling pathways that modulate the expression of genes involved in cell division, ensuring that replication occurs under optimal conditions. The coordination of cell division with other cellular processes like DNA replication is also critical. For example, before a bacterium divides, it must ensure that its DNA is fully replicated and accurately segregated to each daughter cell.

New research into the molecular mechanisms of bacterial cell division provides insights into the roles of proteins like FtsZ, ZipA, and FtsA, along with their interactions with other cellular components during the division process. Understanding these interactions can help identify potential drug targets and improve strategies for combating antibiotic resistance. Moreover, examining the variations of division mechanisms among different bacterial species enhances our understanding of microbial diversity and evolution.

Overall, a comprehensive understanding of bacterial cell division not only illuminates fundamental biological processes but also paves the way for innovative approaches to tackle bacterial infections effectively.

Cell Asymmetry and Extensions

Certain bacteria, like Caulobacter crescentus, generate asymmetric cell types during division, resulting in cellular differentiation and specialization. Moreover, certain species develop extensions and nanotubes for enhanced nutrient acquisition and intercellular communication.