Cell Structure and Function in Microbiology Ch3-2

Cell Structure and Function

Introduction

This chapter explores the structural and functional organization of prokaryotic and eukaryotic cells, focusing on the cytoplasmic membrane, transport mechanisms, cytoplasmic components, and the unique features of bacteria, archaea, and eukaryotes. Understanding these cellular structures is fundamental to microbiology, as they underpin microbial physiology, adaptation, and survival.

Bacterial Cytoplasmic Membranes

Active Transport and Group Translocation

The bacterial cytoplasmic membrane is essential for controlling the movement of substances into and out of the cell. It utilizes both passive and active transport mechanisms:

• Active Transport: Requires energy (usually ATP) to move substances against their electrochemical gradient. Types include unipport, antiport, and symport systems.

• Group Translocation: A unique form of active transport where the substance is chemically modified during its passage across the membrane (e.g., phosphorylation of glucose).

Types of Transport Processes

Transport across bacterial membranes can be classified as passive or active:

• Passive Transport: Does not require cellular energy. Includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires ATP to move substances against their gradient.

• Group Translocation: Substance is chemically altered during transport (e.g., glucose to glucose-6-phosphate).

Table: Transport Processes Across Bacterial Cytoplasmic Membranes

Process	Description	Examples of Transported Substances
Diffusion	Molecules move down their electrochemical gradient through the phospholipid bilayer.	O2, CO2, lipid-soluble chemicals
Facilitated diffusion	Molecules move down their gradient through channels or carrier proteins.	Glucose, fructose, urea, some vitamins
Osmosis	Water moves down its concentration gradient across a selectively permeable membrane.	Water
Active transport	ATP-dependent carrier proteins bring substances into the cell.	Na+, K+, Ca2+, H+, Cl−
Group translocation	Substance is chemically altered during transport.	Glucose, mannose, fructose

Cytoplasm of Bacteria

Cytosol and Inclusions

The cytosol is the liquid portion of the cytoplasm, primarily composed of water and containing the cell's DNA in the nucleoid region. Inclusions are reserve deposits of chemicals, such as polyhydroxybutyrate (PHB) granules, which serve as energy and carbon storage.

Endospores

Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus and Clostridium) as a defense against unfavorable conditions. They are resistant to heat, radiation, and chemicals, allowing survival in extreme environments.

• Formation: Vegetative cells transform into endospores when nutrients are limited.

• Significance: Endospores can remain viable for decades and are a concern in medical and food industries due to their resilience.

Cytoplasm of Prokaryotes

Nonmembranous Organelles

• Ribosomes: Sites of protein synthesis, composed of polypeptides and rRNA. Prokaryotic ribosomes are 70S (smaller than eukaryotic 80S ribosomes).

• Cytoskeleton: Composed of protein fibers, involved in cell division, maintaining cell shape, DNA segregation, and movement.

External Structures of Archaea

Glycocalyces, Flagella, Fimbriae, and Hami

• Glycocalyces: Aid in biofilm formation and cell adhesion.

• Flagella: Structurally distinct from bacterial flagella; used for motility.

• Fimbriae and Hami: Fimbriae are used for attachment; hami are unique, hook-like structures for surface attachment.

Archaeal Cell Walls and Cytoplasmic Membranes

Structure and Function

• Most archaea have cell walls composed of specialized polysaccharides and proteins (not peptidoglycan).

• All archaea have cytoplasmic membranes that maintain gradients and regulate transport.

Cytoplasm of Archaea

Similarities and Differences with Bacteria

• Both have 70S ribosomes, fibrous cytoskeleton, and circular DNA.

• Archaea differ in ribosomal proteins, metabolic enzymes, and genetic code (more similar to eukaryotes).

Table: Some Structural Characteristics of Prokaryotes

Feature	Archaea	Bacteria
Glycocalyx	Polypeptide or polysaccharide	Polypeptide or polysaccharide
Flagella	Present in some; 10–14 nm diameter; grow at base	Present in some; ~20 nm diameter; grow at tip
Fimbriae	Proteinaceous; attachment, biofilms	Proteinaceous; attachment, motility, biofilms
Pili	None discovered	Present in some; DNA exchange
Hami	Present in some; attachment	Absent
Cell Walls	Polysaccharides/proteins (no peptidoglycan)	Peptidoglycan
Cytoplasmic Membrane	Ether-linked lipids; some single layer	Ester-linked phospholipid bilayer
Cytoplasm	Circular DNA, 70S ribosomes (eukaryote-like proteins)	Circular DNA, 70S ribosomes (bacterial proteins)

External Structure of Eukaryotic Cells

Glycocalyces

• Less organized than prokaryotic capsules.

• Functions: anchor cells, strengthen surfaces, prevent dehydration, and mediate cell recognition and communication.

Eukaryotic Cell Walls and Cytoplasmic Membranes

Cell Walls

• Present in fungi, algae, plants, and some protozoa.

• Composed of cellulose (plants), chitin/glucomannan (fungi), or various polysaccharides (algae).

Cytoplasmic Membranes

• Fluid mosaic of phospholipids and proteins.

• Contain steroid lipids for fluidity and membrane rafts for specialized functions.

• Control movement of substances into and out of the cell.

Endocytosis and Exocytosis

• Endocytosis: Uptake of substances via phagocytosis (solids) or pinocytosis (liquids) using pseudopods.

• Exocytosis: Release of substances by fusion of vesicles with the membrane.

Table: Active Transport Processes Found Only in Eukaryotes

Process	Description	Examples
Endocytosis	Substances surrounded by pseudopods and brought into cell	Bacteria, viruses, dead cells, liquid nutrients
Exocytosis	Vesicles fuse with membrane, releasing contents	Wastes, secretions

Cytoplasm of Eukaryotes

Flagella and Cilia

• Flagella: Differ structurally from prokaryotic flagella; composed of microtubules in a "9+2" arrangement, anchored by a basal body, and move by undulation.

• Cilia: Shorter, more numerous, and beat in coordinated waves to move cells or substances.

Other Nonmembranous Organelles

• Ribosomes: Larger (80S) than prokaryotic ribosomes, composed of 60S and 40S subunits.

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments for shape, support, and movement.

• Centrioles and Centrosome: Involved in mitosis, cytokinesis, and formation of flagella/cilia (not present in all eukaryotes).

Membranous Organelles

• Nucleus: Contains most DNA, surrounded by a nuclear envelope with pores; nucleolus synthesizes RNA.

• Endoplasmic Reticulum (ER): Network for transport and synthesis; rough ER (with ribosomes) and smooth ER (lipid synthesis).

• Golgi Body: Processes and packages molecules for export.

• Lysosomes, Peroxisomes, Vacuoles, Vesicles: Storage, digestion, and detoxification.

• Mitochondria: Double-membraned, produce ATP, contain their own DNA and 70S ribosomes.

• Chloroplasts: Found in photosynthetic eukaryotes, site of photosynthesis, contain DNA and 70S ribosomes.

Table: Nonmembranous and Membranous Organelles of Cells

Organelle	General Function	Prokaryotes	Eukaryotes
Ribosomes	Protein synthesis	Present in all	Present in all
Cytoskeleton	Shape, support, movement	Some	All
Centrosome	Mitosis, cytokinesis, flagella/cilia formation	Absent	Animals
Nucleus	Control center	Absent	All
ER	Transport, lipid synthesis	Absent	All
Golgi bodies	Secretion	Absent	Some
Lysosomes	Breakdown, self-destruction	Absent	Some
Peroxisomes	Neutralize toxins	Absent	Some
Vacuoles/Vesicles	Storage, digestion, transport	Absent	Some/All
Mitochondria	ATP production	Absent	Most
Chloroplasts	Photosynthesis	Absent	Plants, algae

Comparison of Archaeal, Bacterial, and Eukaryotic Cells

Key Differences

Characteristic	Archaea	Bacteria	Eukaryotes
Nucleus	Absent	Absent	Present
Organelles	Absent	Few	Various types present
Glycocalyx	Present	Present	Some
Motility	Some	Some	Some
Flagella	Some; rotate	Some; rotate	Some; undulate
Cilia	Absent	Absent	Some
Fimbriae/Pili	Some	Some	Absent
Hami	Some	Absent	Absent
Cell Wall	Most; no peptidoglycan	Most; peptidoglycan	Plants, algae, fungi
Cytoplasmic Membrane	All	All	All
Cytosol	All	All	All
Inclusions	Most	Most	Some
Endospores	Absent	Some	Absent
Ribosomes	70S	70S	80S (cytosol/ER), 70S (mitochondria/chloroplasts)
Chromosomes	Single, circular	Single, circular	Linear, multiple

Endosymbiotic Theory

The endosymbiotic theory proposes that eukaryotic cells originated from a symbiotic relationship between small aerobic prokaryotes (which became mitochondria) and larger anaerobic prokaryotes. Chloroplasts are thought to have a similar origin in photosynthetic eukaryotes.

Summary

Understanding the structural and functional differences among bacteria, archaea, and eukaryotes is crucial for microbiology. These differences impact microbial physiology, adaptation, and the development of antimicrobial strategies.