Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Cell Structure and Function

Major Processes of Living Cells

All living cells share fundamental processes that define life. These include growth, reproduction, responsiveness, metabolism, and cellular structure. Viruses, while often studied in microbiology, lack several of these processes and are considered acellular.

• Growth: Increase in size; occurs in all living cells but not in viruses.

• Reproduction: Increase in number; occurs in all living cells. Viruses replicate only within host cells.

• Responsiveness: Ability to react to environmental stimuli; present in all living cells, viruses respond only in some cases.

• Metabolism: Controlled chemical reactions; occurs in all living cells, viruses lack metabolism.

• Cellular Structure: Membrane-bound structure capable of all life functions; present in all living cells, absent in viruses.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on structural features. Prokaryotes include Bacteria and Archaea, while Eukaryotes include animals, plants, fungi, algae, and protozoa.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; DNA is located in a nucleoid region. Generally smaller and simpler.

• Eukaryotic Cells: Have a true nucleus and membrane-bound organelles (e.g., mitochondria, ER, Golgi bodies). Larger and more complex.

External Structures of Cells

Glycocalyces

The glycocalyx is a sticky, carbohydrate-rich layer outside the cell wall, important for protection, attachment, and immune evasion. It is especially relevant in pathogenic bacteria.

• Capsule: Well-organized, firmly attached; protects against phagocytosis.

• Slime Layer: Loose, irregular; aids in adhesion.

Flagella, Fimbriae, and Pili

• Flagella: Long, whip-like structures for motility. Arrangements include monotrichous, lophotrichous, amphitrichous, and peritrichous.

• Fimbriae: Short, numerous appendages for attachment.

• Pili: Longer, fewer; used for DNA transfer (conjugation).

Shapes and Arrangements of Bacterial Cells

• Cocci: Spherical

• Bacilli: Rod-shaped

• Spirilla/Spirochetes: Spiral-shaped

• Arrangements: Diplo- (pairs), Strepto- (chains), Staphylo- (clusters), Palisades, Tetrads

Bacterial Cell Walls

Bacterial cell walls are primarily composed of peptidoglycan, a mesh-like polymer of sugars and peptides. The structure and composition of the cell wall are critical for bacterial identification and antibiotic targeting.

• Peptidoglycan: Alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars, cross-linked by peptides.

• Gram-Positive: Thick peptidoglycan, teichoic acids, stains purple.

• Gram-Negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), stains pink/red.

• Acid-Fast: Waxy mycolic acids, resistant to staining and chemicals.

Cytoplasmic Membrane and Transport

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, forming a selectively permeable barrier. Transport across the membrane can be passive or active.

• Fluid Mosaic Model: Membrane proteins and lipids move laterally; proteins serve various functions.

• Passive Transport: No energy required; includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires ATP; includes uniport, antiport, symport, and group translocation.

Osmosis and Solution Effects

• Osmosis: Movement of water across a selectively permeable membrane.

• Isotonic: Equal solute concentration inside and outside.

• Hypertonic: Higher solute outside; cell shrinks.

• Hypotonic: Lower solute outside; cell swells.

Endocytosis and Exocytosis in Eukaryotes

Eukaryotic cells use endocytosis and exocytosis for bulk transport of materials.

• Endocytosis: Uptake of substances via vesicle formation; includes phagocytosis (solids) and pinocytosis (liquids).

• Exocytosis: Release of substances via vesicle fusion with the membrane.

Cytoplasm and Organelles

The cytoplasm contains water, enzymes, nutrients, ribosomes, plasmids, inclusions, and organelles (in eukaryotes). Organelles are specialized structures for cellular functions.

• Ribosomes: Protein synthesis; 70S in prokaryotes, 80S in eukaryotes.

• Cytoskeleton: Structural support, movement, and organization.

• Inclusions: Storage granules (e.g., glycogen, polyphosphate).

• Endospores: Highly resistant dormant structures formed by Bacillus and Clostridium.

Comparison of Organelles in Prokaryotes and Eukaryotes

Organelle	General Function	Prokaryotes	Eukaryotes
Ribosome	Protein synthesis	Present in all	Present in all
Cytoskeleton	Support, movement	Present in some	Present in all
Centrosome	Cell division, flagella/cilia formation	Absent	Present in animals
Nucleus	DNA storage	Absent	Present in all
Endoplasmic Reticulum	Protein/lipid synthesis	Absent	Present in all
Golgi Body	Modification, packaging	Absent	Present in some
Lysosome	Digestion	Absent	Present in animals
Peroxisome	Detoxification	Absent	Present in all
Vacuole	Storage	Absent	Present in some
Mitochondrion	ATP production	Absent	Present in all
Chloroplast	Photosynthesis	Absent	Present in plants/algae

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. It proposes that these organelles originated from prokaryotic cells engulfed by ancestral eukaryotes, leading to a symbiotic relationship.

• Evidence: Presence of circular DNA, 70S ribosomes, double membranes, and independent replication in mitochondria and chloroplasts.

Summary Table: Transport Processes

Process	Description	Examples of Transported Substances
Diffusion	Molecules move down their electrochemical gradient through the phospholipid bilayer	Oxygen, carbon dioxide, lipid-soluble chemicals
Facilitated Diffusion	Molecules move down their electrochemical gradient through channels or carrier proteins	Glucose, fructose, urea, some vitamins
Osmosis	Water molecules move down their concentration gradient across a selectively permeable membrane	Water
Active Transport	ATP-dependent carrier proteins bring substances into cell	Na+, K+, Ca2+, H+, Cl-
Group Translocation	The substance is chemically altered during transport	Glucose, mannose, fructose

Summary Table: Endocytosis and Exocytosis

Process	Description	Examples of Transported Substances
Endocytosis (Phagocytosis & Pinocytosis)	Substances are surrounded by pseudopods and brought into the cell	Bacteria, viruses, aged and dead cells, liquid nutrients in extracellular solutions
Exocytosis	Vesicles containing substances fuse with cytoplasmic membrane, dumping their contents to the outside	Waste, secretions

Summary Table: Characteristics of Life

Characteristic	Bacteria, Archaea, Eukaryotes	Viruses
Growth	Occurs in all	Does not occur
Reproduction	Occurs in all	Host cell replicates virus
Responsiveness	Occurs in all	Reaction to host cells seen in some viruses
Metabolism	Occurs in all	Viruses lack metabolism
Cellular Structure	Present in all	Viruses lack cytoplasmic membrane or cellular structure

Additional info:

• Equations for osmosis and diffusion can be represented as: $J = -D \frac{dC}{dx}$, where J is flux, D is diffusion coefficient, and dC/dx is concentration gradient.

• ATP hydrolysis for active transport: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$