Cell Structure and Function

Processes of Life

All living things share four basic processes essential for life. Understanding these processes is foundational for studying microbiology and cell biology.

• Growth: An increase in size.

• Reproduction: The ability to produce more organisms.

• Responsiveness: The ability to react to environmental stimuli.

• Metabolism: All chemical reactions that occur within a cell.

Prokaryotic and Eukaryotic Cells: An Overview

Cells are classified as either prokaryotic or eukaryotic based on their structural differences. This distinction is crucial for understanding microbial diversity.

• Prokaryotic Cells: Lack a nucleus and most internal membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Possess a nucleus and various membrane-bound organelles. Examples include Fungi, Algae, Protozoa, and Animals.

External Structures of Bacterial Cells

Bacterial cells possess several external structures that aid in protection, attachment, and motility.

• Glycocalyces: Gelatinous, sticky substances surrounding the outside of the cell. They can be organized as a capsule (firmly attached) or a slime layer (loosely attached). Glycocalyces protect cells from desiccation and help in adherence to surfaces.

• Flagella: Long, whip-like structures responsible for motility. Bacterial flagella rotate to propel the cell. The arrangement and number of flagella can vary (monotrichous, lophotrichous, amphitrichous, peritrichous).

• Fimbriae and Pili: Short, hair-like appendages. Fimbriae aid in attachment to surfaces and other cells, while pili (especially sex pili) are involved in DNA transfer between cells (conjugation).

Bacterial Cell Walls

The cell wall provides structure, shape, and protection to bacterial cells. It is primarily composed of peptidoglycan, a polymer of sugars and amino acids.

• Gram-Positive Cell Walls: Thick layer of peptidoglycan and teichoic acids. Stain purple in Gram stain.

• Gram-Negative Cell Walls: Thin layer of peptidoglycan, outer membrane containing lipopolysaccharide (LPS). Stain pink in Gram stain.

• Bacteria Without Cell Walls: Some bacteria, such as Mycoplasma, lack cell walls but maintain cell integrity through other means.

Bacterial Cytoplasmic Membranes

The cytoplasmic membrane is a double-layered structure (phospholipid bilayer) that controls the movement of substances into and out of the cell.

• Structure: Composed of phospholipids and proteins. The fluid mosaic model describes the dynamic nature of the membrane.

• Function: Selectively permeable, allowing only certain substances to pass. Involved in energy production and transport.

• Passive Processes: Include diffusion, facilitated diffusion, and osmosis. No energy required.

• Active Processes: Require energy (usually ATP) to move substances against their concentration gradient. Includes active transport and group translocation.

Cytoplasm of Bacteria

The cytoplasm contains the cell's internal components, including the cytosol, inclusions, and sometimes endospores.

• Cytosol: Liquid portion containing water, ions, and dissolved molecules.

• Inclusions: Reserve deposits of nutrients such as starch, glycogen, or lipids.

• Endospores: Highly resistant structures formed by some bacteria (e.g., Bacillus, Clostridium) for survival in harsh conditions.

• Nonmembranous Organelles: Ribosomes (site of protein synthesis) and cytoskeleton (shape and support).

External Structures of Archaea

Archaea possess unique external structures, including glycocalyces, flagella, and fimbriae. Some have a specialized attachment structure called a hamus.

• Glycocalyces: Similar to those in bacteria, providing protection and adherence.

• Flagella: Structurally different from bacterial flagella; not hollow and assembled differently.

• Fimbriae and Hamus: Aid in attachment to surfaces and other cells.

Archaeal Cell Walls and Cytoplasmic Membranes

Archaeal cell walls lack peptidoglycan and may be composed of proteins or polysaccharides. Their cytoplasmic membranes can contain unique lipids not found in bacteria or eukaryotes.

Cytoplasm of Archaea

Similar to bacteria, but may contain unique ribosomal proteins and enzymes.

External Structure of Eukaryotic Cells

Eukaryotic cells may have glycocalyces anchored to their cytoplasmic membranes, aiding in cell recognition and protection.

Eukaryotic Cell Walls and Cytoplasmic Membranes

Eukaryotic cell walls (in fungi, algae, plants) are composed of polysaccharides such as cellulose, chitin, or glucomannan. The cytoplasmic membrane contains sterols for stability.

Cytoplasm of Eukaryotes

The eukaryotic cytoplasm is more complex than that of prokaryotes, containing numerous membrane-bound organelles.

• Flagella: Structurally distinct from prokaryotic flagella; composed of microtubules in a 9+2 arrangement.

• Cilia: Shorter and more numerous than flagella; used for movement and feeding.

• Other Nonmembranous Organelles: Ribosomes, cytoskeleton, centrioles.

• Membranous Organelles: Nucleus, endoplasmic reticulum, Golgi body, lysosomes, peroxisomes, vacuoles, vesicles, mitochondria, chloroplasts.

Membranous Organelles

• Nucleus: Contains DNA; surrounded by a nuclear envelope.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Body: Modifies, packages, and sorts proteins and lipids.

• Lysosomes, Peroxisomes, Vacuoles, Vesicles: Involved in storage and breakdown of substances.

• Mitochondria: Site of ATP production; contains its own DNA.

• Chloroplasts: Site of photosynthesis in plants and algae; contains its own DNA.

Endosymbiotic Theory

This theory proposes that mitochondria and chloroplasts originated from ancient prokaryotic cells engulfed by ancestral eukaryotes, leading to a symbiotic relationship.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Key Equations

• Osmosis: Movement of water across a semipermeable membrane from low solute concentration to high solute concentration.

• Diffusion Rate:

• ATP Hydrolysis:

Examples and Applications

• Antibiotics: Many antibiotics target bacterial cell walls (e.g., penicillin inhibits peptidoglycan synthesis).

• Gram Staining: Used to differentiate between Gram-positive and Gram-negative bacteria in clinical diagnostics.

• Endospores: Important in food safety and sterilization due to their resistance to heat and chemicals.

Additional info: Academic context and definitions have been expanded for clarity and completeness.