Cell Structure and Function

Characteristics of Living Things

All living organisms share certain fundamental characteristics that distinguish them from non-living matter.

• Cellular Organization: All living things are composed of one or more cells.

• Metabolism: Living organisms carry out chemical reactions to obtain energy and build cellular components.

• Growth and Development: Organisms increase in size and complexity over time.

• Reproduction: Ability to produce new individuals, either sexually or asexually.

• Response to Stimuli: Organisms can sense and respond to changes in their environment.

• Homeostasis: Maintenance of a stable internal environment.

• Evolution: Populations of organisms change over generations through genetic variation and natural selection.

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two major types based on their structural features: prokaryotic and eukaryotic.

• Prokaryotic Cells: Lack a membrane-bound nucleus and organelles; genetic material is in the nucleoid region. Examples: Bacteria, Archaea.

• Eukaryotic Cells: Possess a true nucleus enclosed by a nuclear membrane and various membrane-bound organelles. Examples: Fungi, Protozoa, Plants, Animals.

Prokaryotic Cell Structures Targeted by Drugs

Pharmaceutical compounds often target unique structures in prokaryotic cells to treat infections.

• Cell Wall: Many antibiotics (e.g., penicillins) inhibit peptidoglycan synthesis.

• Ribosomes: Prokaryotic ribosomes (70S) differ from eukaryotic ribosomes (80S); drugs like tetracycline inhibit protein synthesis.

• Cell Membrane: Some drugs disrupt membrane integrity.

• DNA Replication Machinery: Quinolones target bacterial DNA gyrase.

Superficial Structures for Desiccation Resistance

Bacteria possess surface structures that help them survive in dry environments.

• Capsule: A polysaccharide layer that retains moisture and protects against desiccation.

• Cell Wall: Provides structural support and limits water loss.

• S-Layer: A proteinaceous surface layer that can aid in environmental resistance.

Flagella: Structure and Function

Flagella are whip-like appendages that provide motility to many bacteria and some eukaryotic cells.

• Function: Enables movement toward nutrients or away from harmful substances (chemotaxis).

• Structure: Composed of a filament, hook, and basal body in prokaryotes.

• Example: Escherichia coli uses flagella for swimming in liquid environments.

Cell Wall: Structure and Function

The cell wall is a rigid layer that surrounds the cell membrane, providing shape and protection.

• Prokaryotic Cell Wall: Mainly composed of peptidoglycan in bacteria; provides structural integrity and prevents osmotic lysis.

• Eukaryotic Cell Wall: Found in plants (cellulose) and fungi (chitin).

Cell Membrane: Structure and Function

The cell membrane is a selectively permeable barrier that regulates the movement of substances in and out of the cell.

• Structure: Phospholipid bilayer with embedded proteins.

• Function: Maintains homeostasis, facilitates communication, and supports metabolic processes.

Mitochondria: Structure and Function

Mitochondria are membrane-bound organelles found in eukaryotic cells, responsible for energy production.

• Function: Site of cellular respiration; converts glucose and oxygen into ATP (energy).

• Structure: Double membrane with inner folds called cristae.

• Equation:

Ribosomes: Structure and Function

Ribosomes are molecular machines responsible for protein synthesis in all living cells.

• Prokaryotic Ribosomes: 70S (smaller, targeted by certain antibiotics).

• Eukaryotic Ribosomes: 80S (larger, found in cytoplasm and on rough ER).

• Function: Translate mRNA into polypeptide chains (proteins).

Diversity of Prokaryotes vs. Eukaryotes

Prokaryotes exhibit greater diversity than eukaryotes due to their adaptability and evolutionary history.

• Rapid Reproduction: Short generation times allow for quick adaptation.

• Genetic Variation: Horizontal gene transfer increases diversity.

• Environmental Range: Prokaryotes inhabit extreme environments (e.g., hot springs, deep sea vents).

• Metabolic Diversity: Wide range of metabolic pathways (e.g., photosynthesis, chemosynthesis).

Additional info: Prokaryotes are the earliest life forms and have evolved to occupy nearly every ecological niche on Earth, contributing to their vast diversity.