Cell Structure, Classification, and Antibiotics: A General Biology Study Guide

The Microscopic World of Cells

Introduction to Cells

Cells are the fundamental units of life. All living things are composed of cells, and all cells arise from pre-existing cells. Organisms can be single-celled (unicellular) or multicellular, and cells vary greatly in structure and function.

• Cell Theory: States that all living things are composed of cells and that all cells come from earlier cells.

• Unicellular organisms: Include most prokaryotes and protists.

• Multicellular organisms: Include plants, animals, and most fungi.

Major Categories of Cells

Biologists classify all life into three major groups called domains. The two primary types of cells are prokaryotic and eukaryotic cells.

• Prokaryotic cells: Organisms in the domains Bacteria and Archaea. These cells lack a nucleus and most organelles. They are usually unicellular.

• Eukaryotic cells: Organisms in the domain Eukarya, including protists, plants, fungi, and animals. These cells have a nucleus and various organelles. They can be unicellular or multicellular.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Prokaryotic cells	Eukaryotic cells
First evolved approximately 3.5 billion years ago	First evolved approximately 2.1 billion years ago
Found in bacteria and archaea	Found in protists, plants, fungi, and animals
Usually smaller and simpler	Usually larger and more complex
Do not have a nucleus	Have a nucleus surrounded by a nuclear envelope
Do not have membrane-bound organelles	Have membrane-bound organelles (e.g., mitochondria, ER)
DNA is in a single circular chromosome	DNA is organized into one or more linear chromosomes
Have ribosomes	Have ribosomes

Cell Structure and Surfaces

Cell Walls and Extracellular Matrix

Cells have specialized structures that provide support and protection.

• Plant cells: Have a cell wall made of cellulose fibers. The cell wall protects the cell, maintains its shape, and helps prevent excessive water uptake.

• Animal cells: Lack cell walls but secrete a sticky coat called the extracellular matrix, which helps hold cells together in tissues and supports cell-to-cell communication.

• Cell junctions: Structures that connect cells together into tissues, allowing them to function in a coordinated way.

Plasma Membrane Structure

The plasma membrane is a critical structure that separates the living cell from its environment and controls the movement of substances in and out of the cell.

• Phospholipid bilayer: The main component of the plasma membrane, consisting of two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

• Fluid mosaic model: Describes the plasma membrane as a flexible layer made of lipid molecules interspersed with proteins. Molecules can move freely past one another within the layer.

• Membrane proteins: Embedded in the bilayer, these proteins serve various functions, including transport, communication, and structural support.

Antibiotics and Bacterial Cells

Antibiotics: Definition and Mechanism

Antibiotics are drugs that disable or kill infectious bacteria. They are designed to target structures unique to bacterial cells, minimizing harm to human cells.

• Goal of antibiotic treatment: To kill invading bacteria while doing minimal damage to the human host.

• Specificity: Most antibiotics are highly specific, binding to structures found only in bacterial cells (e.g., cell walls, ribosomes).

• Research: Scientists exploit unique bacterial structures to design and discover new antibiotics.

• Example: Staphylococcus aureus is a common bacterium targeted by antibiotics.

Case Study: Discovery of the First 21st Century Antibiotic

To address the growing problem of antibiotic resistance, researchers continually seek new antibiotics.

• Background: Medical researchers are trying to produce new antibiotics due to resistance in bacteria such as MRSA (Methicillin-resistant Staphylococcus aureus).

• Method: Soil bacteria were isolated in separate compartments. The devices were then buried in the original soil, allowing bacteria to grow in their natural environment.

• Results: A new species of bacteria was found to produce teixobactin, a new type of antibiotic effective against S. aureus and other pathogens.

Key Points:

• Membranes allowed nutrients into each compartment but kept other bacteria species out.

• Teixobactin was most effective in killing S. aureus.

Microscopy and Cell Movement

Types of Microscopy

Microscopes are essential tools for viewing cells and their structures.

• Light Microscope: Can be used to view living or dead specimens.

• Scanning Electron Microscope (SEM): Provides a view of surface features of cells.

• Transmission Electron Microscope (TEM): Provides a view of internal cell structures.

Cell Motility Structures

• Flagella: Long, whip-like structures that propel cells in a whip-like motion (e.g., sperm cells, some bacteria).

• Cilia: Short, hair-like structures that cover the surface of some protists and move in a coordinated fashion to move the cell or substances across the cell surface.