The Cell

Cell Theory and Cell Properties

The cell is the fundamental unit of life, and all living organisms are composed of one or more cells. Understanding cell structure and function is essential in biology.

• Cell Theory: States that all living things are made of cells, cells are the basic units of life, and all cells arise from pre-existing cells.

• Cell Size: Most cells are microscopic, ranging from 1 to 100 micrometers in diameter.

• Surface Area to Volume Ratio: As a cell increases in size, its volume grows faster than its surface area, limiting the efficiency of transport and exchange. Formula:

• Cell Shape: Varies depending on function and environment; shapes can be spherical, rod-like, or irregular.

• Resolution: The ability to distinguish two close objects as separate; critical for microscopy.

Example: Red blood cells are biconcave to maximize surface area for gas exchange.

Surface Area to Volume Ratio Table

Viewing Cells: Microscopy

Microscope Principles

Microscopes are essential tools for studying cells, allowing visualization of structures not visible to the naked eye.

• Magnification: The process of enlarging the appearance of an object.

• Resolution: The minimum distance at which two points can be distinguished as separate.

• Contrast: The ability to distinguish objects from the background.

Types of Microscopes:

• Light Microscopes: Use visible light to illuminate specimens; suitable for living cells.

• Electron Microscopes: Use beams of electrons for much higher resolution; includes transmission (TEM) and scanning (SEM) electron microscopes.

Example: TEM can reveal internal cell structures, while SEM provides detailed surface images.

Plasma Membrane

Structure and Function

The plasma membrane is a dynamic boundary that separates the cell from its environment and regulates the movement of substances.

• Phospholipid Bilayer: Composed of hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Embedded Proteins: Integral and peripheral proteins perform transport, signaling, and structural roles.

• Selectively Permeable Barrier: Controls entry and exit of molecules.

• Communication: Receptors and signaling molecules allow interaction with the environment.

• Homeostasis: Maintains a stable internal environment.

Example: Aquaporins are membrane proteins that facilitate water transport.

Prokaryotes and Eukaryotes

Cell Types and Characteristics

Cells are classified as prokaryotic or eukaryotic based on their internal organization.

• Prokaryotes: Include Bacteria and Archaea. Lack a membrane-bound nucleus and internal compartments.

• Eukaryotes: Include Animals, Plants, Fungi, and Protists. Possess a membrane-bound nucleus and organelles.

Example: Escherichia coli is a prokaryotic bacterium; human cells are eukaryotic.

Prokaryotic Cells

Basic Structures

Prokaryotic cells have a simple structure but can possess specialized features for survival.

• Plasma Membrane: Encloses the cell and regulates transport.

• Nucleoid: Region containing the cell's DNA.

• Cytoplasm: Contains cytosol and ribosomes for protein synthesis.

Specialized Features

• Cell Wall: Provides structural support; made of peptidoglycan in bacteria.

• Outer Membrane: Additional protective layer in some bacteria.

• Capsule (Glycocalyx): Sticky outer layer for protection and adhesion.

• Internal Membrane System: Folds of plasma membrane for photosynthesis or other functions (e.g., mesosomes).

• Flagella: Tail-like structures for movement.

• Pili: Hair-like structures for attachment and genetic exchange.

Example: The capsule in Streptococcus pneumoniae helps evade the immune system.

Comparison of Prokaryotic and Eukaryotic Cells

Additional info: These notes cover the foundational aspects of cell structure and function, including microscopy, membrane structure, and the differences between prokaryotic and eukaryotic cells, suitable for introductory college biology.