General biology unit 6

A Tour of the Cell

Introduction

The cell is the fundamental unit of life. Understanding its structure and function is essential for all biological sciences. This guide covers the use of microscopy in cell biology, the differences between cell types, and the functions of key organelles.

Microscopy in Cell Biology

Light vs. Electron Microscopes

Microscopes are essential tools for studying cells. Biologists use both light and electron microscopes, each with distinct capabilities.

• Resolution: The ability to distinguish two close objects as separate. Electron microscopes have higher resolution than light microscopes.

• Useful Magnification: Electron microscopes provide much greater magnification, allowing visualization of smaller cell structures.

• Contrast: Techniques such as staining or electron-dense materials enhance contrast, making structures more visible.

• Living Cells: Light microscopes can be used to observe living cells, while electron microscopes require cells to be fixed and dead.

Example: Observing mitochondria in detail requires an electron microscope due to their small size.

Cell Structure and Diversity

What are Cells?

All living organisms are composed of cells. Cells can be classified as prokaryotic or eukaryotic based on their internal organization.

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

• Eukaryotic Cells: Have a nucleus and various membrane-bound organelles. Examples include Animal, Plant, Fungi, and Protist cells.

Example: Escherichia coli is a prokaryotic cell, while a human skin cell is eukaryotic.

Organelles and Their Functions

Eukaryotic cells contain specialized structures called organelles, each with specific functions.

Organelle	Function
Nucleus	Contains genetic material (DNA); controls cell activities
Ribosomes	Site of protein synthesis
Rough Endoplasmic Reticulum (ER)	Protein synthesis and processing
Smooth Endoplasmic Reticulum (ER)	Lipid synthesis and detoxification
Golgi Apparatus	Modification, sorting, and packaging of proteins and lipids
Lysosomes	Digestion of macromolecules and waste
Vacuoles	Storage and structural support (especially in plant cells)
Peroxisomes	Breakdown of fatty acids and detoxification
Mitochondria	ATP production through cellular respiration
Chloroplasts*	Photosynthesis (in plant and algal cells)
Cytoskeleton	Structural support, cell movement, and transport
Plasma Membrane	Selective barrier; regulates entry and exit of substances
Cell Wall*	Rigid outer layer (in plants, fungi, and some protists)
Cytosol	Fluid portion of the cell where metabolic reactions occur

*Present only in plant cells and some protists.

Comparing Animal and Plant Cells

Animal and plant cells share many organelles but also have key differences.

• Plant Cells: Have cell walls, chloroplasts, and large central vacuoles.

• Animal Cells: Lack cell walls and chloroplasts, but have centrioles and smaller vacuoles.

Example: Photosynthesis occurs in chloroplasts, which are found only in plant cells.

Endosymbiont Theory

Origin of Mitochondria and Chloroplasts

The endosymbiont theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence: Both organelles have their own DNA, double membranes, and reproduce independently within the cell.

• Theory vs. Hypothesis: A theory is a well-supported explanation based on evidence; a hypothesis is a testable prediction. The endosymbiont theory is supported by extensive data.

Example: Mitochondria share similarities with certain bacteria, supporting the theory.

Extracellular Components and Connections

Function and Importance

Cells are surrounded by extracellular structures that provide support, protection, and facilitate communication.

• Cell Wall: Provides rigidity and protection in plants, fungi, and some protists.

• Extracellular Matrix (ECM): In animal cells, the ECM is composed of proteins and carbohydrates that support cell structure and mediate cell signaling.

• Cell Junctions: Specialized connections between cells (e.g., tight junctions, desmosomes, gap junctions) allow communication and maintain tissue integrity.

Example: Collagen is a major protein in the animal ECM, providing structural support.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Size	Small (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Equations

• Magnification: $\text{Total Magnification} = \text{Objective Lens Magnification} \times \text{Ocular Lens Magnification}$

• Resolution: $d = \frac{0.61 \lambda}{n \sin \theta}$ where $d$ is the minimum resolvable distance, $\lambda$ is the wavelength of light, $n$ is the refractive index, and $\theta$ is the half-angle of the maximum cone of light that can enter the lens.

Additional info:

• Supplemental materials such as animations and videos can enhance understanding of cell structure and function.

• Further study of mitochondria and chloroplasts will include their roles in respiration and photosynthesis, respectively.