Cell Structure, Organelles, and Microscopy: Study Notes for General Biology

Cell Structure & Organelles

Introduction to Cell Structure

Cells are the fundamental units of life, and their internal organization is essential for proper function. Eukaryotic cells contain specialized structures called organelles that perform distinct tasks necessary for cellular survival and activity.

• Organelles are membrane-bound compartments within eukaryotic cells.

• Each organelle has a unique function, such as energy production, protein synthesis, or waste processing.

• Examples include the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Understanding organelle structure and function is crucial for studying cell biology and disease mechanisms.

Microscopy: Visualizing Cellular Structures

Types of Microscopes

Microscopes are essential tools for observing cells and their components. Different types of microscopes provide varying levels of resolution and detail.

• Light Microscopes: Use lenses and visible light to magnify specimens. Suitable for viewing most plant and animal cells, some organelles, and bacteria.

• Electron Microscopes: Use beams of electrons, which have much shorter wavelengths than light, allowing for higher resolution and magnification. Can visualize viruses, proteins, and detailed organelle structures.

Example: The diagram shows the scale of biological structures, from human height (meters) down to atoms (nanometers).

Light Microscopy Techniques

Light microscopy allows us to study living cells and their internal structures using different optical methods.

• Bright-field optics: Standard method where light passes directly through the specimen.

• Phase contrast optics: Enhances contrast by exploiting differences in refractive indices (how light travels through different media) of cell components.

• Both techniques can be used on the same microscope by changing optical components.

Key Point: Phase contrast is especially useful for visualizing transparent, unstained cells.

Electron Microscopy

Electron microscopes provide much higher resolution than light microscopes, allowing visualization of subcellular structures.

• Transmission Electron Microscope (TEM): Passes electrons through a thin specimen to reveal internal structures. Requires specimens to be stained with electron-dense materials and placed in a vacuum.

• Scanning Electron Microscope (SEM): Scans the surface of a specimen with electrons to produce detailed surface images.

• TEM is used for internal details; SEM is used for surface details.

Example: TEM can resolve structures as small as 1 nm, such as ribosomes and viruses.

Cell Types: Prokaryotes vs. Eukaryotes

Classification of Cells

Cells are classified into two major types based on their internal organization.

• Prokaryotic cells: Small, simple cells without a nucleus or membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotic cells: Larger, complex cells with a nucleus and membrane-bound organelles. Includes plants, animals, fungi, and protists.

Key Point: The presence of a nucleus and organelles distinguishes eukaryotes from prokaryotes.

Eukaryotic Cell Components

Overview of Eukaryotic Cell Structure

Eukaryotic cells have a plasma membrane, nucleus, cytosol, and various organelles, each with specialized functions.

• Plasma membrane: Defines cell boundaries and retains contents.

• Nucleus: Stores genetic material (DNA) and controls cellular activities.

• Mitochondria: Produce energy (ATP) through cellular respiration.

• Chloroplasts (in plants): Harvest solar energy and convert it to chemical energy.

• Endoplasmic reticulum (ER): Synthesizes and transports proteins and lipids.

• Golgi apparatus: Processes, sorts, and ships proteins and lipids.

• Lysosomes: Digest macromolecules and cellular debris.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Provides structural support and enables movement.

Plasma Membrane Structure

The plasma membrane is a semi-permeable barrier composed of a phospholipid bilayer and embedded proteins.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer in water.

• Membrane proteins: Integral or peripheral proteins that assist in transport, signaling, and structural support.

• Cholesterol: Maintains membrane fluidity and stability.

• Glycocalyx: Carbohydrate-rich layer on the cell surface, important for cell recognition and protection.

Transport Across Cell Membranes

Passive and Active Transport

Cells regulate the movement of substances across their membranes using passive and active transport mechanisms.

• Passive transport: Movement of molecules down their concentration gradient without energy input. Includes simple diffusion and facilitated diffusion via channels or transporters.

• Active transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP). Includes pumps such as the Na+/K+ pump.

Equation:

$\text{Rate of diffusion} = k \times [\text{Concentration gradient}]$

Membrane Potential

The membrane potential is the voltage difference across the cell membrane, created by unequal distribution of ions.

• Resting membrane potential: Typically negative inside the cell due to high K+ and low Na+ concentrations.

• Electrochemical gradient: Combination of concentration gradient and electrical potential that drives ion movement.

• Na+/K+ pump: Uses ATP to move 3 Na+ out and 2 K+ in, maintaining gradients.

Equation:

$\text{Na}^+/\text{K}^+ \text{ pump:} \quad 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}}$

Ion Channels and Gating

Ion channels are proteins that allow selective passage of ions across membranes. They can be gated by voltage, ligands, or mechanical forces.

• Voltage-gated channels: Open in response to changes in membrane potential.

• Ligand-gated channels: Open when specific molecules bind to the channel.

• Mechanically-gated channels: Open in response to physical deformation of the membrane.

Example: Neurons use voltage-gated channels to propagate electrical signals.

Table: Comparison of Light and Electron Microscopy

Feature	Light Microscope	Electron Microscope
Source of Illumination	Visible light	Electron beam
Resolution	~200 nm	~1 nm
Specimen Preparation	Simple, can view live cells	Complex, requires thin sections and staining
Applications	Cells, tissues, some organelles	Viruses, proteins, detailed organelle structure

Summary

Understanding cell structure, organelle function, and microscopy techniques is foundational for studying biology. Mastery of these concepts enables students to explore cellular processes, disease mechanisms, and advances in biomedical research.