Cell Structure and Function: Microscopy, Cell Fractionation, Organelles, and Cytoskeleton

Microscopy in Cell Biology

Types of Microscopy

Microscopy is essential for visualizing cells and their components. Different types of microscopy offer varying levels of resolution and visualization techniques.

• Light Microscopy: Uses visible light to illuminate specimens. Light is either reflected or transmitted through the sample.

• Fluorescence Microscopy: Utilizes fluorescent dyes or proteins to label specific cellular components, allowing visualization of structures with high specificity.

• Electron Microscopy (EM): Employs beams of electrons for much higher resolution, enabling detailed views of organelles and macromolecular complexes.

Limits of Resolution

• Light Microscopy: Limit of resolution is approximately 200 nm.

• Fluorescence Microscopy: Similar to light microscopy, but can resolve specific labeled structures within the cell.

• Electron Microscopy: Limit of resolution can be as low as 0.1 nm, allowing visualization of molecular complexes.

Example: Electron microscopy can reveal the detailed structure of mitochondria, while fluorescence microscopy can highlight the location of specific proteins within a cell.

Cell Fractionation

Purpose and Method

Cell fractionation is a laboratory technique used to separate cellular components for individual study.

• Definition: The process of breaking apart cells and separating their components (organelles, proteins) using centrifugation.

• Application: Allows researchers to study the function of each cell part in isolation.

Example: Isolating mitochondria from liver cells to study their role in energy production.

Cell Types: Prokaryotic vs. Eukaryotic

Major Differences

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; cytoplasm is enclosed by a cell wall.

• Eukaryotic Cells: Have a nucleus enclosed by a double membrane and possess membrane-bound organelles; cytoplasm is compartmentalized.

Example: Escherichia coli is a prokaryote; human liver cells are eukaryotes.

Cell Size and Its Limitations

Factors Affecting Cell Size

Most cells are small due to limitations in surface area to volume ratio, which affects nutrient uptake and waste removal.

• Surface Area to Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting efficient exchange with the environment.

• Adaptations: Some cells overcome size limitations by having elongated shapes or specialized structures (e.g., microvilli).

Example: Intestinal epithelial cells have microvilli to increase surface area for absorption.

Major Eukaryotic Organelles and Their Functions

Organelle Functions

Eukaryotic cells contain specialized organelles, each with distinct functions.

• Nucleus: Stores DNA and coordinates cell activities such as growth and reproduction.

• Ribosomes: Synthesize proteins; found free in cytoplasm or attached to rough ER.

• Smooth Endoplasmic Reticulum (ER): Synthesizes lipids, detoxifies drugs and poisons, stores calcium ions.

• Rough Endoplasmic Reticulum (ER): Synthesizes proteins and distributes transport vesicles.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Break down macromolecules, waste, and foreign invaders.

• Vacuoles: Store substances; central vacuole in plants maintains cell rigidity.

• Mitochondria: Site of cellular respiration; converts chemical energy from food into ATP.

• Chloroplasts: Site of photosynthesis; converts light energy, water, and CO2 into chemical energy (glucose) and oxygen.

Example: Muscle cells have many mitochondria to meet high energy demands.

Organelles in Plant and Animal Cells

Comparison Table

Some organelles are found in both plant and animal cells, while others are unique to one type.

Organelle	Plant Cells	Animal Cells
Nucleus	Yes	Yes
Ribosomes	Yes	Yes
Plasma Membrane	Yes	Yes
Central Vacuole	Yes	No
Chloroplasts	Yes	No
Lysosomes	Rare	Yes
Mitochondria	Yes	Yes

Additional info: Plant cells have cell walls and chloroplasts; animal cells have lysosomes and centrioles.

Endosymbiosis Theory

Origin of Mitochondria and Chloroplasts

The endosymbiosis theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

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

Example: Mitochondria are thought to have originated from aerobic bacteria; chloroplasts from photosynthetic bacteria.

Cytoskeleton

Types and Functions of Cytoskeletal Filaments

The cytoskeleton provides structural support, enables movement, and organizes cellular components.

• Microfilaments (Actin Filaments): Located throughout the cell; involved in cell shape, movement, and muscle contraction.

• Microtubules: Form hollow tubes; guide organelle movement, separate chromosomes during cell division, and form cilia/flagella.

• Intermediate Filaments: Provide mechanical strength and maintain cell integrity.

Example: Microtubules form the spindle apparatus during mitosis.

Movement and Transport in Cells

Role of Actin Filaments and Microtubules

Actin filaments and microtubules are essential for cellular movement, transport, and structural changes.

• Actin Filaments: Interact with myosin for muscle contraction; enable cell crawling and changes in cell shape.

• Microtubules: Form the core of cilia and flagella, enabling movement; serve as tracks for motor proteins transporting vesicles.

Example: Motor proteins such as kinesin and dynein move along microtubules to transport cellular cargo.

Additional info: The dynamic nature of the cytoskeleton allows cells to adapt their shape and internal organization in response to environmental signals.