Cell Structure and Microscopy

Limits on Cell Surface Area and Volume

Cells are limited in size due to the relationship between their surface area and volume. As a cell grows, its volume increases faster than its surface area, which restricts the rate at which materials can enter or leave the cell.

• Surface Area-to-Volume Ratio: A high ratio allows efficient exchange of materials; as cells grow, this ratio decreases, limiting cell size.

• Example: Small cells like Escherichia coli have a high surface area-to-volume ratio, facilitating rapid nutrient uptake and waste removal.

Compound Light Microscope

A compound light microscope uses multiple lenses to magnify small objects, allowing visualization of cells and some organelles.

• How it Works: Light passes through the specimen and is magnified by objective and ocular lenses.

• Magnification: Total magnification is the product of the objective and ocular lens magnifications.

• Resolution: The ability to distinguish two close points as separate; higher resolution provides clearer images.

• Formula:

Light vs. Electron Microscopes

Light microscopes use visible light, while electron microscopes use electron beams for much higher resolution.

• Light Microscopes: Suitable for living cells, general cell structure, and larger organelles.

• Electron Microscopes: Used for detailed ultrastructure (e.g., membranes, ribosomes); cannot be used on living specimens.

• When to Use: Use light microscopes for routine cell observation; electron microscopes for detailed internal or surface structures.

Magnification and Resolution

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

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

• Calculation: where is the wavelength of light used.

Interpreting Micrographs

• Light Micrograph: Image produced by a light microscope.

• Electron Micrograph: Image produced by an electron microscope, showing much finer detail.

Staining Techniques

• Purpose: Enhance contrast in light microscopy to visualize structures.

• Common Stains: Gram stain, crystal violet, safranin.

Prokaryotic vs. Eukaryotic Cells

Major Differences

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region.

• Eukaryotic Cells: Have a true nucleus and membrane-bound organelles.

• Size: Eukaryotic cells are generally larger than prokaryotic cells.

Common Structures

• Both cell types have: Plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).

Prokaryotic Cell Features

• Nucleoid: Region containing circular DNA; not membrane-bound.

• Cell Wall: Provides structure and protection; composition differs between Gram-positive and Gram-negative bacteria.

• Gram-Positive vs. Gram-Negative:

  • Gram-positive: Thick peptidoglycan layer, stains purple.

  • Gram-negative: Thin peptidoglycan layer, outer membrane, stains pink.

• Shapes of Bacteria: Bacilli (rod-shaped), cocci (spherical), spirilla (spiral-shaped).

• Flagella: Used for movement; prokaryotic flagella rotate, while eukaryotic flagella have a whip-like motion.

• Pili: Hair-like structures for attachment and conjugation.

• Capsule: Protective outer layer; helps evade immune response.

Antibiotics and Selective Toxicity

• Selective Toxicity: Antibiotics target bacterial structures (e.g., cell wall, ribosomes) not found in human cells, minimizing harm to the host.

Eukaryotic Cell Features

• Nucleus: Contains DNA; surrounded by a nuclear envelope.

• Ribosomes: Sites of protein synthesis; can be free or bound to the endoplasmic reticulum.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and vesicles.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes; break down waste and cellular debris. Three functions: Digestion of macromolecules, recycling of cellular components, and defense against pathogens.

• Vacuoles: Storage organelles; large central vacuole in plant cells stores water and maintains turgor pressure.

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

Mitochondria and Chloroplasts

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Site of photosynthesis in plants and algae; converts solar energy to chemical energy.

• Producers vs. Consumers: Producers (plants, algae) have chloroplasts; consumers (animals) do not.

Endosymbiotic Theory

• Definition: Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, and ribosomes similar to bacteria.

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and enables movement.

• Microfilaments: Composed of actin; involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical support; maintain cell integrity.

• Microtubules: Hollow tubes; involved in cell division, intracellular transport, and movement of cilia and flagella.

• Centrioles: Organize microtubules during cell division in animal cells.

• Cilia and Flagella: Motile structures for movement; cilia are short and numerous, flagella are longer and fewer.

Extracellular Matrix and Cell Junctions

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells; provides support and regulates cell behavior.

• Plant Cell Walls: Rigid structure made of cellulose; provides support and protection.

• Junctions:

  • Tight Junctions: Seal cells together, preventing leakage.

  • Adhering Junctions: Anchor cells to each other.

  • Gap Junctions: Allow communication between animal cells.

  • Plasmodesmata: Channels between plant cells for transport and communication; functionally similar to gap junctions in animals.

Specialized Structures in Plant and Animal Cells

• Plant Cells: Unique structures include cell wall, chloroplasts, and large central vacuole.

• Animal Cells: Unique structures include centrioles and lysosomes.

Cell Specialization

• Definition: The process by which cells develop specific structures and functions.

• Example: Muscle cells have abundant mitochondria for energy; nerve cells have long extensions for signal transmission.

• Structure-Function Relationship: A cell’s structure is closely related to its specialized function.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curriculum.