A Tour of the Cell

Microscopy and Cell Structure

Microscopy is essential for studying cells, the fundamental units of life. Three key parameters define microscopy: magnification, resolution, and contrast. Light microscopes use visible light and glass lenses to magnify specimens, while electron microscopes (EMs) use electron beams for higher resolution, enabling visualization of subcellular structures.

• Magnification: Ratio of image size to real size.

• Resolution: Minimum distance between two distinguishable points.

• Contrast: Visible differences in brightness between parts of the sample.

• Light Microscopes: Effective up to 1,000x magnification; limited resolution for organelles.

• Electron Microscopes: SEMs provide 3-D surface images; TEMs reveal internal cell structures.

• Recent Advances: Fluorescent labeling, confocal, and cryo-EM enhance detail and preserve cellular environment.

Cell fractionation uses centrifugation to separate organelles, correlating cell structure with function.

Prokaryotic vs. Eukaryotic Cells

All living organisms are composed of cells, classified as prokaryotic or eukaryotic. Prokaryotes (Bacteria and Archaea) lack a nucleus and membrane-bound organelles, while eukaryotes (protists, fungi, animals, plants) possess these features.

• Basic features of all cells: Plasma membrane, cytosol, chromosomes (DNA), ribosomes (protein synthesis).

• Prokaryotic cells: DNA in nucleoid, no nucleus, no membrane-bound organelles.

• Eukaryotic cells: DNA in nucleus (double membrane), membrane-bound organelles, larger size.

• Surface area to volume ratio: Limits cell size; adaptations include division, shape changes, and extensions.

Eukaryotic Cell Diversity

Eukaryotic cells have internal membranes and organelles specialized for various functions. Each cell type is unique to its function, as illustrated below:

• Neuron in brain: Specialized for signal transmission.

• Blood cells: Transport oxygen and immune functions.

• Fat cell: Stores energy.

• Bone cell: Provides structural support.

• Smooth muscle cell: Enables movement.

• Cells lining intestinal tract: Absorb nutrients.

• Ovum and sperm: Reproduction.

The Nucleus and Ribosomes

The nucleus is the cell's information center, containing most genetic material. The nuclear envelope (double membrane) separates it from the cytoplasm, with nuclear pores regulating molecular traffic. DNA is organized as chromatin, condensing into chromosomes during cell division. The nucleolus synthesizes ribosomal RNA (rRNA).

• Ribosomes: Complexes of rRNA and protein; sites of protein synthesis.

• Free ribosomes: In cytosol; synthesize proteins for internal use.

• Bound ribosomes: Attached to ER or nuclear envelope; synthesize proteins for membranes or export.

The Endomembrane System

The endomembrane system regulates protein traffic and metabolic functions. It includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, vacuoles, and plasma membrane. Components are connected directly or via vesicles.

• Smooth ER: Synthesizes lipids, detoxifies drugs, stores calcium, carbohydrate metabolism.

• Rough ER: Studded with ribosomes; produces glycoproteins, distributes transport vesicles, membrane factory.

• Golgi apparatus: Modifies, sorts, and packages ER products; manufactures macromolecules.

• Lysosomes: Digestive organelles; hydrolytic enzymes break down macromolecules, recycle cell components (autophagy).

• Peroxisomes: Oxidative organelles; break down fatty acids, detoxify compounds, convert fatty acids to sugars in plants.

• Vacuoles: Diverse functions; food vacuoles (phagocytosis), contractile vacuoles (water regulation), central vacuole (plant cell growth, ion storage).

Mitochondria and Chloroplasts: Energy Conversion

Mitochondria are the sites of cellular respiration, generating ATP. Chloroplasts (in plants and algae) conduct photosynthesis. Both organelles share similarities with bacteria, supporting the endosymbiont theory—they originated from prokaryotic cells engulfed by ancestors of eukaryotes.

• Mitochondria: Double membrane, own DNA and ribosomes, reproduce independently, inner membrane forms cristae.

• Chloroplasts: Contain chlorophyll, thylakoids (stacked as granum), stroma (internal fluid), own DNA and ribosomes.

The Cytoskeleton: Support and Motility

The cytoskeleton is a network of fibers supporting cell shape, organization, and movement. It interacts with motor proteins for intracellular transport and cell motility.

• Microtubules: Hollow rods (tubulin dimers); shape cell, guide organelle movement, separate chromosomes.

• Microfilaments (Actin): Twisted double chains; support cell shape, core of microvilli, involved in motility (myosin interactions).

• Intermediate filaments: Diverse proteins; permanent fixtures, maintain cell shape, anchor organelles.

Extracellular Components and Cell Junctions

Cells synthesize extracellular materials for protection, support, and communication. Plant cells have cell walls (cellulose), while animal cells have an extracellular matrix (ECM) (glycoproteins, integrins).

• Cell wall: Multiple layers (primary, middle lamella, secondary); protects, maintains shape, prevents water uptake.

• ECM: Regulates cell behavior, gene activity, mechanical signaling.

Cell Junctions

• Plasmodesmata (plants): Channels connecting cells, allowing passage of water, solutes, proteins, RNA.

• Tight junctions (animals): Prevent leakage between cells.

• Desmosomes: Anchor cells together.

• Gap junctions: Cytoplasmic channels for communication.

A Cell is Greater Than the Sum of Its Parts

Cellular functions depend on the coordinated activity of all components. For example, macrophages destroy bacteria using cytoskeleton, lysosomes, and plasma membrane, illustrating the integrated nature of cellular processes.

Summary Table: Cytoskeletal Elements

Key Equation: Surface Area to Volume Ratio

The surface area to volume ratio is critical for cell function:

As cell size increases, the ratio decreases, limiting efficient exchange of materials.