Tour of the Cell

The Fundamental Units of Life

Cells are the basic structural and functional units of all living organisms. Understanding their structure and function is essential for studying biology.

• Organisms are made of cells: All living things consist of one or more cells.

• Cells are the simplest collection of matter that can be alive: Organelles within cells are considered nonliving, but the cell as a whole is alive.

• Cells are related by descent from earlier cells: All cells arise from pre-existing cells, supporting the continuity of life.

• Common features: Despite differences, all cells share certain structural features.

Microscopy and Cell Study

Microscopes in Biology

Microscopes are essential tools for visualizing cells and their components, which are too small to be seen with the naked eye.

• Cell size: Most cells are between 1 and 100 μm in diameter.

• Types of Microscopes:

  • Light Microscope (LM): Uses visible light and glass lenses to magnify specimens up to about 1,000 times. Enhances contrast with stains or labels.

  • Electron Microscopes (EM): Use electron beams for much higher resolution.

    • Scanning Electron Microscope (SEM): Produces 3D images of specimen surfaces.

    • Transmission Electron Microscope (TEM): Reveals internal structures by passing electrons through thin sections.

• Key parameters:

  • Magnification: Ratio of image size to real size.

  • Resolution: Minimum distance between two distinguishable points.

  • Contrast: Visible differences in sample parts.

Cell Fractionation: Uses centrifugation to separate cell components by size, allowing study of organelle functions.

Cell Types and Structure

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their internal organization.

• Prokaryotic Cells:

  • No nucleus; DNA is in the nucleoid region.

  • No membrane-bound organelles.

  • Domains: Bacteria and Archaea.

  • Structures: plasma membrane, cytosol, chromosomes, ribosomes, cell wall, sometimes capsule, fimbriae, and flagella.

• Eukaryotic Cells:

  • DNA enclosed in a nucleus with a double membrane.

  • Membrane-bound organelles (e.g., mitochondria, ER, Golgi apparatus).

  • Generally larger than prokaryotic cells.

  • Domains: Protists, Fungi, Animals, Plants.

Plasma Membrane: Selective barrier composed of a phospholipid bilayer with embedded proteins, controlling passage of substances.

Surface Area to Volume Ratio: As cells grow, volume increases faster than surface area, limiting cell size for efficient exchange.

Eukaryotic Cell Organization

Internal Membranes and Organelles

Eukaryotic cells have complex internal structures that compartmentalize functions.

• Nucleus: Contains most genetic material; surrounded by nuclear envelope with pores for molecular exchange.

• Chromatin: DNA-protein complex; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

• Ribosomes: Protein synthesis factories; found free in cytosol or bound to ER.

Endomembrane System

Components and Functions

The endomembrane system coordinates protein traffic and metabolic functions.

• Nuclear Envelope

• Endoplasmic Reticulum (ER):

  • Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs, stores calcium ions.

  • Rough ER: Studded with ribosomes; synthesizes glycoproteins, distributes transport vesicles, produces membranes.

• Golgi Apparatus: Modifies, sorts, and packages proteins and macromolecules into vesicles for transport.

• Lysosomes: Membranous sacs with hydrolytic enzymes for digesting macromolecules and recycling cell components (autophagy).

• Vacuoles: Large vesicles for storage and maintenance; central vacuole in plants stores water and organic compounds.

• Plasma Membrane: Boundary for the cell, involved in transport and communication.

Energy Conversion Organelles

Mitochondria and Chloroplasts

These organelles transform energy for cellular use.

• Mitochondria: Site of cellular respiration; generates ATP using oxygen. Structure includes outer membrane, inner membrane (with cristae), intermembrane space, and matrix.

• Chloroplasts: Site of photosynthesis in plants and algae; contains thylakoids (stacked as grana), stroma, and double membrane.

• Peroxisomes: Oxidative organelles that produce hydrogen peroxide and convert it to water.

Endosymbiont Theory: Mitochondria and chloroplasts originated from prokaryotic cells engulfed by ancestral eukaryotes, explaining their double membranes and independent DNA.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that organizes cell structure and activities, providing support and facilitating movement.

• Microtubules: Thickest fibers, made of tubulin; maintain cell shape, guide organelle movement, separate chromosomes during division.

• Microfilaments (Actin Filaments): Thinnest fibers, made of actin; bear tension, support cell shape, involved in muscle contraction and cell movement.

• Intermediate Filaments: Middle-sized fibers; maintain cell shape, anchor organelles, more permanent than other cytoskeletal elements.

Motor Proteins: Interact with cytoskeletal fibers to move vesicles and organelles within the cell.

Extracellular Components and Cell Junctions

Coordination of Cellular Activities

Cells produce and interact with extracellular structures for protection, support, and communication.

• Plant Cell Walls: Made of cellulose; protect, maintain shape, prevent excessive water uptake. Layers include primary wall, middle lamella, and sometimes secondary wall.

• Plasmodesmata: Channels between plant cells for transport and communication.

• Extracellular Matrix (ECM) in Animals: Composed of glycoproteins (collagen, proteoglycans, fibronectin); provides structural support and regulates cell behavior via integrins.

• Intercellular Junctions in Animals:

  • Tight Junctions: Prevent leakage of extracellular fluid.

  • Desmosomes: Anchor cells together.

  • Gap Junctions: Allow passage of ions and small molecules for communication.

Summary Table: Cell Structures and Functions

Key Equations

• Surface Area of a Cube:

• Volume of a Cube:

• Surface Area to Volume Ratio:

Conclusion

Cells are highly organized units that perform essential life functions. Their structures, from membranes to organelles and cytoskeleton, work together to maintain life, growth, and communication.