Chapter 6 - A Tour of the Cell

Relative Size and Scale of Cells

Cells vary greatly in size and structure, but all share certain fundamental characteristics. Understanding the relative scale of different cell types is essential for appreciating their biological roles.

• Prokaryotic cells (such as bacteria) are generally smaller than eukaryotic cells (such as plant and animal cells).

• Most prokaryotic cells are 1–10 μm in diameter, while most eukaryotic cells are 10–100 μm.

• Organelles within eukaryotic cells (e.g., mitochondria, chloroplasts) are even smaller, typically 1–10 μm.

• Viruses are much smaller than cells, often 20–300 nm.

• Example: A typical human red blood cell is about 8 μm in diameter, while an E. coli bacterium is about 2 μm long.

Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells.

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

• All cells arise from pre-existing cells by division.

• Additional info: Modern cell theory also includes that cells contain hereditary information (DNA) and that all energy flow occurs within cells.

Limits to Cell Size: Surface Area-to-Volume Ratio

Cell size is limited by the relationship between surface area and volume, which affects the ability of the cell to transport materials in and out efficiently.

• As a cell grows, its volume increases faster than its surface area.

• A high surface area-to-volume ratio is necessary for efficient exchange of materials (nutrients, gases, wastes).

• Cells divide to maintain an optimal surface area-to-volume ratio.

• Formula:

• Example: Smaller cells have a larger surface area relative to their volume, allowing for more efficient transport.

Light vs. Electron Microscopes

Microscopes are essential tools for studying cells. There are two main types: light microscopes and electron microscopes.

• Light Microscopes:

  • Use visible light to illuminate specimens.

  • Can view living cells.

  • Resolution limit: ~200 nm.

  • Less expensive and easier to use.

• Electron Microscopes:

  • Use beams of electrons for much higher resolution (up to 0.1 nm).

  • Can reveal detailed cell ultrastructure.

  • Cannot view living cells (specimens must be fixed and dehydrated).

  • More expensive and complex.

• Example: Electron microscopes are used to study organelles like mitochondria and ribosomes in detail.

Prokaryotic vs. Eukaryotic Cells

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

• Prokaryotic Cells:

  • Lack a nucleus and membrane-bound organelles.

  • DNA is located in a region called the nucleoid.

  • Examples: Bacteria and Archaea.

• Eukaryotic Cells:

  • Have a true nucleus surrounded by a nuclear envelope.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Examples: Plants, animals, fungi, and protists.

Cell Organelles: Names, Structures, and Functions

Eukaryotic cells contain various organelles, each with specific functions essential for cell survival.

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: (in plants) Site of photosynthesis; converts solar energy to chemical energy.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

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

• Lysosomes: Contain digestive enzymes; break down waste materials and cellular debris.

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

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

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

• Plasma Membrane: Selectively permeable barrier; controls movement of substances in and out of the cell.

The Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Proteins and lipids are synthesized in the ER, modified in the Golgi apparatus, and transported in vesicles.

• Example: Digestive enzymes are synthesized in the rough ER, processed in the Golgi, and sent to lysosomes.

Structure of Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles found in eukaryotic cells.

• Mitochondria:

  • Double-membraned organelle; inner membrane folded into cristae.

  • Matrix contains enzymes for the Krebs cycle.

  • Site of ATP production via cellular respiration.

• Chloroplasts:

  • Found in plant and algal cells.

  • Double-membraned; contains thylakoids stacked into grana.

  • Stroma contains enzymes for the Calvin cycle.

  • Site of photosynthesis.

The Cytoskeleton: Three Types of Fibers

The cytoskeleton is a network of protein fibers that provides structural support, cell shape, and facilitates movement.

Cell Junctions and Plasmodesmata

Cells interact with each other through specialized structures called cell junctions (in animals) and plasmodesmata (in plants).

• Plasmodesmata: Channels between plant cell walls that allow transport of water, ions, and small molecules.

• Animal Cell Junctions:

  • Tight Junctions: Prevent leakage of extracellular fluid.

  • Desmosomes: Anchor cells together, providing mechanical stability.

  • Gap Junctions: Allow direct communication between cells via channels.

• Example: Gap junctions in heart muscle cells allow rapid transmission of electrical signals.