Chapter 6: A Tour of the Cell

Cell Size and Scale

Cells vary greatly in size, from the smallest bacteria to large eukaryotic cells. Understanding the relative scale of different cell types is important for grasping biological organization.

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

• Order of Magnitude: Viruses < Prokaryotes < Eukaryotes.

• Example: A typical bacterium is about 1-5 μm in diameter, while a typical animal cell is 10-100 μm.

• Application: The small size of cells allows for efficient exchange of materials with the environment.

Cell Theory

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

• Key Points:

  1. All living things are composed of cells.

  2. The cell is the basic unit of life.

  3. All cells arise from pre-existing cells.

• Example: Both unicellular organisms (e.g., Escherichia coli) and multicellular organisms (e.g., humans) are made of 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 to exchange materials.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area.

• Equation:

• Implication: Smaller cells have a higher surface area-to-volume ratio, allowing for more efficient transport of nutrients and waste.

• Why Cells Divide: To maintain a favorable surface area-to-volume ratio and support cellular functions.

Microscopy: 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.

  • Lower magnification and resolution compared to electron microscopes.

• Electron Microscopes:

  • Use beams of electrons for imaging.

  • Higher magnification and resolution; can reveal ultrastructure.

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

• Example: Transmission electron microscopes (TEM) are used to study internal cell structures, while scanning electron microscopes (SEM) show surface details.

Prokaryotic vs Eukaryotic Cells

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

• Prokaryotic Cells:

  • No nucleus; DNA is in the nucleoid region.

  • Lack membrane-bound organelles.

  • Examples: Bacteria and Archaea.

• Eukaryotic Cells:

  • Have a true nucleus containing DNA.

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

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

• Comparison Table:

Cell Organelles (Figure 6.8)

Eukaryotic cells contain specialized structures called organelles, each with distinct functions.

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

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes; break down waste.

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

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

• Plasma Membrane: Controls movement of substances in and out of the cell.

• Example: Animal cells have lysosomes, while plant cells have a large central vacuole and chloroplasts.

Endomembrane System (Figure 6.15)

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

• Components: Nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Function: Synthesis and transport of proteins and lipids; detoxification; waste processing.

• Example: Proteins synthesized in the rough ER are transported to the Golgi apparatus for modification and then sent to their destination via vesicles.

Mitochondria and Chloroplasts (Figures 6.17 & 6.18)

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

• Mitochondria:

  • Site of cellular respiration; converts glucose to ATP.

  • Double membrane structure; inner membrane forms cristae.

  • Contains its own DNA and ribosomes.

• Chloroplasts:

  • Site of photosynthesis in plant cells.

  • Contains thylakoids stacked into grana; surrounded by stroma.

  • Has its own DNA and ribosomes.

• Equation for Cellular Respiration:

• Equation for Photosynthesis:

Cytoskeleton (Table 6.1, p113)

The cytoskeleton is a network of protein fibers that provides structural support, movement, and organization within the cell.

• Three Main Types of Fibers:

Cell Junctions and Plasmodesmata (Figures 6.29 & 6.30)

Cells communicate and adhere to each other through specialized structures called cell junctions (in animals) and plasmodesmata (in plants).

• Plasmodesmata: Channels between plant cell walls that allow transport and communication.

• Animal Cell Junctions:

  • Tight Junctions: Prevent leakage of extracellular fluid.

  • Desmosomes: Anchor cells together.

  • Gap Junctions: Allow passage of ions and small molecules between cells.

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