Chapter 4: A Tour of the Cell – Study Notes

Introduction to the Cell

Historical Development of Cell Theory

The development of improved microscopes and techniques has greatly expanded our understanding of cells. Early scientists such as Robert Hooke and Anton van Leeuwenhoek made significant contributions to cell biology by observing cells and microorganisms using primitive microscopes.

• Robert Hooke (1665): Used a crude microscope to examine cork and coined the term "cell" after observing small compartments he called cellulae.

• Anton van Leeuwenhoek: Improved microscope design and observed living cells in blood, sperm, and pond water.

Cell theory emerged from these studies, stating that:

• All living things are composed of cells.

• All cells come from other cells.

Big Ideas of Chapter 4

• Introduction to the Cell

• The Nucleus and Ribosomes

• The Endomembrane System

• Energy-Converting Organelles

• The Cytoskeleton and Cell Surfaces

Microscopes Reveal the World of the Cell

Types of Microscopes

Microscopes are essential tools for studying cells, allowing scientists to observe structures not visible to the naked eye.

• Light Microscope: Can display living cells and is commonly used in biology labs.

• Electron Microscopes:

  • Scanning Electron Microscope (SEM): Reveals surface details of cells.

  • Transmission Electron Microscope (TEM): Reveals internal ultrastructure of cells.

Magnification is the increase in an object's image size compared with its actual size.

Resolution is a measure of the clarity of an image, or the ability to distinguish two nearby objects as separate.

The Small Size of Cells and the Plasma Membrane

Surface-to-Volume Ratio

The microscopic size of most cells is related to the need to efficiently exchange materials across the plasma membrane. A high surface-to-volume ratio allows for effective transport of nutrients and waste.

• Plasma Membrane: Composed of a phospholipid bilayer with embedded proteins.

• Some proteins form channels for hydrophilic molecules; others act as pumps for active transport.

Formula for Surface Area of a Cube:

$\text{Surface Area} = 6a^2$

Formula for Volume of a Cube:

$\text{Volume} = a^3$

Where a is the length of a side of the cube.

Surface-to-Volume Ratio:

$\text{Surface-to-Volume Ratio} = \frac{6a^2}{a^3} = \frac{6}{a}$

Smaller cells have a greater surface area relative to their volume, facilitating efficient exchange.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

Prokaryotic cells are structurally simpler than eukaryotic cells and lack membrane-bound organelles.

• Domains: Bacteria and Archaea

• Key Features:

  • Plasma membrane

  • DNA located in the nucleoid region

  • Ribosomes

  • Cytosol

  • Cell wall (in most)

  • Capsule (in some)

Eukaryotic Cells

Eukaryotic cells are more complex and contain membrane-bound organelles, including a nucleus.

• Domain: Eukarya (includes animals, plants, fungi, and protists)

• Key Features:

  • Membrane-enclosed nucleus

  • Organelles such as endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes

  • Mitochondria (in all eukaryotes)

  • Chloroplasts (in plants and algae)

  • Cytoskeleton

  • Plasma membrane

  • Cell wall (in plants, fungi, and some protists)

Functional Compartments in Eukaryotic Cells

Organization of Cellular Functions

Membrane-enclosed organelles compartmentalize a cell's activities, allowing for specialized functions.

• Genetic Control: Nucleus and ribosomes

• Manufacture, Distribution, and Breakdown: Endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes

• Energy Processing: Mitochondria (all cells), chloroplasts (plant cells)

• Structural Support, Movement, Communication: Cytoskeleton, plasma membrane, cell wall (plants)

The Nucleus and Ribosomes

The Nucleus

The nucleus contains the cell's genetic instructions and directs protein synthesis via messenger RNA (mRNA).

• DNA: Organized as chromatin within the nucleus

• Nucleolus: Site of ribosome subunit assembly

Ribosomes

Ribosomes are composed of ribosomal RNA (rRNA) and proteins. They synthesize proteins according to instructions from DNA.

• Free Ribosomes: Float in the cytosol and produce proteins for use within the cell

• Bound Ribosomes: Attached to the endoplasmic reticulum or nuclear envelope; produce proteins for export or for membranes

Cells that produce large amounts of protein have many ribosomes.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No (nucleoid region)	Yes (membrane-bound)
Organelles	Absent	Present (membrane-bound)
Size	Smaller	Larger
Domains	Bacteria, Archaea	Eukarya
Cell Wall	Present (most)	Present (plants, fungi, some protists)