Chapter 6: A Tour of the Cell – General Biology Study Notes

Chapter 6: A Tour of the Cell

6.1 Techniques of Microscopy and Biochemistry in Cell Biology

Understanding cells requires specialized techniques to visualize and analyze their structure and function. Microscopy and cell fractionation are essential tools in cell biology.

• Microscopy: Allows scientists to observe cells and their organelles.

  • Magnification: The process of enlarging the appearance of an object.

  • Resolving Power: The ability to distinguish two close objects as separate.

  • Electron Microscopy: Uses beams of electrons for higher resolution than light microscopy.

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

  • Transmission Electron Microscopy (TEM): Provides detailed images of internal cell structures (e.g., sections through a tracheal cell).

• Cell Fractionation: Technique to separate cell components for biochemical analysis.

  • Cells are broken up and centrifuged to isolate organelles.

  • Smallest organelles are typically ribosomes.

Example: Preparing specimens for electron microscopy may kill cells, which is a disadvantage.

6.2 Prokaryotic vs. Eukaryotic Cells; Animal vs. Plant Cells

Cells are classified as prokaryotic or eukaryotic, and as animal or plant cells, based on their structure and organization.

• Prokaryotic Cells: Lack a nucleus; DNA is in a region called the nucleoid.

  • Cell Wall: Rigid structure outside plasma membrane.

  • Plasma Membrane: Encloses cytoplasm.

  • Bacterial Chromosome: Carries genes in the form of DNA.

  • Nucleoid: Region where DNA is located.

  • Ribosomes: Synthesize proteins.

  • Flagella: Organelles for locomotion in some bacteria.

• Eukaryotic Cells: Have a nucleus containing DNA.

  • Animal cells: No cell wall, have lysosomes and centrioles.

  • Plant cells: Have cell wall, chloroplasts, and central vacuole.

Surface Area-to-Volume Ratio: Determines cell's ability to exchange materials with the environment. As cell size increases, the ratio decreases, limiting efficiency.

• Formula: $\text{Surface Area-to-Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

Example: Microvilli in intestinal cells increase surface area without increasing volume, enhancing absorption.

6.3 Structure and Function of the Nucleus, Chromosomes, and Ribosomes

The nucleus is the control center of eukaryotic cells, containing genetic material and coordinating cellular activities.

• Nuclear Envelope: Double membrane enclosing the nucleus; contains nuclear pores for transport.

• Nuclear Lamina: Protein filaments supporting the nuclear envelope.

• Chromatin: DNA and protein complex; condenses to form chromosomes during cell division.

• Nucleolus: Site of ribosome synthesis within the nucleus.

• Ribosomes: Complexes of rRNA and protein; synthesize proteins. Two types:

  Type of Ribosome	Location	Product
  Free ribosomes	Cytosol	Proteins that function within cytosol
  Bound ribosomes	Outside of endoplasmic reticulum	Proteins for insertion into membranes or packaging within organelles

Example: mRNA passes through nuclear pores to reach ribosomes for protein synthesis.

6.4 The Endomembrane System

The endomembrane system regulates protein traffic and performs metabolic functions in the cell.

• Components: Nuclear envelope, Endoplasmic reticulum (ER), Golgi apparatus, Lysosomes, Vesicles, Vacuoles, Plasma membrane.

• Endoplasmic Reticulum (ER):

  • Smooth ER: Synthesizes lipids, detoxifies drugs/poisons, stores calcium ions.

  • Rough ER: Studded with ribosomes; synthesizes proteins and glycoproteins, grows by adding membrane proteins and phospholipids.

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

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion; have neutral pH.

  • Function in phagocytosis (digestion of particles) and autophagy (recycling cellular components).

  • Dysfunction can cause diseases (e.g., Tay-Sachs disease).

• Vacuoles: Diverse functions depending on cell type.

  • Food vacuoles: Formed by phagocytosis.

  • Contractile vacuoles: Pump excess water out of cell.

  • Central vacuole: In plants, maintains turgor pressure and stores nutrients.

Example: The ER and Golgi apparatus function together to secrete proteins and digest cellular components.

6.5 Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles with unique structures and origins.

• Endosymbiont Theory: Mitochondria and plastids originated as prokaryotic cells engulfed by ancestors of eukaryotes.

  • Evidence: Double membranes, circular DNA, ribosomes, and ability to grow/reproduce independently.

• Mitochondria Structure:

  • Outer membrane

  • Inner membrane

  • Intermembrane space

  • Cristae (folds of inner membrane)

  • Matrix (internal fluid)

  • Ribosomes

• Chloroplast Structure:

  • Outer and inner membranes

  • Thylakoid membranes (site of photosynthesis)

  • Stroma (fluid surrounding thylakoids)

  • Ribosomes

Example: Mitochondria are the site of cellular respiration; chloroplasts are the site of photosynthesis in plants.

Additional info:

• Some context and explanations were expanded for clarity and completeness.

• Tables and diagrams referenced in the original were recreated or described in text.