Chapter 6: A Tour of the Cell

Concept 6.1: Microscopy and Biochemistry in Cell Biology

This section introduces the techniques used to study cells, focusing on microscopy and cell fractionation. Understanding these methods is essential for exploring cell structure and function.

• Microscopy: The use of microscopes to magnify and resolve cellular structures. Magnification is the process of enlarging an image, while resolving power is the ability to distinguish two close points as separate entities.

• Light Microscopy: Uses visible light to observe cells. Limited by the wavelength of light, restricting resolution to about 200 nm.

• Electron Microscopy: Uses electron beams for much higher resolution (up to 2 nm). Two main types:

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

  • Transmission Electron Microscopy (TEM): Provides detailed images of internal cell structures.

• Cell Fractionation: Technique to separate cellular components by size and density using centrifugation. Allows study of organelles and their functions.

Example: Using TEM, scientists can observe the arrangement of organelles within a eukaryotic cell.

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

This section compares the two major cell types and highlights the differences between animal and plant cells.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Animals, plants, fungi, and protists.

• Key Differences:

  • DNA location: Nucleoid (prokaryotes) vs. nucleus (eukaryotes)

  • Presence of organelles: Absent in prokaryotes, present in eukaryotes

  • Cell size: Prokaryotes are generally smaller (0.1–5 μm) than eukaryotes (10–100 μm)

• Animal vs. Plant Cells:

  • Plant cells have cell walls, chloroplasts, and large central vacuoles; animal cells do not.

  • Animal cells have lysosomes and centrioles, which are rare in plant cells.

Example: The presence of chloroplasts in plant cells enables photosynthesis, a process absent in animal cells.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Concept: Surface Area-to-Volume Ratio

The surface area-to-volume ratio is crucial for cell function, as it affects the rate of exchange of materials with the environment.

• Formula: For a cube, , , where is the length of a side.

• As a cell increases in size, its volume grows faster than its surface area, reducing the ratio.

• Cells remain small or develop structures like microvilli to maximize surface area for efficient exchange.

Example: Intestinal cells have microvilli to increase surface area for nutrient absorption.

Concept 6.3: The Nucleus, Chromosomes, and Ribosomes

The nucleus contains the cell's genetic material and is the site of DNA replication and transcription. Ribosomes are responsible for protein synthesis.

• Nucleus: Surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for molecular transport.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Nucleolus: Region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Ribosomes: Complexes of rRNA and protein; can be free in the cytosol or bound to the endoplasmic reticulum (ER).

Table: Types of Ribosomes

Concept 6.4: 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.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Endoplasmic Reticulum (ER):

  • Smooth ER: Lacks ribosomes; functions in lipid synthesis, detoxification, and calcium storage.

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion and recycling of cellular components.

• Vacuoles: Storage organelles; types include food vacuoles, contractile vacuoles (in protists), and central vacuoles (in plants).

Example: The rough ER produces glycoproteins, which are then modified and sorted by the Golgi apparatus before being secreted from the cell.

Table: Types of Vacuoles

• Some questions reference figures and diagrams from the textbook; students should consult their textbook for visual aids.

• Understanding the relationship between structure and function is a recurring theme in cell biology.