Chapter 6: A Tour of the Cell – Study Notes

How We Study Cells

Microscopy and Cell Biology

Understanding cell structure and function relies on microscopy and biochemical techniques. The development of these tools has been central to the field of cytology, the study of cell structures.

• Microscopes are essential for visualizing cells and their components.

• Cytology is the study of cell structures, while biochemistry focuses on the molecules and chemical processes within cells.

• The invention and improvement of microscopes in the 17th century enabled the discovery and study of cells.

Types of Microscopes

• Light Microscopes (LMs): Use visible light and glass lenses to magnify specimens. Effective up to about 1,000x magnification; limited by the wavelength of light (~2 microns resolution).

• Electron Microscopes (EMs): Use electron beams and electromagnets for much higher resolution (practical limit ~2 nm). Two main types:

  • Transmission Electron Microscope (TEM): Studies internal cell structures by passing electrons through thin sections stained with heavy metals.

  • Scanning Electron Microscope (SEM): Examines cell surfaces by scanning with electrons; samples are coated with gold.

• Staining and Contrast: Techniques such as dyes and fluorescence enhance contrast and highlight specific cell components.

Advantages and Disadvantages

• Light Microscopes: Lower resolution but can be used on living cells.

• Electron Microscopes: Higher resolution but only for dead cells; may introduce artifacts.

Cell Fractionation

• Purpose: To separate and study individual cell organelles.

• Process: Uses an ultracentrifuge to spin cell homogenates at high speeds, separating components by size and density.

• Steps:

  1. Homogenization: Gently disrupts cells.

  2. Centrifugation: Heavier components form a pellet; lighter remain in the supernatant.

  3. Repeated at increasing speeds to isolate smaller organelles.

• Example: Mitochondria can be isolated and identified as the site of cellular respiration.

A Panoramic View of the Cell

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on structural differences.

• All cells have a plasma membrane that encloses their contents.

• Prokaryotic cells: Found in Bacteria and Archaea; lack a nucleus and membrane-bound organelles.

• Eukaryotic cells: Found in all other organisms; have a nucleus and membrane-bound organelles.

• Internal membranes in eukaryotes compartmentalize cellular functions.

Plant vs. Animal Cells

• Plant cells: Have a cell wall, chloroplasts, and a large central vacuole; usually lack centrioles.

• Animal cells: Lack cell walls and chloroplasts; usually do not have a large central vacuole but have centrioles.

The Nucleus and Ribosomes

Nucleus

The nucleus is the control center of the eukaryotic cell, containing most of the genetic material.

• DNA is organized with proteins into chromatin, which condenses into chromosomes during cell division.

• Nucleolus (plural: nucleoli): Site of ribosome synthesis within the nucleus.

• Exchange of materials between nucleus and cytoplasm occurs through nuclear pores.

Ribosomes

• Function: Protein synthesis.

• Types:

  • Free ribosomes: Suspended in cytosol; make proteins for use within the cell.

  • Bound ribosomes: Attached to the endoplasmic reticulum (ER) or nuclear envelope; make proteins for membranes or export.

The Endomembrane System

Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; synthesizes membrane and secretory proteins, which are transported in vesicles.

• Smooth ER: Lacks ribosomes; synthesizes lipids (including steroids), metabolizes carbohydrates, stores calcium (especially in muscle), and detoxifies poisons (notably in the liver).

Golgi Apparatus

• Structure: Stacks of flattened membranous sacs (cisternae).

• Function: Modifies, sorts, and ships proteins and lipids from the ER.

• Cis face: Receives vesicles from ER; Trans face: Ships modified products.

• Also synthesizes some polysaccharides (e.g., pectin in plants).

Lysosomes

• Structure: Membranous sacs containing hydrolytic enzymes.

• Function: Digests macromolecules, recycles cell components, and breaks down substances ingested by phagocytosis.

Vacuoles

• Function: Storage, waste disposal, cell growth, and protection (especially in plant cells).

• Central vacuole: Prominent in plant cells; maintains turgor pressure.

Other Membranous Organelles

Mitochondria

Mitochondria are the sites of cellular respiration and ATP production in eukaryotic cells.

• Enclosed by two membranes; inner membrane is folded into cristae to increase surface area.

• Some metabolic reactions occur in the mitochondrial matrix; others are catalyzed by enzymes in the inner membrane.

Chloroplasts

• Sites of photosynthesis in plants and algae.

• Enclosed by two membranes; contain stroma (fluid) and thylakoids (membranous sacs) stacked into grana.

Peroxisomes

• Contain enzymes that transfer hydrogen from substrates to oxygen, producing hydrogen peroxide (H2O2), which is then converted to water.

• Detoxify harmful substances (e.g., alcohol in the liver).

• Glyoxysomes: Specialized peroxisomes in plants that convert fatty acids to sugars in seedlings.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers that provides structural support, cell motility, and regulation.

• Microtubules: Hollow rods made of tubulin; shape the cell, guide organelle movement, and separate chromosomes during cell division.

• Centrosome: Microtubule-organizing center near the nucleus; contains two centrioles in animal cells.

• Cilia and Flagella: Motile structures with a core of microtubules; movement powered by the motor protein dynein.

• Microfilaments: Thin rods of actin; involved in muscle contraction, cell movement (amoeboid movement), cytoplasmic streaming, and support for microvilli.

• Intermediate Filaments: Fibrous proteins (e.g., keratin); maintain cell shape and anchor organelles.

Cell Surfaces and Junctions

Cell Walls and Extracellular Matrix

• Plant cell walls: Composed of cellulose, other polysaccharides, and proteins; provide structural support.

• Extracellular Matrix (ECM) in animal cells: Composed of glycoproteins (e.g., collagen, proteoglycans, fibronectin); supports, adheres, and regulates cells.

Intercellular Junctions

• Plasmodesmata: Channels between plant cells for communication and transport.

• Tight Junctions: Seal cells together, preventing leakage (e.g., in intestinal lining).

• Desmosomes (Anchoring Junctions): Bind cells together and anchor cytoskeletal fibers; provide tissue rigidity.

• Gap Junctions (Communicating Junctions): Channels that allow passage of ions and small molecules between animal cells; coordinate cell activity.

Structure	Location	Function	Key Components
Cell Wall	Plant cells	Support, protection	Cellulose, polysaccharides, protein
Extracellular Matrix (ECM)	Animal cells	Support, adhesion, regulation	Collagen, proteoglycans, fibronectin
Plasmodesmata	Plant cells	Communication	Membrane-lined channels
Tight Junctions	Animal cells (epithelia)	Seal cells, prevent leakage	Membrane proteins
Desmosomes	Animal cells	Anchoring, tissue rigidity	Keratin, other proteins
Gap Junctions	Animal cells	Communication	Protein channels

Example: Gap junctions in cardiac muscle allow rapid spread of electrical signals, coordinating heart contractions.

Additional info: The cytoskeleton also plays a role in intracellular transport, with motor proteins such as kinesin and dynein moving vesicles along microtubules.