Chapter 4: A Tour of the Cell

4.1 Microscopes and Biochemistry to Study Cells

Microscopes and biochemical techniques are essential tools for studying cell structure and function. They allow scientists to visualize cells and their components at various levels of detail.

• Robert Hooke discovered cell walls in 1665 from an oak tree.

• Light Microscope: Uses visible light passed through a specimen and glass lenses to magnify images.

  • Lenses refract light, magnifying the specimen.

• Scanning Electron Microscope (SEM): Useful for detailed study of the surface topography of a specimen.

• Transmission Electron Microscope (TEM): Used to study internal cell structure.

  • Aims a beam through a thin section of the specimen.

  • Samples are stained with atoms of heavy metals.

  • Cryo-electron microscopy: A new TEM technique that preserves specimens at extremely low temperatures.

• Cell Fractionation: Technique for studying cell structure and function by breaking up cells and spinning them in a centrifuge.

  • Largest cell components settle to the bottom, forming a pellet.

4.2 Eukaryotic Cells Have Internal Membranes

Eukaryotic cells are characterized by internal membranes that compartmentalize functions and structures, distinguishing them from prokaryotic cells.

• Prokaryotic cells: Lack membrane-bound organelles; DNA is in the nucleoid region.

• Eukaryotic cells: Have membrane-bound organelles, including a nucleus.

• Internal membranes increase surface area for metabolic processes.

4.3 Eukaryotic Cell's Genetic Information

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

• Nucleus: About 5 micrometers in diameter.

• Nuclear Envelope: Double membrane enclosing the nucleus, each a lipid bilayer, lined by the nuclear lamina.

• Chromosomes: Contain histones (basic proteins) and chromatin (complex of DNA and proteins).

• Nucleolus: Prominent structure in nondividing nucleus; synthesizes rRNA.

• Nucleus directs protein synthesis by synthesizing messenger RNA (mRNA).

4.4 Endomembrane System

The endomembrane system includes all organelles that carry out various cellular functions, such as synthesis, transport, and metabolism.

• Endoplasmic Reticulum (ER): Network of membranes accounting for more than half of the total membrane in a cell.

  • Smooth ER: No ribosomes; involved in synthesis of lipids, metabolism, detoxification, and storage of calcium ions.

  • Rough ER: Ribosomes on surface; synthesizes proteins, especially secretory proteins and glycoproteins.

• Transport Vesicles: Move proteins and other molecules between organelles.

• Golgi Apparatus: Warehouse for receiving, sorting, shipping, and manufacturing.

  • Consists of flattened membranous sacs called cisternae.

  • Cis face: Receiving side; Trans face: Shipping side.

• Lysosomes: Membranous sacs of hydrolytic enzymes; digest macromolecules and recycle cell's organic material (autophagy).

• Vacuoles: Large vesicles derived from ER and Golgi; include food vacuoles, contractile vacuoles, and central vacuole in plants.

4.5 Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles, displaying similarities with bacteria and supporting the endosymbiont theory.

• Mitochondria: Site of cellular respiration; uses oxygen to generate ATP.

  • Outer membrane is smooth; inner membrane is folded into cristae.

  • Inner membrane divides mitochondrion into intermembrane space and mitochondrial matrix.

• Chloroplasts: Site of photosynthesis; converts solar energy to chemical energy.

  • Contains green pigment chlorophyll and enzymes for sugar production.

  • Contains thylakoids (stacked into granum), stroma (fluid outside thylakoids), and own DNA/ribosomes.

  • Specialized members of the plastid family.

• Endosymbiont Theory: Eukaryotic cells evolved from symbiosis between ancestral prokaryotes.

• Peroxisomes: Specialized metabolic compartments; contain enzymes that transfer hydrogen to oxygen, producing hydrogen peroxide (), which is then converted to water.

4.6 Cytoskeleton

The cytoskeleton is a network of fibers that organizes cell structure and activities, providing mechanical support and enabling cell movement.

• Microtubules: Hollow rods; maintain cell shape, guide organelle movement, and separate chromosomes during cell division.

• Microfilaments: Thin solid rods; built from actin; involved in cell shape, movement, and muscle contraction.

• Intermediate Filaments: Fibrous proteins; maintain cell shape and anchor organelles.

• Cytoskeleton works with motor proteins to enable cell motility.

4.7 Extracellular Components and Connections

Cells have extracellular structures that provide support, protection, and facilitate communication between cells.

• Cell Wall: Found in plants, fungi, and some protists; protects and maintains cell shape.

• Extracellular Matrix (ECM): Found in animal cells; composed of glycoproteins like collagen and proteoglycans; provides structural support and regulates cell behavior.

• Intercellular Junctions:

  • Plasmodesmata: Channels in plant cell walls for communication.

  • Tight Junctions, Desmosomes, Gap Junctions: Common in animal tissues; facilitate adhesion and communication.

Key Terms Table

Important Equations

• Surface Area to Volume Ratio: Determines efficiency of cell transport and metabolism.

• Peroxisome Reaction:

Summary Table: Prokaryotic vs. Eukaryotic Cells

Additional info: Academic context and definitions have been expanded for clarity and completeness. All major topics from the original notes have been included and organized for effective exam preparation.