Chapter 4: Cell Structure and Function

4.1 Microscopy and Cell Types

Understanding microscopy is essential for studying cells and their components. This section covers the differences between light and electron microscopes, and introduces the general classes of cells and biological molecules.

• Resolving Power and Magnification: Light microscopes use visible light to magnify specimens, typically up to 1000x, with a resolving power of about 200 nm. Electron microscopes use electron beams, achieving much higher magnification (up to 1,000,000x) and resolving power (as low as 0.1 nm).

• Types of Electron Microscopes: Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) are the two main types. TEM provides detailed internal structures, while SEM gives 3D images of surfaces.

• General Classes of Cells: Cells are classified as prokaryotic (e.g., bacteria) or eukaryotic (e.g., plants, animals, fungi, protists).

• Biological Molecules: Includes viruses, macromolecules (proteins, nucleic acids, carbohydrates, lipids), and small molecules (water, ions).

4.2 Basic Features Shared by All Cell Types

All cells share certain structural features, but also have unique components depending on their classification. This section explores these features and compares prokaryotic and eukaryotic cells.

• Plasma Membrane: A selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It regulates the movement of substances in and out of the cell.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing cytosol and organelles.

• Ribosomes: Structures responsible for protein synthesis. Found in both prokaryotic and eukaryotic cells.

• Genetic Material: DNA is the hereditary material in all cells.

• Additional Structures: Some cells have specialized structures such as flagella (for movement), cilia, and cell walls (in plants, fungi, and some prokaryotes).

Prokaryotic vs. Eukaryotic Cells:

• Prokaryotic Cells: No nucleus; DNA is located in the nucleoid region. Lack membrane-bound organelles.

• Eukaryotic Cells: Have a true nucleus and various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

Comparison Table:

4.3 Eukaryotic Cell Structure

Eukaryotic cells contain various organelles, each with specific functions. This section describes the nucleus and other key organelles.

• Nucleus: Contains the cell's genetic material (DNA) and is surrounded by a double membrane called the nuclear envelope. The nuclear lamina provides structural support.

• Nucleolus: Site of ribosome synthesis within the nucleus.

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

• Other Organelles: Mitochondria (energy production), chloroplasts (photosynthesis in plants), endoplasmic reticulum (protein and lipid synthesis), Golgi apparatus (modification and transport of molecules).

4.4 Endomembrane System

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins. This section explains its components and functions.

• Components: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and vesicles.

• Functions: Protein synthesis, lipid metabolism, detoxification, and transport of materials within the cell.

• Protein Synthesis and Transport: Proteins synthesized in the rough ER are transported to the Golgi apparatus for modification and sorting, then sent to their destinations via vesicles.

• Vesicle Transport: Vesicles bud from one organelle and fuse with another, facilitating the movement of molecules.

Example: Secretory proteins are synthesized in the rough ER, processed in the Golgi apparatus, and exported from the cell via exocytosis.

4.5 Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles found in eukaryotic cells. This section discusses their structure, function, and evolutionary origins.

• Mitochondria: Site of cellular respiration, converting glucose and oxygen into ATP (energy). Contains its own DNA and ribosomes.

• Chloroplasts: Site of photosynthesis in plants and algae. Contains its own DNA and ribosomes.

• Endosymbiotic Theory: Proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Evidence includes their double membranes, circular DNA, and similarity to certain bacteria.

Equation for Cellular Respiration:

Equation for Photosynthesis:

4.6 Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, facilitates cell movement, and organizes cellular components.

• Types of Cytoskeletal Fibers:

  • Microfilaments (actin filaments): Involved in cell movement and shape changes.

  • Intermediate Filaments: Provide mechanical strength.

  • Microtubules: Involved in cell division, intracellular transport, and movement of cilia and flagella.

• Centrosomes and Centrioles: Organize microtubules and are important in cell division.

• Cilia and Flagella: Structures for cell movement. Cilia are short and numerous; flagella are longer and usually singular.

Example: Sperm cells use flagella for motility; respiratory tract cells use cilia to move mucus.

4.7 Cell Walls, Extracellular Matrix, and Cell Junctions

Cells have specialized structures for support, protection, and communication. This section describes cell walls, the extracellular matrix, and cell junctions.

• Cell Walls: Found in plants, fungi, and some prokaryotes. Provide structural support and protection. Plant cell walls are primarily composed of cellulose.

• Extracellular Matrix (ECM): Complex network of proteins and carbohydrates outside animal cells. Provides structural support and regulates cell behavior.

• Cell Junctions:

  • Tight Junctions: Prevent leakage of materials between cells.

  • Desmosomes: Anchor cells together, providing mechanical strength.

  • Gap Junctions: Allow direct communication between cells through channels.

  • Plasmodesmata: Channels between plant cells that allow transport and communication.

Example: Gap junctions in cardiac muscle cells allow rapid transmission of electrical signals for synchronized contraction.

Additional info: The table summarizing organelles (referenced on page 98) is recommended for further study, but is not included in this guide.