Chapter 4: Cell Structure and Function

4.1 Microscopy and Cell Types

Understanding the tools used to study cells is fundamental in biology. Microscopes allow scientists to observe cells and their components at various levels of detail.

• Light Microscope: Uses visible light to magnify specimens. Resolving power is limited to about 200 nm.

• Electron Microscope: Uses beams of electrons for much higher resolution (up to 0.1 nm), allowing visualization of organelles, viruses, and macromolecules.

• General Sizes: Cells (1-100 μm), organelles (up to several μm), viruses (20-300 nm), macromolecules (1-10 nm), small molecules (less than 1 nm).

Example: The nucleus is visible with a light microscope, but ribosomes require electron microscopy.

4.2 Basic Features Shared by All Cell Types

All cells share certain structural features, but there are key differences between prokaryotic and eukaryotic cells.

• Plasma Membrane: A phospholipid bilayer that controls entry and exit of substances.

• Cytoplasm: The region between the plasma membrane and the nucleus (or nucleoid).

• DNA: Genetic material; in prokaryotes, found in the nucleoid, in eukaryotes, within the nucleus.

• Ribosomes: Sites of protein synthesis.

Prokaryotic Cell Structures:

• Plasma membrane

• Cell wall (peptidoglycan)

• Nucleoid (region containing DNA)

• Ribosomes

• Flagella (for movement)

Eukaryotic Cell Structures:

• Plasma membrane

• Nucleus (with nuclear envelope)

• Membrane-bound organelles (mitochondria, chloroplasts, ER, Golgi apparatus, etc.)

• Cytoskeleton

• Ribosomes

Comparison: Eukaryotic cells are generally larger and more complex than prokaryotic cells.

Example: Animal cells lack cell walls, while plant cells have cell walls made of cellulose.

4.3 Eukaryotic Cell Structure

Eukaryotic cells contain a variety of organelles, each with specialized functions.

• Nucleus: Contains DNA; surrounded by nuclear envelope and nuclear lamina.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins.

• Chromosomes: Condensed chromatin during cell division.

Functions:

• Genetic information storage and processing

• Regulation of gene expression

Example: The nuclear envelope separates the nucleus from the cytoplasm, allowing compartmentalization of genetic processes.

4.4 Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that manages protein and lipid synthesis, transport, and processing.

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

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Vesicles: Transport materials between organelles.

Protein Synthesis and Transport:

• Proteins synthesized in the rough ER are transported to the Golgi apparatus for modification.

• Vesicles carry proteins to their final destinations.

Equation:

Example: Insulin is synthesized in the rough ER, processed in the Golgi, and secreted via vesicles.

4.5 Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles found in eukaryotic cells. They are central to cellular metabolism and energy production.

• Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP.

• Chloroplasts: Found in plants and algae; site of photosynthesis, converting light energy into chemical energy.

• Endosymbiotic Theory: Suggests mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Equation for Cellular Respiration:

Equation for Photosynthesis:

Example: Chloroplasts contain their own DNA and ribosomes, supporting the endosymbiotic theory.

4.6 Cytoskeleton

The cytoskeleton provides structural support, facilitates cell movement, and organizes cellular components.

• Microfilaments (Actin Filaments): Involved in cell movement and shape changes.

• Intermediate Filaments: Provide mechanical strength.

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

• Centrosomes/Centrioles: Organize microtubules during cell division.

• Cilia and Flagella: Structures for cell movement; powered by microtubules.

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

4.7 Cell Walls, Extracellular Matrix, and Cell Junctions

Cells are supported and connected by external structures and specialized junctions.

• Cell Walls: Found in plants, fungi, and some protists; provide rigidity and protection.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells; provides structural support and regulates cell behavior.

• Cell Junctions: Specialized structures that connect cells and facilitate communication.

Example: Gap junctions enable electrical signaling between heart muscle cells.

Additional info: The table summarizing organelles (referenced on page 98) is a recommended study tool for further review.