9. Cells & Their Parts: Eukaryotic Cells

Overview of Eukaryotic Cells

Eukaryotic cells are complex cells characterized by the presence of membrane-bound organelles, including a nucleus. They are found in animals, plants, fungi, and protists. Understanding their structure and function is fundamental to general biology.

• Eukaryotic cells have a true nucleus and various organelles that perform specialized functions.

• Animal and plant cells share many features but also have distinct differences (e.g., plant cells have cell walls and chloroplasts).

Structure and Function of the Nucleus, Chromosomes, and Ribosomes

The nucleus, chromosomes, and ribosomes are essential components involved in storing, transmitting, and expressing genetic information.

• Nucleus: The control center of the cell, surrounded by a double membrane called the nuclear envelope. Contains the cell's genetic material.

• Nuclear Envelope: Double membrane that encloses the nucleus, separating it from the cytoplasm and regulating transport of molecules.

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

• Chromosomes: Structures composed of DNA and proteins (histones) that carry genetic information. Humans have 46 chromosomes in each somatic cell.

• Ribosomes: Molecular machines responsible for protein synthesis. Found free in the cytoplasm or attached to the endoplasmic reticulum.

Function: Store, transmit, and express genetic information.

• Example: During cell division, chromosomes ensure accurate distribution of genetic material to daughter cells.

Endomembrane System: Structure and Roles

The endomembrane system is a network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Nuclear Envelope: Separates the nucleus from the cytoplasm and is continuous with the endoplasmic reticulum.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes and processes proteins.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

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

• Lysosomes: Contain digestive enzymes to break down macromolecules, old organelles, and foreign substances.

• Vacuoles: Storage organelles; large central vacuole in plant cells stores water, nutrients, and waste products.

• Plasma Membrane: Phospholipid bilayer that controls entry and exit of substances, maintaining homeostasis.

Example: Proteins synthesized in the rough ER are transported to the Golgi apparatus for modification and then sent to their final destinations.

Comparison: Animal vs. Plant Eukaryotic Cells

Animal and plant cells share many organelles but also have unique structures.

• Plant cells have a cell wall, chloroplasts, and a large central vacuole.

• Animal cells lack cell walls and chloroplasts but have centrioles.

Summary of Key Terms

• Eukaryotic cell: Cell with a nucleus and membrane-bound organelles.

• Nucleus: Organelle containing DNA.

• Chromosome: DNA-protein complex carrying genetic information.

• Ribosome: Site of protein synthesis.

• Endomembrane system: Network of membranes involved in transport and processing.

Equations and Scientific Principles

• Central Dogma of Molecular Biology:

• Surface Area to Volume Ratio: Important for cell efficiency.

How Parts of the Cell Work Together

Cellular organelles interact to maintain homeostasis and perform life functions. For example, the nucleus directs protein synthesis, which occurs at ribosomes, and proteins are processed and transported by the endomembrane system.

• Example: Hormone production in endocrine cells involves transcription in the nucleus, translation at ribosomes, modification in the ER and Golgi, and secretion via vesicles.