Inside the Cell

Overview of Cell Theory

Cell theory is a foundational concept in biology, stating that all living organisms are composed of cells. Cells are the basic units of structure and function in all organisms.

• Proteins: Perform most cellular functions, including catalysis, structure, and signaling.

• Nucleic acids: Store, transmit, and process genetic information (e.g., DNA and RNA).

• Carbohydrates: Provide chemical energy, carbon, structural support, and cellular identity.

• Plasma membrane: A selectively permeable barrier that separates the cell from its environment.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on their morphology and evolutionary history:

• Prokaryotes: Lack a membrane-bound nucleus. Includes Bacteria and Archaea.

• Eukaryotes: Possess a membrane-bound nucleus. Includes Protists, Fungi, Plants, and Animals.

Organisms are divided into three domains:

• Bacteria (prokaryotic)

• Archaea (prokaryotic)

• Eukarya (eukaryotic)

Prokaryotic Cell Structure

Key Components

• Chromosome: Usually a single, circular DNA molecule associated with proteins, located in the nucleoid region.

• Plasmids: Small, circular, supercoiled DNA molecules that carry additional genes.

• Ribosomes: Macromolecular complexes of RNA and protein, responsible for protein synthesis.

• Phospholipids: Bacterial membranes have fatty acids bound to glycerol; archaeal membranes have branched isoprenoid chains bound to glycerol.

• Cytoplasm: All contents within the plasma membrane.

• Cytoskeleton: Protein filaments that maintain cell shape and are essential for cell division.

• Internal Membranes (in photosynthetic species): Membrane infoldings containing enzymes and pigments for photosynthesis.

• External Structures:

  • Flagella: Long filaments that rotate to propel the cell.

  • Fimbriae: Needlelike projections for attachment to surfaces or other cells.

Eukaryotic Cell Structure

General Features

• Range in size from microscopic algae to large plants and animals.

• May be unicellular or multicellular.

• Typically larger and more complex than prokaryotic cells.

Benefits of Compartmentalization

• Separation of incompatible chemical reactions.

• Increased efficiency of cellular processes.

Major Eukaryotic Cell Components

The Endomembrane System

The endomembrane system includes the ER, Golgi apparatus, and lysosomes. It is responsible for the production, processing, and transport of proteins, carbohydrates, and lipids.

• Proteins are synthesized in the ER, processed in the Golgi apparatus, and shipped to their destinations via vesicles.

• Each protein contains a molecular "zip code" that directs it to the correct location.

Protein Targeting and Transport

• Nuclear Localization Signal (NLS): A 17-amino-acid sequence that directs proteins to the nucleus through nuclear pores.

• Proteins destined for the endomembrane system are folded in the ER lumen, modified in the Golgi, and sorted into vesicles for delivery.

• Exocytosis is the process by which vesicles fuse with the plasma membrane to secrete contents outside the cell.

Lysosomal Recycling Pathways

• Endocytosis: Uptake of external materials by pinching off the plasma membrane.

  • Receptor-mediated endocytosis: Specific molecules bind to receptors, are internalized, and delivered to early endosomes, which mature into lysosomes.

  • Phagocytosis: The cell engulfs large particles or cells, forming a phagosome that fuses with a lysosome for digestion.

• Autophagy: Damaged organelles are enclosed in a membrane (autophagosome) and delivered to lysosomes for degradation.

The Cytoskeleton

Structure and Function

The cytoskeleton is a dynamic network of protein fibers that provides structural support, maintains cell shape, and facilitates movement and transport within the cell.

• Not a static structure; components can assemble and disassemble as needed.

• Responsible for changes in cell shape, movement of organelles, and cell motility.

Major Cytoskeletal Elements

• Actin filaments (microfilaments): Smallest fibers, composed of actin protein subunits; abundant in animal cells.

• Intermediate filaments: Provide mechanical strength; composition varies by cell type.

• Microtubules: Largest fibers; hollow tubes made of tubulin; involved in cell division, intracellular transport, and cell movement.

Example:

• Actin filaments are essential for muscle contraction, cell movement, and cytokinesis during cell division.

• Microtubules form the mitotic spindle during cell division and serve as tracks for vesicle transport.

Additional info: The dynamic nature of the cytoskeleton allows cells to rapidly reorganize their internal structure in response to environmental signals or developmental cues.