Cell Theory and Types

Cell Theory

• Cells are the fundamental units of life, responsible for structure and function.
• The activity of an organism depends on the individual and collective activities of its cells.
• The activity of the cell is governed by its subcellular structures.
• All cells originate from pre-existing cells.

Cell Types

• Prokaryotic Cells:

  • Lack a nucleus.
  • DNA is located in a region called the nucleoid.
  • No membrane-bound organelles.
  • Examples: bacteria and archaea.

• Eukaryotic Cells:

  • DNA is enclosed within a membrane-bound nucleus.
  • Contain membrane-bound organelles.
  • Generally larger than prokaryotic cells.
  • Examples: animal, plant, and fungal cells.

Plasma Membrane

• A phospholipid bilayer with embedded proteins, cholesterol, and glycolipids.
• Separates the intracellular (inside the cell) environment from the extracellular (outside the cell) environment.
• Regulates the passage of substances into and out of the cell.
• Plays a role in cell communication.

Cell Wall

• Found in plants and fungi.
• Provides structural support, protection, and shape to the cell.
• Composed of carbohydrate fibers embedded in a carbohydrate or protein matrix.

Cilia and Flagella

• Extensions of the plasma membrane, containing a complex of proteins that support and move the structure.
• Differences in structure lead to different movement patterns.
• Flagella: Usually one or two per cell, responsible for locomotion (moving the cell through fluid).
• Cilia: Usually numerous per cell, responsible for movement of fluid past the cell.

Cell Junctions

• Tight junctions (animal cells):

  • Form an impermeable barrier, preventing extracellular fluids from passing between cells.

• Desmosomes (animal cells):

  • Act like rivets, forming strong connections between adjacent cells.

• Gap junctions (animal cells):

  • Channels connecting the cytoplasm of adjacent cells, allowing for communication and exchange of small molecules.

• Plasmodesmata (plant cells):

  • Similar to gap junctions, connecting the cytoplasm of adjacent cells.

Nuclear Structures

• Nuclear envelope:

  • Double-layered membrane surrounding the nucleus.

• Chromosomes/Chromatin:

  • Contain genetic information (DNA) that directs protein synthesis.

• Nucleolus:

  • Contains structures for assembling rRNA and proteins into ribosomal subunits.

Cytoplasm

• Cytosol:

  • A watery solution of proteins, salts, sugars, and other solutes.

• Cytoplasm:

  • The fluid (cytosol) and all the structures within the cell, excluding the nucleus.

Transcription

• The process of copying genetic information (DNA) into RNA.
• This process takes place in the nucleus.