Lecture Notes: Cell Theory and Cell Structure

Introduction to Biological Theories

• Theory of Evolution: By natural selection.
• Cell Theory: Consists of three main components:
  • Cells are the basic unit of life.
  • All cells originate from other cells.
  • Viruses, lacking a membrane, are not considered alive under cell theory despite showing other life properties.

Discovery and Basic Structure of Cells

• Early cell discovery was limited by rudimentary microscopes.
• Cell Membrane: Sac-like structure containing the cell.
• Organelles: Structures within cells with specific functions, similar to organs.
• Cytoplasm: Medium in which organelles are suspended.

Cell Size and Surface Area to Volume Ratio

• Cells must interact with their environment (e.g., gas exchange, nutrient absorption).
• The surface area to volume ratio limits cell size; if too large, cells must divide or stop growing.

Types of Cells

• Prokaryotic Cells: Lack a membrane-bound nucleus, have a circular DNA plasmid.
  • Examples: Bacteria, cyanobacteria.
  • Shapes: Rod-shaped, spherical, spiral.
  • Division: Binary fission.
• Eukaryotic Cells: Have a membrane-bound nucleus, linear DNA.
  • Present in all kingdoms except Monera.
  • Can be single-celled or multicellular.
  • Contain membrane-bound organelles for metabolism and energy conversion.

Key Organelles and Their Functions

Nucleus

• DNA Storage: Contains the genetic material (DNA).
• Nuclear Envelope and Pores: Allow passage of materials.
• Chromatin: DNA in non-reproducing cells.
• Nucleolus: Produces ribosomes.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes, synthesizes proteins.
• Smooth ER: Synthesizes lipids, steroids, and metabolizes carbohydrates.

Golgi Apparatus

• Modifies, sorts, and packages protein and lipid molecules for secretion or use within the cell.

Lysosomes

• Break down waste materials and cellular debris (autophagy).
• Known as the "stomach" of the cell.

Vacuoles

• Enclosed compartments for storage of water, nutrients, and waste.
• Prominent in plant cells for long-term water storage.

Mitochondria

• Powerhouse of the cell, generates ATP.
• Double-membraned, semi-autonomous with its own DNA.

Cytoskeleton

• Provides structural support, intracellular transport, and assists in cell division.

Chloroplasts (Plant Cells)

• Conduct photosynthesis, contain chlorophyll making them green.
• Thought to originate from cyanobacteria.

Cell Wall

• Structural support and protection, present in plants, fungi, and some bacteria.

Conclusion

• Cells are fundamental units of life with complex structures and diverse functions across different organisms.
• Understanding cell components and their roles is crucial in biology.