Biology Lecture: The Cell as the Basis of Life

Introduction

• Lecturer: Andy from Med School EU
• Focus: Second lecture on biology, covering cell theory and cell size.

Cell Theory

• Origin: Proposed by Robert Hook, using an early microscope.
• Three Main Tenets:
  1. All organisms are composed of one or more cells.
     • Includes multicellular and unicellular organisms: prokaryotes, eukaryotes, plant cells, animal cells, fungal cells, protists, bacteria.
     • No living organism is composed solely of inorganic substances.
  2. The cell is the basic structural and functional unit of all living organisms.
     • Cells, if sliced or broken, lose the property of life (cannot grow, reproduce, or respond independently).
     • Life does not exist in simpler units than cells.
  3. Cells arise only from the division of pre-existing cells.
     • Occurs through asexual or sexual reproduction (mitosis, meiosis).
     • Cells do not spontaneously generate from inorganic matter.

Cell Size

• Units of Measurement:
  • Meter (m), Centimeter (cm), Millimeter (mm), Micrometer (μm), Nanometer (nm).
  • Conversions are crucial for exams (e.g., knowing how many nm are in a μm).

Examples of Cell Sizes:

• Oxygen Atom (O2):
  • Diameter: 0.07 nm.
  • Illustrates the small scale (0.07 billionth of a meter).
• DNA Double Helix:
  • Width: 2 nm.
  • Larger than O2 but still tiny.
• Cell Membrane:
  • Thickness: 9 nm.
  • Several times larger than DNA width.
• Virus:
  • Example: HIV virus, about 100 nm.
  • Larger viruses include influenza.
• E. coli Bacteria:
  • Width: 1 μm.
  • 10 times the size of a large virus.
• Mitochondrion:
  • Size: 1-5 μm.
  • Comparable to E. coli, possibly evolved from independent bacteria.
• Animal Cell:
  • Typical size: 5-30 μm.
  • Human egg: around 100 μm.
• Plant Cell:
  • Size: 10-100 μm.
  • Larger than animal cells, tightly packed with distinct structures.

Visibility:

• Human eye can typically see 1 mm, which is much larger than the structures mentioned.

Exam Preparation Tips

• Potential Questions:
  • Ranking structures from smallest to largest, or vice-versa.
  • Understanding relative sizes: plant cells > animal cells, bacteria smaller than either, viruses smaller than bacteria, and atoms as the smallest measurable structures.

Next Lecture

• Focus on prokaryotic and eukaryotic cells, as well as differences between animal and plant cells.