Lecture Notes: Cell Size and Surface Area to Volume Ratios

Introduction

• Bacterial Cell Size: Smallest cells can be as small as 0.00001 meters, appearing as small dots under light microscopes.
• Frog Cells: Very large, nearly 1 cm across; 10,000 times bigger than bacterial cells but composed of the same subcellular components.

Topics Covered

1. Shape and size of cells related to surface area to volume ratios.
2. Calculating surface area to volume ratios using geometry.
3. Limitations of cell size due to surface area to volume ratios.
4. Importance of cell size for nutrient uptake and waste elimination.

Importance of Surface Area to Volume Ratios

• Critical for cells to obtain nutrients and gather energy.
• Smallest cells are ~1 micrometer; e.g., bacterial cells similar in size to chloroplasts and mitochondria.
• Bacterial Cell Size Limitation: Max size ~100 micrometers due to lack of internal organelles.
• Eukaryotic Cells: Can be up to 1 cm; average ~100 micrometers.

Factors Affecting Cell Size

• Surface Area to Volume Ratio: Affects efficiency in exchanging macromolecules, gases, and water.
• Cell Membrane: Composed of phospholipids and proteins for substance exchange.
• As cells grow, volume increases faster than surface area, requiring more proteins.
• Limits:
  • Upper limit: Cannot pack enough proteins into the membrane.
  • Lower limit: Insufficient volume for DNA replication and functions of life.

Visualizing Cell Sizes

• Comparisons: Coffee bean, grain of rice, sesame seed, human egg, skin cells, and bacteria.
• Bacteria are larger than viruses but much smaller than human cells.

Calculating Surface Area to Volume Ratios

• Cube Model: Volume = 2, Surface Area = 10 → Ratio = 5
• Sphere Model: Diameter = 2 mm, Volume ≈ 4.2 mm³, Surface Area ≈ 12.6 mm² → Ratio = 3
• Conclusion: Shape affects the ratio; cubes have higher ratios than spheres.

Examples

• Eggs: Large volume, less surface area; cells reduce size for efficiency post-fertilization.
• Bacteria: High surface area to volume ratio; limited by DNA housing and protein space.

Enhancing Efficiency in Organisms

• Folding in Tissues: Increases efficiency, e.g., alveoli in lungs and small intestine.
• Alveoli: Small sacs that increase surface area for efficient gas exchange.
• Small Intestine: Multiple levels of folding (lining, villi, microvilli) to maximize nutrient absorption.

Other Functions

• Heat Dissipation: Small cells on skin/lungs transfer heat out.
• Heat Retention: Large fat cells in seals retain heat in cold environments.

Conclusion

• Surface area to volume ratio is crucial for efficient cell function.
• Understanding this concept is important for biology studies.
• Resources: Video, quizzes, and additional resources available for exam prep.

Note: These notes summarize key points from the lecture on cell size and surface area to volume ratios, aimed at helping students understand the biological significance of these concepts.