Edexcel iGCSE Biology-3.1 Mode of action of enzymes- Study Notes- New Syllabus

Diffusion in Simple Unicellular Organisms

📝 Introduction

All organisms need to exchange substances (like oxygen, carbon dioxide, nutrients, and wastes) with their surroundings.
The method depends on their size and structure.

🌱 Why Unicellular Organisms Can Rely on Diffusion

Small Size → Large Surface Area to Volume Ratio (SA:V)

• Unicellular organisms (e.g., Amoeba, bacteria) are very small.
• They have a large surface area compared to volume.
• Diffusion across the cell membrane is fast enough to meet their needs.

Short Diffusion Distance

• The whole cell is only a few micrometres across.
• Substances travel only a tiny distance → diffusion is quick.

Low Metabolic Demand

• Simple unicellular organisms don’t need as much oxygen/nutrients as bigger multicellular ones.
• Diffusion alone is sufficient to supply them.

Direct Exchange with Environment

• They usually live in watery environments.
• Gases (O₂, CO₂) and nutrients diffuse directly through the cell membrane.
• No need for blood or complex transport systems.

📊 Summary Table

⚡ Quick Recap
Unicellular = large SA:V ratio + short distance → diffusion is enough.
Multicellular = small SA:V ratio + high demand → need transport systems.