B3.1.1 – GAS EXCHANGE IN SINGLE-CELLED AND SIMPLE MULTICELLULAR ORGANISMS
📌Definition Table
| Term | Definition |
|---|---|
| Gas exchange | The process by which oxygen is absorbed and carbon dioxide is released across a respiratory surface. |
| Diffusion | The passive movement of molecules from an area of high concentration to low concentration. |
| SA:V ratio | The surface area-to-volume ratio, a key factor determining efficiency of gas exchange. |
| Simple diffusion | Gas exchange directly across a cell membrane without the need for specialised organs. |
| Unicellular organism | A living organism consisting of a single cell (e.g., Amoeba). |
📌Introduction
Single-celled organisms and simple multicellular organisms rely primarily on diffusion across their external surfaces for gas exchange. Their high surface area-to-volume ratio, short diffusion distances, and relatively low metabolic demands allow passive diffusion to meet their oxygen needs and remove carbon dioxide effectively. However, as organisms increase in size and complexity, this method becomes insufficient, necessitating adaptations for more efficient gas exchange.
📌 Gas Exchange in Unicellular Organisms
- Unicellular organisms such as Amoeba and Paramecium have a very high SA:V ratio, ensuring that diffusion provides sufficient oxygen for cellular respiration.
- The thin plasma membrane reduces diffusion distance, allowing gases to move in and out rapidly.
- Their relatively low metabolic demands compared to large animals mean diffusion alone can meet oxygen requirements.
- Carbon dioxide, produced as a waste product of respiration, diffuses out of the cell along its concentration gradient.
- Living in aqueous environments facilitates diffusion since gases can dissolve and move efficiently across moist membranes.
🧠 Examiner Tip: Do not confuse gas exchange (the physical diffusion of gases) with respiration (the chemical process of energy release in cells). This is a common exam error.
📌 Gas Exchange in Simple Multicellular Organisms
- Flatworms and cnidarians maintain high SA:V ratios due to flattened or simple body plans, enabling gases to diffuse directly across the body surface.
- Their thin epithelial tissues reduce diffusion distance, further facilitating oxygen uptake.
- However, increased body size reduces efficiency of simple diffusion, creating a limit on organismal complexity.
- Many simple animals live in aquatic habitats, as moisture is essential for gas diffusion across membranes.
🧬 IA Tips & Guidance: Students could investigate diffusion using agar blocks of varying sizes soaked with an indicator such as phenolphthalein, measuring how surface area-to-volume ratio affects diffusion rates — a model for constraints on unicellular vs multicellular organisms.
📌 Limitations of Diffusion Alone
- As body size increases, SA:V ratio decreases, reducing efficiency of gas exchange across the surface.
- Protective external structures (e.g., cuticles in some organisms) may reduce permeability.
- Diffusion alone cannot meet oxygen demands of highly active or large organisms, leading to evolution of specialised respiratory organs.
🌐 EE Focus: An EE could explore how SA:V ratios affect survival in different environments, for example comparing aquatic vs terrestrial simple organisms, or experimentally modelling diffusion rates in artificial systems of different sizes.
📌 Summary
- Small organisms rely on diffusion due to high SA:V ratio and low metabolic needs.
- Larger, more active organisms evolve respiratory surfaces and circulatory systems to overcome diffusion limitations.
❤️ CAS Link: Students could create models or simple classroom demonstrations of diffusion using coloured dyes in water to illustrate why diffusion is fast in small cells but inadequate in larger bodies.
🌍 Real-World Connection: Understanding diffusion limitations explains why parasites like flatworms remain small and flat, while more active organisms evolve lungs or gills. It also informs biomedical design, such as drug delivery systems where diffusion constraints must be considered.
🔍 TOK Perspective: This topic raises the TOK question: how do physical laws like diffusion limit the possibilities of biology? It shows how scientific models of diffusion not only describe but constrain the evolution of life.