Gaseous exchange takes place through the plasma membrane in

1. Analyze the question The question asks which organism exchanges gases directly across the plasma membrane. This is only possible for an organism that is a single cell, so the plasma membrane is the only surface between it and the environment.

2. Explain the concept

• Unicellular organisms (e.g. Amoeba, Paramecium) have no specialised respiratory organs. Oxygen diffuses in and carbon dioxide diffuses out directly through the plasma membrane (cell membrane).
• Multicellular organisms (even simple ones) exchange gases across body surfaces made of many cells. This is gaseous exchange through the body surface or skin, not "through the plasma membrane" of the whole organism.

3. Step-by-step solution

• A. Hydra: multicellular (a cnidarian). It exchanges gases by diffusion across its body wall, which is made of two cell layers. ✗
• B. Paramecium: a unicellular protozoan. Gases diffuse straight across its plasma membrane. ✓
• C. Flatworm: multicellular. It uses its flattened body surface, which has a large surface area. ✗
• D. Earthworm: multicellular. It uses its moist skin (cuticle) with a blood supply beneath it. ✗

4. Answer The correct option is B. Paramecium.

5. Exam tip Whenever you see "through the plasma membrane" or "cell membrane" for gas exchange, look for the unicellular organism (Amoeba, Paramecium, Euglena). Remember the pattern:

• Unicellular → plasma membrane
• Simple multicellular (hydra, flatworm, earthworm) → body surface/skin
• Insects → tracheae
• Fish → gills
• Mammals → lungs