OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

How do single-celled organisms effectively exchange substances?

1. Through active transport

2. By diffusion across cell membrane

3. Via specialized structures

4. By cellular respiration

The correct answer is: By diffusion across cell membrane

Single-celled organisms, such as bacteria and protozoa, exchange substances primarily through diffusion across their cell membranes. This method is effective for these organisms due to their small size and large surface area-to-volume ratio. When substances such as nutrients, gases, and waste products need to enter or leave the cell, they move down their concentration gradient through the selectively permeable cell membrane. Diffusion can occur directly through the lipid bilayer or via protein channels, allowing essential molecules to pass freely without the need for energy input. This passive process is efficient for single-celled organisms that are surrounded by their environment and have all necessary substances dissolved in their surroundings, making it an ideal strategy for sustenance and waste management. Other methods mentioned, such as active transport, involve energy consumption to move substances against their concentration gradient and are typically more relevant in multicellular organisms. Specialized structures also do not apply to single-celled organisms in the same way they might to eukaryotic cells with complex systems. Cellular respiration is a metabolic process that does not pertain directly to the mechanism of substance exchange across cell membranes.