Why can a single-celled organism rely on diffusion alone?
Reveal answer
Large surface area to volume ratio → short diffusion distance → diffusion is fast enough.
Key words: surface area · distance
3(h) TRANSPORT · RECALL
3.1 Diffusion in unicellular organisms
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Which ratio falls as an organism gets bigger?
Reveal answer
The surface area to volume ratio.
Key words: surface area to volume
What is meant by the term unicellular?
Reveal answer
Consisting of one cell.
Key words: unicellular
What is meant by the term exchange?
Reveal answer
Movement of materials between an organism and its environment.
Key words: exchange
Size and exchange: for Surface area : volume, give the small unicellular organism and the larger body.
Reveal answer
Small unicellular organism: High. Larger body: Lower.
Key words: Surface area : volume
Size and exchange: for Internal distance, give the small unicellular organism and the larger body.
Reveal answer
Small unicellular organism: Very short. Larger body: Longer.
Key words: Internal distance
Size and exchange: for Diffusion sufficiency, give the small unicellular organism and the larger body.
Reveal answer
Small unicellular organism: Often sufficient. Larger body: Too slow for all cells.
Key words: Diffusion sufficiency
Size and exchange: for Transport system, give the small unicellular organism and the larger body.
Reveal answer
Small unicellular organism: Not required. Larger body: Usually required.
Key words: Transport system
True or false, and why? “Small cells need fewer substances because they are inactive.”
Reveal answer
False. They may be highly active; geometry makes exchange efficient relative to demand.
True or false, and why? “Diffusion stops once concentrations become equal.”
Reveal answer
False. Random movement continues but net movement becomes zero.
Name a unicellular organism that relies on diffusion alone.
Reveal answer
Amoeba (or another single-celled protoctist or a bacterium).
Key words: amoeba
A cube-shaped cell has sides of 2 µm. Calculate its surface area to volume ratio.
Reveal answer
Surface area 6 × 4 = 24 µm², volume 8 µm³ → 3 : 1.
Key words: 3
How does oxygen reach the inside of an amoeba?
Reveal answer
Diffuses across the cell membrane from a higher concentration in the water to a lower concentration in the cytoplasm.
Key words: cell membrane
Which waste gas does a single-celled organism lose by diffusion across its membrane?
Reveal answer
Carbon dioxide.
Key words: carbon dioxide
Suggest why a single-celled organism cannot grow very large.
Reveal answer
As it grows, SA:V falls → diffusion becomes too slow to supply the centre of the cell.
Key words: centre