The diagram shows carbon transfers in an indoor fish-rearing tank. A student adds a measured mass of dried algae each day. Tilapia fish eat some algae, whil...
Question 1 Report
The diagram shows carbon transfers in an indoor fish-rearing tank. A student adds a measured mass of dried algae each day. Tilapia fish eat some algae, while bacteria in the filter act on fish waste and dead cells. Air is bubbled through the water so that carbon dioxide can move between the water and the atmosphere. The student is considering whether growing more living plants in the tank would reduce carbon dioxide in the water.
(a) Name the process by which algae transfer carbon dioxide into their cells. [2]
(b) Describe how carbon in the algae can reach the filter bacteria. [3]
(c) Explain why a large increase in fish mass could cause carbon dioxide concentration in the water to rise at night, even if the same mass of algae is added. [4]
Answer Details
(a) The process is photosynthesis [1]. In photosynthesis, algae use carbon dioxide to make carbohydrate, which becomes algal biomass [1]. [2]
(b) The carbon transfer follows the food and waste pathway: tilapia fish eat algae [1]; the fish produce waste or die [1]; filter bacteria feed on or decompose this waste and the dead cells [1]. Carbon originally in algae has therefore reached the bacteria. [3]
(c) A larger fish mass means more fish cells respire [1]. Respiration releases carbon dioxide [1]. At night, algae and plants do not photosynthesise in darkness [1], so they do not remove carbon dioxide by photosynthesis. Consequently, less carbon dioxide is removed than released, so its concentration in the water rises [1]. [4]
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