Identify inputs, products and energy transfers of the main cellular pathways.
Method and worked examples
Most autotrophs use light energy to make organic compounds from carbon dioxide and water. Photosynthesis stores energy in chemical form and releases oxygen. Heterotrophs obtain organic materials from other organisms. Both groups require respiration: cells transfer some chemical energy from fuels to ATP for cellular work. Aerobic respiration uses oxygen and produces carbon dioxide and water.
Fermentation regenerates molecules needed for glycolysis when oxygen is unavailable or insufficient for aerobic demand. Yeast can form ethanol and carbon dioxide; animal muscle cells can form lactate. Fermentation yields much less ATP per glucose than aerobic respiration. Plants respire in light and darkness, so measured leaf gas exchange in light is the net effect of photosynthesis and respiration.
Example 1
A leaf uses 9 units of CO2 in photosynthesis and releases 3 through respiration over the same period. Find net uptake.
Net uptake =9−3=6 units of CO2.
Example 2
Yeast supplied glucose without oxygen releases gas that turns limewater cloudy. Identify the gas and pathway.
The gas is carbon dioxide, produced with ethanol in alcoholic fermentation.
Watch out:Plant cells respire in light and darkness; photosynthesis requires suitable light and can mask respiratory gas exchange.
Photosynthesis, respiration and fermentation | Year 11 Biology | Ascent Arcade