Plants use photosynthesis to convert light energy into chemical energy, but not all plants capture carbon dioxide in the same way. C3, C4, and CAM photosynthesis are three pathways that help plants balance sugar production with water loss and heat stress. These differences matter because a plant's photosynthetic pathway affects where it grows best, how much water it needs, and how efficiently it can make food.
Comparing them helps explain why rice, corn, and cactus thrive in very different environments.
C3 plants use the simplest pathway, in which carbon dioxide enters the Calvin cycle directly in mesophyll cells. C4 plants first capture carbon dioxide into a 4-carbon compound, then deliver it to bundle sheath cells where the Calvin cycle runs with less photorespiration. CAM plants also make 4-carbon acids, but they separate steps by time, opening stomata at night and running the Calvin cycle during the day.
These adaptations reduce water loss and photorespiration in hot or dry conditions, but they require different amounts of energy.
Key Facts
- C3 photosynthesis: CO2 + RuBP forms a 3-carbon molecule called 3-PGA using the enzyme Rubisco.
- C4 photosynthesis: CO2 + PEP forms oxaloacetate using PEP carboxylase, then CO2 is released near Rubisco.
- CAM photosynthesis: stomata open at night to take in CO2, which is stored as organic acids for use during the day.
- Overall photosynthesis equation: 6 CO2 + 6 H2O + light energy -> C6H12O6 + 6 O2.
- Photorespiration increases when O2 competes with CO2 at Rubisco, especially in hot, dry conditions.
- C4 and CAM pathways reduce photorespiration but require extra ATP compared with the basic C3 pathway.
Vocabulary
- Calvin cycle
- The set of reactions that uses carbon dioxide, ATP, and NADPH to build sugar molecules in photosynthesis.
- Rubisco
- The enzyme that attaches carbon dioxide to RuBP during carbon fixation, but can also bind oxygen and cause photorespiration.
- Photorespiration
- A wasteful process in which Rubisco reacts with oxygen instead of carbon dioxide, reducing sugar production.
- PEP carboxylase
- An enzyme used in C4 and CAM plants to capture carbon dioxide efficiently and form 4-carbon compounds.
- Stomata
- Small pores on leaves that open and close to control gas exchange and water loss.
Common Mistakes to Avoid
- Saying C4 plants do not use the Calvin cycle is wrong because C4 plants still use the Calvin cycle, but they run it mainly in bundle sheath cells.
- Thinking CAM plants photosynthesize only at night is wrong because CAM plants take in CO2 at night but use light reactions and the Calvin cycle mainly during the day.
- Assuming C3 plants are always less efficient is wrong because C3 plants can be very efficient in cool, moist conditions where photorespiration is low.
- Confusing water conservation with higher sugar output is wrong because CAM plants save water well but often grow slowly due to limited CO2 intake.
Practice Questions
- 1 A C3 plant fixes 18 molecules of CO2 in the Calvin cycle. If 3 CO2 are needed to make one net G3P, how many net G3P molecules can be produced?
- 2 A CAM plant opens its stomata for 10 hours at night and keeps them closed for 14 hours during the day. What fraction of a 24-hour day are the stomata open, and what is this as a percent?
- 3 A farmer is choosing crops for a hot, dry field with strong sunlight and frequent drought. Explain whether a C3, C4, or CAM plant would likely be best and justify your choice using photorespiration and water loss.