This cheat sheet compares the three major photosynthetic pathways plants use to capture carbon dioxide: C3, C4, and CAM photosynthesis. Students need this reference because these pathways explain how plants survive in different climates and manage tradeoffs between growth, water loss, and photorespiration. It is especially useful for biology units on plant physiology, ecosystems, climate adaptation, and evolution.
Key Facts
- C3 photosynthesis fixes CO2 directly with Rubisco in the Calvin cycle, forming a 3-carbon molecule called 3-PGA.
- C4 photosynthesis first fixes CO2 with PEP carboxylase into a 4-carbon molecule, then delivers CO2 to bundle sheath cells for the Calvin cycle.
- CAM photosynthesis opens stomata at night, stores CO2 as organic acids, and releases CO2 during the day for the Calvin cycle.
- Rubisco can bind CO2 or O2, and binding O2 causes photorespiration, which wastes energy and reduces sugar production.
- PEP carboxylase has a high affinity for CO2 and does not bind O2, so C4 and CAM plants reduce photorespiration.
- C3 plants are usually most efficient in cool, moist conditions because photorespiration is lower and stomata can stay open.
- C4 plants are adapted to hot, sunny environments because they separate initial carbon fixation and the Calvin cycle in different cell types.
- CAM plants are adapted to dry environments because they separate initial carbon fixation and the Calvin cycle by time, opening stomata mostly at night.
Vocabulary
- C3 photosynthesis
- A photosynthetic pathway in which CO2 is fixed directly by Rubisco into a 3-carbon compound during the Calvin cycle.
- C4 photosynthesis
- A photosynthetic pathway that first fixes CO2 into a 4-carbon compound before sending concentrated CO2 to the Calvin cycle.
- CAM photosynthesis
- A photosynthetic pathway in which plants take in CO2 at night and use the stored carbon during the day.
- Rubisco
- The enzyme that adds CO2 to RuBP in the Calvin cycle but can also bind O2 and cause photorespiration.
- Photorespiration
- A wasteful process that occurs when Rubisco binds O2 instead of CO2, reducing the efficiency of photosynthesis.
- Stomata
- Tiny pores on leaves that open and close to control gas exchange and water loss.
Common Mistakes to Avoid
- Saying C4 and CAM plants do not use the Calvin cycle is wrong because all three pathways use the Calvin cycle to make sugars.
- Confusing C4 with CAM is wrong because C4 plants separate carbon fixation by location, while CAM plants separate it by time.
- Assuming C3 plants are always less efficient is wrong because C3 plants can be very efficient in cool, moist conditions.
- Thinking stomata only control CO2 intake is wrong because stomata also affect water loss through transpiration.
- Saying photorespiration helps plants make glucose is wrong because photorespiration uses energy and releases CO2 instead of building sugar.
Practice Questions
- 1 A plant fixes CO2 into a 3-carbon compound using Rubisco as the first step. Which photosynthetic pathway is it using?
- 2 A desert plant opens its stomata mostly at night and stores CO2 as organic acids. Which pathway does it use, and why does this save water?
- 3 A corn plant uses PEP carboxylase to form a 4-carbon compound before releasing CO2 near Rubisco. Which pathway is this, and how does it reduce photorespiration?
- 4 Explain why hot, dry conditions increase photorespiration in C3 plants but are less harmful to C4 and CAM plants.