Premed · Premed · General Biology 1
Lecture 13: Photosynthesis — Calvin Cycle
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the three phases of the Calvin cycle and the role of RuBisCO
- Explain the inputs and outputs of the Calvin cycle per CO2 fixed
- Calculate the total ATP and NADPH cost to produce one molecule of glucose
- Describe photorespiration and explain why it reduces photosynthetic efficiency
- Compare C3, C4, and CAM photosynthesis as adaptations to minimize photorespiration
Lecture Content
I. Overview of the Calvin Cycle
The Calvin cycle--also known as the light-independent reactions or carbon fixation--takes place in the stroma of the chloroplast and uses the ATP and NADPH produced by the light reactions to convert atmospheric CO2 into organic molecules. The pathway was elucidated by Melvin Calvin using radioactive carbon-14 as a tracer, work that earned him the Nobel Prize in 1961. Despite its designation as "light-independent," the Calvin cycle is not truly independent of light: it depends entirely on ATP and NADPH, which are produced only when photosystems are actively absorbing light. The cycle proceeds through three phases: carbon fixation, reduction, and regeneration of ribulose-1,5-bisphosphate (RuBP).
II. The Three Phases of the Calvin Cycle
Phase 1: Carbon Fixation
The cycle begins when the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the attachment of CO2 to the five-carbon sugar RuBP. The resulting unstable six-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. RuBisCO is the most abundant protein on Earth--plants produce enormous quantities of it to compensate for the fact that it is also one of the slowest enzymes known, catalyzing only about 3 to 10 reactions per second.
Phase 2: Reduction
In this energy-consuming phase, 3-PGA is phosphorylated by ATP and then reduced by NADPH to produce glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that is the direct product of the Calvin cycle. For every three molecules of CO2 fixed, six molecules of G3P are produced, but only one G3P represents a net gain. The other five must be recycled to regenerate the CO2 acceptor, RuBP.
Phase 3: Regeneration of RuBP
The five remaining G3P molecules (containing a total of 15 carbons) are rearranged through a complex series of reactions involving enzymes such as transketolase, aldolase, and phosphoribulokinase to regenerate three molecules of RuBP (also totaling 15 carbons). This phase consumes an additional 3 ATP for the phosphorylation step that converts ribulose-5-phosphate to RuBP, completing the cycle and preparing it for another round of carbon fixation.
III. Stoichiometry of the Calvin Cycle
To fix three molecules of CO2 and produce one net molecule of G3P, the cycle consumes 9 ATP and 6 NADPH. Since two G3P molecules are needed to build one glucose molecule, producing a single glucose requires six turns of the cycle, consuming a total of 18 ATP and 12 NADPH and fixing 6 CO2.
G3P is a remarkably versatile intermediate. Two molecules can be combined to form glucose, fructose, or sucrose. G3P can also be channeled into the synthesis of amino acids, fatty acids, and starch, making it the central hub from which a plant constructs virtually all of its organic molecules.
<image>A circular diagram of the Calvin cycle divided into three colored phases. Phase 1 (Carbon Fixation, blue): 3 CO2 molecules enter, each combining with RuBP (5C) via RuBisCO to produce 6 molecules of 3-PGA (3C). Phase 2 (Reduction, green): 6 ATP and 6 NADPH are consumed to reduce 6 3-PGA into 6 G3P (3C). One G3P exits the cycle as the net product (highlighted with an arrow pointing to "glucose and other organic molecules"). Phase 3 (Regeneration of RuBP, orange): The remaining 5 G3P molecules are rearranged, consuming 3 ATP, to regenerate 3 RuBP molecules. Input/output boxes around the cycle show: 3 CO2 in, 9 ATP consumed, 6 NADPH consumed, 1 G3P net output.</image>
IV. Photorespiration
RuBisCO harbors a significant evolutionary liability: in addition to its carboxylase activity (fixing CO2), it also possesses oxygenase activity, meaning it can bind O2 instead of CO2. When RuBisCO fixes oxygen, it produces one molecule of 3-PGA and one molecule of 2-phosphoglycolate, a two-carbon compound that is toxic and metabolically useless. The cell must salvage 2-phosphoglycolate through the photorespiratory pathway, a wasteful detour that involves the chloroplast, peroxisome, and mitochondrion, consuming ATP and releasing previously fixed CO2.
Photorespiration produces no ATP and no sugar, and it can reduce photosynthetic efficiency by 25-50% in C3 plants. It is favored by hot, dry conditions: when temperatures rise, stomata close to conserve water, trapping O2 inside the leaf while CO2 is consumed. As the O2-to-CO2 ratio climbs, RuBisCO increasingly fixes oxygen rather than carbon dioxide. This design flaw is a legacy of evolution--RuBisCO evolved when atmospheric oxygen levels were far lower than today, and its oxygenase activity was not a significant problem.
V. C3 Photosynthesis
The standard Calvin cycle pathway described above defines C3 photosynthesis, so named because the first stable product of carbon fixation is the three-carbon compound 3-PGA. The majority of plant species--including rice, wheat, soybeans, and most trees--are C3 plants. While perfectly effective in cool, moist climates, C3 plants are vulnerable to the inefficiency of photorespiration in hot, arid environments.
VI. C4 Photosynthesis
C4 photosynthesis is an adaptation that minimizes photorespiration through spatial separation of initial carbon fixation and the Calvin cycle. C4 plants display a distinctive Kranz anatomy, in which two types of photosynthetic cells are arranged concentrically around the leaf veins. In the outer mesophyll cells, the enzyme PEP carboxylase fixes CO2 into the four-carbon compound oxaloacetate (hence "C4"). PEP carboxylase has no oxygenase activity and a much higher affinity for CO2 than RuBisCO, making it an efficient initial carbon trap. Oxaloacetate is converted to malate, which is transported to the inner bundle-sheath cells. There, malate is decarboxylated, releasing CO2 at a high concentration directly around RuBisCO, which can then fix carbon through the Calvin cycle with minimal photorespiration. The three-carbon pyruvate left behind returns to the mesophyll cells and is regenerated to PEP at a cost of 2 additional ATP per CO2.
C4 plants--including corn, sugarcane, sorghum, and crabgrass--are more efficient than C3 plants under hot, sunny conditions. The extra ATP cost (2 ATP per CO2 beyond the Calvin cycle requirement) is a worthwhile investment when photorespiration would otherwise squander a significant fraction of fixed carbon.
VII. CAM Photosynthesis
Crassulacean Acid Metabolism (CAM), named for the succulent family Crassulaceae in which it was first described, represents an adaptation to extreme aridity that uses temporal separation rather than spatial separation. CAM plants open their stomata at night, when temperatures are cooler and humidity is higher, minimizing water loss. During the night, PEP carboxylase fixes CO2 into organic acids (primarily malate), which are stored in large vacuoles. During the day, stomata close to conserve water. The stored malate is released from the vacuoles and decarboxylated, releasing CO2 that feeds the Calvin cycle in the same cell, powered by ATP and NADPH from the light reactions operating simultaneously.
CAM plants--including cacti, pineapple, agave, jade plants, and many epiphytic orchids--sacrifice photosynthetic speed for water conservation. Both C4 and CAM plants use PEP carboxylase for initial carbon fixation, but the key distinction is the mode of separation: C4 plants separate the two fixation steps spatially between two cell types, while CAM plants separate them temporally between night and day within the same cell.
<image>A three-panel comparison figure. Panel A (C3): A single mesophyll cell where CO2 enters the stoma and is directly fixed by RuBisCO into 3-PGA; the Calvin cycle occurs in the same cell. Panel B (C4): Two cell types — CO2 is fixed by PEP carboxylase in the mesophyll cell into a 4C acid (malate), which is transported to the bundle-sheath cell where it releases CO2 for the Calvin cycle (RuBisCO); pyruvate returns to the mesophyll cell. Kranz anatomy is illustrated with concentric cell layers around the vein. Panel C (CAM): A single cell shown at two time points — at night (stomata open, CO2 fixed by PEP carboxylase into malate, stored in vacuole) and during the day (stomata closed, malate released from vacuole, CO2 feeds the Calvin cycle). A clock icon indicates temporal separation.</image>
VIII. Integration: Photosynthesis and Cellular Respiration
Photosynthesis and cellular respiration are complementary processes that together sustain the flow of energy and carbon through the biosphere. Photosynthesis captures light energy and stores it in glucose (an endergonic process), while respiration breaks down glucose to release that energy as ATP (an exergonic process). Both rely on the principle of chemiosmosis--proton gradients driving ATP synthesis through ATP synthase--underscoring a deep mechanistic unity in bioenergetics. Both employ electron transport chains with mobile carriers. Plants carry out both processes: photosynthesis in chloroplasts during the day and respiration in mitochondria continuously. For a plant to grow, its net rate of photosynthesis must exceed its rate of respiration, ensuring a surplus of fixed carbon that can be invested in new biomass.
