Difficulty: Intermediate | Prerequisites: Basic cell biology, cellular respiration (ATP synthase, electron transport chain, chemiosmosis).
Photosynthesis sits at the heart of energy flow in biology. If cellular respiration was how cells cash in on glucose, photosynthesis is how that glucose gets built in the first place, using light energy to fix carbon dioxide into sugar. This worksheet (DSW 10) from BIO 311C at UT Austin covers the full process: light reactions, the Calvin Cycle, and the three carbon-fixation strategies plants use (C3, C4, CAM). You should already be comfortable with how the electron transport chain and ATP synthase work in mitochondria, because the chloroplast runs a strikingly parallel system.
Plants use light energy to split water, generating ATP and NADPH in the thylakoid membrane (light reactions), then feed those into the Calvin Cycle in the stroma to fix CO₂ into sugar. All plants run the Calvin Cycle, but C4 and CAM plants add an extra carbon-fixation step using PEP carboxylase to concentrate CO₂ and avoid photorespiration. C4 plants separate the two fixation steps by cell type (space); CAM plants separate them by time of day (time).
Photosynthesis
The process by which plants, algae, and some bacteria convert light energy, water, and CO₂ into glucose and O₂. Think of it as the reverse of cellular respiration: instead of breaking fuel down to release energy, it builds fuel using energy from sunlight.
Light reactions (light-dependent reactions)
The stage of photosynthesis that occurs in the thylakoid membranes, where light energy splits water, generates ATP and NADPH, and releases O₂. In simple terms, this is where sunlight gets converted into chemical energy carriers.
Calvin Cycle (light-independent reactions / carbon fixation cycle)
The stage in the stroma where CO₂ is fixed into a three-carbon sugar (G3P) using ATP and NADPH from the light reactions. In simple terms, this is where the plant actually builds sugar from carbon dioxide.
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase)
The enzyme in C3 plants that catalyses the first step of carbon fixation, attaching CO₂ to RuBP (a 5-carbon molecule) to produce two 3-carbon compounds (3-PGA). It is the most abundant protein on Earth, but it has a flaw: it also binds O₂.
PEP carboxylase
The enzyme used by C4 and CAM plants for initial carbon fixation. It fixes CO₂ onto phosphoenolpyruvate (PEP) to form oxaloacetate, a 4-carbon compound. It has a much higher affinity for CO₂ than RuBisCO does, which is why it works well even at low CO₂ concentrations.
C3 plants
Plants that use only RuBisCO for carbon fixation. The first stable product is a 3-carbon compound (3-PGA), which is where the name comes from. Examples: beans, wheat, rice.
C4 plants
Plants that use PEP carboxylase for an initial fixation step, producing a 4-carbon compound (oxaloacetate), then shuttle the CO₂ to bundle sheath cells for the Calvin Cycle. This separates fixation in space. Examples: maize, sugarcane.
CAM plants (Crassulacean Acid Metabolism)
Plants that fix CO₂ at night using PEP carboxylase (stomata open), then run the Calvin Cycle during the day (stomata closed). This separates fixation in time. Examples: pineapple, cacti, succulents.
Photorespiration
A wasteful process that occurs when RuBisCO binds O₂ instead of CO₂, producing a 2-carbon compound that must be recycled at an energy cost. It does not produce useful ATP or sugar for the plant.
Chemiosmosis
The process by which a proton (H⁺) gradient across a membrane drives ATP synthase to produce ATP. It works the same way in both mitochondria (inner membrane) and chloroplasts (thylakoid membrane).
Thylakoid space (thylakoid lumen)
The interior compartment of the thylakoid where protons accumulate during the light reactions, creating the gradient that drives ATP synthase. Analogous to the intermembrane space of mitochondria.
Km (Michaelis constant)
The substrate concentration at which an enzyme operates at half its maximum rate (Vmax). A lower Km means higher affinity for the substrate. In simple terms, a low Km enzyme grabs its substrate more tightly.
Vmax (maximum reaction rate)
The fastest rate an enzyme can catalyse a reaction when fully saturated with substrate. A higher Vmax means the enzyme can process more substrate per unit time when there is plenty available.
Main inputs (reactants) of photosynthesis
CO₂: provides the carbon that gets fixed into sugar. Used in the Calvin Cycle.
H₂O: provides the electrons for the electron transport chain in the light reactions. Water is split (photolysis), releasing O₂ as a by-product.
Light energy: excites electrons in the reaction centre pigments (chlorophyll), driving them through the electron transport chain.
Why light is required
Light energy is needed to excite electrons in the reaction centre of photosystems I and II. These excited electrons travel through the electron transport chain, and the energy they release is used to pump protons (H⁺) into the thylakoid space, building a concentration gradient. This is directly analogous to how cellular respiration uses chemical energy to pump protons into the intermembrane space of mitochondria. In both cases, the proton gradient drives ATP synthase via chemiosmosis.
The parallel between chloroplasts and mitochondria
The logic is the same in both organelles: energy input concentrates protons on one side of a membrane, and ATP synthase harvests that gradient to make ATP. The difference is the energy source: light in chloroplasts, chemical bond energy (from NADH/FADH₂) in mitochondria.
Outputs of the light reactions
ATP and NADPH: both are used in the Calvin Cycle to fix CO₂ into sugar.
O₂: released as a by-product of water splitting.
The Calvin Cycle
All plants use the Calvin Cycle. It takes place in the stroma of the chloroplast and uses ATP and NADPH from the light reactions to reduce CO₂ into G3P (glyceraldehyde-3-phosphate), which can then be used to build glucose and other organic molecules.
C3 pathway
Uses RuBisCO as the sole carbon-fixing enzyme.
CO₂ is fixed directly onto RuBP (a 5-carbon molecule), producing two molecules of 3-PGA (a 3-carbon compound, hence "C3").
Works well in cool, moist conditions where stomata can stay open without excessive water loss.
Vulnerable to photorespiration when O₂ levels are high relative to CO₂ (e.g. hot, dry days when stomata close).
C4 pathway
Uses PEP carboxylase for the initial fixation step in mesophyll cells. CO₂ is fixed onto PEP, forming oxaloacetate (a 4-carbon compound, hence "C4").
The 4-carbon compound is then shuttled to bundle sheath cells, where it releases CO₂ for the Calvin Cycle (run by RuBisCO in these cells).
This spatial separation concentrates CO₂ around RuBisCO, effectively eliminating photorespiration.
Examples: maize, sugarcane, sorghum.
CAM pathway
Also uses PEP carboxylase for initial fixation, but separates the two steps in time rather than space.
At night: stomata open, CO₂ is fixed into organic acids (stored in vacuoles).
During the day: stomata close (conserving water), the stored acids release CO₂ for the Calvin Cycle.
This is an adaptation to extremely dry environments. Examples: pineapple, cacti, jade plants.
The key distinction
"C4 plants separate C3 and C4 processes in space; CAM plants separate them in time." C4 plants run PEP carboxylase in one cell type (mesophyll) and the Calvin Cycle in another (bundle sheath). CAM plants run PEP carboxylase at night and the Calvin Cycle during the day, all in the same cell.
Which plants use which cycle?
All plants use the Calvin Cycle to actually reduce CO₂ into sugar.
C4 and CAM plants add the extra PEP carboxylase step as a CO₂-concentrating mechanism.
C3 plants skip that extra step and fix CO₂ directly with RuBisCO.
Why RuBisCO binds O₂ (an evolutionary accident)
When RuBisCO first evolved, Earth's atmosphere had almost no oxygen. There was no selection pressure to make the enzyme distinguish between CO₂ and O₂. The O₂ in our atmosphere was produced by those very early photosynthetic organisms. By the time O₂ accumulated, RuBisCO's dual affinity was already entrenched. C4 and CAM pathways evolved later as workarounds.
Photorespiration
When RuBisCO binds O₂ instead of CO₂, it produces a 2-carbon compound that must be salvaged through an energy-costly recycling pathway. This process consumes ATP and releases CO₂ without producing useful sugar. It increases in hot, dry conditions when stomata close (CO₂ drops inside the leaf, O₂ concentration rises relative to CO₂).
Enzyme kinetics: PEP carboxylase vs. RuBisCO
The DSW 10 graph compares photosynthetic rate vs. CO₂ concentration for a C4 plant (maize) and a C3 plant (bean).
Km (affinity): PEP carboxylase (orange curve, maize) has a lower Km than RuBisCO (blue curve, bean). A lower Km means the enzyme reaches half its maximum rate at a lower substrate concentration, so PEP carboxylase has a higher affinity for CO₂. It works efficiently even when CO₂ is scarce.
Vmax (maximum rate): RuBisCO (blue curve, bean) has a higher Vmax. This means C3 plants can fix carbon faster than C4 plants when CO₂ is abundant, because the enzyme can process more substrate at saturation.
What this means for rising CO₂ levels
C3 plants stand to gain the most from higher atmospheric CO₂, because RuBisCO's higher Vmax means it can ramp up fixation rate as CO₂ increases. C4 plants are already running near their maximum efficiency at current CO₂ levels.
What this means for rising temperatures
Higher temperatures increase photorespiration in C3 plants (RuBisCO binds O₂ more readily in heat). C4 and CAM plants, which concentrate CO₂ around RuBisCO using PEP carboxylase, are largely protected from this. CAM plants, which fix CO₂ at night when temperatures are coolest, are likely the least affected by warming.
Plant cells contain both chloroplasts and mitochondria, and the outputs of one process feed directly into the other.
Photosynthesis produces glucose and O₂. Both are inputs for cellular respiration.
Cellular respiration produces CO₂ and H₂O. Both are inputs for photosynthesis.
This creates a cycle: light energy is captured and stored as chemical energy in glucose (photosynthesis), then that chemical energy is released to power the cell's work (respiration). The two processes are complementary, not opposites. Photosynthesis is an energy-storing, carbon-fixing process; respiration is an energy-releasing, carbon-oxidising process.
Overall equation for photosynthesis:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Overall equation for cellular respiration (for comparison):
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (+ heat)
Notice these are essentially the reverse of each other.
Enzyme kinetics (from the DSW 10 graph):
Km = the [CO₂] at which the enzyme reaches half its Vmax. Lower Km = higher affinity.
PEP carboxylase: lower Km (higher CO₂ affinity), lower Vmax.
RuBisCO: higher Km (lower CO₂ affinity), higher Vmax.
The C3/C4/CAM distinction matters for agriculture and climate adaptation. Crop scientists breed and select C4 crops (maize, sorghum, sugarcane) for hot climates because they resist photorespiration. There is active research into engineering C4-like pathways into C3 crops such as rice to improve yields in warming tropical regions.
CAM physiology is why succulents and cacti thrive in deserts: their night-time CO₂ fixation lets them keep stomata closed all day, losing almost no water.
Students often think C4 and CAM plants skip the Calvin Cycle. They do not. All plants run the Calvin Cycle. C4 and CAM plants simply add an extra fixation step before it.
Students confuse "light-independent" with "happens in the dark." The Calvin Cycle does not require light directly, but it depends on ATP and NADPH from the light reactions, so it runs mainly during the day.
Students sometimes think photorespiration is a form of cellular respiration. It is not. Photorespiration is a wasteful side reaction of RuBisCO, not an energy-producing pathway.
Students mix up which enzyme has the lower Km. PEP carboxylase has the lower Km (higher affinity for CO₂). RuBisCO has the higher Vmax (higher maximum rate). These are two different properties.
Know the inputs and outputs of both the light reactions and the Calvin Cycle, and where each occurs (thylakoid membrane vs. stroma).
Be able to explain why light is necessary (proton gradient for chemiosmosis), not just state it.
Understand the difference between C3, C4, and CAM: which enzyme, which cells, which timing.
The "space vs. time" distinction (C4 = spatial separation, CAM = temporal separation) is a classic exam question.
Be prepared to interpret a Km/Vmax graph comparing PEP carboxylase and RuBisCO. Know what lower Km and higher Vmax each mean.
The evolutionary explanation for RuBisCO's O₂ binding (no selection pressure against it on early Earth) is likely testable.
The relationship between photosynthesis outputs and respiration inputs (and vice versa) is a standard integrative question.
True or False: Only C3 plants use the Calvin Cycle. (False. All plants use the Calvin Cycle.)
Fill in the blank: Water acts as the source of _______ for the electron transport chain in the light reactions. (electrons)
True or False: PEP carboxylase has a higher Vmax than RuBisCO. (False. RuBisCO has the higher Vmax.)
Fill in the blank: CAM plants fix CO₂ at _______ and run the Calvin Cycle during the _______. (night; day)
True or False: Photorespiration produces useful ATP for the plant. (False. It is a wasteful process.)
Q: What are the main reactants of photosynthesis, and where is each used?
A: CO₂ is used in the Calvin Cycle (carbon fixation). H₂O is used in the light reactions as the electron source for the electron transport chain. Light energy excites electrons in the reaction centre to drive the ETC.
Q: Explain why RuBisCO also binds O₂, even though this causes photorespiration.
A: RuBisCO evolved when Earth's atmosphere had little to no O₂, so there was no evolutionary pressure to develop high specificity for CO₂ over O₂. The atmospheric O₂ was itself produced by early photosynthetic organisms. By the time O₂ accumulated, the enzyme's dual affinity was already fixed.
Q: What does it mean that "C4 plants separate C3 and C4 processes in space"?
A: C4 plants run PEP carboxylase (the C4 fixation step) in mesophyll cells and the Calvin Cycle (the C3 step) in bundle sheath cells. The two processes occur in different cell types within the same leaf.
Q: On a graph of photosynthetic rate vs. [CO₂], which curve (C3 or C4) has the lower Km, and what does this mean?
A: The C4 curve (PEP carboxylase) has the lower Km. This means PEP carboxylase reaches half its maximum rate at a lower CO₂ concentration, so it has a higher affinity for CO₂ than RuBisCO does.
Q: Which type of plant is likely to benefit least from rising global temperatures, and why?
A: C3 plants. Higher temperatures increase the rate of photorespiration because RuBisCO binds O₂ more readily in heat. C4 and CAM plants, which use PEP carboxylase to concentrate CO₂, avoid this problem. CAM plants may be least affected because they fix CO₂ at night when temperatures are lowest.
Q: Name two outputs of photosynthesis that serve as inputs for cellular respiration, and two outputs of respiration that serve as inputs for photosynthesis.
A: Photosynthesis outputs glucose and O₂, both used in respiration. Respiration outputs CO₂ and H₂O, both used in photosynthesis.
This material connects directly to cellular respiration (the proton gradient, ATP synthase, and chemiosmosis work the same way in both organelles, so understanding one reinforces the other). It also ties into enzyme kinetics (Km, Vmax) from earlier in the course, applied here to RuBisCO and PEP carboxylase. The evolutionary discussion around RuBisCO's O₂ affinity links to natural selection and adaptation, showing how environmental changes (rising O₂) can turn a neutral trait into a liability.
photosynthesis, light reactions, light-dependent reactions, Calvin Cycle, light-independent reactions, dark reactions, carbon fixation, RuBisCO, ribulose bisphosphate carboxylase oxygenase, PEP carboxylase, C3 plants, C4 plants, CAM plants, crassulacean acid metabolism, photorespiration, chemiosmosis, ATP synthase, thylakoid, stroma, chloroplast, electron transport chain, photosystem I, photosystem II, NADPH, oxaloacetate, phosphoenolpyruvate, bundle sheath cells, mesophyll cells, Km, Vmax, enzyme kinetics, Michaelis-Menten, BIO 311C, Sathasivan, UT Austin introductory biology