Photosynthesis
Energy from Light
Explore how plants convert light energy into chemical energy, powering virtually all life on Earth through the elegant chemistry of \( 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \).
AP Biology
What Is Photosynthesis?
Photosynthesis is the process by which autotrophs (primarily plants, algae, and cyanobacteria) capture light energy and convert it into chemical energy stored in glucose. This process is the foundation of nearly every food web on Earth.
Key Molecules in Photosynthesis
Chlorophyll
Primary pigment that absorbs red and blue light, reflects green. Found in thylakoid membranes.
\(\text{ATP}\)
Adenosine triphosphate: the energy currency produced during the light reactions and consumed in the Calvin cycle.
\(\text{NADPH}\)
An electron carrier that stores high-energy electrons from the light reactions and delivers them to the Calvin cycle.
Inside the Chloroplast
Two Stages: Light and Dark Reactions
The Light Reactions in Detail
1
Photosystem II (PSII) absorbs light at 680 nm. Water is split (photolysis): \( 2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2 \). Oxygen is released.
2
Electron transport chain (ETC) passes electrons through plastoquinone, cytochrome \(b_6f\) complex, and plastocyanin. Protons are pumped into the thylakoid lumen.
3
Photosystem I (PSI) re-energizes electrons at 700 nm. Ferredoxin passes them to \(\text{NADP}^+\) reductase, producing \(\text{NADPH}\).
4
Chemiosmosis: The proton gradient drives ATP synthase, converting \(\text{ADP} + \text{P}_i\) into \(\text{ATP}\) (photophosphorylation).
Thylakoid membrane couples electron flow to proton gradient
Water supplies electrons to the light reactions. Electron transport builds a proton gradient, and proton return from the thylakoid lumen to the stroma through ATP synthase powers ATP production.Thylakoid: gradient to ATP
Water supplies electrons to the light reactions. Electron transport builds a proton gradient, and proton return from the thylakoid lumen to the stroma through ATP synthase powers ATP production.The Calvin Cycle (Light-Independent)
The Overall Equation Decoded
\( 6\text{CO}_2 \)
Carbon dioxide from the atmosphere enters through stomata and is fixed by RuBisCO in the Calvin cycle.
\( 6\text{H}_2\text{O} \)
Water is split during photolysis in PSII. Its electrons replace those lost by chlorophyll.
\( \text{C}_6\text{H}_{12}\text{O}_6 \)
Glucose: the primary energy-storing product. Built from two G3P molecules after 6 Calvin cycle turns.
\( 6\text{O}_2 \)
Oxygen is a byproduct of water splitting. It exits through stomata: the oxygen we breathe.
Photosynthesis vs. Cellular Respiration
| Photosynthesis | Cellular Respiration | |
|---|---|---|
| Location | Chloroplasts | Mitochondria |
| Reactants | \(\text{CO}_2 + \text{H}_2\text{O}\) | \(\text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2\) |
| Products | \(\text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2\) | \(\text{CO}_2 + \text{H}_2\text{O}\) |
| Energy | Light energy stored | Chemical energy released |
| Organisms | Autotrophs only | All living cells |
| When? | Requires light | Occurs continuously |
Light Absorption and Pigments
Factors Affecting Rate
Light Intensity
Higher light increases the rate until the light saturation point: all chlorophyll molecules are excited and the rate plateaus.
\( \text{CO}_2 \) Concentration
More \( \text{CO}_2 \) accelerates carbon fixation by RuBisCO until enzyme saturation. After that, other factors become limiting.
Temperature
Enzymes (especially RuBisCO) have optimal temperatures (\( \sim 25-35^\circ\text{C} \)). Too hot denatures them; too cold slows kinetics.
Photorespiration and Adaptations
When \( \text{O}_2 \) concentration is high relative to \( \text{CO}_2 \), RuBisCO binds \( \text{O}_2 \) instead of \( \text{CO}_2 \), producing a toxic 2-carbon compound. This wasteful process is called photorespiration: it consumes ATP without producing sugar.
C4 Plants
Corn, sugarcane. Use PEP carboxylase to pre-fix \( \text{CO}_2 \) in mesophyll cells, then shuttle it to bundle sheath cells where RuBisCO operates at high \( \text{CO}_2 \).
CAM Plants
Cacti, succulents. Open stomata at night to fix \( \text{CO}_2 \) into malate, then release it during the day for the Calvin cycle. Minimizes water loss.
C4 and CAM separate initial fixation from Calvin chemistry
C₄ plants separate initial carbon fixation and the Calvin cycle between cell types. CAM plants separate them between night and day. Both concentrate carbon dioxide while retaining Calvin-cycle chemistry.Three routes to carbon fixation
C₄ plants separate initial carbon fixation and the Calvin cycle between cell types. CAM plants separate them between night and day. Both concentrate carbon dioxide while retaining Calvin-cycle chemistry.How to Trace Atoms Through Photosynthesis
1
Identify the atom. Are you tracking carbon (C), oxygen (O), or hydrogen (H)?
2
Carbon: \( \text{CO}_2 \) enters the Calvin cycle, is fixed by RuBisCO into \(\text{3-PGA}\), reduced to \(\text{G3P}\), and built into \( \text{C}_6\text{H}_{12}\text{O}_6 \).
3
Oxygen from \( \text{H}_2\text{O} \): Water is split in PSII. The oxygen atoms become \( \text{O}_2 \) gas released to the atmosphere.
4
Hydrogen from \( \text{H}_2\text{O} \): The \( \text{H}^+ \) ions contribute to the proton gradient and are carried by NADPH to the Calvin cycle, ending up in glucose.
5
Oxygen from \( \text{CO}_2 \): These oxygens remain in the sugar backbone (\( \text{C}_6\text{H}_{12}\text{O}_6 \)), NOT in the \( \text{O}_2 \) released.
Example: Counting ATP and NADPH
Question: How many ATP and NADPH molecules are consumed to produce one molecule of glucose via the Calvin cycle?
Example: Isotope Tracing with \( ^{18}\text{O} \)
Question: If you supply a plant with water labeled \( \text{H}_2{^{18}\text{O}} \), where does the \( ^{18}\text{O} \) end up: in glucose or in \( \text{O}_2 \)?
1
Water is split in PSII: \( 2\text{H}_2{^{18}\text{O}} \rightarrow 4\text{H}^+ + 4e^- + {^{18}\text{O}}_2 \)
2
The oxygen released comes from water. Therefore, the \( ^{18}\text{O} \) label appears in the \( \text{O}_2 \) gas.
3
Conversely, if you label the \( \text{CO}_2 \) with \( ^{18}\text{O} \), those oxygens end up in glucose, not in \( \text{O}_2 \).
Common Mistakes
✕ "Dark reactions need dark"
The Calvin cycle is light-independent, but it still occurs during the day. It needs ATP and NADPH from the light reactions, which require light.
✕ "\( \text{O}_2 \) comes from \( \text{CO}_2 \)"
The oxygen gas released comes from the splitting of water, not from carbon dioxide. The oxygen atoms of \( \text{CO}_2 \) end up in glucose.
✕ "Plants don't do respiration"
Plants perform both photosynthesis AND cellular respiration. They photosynthesize during the day and respire continuously, 24/7.
Question 1 of 5
In the light reactions, which molecule is the primary electron donor that replaces electrons lost by Photosystem II?
Water (\( \text{H}_2\text{O} \)) is split by photolysis in PSII, providing electrons to replace those excited by light. This process also releases \( \text{O}_2 \) and \( \text{H}^+ \) ions.
Question 2 of 5
Where does the Calvin cycle take place within the chloroplast?
The Calvin cycle (light-independent reactions) occurs in the stroma, the fluid-filled space surrounding the thylakoids. The light reactions occur on the thylakoid membrane.
Question 3 of 5
A plant is supplied with water labeled \( \text{H}_2{^{18}\text{O}} \). Where will the \( ^{18}\text{O} \) isotope primarily appear?
The oxygen in \( \text{O}_2 \) released during photosynthesis comes from the splitting of water, not \( \text{CO}_2 \). Therefore, the \( ^{18}\text{O} \) from labeled water appears in the \( \text{O}_2 \) gas.
Question 4 of 5
C4 plants minimize photorespiration by:
C4 plants use PEP carboxylase (which has no affinity for \( \text{O}_2 \)) to initially fix \( \text{CO}_2 \) in mesophyll cells. The 4-carbon compound is then shuttled to bundle sheath cells, where \( \text{CO}_2 \) is released at high concentration for RuBisCO. Option A describes CAM plants.
Question 5 of 5
How many molecules of ATP and NADPH are consumed by the Calvin cycle to produce one molecule of glucose?
Each turn of the Calvin cycle uses 3 ATP and 2 NADPH. To fix 6 \( \text{CO}_2 \) (needed for 1 glucose), the cycle turns 6 times: \( 6 \times 3 = 18 \) ATP and \( 6 \times 2 = 12 \) NADPH. Option B (9 ATP, 6 NADPH) is the cost per 3 turns (one G3P net), not one full glucose.