IB DP Biology 2025: Interaction and Interdependence
C1.3 — Photosynthesis
Theme C: Interdependence · Standard & Higher Level
C1.3.1 – C1.3.3
Energy Transformation
Key UnderstandingPhotosynthesis is the conversion of light energy into chemical energy stored in carbon compounds.
Plants, algae, and certain groups of bacteria (like cyanobacteria) possess the remarkable ability to capture energy from sunlight and convert it into chemical energy. This energy transformation involves complex metabolic pathways that take place within specialized cellular structures.
This process is foundational to almost all life on Earth, as it provides the organic compounds needed by heterotrophs for cellular respiration and building biomass. The overall balanced chemical equation summarizes the net transformation of reactants into products:
6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂
The transformation occurs in fundamentally distinct but interconnected stages:
Light energy is absorbed by photosynthetic pigments (like chlorophyll)
↓
Water molecules are split (photolysis) to provide electrons, releasing O₂ as a byproduct
↓
Carbon dioxide is fixed and reduced to synthesize carbohydrates (sugars)
Photolysis
The splitting of a molecule using light energy. In photosynthesis, water is split to yield electrons, protons, and oxygen gas.
Carbon Fixation
The conversion of inorganic carbon (CO₂) into organic compounds by living organisms, typically driven by photosynthesis.
C1.3.4 & C1.3.5
Photosynthetic Pigments & Spectra
Key UnderstandingDifferent pigments absorb specific wavelengths of light, exciting their electrons to drive photosynthetic reactions.
Light from the sun appears white, but it is actually composed of a spectrum of different wavelengths (colors). Photosynthetic organisms contain specialized molecules called pigments that absorb some wavelengths and reflect others.
Chlorophyll a and b are the primary photosynthetic pigments. They strongly absorb in the blue (around 430-450 nm) and red (around 640-680 nm) regions of the visible spectrum. They reflect green light (around 500-550 nm), which is why most leaves appear green to our eyes.
To maximize light absorption, organisms also possess accessory pigments like carotenoids (which absorb blue/green light and appear yellow/orange) and xanthophylls. This broadens the total spectrum of light that can be utilized for photosynthesis.
Skill: Chromatography
Photosynthetic pigments can be extracted and separated using paper or thin-layer chromatography. A solvent moves up the paper, carrying different pigments at different speeds based on their solubility and size. The retention factor ($R_f$) is calculated as:
$$ R_f = \frac{\text{distance travelled by pigment}}{\text{distance travelled by solvent}} $$
Absorption Spectrum
A graph that displays the specific wavelengths of light absorbed by different individual pigments (e.g., chlorophyll a, chlorophyll b, carotenoids).
Action Spectrum
A graph that shows the overall physiological rate of photosynthesis of the whole organism across different wavelengths of light. It closely matches the combined absorption spectrum.
C1.3.7 & C1.3.8
Limiting Factors of Photosynthesis
Key UnderstandingThe rate of photosynthesis is determined by the specific factor that is nearest to its minimum value, known as the limiting factor.
Photosynthesis is a complex pathway involving many steps. According to Blackman's Law of Limiting Factors, if a chemical process is affected by more than one factor, its rate is limited by the pace of the slowest factor. The three main environmental limiting factors are:
- Light Intensity: Initially, as light intensity increases, the rate of photosynthesis increases linearly because more ATP and reduced NADP are produced. Eventually, the rate plateaus when all available pigments are saturated, or another factor becomes limiting.
- CO₂ Concentration: Carbon dioxide is a raw material for the Calvin cycle. Increasing its concentration increases the rate of carbon fixation up to a plateau, beyond which the enzymes processing the CO₂ are working at maximum capacity.
- Temperature: Photosynthesis relies on enzymes (especially Rubisco). As temperature increases, molecular collisions increase, raising the rate to an optimum point (often 25-35°C). Above this optimum, enzymes begin to denature, and the rate drops rapidly.
Application: FACE Experiments
Free-Air Concentration Enrichment (FACE) experiments are large-scale ecological studies. Researchers artificially elevate CO₂ levels in open-air natural environments without altering other factors like sunlight or rain. This allows scientists to study how different plant ecosystems will respond to future predicted atmospheric CO₂ changes without the artificial constraints of a greenhouse.
C1.3.9 HL Only
Chloroplast Structure & Function
The chloroplast is a highly compartmentalized organelle. Its complex internal structure is perfectly adapted to optimize the efficiency of the two main stages of photosynthesis: the light-dependent and light-independent reactions.
| Structure | Adaptive Function |
|---|
| Thylakoid Membrane |
Highly folded into stacks (grana) to provide a massive surface area for light absorption. It is densely embedded with photosystems, electron carriers, and ATP synthase needed for the light-dependent reactions. |
| Thylakoid Space (Lumen) |
Has a very small internal volume. This allows a steep proton (H⁺) gradient to be established rapidly after photolysis and proton pumping, which is essential for chemiosmosis and ATP generation. |
| Stroma |
A fluid-filled matrix surrounding the thylakoids. It contains a high concentration of the enzymes required for the Calvin cycle (like Rubisco), as well as optimal pH, naked DNA, and 70S ribosomes for synthesizing essential chloroplast proteins. |
| Double Membrane |
The outer and inner membranes isolate the working enzymes and substrates of the chloroplast from the rest of the cytoplasm, compartmentalizing the reactions. |
C1.3.10 – C1.3.14 HL Only
Light-Dependent Reactions
Key Understanding (HL) Light energy is utilized in the thylakoid membranes to generate ATP and reduced NADP, which carry energy and reducing power to the next stage.
The light-dependent reactions occur within and across the thylakoid membrane. They utilize light energy to create the biochemical energy carriers required for synthesizing sugar later.
1. Photoactivation: Photosystem II (PSII) and Photosystem I (PSI) absorb photons of light, exciting electrons in their chlorophyll reaction centers to a higher energy state.
↓
2. Photolysis of Water: To replace the electrons lost by PSII, an enzyme splits water molecules:
2H₂O → 4H⁺ + 4e⁻ + O₂. Oxygen is released as a waste gas.
↓
3. Electron Transport Chain (ETC): The excited electrons from PSII are passed down a chain of electron carriers in the membrane, losing energy. This energy is used to pump H⁺ ions from the stroma into the thylakoid space.
↓
4. Chemiosmosis & Photophosphorylation: The accumulation of H⁺ in the thylakoid space creates an electrochemical gradient. Protons diffuse back into the stroma through the enzyme ATP Synthase, driving the phosphorylation of ADP into ATP.
↓
5. NADP Reduction: Electrons re-excited in PSI are passed to a final electron acceptor, NADP⁺, combining with a proton to form reduced NADP (NADPH).
C1.3.15 – C1.3.19 HL Only
The Calvin Cycle
Key Understanding (HL) The energy (ATP) and reducing power (reduced NADP) generated by the light-dependent reactions are used to fix carbon dioxide into complex organic molecules within the stroma.
The light-independent reactions, often called the Calvin Cycle, do not require light directly but rely absolutely on the products of the light-dependent reactions. The cycle consists of three main phases:
- Carbon Fixation (Carboxylation): The crucial enzyme Rubisco (ribulose bisphosphate carboxylase) catalyzes the attachment of a CO₂ molecule to a 5-carbon compound called RuBP (ribulose bisphosphate).
- Reduction: The resulting 6-carbon compound is highly unstable and immediately splits into two 3-carbon molecules called glycerate 3-phosphate (GP). These GP molecules are then reduced using ATP (for energy) and reduced NADP (for hydrogen/electrons) to form triose phosphate (TP), a 3-carbon sugar.
- Regeneration: For every six molecules of TP produced, one exits the cycle to be used in synthesizing larger carbohydrates (like glucose, starch, and cellulose), while the remaining five TP molecules are utilized to regenerate three molecules of RuBP, consuming more ATP in the process so the cycle can continue.
Exam Tip: Be sure to emphasize the cyclical interdependence of these reactions. The Calvin cycle will rapidly halt in the dark because it runs out of ATP and NADPH. Conversely, the light-dependent reactions will halt if the Calvin cycle isn't running to return ADP and NADP⁺ back to the thylakoid.
Rubisco
Arguably the most abundant enzyme on Earth, responsible for fixing CO₂ onto RuBP in the first step of the Calvin Cycle.
Triose Phosphate (TP)
A 3-carbon sugar product of the Calvin cycle. Used to regenerate RuBP and to synthesize glucose, lipids, and amino acids.
Self-Assessment
Check Your Understanding
Test your knowledge of Topic C1.3 Photosynthesis by answering the questions below. Click to reveal the correct answers.
What is the primary function of the photolysis of water in the light-dependent reactions?
The photolysis of water splits H₂O molecules using light energy to provide a source of replacement electrons for Photosystem II. It also generates protons (H⁺) that contribute to the chemiosmotic gradient and releases oxygen gas (O₂) as a byproduct.
How does an action spectrum differ from an absorption spectrum?
An absorption spectrum shows the specific wavelengths of light absorbed by individual pigments (like chlorophyll a). An action spectrum shows the overall rate of photosynthesis of the whole plant at different wavelengths. They typically mirror each other, indicating that the absorbed light is what drives photosynthesis.
Explain why temperature is a limiting factor for photosynthesis.
Photosynthesis involves many enzyme-catalyzed reactions, particularly the Calvin cycle which relies heavily on Rubisco. As temperature increases, reaction rates increase up to an optimum due to more kinetic energy and collisions. Beyond the optimum, the enzymes denature, altering their active sites, and the rate of photosynthesis sharply declines.
(HL) Outline the relationship between the structure of the thylakoid space and its function.
The thylakoid space (or lumen) has a very small internal volume. This is an adaptation that allows a small number of protons (H⁺) pumped across the membrane to rapidly create a steep concentration gradient. This gradient is essential for driving chemiosmosis and generating ATP via ATP synthase.
✅
C1.3 Photosynthesis Complete!
You have successfully reviewed all SL and HL understandings for Biology Theme C1.3.
