C1.3.1 – Transformation of Light Energy to Chemical Energy in Photosynthesis
🔁 What Is Energy Transformation in Photosynthesis?
- Photosynthesis is the process where light energy (from the sun) is converted into chemical energy stored in carbon compounds (like glucose).
- This process powers most ecosystems by supplying energy to producers and, indirectly, to consumers and decomposers.
⚡ Light Energy ➡️ Chemical Energy
Chlorophyll (in chloroplasts) absorbs light energy, especially red and blue wavelengths.
This energy is used to:
- Split water molecules (photolysis) → releases electrons, protons, and oxygen.
- Fix carbon dioxide (CO₂) into glucose and other carbon compounds.
🌱 Why This Energy Transformation Matters
Glucose stores chemical energy in C–H bonds.
This energy can be:
- Used directly in cell respiration by plants.
- Passed to herbivores and carnivores through food chains.
Without this transformation, life would not have a continuous energy source.
🧬 Key Reactions Involved
- Light-dependent reactions: Convert light energy into ATP and reduced NADP (NADPH).
- Light-independent reactions (Calvin cycle): Use ATP and NADPH to fix CO₂ into organic molecules like glucose.
🌍 Ecosystem Impact
Nearly all energy in food chains originates from this photosynthetic process.
It is the foundation of energy flow in ecosystems.
Photosynthesis transforms light energy into chemical energy stored in carbon compounds (like glucose).
This chemical energy fuels life in almost all ecosystems by supporting food chains and metabolic processes.
C1.3.2 – Conversion of Carbon Dioxide to Glucose in Photosynthesis Using Hydrogen from Water
🧪 Photosynthesis in Simple Terms
Photosynthesis is the process where plants, algae, and some bacteria convert:
- Carbon dioxide (CO₂) from the air
- Water (H₂O) from the soil
- Light energy from the sun
→ into glucose (a sugar) and oxygen (O₂)
🧾 Simple Word Equation
Carbon dioxide + Water → Glucose + Oxygen
(Using light energy and chlorophyll)
🔬 Where Does the Hydrogen Come From?
In the light-dependent stage, water is split by light energy in a process called photolysis:
2H₂O → 4H⁺ + 4e⁻ + O₂
The hydrogen ions (H⁺) and electrons (e⁻) are used in the light-independent stage (Calvin cycle) to help convert CO₂ into glucose.
How CO₂ Becomes Glucose
- In the Calvin cycle:
- CO₂ is fixed (attached to a 5-carbon compound).
- The carbon compound is reduced (gains hydrogen and electrons).
- Hydrogen from water (via NADPH) helps build up glucose (C₆H₁₂O₆).
🌱 Why Glucose Is Important
- It’s a source of energy for the plant (via respiration).
- It can be stored as starch or used to build cellulose, proteins, and other compounds.
- It’s the starting point for all food chains.
🔁 Key Idea:
In photosynthesis, CO₂ is converted into glucose using hydrogen from water, which is released during photolysis. The overall process can be summarized in a simple word equation:
Carbon dioxide + Water → Glucose + Oxygen
This reaction powers plant growth and supports life in ecosystems.
C1.3.3 – Oxygen as a By-Product of Photosynthesis in Plants, Algae and Cyanobacteria
🧾 Simple Word Equation for Photosynthesis
Carbon dioxide + Water → Glucose + Oxygen
(Using light energy and chlorophyll)
💧Where Does the Oxygen Come From?
During the light-dependent reactions of photosynthesis, water (H₂O) is split in a process called photolysis.
This reaction uses light energy and happens in the chloroplasts:
2H₂O → 4H⁺ + 4e⁻ + O₂
The O₂ (oxygen gas) released is a by-product — it is not used by the plant in photosynthesis but diffuses out into the atmosphere.
🧪 Who Does This?
- Plants (leaves and green parts)
- Algae (unicellular or multicellular in water)
- Cyanobacteria (photosynthetic prokaryotes)
All of these organisms:
- Use chlorophyll or similar pigments to capture light
- Carry out oxygenic photosynthesis (oxygen is produced)
🌎 Why It Matters
- The oxygen we breathe is mainly from photosynthesis.
- This process helped form Earth’s oxygen-rich atmosphere billions of years ago (thanks to cyanobacteria!).
Photosynthetic organisms like plants, algae, and cyanobacteria produce oxygen as a by-product when they split water molecules during photosynthesis. This oxygen is released into the environment and supports life on Earth.
C1.3.4 – Separation and Identification of Photosynthetic Pigments by Chromatography
🧪 What Is Chromatography?
Chromatography is a technique used to separate and identify substances in a mixture based on their movement through a medium.
In this case, we’re separating photosynthetic pigments like:
- Chlorophyll a
- Chlorophyll b
- Carotene
- Xanthophyll
🧫 How It Works (Paper or Thin-Layer Chromatography)
- Extract pigments from a leaf (grind it with solvent like acetone or ethanol).
- Apply a small spot of the pigment extract onto chromatography paper or a TLC plate.
- Place the bottom of the paper/plate into a solvent (but not the pigment spot!).
- The solvent moves upward by capillary action, carrying pigments with it.
Pigments separate based on:- Solubility in the solvent
- Attraction to the paper/plate
🎯 Rf Values (Retention Factor)
To identify pigments, calculate their Rf values:
Rf = Distance travelled by pigment ÷ Distance travelled by solvent front
Each pigment has a characteristic Rf value (in a specific solvent), allowing identification.
🌈 Typical Pigment Colours and Positions
| Pigment | Colour | Rf (approx.) |
|---|---|---|
| Chlorophyll a | Blue-green | ~0.3 – 0.4 |
| Chlorophyll b | Yellow-green | ~0.2 – 0.3 |
| Carotene | Orange | ~0.9 |
| Xanthophyll | Yellow | ~0.5 – 0.7 |
Rf values vary with solvent and conditions — these are rough guides.
🧠 Applications and Skills
- Run a chromatography experiment
- Measure distances with a ruler
- Calculate Rf values
- Identify pigments using both colour and Rf
Chromatography separates photosynthetic pigments based on their solubility and interaction with the medium. Rf values help identify each pigment, supporting analysis of the types and amounts of pigments in leaves.
C1.3.5 – Absorption of Specific Wavelengths of Light by Photosynthetic Pigments
🌿 What Are Photosynthetic Pigments?
Photosynthetic pigments (like chlorophyll and carotenoids) are molecules in plants, algae, and cyanobacteria that absorb light energy for photosynthesis.
Different pigments absorb different wavelengths of light.
🌈 Why Only Some Wavelengths Are Absorbed
Light = electromagnetic radiation that travels in waves (measured in nanometers, nm).
The visible light spectrum ranges from about 400 nm (violet) to 700 nm (red).
Pigments absorb specific wavelengths and reflect others (the reflected ones are what we see!).
| Colour of Light | Wavelength (nm) |
|---|---|
| Violet | ~400–430 |
| Blue | ~430–500 |
| Green | ~500–570 |
| Yellow | ~570–590 |
| Orange | ~590–620 |
| Red | ~620–700 |
📈 Absorption Spectra
An absorption spectrum shows how much light is absorbed by a pigment at each wavelength.
- 🟢 Chlorophyll a: Absorbs blue (≈430 nm) and red (≈665 nm). Reflects green → why plants look green.
- 🟡 Chlorophyll b: Absorbs blue (≈455 nm) and orange-red (≈640 nm).
- 🟠 Carotenoids: Absorb blue-violet light (400–500 nm). Reflect yellow/orange.
⚡ What Happens When Light Is Absorbed?
- A photon (light particle) hits a pigment.
- An electron in the pigment molecule gets excited (jumps to a higher energy level).
- This excitation is the first step in converting light energy into chemical energy.
🌱 Why It Matters for Photosynthesis
- Only absorbed light can be used in photosynthesis.
- Reflected light (like green in chlorophyll) does not contribute to energy conversion.
- This is why the combination of pigments helps plants capture a broader range of light.
Photosynthetic pigments absorb specific wavelengths of visible light. These wavelengths excite electrons, transforming light energy into chemical energy. Absorption spectra show how efficiently pigments absorb light across the spectrum.
C1.3.6 – Similarities and Differences of Absorption and Action Spectra
🧪 What Are Absorption and Action Spectra?
| Term | Definition |
|---|---|
| Absorption Spectrum | A graph showing how much light a pigment absorbs at different wavelengths. |
| Action Spectrum | A graph showing the rate of photosynthesis at different wavelengths of light. |
✅ Similarities Between Them
- Both relate to wavelengths of light (400–700 nm).
- Both use colour and wavelength on the x-axis.
- Both show peaks in the blue and red regions of the spectrum.
- Both help explain which light wavelengths are most useful for photosynthesis.
❌ Differences Between Them
| Feature | Absorption Spectrum | Action Spectrum |
|---|---|---|
| What it shows | Light absorbed by pigments (e.g., chlorophyll a/b) | Effectiveness of light in driving photosynthesis |
| Y-axis | % of light absorbed | Rate of photosynthesis |
| Data source | Physical absorption of light by pigments | Oxygen produced or CO₂ consumed |
| Biological meaning | Potential for energy capture | Actual energy conversion in photosynthesis |
Typical Graph Comparison
- Absorption spectrum shows high absorption at ~430 nm (blue) and ~660 nm (red).
- Action spectrum also peaks in blue and red, showing maximum photosynthesis rates there.
- Green light (~500–550 nm) is low in both → not absorbed well and not effective for photosynthesis.
📈 Skill: Interpreting and Plotting an Action Spectrum
- How to determine photosynthesis rates:
- Use data from:
- O₂ production (e.g., bubbles in aquatic plants)
- CO₂ consumption (e.g., pH indicators in solution)
- Plot rate vs. wavelength to create an action spectrum
- Use data from:
- This lets you see which wavelengths drive the highest photosynthesis rate.
The absorption spectrum shows how much light pigments absorb. The action spectrum shows how much light actually contributes to photosynthesis. Both peak in blue and red regions, helping explain why those colors are most effective for energy capture in plants.
C1.3.7 – Investigating Limiting Factors in Photosynthesis
🌡️Limiting Factors in Photosynthesis
A limiting factor is any environmental condition that directly restricts the rate of photosynthesis when in short supply. The three main ones are:
- Carbon dioxide concentration (CO₂)
- Light intensity
- Temperature
🧪 Experimental Techniques to Investigate Limiting Factors
| Factor | How to Vary It | How to Measure Rate |
|---|---|---|
| CO₂ concentration | Add sodium hydrogencarbonate (NaHCO₃) to water | Count bubbles from pondweed (e.g., Elodea), or use O₂ sensors |
| Light intensity | Move a lamp closer or farther from the plant | Measure rate of O₂ production |
| Temperature | Use a water bath or thermostat to maintain specific temperatures | Observe changes in bubble rate or O₂ output |
🧠 Application of Scientific Skills
Identifying Variables
| Type | Example |
|---|---|
| Independent | Light intensity / CO₂ concentration / temperature |
| Dependent | Rate of photosynthesis (O₂ production or bubble count) |
| Controlled | Species of plant, light wavelength, water pH, volume |
Forming Hypotheses
A hypothesis is a provisional explanation that can be tested by experiment.
Examples:
- If light intensity increases, then the rate of photosynthesis will increase until a plateau is reached.
- If CO₂ concentration is low, then it will limit the rate of photosynthesis even if light is abundant.
🧬 Nature of Science (NOS):
Hypotheses can be made before or after experiments, depending on the approach. They should be based on prior evidence or theory and must be repeatedly tested.
📊 Common Graph Patterns
| Limiting Factor | Graph Shape (rate vs. factor) | Reason |
|---|---|---|
| CO₂ or Light | Increases then levels off (saturation) | Another factor becomes limiting |
| Temperature | Bell-shaped curve | Enzyme activity increases, then declines due to denaturation |
Photosynthesis is limited by factors like CO₂, light, and temperature. Experiments help test hypotheses about these effects by changing one factor at a time. Understanding variables and patterns helps reveal how plants optimize energy production.
C1.3.8 – Carbon Dioxide Enrichment Experiments & Future Photosynthesis
🧪What Are CO₂ Enrichment Experiments?
CO₂ enrichment experiments test how increased carbon dioxide levels affect photosynthesis and plant growth. They help scientists predict how plants might respond to future atmospheric CO₂ levels due to climate change.
🔬 Two Main Types of CO₂ Enrichment Experiments
| Type | Description | Example Setup | Advantages | Disadvantages |
|---|---|---|---|---|
| Greenhouse Experiments | Plants grown in enclosed environments with controlled CO₂ levels | Glass or plastic structures | Precise control of CO₂, temperature, light, and humidity | Not truly natural conditions |
| FACE (Free-Air Carbon dioxide Enrichment) | CO₂ released around open-air plots in natural ecosystems | Open fields with CO₂ pipes and sensors | Real-world growing conditions | Harder to control all variables |
🧠 Application of Scientific Skills
Identifying a Controlled Variable
In experiments, some variables must be kept constant to make sure the independent variable (CO₂) is the one affecting results.
Examples of controlled variables:
- Temperature
- Light intensity
- Water availability
- Soil nutrients
- Plant species
🔬 Nature of Science (NOS)
- Careful experimental design requires controlling variables as much as possible.
- This is easier in labs (e.g. greenhouses) but not always realistic for understanding natural ecosystems.
- Field experiments, like FACE, are crucial for studying real plant responses to changing environments.
🌿 Why These Experiments Matter
- Predict future crop yields in elevated CO₂ environments
- Understand carbon cycling in ecosystems
- Guide climate change models and agricultural planning
CO₂ enrichment experiments help scientists explore how rising CO₂ will affect photosynthesis and plant growth. Laboratory setups offer control, while field studies like FACE provide realistic insight. Identifying controlled variables is essential for accurate experimental results.
Additional Higher Level
C1.3.9 – Photosystems & Excited Electrons
📚 What Are Photosystems?
Photosystems are molecular arrays of pigments (like chlorophyll) found in photosynthetic organisms. They are essential for capturing light energy and starting the light-dependent reactions of photosynthesis.
🌱 Where Are Photosystems Found?
- Always located in membranes
- In chloroplasts of plants and algae → in the thylakoid membrane
- In cyanobacteria → in their plasma membrane or internal membrane systems
🧪 Structure of a Photosystem
| Component | Function |
|---|---|
| Chlorophyll pigments | Absorb light at specific wavelengths |
| Accessory pigments | Broaden the range of light absorbed (e.g. carotenoids) |
| Reaction centre | A special chlorophyll molecule that emits the excited electron |
| Protein framework | Holds the pigments in place within the membrane |
📌 A photosystem = antenna pigments + reaction centre
⚡ How Do Photosystems Work?
- Light energy is absorbed by pigment molecules.
- Energy is passed from one pigment to another (like a relay).
- It reaches the reaction centre chlorophyll.
- An electron gets excited (gains energy) and is emitted from the reaction centre.
- This excited electron is passed to an electron transport chain → leads to ATP and NADPH formation.
🧬 Types of Photosystems
| Photosystem | Found in | Main Role |
|---|---|---|
| Photosystem II (PSII) | Plants, algae, cyanobacteria | Starts the light-dependent reactions by splitting water |
| Photosystem I (PSI) | Same as above | Helps form NADPH for the Calvin cycle |
Photosystems are membrane-bound pigment complexes found in cyanobacteria and chloroplasts. They capture light energy and transfer it to a reaction centre chlorophyll, which emits an excited electron to power photosynthesis.
C1.3.10 – Advantages of Structured Pigment Arrays in Photosystems
🧬 Why Just One Pigment Isn’t Enough
A single pigment molecule (like one chlorophyll) cannot:
- Absorb all wavelengths of light
- Pass on enough energy efficiently
- Sustain photosynthesis alone
So, plants group different pigments into photosystems, which makes light capture and energy transfer much more efficient.
🧩 What Is a Structured Pigment Array?
A photosystem is a highly organized structure made of:
- Multiple chlorophyll a molecules
- Accessory pigments (like chlorophyll b, xanthophylls, carotenoids)
- A protein matrix that positions them properly
Together, they form an antenna complex surrounding a reaction centre.
Advantages of This Structured Arrangement
| Advantage | Explanation |
|---|---|
| Wider light absorption | Different pigments absorb different wavelengths of light → More light energy captured |
| Efficient energy transfer | Excitation energy is passed from one pigment to another until it reaches the reaction centre |
| Boosts rate of photosynthesis | More efficient light capture = more excited electrons = more ATP and NADPH |
| Stable and consistent | Pigments held in a fixed structure → ensures the energy gets to the right place every time |
A structured array of multiple pigments in a photosystem allows plants to absorb more light, use energy more efficiently, and perform photosynthesis far better than any single pigment could alone.
C1.3.11 – Generation of Oxygen by the Photolysis of Water in Photosystem II
⚡ What Is Photolysis?
Photolysis = Splitting of water molecules using light energy.
- Occurs in Photosystem II (PSII) in the thylakoid membrane.
- Catalyzed by a specific enzyme when PSII absorbs light.
Equation:
2H2O → 4H+ + 4e− + O2
🧪 What’s Made During Photolysis?
| Product | Role |
|---|---|
| Protons (H⁺) | Used to generate a proton gradient → drives ATP synthesis (chemiosmosis) |
| Electrons (e⁻) | Replace lost electrons in Photosystem II → keeps the electron transport chain going |
| Oxygen (O₂) | Waste product → diffuses out of the chloroplast and exits through the stomata |
🌍 Why Was This a Big Deal?
Oxygenation of Earth’s Atmosphere
Early Earth had little free oxygen.
Over billions of years, photosynthesis by cyanobacteria, algae, and plants:
- Increased atmospheric oxygen
- Enabled the evolution of aerobic respiration → much more efficient than anaerobic
Formation of Rocks
Oxygen reacted with dissolved iron in oceans → formed banded iron formations
Changed ocean chemistry and geological processes
Photolysis in Photosystem II splits water into protons, electrons, and oxygen. The H⁺ and e⁻ are used in photosynthesis, but O₂ is released as a waste product—a change that transformed Earth’s atmosphere and enabled complex life.
C1.3.12 – ATP Production by Chemiosmosis in Thylakoids
⚡ What Is Chemiosmosis?
Chemiosmosis = the movement of protons (H⁺) down a concentration gradient through ATP synthase, driving the production of ATP from ADP + Pi.
🌀 Where Does This Happen?
- In the thylakoid membranes of chloroplasts (site of the light-dependent reactions).
- Involves the electron transport chain (ETC) and ATP synthase enzyme.
🔄 Electron Flow and Proton Pumping
| Process | Description |
|---|---|
| Electron transport | Electrons are passed through a chain of electron carriers in the thylakoid membrane. |
| Proton pumping | Energy from electrons is used to pump protons (H⁺) into the thylakoid space, creating a proton gradient. |
| Proton accumulation | High H⁺ concentration builds up inside the thylakoid. |
| Proton diffusion | H⁺ diffuses back into the stroma through ATP synthase, releasing energy. |
| ATP synthesis | ATP synthase uses this energy to phosphorylate ADP → ATP. |
🔁 Types of Photophosphorylation
| Type | Electron Source | Path | Products |
|---|---|---|---|
| Non-cyclic | Photosystem II (PSII) → Photosystem I (PSI) | Electrons go from water → NADP⁺ | ATP, NADPH, and O₂ |
| Cyclic | Photosystem I only | Electrons cycle back to PSI | ATP only (no NADPH, no O₂) |
Photophosphorylation = the use of light to add a phosphate to ADP → ATP.
In chemiosmosis, electrons from PSII or PSI flow through an electron transport chain, powering proton pumps. The resulting proton gradient drives ATP synthase, which produces ATP—the cell’s energy currency during photosynthesis.
C1.3.13 – Reduction of NADP⁺ by Photosystem I
💡 What Is NADP⁺?
NADP⁺ = Nicotinamide adenine dinucleotide phosphate
- It is a hydrogen and electron carrier.
- Reduced NADP (NADPH) carries energy-rich electrons and H⁺ to the Calvin cycle.
🔋 How Is NADP⁺ Reduced?
| Step | Description |
|---|---|
| 1. | Light excites electrons in Photosystem I (PSI). |
| 2. | These high-energy electrons are transferred along proteins in the electron transport chain. |
| 3. | NADP⁺ in the stroma accepts: • 2 electrons from PSI • 1 H⁺ ion from the stroma |
| This reduces NADP⁺ → NADPH (reduced NADP). |
🧬 Why It Matters
- NADPH is used in the Calvin cycle to help convert CO₂ into glucose.
- It acts as a reducing agent, donating electrons and H⁺ to build carbon compounds.
🔁 Summary of the Reaction
NADP⁺ + 2e⁻ + H⁺ → NADPH
⚠️ Consistent Terminology Tip
- Use either: “NADP⁺ and NADPH” or “NADP and reduced NADP”
- Don’t mix both in the same explanation.
Light energy from Photosystem I is used to reduce NADP⁺ to NADPH by adding 2 electrons and 1 proton. This NADPH is then used in the Calvin cycle to help make glucose.
C1.3.14 – Thylakoids as Systems for Light-Dependent Reactions
🧩 What Are Thylakoids?
Thylakoids are flattened membrane sacs found inside chloroplasts.
- They are stacked into grana and surrounded by the stroma.
- Their membranes are the site of light-dependent reactions of photosynthesis.
⚡️ Key Light-Dependent Reactions in Thylakoids
| Process | Location | Description |
|---|---|---|
| Photolysis of Water | In Photosystem II, embedded in the thylakoid membrane | Light splits water into: • 2 electrons (used in ETC) • 2 H⁺ (stay in thylakoid lumen) • ½ O₂ (waste gas) |
| ATP Synthesis (Chemiosmosis) | Across the thylakoid membrane | Proton gradient builds up in lumen → protons flow back into stroma via ATP synthase, forming ATP |
| Reduction of NADP⁺ | On the stromal side of the thylakoid membrane | Electrons from Photosystem I + H⁺ from stroma reduce NADP⁺ to NADPH |
🧪 Summary of Thylakoid Functions
| Component | Function |
|---|---|
| Photosystem II | Captures light and splits water (photolysis) |
| Electron transport chain | Transfers electrons and pumps protons into lumen |
| ATP synthase | Uses proton gradient to synthesize ATP |
| Photosystem I | Excites electrons used to reduce NADP⁺ |
🔁 Overall Products of Light-Dependent Reactions
- Oxygen (O₂) – released as a waste product from water.
- ATP – energy currency used in the Calvin cycle.
- NADPH – reducing power for converting CO₂ to glucose.
Thylakoids are specialized for light-dependent reactions of photosynthesis. Inside them, photolysis of water, ATP production by chemiosmosis, and NADP⁺ reduction all occur producing the energy and electrons needed for glucose synthesis.
C1.3.15 – Carbon Fixation by Rubisco
🧬 What Is Carbon Fixation?
Carbon fixation is the first step of the Calvin cycle, part of light-independent reactions of photosynthesis.
It involves converting inorganic CO₂ into an organic compound – glycerate 3-phosphate (GP).
🔑 Key Components
| Term | Meaning |
|---|---|
| RuBP (ribulose bisphosphate) | A 5-carbon compound that reacts with CO₂ |
| CO₂ | Carbon dioxide from the atmosphere |
| Rubisco | Enzyme that catalyzes the fixation of CO₂ |
| Glycerate 3-phosphate (GP) | A 3-carbon compound produced from the reaction |
⚙️ Step-by-Step: How Carbon Fixation Happens
- Rubisco binds CO₂ to a RuBP molecule (5C).
- This forms an unstable 6-carbon intermediate.
- The unstable intermediate immediately splits into two molecules of GP (each 3C).
🧪 About Rubisco
- Full name: Ribulose bisphosphate carboxylase/oxygenase.
- Most abundant enzyme on Earth.
- Found in high concentrations in the stroma of chloroplasts.
- Works slowly, so large amounts are needed.
- Inefficient in low CO₂ levels – struggles to fix carbon efficiently in such conditions.
Carbon fixation is the first step of the Calvin cycle, where Rubisco catalyzes the reaction between CO₂ and RuBP to produce glycerate 3-phosphate. Despite being essential, Rubisco is slow and inefficient, so plants need a lot of it in the stroma to keep photosynthesis going.
C1.3.16 – Synthesis of Triose Phosphate Using Reduced NADP and ATP
🔁 Where Are We?
This step is part of the Calvin cycle, during the light-independent reactions of photosynthesis.
Follows carbon fixation (C1.3.15), where glycerate-3-phosphate (GP) is formed.
🔄 Converting GP into TP
| Substance | Role |
|---|---|
| GP (glycerate-3-phosphate) | The 3-carbon compound formed from CO₂ and RuBP |
| ATP | Provides energy for the reaction |
| NADPH (reduced NADP) | Provides hydrogen (reducing power) |
| TP (triose phosphate) | A 3-carbon sugar — building block for glucose, starch, etc. |
⚙️ Step-by-Step Process
- Each GP molecule receives energy from ATP (from the light-dependent reaction).
- Hydrogen is transferred from NADPH to reduce GP.
- This produces triose phosphate (TP).
🔁 Balanced Overview:
GP + ATP + NADPH → TP + ADP + Pi + NADP⁺
ADP and NADP⁺ are recycled back to the light-dependent reactions.
TP is either:
- Used to regenerate RuBP (to keep the cycle going), or
- Used to make glucose and other organic molecules.
🧪 Quick Facts
- Two molecules of TP are needed to make 1 glucose (C₆H₁₂O₆).
- This stage is energy-consuming – ATP and NADPH are essential.
In the Calvin cycle, glycerate-3-phosphate (GP) is converted to triose phosphate (TP) using ATP (for energy) and NADPH (for hydrogen). TP is a key product used to build carbohydrates and regenerate RuBP.
C1.3.17 – Regeneration of RuBP in the Calvin Cycle Using ATP
🔬 Why Regenerate RuBP?
The Calvin cycle is cyclical – for it to keep going, RuBP (ribulose bisphosphate) must be regenerated.
RuBP (5C) is the compound that captures CO₂, so without it, carbon fixation stops.
🔄 How Is RuBP Regenerated?
Five molecules of triose phosphate (TP) (3C) are used to make three molecules of RuBP (5C).
This process requires ATP to provide the energy for rearranging carbon atoms and forming new bonds.
📊 Triose Phosphate Fate
| TP Molecule Use | Description |
|---|---|
| 1/6 TP | Used to make glucose and other carbohydrates |
| 5/6 TP | Recycled to regenerate RuBP |
🔁 This recycling is essential – otherwise the Calvin cycle would stop after one round.
⚙️ Summary of Inputs & Outputs
| Compound | Role |
|---|---|
| ATP | Provides energy for regenerating RuBP |
| Triose phosphate (TP) | Carbon source for RuBP |
| RuBP | Re-formed to fix more CO₂ |
To keep the Calvin cycle running, five out of six TP molecules are recycled using ATP to regenerate RuBP, the CO₂-acceptor molecule. This ensures continuous carbon fixation in photosynthesis.
C1.3.18 – Synthesis of Carbohydrates, Amino Acids, and Other Carbon Compounds Using Calvin Cycle Products
🌱 All Carbon Starts with the Calvin Cycle
In photosynthetic organisms, all carbon-containing compounds (like sugars, proteins, lipids) originate from the Calvin cycle.
The carbon atoms used to make these compounds are fixed from CO₂ during the cycle.
🧪 Products of the Calvin Cycle
The key product is triose phosphate (TP) – a 3-carbon sugar.
TP can be used to make:
- Glucose and other carbohydrates
- Fatty acids and glycerol (lipids)
- Amino acids (with the help of nitrate or ammonium ions from the soil)
🔁 Role of Mineral Nutrients
| Nutrient | Role |
|---|---|
| Nitrate (NO₃⁻) or Ammonium (NH₄⁺) | Provide nitrogen to form amino groups in amino acids |
| Phosphate (PO₄³⁻) | Needed for ATP, DNA, and phospholipids |
| Magnesium (Mg²⁺) | Essential for making chlorophyll |
| Sulfur (SO₄²⁻) | Used in some amino acids (e.g. cysteine) |
🛠️ From TP to Other Molecules
- Glucose: Made by joining two TP molecules, can be stored as starch or used in cell respiration.
- Cellulose: Built from glucose; used for cell walls.
- Amino acids: TP + nitrate/ammonium = amino acids → proteins.
- Lipids: Made by converting TP into fatty acids and glycerol.
All carbon compounds in plants – including sugars, amino acids, and lipids are made using Calvin cycle products, especially triose phosphate (TP). These transformations require mineral nutrients such as nitrates and phosphates.
C1.3.19 – Interdependence of the Light-Dependent and Light-Independent Reactions
🌞 The Two Stages of Photosynthesis
Photosynthesis has two interconnected stages:
- Light-dependent reactions (in the thylakoids)
– Need light to occur.
– Produce:- ATP
- Reduced NADP (NADPH)
- Oxygen (from photolysis of water)
- Light-independent reactions (aka the Calvin cycle, in the stroma)
– Use ATP and NADPH to fix CO₂ and make sugars (like glucose).
🔗 How They’re Interdependent
| 🔄 Connection | What Happens |
|---|---|
| From light-dependent to light-independent | ATP and NADPH are supplied to the Calvin cycle for making carbohydrates |
| From light-independent to light-dependent | The cycle regenerates NADP⁺ and ADP, which go back to the thylakoids to be reused |
🚫 What Happens if One Stops?
- No light → Light-dependent reactions stop:
– No ATP or NADPH made → Calvin cycle halts - No CO₂ → Calvin cycle stops:
– NADPH not used → builds up
– NADP⁺ not regenerated → electron flow in Photosystem II slows → less ATP and less photolysis
🧠 Key Insight: Even though only one stage may be directly affected, both are disrupted due to their reliance on each other.
– The Calvin cycle needs ATP and NADPH from the light-dependent reactions.
– The light-dependent reactions need NADP⁺ and ADP, which are regenerated in the Calvin cycle.
– A lack of light or CO₂ stalls both reactions, showing their tight interdependence.