CIE AS/A Level Biology -13.1 Photosynthesis as an energy transfer process- Study Notes- New Syllabus
CIE AS/A Level Biology -Photosynthesis as an energy transfer process- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -Photosynthesis as an energy transfer process- Study Notes- New Syllabus
Key Concepts:
- describe the relationship between the structure of chloroplasts, as shown in diagrams and electron micrographs, and their function
- explain that energy transferred as ATP and reduced NADP from the light-dependent stage is used during the light-independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules
- state that within a chloroplast, the thylakoids (thylakoid membranes and thylakoid spaces), which occur in stacks called grana, are the site of the light-dependent stage and the stroma is the site of the light-independent stage
- describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and xanthophyll) in light absorption in thylakoids
- interpret absorption spectra of chloroplast pigments and action spectra for photosynthesis
- describe and use chromatography to separate and identify chloroplast pigments (reference should be made to Rf values in identification of chloroplast pigments)
- state that cyclic photophosphorylation and non-cyclic photophosphorylation occur during the light-dependent stage of photosynthesis
- explain that in cyclic photophosphorylation:
• only photosystem I (PSI) is involved
• photoactivation of chlorophyll occurs
• ATP is synthesised - explain that in non-cyclic photophosphorylation:
• photosystem I (PSI) and photosystem II (PSII) are both involved
• photoactivation of chlorophyll occurs
• the oxygen-evolving complex catalyses the photolysis of water
• ATP and reduced NADP are synthesised - explain that during photophosphorylation:
• energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected)
• the released energy is used to transfer protons across the thylakoid membrane
• protons return to the stroma from the thylakoid space by facilitated diffusion through ATP synthase, providing energy for ATP synthesis (details of ATP synthase are not expected) - outline the three main stages of the Calvin cycle:
• rubisco catalyses the fixation of carbon dioxide by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound
• GP is reduced to triose phosphate (TP) in reactions involving reduced NADP and ATP
• RuBP is regenerated from TP in reactions that use ATP - state that Calvin cycle intermediates are used to produce other molecules, limited to GP to produce some amino acids and TP to produce carbohydrates, lipids and amino acids
Chloroplast Structure and Function
📌 Overview
- Chloroplasts are plastids responsible for photosynthesis.
- Found mainly in mesophyll cells of leaves.
- Structure is adapted to maximize light capture and energy conversion.
🌱 Key Structural Features
| Structure | Function |
|---|---|
| Double membrane | Outer membrane encloses chloroplast; inner membrane regulates transport of materials. |
| Thylakoids | Flattened sacs containing chlorophyll and pigments → site of light-dependent reactions. |
| Grana (stacks of thylakoids) | Increases surface area for light absorption and electron transport. |
| Stroma | Fluid-filled matrix → site of light-independent reactions (Calvin cycle); contains enzymes, starch grains, and DNA/ribosomes. |
| Chloroplast DNA & ribosomes | Allow synthesis of some proteins and enzymes independently of the nucleus. |
| Lamellae | Connect grana, facilitating transport of molecules and electrons between thylakoids. |
🔬 Diagrams & Electron Micrographs
- Diagrams: Show outer membrane, stroma, grana, thylakoids, and lamellae. Useful for labeling and understanding functional regions.
- Electron micrographs: Show actual thylakoid stacks and grana density. Denser grana → more photosynthetic capacity.
🧠 Structure → Function Relationship:
– Thylakoid membranes and grana: Maximize light capture and ATP/NADPH production.
– Stroma: Contains enzymes for carbon fixation; site for synthesis of sugars.
– Double membrane & lamellae: Efficient molecule transport and compartmentalization.
– DNA & ribosomes: Support protein synthesis for chloroplast function.
– Thylakoid membranes and grana: Maximize light capture and ATP/NADPH production.
– Stroma: Contains enzymes for carbon fixation; site for synthesis of sugars.
– Double membrane & lamellae: Efficient molecule transport and compartmentalization.
– DNA & ribosomes: Support protein synthesis for chloroplast function.
Photosynthesis: ATP and NADPH in the Calvin Cycle
📌 Overview
- Photosynthesis occurs in two linked stages:
- Light-dependent reactions → in thylakoid membranes
- Light-independent reactions (Calvin cycle) → in stroma
- Energy captured from light is temporarily stored in ATP and reduced NADP (NADPH).
🌱 Role of ATP and NADPH in the Calvin Cycle
- ATP
- Provides energy to drive enzymatic reactions in the Calvin cycle.
- Powers conversion of 3-carbon molecules into higher-energy sugars.
- Reduced NADP (NADPH)
- Provides reducing power (electrons).
- Used to reduce 3-carbon intermediates into glyceraldehyde-3-phosphate (G3P).
🌱 Summary of the Process
- Light-dependent reactions: Light energy → ATP + NADPH + O₂
- Calvin cycle (light-independent): Uses ATP (energy) and NADPH (electrons); CO₂ → 3-carbon sugars → eventually glucose and other complex organic molecules
- Key point: Calvin cycle does not require light directly, but depends on products of the light-dependent stage.
🧠 Key Points
ATP = energy source
NADPH = electron donor / reducing power
Light-independent stage produces complex organic molecules for growth and storage
Energy flow: Sunlight → ATP/NADPH → Carbon fixation → Sugars
ATP = energy source
NADPH = electron donor / reducing power
Light-independent stage produces complex organic molecules for growth and storage
Energy flow: Sunlight → ATP/NADPH → Carbon fixation → Sugars
Sites of Photosynthesis Within Chloroplasts
📌 Key Points
- Thylakoids
- Flattened membrane sacs within chloroplasts.
- Contain thylakoid membranes and thylakoid spaces.
- Often stacked into grana.
- Site of the light-dependent stage of photosynthesis.
- Function: Capture light energy to produce ATP and NADPH.
- Stroma
- Fluid-filled matrix surrounding the grana.
- Site of the light-independent stage (Calvin cycle).
- Function: Uses ATP and NADPH to fix CO₂ into sugars.
🧠 Summary:
– Thylakoids/grana → light-dependent reactions
– Stroma → light-independent reactions (Calvin cycle)
– Energy captured in the thylakoids is transferred to the stroma to drive the synthesis of complex organic molecules.
– Thylakoids/grana → light-dependent reactions
– Stroma → light-independent reactions (Calvin cycle)
– Energy captured in the thylakoids is transferred to the stroma to drive the synthesis of complex organic molecules.
Chloroplast Pigments and Light Absorption
📌 Overview
- Photosynthesis depends on pigments in thylakoid membranes to capture light energy.
- Different pigments absorb different wavelengths of light, maximizing energy capture.
🌱 Key Chloroplast Pigments
| Pigment | Type | Light Absorbed | Role in Photosynthesis |
|---|---|---|---|
| Chlorophyll a | Main pigment | Blue-violet & red | Primary pigment; converts light energy into chemical energy |
| Chlorophyll b | Accessory | Blue & red-orange | Expands range of light absorption; transfers energy to chlorophyll a |
| Carotene | Accessory | Blue & blue-green | Protects chlorophyll from photooxidation; passes energy to chlorophyll a |
| Xanthophyll | Accessory | Blue & blue-green | Protects chlorophyll; funnels light energy to chlorophyll a |
🌱 Function in Thylakoids
- Located in thylakoid membranes, forming photosystems.
- Harvest light energy → excite electrons → drive ATP and NADPH production.
- Accessory pigments broaden the spectrum of light absorbed and protect the chloroplast from damage by excess light.
🧠 Key Points
Chlorophyll a: main pigment for energy conversion.
Accessory pigments (chlorophyll b, carotene, xanthophyll): increase light absorption range & protect chloroplasts.
Together, they ensure efficient light harvesting in the thylakoids.
Chlorophyll a: main pigment for energy conversion.
Accessory pigments (chlorophyll b, carotene, xanthophyll): increase light absorption range & protect chloroplasts.
Together, they ensure efficient light harvesting in the thylakoids.
Chloroplast Pigments: Absorption and Action Spectra
📌 Overview
- Absorption spectrum: Shows the wavelengths of light absorbed by a pigment.
- Action spectrum: Shows the rate of photosynthesis at different wavelengths of light.
- Comparing the two helps identify which pigments are responsible for photosynthesis.
🌱 Absorption Spectra of Chloroplast Pigments
| Pigment | Light Absorbed |
|---|---|
| Chlorophyll a | Blue-violet & red |
| Chlorophyll b | Blue & red-orange |
| Carotene | Blue & blue-green |
| Xanthophyll | Blue & blue-green |
Interpretation:
- Peaks in absorption spectra indicate wavelengths efficiently absorbed.
- Chlorophyll a shows major peaks in red and blue → main driver of photosynthesis.
- Accessory pigments absorb other wavelengths and transfer energy to chlorophyll a.
🌱 Action Spectrum for Photosynthesis
- Measures rate of photosynthesis (e.g., O₂ production) at different wavelengths.
- Peaks in the action spectrum correspond closely to absorption peaks of chlorophyll a.
- Slightly broader peaks than chlorophyll a absorption due to accessory pigments contributing light energy.
🔍 Key Points
Absorption spectrum: pigment-specific light absorption.
Action spectrum: overall photosynthetic efficiency at each wavelength.
Relationship: Peaks in the action spectrum = wavelengths effectively used in photosynthesis.
Accessory pigments expand the range of usable light.
Conclusion: Chlorophyll a is the primary pigment, accessory pigments increase efficiency.
Absorption spectrum: pigment-specific light absorption.
Action spectrum: overall photosynthetic efficiency at each wavelength.
Relationship: Peaks in the action spectrum = wavelengths effectively used in photosynthesis.
Accessory pigments expand the range of usable light.
Conclusion: Chlorophyll a is the primary pigment, accessory pigments increase efficiency.
Separation and Identification of Chloroplast Pigments Using Chromatography
📌 Overview
- Chromatography separates pigments based on their solubility and affinity for the stationary phase.
- Commonly used to identify chlorophyll a, chlorophyll b, carotene, and xanthophyll.
🌱 Principle
- Pigments dissolve in a solvent (mobile phase) and move along chromatography paper or thin-layer chromatography plate (stationary phase).
- Pigments with higher solubility in the solvent travel further, while pigments more attracted to the paper travel less.
- Rf value = ratio of distance traveled by pigment ÷ distance traveled by solvent.
- Pigments can be identified by comparing Rf values to known standards.
🌱 Procedure (Paper Chromatography)
- Grind fresh spinach leaves in acetone or alcohol to extract pigments.
- Place a spot of extract near the base of chromatography paper.
- Place the paper in a solvent (e.g., mixture of petroleum ether and acetone) without submerging the spot.
- Allow solvent to rise up the paper, carrying pigments.
- Remove paper, mark pigment bands, and measure distances.
- Calculate Rf values:
Rf = Distance traveled by pigment ÷ Distance traveled by solvent front - Compare Rf values with known standards to identify pigments.
🌱 Expected Pigments and Rf Values
| Pigment | Approx. Rf Value | Color |
|---|---|---|
| Carotene | Highest (0.9) | Orange |
| Xanthophyll | 0.8 | Yellow |
| Chlorophyll a | 0.6–0.7 | Blue-green |
| Chlorophyll b | 0.5–0.6 | Yellow-green |
🧠 Key Points
Chromatography separates pigments based on solubility and affinity.
Rf values allow identification of pigments.
Pigments have different colors and contribute to photosynthesis efficiency by absorbing different wavelengths.
Chromatography separates pigments based on solubility and affinity.
Rf values allow identification of pigments.
Pigments have different colors and contribute to photosynthesis efficiency by absorbing different wavelengths.
Photophosphorylation in Photosynthesis
📌 Key Points
- During the light-dependent stage, chloroplasts use light energy to produce ATP and NADPH.
- There are two types of photophosphorylation:
Cyclic Photophosphorylation
- Electrons cycle back to chlorophyll.
- Produces ATP only.
- No NADPH or O₂ is produced.
Non-Cyclic Photophosphorylation
- Electrons move from water → photosystem II → photosystem I → NADP⁺.
- Produces ATP, NADPH, and O₂.
- Water is split (photolysis) to provide electrons and release oxygen.
🧠 Summary
Cyclic: ATP only; electrons recycled.
Non-cyclic: ATP + NADPH + O₂; electrons flow linearly.
Both occur in the thylakoid membranes during the light-dependent stage.
Cyclic: ATP only; electrons recycled.
Non-cyclic: ATP + NADPH + O₂; electrons flow linearly.
Both occur in the thylakoid membranes during the light-dependent stage.
Cyclic Photophosphorylation
📌 Key Points
- Occurs during the light-dependent stage of photosynthesis.
- Involves only Photosystem I (PSI).
🌱 Steps
- Photoactivation of chlorophyll
Light energy excites electrons in PSI chlorophyll.
Electrons become high-energy. - Electron flow
Excited electrons pass along an electron transport chain.
Electrons eventually return to PSI (hence “cyclic”). - ATP synthesis
Energy released during electron transport is used to pump protons into the thylakoid space.
Protons flow back through ATP synthase, producing ATP.
🧠 Key Points
Only PSI is involved.
No NADPH or O₂ is produced.
Purpose: Provides extra ATP needed for the Calvin cycle.
Only PSI is involved.
No NADPH or O₂ is produced.
Purpose: Provides extra ATP needed for the Calvin cycle.
Non-Cyclic Photophosphorylation
📌 Key Points
- Occurs during the light-dependent stage of photosynthesis.
- Involves both Photosystem II (PSII) and Photosystem I (PSI).
🌱 Steps
- Photoactivation of chlorophyll
Light excites electrons in PSII and PSI chlorophyll.
Electrons gain high energy. - Photolysis of water
Oxygen-evolving complex in PSII splits water:
2 H2O → 4 H+ + 4 e− + O2
Electrons replace those lost by PSII; oxygen is released as a by-product. - Electron flow
Electrons move from PSII → electron transport chain → PSI → NADP⁺, forming reduced NADP (NADPH). - ATP synthesis
Energy released during electron transport pumps protons into the thylakoid space.
Protons return via ATP synthase, producing ATP.
🧠 Key Points
PSII and PSI both involved.
Produces ATP, NADPH, and O₂.
Electrons flow linearly (non-cyclic).
Provides energy and reducing power for the Calvin cycle.
PSII and PSI both involved.
Produces ATP, NADPH, and O₂.
Electrons flow linearly (non-cyclic).
Provides energy and reducing power for the Calvin cycle.
ATP Synthesis in Photophosphorylation
📌 Key Points
- Occurs in the thylakoid membranes of chloroplasts during the light-dependent stage.
🌱 Steps
- Electron Excitation
Light energy excites electrons in chlorophyll, producing energetic electrons. - Electron Transport Chain (ETC)
Energetic electrons pass along the ETC.
Energy is released as electrons move through the chain. - Proton Pumping
Released energy is used to pump protons (H⁺) from the stroma into the thylakoid space.
Creates a proton gradient across the thylakoid membrane. - ATP Formation
Protons return to the stroma via facilitated diffusion through ATP synthase.
Energy from proton flow drives ATP synthesis from ADP + Pi.
🧠 Key Points
Electron energy → proton gradient → ATP production.
Process is common to both cyclic and non-cyclic photophosphorylation.
Provides ATP needed for the Calvin cycle in the stroma.
Electron energy → proton gradient → ATP production.
Process is common to both cyclic and non-cyclic photophosphorylation.
Provides ATP needed for the Calvin cycle in the stroma.
Calvin Cycle (Light-Independent Stage of Photosynthesis)
📌 Overview
- Occurs in the stroma of chloroplasts.
- Uses ATP and NADPH from the light-dependent stage to fix CO₂ into sugars.
- Three main stages: Carbon fixation, Reduction, Regeneration of RuBP.
🌱 Stages of the Calvin Cycle
- Carbon Fixation
Enzyme: Rubisco
Process: CO₂ combines with ribulose bisphosphate (RuBP, 5C) → forms two molecules of glycerate 3-phosphate (GP, 3C). - Reduction
GP is reduced to triose phosphate (TP, 3C).
Requires ATP and reduced NADP (NADPH) from light-dependent reactions.
TP is a 3-carbon sugar used to form glucose and other organic molecules. - Regeneration of RuBP
Some TP molecules are used to regenerate RuBP.
ATP is required for the regeneration process.
Ensures the cycle can continue fixing CO₂.
🧠 Key Points
Rubisco: key enzyme for carbon fixation.
GP → TP: reduction using ATP + NADPH.
TP → RuBP: regeneration using ATP.
The cycle links energy capture (light-dependent) to carbon fixation and sugar production.
Rubisco: key enzyme for carbon fixation.
GP → TP: reduction using ATP + NADPH.
TP → RuBP: regeneration using ATP.
The cycle links energy capture (light-dependent) to carbon fixation and sugar production.
Calvin Cycle Intermediates and Their Uses
📌 Key Points
- Intermediates of the Calvin cycle are not just recycled to regenerate RuBP, they also serve as building blocks for other biomolecules.
🌱 Uses of Intermediates
| Intermediate | Used to produce |
|---|---|
| Glycerate 3-phosphate (GP) | Some amino acids (via transamination reactions) |
| Triose phosphate (TP) | Carbohydrates (e.g., glucose, starch, sucrose) Lipids (after conversion to glycerol and fatty acids) Amino acids (via further metabolic pathways) |
🧠 Key Points
Calvin cycle intermediates link photosynthesis to biosynthesis.
GP and TP provide carbon skeletons for a variety of organic molecules essential for plant growth.
Highlights the role of the Calvin cycle in both energy storage and metabolism.
Calvin cycle intermediates link photosynthesis to biosynthesis.
GP and TP provide carbon skeletons for a variety of organic molecules essential for plant growth.
Highlights the role of the Calvin cycle in both energy storage and metabolism.