Edexcel A Level (IAL) Biology -5.1-5.4 Photosynthesis: Overview- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -5.1-5.4 Photosynthesis: Overview- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -5.1-5.4 Photosynthesis: Overview- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 5.1 understand the overall reaction of photosynthesis as requiring energy from light to split apart the strong bonds in water molecules, storing the hydrogen in a fuel (glucose) by combining it with carbon dioxide and releasing oxygen into the atmosphere
- 5.2 understand how photophosphorylation of ADP requires energy and that hydrolysis of ATP provides an immediate supply of energy for biological processes
- 5.3 understand the light-dependent reactions of photosynthesis, including how light energy is trapped by exciting electrons in chlorophyll and the role of these electrons in generating ATP, reducing NADP in cyclic and non-cyclic photophosphorylation and producing oxygen through photolysis of water
- 5.4 (i) understand the light-independent reactions as reduction of carbon dioxide using the products of the light-dependent reactions (carbon fixation in the Calvin cycle, the role of GP, GALP, RuBP and RUBISCO)
(ii) know that the products are simple sugars that are used by plants, animals and other organisms in respiration and the synthesis of new biological molecules (polysaccharides, amino acids, proteins, lipids and nucleic acids)
Photosynthesis: Overall Reaction & Energy Requirement
🌱 Introduction
Photosynthesis is the process by which green plants, algae, and some bacteria use light energy to make glucose (a chemical fuel) from carbon dioxide (CO₂) and water (H₂O). It is the foundation of life on Earth converting light energy → chemical energy.
💡 The Overall Reaction
6CO₂ + 6H₂O → (light, chlorophyll) → C₆H₁₂O₆ + 6O₂
- Carbon dioxide comes from the air (via stomata).
- Water comes from the soil (via roots).
- Glucose (C₆H₁₂O₆) stores chemical energy.
- Oxygen (O₂) is released as a by-product into the atmosphere.
⚡ Energy from Light
Light energy (from the Sun) is absorbed by chlorophyll in chloroplasts. This energy is used to split water molecules a process called photolysis:
2H₂O → 4H⁺ + 4e⁻ + O₂
- The hydrogen (H⁺) and electrons (e⁻) are used later to reduce CO₂ into glucose.
- The oxygen (O₂) formed is released into the air.
Key idea: Light energy breaks strong O-H bonds in water → Hydrogen is captured → Oxygen is released.
🌾 Energy Storage in Glucose
Hydrogen (from water) combines with carbon dioxide to form glucose. This stores solar energy in C–H bonds of glucose making it a stable chemical fuel.
Plants use glucose for:
- Respiration → to release energy.
- Growth → building cellulose, starch, and proteins.
🔬 Summary of Roles
| Component | Function |
|---|---|
| Light energy | Provides energy to split water molecules |
| Water (H₂O) | Source of hydrogen and oxygen |
| Carbon dioxide (CO₂) | Source of carbon for glucose |
| Chlorophyll | Absorbs light and transfers energy |
| Glucose (C₆H₁₂O₆) | Energy-rich fuel formed |
| Oxygen (O₂) | Released as a by-product |
📘 Importance of the Process
- Primary source of energy for nearly all life on Earth.
- Maintains atmospheric O₂ and CO₂ balance.
- Forms the base of food chains in all ecosystems.
⚡ Quick Recap
Step 1: Light absorbed by chlorophyll → Splits H₂O → O₂ released.
Step 2: Hydrogen from H₂O + CO₂ → Glucose (C₆H₁₂O₆) formed.
Energy Conversion: Light → Chemical (stored as glucose bonds)
Main idea: Light energy breaks water bonds, hydrogen is stored in glucose, and oxygen is released — converting solar energy into fuel for life.
Photophosphorylation & ATP: The Cell’s Energy Currency
🌱 Introduction
Every living cell needs a quick, reliable source of energy to stay alive and perform vital functions.
That energy comes from ATP (Adenosine Triphosphate) a molecule that acts like a rechargeable battery inside the cell.
💡 What is ATP?
Full form: Adenosine Triphosphate
Structure:
- Adenine (nitrogen base)
- Ribose (sugar)
- 3 phosphate groups (-P)
Energy is stored in the high-energy bonds between the phosphate groups especially the last bond (between the 2nd and 3rd phosphate).
☀️ Photophosphorylation – Making ATP (Energy Storage)
Definition:
The process by which ADP + Pi → ATP using energy from light absorbed during photosynthesis.
ADP + Pi + light energy → ATP
Where it happens:
In the thylakoid membranes of chloroplasts (during light-dependent reactions).
How it works (in short):
- Light energy excites electrons in chlorophyll.
- The electrons move through the electron transport chain (ETC).
- This movement drives H⁺ ions across the thylakoid membrane.
- The proton gradient powers ATP synthase enzyme, which joins ADP and Pi to form ATP.
Result: Light energy → converted into chemical energy (ATP) that can be used by the cell.
⚙️ Hydrolysis of ATP – Energy Release (Instant Use)
Definition:
When a cell needs energy, ATP is broken down into ADP + Pi, releasing energy for immediate use.
ATP → ADP + Pi + Energy
Features:
- Happens quickly and is reversible.
- Controlled by the enzyme ATPase.
- Energy released is small, manageable, and prevents wastage as heat.
🔋 Why ATP is the Perfect Energy Molecule
| Property | Advantage |
|---|---|
| Small & soluble | Easy to transport within cells |
| Releases just enough energy | Avoids damage or waste |
| Easily regenerated | Can be recharged through respiration or photosynthesis |
| Universal energy carrier | Used by all types of cells |
🌿 Uses of ATP in Cells
| Biological Process | Function of ATP |
|---|---|
| Active transport | Powers ion pumps and carrier proteins |
| Muscle contraction | Provides energy for actin-myosin movement |
| Protein synthesis | Forms peptide bonds |
| DNA replication | Powers DNA polymerase |
| Cell signaling | Phosphorylates proteins to activate enzymes |
🧩 Summary Flow
| Step | Reaction | Role of Energy |
|---|---|---|
| 1. Photophosphorylation | ADP + Pi + light → ATP | Energy stored |
| 2. Hydrolysis | ATP → ADP + Pi + Energy | Energy released |
| 3. Use | Drives metabolic processes | Energy used immediately |
⚡ Quick Recap
ATP → Universal energy currency of the cell
Photophosphorylation → Light energy used to make ATP
Hydrolysis of ATP → Immediate energy release for cellular work
Enzymes involved → ATP synthase (formation), ATPase (breakdown)
Main idea: ATP stores energy → hydrolysis releases it instantly when needed.
Light-Dependent Reactions of Photosynthesis
🌱 Introduction
Photosynthesis begins with the light-dependent reactions, which take place in the thylakoid membranes of the chloroplast.
These reactions convert light energy → chemical energy, producing ATP, reduced NADP, and oxygen.
The products of these reactions (ATP + reduced NADP) are then used in the light-independent stage (Calvin Cycle).
💡 What Happens in the Light-Dependent Stage?
Overall purpose:
- Trap light energy using chlorophyll.
- Make ATP (energy currency).
- Make reduced NADP (hydrogen carrier).
- Release oxygen from the splitting of water.
🌞 Trapping Light Energy
Chlorophyll molecules in photosystems (PSI & PSII) absorb light energy.
- This excites electrons to a higher energy level.
- The high-energy electrons are passed along an electron transport chain (ETC) in the thylakoid membrane.
The energy from these electrons is used to:
- Make ATP (via photophosphorylation)
- Reduce NADP to NADPH
- Split water (photolysis)
💧 Photolysis of Water
2H₂O → 4H⁺ + 4e⁻ + O₂
Occurs in Photosystem II (PSII).
- Light energy splits water into:
- Protons (H⁺) – used to reduce NADP
- Electrons (e⁻) – replace those lost from chlorophyll in PSII
- Oxygen (O₂) – released as a by-product into the atmosphere
This is the source of oxygen in photosynthesis!
⚙️ Photophosphorylation – Making ATP
Two types occur: cyclic and non-cyclic.
🌿 (a) Non-Cyclic Photophosphorylation
Involves both Photosystem II (PSII) and Photosystem I (PSI)
Step-by-step:
- Light hits PSII → electrons get excited and leave chlorophyll.
- Electrons move through the electron transport chain (ETC).
- Energy from electron flow pumps H⁺ ions into the thylakoid lumen.
- H⁺ ions move back through ATP synthase → ATP is produced (chemiosmosis).
- Light hits PSI, exciting new electrons → used to reduce NADP to NADPH.
- Electrons lost from PSII are replaced by photolysis of water.
Products: ATP, reduced NADP, and O₂.
🔁 (b) Cyclic Photophosphorylation
Involves only Photosystem I (PSI)
- Light excites electrons in PSI → they pass along the ETC.
- Instead of reducing NADP, the electrons return to PSI.
- Energy released is used to make ATP only (no NADPH, no O₂).
Purpose: Provides extra ATP for the Calvin cycle when NADPH is already sufficient.
⚡ Summary of Products
| Process | Photosystem(s) Used | Products Formed | Oxygen Released? |
|---|---|---|---|
| Non-cyclic | PSII & PSI | ATP, NADPH, O₂ | Yes |
| Cyclic | PSI only | ATP only | No |
🌿 Role of Key Products
| Product | Role |
|---|---|
| ATP | Provides energy for reactions in the Calvin cycle |
| Reduced NADP (NADPH) | Supplies hydrogen for reducing CO₂ to glucose |
| Oxygen (O₂) | By-product, diffuses out for respiration or released into air |
⚡ Quick Recap
Photolysis → Light splits water → O₂ + H⁺ + e⁻
Excited electrons → Drive ATP & NADPH formation
Non-cyclic photophosphorylation → ATP + NADPH + O₂ made
Cyclic photophosphorylation → Only ATP made
Main site → Thylakoid membranes of chloroplasts
ATP & NADPH → Used in the light-independent (Calvin) stage
The Light-Independent Reactions (Calvin Cycle)
🌱 Introduction
After the light-dependent reactions make ATP and reduced NADP, the chloroplast uses them in the light-independent reactions (also called the Calvin Cycle) to fix carbon dioxide and make sugars.
These reactions do not need light directly, but they depend on products from the light-dependent stage.
They occur in the stroma of the chloroplast.
💡 Aim of the Light-Independent Stage
To convert carbon dioxide (CO₂) into carbohydrates (glucose) by using:
- ATP (energy supply)
- Reduced NADP (provides hydrogen for reduction)
🔁 Overview of the Calvin Cycle
| Step | Main Process | Key Molecules Involved |
|---|---|---|
| 1 | Carbon fixation | CO₂ + RuBP → GP (via RuBisCO) |
| 2 | Reduction | GP → GALP using ATP & NADPH |
| 3 | Regeneration | GALP → RuBP (using ATP) |
🍃 Step-by-Step Process
Step 1: Carbon Fixation (Carboxylation)
- Enzyme: RuBisCO (Ribulose Bisphosphate Carboxylase/Oxygenase).
- CO₂ from the air combines with RuBP (5C) → forms an unstable 6C compound.
- This instantly splits into two molecules of GP (3C) – glycerate-3-phosphate.
CO₂ + RuBP → 2 × GP
This is the “carbon fixation” step – inorganic carbon is fixed into an organic form.
Step 2: Reduction (Using Products of Light-Dependent Stage)
- Each GP (3C) is reduced to GALP (3C) (glyceraldehyde-3-phosphate).
- Requires:
- ATP → provides energy.
- Reduced NADP (NADPH) → provides hydrogen.
GP + ATP + NADPH → GALP + ADP + Pi + NADP⁺
GALP is an energy-rich molecule – used to build sugars and other compounds.
Step 3: Regeneration of RuBP
- Out of every 6 molecules of GALP produced:
- 1 is used to make glucose (or other organic molecules).
- 5 are recycled to regenerate RuBP.
- Regeneration requires ATP to rearrange carbon atoms back into RuBP (5C).
GALP + ATP → RuBP
Cycle continues – using CO₂, ATP, and NADPH to keep producing GALP.
🍯 Products of the Calvin Cycle
| Product | Source | Role / Fate |
|---|---|---|
| GALP (3C) | Reduced GP | Used to make glucose |
| Glucose | From GALP | Used in respiration or stored as starch |
| RuBP (5C) | Regenerated from GALP | Accepts new CO₂ to restart cycle |
🧬 How the Products Are Used
| Molecule Formed | Use / Conversion |
|---|---|
| Glucose | Used in respiration to release energy |
| Sucrose, starch, cellulose | Polysaccharides for transport or structure |
| Amino acids & proteins | Made using nitrogen (from nitrates) and GALP |
| Lipids | Made from GALP → glycerol + fatty acids |
| Nucleic acids | Made from sugars (deoxyribose/ribose) derived from GALP |
So, the Calvin Cycle provides the raw carbon skeletons needed to build all essential biomolecules.
🌿 Key Roles of Main Molecules
| Molecule | Function |
|---|---|
| RuBisCO | Catalyses fixation of CO₂ to RuBP |
| RuBP (5C) | CO₂ acceptor molecule |
| GP (3C) | First stable product of CO₂ fixation |
| GALP (3C) | Reduced sugar – can make glucose |
| ATP | Provides energy for reduction and regeneration |
| Reduced NADP | Supplies H⁺ ions for reduction of GP |
🧩 Overall Summary
6CO₂ + 12NADPH + 18ATP → C₆H₁₂O₆ + 12NADP⁺ + 18ADP + 18Pi + 6H₂O
End Products:
- Glucose (C₆H₁₂O₆) → energy source
- NADP⁺, ADP, Pi → recycled back to light-dependent reactions
⚡ Quick Recap
| Step | Process | Inputs | Outputs |
|---|---|---|---|
| 1 | Carbon fixation | CO₂, RuBP | GP |
| 2 | Reduction | ATP, NADPH | GALP |
| 3 | Regeneration | ATP | RuBP |
| – | Final products | – | Glucose, NADP⁺, ADP, Pi |