CIE iGCSE Biology-6.1 Photosynthesis- Study Notes- New Syllabus
CIE iGCSE Biology-6.1 Photosynthesis- Study Notes – New syllabus
CIE iGCSE Biology-6.1 Photosynthesis- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe photosynthesis as the process by which plants synthesise carbohydrates from raw materials using energy from light
- State the word equation for photosynthesis as: carbon dioxide + water → glucose + oxygen in the presence of light and chlorophyll
- State that chlorophyll is a green pigment that is found in chloroplasts
- State that chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates
- Outline the subsequent use and storage of the
carbohydrates made in photosynthesis, limited to:
(a) starch as an energy store
(b) cellulose to build cell walls
(c) glucose used in respiration to provide energy
(d) sucrose for transport in the phloem
(e) nectar to attract insects for pollination - Explain the importance of:
(a) nitrate ions for making amino acids
(b) magnesium ions for making chlorophyll - Investigate the need for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls
- Investigate and describe the effects of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis
- Investigate and describe the effect of light and dark conditions on gas exchange in an aquatic plant using hydrogencarbonate indicator solution
Supplement
- State the balanced chemical equation for photosynthesis as:
6CO2 + 6H2O → C6H12O6 + 6O2 - Identify and explain the limiting factors of photosynthesis in different environmental conditions
What Is Photosynthesis?
📘 Core Idea:
Photosynthesis is the process by which green plants make their own food using light energy. It mainly takes place in the leaves of the plant.
Key Definition:
Photosynthesis is the process by which plants synthesise carbohydrates (like glucose) from carbon dioxide and water, using energy from sunlight.
🌞 Where Does the Energy Come From?
Light energy is absorbed by chlorophyll, a green pigment found in chloroplasts. This energy drives the chemical reaction to make glucose.
Raw Materials Needed:
| Raw Material | Source |
|---|---|
| Carbon dioxide | From the air, enters through stomata |
| Water | From the soil, absorbed by roots |
| Light energy | From the sun |
🧪 End Products of Photosynthesis:
- Glucose – used for energy or stored as starch
- Oxygen – released as a by-product into the air
📌 Word Equation for Photosynthesis:
Carbon dioxide + Water → Glucose + Oxygen
(in the presence of light energy and chlorophyll)
Photosynthesis – Word Equation & Explanation
Word Equation for Photosynthesis:
Carbon dioxide + Water → Glucose + Oxygen
(in the presence of light and chlorophyll)
🔍 Understanding Each Part of the Equation
| Component | Function in Photosynthesis |
|---|---|
| Carbon dioxide | Enters through stomata from the air |
| Water | Absorbed by roots and carried to leaves via xylem |
| Light energy | Provided by the sun, absorbed by chlorophyll |
| Chlorophyll | Green pigment in chloroplasts that captures light energy |
| Glucose | Used for energy, growth, or stored as starch |
| Oxygen | Released into the air through stomata as a by-product |
Where Does It Occur?
- In the leaves of green plants
- Inside chloroplasts, which contain chlorophyll
🧪 Why Is This Equation Important?
- Shows how plants make food using natural raw materials
- Highlights the role of light and chlorophyll in driving the process
- Explains why plants release oxygen, which is vital for life
📌 Key Points to Remember
- Photosynthesis is light-dependent
- Needs chlorophyll (green pigment)
- Produces glucose (food) and oxygen (waste gas)
- Occurs in chloroplasts inside leaf cells
Chlorophyll – The Green Pigment of Life
📘 What Is Chlorophyll?
- Chlorophyll is the substance that gives plants their green colour.
- It is a pigment – it absorbs light (especially red and blue wavelengths) and reflects green.
- It is found in specialised structures called chloroplasts, located in the cells of green parts of the plant.
🧪 Where Is Chlorophyll Found?
- In chloroplasts, found inside green plant cells
- Especially in palisade mesophyll cells (top layer of leaves)
- Also in guard cells (around stomata)
- Inside chloroplasts, chlorophyll is stored in thylakoid membranes
☀️ What Does Chlorophyll Do?
- Chlorophyll is essential for photosynthesis. It captures light energy from the sun and uses it to make glucose from carbon dioxide and water.
- Without chlorophyll, plants couldn’t make food – and life on Earth wouldn’t exist as we know it.
Summary:
| Term | Description |
|---|---|
| Chlorophyll | Green pigment that absorbs light |
| Location | Inside chloroplasts in leaf cells |
| Function | Traps light for photosynthesis (makes food for the plant) |
| Why green? | Reflects green light and absorbs red & blue light |
💡 Fun Analogy:
Think of chloroplasts as solar panels and chlorophyll as the green coating that captures sunlight.
Just like solar panels convert sunlight into electricity, chlorophyll converts sunlight into plant food (glucose).
Role of Chlorophyll in Photosynthesis
🌞 What Is Chlorophyll?
Chlorophyll is a green pigment found in chloroplasts, mainly in leaf cells. It absorbs light energy from the sun – especially red and blue wavelengths and is essential for photosynthesis.
🔬 What Does Chlorophyll Actually Do?
Chlorophyll plays a central role in converting light energy into a usable form of energy for the plant.
- Captures light energy from sunlight
- Converts this energy into chemical energy
- Chemical energy is used to combine carbon dioxide and water into glucose
- Glucose is then used for respiration, growth, or stored as starch
🧪 Summary Table
| Feature | Chlorophyll’s Role |
|---|---|
| Found in | Chloroplasts (mainly in leaf cells) |
| Absorbs | Light energy from the sun |
| Converts | Light energy → chemical energy |
| Used for | Making carbohydrates (e.g. glucose) from CO₂ and H₂O |
What Happens to the Carbohydrates Made in Photosynthesis?
📘 Core Idea:
The glucose produced during photosynthesis can be converted into other useful forms depending on the plant’s needs. These products are used for energy, growth, storage, transport, and even pollination.
(a) Starch – Storage Form of Energy
💡Analogy:
Starch is the plant’s “savings account” of energy – stored away for future use.
Some glucose is joined together to form starch, a large insoluble carbohydrate.
Starch is stored:
- Inside chloroplasts (short-term)
- In roots, seeds, and tubers (long-term)
Because starch is insoluble, it does not affect osmosis – making it safe for storage.
(b) Cellulose – For Building Cell Walls
Glucose can also be rearranged into cellulose, a tough, fibrous carbohydrate.
It forms the rigid outer wall of plant cells, giving each cell:
- Strength
- Structure
- Protection
(c) Glucose – For Respiration
⚡Note:
Glucose is the plant’s “fuel” – instantly usable energy for daily work.
Glucose is broken down in cellular respiration to release energy (ATP).
This energy is used for:
- Growth
- Cell division
- Active transport
- Repair
(d) Sucrose – For Transport
Glucose is converted into sucrose, a soluble sugar for easy transport.
Sucrose travels through the phloem to:
- Growing regions (buds, new leaves)
- Storage tissues (roots, fruits)
(e) Nectar – To Attract Pollinators
Some glucose is used to make nectar, a sugary liquid secreted by flowers. Nectar attracts insects like bees and butterflies, helping with pollination.
🌸 Pollination Helper: Nectar is the plant’s “sweet deal” to attract pollinators.
📌 Summary Table
| Carbohydrate Form | Function | Where/How Used |
|---|---|---|
| Starch | Energy storage | Roots, leaves, seeds, tubers |
| Cellulose | Structure | Builds strong plant cell walls |
| Glucose | Energy release | Used in respiration by all living cells |
| Sucrose | Transport | Moved in phloem to growing/storage areas |
| Nectar | Pollination | Secreted by flowers to attract insects |
Importance of Mineral Ions in Plants
Plants need mineral ions from the soil to stay healthy and grow properly. Two essential ions are nitrate and magnesium.
(a) Nitrate Ions – For Making Amino Acids
Nitrate ions (NO₃⁻) are absorbed by plant roots from the soil. They are essential for making amino acids, which are used to build proteins.
🌿 In Short:
No nitrate = no protein = poor growth
- Proteins are needed for growth (new cells), repair, and enzyme production.
⚠️ Nitrate Deficiency:
A plant without enough nitrate:
- Can’t make proteins → stunted growth
- Leaves turn pale yellow
(b) Magnesium Ions – For Making Chlorophyll
Magnesium ions (Mg²⁺) are needed to make chlorophyll – the green pigment that absorbs light energy for photosynthesis.
🌞 In Short:
No magnesium = no chlorophyll = no food
- Without magnesium, plants can’t make chlorophyll → photosynthesis slows down
- This affects glucose production and energy for growth
⚠️ Magnesium Deficiency:
A plant without enough magnesium:
- Shows yellowing between veins (called chlorosis)
- Grows slowly due to less glucose and energy
📌 Summary Table
| Mineral Ion | Use in Plant | Deficiency Symptom |
|---|---|---|
| Nitrate (NO₃⁻) | Makes amino acids → proteins | Stunted growth, yellow leaves |
| Magnesium (Mg²⁺) | Makes chlorophyll → photosynthesis | Yellowing between leaf veins |
Investigating the Need for Chlorophyll, Light, and Carbon Dioxide in Photosynthesis
General Strategy:
- We test whether starch is formed (a product of photosynthesis) using iodine solution.
- If no starch is present, it means photosynthesis did not occur.
🧪 1. Testing the Need for Chlorophyll
Method:
- Use a variegated leaf (with green and white areas)
- Expose plant to sunlight for several hours
- Boil leaf in water (kills cells)
- Boil in ethanol (removes chlorophyll)
- Rinse in warm water
- Add iodine solution
Observation: Only green areas turn blue-black (starch present), white areas remain unchanged.
Conclusion: Chlorophyll is needed for photosynthesis. Only green areas (with chlorophyll) produce starch.
🔦 2. Testing the Need for Light
Method:
- Cover part of a green leaf with black paper or foil
- Expose to sunlight for a few hours
- Remove the leaf and test with iodine
Observation: Exposed areas turn blue-black; covered areas remain brown/yellow.
Conclusion: Light is necessary for photosynthesis to occur.
🫧 3. Testing the Need for Carbon Dioxide
Method:
- Place a plant in a sealed bell jar with sodium hydroxide (NaOH) to absorb CO₂
- Set up another jar with water as a control
- Expose both to sunlight for several hours
- Test leaves from both jars using iodine
Observation:
- Leaf from jar with NaOH: No starch (no colour change)
- Leaf from control jar: Turns blue-black (starch present)
Conclusion: Carbon dioxide is essential for photosynthesis.
📌 Summary Table
| Factor Tested | How It’s Tested | Control/Setup | What Shows Photosynthesis? |
|---|---|---|---|
| Chlorophyll | Use variegated leaf, test for starch | Green vs. white areas | Only green areas turn blue-black |
| Light | Block part of leaf from light | Exposed vs. covered areas | Only light-exposed area turns black |
| Carbon dioxide | Remove CO₂ using NaOH in bell jar | Compare with normal setup | No CO₂ = no starch = no photosynthesis |
Investigating Factors Affecting Photosynthesis
🔍 What Is Being Measured?
The rate of photosynthesis is estimated by measuring how quickly a plant produces oxygen bubbles in water.
🔦 1. Effect of Light Intensity
Method:
- Place a lamp at different distances from the plant in water.
- Count the number of oxygen bubbles released per minute.
- Keep carbon dioxide level and temperature constant.
Observation: More light → more bubbles (faster photosynthesis), up to a limit.
Conclusion: Light intensity increases the rate of photosynthesis until another factor becomes limiting.
🌬️ 2. Effect of Carbon Dioxide Concentration
Method:
- Add different amounts of sodium hydrogen carbonate (NaHCO₃) to the water to vary CO₂ levels.
- Keep temperature and light constant.
- Count the number of bubbles released per minute.
Observation: More CO₂ → more photosynthesis, up to a certain level.
Conclusion: Increased CO₂ speeds up photosynthesis until another factor (e.g. light) limits the rate.
🌡️ 3. Effect of Temperature
Method:
- Place the setup in warm or cool water baths or controlled-temperature rooms.
- Keep light and CO₂ constant.
- Measure the rate of bubble production.
Observation: Rate increases with temperature up to 30–35°C, then drops due to denaturation.
Conclusion: Temperature affects enzyme activity. Too low = slow. Too high = denaturation.
📊 Summary Table
| Factor | How It’s Changed | Effect on Photosynthesis |
|---|---|---|
| Light intensity | Move lamp closer/farther | More light = faster rate, until a plateau |
| CO₂ concentration | Add sodium hydrogen carbonate | More CO₂ = faster rate, until limited by another factor |
| Temperature | Use warm/cool water or thermostatic bath | Rate rises to optimum, then falls due to denaturation |
Investigating Gas Exchange in Plants Using Hydrogencarbonate Indicator
🧪 Key Idea:
Plants carry out both photosynthesis and respiration, which involve the exchange of gases like oxygen and carbon dioxide. Light affects the balance between the two processes.
🌡️ What Is Hydrogencarbonate Indicator?
Hydrogencarbonate indicator is a pH indicator that changes colour based on the level of carbon dioxide in water.
| Colour | CO₂ Level | Condition in Water |
|---|---|---|
| Red/Orange | Normal atmospheric CO₂ | Neutral |
| Yellow | High CO₂ | More acidic → More respiration |
| Purple | Low CO₂ | More alkaline → More photosynthesis |
Method (Light vs Dark)
- Place equal-sized aquatic plants (e.g. Elodea) into test tubes with hydrogencarbonate indicator.
- Cover one tube with foil (dark); leave the other uncovered in light.
- Leave the setup for 1–2 hours.
- Observe and record the indicator colour in each tube.
Observations
| Condition | Colour Change | Explanation |
|---|---|---|
| Light | Purple | Photosynthesis > Respiration → CO₂ absorbed |
| Dark | Yellow | Only respiration occurs → CO₂ released |
Conclusions:
In light, plants absorb CO₂ for photosynthesis → CO₂ levels fall → indicator turns purple.
In dark, plants still respire but can’t photosynthesise → CO₂ builds up → indicator turns yellow.
📌 Summary
| Condition | What Happens | Indicator Colour |
|---|---|---|
| Light | CO₂ used up in photosynthesis | Purple (low CO₂) |
| Dark | CO₂ released by respiration only | Yellow (high CO₂) |
Balanced Chemical Equation for Photosynthesis
Balanced Chemical Equation:
\[
6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2
\]
(in the presence of light and chlorophyll)
🔍 What Does Each Part Mean?
| Substance | Role in Photosynthesis |
|---|---|
| 6CO₂ (Carbon dioxide) | Enters the leaf from the air through stomata |
| 6H₂O (Water) | Absorbed by the roots from the soil |
| Light energy | Trapped by chlorophyll in the chloroplasts |
| C₆H₁₂O₆ (Glucose) | Used for energy, growth, or stored as starch |
| 6O₂ (Oxygen) | Released as a waste gas through stomata |
🌞 Conditions Needed
- Light – energy source for the reaction
- Chlorophyll – green pigment that traps light energy
- Occurs in chloroplasts – found in green leaf cells
Key Features of the Equation:
- It is balanced – same number of atoms on both sides
- Shows reactants (CO₂ + H₂O) and products (glucose + oxygen)
- Demonstrates energy conversion: light energy → chemical energy in glucose
Limiting Factors of Photosynthesis
📘 What Is a Limiting Factor?
A limiting factor is something that slows down or limits the rate of photosynthesis when it is in short supply. Even if all other conditions are perfect, photosynthesis can only go as fast as the most limiting factor allows.
The 3 Main Limiting Factors:
| Factor | Why It Matters |
|---|---|
| Light intensity | Provides energy for the reaction. More light = faster rate (up to a point). |
| Carbon dioxide (CO₂) | Raw material for making glucose. More CO₂ = faster rate (until saturation). |
| Temperature | Affects enzyme activity. Too cold = slow enzymes. Too hot = enzymes denature. |
🔍 How Limiting Factors Change in Different Conditions
1. Light Intensity:
In low light (e.g., cloudy days or underwater), photosynthesis is slow. As light increases, the rate increases — but only up to a certain point. After that, another factor becomes limiting.
2. Carbon Dioxide Concentration:
More CO₂ leads to more glucose production, until saturation. In greenhouses, CO₂ is added to boost plant growth.
3. Temperature:
Affects the enzymes driving photosynthesis. Too low → slow rate. Too high → enzymes denature, and the rate drops.
Example: On very hot days, photosynthesis may slow due to enzyme damage despite high light and CO₂.
📊 Combined Limiting Effects
Sometimes, more than one factor is low, but only the most limiting one matters at that time.
Example Scenario:
CO₂ = low
Light = high
Temp = ideal
🡺 CO₂ is the limiting factor – increasing light won’t help until CO₂ is increased.
📌 Summary Table
| Factor | Low Level Effect | High Level Effect |
|---|---|---|
| Light intensity | Slows photosynthesis (less energy) | Faster rate until plateau |
| CO₂ concentration | Less glucose made | Faster photosynthesis until limited by another factor |
| Temperature | Enzymes too slow → low rate | Optimum = fast; too hot = denatured enzymes |