Where Exams Make Sense
Premium academic resources for international exam students
www.examslogic.com
Ordinary Level Revision Notes
Leaving Cert Ordinary Level Biology
Chapter 9: Photosynthesis
Food production in plants
Subtopics Covered
- Learning Outcomes
- Big Picture: Why Photosynthesis Matters
- Definition and Equation
- Chloroplast Structure and Pigments
- Leaf Adaptations for Photosynthesis
- Two Stages of Photosynthesis
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- Define photosynthesis as the process by which green plants make food using light energy.
- Identify chloroplasts as the site of photosynthesis.
- Explain the role of chlorophyll and other pigments in light absorption.
- Write the word equation and balanced chemical equation for photosynthesis.
- Describe the light-dependent stage at Ordinary Level: light is absorbed, oxygen is produced, and energy carriers are formed.
- Describe the light-independent stage at Ordinary Level: carbon dioxide is fixed and glucose is produced using energy carriers.
- Explain the roles of carbon dioxide, water, light, chlorophyll, ATP and NADPH in simple terms.
- Understand that detailed cyclic and non-cyclic electron pathways are Higher Level only.
- Interpret basic photosynthesis graphs and experiment results.
Ordinary Level focus: You need the big picture, key substances and correct order. You do not need detailed electron pathways.
2. Big Picture: Why Photosynthesis Matters
Food
Photosynthesis makes glucose, a sugar used as food by plants.
Oxygen
Oxygen is released as a useful by-product during the light-dependent stage.
Energy
Light energy is converted into chemical energy stored in glucose.
Biology link: Photosynthesis stores energy in glucose. Respiration releases energy from glucose. These two processes are connected in ecosystems.
3. Definition and Equation
Photosynthesis is the process by which green plants use light energy to make glucose from carbon dioxide and water.
Carbon dioxide + Water + Light energy → Glucose + Oxygen
6CO2 + 6H2O → C6H12O6 + 6O2
| Substance | Role in photosynthesis | Where it comes from / goes |
|---|---|---|
| Carbon dioxide | Raw material used to make glucose. | Diffuses into leaves through stomata. |
| Water | Raw material; source of hydrogen and oxygen. | Absorbed by roots and carried in xylem. |
| Light | Energy source. | Absorbed by chlorophyll in chloroplasts. |
| Glucose | Main food product. | Used in respiration, stored as starch, or converted to other substances. |
| Oxygen | By-product. | Diffuses out through stomata. |
Exam Tip 1: In definitions, include light energy, carbon dioxide, water and glucose. These are high-value marking points.
Common Mistake: Saying plants “eat from the soil.” Plants absorb minerals and water from soil, but they make glucose by photosynthesis.
4. Chloroplast Structure and Pigments
Photosynthesis happens in chloroplasts. Chloroplasts are found mainly in the cells of leaves, especially palisade mesophyll cells.
| Part | Simple OL function |
|---|---|
| Chloroplast | Organelle where photosynthesis occurs. |
| Chlorophyll | Green pigment that absorbs light energy. |
| Thylakoid membranes | Contain pigments; light is absorbed here. |
| Grana | Stacks of thylakoids, increasing surface area for light absorption. |
| Stroma | Fluid part of the chloroplast where glucose is made during the light-independent stage. |
Examiner Trap 1: Chlorophyll is a pigment, not the whole organelle. The whole organelle is the chloroplast.
5. Leaf Adaptations for Photosynthesis
| Leaf feature | How it helps photosynthesis |
|---|---|
| Broad, flat shape | Large surface area to absorb light. |
| Thin leaf | Short diffusion distance for gases. |
| Transparent upper epidermis | Allows light to reach palisade cells. |
| Palisade mesophyll cells | Contain many chloroplasts for maximum light absorption. |
| Spongy mesophyll air spaces | Allow carbon dioxide and oxygen to diffuse easily. |
| Stomata | Allow carbon dioxide in and oxygen out. |
| Xylem | Transports water to the leaf. |
| Phloem | Transports sugars away from the leaf. |
6. Two Stages of Photosynthesis
Photosynthesis can be divided into two main stages. At Ordinary Level, focus on the inputs and outputs of each stage.
Light-Dependent Stage
- Requires light.
- Occurs on thylakoid membranes in chloroplasts.
- Light energy is absorbed by chlorophyll.
- Water is split.
- Oxygen is released.
- ATP and NADPH are formed as energy carriers.
Light-Independent Stage
- Does not directly require light.
- Occurs in the stroma of chloroplasts.
- Carbon dioxide is fixed.
- ATP and NADPH provide energy and hydrogen.
- Glucose is produced.
HL Only — not needed for OL: detailed cyclic electron flow, non-cyclic electron flow, named carriers, and detailed Calvin cycle steps.
7. Light-Dependent Stage
The light-dependent stage uses light energy. The plant absorbs light using chlorophyll in the thylakoid membranes.
Light absorbed→Water split→Oxygen released→ATP + NADPH formed
| Feature | Ordinary Level explanation |
|---|---|
| Needs light? | Yes. |
| Where? | Thylakoid membranes / grana of chloroplasts. |
| Input | Light energy and water. |
| Output | Oxygen, ATP and NADPH. |
| Importance | Provides energy carriers needed for the dark stage. |
Examiner Trap 2: The oxygen released in photosynthesis comes from water splitting, not from carbon dioxide.
8. Light-Independent Stage / Dark Stage
The light-independent stage is often called the dark stage. This does not mean it only happens at night. It means light is not used directly in this stage.
Carbon dioxide enters→Carbon fixation→ATP + NADPH used→Glucose made
| Feature | Ordinary Level explanation |
|---|---|
| Needs light directly? | No, but it depends on ATP and NADPH from the light stage. |
| Where? | Stroma of the chloroplast. |
| Input | Carbon dioxide, ATP and NADPH. |
| Output | Glucose. |
| Meaning of carbon fixation | Carbon dioxide is built into an organic molecule. |
Examiner Secret 1: A strong OL answer says: light stage makes ATP and NADPH; dark stage uses ATP and NADPH to make glucose from carbon dioxide.
9. Photosynthesis Full Overview
10. ATP and NADPH in Photosynthesis
ATP
ATP is an energy carrier. It transfers energy from the light-dependent stage to the light-independent stage.
Energy transferMade in light stageUsed in dark stageNADPH
NADPH carries hydrogen and high-energy electrons. At OL, remember it helps convert carbon dioxide into glucose.
Hydrogen carrierMade in light stageUsed to make glucoseExam Tip 2: ATP and NADPH are not final food products. They are temporary carriers used to help make glucose.
11. Factors Affecting the Rate of Photosynthesis
The rate of photosynthesis can be limited by whichever required factor is in shortest supply.
| Factor | Effect | Explanation |
|---|---|---|
| Light intensity | Increasing light usually increases photosynthesis up to a limit. | More light energy is available for the light-dependent stage. |
| Carbon dioxide concentration | Increasing CO2 usually increases photosynthesis up to a limit. | More raw material is available for glucose formation. |
| Temperature | Rate rises to an optimum, then falls if enzymes denature. | Photosynthesis uses enzyme-controlled reactions. |
| Water availability | Lack of water reduces photosynthesis. | Stomata may close and water is a raw material. |
| Chlorophyll | Less chlorophyll reduces light absorption. | Less light energy can be captured. |
Examiner Trap 3: A graph that levels off does not mean photosynthesis has stopped. It means another factor is now limiting the rate.
12. Testing a Leaf for Starch
Glucose made by photosynthesis is often stored as starch. Testing for starch shows that photosynthesis has taken place.
| Step | Purpose |
|---|---|
| Boil the leaf in water. | Kills the leaf and stops reactions. |
| Place leaf in hot alcohol. | Removes chlorophyll so the colour change can be seen. |
| Rinse in warm water. | Softens the leaf. |
| Add iodine solution. | Tests for starch. |
| Blue-black colour appears. | Starch is present. |
Common Mistake 2: Forgetting that alcohol is flammable. In school practicals, alcohol is heated safely using a water bath, not directly over a flame.
13. Experiments Linked to Photosynthesis
| Question | Experiment idea | Expected result |
|---|---|---|
| Is light needed? | Cover part of a destarched leaf with black paper, expose to light, test for starch. | Only uncovered part turns blue-black. |
| Is carbon dioxide needed? | Use a plant with CO2 absorbed by potassium hydroxide/soda lime, then test leaf for starch. | Leaf without CO2 does not make starch. |
| Is chlorophyll needed? | Use a variegated leaf and test for starch after light exposure. | Only green parts turn blue-black. |
| Is oxygen produced? | Use pondweed in light and collect gas bubbles. | Gas relights a glowing splint if enough oxygen is collected. |
Exam Tip 3: Practical answers should include the variable being tested, the control or comparison, and the result.
14. Interactive Simulator: Photosynthesis Stage Detective
Choose the Stage
Clue: Oxygen is produced when water is split.
Feedback
Ready
Look for oxygen, light, ATP, NADPH, carbon dioxide or glucose.
15. Interactive Simulator: Limiting Factor Predictor
Adjust the Conditions
Predicted Rate
Rate: medium
The limiting factor is the condition in shortest supply.
16. High-Value Exam Guidance
Exam Tip 1: Learn the word equation and balanced equation.
Exam Tip 2: State that chlorophyll absorbs light energy.
Exam Tip 3: Mention that glucose can be stored as starch.
Examiner Trap 1: The dark stage does not mean photosynthesis happens only in darkness.
Examiner Trap 2: Oxygen released comes from water, not carbon dioxide.
Examiner Trap 3: ATP and NADPH are energy carriers, not the final food made.
Common Mistake 1: Writing respiration equations instead of photosynthesis equations.
Common Mistake 2: Saying chloroplasts are found in animal cells.
Examiner Secret 1: For stage questions, oxygen points to the light stage; glucose points to the dark stage.
Examiner Secret 2: In graph questions, use the phrase “limiting factor” when the rate stops increasing.
17. MCQs with Instant Answers
1. Photosynthesis mainly occurs in:
Answer: B. Chloroplasts are the sites of photosynthesis.
2. Chlorophyll is important because it:
Answer: A. Chlorophyll absorbs light energy for photosynthesis.
3. Oxygen is produced during the:
Answer: A. Oxygen is released during the light-dependent stage.
4. Carbon dioxide is fixed during the:
Answer: A. Carbon dioxide is used to make glucose in the light-independent stage.
5. A blue-black colour with iodine shows the presence of:
Answer: C. Iodine turns blue-black when starch is present.
6. The main product of photosynthesis used as food is:
Answer: A. Glucose is the food made during photosynthesis.
18. Structured Questions
- Define photosynthesis. [3]
- Write the word equation for photosynthesis. [5]
- Name the organelle where photosynthesis occurs and state the role of chlorophyll. [4]
- Describe the light-dependent stage at Ordinary Level. [5]
- Describe the light-independent stage at Ordinary Level. [5]
- Explain how a leaf is tested for starch. [7]
- Explain what is meant by a limiting factor in photosynthesis. [4]
- Compare the light-dependent and light-independent stages using three matched points. [6]
19. Mark Scheme
Q1. Process by which green plants make glucose/food [1] from carbon dioxide and water [1] using light energy/chlorophyll [1].
Q2. Carbon dioxide [1] + water [1] + light energy [1] -> glucose [1] + oxygen [1].
Q3. Chloroplast [1]. Chlorophyll is a green pigment [1] that absorbs light energy [1] for photosynthesis [1].
Q4. Requires light [1]. Occurs in chloroplast/thylakoids [1]. Light is absorbed by chlorophyll [1]. Water is split and oxygen is produced [1]. ATP and NADPH are formed [1].
Q5. Does not directly require light [1]. Occurs in stroma/chloroplast [1]. Carbon dioxide is fixed/used [1]. ATP and NADPH are used [1]. Glucose is produced [1].
Q6. Boil leaf in water [1]. Heat in alcohol/water bath [1] to remove chlorophyll [1]. Rinse in warm water [1]. Add iodine [1]. Blue-black colour [1] shows starch present [1].
Q7. A factor in shortest supply [1] that prevents the rate from increasing [1]. Example: light intensity/CO2/temperature [1]. When it is increased, rate may increase until another factor becomes limiting [1].
Q8. Light stage requires light [1], dark stage does not directly require light [1]. Light stage produces oxygen [1], dark stage produces glucose [1]. Light stage makes ATP/NADPH [1], dark stage uses ATP/NADPH [1].
Q2. Carbon dioxide [1] + water [1] + light energy [1] -> glucose [1] + oxygen [1].
Q3. Chloroplast [1]. Chlorophyll is a green pigment [1] that absorbs light energy [1] for photosynthesis [1].
Q4. Requires light [1]. Occurs in chloroplast/thylakoids [1]. Light is absorbed by chlorophyll [1]. Water is split and oxygen is produced [1]. ATP and NADPH are formed [1].
Q5. Does not directly require light [1]. Occurs in stroma/chloroplast [1]. Carbon dioxide is fixed/used [1]. ATP and NADPH are used [1]. Glucose is produced [1].
Q6. Boil leaf in water [1]. Heat in alcohol/water bath [1] to remove chlorophyll [1]. Rinse in warm water [1]. Add iodine [1]. Blue-black colour [1] shows starch present [1].
Q7. A factor in shortest supply [1] that prevents the rate from increasing [1]. Example: light intensity/CO2/temperature [1]. When it is increased, rate may increase until another factor becomes limiting [1].
Q8. Light stage requires light [1], dark stage does not directly require light [1]. Light stage produces oxygen [1], dark stage produces glucose [1]. Light stage makes ATP/NADPH [1], dark stage uses ATP/NADPH [1].
20. Mastery Checklist
- I can define photosynthesis accurately.
- I can write the word equation for photosynthesis.
- I can identify chloroplasts as the site of photosynthesis.
- I can explain the role of chlorophyll.
- I can describe the light-dependent stage at OL standard.
- I can describe the light-independent stage at OL standard.
- I know that detailed electron pathways are HL only.
- I can explain the role of ATP and NADPH simply.
- I can describe the starch test for a leaf.
- I can explain limiting factor graphs.
- I can answer exam questions using correct biological terms.