Photosynthesis

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Lesson notes

What is Photosynthesis?

Photosynthesis is the process by which green plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into glucose and oxygen.
It occurs in chloroplasts, which contain the green pigment chlorophyll that absorbs light energy.
The energy from the sun is transformed into chemical potential energy stored in glucose.
Photosynthesis is essential for life on Earth because it provides food (glucose) and oxygen for other organisms.

Diagram showing the inputs and outputs of photosynthesis in a plant

Requirements for Photosynthesis

Light energy from the sun (radiant energy).
Carbon dioxide from the air.
Water from the soil.
Chlorophyll (the green pigment) and chloroplasts in plant cells.

Products of Photosynthesis

Glucose (a sugar) is the main product, used for energy and building materials.
Oxygen is released as a byproduct.
Glucose can be converted into starch for storage (insoluble in water).
Glucose can also be converted into cellulose for structural support.

The Chemical Equation

The simplified equation is: carbon dioxide + water → (in presence of chlorophyll and sunlight) glucose + oxygen.
The balanced chemical equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
This equation shows the transformation of matter (CO₂ and H₂O) into glucose and oxygen using light energy.

The chemical equation for photosynthesis, showing light energy converted to chemical energy stored in glucose

Glucose Storage and Use

Plants use glucose for respiration to release energy for life processes.
Excess glucose is stored as starch in leaves, stems, roots, and fruits.
Starch is insoluble in water, making it suitable for storage.
Glucose is also converted into other compounds like proteins, oils, and pigments.

Testing for Starch

The iodine test is used to detect starch: iodine solution turns from brownish-orange to dark blue-black in the presence of starch.
To test a leaf, first remove chlorophyll by boiling in alcohol, then add iodine solution.
A positive result (blue-black color) indicates starch is present, showing photosynthesis occurred.
Leaves kept in the dark for 24-48 hours show little or no starch, demonstrating that light is needed for photosynthesis.

Iodine test for starch showing positive (dark blue-black) and negative (brownish-orange) results

Historical Discoveries

Jan Baptista van Helmont (1600s) showed that a tree's mass comes from water, not soil, but didn't know about carbon dioxide.
Joseph Priestley (1770s) showed that plants can 'fix' air that candles or mice have 'injured', indicating oxygen production.
Jan Ingenhousz (1779) showed that plants produce oxygen only in sunlight, linking light to photosynthesis.

Importance of Photosynthesis for Life

Photosynthesis is the basis of most food chains, as plants are producers.
It releases oxygen into the atmosphere, essential for aerobic respiration.
Without photosynthesis, life on Earth would not survive, as energy from the sun would not be captured into food.
All organisms ultimately depend on the sun's energy, either directly (plants) or indirectly (herbivores and carnivores).

Flow diagram showing how energy from the sun is captured by wheat through photosynthesis and transferred to humans through bread.

Photosynthesis vs. Respiration

Photosynthesis uses carbon dioxide and water, produces glucose and oxygen, and requires light energy.
Respiration uses glucose and oxygen, produces carbon dioxide and water, and releases energy.
Photosynthesis occurs only in plants, algae, and some bacteria; respiration occurs in all living organisms.
Photosynthesis happens mainly in daylight; respiration happens all the time (day and night).

Experiments and Investigations

To show that light is needed for photosynthesis, compare leaves from plants kept in light vs. dark using the starch test.
Variegated leaves (with green and white parts) can show that only green parts photosynthesize because they contain chlorophyll.
The lima bean experiment demonstrates that plants need light energy to grow; plants with light grow tall, while those without light die.
In the yeast experiment, carbon dioxide is produced by yeast and sugar, which can be detected by limewater turning milky.

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Practice questions

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1.What is the name of the green pigment that absorbs light energy during photosynthesis?
Easy
- Achlorophyll
- Bchloroplast
- Cglucose
- Dstarch
2.Which of the following are raw materials (inputs) for photosynthesis?
Easy
- Acarbon dioxide and water
- Boxygen and glucose
- Csunlight and oxygen
- Dglucose and water
3.Photosynthesis converts radiant energy from the Sun into chemical potential energy stored in glucose.
Easy
True or false?
4.In which part of a plant cell does photosynthesis take place?
Easy
- Achloroplast
- Bnucleus
- Ccell wall
- Dvacuole
5.Plants only photosynthesise during the day, but they respire all the time.
Easy
True or false?
6.Why do plants convert glucose into starch?
Easy
- AStarch is soluble in water and can be transported easily.
- BStarch is insoluble and can be stored without affecting water balance.
- CStarch provides more energy than glucose.
- DStarch is used directly in respiration.
7.A plant is kept in a dark cupboard for 48 hours. When a leaf is tested with iodine solution, what colour would you expect and why?
Easy
- ABlue-black; starch is present because photosynthesis continued in the dark.
- BBrown-orange; no starch is present because photosynthesis could not occur without light.
- CGreen; chlorophyll is still present.
- DClear; the leaf has no starch.
8.Match each term with its correct description.
Medium
- Chlorophyll
- Glucose
- Starch
- Green pigment that absorbs light
- Soluble sugar produced in photosynthesis
- Insoluble storage form of glucose

Revision notes

## Photosynthesis: Night-Before Exam Notes ### What is Photosynthesis? - Process where plants, algae, and cyanobacteria convert light energy (sunlight) into chemical energy stored in glucose. - Releases oxygen as a byproduct (oxygenic photosynthesis). - Opposite of cellular respiration: photosynthesis uses CO₂ and H₂O to make glucose; respiration breaks glucose to release energy. ### Key Equation (simplified) 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ (glucose) + 6O₂ ### Where Does It Happen? - In plant cells: **chloroplasts** (contain chlorophyll, the green pigment that absorbs red and blue light, reflects green). - Chlorophyll is found in leaf cells. ### Two Main Stages 1. **Light-Dependent Reactions** (need light) - Light energy splits water (H₂O) → releases O₂, H⁺, and electrons. - Energy is used to make ATP and NADPH (energy carriers). 2. **Light-Independent Reactions (Calvin Cycle)** (do not need light directly) - Uses ATP and NADPH from stage 1. - Fixes CO₂ from air into organic molecules (carbon fixation). - Produces glucose and other carbohydrates. ### Why Is Photosynthesis Important? - Provides food (glucose) for plants and, indirectly, for animals. - Produces most of the oxygen in Earth's atmosphere. - Stores energy from the sun that powers nearly all life. - Removes CO₂ from the atmosphere. ### Key Facts to Remember - Inputs: carbon dioxide, water, sunlight. - Outputs: glucose (sugar), oxygen. - Plants are **photoautotrophs** – they make their own food using light. - Photosynthesis is an **endothermic** reaction (needs energy input). - Global photosynthesis captures ~130 terawatts of energy and converts ~100 billion tons of carbon into biomass per year. ### Quick Comparison | Process | Inputs | Outputs | |---------|--------|--------| | Photosynthesis | CO₂, H₂O, light | Glucose, O₂ | | Cellular Respiration | Glucose, O₂ | CO₂, H₂O, energy |

Photosynthesis: The process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose, using carbon dioxide and water, and releasing oxygen.
Chloroplast: Organelle in plant cells where photosynthesis occurs; contains chlorophyll.
Chlorophyll: Green pigment in chloroplasts that absorbs light energy (mainly red and blue) for photosynthesis.
Glucose: A simple sugar (C₆H₁₂O₆) produced by photosynthesis; used as an energy source by organisms.
Light-dependent reactions: First stage of photosynthesis; requires light, splits water, produces ATP, NADPH, and oxygen.
Calvin cycle: Light-independent reactions of photosynthesis; uses ATP and NADPH to convert CO₂ into glucose.
Carbon fixation: The process of incorporating carbon dioxide into organic molecules, such as during the Calvin cycle.
Photoautotroph: An organism that uses light energy to synthesize food from carbon dioxide and water (e.g., plants, algae).
Cellular respiration: The process that breaks down glucose to release energy, using oxygen and producing carbon dioxide and water; opposite of photosynthesis.

Flashcards

What is photosynthesis?

Photosynthesis is a process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy stored in sugars like glucose.

What are the inputs (reactants) of photosynthesis?

The inputs are carbon dioxide (CO₂), water (H₂O), and light energy (usually from sunlight).

What are the outputs (products) of photosynthesis?

The outputs are glucose (a sugar) and oxygen (O₂).

Write the balanced chemical equation for photosynthesis.

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

In which organelle does photosynthesis occur in plants?

Photosynthesis occurs in the chloroplasts, which contain chlorophyll.

What is chlorophyll and what does it do?

Chlorophyll is a green pigment in chloroplasts that absorbs light energy, mainly from the red and blue parts of the spectrum, and reflects green light.

Why do leaves appear green?

Leaves appear green because chlorophyll reflects green light and absorbs red and blue light.

What are the two main stages of photosynthesis?

The two stages are the light-dependent reactions and the light-independent reactions (Calvin cycle).

What happens during the light-dependent reactions?

Light energy is absorbed by chlorophyll, water is split (releasing oxygen), and energy is stored in ATP and NADPH.

What happens during the Calvin cycle (light-independent reactions)?

Carbon dioxide is fixed into organic molecules using ATP and NADPH, producing glucose.

What is the role of water in photosynthesis?

Water is split during the light-dependent reactions, providing electrons and hydrogen ions, and releasing oxygen as a byproduct.

What is the role of carbon dioxide in photosynthesis?

Carbon dioxide is used in the Calvin cycle to build glucose molecules through carbon fixation.

What is the energy currency molecule produced during the light-dependent reactions?

ATP (adenosine triphosphate) is produced and used to power the Calvin cycle.

What is NADPH and why is it important?

NADPH is a carrier molecule that stores high-energy electrons and hydrogen; it is used in the Calvin cycle to help build glucose.

How does photosynthesis relate to cellular respiration?

Photosynthesis produces glucose and oxygen, which are used in cellular respiration to release energy, producing carbon dioxide and water.

Why is photosynthesis important for life on Earth?

It produces oxygen for the atmosphere and provides the organic compounds that serve as food for nearly all living things.

What is a photoautotroph?

A photoautotroph is an organism that uses light energy to synthesize its own food from carbon dioxide and water (e.g., plants, algae, cyanobacteria).

What is oxygenic photosynthesis?

Oxygenic photosynthesis is the type of photosynthesis that produces oxygen as a byproduct; it is performed by plants, algae, and cyanobacteria.

What is anoxygenic photosynthesis?

Anoxygenic photosynthesis does not produce oxygen; some bacteria use hydrogen sulfide instead of water, releasing sulfur.

What is carbon fixation?

Carbon fixation is the process of converting inorganic carbon dioxide into organic compounds like glucose during the Calvin cycle.

Where do the light-dependent reactions take place?

They take place in the thylakoid membranes inside the chloroplast.

Where does the Calvin cycle occur?

The Calvin cycle occurs in the stroma, the fluid-filled space around the thylakoids in the chloroplast.

What is the main pigment that captures light energy in plants?

Chlorophyll a is the primary pigment, but accessory pigments like chlorophyll b and carotenoids also help capture light.

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