• Plants absorb water through their roots and carbon dioxide through their leaves and covert these into carbohydrate molecules, initially in the form of glucose, the waste product is oxygen! handy for us!

• The carbon dioxide in air diffuses into leaves through the stomata, water comes up from the roots via the xylem tubes, oxygen diffuses out and sugars are transported around the plant by the phloem tubes.

  • For more on plant structure & function including gas exchange and leaf adaptations see Part 3

  • and also ....

  • Plant cells, transport, gas exchange, transpiration, leaf and root structure

• Carbon dioxide into leaves and oxygen out of the leaves is an example of gas exchange system on the surface pores (stomata) of the leaves.

• The biochemical process of photosynthesis takes place in the chloroplasts of plant cells in the green leaves and stems with the help of green molecules called chlorophyll.

  • It is the green pigment chlorophyll that absorbs the light energy to power photosynthesis.

• The chemistry of photosynthesis

• Photosynthesis is summarised by the equation:

  •  carbon dioxide + water == light + chlorophyll  ==> glucose + oxygen

  •  6H₂O_(l) + 6CO_2(g) == sunlight ==> C₆H₁₂O_6(aq) + 6O_2(g)

  • This is overall an endothermic chemical reaction, energy is taken in, i.e. sunlight energy is absorbed in the process of photosynthesis.

  • Photosynthesis is the process by which plants make food, initially in the form of glucose, for themselves, and for most animal life, including us too via food chains!

    • The plant will use some of the glucose immediately to fuel all the necessary life maintaining processes.

    • The plant converts some of the glucose to starch - a chemical potential energy food store for the plant and animals like us too!

  • Photosynthesis utilises sunlight energy to convert carbon dioxide and water into glucose (basis of food) and oxygen.

    • Most of the oxygen is a waste gas by-product to plants, but vital for respiration for us and other animals!

    • See simple experiments to show light and carbon dioxide are needed for photosynthesis.

  • The green pigment chlorophyll is in the subcellular structures called chloroplasts, where photosynthesis takes place in green plant cells.

    • All of the photosynthetic chemistry facilitated by enzymes (biological catalysts).

    • The chemistry of photosynthesis is very complicated but it takes place in two main stages.

    • 1. Chlorophyll absorbs a photon of light energy. This sunlight energy (visible light photons) splits water (H₂O) into hydrogen ions (H⁺) and oxygen (O₂).

    • From the plants point of view, the oxygen gas is given out as a waste material in the day except for some used in respiration, but the plant needs oxygen for respiration at night too, when carbon dioxide becomes the waste product.

    • 2. The hydrogen ions combine with carbon dioxide (CO₂) to form glucose molecules (C₆H₁₂O₆).

  • The carbon dioxide diffuses in through the stomata of the guard cells - effectively pores that can open and close i.e. CO₂ in, and oxygen O₂ out in the day and O₂ in at night.

    • In daylight the rate of photosynthesis will exceed the rate of respiration.

    • At night the rate of respiration will exceed that of photosynthesis.

    • Both processes are need to keep the plant alive.

  • During photosynthesis light energy is absorbed by the green chlorophyll, which is found in chloroplasts in some plant cells and algae.

    • Chlorophyll looks green because it absorbs in the violet-blue and orange-red regions of visible light, so plants can absorb use the energy from visible electromagnetic radiation.

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See also states of matter about diffusion in Sections ...

4. What is diffusion? Particle model and examples of demonstrating diffusion in gases

6. Particle model of a liquid and diffusion experiments and associated phenomena

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Key points on the chemical process of photosynthesis

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

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The Chemical Process of Photosynthesis

1. Definition of Photosynthesis

Photosynthesis is a chemical reaction that occurs in green plants and algae, using light energy to convert carbon dioxide and water into glucose  and oxygen (O₂).

It is an endothermic reaction, meaning it absorbs energy.

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2. Word and Symbol Equation

Carbon dioxide + Water → Glucose + Oxygen

Balanced symbol equation:

 6H₂O_(l) + 6CO_2(g) == sunlight ==> C₆H₁₂O_6(aq) + 6O_2(g)

Light energy is absorbed by chlorophyll, a green pigment found in the chloroplasts of plant cells.

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3. Where Does It Happen?  see section (3)

• Organism: Green plants, algae, and some bacteria.

• Cell structure: In the chloroplasts.

• Tissue: Mainly in the palisade mesophyll cells of leaves.

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4. Raw Materials (Reactants)

• Carbon dioxide: Enters the plant from the air via stomata.

• Water: Absorbed by the roots and transported via xylem vessels.

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5. What's Produced? (Products)

• Glucose

  • Used for respiration to release energy.

  • Stored as starch.

  • Converted into cellulose, fats, and proteins.

• Oxygen

  • Released as a by-product through the stomata.

  • Used by plants and animals for aerobic respiration.

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6. Why Is It Endothermic?

Photosynthesis requires energy input to drive the reaction.

• Light energy is absorbed by chlorophyll and converted into chemical energy in glucose.

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7. Importance of Photosynthesis - see section (1)

• Provides the energy and organic molecules for nearly all living things.

• Releases oxygen, essential for most life.

• Removes carbon dioxide from the atmosphere, helping to regulate global climate.

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Exam Tips

• Memorise the word and symbol equations.

• Understand the roles of light, chlorophyll, CO₂, and H₂O.

• Be able to explain why the reaction is endothermic.

• Link photosynthesis to aerobic respiration, plant nutrition, and the carbon cycle.

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Keywords, phrases and learning objectives for this part on photosynthesis

Be able to describe in words and equations the chemical process of photosynthesis?

Know hat is needed for photosynthesis i.e. water carbon dioxide sunlight chlorophyll and what is produced i.e. glucose and the 'waste' product oxygen gas.

Know the importance and role of diffusion in the gas exchanges associated with photosynthesis.

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