Ages 13–16 · Key Stage 3 to Sixth form · Science · 6 minutes
Photosynthesis and Respiration Explained
How do photosynthesis and cellular respiration connect? Follow Joseph Priestley's mint experiment, track carbon from air into trees, and discover why plants respire too. Includes a worked gas-exchange question and the difference between a carbon store and a carbon sink.
Key terms
- Photosynthesis
- Plants use light energy, carbon dioxide and water to make sugars and release oxygen.
- Aerobic cellular respiration
- Cells use oxygen to break down glucose, producing carbon dioxide and water and transferring energy to ATP and heat.
- ATP
- A molecule that powers cellular work.
- Chloroplast
- A structure in plant cells where photosynthesis takes place.
- Carbon store
- Carbon held in a system, including forest trunks, roots, dead wood and soil.
- Carbon sink
- A system that gains carbon overall because more enters than leaves over a particular period.
Transcript
A mouse. A sprig of mint. And a puzzle about the air. In seventeen seventy one, Joseph Priestley tested air that mice had already breathed. He split it into two samples, adding mint to just one. Days later, a mouse could survive in the sample with the plant. What had the mint changed? The answer connects the oxygen you breathe, the food you eat, and the carbon stored in a forest.
First, follow the light. Priestley also found that mint could restore air in which a candle had gone out. But he had not yet worked out the whole explanation. We now know that green plants use light to make sugars through photosynthesis. The plant releases oxygen, which supports a flame and aerobic respiration. So the mint was not magically cleaning every kind of pollution. It was changing particular gases. And the light matters.
Look inside a leaf. Its cells contain chloroplasts, with a green pigment called chlorophyll that absorbs light. Carbon dioxide enters through tiny pores called stomata. Water arrives from the roots. Using light energy, the plant makes sugars, including glucose, and releases oxygen. Here is the usual summary: carbon dioxide plus water, with light energy, gives glucose plus oxygen. The arrow you see stands for many reactions. It is a recipe summary, not one instant chemical trick.
Now track just the carbon. The carbon atom in carbon dioxide becomes part of the sugar. In the balanced summary, six carbon dioxide molecules provide the six carbon atoms in one glucose molecule. In this summary, six water molecules go in, and six oxygen molecules come out. The atoms are rearranged, not created. Here is the surprise: much of a tree's dry mass comes from carbon dioxide in the air. Roots supply water and mineral nutrients, but a tree is not mostly soil pulled up a straw.
Second, unlock the sugar. A mouse cannot run its cells on sunlight. It gets chemical energy from food. In aerobic cellular respiration, glucose reacts with oxygen, producing carbon dioxide and water. Some of the energy transferred is used to make ATP, a molecule that powers cellular work; some becomes heat. In plants and animals, much of this process happens in mitochondria. Breathing moves gases into and out of an animal. Cellular respiration is the chemistry using them inside cells.
Here is the trap: plants do this too. A leaf has chloroplasts for photosynthesis and mitochondria for respiration. Living root cells respire as well, even though they are underground. In daylight, both processes can happen at once. In darkness, photosynthesis stops, but respiration continues. The plant uses sugars made earlier. The two summary equations look reversed, but the processes use different machinery. Matter can cycle between them. Energy flows from light, through living things, and eventually out as heat.
Try a quick prediction. Imagine a plant in bright light, making glucose faster than it breaks glucose down in respiration. Overall, is it taking in carbon dioxide or releasing it? Pause if you want to work it out. The plant is taking it in. Photosynthesis uses more carbon dioxide than respiration releases. Now turn off the light. Photosynthesis stops, while respiration keeps releasing carbon dioxide. A measurement outside the leaf shows the balance of the two processes, not each process separately.
Third, scale up to a forest. Sugar is not only fuel. Plants use it to build materials such as cellulose in wood, or store it as starch. Carbon can remain in trunks, roots, dead wood, and soil. That makes a forest a carbon store. But the forest also returns carbon dioxide to the air. Trees respire. Fungi and other decomposers break down dead material and respire. Fire can release stored carbon too. A forest stores carbon in living and dead material, with carbon moving in and out.
A carbon store and a carbon sink are not quite the same. Store means carbon held there. Sink means that, over a particular period, more carbon enters than leaves. A growing forest can be both. If losses exceed uptake, it becomes a net source, even while plenty of carbon remains in its trees and soil. So, to judge a forest, ask two questions: how much carbon is stored, and is that store increasing or decreasing? A store can remain large even when it is not growing.
Back to Priestley's mint. In light, photosynthesis could replace oxygen used by the mice. The plant was also respiring. It was the balance that made the difference. Keep these phrases: photosynthesis stores light energy in sugars; aerobic respiration transfers energy from glucose to ATP; plants carry out both; carbon stores hold carbon, while carbon sinks gain it overall. From one sprig under glass to a whole forest, the same question keeps working: what is going in, what is coming out, and what stays?
Sources
Every fact in the video, and where it comes from.
- 1gutenberg.org/files/29734/29734-h/29734-h.htm
- Priestley described splitting air impaired by mice into two samples in August 1771, adding mint to one, and after eight or nine days a mouse survived in the plant sample but not the other.
- Priestley added mint to air in which a candle had burned out on 17 August 1771; another candle burned in it on 27 August. His experiments did not establish the complete modern photosynthesis explanation.
- 2openstax.org/books/biology-2e/pages/8-1-overview-of-photosy…
- Photosynthesis uses light, carbon dioxide and water to make carbohydrates, releases oxygen, and occurs in chloroplasts containing light-absorbing chlorophyll; carbon dioxide enters through stomata.
- The conventional net photosynthesis summary is 6 CO2 + 6 H2O + light energy -> C6H12O6 + 6 O2, representing multiple reactions; sugars made during photosynthesis can be used to make glucose.
- 3openstax.org/books/biology-2e/pages/30-1-the-plant-body
- Carbon in plant carbohydrates comes from atmospheric carbon dioxide; roots supply water and mineral nutrients, and carbohydrates contribute to plant dry mass and cellulose.
- 4openstax.org/books/biology-2e/pages/7-1-energy-in-living-sy…
- Aerobic cellular respiration breaks down glucose using oxygen, yielding carbon dioxide and water and transferring energy to ATP and heat. ATP powers cellular work.
- 5openstax.org/books/biology-2e/pages/7-chapter-summary
- Glycolysis occurs in cytoplasm; later stages of aerobic respiration in plant and animal cells occur in mitochondria. Breathing and cellular respiration are distinct.
- 6openstax.org/books/biology-2e/pages/6-1-energy-and-metaboli…
- Plants carry out photosynthesis and respiration. Photosynthesis requires light while respiration continues in darkness. Matter cycles and energy flows, eventually dissipating as heat.
- 7openstax.org/books/biology-2e/pages/8-3-using-light-energy-…
- Gas exchange measured around a photosynthesizing leaf reflects net photosynthetic carbon uptake minus respiration; the worked example assumes photosynthesis exceeds respiration.
- 8climatehubs.usda.gov/image/carbon-flux
- Forest carbon is stored in live biomass including roots, dead wood, litter and soils; respiration, decomposition and combustion return carbon to the atmosphere.
- 9noaa.gov/education/resource-collections/climate/carbon-…
- Carbon cycles through living organisms; photosynthesis removes atmospheric carbon dioxide while respiration and decomposition return carbon.
- 10fs.usda.gov/sites/default/files/fs_media/fs_document/GHG-e…
- A carbon sink absorbs more carbon than it releases over a time period; a carbon source releases more than it absorbs; a carbon stock is the quantity stored.
- 11acs.org/education/whatischemistry/landmarks/josephprie…
- Oxygen supports combustion and is used in aerobic respiration.
- 12fs.usda.gov/sites/default/files/Forest-Carbon-FAQs.pdf
- Tree dry material is approximately half carbon by mass; carbon from atmospheric CO2 enters sugars and wood, with carbon also stored in roots, dead wood and soils.
- 13fs.usda.gov/sites/default/files/factsheets-forestcarbon.pdf
- Carbon sink means a system that absorbs more carbon than it releases; positive stock change indicates a sink, negative stock change a source.
- 14openstax.org/books/biology-2e/pages/3-2-carbohydrates
- Plant sugars can form starch for storage and cellulose for structure.
- 15extension.wvu.edu/lawn-gardening-pests/news/2021/09/01/putting-d…
- Root cells respire to supply the energy needed for growth and mineral absorption.
Image and sound credits
- “Joseph Priestley's Chemical apparatus. 18th C” — Wellcome Library, London, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) — https://commons.wikimedia.org/wiki/File:Joseph_Priestley%27s_Chemical_apparatus._18th_C_Wellcome_L0000728.jpg; cropped/panned in video.
- “Joseph Priestley's Chemical apparatus. 18th C” — Wellcome Library, London, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) — https://commons.wikimedia.org/wiki/File:Joseph_Priestley%27s_Chemical_apparatus._18th_C_Wellcome_L0000729.jpg; cropped/panned in video.
- “Free mint leaves black background” by Unknown (CC0) — https://www.rawpixel.com/image/5905553/photo-image-background-public-domain-leaves; cropped/panned in video.
- “Soil turbation” by Prof. Dr. Alessandro Samuel-Rosa (CC0) — https://commons.wikimedia.org/wiki/File:Soil_turbation.jpg; cropped/panned in video.
- “'FASTENING ROOTS' of the 'Mangroove tree' that holds the tree firmly to the soil, also controlling 'SOIL EROSION'.” by Rudolph.A.furtado (Public domain) — https://commons.wikimedia.org/wiki/File:%27FASTENING_ROOTS%27_of_the_%27Mangroove_tree%27_that_holds_the_tree_firmly_to_the_soil,_also_controlling_%27SOIL_EROSION%27..JPG; cropped/panned in video.
- “Grosmannia clavigera (blue stain fungus)” by James St. John, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) — https://commons.wikimedia.org/w/index.php?curid=95220887; cropped/panned in video.
- “Mus musculus 131536695” by kcthetc1 (CC0) — https://commons.wikimedia.org/wiki/File:Mus_musculus_131536695.jpg; cropped/panned in video.
- “Conducting Prescribed Fire, Lake Como, Darby Sula R.D. Bitterroot N.F” by Forest Service Northern Region (Public domain) — https://commons.wikimedia.org/wiki/File:Conducting_Prescribed_Fire,_Lake_Como,_Darby_Sula_R.D._Bitterroot_N.F.jpg; cropped/panned in video.
- “File:Priestley.jpg” by Ellen Sharples (Public domain) — https://commons.wikimedia.org/wiki/File:Priestley.jpg; cropped/panned in video.
- “Havelock Island, Canopy of tropical moist evergreen forest, Andaman Islands” by Vyacheslav Argenberg, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) — https://commons.wikimedia.org/wiki/File:Havelock_Island,_Canopy_of_tropical_moist_evergreen_forest,_Andaman_Islands.jpg; cropped/panned in video.
- “Boxwood and beech relict forest canopy, Epiphytes, Mezmay, Russia” by Vyacheslav Argenberg, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) — https://commons.wikimedia.org/wiki/File:Boxwood_and_beech_relict_forest_canopy,_Epiphytes,_Mezmay,_Russia.jpg; cropped/panned in video.
- “Macro view of a single green leaf fragment on a mossy rock” by Krishna k. sahh (CC0) — https://commons.wikimedia.org/wiki/File:Macro_view_of_a_single_green_leaf_fragment_on_a_mossy_rock.jpg; cropped/panned in video.
- “Translucent leaf in macro (Unsplash)” by Joe Wroten sharpshark28 (CC0) — https://commons.wikimedia.org/wiki/File:Translucent_leaf_in_macro_(Unsplash).jpg; cropped/panned in video.
- “Red Apple” by Abhijit Tembhekar from Mumbai, India, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) — https://commons.wikimedia.org/wiki/File:Red_Apple.jpg; cropped/panned in video.
- “Tree with red apples in Barkedal 4” by W.carter (CC0) — https://commons.wikimedia.org/wiki/File:Tree_with_red_apples_in_Barkedal_4.jpg; cropped/panned in video.
- “Mushrooms on dead log” by Sheila Sund, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) — https://commons.wikimedia.org/wiki/File:Mushrooms_on_dead_log.jpg; cropped/panned in video.
- Paper textures: ambientCG (CC0).
- Sound effects: Freesound contributors (CC0); see studio kit SFX credits.
- Narration: Gemini TTS, Charon. Music: studio collage bed generated with ACE-Step.
- Thumbnail images: GPT Image 2.6; fictional scientist in variant B, not a portrait of Priestley.
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