Stomata and transpiration
Up to 90 percent of the water a plant takes up through its roots can leave again as vapour through the stomata.
A trade every leaf has to make
A leaf needs carbon dioxide, and carbon dioxide is a gas in the air. To let it in, the leaf has to open holes in its waterproof skin. The same holes let water vapour out. OpenStax describes it as a balance between efficient photosynthesis and water loss. The holes are the stomata, and the plant controls them with pairs of guard cells.
Guard cells open and close in response to light, the leaf’s water status and the carbon dioxide level. When they take up water they swell, bow apart and open the pore. When they lose water they go limp and the pore closes. On a sunny morning with moist soil the stomata open wide. If the soil dries out, the guard cells close the pores even in bright light, saving water at the cost of taking in less carbon dioxide.
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How water climbs a tree
Transpiration is the loss of water vapour from the plant, mostly through the stomata. It is also the engine that lifts water. Inside the leaf, water evaporates from the wet walls of the mesophyll cells into the air spaces and escapes through the open stomata. That evaporation pulls on the water left behind. Water molecules stick to each other (cohesion) and to the walls of the xylem (adhesion), so the pull passes down an unbroken column of water through the stem to the roots. This is the cohesion-tension theory of sap ascent. Transpiration is passive: the plant spends no ATP to lift the water, and the energy comes from the difference between wet soil and drier air.
The numbers are large. OpenStax gives a tension of about −2 megapascals at the leaf surface. Lifting water against gravity requires about 0.0098 MPa per metre of height, so a 116-metre coast redwood needs about 1.14 MPa just to overcome gravity. Inside living cells the opposite force, turgor pressure, is usually 0.6 to 0.8 MPa and can reach 1.5 MPa in a well-watered plant. That is 1.5 × 140 = 210 pounds per square inch, against the 30 to 34 psi of a typical car tyre. When a plant loses more water than it takes in, turgor falls towards zero and the leaves wilt.
Saving water
Plants from dry places have several ways to cut transpiration. Desert plants (xerophytes) and plants that grow on other plants (epiphytes) usually have a much thicker cuticle. Many have a thick coat of hairs or stomata sunk below the leaf surface, which slows the air moving over the pores. Cacti go further. They open their stomata in the cool night and store the carbon dioxide they take in, then keep the stomata shut during the hot day and use the stored carbon for photosynthesis. This is called CAM. In the lab, switch the plant to “Cactus (CAM)” and press Night.
| Lab preset | Opening | CO₂ in | Water per CO₂ |
|---|---|---|---|
| Sunny morning | 100% | 100 | 0.67 |
| Hot, dry afternoon | 29% | 29 | 1.25 |
| Night | 10% | 0 | none |
| Cactus at night | 100% | 100 | 0.33 |
- Stomata let CO₂ in and water vapour out; guard cells set the balance.
- Light opens stomata; dry soil closes them.
- Evaporation pulls a cohesive water column up the xylem (cohesion-tension).
- CAM plants such as cacti open their stomata at night.
- What drives most water movement up the xylem?
- Transpiration from the leaves
- ATP pumps in the roots
- Gravity
- Phloem pressure
- Transpiration is passive. That means…
- the plant spends no ATP moving the water
- it only happens at night
- it needs no stomata
- water moves from air into the leaf
- Up to what percentage of the water taken up by roots can be lost by transpiration?
- Lifting water requires about 0.0098 MPa per metre. How many MPa are needed for 116 metres? (one decimal place)
- A turgor pressure of 1.5 MPa, with 1 MPa = 140 psi, is how many psi?
- What makes guard cells open a stoma?
- They take up water and swell
- They lose water and shrink
- They divide
- They die
- In the lab, what happens on the hot, dry afternoon compared with the sunny morning?
- The pore partly closes and more water is lost per CO₂ gained
- The pore opens wider
- No water is lost
- CO₂ intake rises
- When do cacti (CAM plants) open their stomata?
- At night
- At midday
- Never
- Only when it rains
- Which feature helps a desert plant reduce transpiration?
- A thick cuticle and sunken stomata
- Very large, thin leaves
- Stomata only on the top surface
- No cuticle
- What holds the water column together in the xylem?
- Cohesion between water molecules and adhesion to the walls
- Sugar in the water
- Air bubbles
- Root pressure only
Sources for this course (16)
- -fix5: gravity calculation corrected using rho*g*h = 1000*9.8*h Pa
- the cited textbook has a tenfold coefficient error. Sources (checked 2026-10-17): OpenStax Biology 2e 8.1 Overview of Photosynthesis (photoautotrophs, mesophyll, stomata on the underside, guard cells, chloroplast, thylakoid, granum, stroma, two stages; openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis)
- 8.2 The Light-Dependent Reactions (400 to 700 nm, chlorophyll a absorbs blue and red, carotenoids, 300 to 400 chlorophylls per antenna, water split at PSII, about 10 percent of the oxygen used by the leaf's mitochondria; .../8-2-the-light-dependent-reactions-of-photosynthesis)
- 8.3 Using Light Energy to Make Organic Molecules (RuBisCO, 3-PGA, G3P, three turns per G3P and six per glucose, "dark reaction" outdated, cacti fix CO2 at night; .../8-3-using-light-energy-to-make-organic-molecules)
- 30.3 Roots (tap and fibrous roots, root cap, three zones in the first centimetre, root hairs, endodermis and Casparian strip; .../30-3-roots)
- 30.4 Leaves (lamina, petiole, midrib, parallel and reticulate venation, Ginkgo, cuticle, trichomes, palisade in one to three layers, spongy mesophyll, guard cells the only epidermal cells with chloroplasts, xylem and phloem, needles and spines; .../30-4-leaves)
- 30.5 Transport of Water and Solutes (redwoods near 116 m, turgor 0.6 to 0.8 MPa and up to 1.5 MPa = 210 psi vs tyres 30 to 34 psi, gravity -0.1 MPa per metre, -2 MPa at the leaf surface, cohesion-tension, up to 90 percent of the water lost, sources and sinks, phloem sap up to 30 percent sugar; .../30-5-transport-of-water-and-solutes-in-plants)
- 31.3 Nutritional Adaptations (rhizobia, nodules, nitrogenase, biological fixation 65 percent of nitrogen used in agriculture, mycorrhizae in more than 80 percent of land plants, up to 20 percent of the carbon; .../31-3-nutritional-adaptations-of-plants)
- 32.2 Pollination and Fertilization (self and cross pollination, bees cannot see red, nectar guides, bats, birds, wind, water, at least 25,000 orchid species, double fertilization 2n zygote and 3n endosperm, seed parts, scutellum, fruit types, dispersal, dormancy; .../32-2-pollination-and-fertilization)
- Britannica, Photosynthesis: history (Priestley 1771, Ingenhousz 1779, 1782: oxygen formed at the expense of fixed air, identified the year before as carbon dioxide) (britannica.com/science/photosynthesis/History-of-the-study-of-photosynthesis)
- Smithsonian Libraries, Ingenhousz, Experiments upon Vegetables, 1779 (library.si.edu/digital-library/book/experimentsuponv00inge)
- van Helmont's account (200 pounds of earth, 5 pound willow, 169 pounds and about 3 ounces after five years, soil short by about 2 ounces)
- quoted from Howe 1965 by S. G. Saupe, College of St. Benedict and St. John's University (employees.csbsju.edu/ssaupe/biol115/van_helmont.htm)
- F. F. Blackman, Optima and Limiting Factors, Annals of Botany 19:281-296, April 1905 (doi.org/10.1093/oxfordjournals.aob.a089000)
- NOAA Global Monitoring Laboratory, Trends in CO2 (gml.noaa.gov/ccgg/trends/: Mauna Loa September 2026 monthly mean 426.07 ppm)
- NOAA Climate.gov, Climate change: atmospheric carbon dioxide (280 ppm or less before the Industrial Revolution of the mid-1700s, from ice cores; climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide).
Checked: 2026-10-17