# Stomata (DD-715)

> Stomata are microscopic pores, each bordered by a pair of guard cells, that control a plant's intake of carbon dioxide and its loss of water vapor. Guard cells open the pore by taking up ions and water and swelling, and they close it when they lose turgor, for example in response to the drought hormone abscisic acid (ABA). Open stomata cover only about 5% of the leaf surface, but they're the main route of water loss (transpiration). Stomatal behavior shapes the global water and carbon cycles, and stomata appear in fossils from about 400 million years ago.

- Department: Science (also Nature)
- First published: 2026-04-29
- Last updated: 2026-04-29
- Permalink: https://detailenjoyer.com/case/stomata/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## Quick facts

- **DD-715-F01**: Open stomatal pores cover only about 5% of a leaf's surface. _(Write-up · Exhibit C (Small area, big effect))_
- **DD-715-F02**: The drought hormone ABA makes stomata close. _(Write-up · Exhibit D (Signals))_

## Write-up

The tiny pores that let leaves trade gases. Stomata cover only a few percent of a leaf, yet most of its water loss goes through them.

1. **A two-cell valve**: Each stoma is a pore between two guard cells. When they take up potassium ions and water, they swell and bow apart to open the pore, and when they lose turgor the pore closes.
2. **The trade-off**: Opening the pore lets in CO2 for photosynthesis but also lets water vapor out. Plants constantly balance carbon gain against water loss.
3. **Small area, big effect**: Stomatal pores cover only about 5% of the leaf surface when open. Even so, water loss through them can reach up to about 70% of what a leaf with no waxy cuticle would lose.
4. **Signals**: Guard cells respond to light, CO2, humidity and the drought hormone abscisic acid (ABA), which triggers closure when water is short.
5. **Planet-scale**: Transpiration through stomata moves huge amounts of water from soil to air and ties plant physiology to climate. Stomata appear in fossil plants from about 400 million years ago.

## Related case files

- [Algaecultural (DD-724)](https://detailenjoyer.com/case/algaecultural/): Another photosynthesis case

## Reel vs. record

- **Note**: Missing the key trade-off: open stomata also let water vapor out (transpiration). That's the main route of a plant's water loss and the reason guard cells close in drought.
- **Missing signal**: the drought hormone abscisic acid (ABA) is a major trigger for closing the pore, alongside light, CO2 and humidity.
- **Mechanism**: guard cells open the pore by taking up potassium ions and water and swelling, and close it when they lose turgor.
- **Precision**: in many plants, stomata are most numerous on the underside of leaves.
- **Scale**: stomatal pores cover only about 5% of a leaf's surface when open, yet they control most of its gas exchange (Hetherington & Woodward, 2003).

## Video: Stomata: The Tiny Pores That Let Leaves Trade Gases

- Duration: 22.4 s · Published: 2026-04-29
- File: https://detailenjoyer.com/assets/videos/DXuewR3ESZb.mp4

### Transcript

DID YOU KNOW? A plant stoma is a tiny pore found primarily on the surface of leaves and stems that regulates gas exchange between the plant and its environment. Each stoma is flanked by two specialized guard cells that can open or close the pore in response to light, humidity, and carbon dioxide levels. When open, stomata allow carbon dioxide to enter for photosynthesis and release oxygen as a byproduct. NOW YOU KNOW!

## Questions & answers

**Q: How many guard cells surround a stoma?**  
A: Two guard cells. _(Source: Write-up · Exhibit A (A two-cell valve) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Wikipedia: Stoma)_

**Q: Besides CO2 and O2, what else passes through stomata?**  
A: Water vapor leaves through them, which is transpiration. _(Source: Write-up · Exhibit B (The trade-off) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Schroeder et al. / Plant Physiology (2006), The Biology of Transpiration. From Guard Cells to Globe)_

**Q: Which hormone makes stomata close during drought?**  
A: ABA. _(Source: Write-up · Exhibit D (Signals) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Schroeder et al. / Plant Physiology (2006), The Biology of Transpiration. From Guard Cells to Globe)_

**Q: About how much of a leaf's surface do open stomatal pores cover?**  
A: About 5%. _(Source: Write-up · Exhibit C (Small area, big effect) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature)_

**Q: What makes guard cells swell to open the pore?**  
A: Taking up ions and water raises their turgor. _(Source: Write-up · Exhibit C (Small area, big effect) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature)_

## Sources

- [Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature](https://www.nature.com/articles/nature01843)
- [Schroeder et al. / Plant Physiology (2006), The Biology of Transpiration. From Guard Cells to Globe](https://pmc.ncbi.nlm.nih.gov/articles/PMC1761997/)
- [Wikipedia: Stoma](https://en.wikipedia.org/wiki/Stoma)

## Cite this case file

Easterling, Marcus. "Stomata." The Detail Department (DetailEnjoyer), case file DD-715, last updated Apr 29, 2026. https://detailenjoyer.com/case/stomata/