Gas Exchange in Plants

Properties shared by ALL gas-exchange surfaces (in plants and animals alike):

• Large surface area — more area for diffusion of gases.
• Thin (short diffusion distance) — gases cross quickly.
• Moist — gases dissolve before diffusing across the membrane.
• Permeable — allows O₂ and CO₂ to pass.
• Concentration gradient maintained — keeps diffusion going (in plants, this is sustained by photosynthesis and respiration constantly using/producing gases).

The leaf as a gas-exchange organ. A leaf is adapted to exchange CO₂ and O₂ with the air efficiently:

• Thin and flat — short diffusion distance and a large surface area for absorbing light and exchanging gases.
• Stomata — pores in the epidermis (mostly the lower surface) that let gases diffuse in and out; opening is adjustable.
• Air spaces in the spongy mesophyll — interconnected spaces let gases diffuse freely between stomata and the photosynthetic cells.
• Large internal surface area of mesophyll cells — the moist cell walls lining the air spaces provide a huge area for gases to dissolve and diffuse into/out of cells.
• Moist cell walls — gases dissolve before crossing the cell membrane.

So gases reach the photosynthesising cells passively: CO₂ diffuses in through stomata → through air spaces → dissolves at the moist mesophyll cell walls → into the cells; O₂ produced by photosynthesis takes the reverse path out.

Stomata and guard cells. Each stoma (plural: stomata) is a pore bordered by two guard cells. Guard cells control the size of the pore — opening it for gas exchange and closing it to limit water loss.

Opening mechanism (turgor changes driven by K⁺):

1. In light, guard cells actively pump K⁺ ions into themselves (using ATP from photosynthesis).
2. The rise in solute concentration lowers water potential, so water enters by osmosis.
3. Guard cells become turgid. Because their inner wall (next to the pore) is thicker/less elastic than the outer wall, they bow apart → the stoma OPENS.

Closing mechanism:

1. K⁺ ions leave the guard cells.
2. Water potential rises, so water leaves by osmosis.
3. Guard cells become flaccid and straighten → the stoma CLOSES (reducing water loss, e.g. at night or when water is scarce).

The net direction of gas exchange changes with light. Both photosynthesis and respiration occur, but their relative rates determine what NET exchange you measure:

• In LIGHT (bright): photosynthesis rate > respiration rate.

  • The plant fixes more CO₂ than it produces and releases more O₂ than it uses.
  • Net result: CO₂ taken UP, O₂ released.

• In DARK: photosynthesis stops; only respiration occurs.

  • The plant uses O₂ and produces CO₂.
  • Net result: O₂ taken UP, CO₂ released — the same as in animals.

• At the COMPENSATION POINT: at low/intermediate light, there is one light intensity where the rate of photosynthesis exactly equals the rate of respiration. The gases produced by one process are entirely used by the other, so there is NO NET gas exchange with the atmosphere.

Crucial contrast. Cellular respiration NEVER stops — it happens in every living plant cell, day and night. Photosynthesis only occurs in light (in chlorophyll-containing cells). So "net O₂ release" in the light does NOT mean the plant has stopped respiring; it means photosynthesis is producing O₂ faster than respiration consumes it.

Measuring gas exchange / photosynthesis rate. The rate of photosynthesis can be estimated by measuring gas exchange — for example, the volume of O₂ released (e.g. bubbles from a pondweed such as Elodea) or CO₂ uptake per unit time. Because respiration runs at the same time, what you measure in the light is NET photosynthesis (gross photosynthesis − respiration); adding the dark respiration rate gives the gross rate.