5.5 gas exchange in plants

Why do plants need gas exchange?

For respiration (uses O₂, releases CO₂) and photosynthesis (uses CO₂, releases O₂). Rates of each process vary with light.

Why is gas exchange slower in plants compared to animals?

Plants have a lower metabolic rate, so lower demand for O₂ and CO₂ than animals.

Which leaf adaptations increase the efficiency of gas exchange?

Thin leaves → short diffusion pathway
• Large surface area
• Air spaces in spongy mesophyll → diffusion of gases
• Stomata → control gas entry and exit
• Moist internal surfaces → allow gases to dissolve before diffusion

What is the role of the spongy mesophyll in gas exchange?

Contains large intercellular air spaces allowing diffusion of CO₂ and O₂ between stomata and photosynthetic cells.

What are stomata?

Pores in the epidermis (usually lower surface) that allow gas exchange and regulate water loss.

How do guard cells open the stomata?

• K⁺ ions actively transported into guard cells.

• Water enters by osmosis.

• Guard cells become turgid, thinning the outer wall.

• Pore opens.

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How do guard cells close stomata?

K⁺ ions leave guard cells → water exits → cells become flaccid → pore closes.

What environmental factors cause stomatal opening?

Light, low CO₂ inside leaf, adequate water.

What environmental factors cause stomatal closure?

Darkness, water stress, high temperature, high wind, high leaf CO₂.

What happens to gas exchange at high light intensities?

Photosynthesis > respiration → net uptake of CO₂ and release of O₂.

What happens at night?

No photosynthesis, so only respiration occurs → net uptake of O₂ and release of CO₂.

What are xerophytes?

Plants adapted to dry habitats, reducing water loss.

Stomatal adaptations in xerophytes?

Sunken stomata
• Hairy leaves
• Stomata on underside only
• Reduced stomatal density

Structural adaptations of xerophytes to reduce water loss?

Thick cuticle
• Rolled leaves
• Small leaves/needles
• Fewer air spaces in mesophyll

How do gases move when stomata are open?

Diffusion down concentration gradients, maintained by photosynthesis and respiration.

name the tissues and cells of a leaf