Gas Exchange - Plants

Epidermis

• single layer of tightly packed cells

• lower epidermis — stomata

• covered by a waxy layer — cuticle

Stomata

• tiny pores through which gas exchange occurs in a leaf

  • allow for diffusion of oxygen and carbon dioxide into and out of leaf

• stoma — controls the openinng and closure of the pore

  • turgid when water moves in — flaccid when water is lost

Mesophyll tissue

• formed by parenchyma cells — contain chloroplasts

• makes up bulk of the internal structure of leaf

  • has palisade & spongy mesophyll

Palisade Mesophyll

• forms a layer beneath the upper epidermis and contains many chloroplasts for maximum photosynthesis

Spongy Mesophyll

• contains large air spaces between the cells for gas exchange to occur

  • increases surface area for gas exchange

Vascular Tissue

• arranged in vascular bundles

• responsible for transport of substances around the plant

  • form veins in leaves

  • xylem transports wateer and mineral ions from roots to leaves

  • phloem transports products of photosynthesis from leaves to other parts

Waxy Cuticle

• prevents gases and water vapour from leavin gthrough the epidermis - gas exchange must occur through stomata

• allows gas exchange and water loss to be controlled

Air spaces

• maintain a concentration gradient of gases between the air and spongy mesophyll cells by allowing movement of gases

Guard cells

• control gas exchange and water loss by opening or closing stomata

Veins

• xylem vessels bring water to leaf —required for photosynthesis & transpiration

• necessary for transport

Transpiration

• inevitable consequence of gas exchange in the leaf

  • provides a means of cooling via evaporation

  • helpful in the uptake of mineral ions

  • turgor pressure of cells provides support to leaves

    • high turgor pressure — leaves do not wilt

Air movement

• leads to increased transpiration

  • lower concentration of water vapour outside of leaf

  • air currents/wind can carry water molecules away from leaf surface — increasing the concentration gradient

Temperature

• higher temps lead to higher rates of transpiration

  • increases kinetic energy

  • increases rate of transpiration

  • temp too high — stomata closes to prevent excess water loss — reduces rate of transpiration

Light Intensity

• higher light intensities — increase rate of transpiration

  • stomata close in dark — reduces rate of transpiration

  • stomata open in light — increases rate of transpiration

Humidity

• higher humidity — reduces rate of transpiration

  • air outside of leaf is saturated with water

  • decreases the rate of transpiration — no concentration gradient