Plant gas exchange

• The waxy cuticle of a leaf prevents water loss
• The cells in the upper epidermis have a giant vacuole to allow light in
• The palisade mesophyll cells contain lots of chloroplasts for photosynthesis
• The spongy mesophyll contains air pockets for gas exchange
• The lower epidermis contains stomata to let gas in and out
• Transpiration is the loss of water from the plant through the stomata
• Plants in dry environments will have fewer stomata to help reduce water loss
• Plants have to minimise the amount of water they lose but still have to exchange gases
• When plants have enough water guard cells are turgid which keeps the pores open. When plants are dehydrates the guard cells become flaccid, causing the hole to close
• K+ ions are actively transported into guard cells. This changes the water potential and swells the guard cells
• All gas exchange surfaces have a large surface area, short diffusion distance and high concentration gradient
• The main gas exchange surface in plants is the spongy mesophyll
• Plants in dry, hot or windy climates (xerophytes) have adaptations to reduce water loss
• Xerophytes have a small leaf surface area to reduce surface area for evaporation. e.g conifer needles, cacti spines
• Xerophytes can have sunken stomata, which maintains humid air around the stomata to reduce the water potential gradient. e.g. marram grass, cacti
• Xerophytes can have stomatal hairs (trichores) to maintain humid air around the stomata to reduce the water potential gradient and reduce evaporation. e.g. marram grass, couch grass
• Xerophytes can have rolled leaves to reduce the effects of wind to reduce the water potential gradient and reduce evaporation. e.g. marram grass
• Xerophytes can have extensive root systems which maximises water uptake and helps to increase chances of contact with water. They are often shallow but wide to absorb rainfall. They often have swollen stems to store the collected water (succulents) e.g. cacti, agave, yucca
• Xerophytes can have a reduced number of stomata to reduce the amount of places water can evaporate from
• Xerophytes often have a thick waxy cuticle to waterproof leaves and stems to reduce evaporation. e.g. yucca, agave, citrus plant
• You can investigate stomatal density by taking thin layers of epidermis or 'imprints' of the leaf surface and examining under the microscope the number of stomata in an area of leaf tissue (stomatal density per mm2)
• When investigating stomatal density multiple fields of view or leaf sections should be examined and a mean taken so the sample is representative
• Label the leaf:

  A) Upper epidermis

  B) Mesophyll cells

  C) Lower epidermis

  D) Stoma

  E) Guard cell

  F) Phloem

  G) Xylem

  H) Cuticle