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J. structure of angiosperm leaf
Cuticle
Upper epidermis
Palisade mesophyll
Spongy mesophyll + air spaces
Lower epidermis: stomata + guard cell
K. Adaptations
light absorption
leaves
- plant can orientate towards light: expose greater SA to sun – absorb max light
- flat: large SA – absorb max light
- thin – allow light to penetrate
transparent cuticle & epidermis – allow light to penetrate
palisade cells
- elongated – reduce number of cell walls absorbing light (prevent it reaching chloroplasts)
- lots chloroplasts – absorb max light
o move inside cells – best position to absorb max light
gas exchange with air spaces & stomata
leaves
- thin: short diffusion distance
- flat: large SA – many stomata
spongy mesophyll
- large SA for gas exchange
- air spaces – allow gases to diffuse between stomata & cells
- moist
reduce water loss – moist gas exchange surface (dehydrated with water loss)
- Waxy cuticle – reduce water loss by evaporation
- Guard cells: change shape to close stomata at night + only on bottom (less sunlight for evaporation)
L. role of leaf as organ of gas exchange
Rely entirely on diffusion
Respiration: all day
Photosynthesis: sunlight hours
stomatal opening
- light: guard cells photosynthesise = ATP – actively transport K + in
- triggers conversion: starch (insoluble) to malate ions (soluble)
- K+ & malate ions: lower water potential – water enters by osmosis = turgid
- outer wall: thinner – stretches more than inner wall = pore between guard cells
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