Plant Mass Transport

• Plants need carbon dioxide for photosynthesis and also oxygen for respiration, with both processes producing the other as waste
• Depending on the time of day, the balance of photosynthesis to respiration will create different concentration gradients, which cause gases to diffuse in or out of the leaf
• Which layer of the leaf are gases exchanged with the atmosphere?

  The mesophyll layer
• How has the mesophyll layer of the leaf adapted for gas exchange?

  Has a large surface area due to the large air spaces in the spongy mesophyll
• Why are leaves thin?

  To reduce the distance gases have to diffuse in and out of the leaf
• Gases move in and out of the leaf through the stomata, which are pores in the lower epidermis of the leaf
• Where are the stomata located in a leaf?

  In the lower epidermis
• The waxy cuticle is a waxy layer at the top of the leaf which stops water loss through evaporation
• The upper epidermis is a transparent layer on the top of the leaf
• The palisade mesophyll is a layer of long cylindrical cells, which is where most of the photosynthesis in the leaf occurs as it contains lots of chloroplasts
• The spongy mesophyll is a layer of cells with air spaces between them, which is where gas exchange in the leaf occurs
• Veins in a leaf contain the xylem and phloem tubes
• The xylem transports water and mineral ions
• The phloem transports dissolved sugars in the plant
• The lower epidermis is a thin underside layer of cells in the leaf, and is where the stomata are found
• What is transpiration?

  Transpiration is the loss of water vapor into the atmosphere through a plants leaves or stem
• Transpiration is controlled by the opening and closing of stomata
• Plants in dry environments have fewer stomata to reduce water loss while still allowing for gas exchange
• Water evaporates from the mesophyll cells into the air spaces in the leaf, and then water vapour diffuses through the stomata
• What cells control stomata?

  Guard cells
• When guard cells are turgid with water, stomata stay open
• When guard cells are flaccid, they close the stomata
• Stomata close at night, when little photosynthesis is occurring as less carbon dioxide and water is needed, so gas exchange and water loss is reduced
• When light intensity is high, potassium ions move into guard cells, lowering the water potential and making water move into them by osmosis, making them turgid and causing the stomata to be open
• In organisms that adaptations reduce water loss, gas exchange is negatively affected (and vice versa)
• Organisms have made compromises to ensure that there is a sufficient supply of both water and gases
• What are some examples of organisms that had to make compromises between gas exchange and water loss?

  Terrestrial insects and xerophytic plants
• As terrestrial insects are prone to drying out, they have a waterproof exoskeleton that prevents water loss
• In terrestrial insects, the waterproof exoskeleton makes gas exchange by diffusion difficult, so they have evolved a breathing system known as the tracheal system
• The tracheal system in terrestrial insects consists of many tubes that carry oxygen directly to all tissues and cells of the body.
• Xerophytes are plants in hot, dray or windy environments have extra adaptations to prevent too much water loss when their stomata are open
• The leaves of xerophytes have a small surface area to reduce evaporation, as there are fewer stomata
• What are some examples of small surfaced area leaves on xerophytes?

  Conifers, needles and cactus spines
• Xerophytes have sunken stomata and Stomatal hairs to maintain the humid air around the stomata, to reduce the water potential gradient and reduce evaporation
• Xerophytes have rolled leaves to reduce the effects of wind to reduce the water potential gradient and reduce evaporation
• Xerophytes have extensive root systems to maximise water uptake, as it increases the chances of contact with water
• Xerophytes typically have a shallow and wide area to absorb rainfall and often have swollen stems to store the collected water
• Xerophytes may be coated in a thick, waxy cuticle on the stems and leaves to reduce evaporation.
• How are mineral ions and organic compounds transported in a plant?

  Dissolved in water
• What is the structure of every xylem vessel?

  Elongated tubes formed from dead cells, which are joined together in bundles known as vessel elements

  No ends between cell walls

  Thick walls made up of lignin

  Gaps in the cell wall