4.4 Transport in plants Flashcards
what are the main transport tissues in plants
xylem and phloem
what is the role of each of the main transport tissues in plants
- xylem carries water and dissolved minerals from the roots to the shoots, up the shoot
- phloem carries the dissolved product of photosynthesis (sucrose) from the leaves to where it is needed for growth/ storage, up and down the shoot
how does the structure of the xylem relate to its role in transport
- cellulose microfibrils in the walls of xylem vessels increase strength of the tube so it can withstand compression forces from the weight of the plant
- lignin in the cell walls allows the cells to become impermeable to water and other substances
what is the lignified tissue of the xylem called
metaxylem
how is water transported through the xylem
- water is moved from the roots to the leaves and leaves the shoots in the transpiration stream
- water moves out of the xylem through unlignified areas or specialised pits in the walls of the xylem vessel
how does the structure of the phloem relate to its role in transport
- they do not become lignified and the walls become perforated tp form specialised sieve plates as phloem contents flow through the pores in the sieve plates
- they have no nucleus, kept alive by companion cells which are very active and supply phloem with needed ATP
- cytoplasm of sieve tubes and companion cells are linked by plasmodesmata, allows exchange of substances
what are the components of the phloem vessel called
phloem sieve tubes made up of phloem sieve plates
how does water move from soil to the xylem
- water is absorbed from the soil byv the root hairs
- water moves from soil into a root hair down a concentration gradient by osmosis
- this makes the root hair cell more dilute than its neighbour, so water moves into neighbouring root hair cell down a concentration gradient by osmosis until it reaches the xylem
state the two ways water can move into the xylem (2)
- symplast pathway
- apoplast pathway
what occurs in the symplast pathway
- water moves by diffusion down a concentration gradient from the root hair cells to the xylem through plasmodesmata (gaps in the cellulose cell wall) that allow strands of cytoplasm to pass through
what occurs in the apoplast pathway
- water is pulled by the attraction between water molecules across adjacent cell walls from root hair to xylem
- because of the loose structure of the cell wall
which factors affect the rate of transpiration
- temperature
- light
- humidity
- and the movement of air
how does light affect transpiration rate
- causes stomata to open for gas exchange and most are closed in the dark
- as a result, transpiration rate increases with light intensity until all of the stomata are open and transpiration is at its maximum
how does temperature affect transpiration rate
- increase in temperature increases the water evaporation from given surfaces of the spongy mesophyll cells
- increases the amount of water that the air can take before it becomes saturated
- these both increase the concentration gradient between the air and the leaf, increasing transpiration rate
- temperature also increases the speed at which molecules move
- as a result, transpiration rate increases with temperature up until a point where something else e.g. light intensity becomes a limiting factor
how does air movement/ wind affect transpiration rate
- air movement reduces the shell of still air around the stomata, this increases the concentration gradient between the inside and outside the leaf
- transpiration rate increases with air movement/ wind
