7.7 Flashcards
Movement of water out through stomata
-humidity of external air is less than that of air spaces next to stomata
-so wpt grad from air spaces through stomata to air
-water vapour molecules diffuse out of air spaces into atmosphere through open stomata
water lost by diffusion through stomata
replaced by water evaporating form cell walls of surrounding ding mesophyll cells
movement of water across cells of leaf
- mesophyll cells lose water to air spaces by evaporation due to heat supplied by sun
- cells no have lower wpt so water enters by osmosis from neighbouring cells
- loss water from neighbouring cells lowers their wpt
-process repeats
movement of water up stem in xylem
- water evaporates from mesophyll cells due to heat from sun leading to transpiration
-water molecules form hydrogen bonds between one another and tend to stick together(cohesion) - water forms continuous unbroken column across mesophyll cells and down xylem, more water molecules are drawn up behind as result of cohesion
- the column of water is pulled up xylem due to transpiration pull as it puts xylem under tension(negative pressure within xylem)
-water molecules also adhere to walls of xylem due to dipolar molecules(opposite charges attract)
3 pieces of evidence for cohesion tension theory
- change in diameter of tree trunks according to rate of transpiration(more tension in xylem during day cause of greater transpiration so diameter shrinks)
- when xylem vessel breaks, air bubble enters because water column is broken and h20 molecules can no longer stick together
- when xylem vessel breaks, water doesn’t leak out but air is drawn in- consistent with tension
transpiration
- the loss of water vapour through the stomata by evaporation
- passive process
translocation
process by which organic molecules and mineral ions are transported from one part of plant(source) to another(sink)
xylem cells
dead and hollow
made up of lignin
sources
site of production os sugars during photosynthesis
sinks
site where sugars will be directly used or stored for future use
1st phase of mass flow theory
- sucrose is manufactured from products of photosynthesis in cells with chloroplasts
- sucrose diffuses down concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
-hydrogen ions are actively transported from companion cells into spaces within cells walls using ATP - hydrogen ions then diffuse down concentration gradient through carrier proteins into sieve tube elements
- sucrose molecules are transported with hydrogen ions in co-transport
2nd phase of mass flow theory
-the sieve tube now has a higher concentration of sucrose so low wpt
- xylem has much higher wpt so water moves from xylem into sieve tubes by osmosis, creating high hydrostatic pressure within them
- at respiring cells, sucrose is used up during respiration or converted to starch for storage
- these cells have low sucrose content so sucrose is actively transported into them from sieve tubes, lowering their wpt
-this lowered wpt means water also then moves into these respiring cells via osmosis
- hydrostatic pressure of sieve tubes is lowered
-water then moves from high hydrostatic pressure at source down pressure gradient to low pressure at sink down sieve tube(causing mass flow of sucrose)
6 pieces of evidence to support mass flow
- there is pressure in sieve tube as sap is released when cut
- conc of sucrose is higher in leaves than in roots
- downward flow in phloem occurs in daylight but not in dark
- increase in sucrose levels in leaf are followed by similar increase in phloem
- metabolic poisons or lack of oxygen stops translocation in phloem
- companion cells possess many mitochondria and readily produce ATP
3 pieces of evidence questioning mass flow
- function of sieve plates is unclear(hinder mass flow)
- not all solutes move as same speed
- sucrose is delivered at more or less same rate to all regions rather than going more quickly to ones with lowest sucrose concentration
