BIOA02 MODULE 1 LEC 9: Transport in Plants

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Published on 29 Jan 2012
Biological Sciences
BIOA02 MODULE 1 LEC 9: Transport in Plants
January 27, 2012
-plants have basic needs for doing photosynthesis they need: carbon
dioxide, water, oxygen, light and minerals
-water moves through the xylem (of the year) through the roots up;
sugars move through phloem (of the year closest to the cambium)
-there are three hypothesis as to how water moves through plant:
1. Capillary action
2. Root osmotic push
3. Transpiration
Capillary action
-due to two main foces: cohesion between water molecules and
adhesion between the molecules to the side of the capillaries pulls
water up
-the more narrow the capillary, the higher the water moves
-the question: are these forces enough to push water to the top of a 30
m tree?
-the answer is no, the maximum amount of water that can be pulled up
by capillary action is 1 m
-it’s still very important however, to maintain a connected string of
water molecule once the water has reached the xylem
Root Osmotic Push
-semi-permeable membrane between water and minerals
opening is only big enough for water to go through but not
concentration of minerals on one side is higher than on the other
-laws of physics wants concentration of minerals to be the same
on both sides -due to the semipermeable membrane, the
minerals cannot move through therefore the water has to move
to make the concentration of minerals the same on both sides
this causes the water to go higher osmotic push
-the question: is the osmotic push enough to push the water all the
way to the top of trees?
-the answer is no can only push the water up 3 m
-osmosis process is still important for the plants nonetheless
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-root hairs use osmosis concentration of minerals in root hairs much
larger than concentration of minerals outside of the root hairs in the
-root acts as a semi-permeable membrane and the it tries to even out
the pressure differences inside and outside as it lets water in
-this is the mechanism of how water is taken up by roots
-uptake of water active process plant pays ATP to actively create
this pressure gradient cells pump molecules into root hair and take
up water
-can take water up 100+ m
-top of the tree and connected to the sun heat causes water in the
leaves from liquid to gas (evaporation) happens in the intercellular
leaf space
-in the air there are fewer gas molecules than in the intracellular space
laws of physics wants there to be same partial pressures of inside
the leaves and outside
-the gaseous molecules move outside the leaves into the air where the
wind picks it up and the pressure gradient is maintained diffusion of
water outside of stomata
-water moves from parenchyma cells to intracellular space again due
to the differential water pressure and driven through the heat of the
continuing chain reaction because water moves from the
parenchyma to the intracellular space, water is sucked from the veins,
xylem into the parenchyma because water molecules stick together by
-therefore once water leaves, it is replaced by the string of water
in the veins
-water transpiration works due to the tension of removing water
molecules through evaporation and the cohesion of water molecules
inside the veins
-this then creates a pull through the xylem connecting to
branches through the rest of the plant
-sun sucks out the water in the upper end, because of continuous
xylem tissue, water gets sucked from the root to the top of the plant
-the limit is that you need the sun and the tension/pull needs to
stronger than gravity
-transpiration pull maximum is 130 m why trees can’t continue to
grow higher and higher
-the water molecules can move sideways between xylem veins
-vessel members place where adhsion/cohesion of the water string
works and pulls up the water
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