WATER AND TRANSPORT
o Water is essential for firmness and growth, transport and it is a solvent
o It is easily lost due to CO2 uptake for photosynthesis.
o Primary production -The amount of light energy converted to chemical energy (organic
compounds) by autotrophs in an ecosystem during a given time period
o Small decreases in water content lead to high amount of stress
o Higher water availability leads to higher primary production in agriculture and in ecosystem (
objasni zasto )
o Water is a suitable solvent and easily uptaken and transported
because it is a polar molecule and polar molecule like ions and
sugars dissolve easily in it , hydrogen bond cause adhesion and
cohesion which will help in transport
o Parts of the plant that do transport: xylem- water and nutrients
from root to all parts of plant , only upwards and phloem-
carbohydrates
o All parts of plant have respiration
TRANSPORT OF WATER
Driven by water potential – potential energy of water or its
ability to perform mechanical work
o Water potential : Ψw = Ψs + Ψp + Ψg
o osmotic potential Ψs = pressure resulting from the property of water to move from hypotonic to
hypertonic solutions
o inside plants Ψs always < 0
o in pure water Ψs = 0
o Hydrostatic pressure Ψp = physical pressure resulting from turgor (Ψp > 0) or tension caused by
cohesion of the rising water in a vessel (Ψp < 0)
o Gravitational potential Ψg = pressure exerted by the position of the water column in the field of
gravity (Ψg > 0)
o Unit is Pa = J m-3 = N m-2
o Turgor- cell has more water then it can hold but cell wall will keep in place- pressure on the wall
o Tension- the pressure exerted by a fluid at rest due to the force of gravity, makin negative
hydrostatic pressure
o Ψs = - (n/V) · R ·T
o R=8.314 J K mol
o T=temperature
o V= volume of solution
o Ψg = ρw · g · h
o with water ρw = density of the liquid (in kg m-3) = 0.998 · 103 kg m-3
o g = Gravitational acceleration = 9.8 m s-2
o h = height (in m)
o = 0.998 · 103 kg m-3 · 9.8 m s-2 · 1 m = 9.8 · 103 kg m-1 s-2 = 9.8 · 103 Pa ≈ 0.01 MPa
,o Movement of water towards the cell and from the cell depends on the osmotic potential ,
hydrostatic pressure and gravitation potential ( height in tree)
o When uptake and evaporation are unbalanced turgor loss will happen quickly
o Transport inside the cell is mostly due to diffusion and it is calculated tc=½ = K · d2/Ds
o Tc=1/2- time it takes until half the concentration of the starting point is reached ( znaci pola stvari
iz jednog u drugo )
o K= 1 at the room temperature
o d – distance
o Ds= diffusion conctant
o However this is mostly in algae large distance transport is not done by diffusion because for the
10 cm leafe would nee around 116 days to reach the destination
o Water is moved by the diffusion through the channels in the membrane = aquaporins , it is passive
and movement depends on water potential , also can move by diffusion through the cellular
membrane
o Water uptake is mostly in the roots and it is lost through evaporation , Driver of water transport
from the roots, through the xylem, to the leaf is the difference in Ψp, it is caused by hydrostatic
pressure
o The waxy layer on the leaf is the cuticle and it reduce3s the rate of water loss
o The atmosphere usually has a low water vapour pressure which leads to diffusion of water from
the leaf to the atmosphere ( unutura biljke je manje negativno nego u atmosphere I voda ide ka
vise negativnom I zato izlazi iz biljke ka atmospferi)
,o Diffusion of water vapour from the leaf to the atmosphere depends
mostly on the resistance of the stomata( closing stomata increases
the resistance) and how far they are open is partly determined by the
CO2 concentration
o Increase in hydrostatic pressure leads to opening of guard cells also
the asymmetrical enforcements of the cells walls
o Evaporation depends also on the relative humidity ,(direct)
temperature,(direct) wind,(direct) humidity of the soil ( indirect) and
light (photosynthesis)
o More wind faster the evaporation , concentration gradient around the leave of high water
concertation= humidity
o Sunflowers have large vessels that transport water, while the conifers have a thin vascular system.
Therefore if there is a lot of water loss there is a lot of pressure that can't keep up with the water
loss
o Water is transpored to the leaves through the xylem
o Types of transport vessels in the xylem
1. Vessels- angiosperms
2. Tracheids- gymnosperms, angiosperms and ferns
o Xylem vessels have a large diameter and as such very little resistance against water transport but
are small enough to have a capillary function
o Trachieds: smaller/thinner, water moves through pores, more resistance in the system, stronger
negative hydrostatic pressure, shorter
o Vessels: larger, contain plates
o Both have a lot of lignin to keep them upright
o No living cytoplasm( they die )
o In vines the thickness of the stem is limiting -> less xylem vessels, but larger diameter of xylem
vessels. Allow an easier transport of water up, because there is only a smaller area, so there is less
resistance to get the water all the way up
o Disadvantage of having larger diameter of vessels is that capillary function will decrease
o Capillary action helps bring water up into the roots.
o Wood is completely consisten of xylem vessels and tracheids
o Gravitational potential increases with the plant height and hydrostatic pressure works in he
oppositw direction as a result of the evaporaton in the leaves ( water transport to the leafs is only
possible if the Ψp + Ψg < 0)
o Due to force on the water column in a plant , caviation can occur = water column breaks and
xylem vessel fills with air , water in this vessels stop
o Diamtere of the xylem vessels- the smaller they are the less susceptible they are to this
o It also decreases the strength of wood
o Root pressure can resolve the air bubbles in the xylem vessels , this pressure is caused by the
active transport of ions to the root xylem and the osmotic pressure increases between the cortex of
the root and the vessels which leads to tranposrt of water to the xylem vessels and increase in
hysrostatic pressure = mass flow of water through the xylem vessels to the leaves , decreasing the
caviation bubbles ( embolies) – under influence of the pressure / also present after winter when
there are no leaves
o This only happens when the humidity of the air is sufficiently high or when stomata is closed
because evaporation prevents the buildup of ion concentration in the xylem
, o Root pressure can cause guttation ( voda an krajevima lista) , droplets through hydathodes
o Guttation- The exudation of water droplets from leaves, caused by root pressure in certain plants
o Transport of water through the cortex happens through:
• the cytoplasm of cells (symplast)
• the cell walls(apoplast)
o Water taken by root hair from the soil , by the symplasmic pathway it is taken by the cytoplasm
through endodermis to the vascular bundles
o Water taken by the apoplastic pathway and then cell wall
o However, transport to the vessels is always through the symplast of the endodermis – the
Casparian strip is impenetrable for water
o Uptake of water from the soil:
• Through the root surface
• Root hairs increase surface area
• Passive flux
• Depends on the continuity of the H-bridges in the soil
• Depends on the matrix potential of the soil particles = their affinity to bind water through
adhesive and capillary forces (Ψp soil)
o Water uptake is easier from the sand then from the clay (hold nore water)
o Water is sometimes uptaken only from the root tip ( less suberized) and other times from the
entire length of the root it depends on the extent of suberin deposition in the cell walls of the
exodermis
o It depends on the extent of suberin deposition
in the cell walls of the exodermis
Questions
1. Why is water essential for plants?
2. Why can a plant not live without water loss?
3. Why is homeostasis in water content so
important?- Homeostasis is essential because it
provides a balance between fluid loss and fluid
intake
4. What are the most important transport routes
for water, nutrients and carbohydrates?
5. What is the definition of water potential?
6. What are the components of the water
potential?
7. What is osmotic potential?
8. What is hydrostatic pressure?
9. What is the gravitational potential?
10. Under which circumstances are the
aforementioned components important, and
when are they negligible?
11. How can you determine the unit for water potential?
12. Which factors influence the osmotic potential?
