Plants need water and minerals
Plants need to obtain raw materials from their environment. The roots of the
plant are adapted to absorb minerals and water from the soil.
Water is a solvent for biochemical reactions.
Mineral ions are needed for chlorophyll synthesis and healthy plant growth.
Transport of Water
Water enters plants through their root hair cells.
Root hair cells increase the surface area for absorption and speed up the
water and mineral uptake into the plant.
 Water moves from high water potential to low water potential by
osmosis.
The soil around the roots has a higher water potential than leaves,
because water evaporates from the leaves by transpiration. Therefore,
water moves from roots to leaves down the concentration gradient.
Plants transport ions such as nitrates to the root hair cells by active
transport against the concentration gradient. This lowers the water
potential, so more water moves in by osmosis.
Root structure
There are 3 root tissues:
1) Epidermis:
This is the outer layer of the cells with root hair cells. The cells are
closely-packed, with no cuticle, chloroplast and stomata.
Functions:
-Protects the underlying tissues of the root.
-Epidermal cells and root hairs absorb water and mineral ions.
2) Cortex:
This is the inner side of the epidermis and surrounded by larger storage
parenchyma cells. Parenchyma cells store organic products in the plant.
Functions:
-The cortex allows diffusion of water, mineral salts and oxygen
-Stores reserve food, especially starch.
-Transports water and salts from the root hairs to the centre of the root.
3) Endodermis:
This is the innermost layer of the cortex. It contains a waterproof layer
which is called the Casparian strip.
Function:
-The Casparian strip prevents
movement of water between the
cells and facilitates the movement
of water from cortex to xylem.
-When the root hairs absorb
water, it gets through the cortex
and endodermis.
Xylem: Transports water and minerals from the roots to the upper
parts of the plant.
Xylem tissue contains long xylem vessels which are adapted for the
rapid transport of water and dissolved mineral ions.
 Water and mineral ions only move up the
stem. Evaporation from the leaves creates
suction. This suction effect pulls the water
up the xylem vessels.
 Xylem does not have any organelles and
the cells are dead, so there is no
obstruction to the flow of water and mineral
ions.
 Xylem vessels do not have end walls which
prevent water flowing through them freely.
The routes of water that can take through the root
1) Apoplast Pathway
2) Symplast Pathway
1) Apoplast Pathway:
Water goes through the non-living parts of the root; the cell walls. A
cell wall is very absorbent, so water can diffuse through it easily.
2)Symplast Pathway:
Water goes through the living cytoplasm of the cells. The cytoplasm
of all the cells connects through the plasmodesmata.
Plasmodesmata: are little strands that pass through small holes in the cell
walls.
The main pathway is an apoplast pathway because it provides less
resistance than a symplast pathway.
When the water reaches the endodermis, water flow is stopped by a waxy
strip which is called the casparian strip and it is impenetrable. Therefore,
water cannot pass through this strip.
As the water is stopped, it has to go through a cell membrane by another
pathway.
The cell membrane controls the movement of substances in the water.
Once it has moved across the endodermis, it continues down the water
potential gradient until it reaches a xylem vessel. Then, it moves up towards
the leaves.
Transport of Water by Xylem
There are 3 ways that xylem transports
water against the force of gravity:
1)Cohesion –Tension Theory
2)Root Pressure
3)Capillarity
1) Cohesion-Tension Theory:
It explains the movement of water from roots to leaves,
against the force of gravity.
Tension is needed to pull more water into the leaf. It is
created by the evaporation of water from the leaves at the top
of the xylem.
Water molecules stick together by using hydrogen bonds to
create the cohesion, so when some are pulled into the leaf
others follow.
Therefore, the whole column of water in the xylem moves
upwards (from the roots down to the leaves) by mass flow.
2) Root Pressure:
Water moves from the roots into the xylem, this creates a
pressure.
Root pressure is created by the osmotic pressure.
Root pressure is a force that moves the water upwards in the
xylem by active transport.
Root pressure is less important than the simple movement of
water (down the water potential gradient), because energy is
required for the active transport.
This pressure is weak, so water cannot reach to the top of bigger
plants
It moves water in young, small plants where the leaves are still
developing.
3)Capillarity:
It helps to move water up in a thin tube.
It happens because molecular attraction occurs between
water molecules and the inside of the xylem tube. This is
known as adhesion.
Movement of water takes place through a combination of
these 3 methods. They are not powerful enough to move
water on their own.
http://academic.kellogg.edu/herbrandsonc/bio111/animations/0031.swf
Water uptake can be
measured by Potometer
Potometer: Is an apparatus that is used to measure the rate
of water uptake.
It contains;
- A part that stores water. This part prevents air bubbles entering
the plant and moves the bubble back along the capillary tube.
- The capillary tube with a scale to measure the rate of water
uptake. It measures by timing how long the water level takes to
move a certain distance along the scales.
- A tap which is closed during the experiment and prevents the
water flow.
 Water uptake is closely linked to the amount of water that is lost by transpiration.
Transpiration: Is the loss of water from the parts of plants by evaporation.
It happens because there are holes (stomata) in the leaves to let gases
(carbon dioxide and oxygen) exchange during photosynthesis and
respiration.
Water evaporates from the moist cell walls and accumulates in the spaces
between cells in the leaf.
Then, it diffuses out of the stomata when they are open. This happens
because there is a diffusion gradient which means there is more water
inside the leaf than in the outside of the air.
Potometer can be used to investigate the effect of abiotic factors on
the rates of transpiration
1) High temperature; increases the water-holding capacity of the
air and increases the transpiration.
2) Wind; moves humid air away from the leaf surface, so
transpiration increases.
3) Low humidity; increases the water potential gradient between
leaf and atmosphere, so it increases the rate of transpiration.
4) High light intensity; causes stomata to open for
photosynthesis which allows transpiration to occur.
Transport of Mineral Ions
Plants absorb dissolved mineral ions from the soil by diffusion and active
transport.
If the concentration of ions is higher inside the cell than the outside of the
cell ions move up by active transport. Moreover, plants can pump out the
ions which are not needed.
Mineral ions travel in the xylem with water by mass flow.
 Scientists have proved this by using radioactive tracers. This
experiment uses radioactive forms of ions, so radioactivity can be
detected. Therefore, it shows where the ions have moved to.
Functions of Different Mineral Ions
1) Nitrate:
It is needed to make amino acids which are needed for protein synthesis.
Also, amino acids are needed for organic bases and nucleic acids.
2) Phosphate:
It is needed in photosynthesis and respiration reactions.
For example; there are phosphate groups in ATP and NADP.
It is also needed for phospholipids and in the backbones of DNA and
RNA.
3) Magnesium:
It is needed to make chlorophyll which is essential for photosynthesis.
Questions
1) Explain how water enters a plant root from the soil and
travels through to the endodermis.
2) From the root, water is transported upwards through the
stem. Explain how evaporation from the leaves can cause
the water to move upwards.
3) In daylight, most of the water evaporates from the
leaves but some is used by the plant. Describe the
ways in which this water could be used by the plant.
Answers
1) Water enters into root hair cells by osmosis, because uptake of
mineral ions has created a water potential gradient. Water moves
through the cortex down a water potential gradient by osmosis.
Then, water goes through the cell walls (apoplast pathway) and
cytoplasm (symplast pathway).
2) Water lost by evaporation, so water potential decreases in leaf
cells. Therefore, water moves out of the xylem by osmosis. This
creates a tension. Tension is needed to pull more water into the
leaf. Water molecules stick together by hydrogen bonds to create
the cohesion. Therefore the whole column of water moves upwards
in the xylem.
3) Water is used in light-dependent reactions of photosynthesis.
Electrons from water produce ATP. Water can be used in hydrolysis
reactions within the plant. It is used as a solvent for transport and
biochemical reactions. Also, it used to create a turgor pressure.
References
1) A2-Level Biology. Editors; Ellen Bowness, Kate Houghton,
Simon Little, Kate Redmond, Rachel Selway, Jennifer
Underwood. Published by Coordination Group Publications
Ltd 2005. Pages 60, 90, 91. ISBN-10: 18146 84146 365
2) www.bcb.uwc.ac.za/Sci_Ed/grade10/anatomy/roots.html
3) htttp:// extension .oregonstate.edu/mg/botany/roots.html
4) http://sharon-taxonomy2009p2.wikispaces.com/Gymnosperms
5) http://www.dr-evans.com/advancedbiology/potometer.html
accessed by 11/03/2011
Any questions
Thanks for listening

Prepared by
Neslihan Ozekman