TRANSPORT OF MATERIALS IN A FLOWERING PLANT
Water Uptake By Roots Water Movement
Up A Plant Control of Transpiration in the Leaves Transverse
Section of a Leaf
Stomata
Opening Factors Mineral Uptake by Roots Carbon Dioxide Uptake and Transport Products of
Photosynthesis
Food
Storage in Plants Gas Exchange in Stems Internet Links
Plants
must:
1. Take in water, mineral salts, and carbon dioxide.
2. Eliminate waste water and oxygen.
3. Distribute food within the plant after it is made in the
leaves.
The following will discuss how the plant does these
things.
1. Water Uptake By Roots:
A. Carried
out by root hairs located at the zone of differentiation.
B. Many root hairs increase the surface
area available for water absorption.
C. Fugal hyphae attached to roots also absorb
water and pass it on to roots.
D. The
process of water intake at the roots is called osmosis. Osmosis is the
movement of a substance through a membrane. Water moves because the
overall water potential (amount of
water) in the soil is higher than the water potential in the roots and plant
parts. Water continues to diffuse from the inside of the root hairs, through
the ground tissue and into the xylem of the root. The water can then travel up through the xylem of
the root and stem, into the petiole, and into the leaves of
the plant.
UPWARD MOVEMENT OF WATER WITHIN THE PLANT
There are 2 processes that
enable the water to move up a plant. They are root pressure and transpiration.
1. Root Pressure: Water
moves into the roots. As new water moves into the roots it causes the water to
move up the plant. Root pressure is capable, under ideal atmospheric
conditions, of pushing water one or two feet above the ground.Since root
pressure is not strong enough to move water up very high another process is
needed to enable the water to continue up the plant. This is transpiration.
2. Transpiration- Transpiration
is the loss of water through the leaves and other parts of the plant. Most
transpiration occurs through openings, called stomata, on the underside
of the leaves. Water moves, because of root pressure, up into the stem.
Because water is being lost out of the stomata of the leaves the water in the
stems is being pulled up. Why is this? Because water molecule cling to each
other by cohesion. As water molecules cling to each other as they move
up the stem and into the leaves they pull the molecules up as they tranpire out
of the stomata. This is called The Cohesion-Tension Model of water
transport in xylem. As water molecules are stuck together by cohesion the
entire column of water in the xylem adheres to the sides of the xylem.
It is said that the water in under tension as the column moves up the
xylem. At the same time, the xylem tube narrows because of the tension.
In Summary…
The Cohesion Tension Theory
Water is a polar molecule. When water molecules approach they form a Hydrogen Bond. The negatively charged oxygen atom of one water molecule forms a hydrogen bond with the positively charged hydrogen atom in another water molecule.
Water enters the xylem in the roots by Osmosis. Once in the xylem the water molecules hydrogen bond forming a continuous string of water molecules up to the leaf.
Water is constantly lost by Transpiration in the leaf. When one water molecule is lost another is pulled along. Transpiration pull is the main cause of water movement.
Control of
Transpiration in the Leaves
Plants must have the ability
to prevent too much water from transpiring out of the the stomata
(stoma, singular) of the leaves. If they didn’t do so they would lose too
much water and the plant would shrivel up and die.
They accomplish this in the
following ways:
1. Plants have a waxy coating called a cuticle on the
upper surface. This stops transpiration through the upper surface. The rate of
evaporation would be high on the upper surface because that is the surface that
faces the sun. The stomata are located on the lower surface.
2. The stomata of plants have 2 guard cells that
surround the opening (stomata). These 2 cells are capable of swelling or
shrivelling. When they swell the stomata are open and exchange of water and
gases can occur. When they shrivel the stomata are closed. Materials can not
pass through the stomata.
When water enters the guard
cells the cells become swollen or turgid. The guard cells are joined at
the tips. The turgidity makes the guard cells buckle in the middle and
thus open the stoma. When the guard cells lose water they shrink and this
causes the stoma to close.
The amount of CO2 present is
another factor in the opening of the stomata. High levels of CO2 in the air
spaces of the leaf causes the stomata to close. This occus at night. In the
daytime when the CO2 is entering the mesophyll and to carry out photosynthesis
the stomata open. (see page 247, fig. 25.17 of book.)
Stomata are usually open in
the days when photosynthesis is occurring and closed at night when
photosynthesis stops.
Stomata also close in the day
if the plant is losing too much water and in high temperatures. This is why a
plant grows more in cooler temperatures than in the hot summer. Photsynthesis
ceases when the stomata are closed.
Minerals enter the root by
the root hairs. The entry of these minerals requires energy. This is called active
transport of materials. Root hairs contain many mitochondria to
supply this energy. Mitochondria are organelles within the cytoplasm of
cells which convery carbohydrates into energy by cellular respiration.
Carbon
Dioxide Uptake and Transport
Photosynthesis occurs mostly
in the mesophyll layer of the leaf. Carbon dioxide must get to this layer.
There are 2 sources of carbon doxide for the mesophyll:
1. Most of the CO2 come in through the stomata. It then
diffuses through the air spaces and into the spongy and palisade cells.
The rate of CO2 uptake is
called the apparent rate of photosynthesis.
2. CO2 is produced in the cells of the leaf through cellular respiration within the leaf. The leaf carries on cellular respiration in
order to obttain energy for its life processes. More CO2 is produced in this
way during high temperatures.
The real or true rate of
photosynthesis = rate of CO2 through stomata +
rate of CO2 produced by respiration.
Photosynthesis takes place in the ground tissue of the
leaf. (Mesophyll layer). It occurs as chlorophyll is used with the sun’s energy
and CO2. The 2 main products of photosynthesis are Oxygen and Glucose.
What Happens To These 2 Products of Photosynthesis?
1. Oxygen- a. Diffuses through the air spaces of the
leaf and then out of the stomata.
b. Some of the oxygen is used by the leaf
for cellular respiration.
2. Glucose- a. Some of the glucose made at the leaf is
used by the leaf for respiration.
b. Most is converted into other
carbohydrates and used by parts of the
plant.
1. Starch stored in the leaves is food and
energy for leaf eating animals.
2. Some glucose is converted into sucrose
and carried by the phloem sieve tube cells to other parts of the plant.
This sugary water carried by the phloem is called phloem sap. This is
carried to growth areas of the
plant where it is needed for new
growth, for respiration, or is stored as starch
Plants store food in their roots, stems, and leaves by
modifying or changing them.
1. Modified roots- Some
dicots produce large, v-shaped taproots. In some plants such as the
carrot or turnip the tap root becomes swollen and fleshy with stored food. The food is used by the plant
later in its life cycle, People harvest the root before the food is used up so
that we can eat it.
2. Modified stems- Potato
plants produce and underground stem system. The tips of the underground stem
become swollen with stored starch. These swollen tips are called tubers.
The tuber is what we know as the potato.
If not harvested and eaten the potato grows new potato
plants from buds on the potato called the “eyes” of the potato.
3. Modifies leaves- Bulbs
contain and underground stem that is reduced in size/ Swollen, fleshy leaves
which are modified to store food are attached to the stem. There is an apical
bud and many lateral buds. Celery and rhubarb are leaf petioles
that are modified to store food. (See page 244, fig. 25.11 of book).
Cells within the stem of
plants need oxygen to carry on respiration. As a result, they produce carbon
dioxide and water as a waste product of respiration.
The bark of the stems of trees and shrubs have openings called lenticels
which allow for the movement of oxygen into and CO2 and water out of them.
Quiz Yourself On The Parts
Of The Cross Section Of A Leaf
View An
Animation of sucrose moving through phloem.
View
An Animation of sugar and water moving through xylem and phloem
View An
Animation of a general view of photosynthesis
Take
an online quiz on the difference between monocot and dicot roots