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Transport
in plants
Adrian
Whyte and Joanna Johnson, Contributor
 JUST
LIKE animals, plants also need to
have substances transported throughout
their structure. The system in plants
is much simpler than in animals. Here
are a few of the differences:
-
Plants do not have a pump.
- Substances
are carried in solution in water.
- There
are separate vessels for carrying
water and minerals and for food.
- It
is not a circulatory system.
CARRIAGE
OF WATER
Water
and mineral salts are carried in plants
through special vessels called xylem.
Xylem
vessels are well suited for this job
as they are dead, elongated cells
that are hollow and connect end to
end. Also, the ends where the cells
meet are completely eroded. Therefore,
xylem looks like pipes attached end
to end throughout the entire plant.
Water
passes through the root hairs in the
soil by the process of osmosis. Minerals
also enter through the same way but
by the process of active transport.
Once inside the root hairs, the water
makes its way to the centre of the
root where the xylem vessels are.
The
water then rises up the xylem and
makes it way to the leaves. The stream
of water from root to leaves is known
as the transpiration stream. The process
by which the excess water exits the
leaves is known as transpiration.
It is affected by a few environmental
factors.
Various
factors will increase the rate of
transpiration.
Increasing light intensity causing
full stomata opening.
Increasing temperature increases kinetic
energy of H2O molecules causing faster
movement.
Decreasing humidity means air can
hold more water and so increases the
concentration gradient between air
and the leaves.
Increased air movement (wind) remove
saturated air from around leaves.
Decreased stomata density prevents
overlap of diffusion cells.
CARRIAGE
OF FOOD
 Food,
primarily sucrose, is transported
by the vascular tissue called phloem
from a source to a sink.
Unlike
transpiration's one-way flow of water
sap, food in phloem sap can be transported
in any direction needed so long as
there is a source of sugar and a sink
able to use, store or remove the sugar.
The
source and sink may be reversed, depending
on the season, or the plant's needs.
Sugar stored in roots may be mobilised
to become a source of food in the
early spring when the buds of trees,
the sink, need energy for growth and
development of the photosynthetic
apparatus.
Phloem
sap is mainly water and sucrose, but
other sugars, hormones and amino acids
are also transported. The movement
of such substances in the plant is
called translocation.
As
glucose is made at the source (by
photosynthesis, for example), it is
converted to sucrose (a dissacharide).
The sugar is then moved into companion
cells and into the living phloem sieve
tubes by active transport.
Water
moves into the phloem by osmosis from
the adjacent xylem.
Again
active transport is necessary to move
the sucrose out of the phloem sap
and into the cells which will use
the sugar converting it into
energy, starch, or cellulose.
PHLOEM
TISSUE
Phloem
tissue is composed of sieve tube cells,
which form long columns with holes
in their end walls called sieve plates.
These cells are alive, but they lose
their nuclei and other organelles,
and their cytoplasm is reduced to
strands around the edge of the cells.
The
centre of these tubes is empty. Each
sieve tube cell is associated with
one or more companion cells, normal
cells with nuclei and organelles.
These companion cells are connected
to the sieve tube cells by special
pores, and provide them with proteins,
ATP and other nutrients.
*
Adrian Whyte and Joanna Johnson
teach Biology at Ardenne High School
masterbio@gmail.com.
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