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Transport
in plants
Adrian
Whyte and Joanna Johnson, Contributor
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| Violin
soloist Jodi-Ann Wray performs
the solo from 'Meditation' by
Jules Massenet with the Immaculate
Conception High School orchestra
at Devon House, St. Andrew, on
Saturday, February 19.
- Andew Smith Photo |
JUST
LIKE animals, plants also need to have
substances transported throughout its
structure. The system in plants is much
simpler than in animals. Here are a
few of the differences:
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Plants do not have a pump.
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Substances are carried in solution
in water.
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There are separate vessels for carrying
water and minerals and for food.
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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 connected 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) removes
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 pholem 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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