Reminder of the two types of
transportation
Other than for active
transport, diffusion, naturally occurs in gases and liquids,
because all the particles (molecules or ions) have sufficient kinetic energy to move around freely
at random from a region of higher concentration to a region of lower
concentration.
Osmosis
is a particular spontaneous diffusion through a membrane from a
higher water potential region (solution less concentrated in
solutes) to a lower water potential region (solution more
concentrated in solutes).
Active transport is the movement of particles (molecules or
ions) through a cell membrane from a region of lower concentration to a
region of higher concentration using energy from respiration.
Within cell membranes there are carrier proteins
which
use energy from respiration to transport molecules or ions across
the membrane, against the natural concentration gradient, therefore cells
that use active transport usually have more mitochondria for
respiration compared to other cells.
The structure and function of plant
xylem and phloem cells - the transport tissues
Plants have two networks of
'fine tubes' to transport molecules and ions (xylem tubes and phloem tubes).
The
xylem vessel tissue
transports water and minerals from the root hairs to all the rest of plant
i.e. through the roots and stem to the very tips of all the leaves.
This xylem vessel process is
driven by transpiration.
The
phloem vessel tissue
transports dissolved sugars from the leaves (where they are made from photosynthesis)
to all parts of the plant e.g. for growth of new cells or to storage tissue
where they are converted to starch.
This function of the phloem to
move sugar molecules (mainly dissolve sucrose), amino acid molecules
and mineral ions around the plant is called
translocation.
These systems are essential for a
plant to be healthy.
In some trees the transport
systems run through the bark.
Unfortunately, some animals like
to chew this bark.
The transport systems are
disrupted and the tree sadly dies!
Consider the xylem first
The xylem tissue transports water and mineral ions
from the roots to the stem and leaves.
Xylem tubes are made of
lignified dead
cells joined together 'end to end' in such a way they form a complete fine tube through
which water and mineral ions are freely transported from the roots, up through
the stem to the leaves.
The xylem cells have no end wall but a hole down the
middle (lumen) allows the free movement of fluid - but in only one
direction.
The strong xylem cell walls are made from cellulose
and are
strengthened-stiffened by a material called lignin - these give the plant
support.
The movement of water from the roots
through the xylem and out of the leaves is called the transpiration stream
and is caused by the diffusion of water and its subsequent evaporation. The
transpiration stream only flows in one direction - up through the
plant.
Water evaporating through
the stomata in the leaves, causes it to be replaced by water absorbed by the roots and this
water moves up via the
xylem tube system through the stem and to all the leaves - from the roots to
ALL of the plant and carrying essential mineral ions too.
If the stomata pores are open, evaporation of water will
always take place because the concentration of water in the air is less than
the concentration off water in outer layers of a leaf.
The diffusion and evaporation of
water from the leaves produces a water deficiency in the plant, so water is
automatically (if available) drawn up through the xylem tube system, so the
transpiration stream is driven by this evaporation of moisture from the
leaves.
Water is essential to the plant
for both transportation and photosynthesis.
Evolution adaptation notes:
Transpiration is a necessary
adaptation to work in conjunction with photosynthesising leaves -
the stomata allow the gas exchange - carbon dioxide in, and water
vapour and oxygen out.
Even the narrow roots of plants
are further covered in tiny root hairs that greatly increase the
surface area even more, and hence increase the efficiency of water
absorption.
This adaptation means the water
has only got to move a short distance to the xylem to be transported
up through the whole of the plant.
Now consider the phloem cells and
compare their structure and function with xylem cells
Phloem cells are
elongated living cells
and the phloem tube tissues carry dissolved sugars
(food - glucose, sucrose) from the leaves to the rest of the
plant, including the tissue growing regions and the storage organs.
The phloem cells also transport other
important materials like amino acids for protein synthesis and mineral
ions for the function of chlorophyll and enzymes.
This process is called
translocation
and can operate fluid flow in
both directions - another useful
adaptation.
Two of the principal substances transported by
translocation are sucrose and amino acids from regions of
production to regions of storage, respiration (sugars) or growth (e.g.
protein synthesis).
Note that some parts of a plant may act as a source or a sink at different times during the life of a plan
The sugars from photosynthesis enter
the phloem system by active transport and transported around by
water which enter the phloem cells by osmosis.
Reminder: Osmosis as the net movement of
water molecules from a region of higher water potential (from a more
dilute solution) to a region of lower water potential (a more
concentrated solution), through a partially permeable membrane
(concentrated refers to dissolved molecules or ions).
Phloem cells are elongated with end
walls that have pores in them to allow fluids to flow through and the
phloem system allows transportation in both directions.
Phloem tubes are sometimes called sieve tube elements and the perforated end-plates allow fluids to
pass through.
The phloem cells (sieve tube
elements) have no nucleus and can't survive on their own, so each one
has a companion cell (not shown, but has nucleus) that controls the living functions for
both cells.
The companion cell, with a
nucleus, has lots of mitochondria to provide energy for the
phloem's transportation function.
They allow the transport of water and dissolved substances
to all part of the plants where nutrients are needed for immediate use in
growth or converted to starch for storage
The phloem tubes mainly
carry the sugars made in the leaves from photosynthesis up and down
the stem to all parts of the plant for immediate use in respiration, new
growth or to the food storage organs to form starch.
Phloem cells contain a little cytoplasm, but no nucleus and have very few
sub-cellular structures, this adaptations increase the efficiency of
transportation - also aided by the sieve plates of adjoining
cells.
