Download Transport in Plants

Transport in Plants
Looking at the role of xylem, phloem, root
hairs and stomata and the effect of
transpiration has on these
The transport system
• The transport system is far less elaborate
than in mammals due to:
– plants are less active and therefore ‘supplies’
don’t run out so quickly
– because of the branching system, gases for
respiration and photosynthesis Can be
obtained from diffusion in the air
– two separate systems; xylem and phloem
Xylem
• Xylem carries the water and minerals
• It is made of many hollow dead cells joined end to end of
which the end cell wall has disappeared to form a long
tube
• Xylem vessels run from root to every leaf
• Xylem vessels contain no cytoplasm or nuclei
• Their walls are made of cellulose and lignin
• Lignin is very strong and so xylem help keep the plant
upright
Lignified cell wall of Xylem
Close up of Hibiscus rosa xylem
Vascular bundles
• Xylem and phloem tubes are normally found close
together, when they are this is called a vascular bundle
• In a root vascular tissue is found at the centre
• In a shoot they are found near the outside edge to help
support the plant
Transverse section of a stem
The transport of water
• Plants take in water from the soil through the root hairs
and is carried in the xylem throughout the plant
• Water gets into root hair by osmosis.
• The cytoplasm and cell sap inside it are quite concentrated
solutions and the water in soil is normally quite dilute
• Water therefore diffuses down its concentration
gradient through a partially permeable membrane
A root tip showing root hairs
OSMOSIS
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.
TRANSPIRATION PULL
The Transpiration Stream
Water enters the xylem
• The root hairs are on the edge of the root and the xylem is
in the centre
• It travels by osmosis through the cortex from cell to cell or
in the spaces between the cells by the following process
– water passes across the root, from cell to cell, by
osmosis, it also seeps between the cells
– water enters root hairs by osmosis
– water is drawn up the xylem vessel because water is
continually being removed by transpiration
TRANSPIRATION PULL
Transpiration
• The evaporation of water from the plant
• Most of which takes place from the leaves through the
stomata
• Guard cells around the stomata control the rate of
transpiration by opening and closing
Transpiration
• When water is lost through transpiration water from the
xylem vessel in the leaf will travel to the cells to replace it
• Water is constantly being taken from the top of the xylem
vessel to supply the cells in the leaves
• This reduces the pressure at the top of the xylem so water
flows up
• This process is known as the transpiration stream, or
transpiration ‘pull’
Definition of transpiration
Evaporation of water at the surfaces of the
mesophyll cells followed by loss of water
vapour from plant leaves, through the stomata
TRANSPIRATION
1. Water leaves the xylem vessels. Some
enters the mesophyll cells.
2. Water evaporates into the air spaces and
forms water vapour.
3. Water vapour diffuses out of the open
stomata into the air.
4. Air movements take the water vapour
away.
Conditions that affect
transpiration rate
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•
•
•
•
Temperature
Humidity
Wind speed
Light intensity
Water supply
THE TRANSPORT OF FOOD
Phloem Translocates organic
food
• TRANSLOCATION is the movement of
sucrose and amino acids in phloem, from
regions of production to regions of storage,
or to regions of utilization in respiration or
growth.
Source and Sink
• The part of the plant from which sucrose and
amino acids are being translocated is called a
SOURCE.
• The part of the plant to which they being
translocated is called a SINK.
• Leaves are generally the major sources of
translocated material.
• Sinks include the roots, flowers and fruits.
TRANSLOCATION
1. Sugar is made in the
leaves by
photosynthesis.
2. Sugar moves into the
phloem tubes.
3a. The phloem tubes carry
sugar to growing buds,
which need it for energy.
3b. The phloem tubes carry
sugar to the roots, where it
may be used for energy or
changed into starch and
stored.
Transport of Food
in plants
Transport the ‘food’ (sucrose
and amino acids)
They are also made of many cells
joined end to end, however their
end wall is not completely broken
down; instead they form sieve
plates. The cells are called Sieve
tube elements.
The cells contain cytoplasm but
no nucleus and they do not have
lignin in their cell walls.
Each sieve cell has a companion
cell next to it which does contain
a nucleus and many other
organelles and supply sieve tube
elements with some of their
requirements.
Movement of water and food