5. plant transport Flashcards
why do plants need a transport system?
A plant transport system ensures all areas of the plant receives a sufficient amount of nutrients to survive.
why do larger plants require specialised transports systems?
- increasing transport distance
- surface area:volume ratio
- a higher metabolic rate meaning a higher metabolic demand
list some structural differences between a monocot and a dicot plant
what are dicot plants?
plants that make seeds that contain 2 cotyledons
what are the two main groups of dicot plants?
1) herbaceous dicots (non woody stem) e.g. daises
2) woody dicots e.g. oak
what is the vascular system?
A plant has a series of transport vessels running through the roots, stems, and leaves
what are the two vascular systems?
xylem phloem
what are the two different types of plant transport systems?
- Transpiration System
- Translocation System
what is the transpiration system?
- The movement of water molecules and dissolved minerals
ions - Xylem vessels
- Passive process
what is the translocation system?
- The movement of sugars (Sucrose) & amino acids
- Phloem vessel – sieve & companion cells
- Active process
What is a vascular bundle?
- Xylem and Phloem are arranged in vascular bundles in the roots, stems
and leaves. The arrangement of xylem and phloem is different in different
organs. - There is a layer of cambium in between xylem and phloem, that is
meristem cells which are involved in production of new xylem and phloem
tissue.
Vascular Bundles in roots
- This provides a ‘drill’ like
structure - This enables the plant to push
down into the root - Xylem tissues is the strongest
so is in the centre – X structure - Phloem in four separate
sections
Vascular Bundles in leaf
- Xylem is located on top of the phloem
- This only applies to dicotyledonous plants, other plants
types have a different structure – you don’t need to know
these
Vascular Bundles in Stem
- Xylem is located on the
inside – in non-wooded
plants - This provides additional
support to the stem - The cambium layer
contains meristem cells
Structure of Xylem
Structure:
*A dead tissue - there is no cytoplasm and no
nuclei in xylem tissue.
* Hollow tubes
*Cell wall contains spiralised lignin that gives
the tissue high strength.
*Pits in wall (non-lignified areas)
Function:
*Transports water and dissolved minerals
upwards from the root hair cells to the
leaves.
*This is called the transpiration stream.
Adaptations of Xylem tissue
Structure of phloem tissue
Structure:
*A living tissue.
* Composed of tubes of elongated cells called phloem sieve
tubes.
* Transports assimilates (sucrose, amino acids etc) from the
leaves (source) to other parts of the plant (sinks).
*Process is called translocation.
* Assimilates move from one phloem cell to the next
through pores in the end walls called sieve plates.
* Each sieve tube has an associated companion cell (with a
nucleus, organelles, enzymes.
Function:
*Transports food (in the form of sucrose) upwards and
downwards, depending on where food is needed –
bidirectional transport.
*This is called Translocation
Phloem tissue adaptations
Comparison of plant tissues
Need for water
in plants
- Mineral ions and sugars
are transported in
aqueous solution - Water is a raw material
of photosynthesis - Cooling effect (by
transpiration) - Turgor pressure –
hydrostatic skeleton
Adaptations of root hair cells
- Very thin cellulose walls, that are readily permeable
to water and dissolved mineral ions. - Microscopic in size
- Large SA:V ratio
- Concentration of solutes in the cytoplasm of root
hair cells maintains a water potential gradient
between the soil water and the cell.
Structure of a root
can you label
Water movement pathways
- Symplast Pathway (through cytoplasm)
- Vacuolar Pathway (through vacuoles)
- Apoplast Pathway (through cell walls)
What is transpiration
Transpiration is the loss of water vapour (by evaporation and diffusion) from the
surface of leaves and stems of a plant.
How is the rate of transpiration controlled?
- 1) Waxy cuticle (=waterproof layer)
- 2) Guard cells can open or close stomata
- 3) Very few stomata on upper surface of leaf
Transpiration stream
the flow of water (in continuous
columns), up the xylem vessels from roots to leaves.
Adhesion
- Adhesion is the force of attraction between two particles of
different substances (e.g. water molecule and xylem wall). - The xylem wall is also polar and hence can form intermolecular
associations with water molecules. - As water molecules move up the xylem via capillary action, they
pull inward on the xylem walls to generate further tension.
Cohesion
- Cohesion is the force of attraction between two particles of the
same substance (e.g. between two water molecules). - Water molecules are polar and can form a type of intermolecular
association called a hydrogen bond. - This cohesive property causes water molecules to be dragged up
the xylem towards the leaves in a continuous stream.
Cohesion-tension theory
- Water vapour diffuses/evaporates out of the leaf via the
stomata (transpiration) from an area of high ψ to an area
of low ψ. - This loss of water vapour creates a low hydrostatic
pressure at the top of the xylem (i.e. in the leaf). - Water is drawn into the xylem in the root (higher
hydrostatic pressure). Pressure gradient is created. - This creates a tension (suction) in the xylem which pulls
up water in a continuous column. - Within the xylem vessels the columns of water are held
together by cohesion (the molecules are hydrogen
bonded to each other) and by adhesion (the attraction
between a water molecule and the walls of the xylem
vessels). - Column (of water) is pulled up by tension.
Stomata
Guard cells control the opening and closing of stomata.
Guard cells are turgid – Stomata Open
* Water moves into the vacuoles by osmosis.
* Outer wall is more flexible than the inner wall,
so to cell bends and opens the stoma.
Guard cells are flaccid – Stomata closed
* Water moves out of the vacuoles by osmosis.
* Outer wall is more flexible than the inner wall,
so to cell bends back and closes the stoma.
Conditions for stomatal opening
- Stomata open during the day and close during the night.
- High water potential outside the stomata
- Low concentrations of CO2 cause stomata to open. High CO2
causes stomata to close. - Light causes stomata to open.
How are guard cells adapted?
- Unevenly thickened (cell) wall – wall beside the pore is thicker.
- Able to change shape/bend
- Transport proteins/ion channels in the plasma membrane. Absorption
of K+
ions by the guard cells. - K+
ions decrease the water potential hence water enters by osmosis and
guard cells can become turgid. - Presence of chloroplasts & mitochondria to provide ATP energy.
Mesophytes
able to take up sufficient water
to replace transpiration (most plants).
Hydrophytes
live either partially or
completely submerged in water – problems
with O2 uptake.
Xerophytes
live in areas where water lost via
transpiration is greater than taken up by roots.
Translocation
- Translocation is the movement of assimilates within phloem sieve
tubes (e.g. sucrose/amino acids, hormones etc) from where they are
made (source) to where they are required (sink). It is an active
process. - Translocation occurs in phloem vessels.
- Requires ATP energy to create a pressure difference.
- Movement is bidirectional (from source to sink).
- Liquid being transported is called ‘phloem sap’
- Glucose is transported as sucrose in the phloem sap (20-30%)
Sources
This is the site where
sucrose/assimilates are made and
loaded into the phloem. (high
concentration).
E.g.
* Green leaves and green stem
(photosynthesis produces glucose
which is transported as sucrose, as
sucrose has less of an osmotic effect
than glucose).
* Storage organs eg. tubers and tap
roots (unloading their stored
substances at the beginning of a
growth period).
* Food stores in seeds (which are
germinating).
Sinks
- The site where sucrose /assimilates are
unloaded from the phloem for use or
storage
E.g. - Meristems (apical or lateral) that are
actively dividing. - Roots that are growing and / or actively
absorbing mineral ions. - Any part of the plant where the
assimilates are being stored (eg.
developing seeds, fruits or storage
organs).
Process of translocation
There are 3 stages to the movement of sucrose and
assimilates from source to sink.
1) Active loading at the source into the phloem sieve
tube.
2) Mass flow of sucrose through the sieve tube
elements (involves water from xylem).
3) Active unloading of sucrose at the sink
Loading of assimilates – pathways
- The pathway that sucrose molecules use to travel to the sieve
tubes is not fully understood yet. - The molecules may move by the:
Symplastic pathway (through the cytoplasm and
plasmodesmata) which is a passive process as the sucrose
molecules move by diffusion.
Apoplastic pathway (through the cell walls) which is an active
process.
- Active loading at the source
Adaptations for active loading
- Companion cells have infoldings in their cell surface membrane to increase the available
surface area for the active transport of solutes and many mitochondria to provide the
energy for the proton pump. - This mechanism permits some plants to build up the sucrose in the phloem to up to three
times the concentration of that in the mesophyll.
- Mass flow of sucrose through phloem sieve tubes.
- Sugars/sucrose/assimilates enter the sieve tube element (at the
source), this lowers the water potential in the sieve tube. - Water enters the sieve tube by osmosis from the xylem.
- This raises the hydrostatic pressure at the source.
- When assimilates leave the sieve tube at the sink, this
increases the water potential inside the sieve tube. - Water leaves the sieve tube by osmosis, down a water potential
gradient and lowers the hydrostatic pressure (at the sink). - Water moves down the hydrostatic pressure gradient (from
high to low) towards the sink, also moving sucrose (and other
assimilates) along the phloem. - This is called mass flow.
- Active unloading of sucrose at the sink
- Sucrose needs to be actively unloaded into the sink where it is needed.
- Unloading of sucrose is thought to be similar to the loading of sucrose.
- Sucrose is actively transported out of the companion cells and then moves
out of the phloem sieve tubes into the sinks via the apoplastic or symplastic
pathways. - In the sink sucrose is converted into other molecules e.g. starch. This helps
to maintain a concentration gradient. - When sucrose diffuses out of the sieve tubes, this increases the water
potential of the tube. - Water therefore moves out of the sieve tube (back into the xylem
vessels) by osmosis. - This creates a low hydrostatic pressure at the sink, compared to the
higher hydrostatic pressure at the source.
