Chapter 9 Transport in Plants Flashcards
What is the need for plant transport systems?
Metabolic demands - many parts of the cell don’t photosynthesise. There is a need for transporting of minerals to areas where they are needed.
Size - some plants are really large so there is a need for the nutrients to be passed through out the whole plant.
SA:V - the roots and stems of plants have a bad SA:V so they cant rely on diffusion.
What is a dicotyledonous plant
they make seeds that contain two cotyledons, and contain the food stores for the developing embryo and form the first two leaves when a seed germinates.
What are the two types of dicotyledonous plants?
herbaceous - soft tissues and short life cycles.
arborscent dicots - have hard lignified tissues and long life cycles.
Where are the vascular bundles in the leaf, stem and root
Leaf - the midrib has the vascular bundle
Stem - vascular bundles are around the edge to give strength and support.
Root - They are in the middle to help with stand tugging strains that result of the wind blowing the stem and leaves.
What is the structure and function of the xylem?
mostly non-living tissue
transports water and mineral ions
flow of materials is from the roots to the shoots and leaves.
xylem vessels are the main part which are long hollow structures made of several columns of cells which fuse together end to end.
they also have xylem parenchyma packs around the vessels which have tannin deposits and store food.
Tannin is a bitter chemical that protects from attacks from herbivores.
xylem fibres are long cells with lignified secondary walls that provide mechanical strength but are insolluble.
Lignin can be in rings, spiral or solid tubes with lots of small unlignified areas called bordered pits.
What is the structure and function of the phloem?
is a living tissue that transports food in the form or organic solutes around the plant.
They have sieve tubes which let phloem content flow through. No lignin.
phloem cells have no nucleus.
close to them are companion cells which are linked to sieve tubes by plasmodesmata.
contains supporting tissues including fibres and sclereids,
Why is water is essential for metabolism and structure of plants
There is turgor pressure as a result of osmosis in plant leaves.
turgor also causes cell expansion
the loss of water by evaporation helps to keep plants cool
mineral ions and the products of photosynthesis are transported in aqueous solutions
water is a raw material of photosynthesis.
How does water move into the root
root hair cells are the exchange surface
microscopic size allows them to penetrate easily between soil particles
each microscopic hair has a large SA:V ratio and there are 1000s
each hair has a thin surface layer through which diffusion and osmosis can take place quickly.
conc. of soluties in the cytoplasm of root hair cells maintains water potential gradeint
The symplast pathway
the water travels continuously between cells through the cytoplasm. At cell boundaries the water travels through the plasmodesmata.
The apoplast pathway
water travels through the loose gaps between the network of fibres in the cellulose cell wall. The water is pulled through because of its cohesive properties.
What is the casparian strip
It is present in the endodermis which is a group of cells surrounding the vascular bundle. It is a band of waxy material called suberin.
Water in the apoplast pathway cant go any further so is redirected to the symplast pathway.
Water has to go through the partially permeable membrane.
The endodermal cells will also send mineral ions into the xylem by active transport so the xylem will have a low water potential.
This increases the rate of water moving into the xylem because of gradient.
Evidence for the role of active transport in root pressure
some poisons such as cyanide stop ATP production in mitochondria. If cyanide was added to root hair cell the root pressure dissappears.
Root pressure increases with a rise in temperature and falls with a fall in temperature.
if levels of oxygen or respiratory substrates fall, root pressure falls.
Root pressure
The active pumping of minerals into the xylem, results in root pressure.
It gives water a push up the xylem
The process of transpiration
The leaves are used in capturing sunlight for photosynthesis. They have waxy cuticles which prevents water from being lost.
CO2 moves out of the leaves and O2 moves into the leaves at the stomata along the conc. gradient. The stomata is opened and closed by guard cells, which surround the stomatal opening.
This also allows water to diffuse out when the stomata are open.
Transpiration is the loss of water vapour from the leaves and stems of plants.
Stomata open and close to control the amount of water lost by a plant, some stomata are open at all times
The transpiration stream
The transpiration stream is the process where water enters the roots of the plant by osmosis and is transported up in the xylem until it reaches the leaves. It moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem through the cells of the leaf where it evaporates from the freely permeable cellulose cell walls of the mesophyl cells into the leaves into the air spaces. The water vapour then moves into the external air through the stomata along a diffusion gradient. This is the transpiration stream.
How is the transpiration stream a passive process if the xylem are non-living
Water molecules evaporate from the surface of mesophyl cells and into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient.
This lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis
This is repeated right across the leaf and water moves out of the xylem into the cells of the leaf by osmosis.
Water molecules form hydrogen bonds with eachother -cohesion and with carbohydrates in the xylem walls - adhesion and this is transpiration pull which works against gravity. Known as cohesion-tension theory.
This creates tension in the xylem and helps to move water across the roots from the soil.
Evidence for the cohesion-tension theory
Changes in diameter of trees - when transpiration is at its height during the day, the tension in the xylem vessels is at its highest too. At night tension is low because low levels of transpiration - you can measure the diameter of the trunks at night and day.
When xylem vessel is broken - when cutting flowers - water is drawn up not leaking out and air is pulled in.
If air is pulled in then the plant can no longer move water up the stem as the continuous stream of water molecules is broken.
Factors effecting transpiration
Light intensity - if it is higher then more stomata will be open, if it is low then less stomata will be open.
Relative humidity - if it is humid then conc. gradient is low so there is lower transpiration. If it isn’t as relatively humid then conc. gradient is high then there is more transpiration.
Temperature - an increase in temp will increase the kinetic energy of molecules so they will evaporate out faster. It will also increase the concentration of water vapour that the external air can hold before it becomes saturated.
Air movement - if there are high levels of wind then the water vapour outside the stomata will be constantly moving and this will maximise the conc. gradient causing an increase in transpiration
Soil-water availability - if the amount of water available in the soil is low then the plant will be under water stress and the rate of transpiration will be reduced.
What are sources and sinks?
Photosynthesis produces glucose but this breaks down into sucrose and this is transported as assimilates from the sources to the sinks
sources of assimilates include:
- green leaves and stems
- storage organs such as tuber and tap roots
Sinks that require assimilates include:
- roots that are growing
- meristems that are actively dividing
-any part of the plant that are laying down food stores.
Phloem loading - symplast route
The sucrose diffuses across cells of the sources to the phloem cells through plasmodesmata. This is a passive process.
Phloem loading - apoplast route
The sucrose travels down the cell walls and inter-cell spaces to the companion cell by diffusion which is maintained by the removal of sucrose into the the phloem vessels.
Hydrogen ions are pumped out of the companion cell by a proton pump. This hydrogen ion returns to the companion cell down a cotransporter which also moves sucrose with it into the companion cell.
What happens in the phloem when the assimilates enter?
As the assimilates enter the companion cell they will cause a decrease in water potential, and this will cause water to move in by osmosis. This will causes a build up of turgor pressure. This causes the assimilates to move into the phloem and move up or down the plant by mass flow to areas of lower pressure.
Phloem unloading
The sucrose is unloaded into the phloem at any point into the cells that need it. The phloem unloading seems to be by diffusion of the sucrose from the phloem into the surrounding cells. The sucrose move on into other cells by diffusion or is converted into another substance , e.g. glucose for respiration, starch for storage, so that the conc. gradient can be maintained.
The loss of assimilates from the phloem leads to a rise in water potential of the phloem. This causes the water to move out into the surrounding cells by osmosis.
Evidence for Translocation
Advances in mircoscopy allow us to see the adaptations of the companion cells for active transport.
If the mitochondria of the companion cells are poisoned translocation will stop.
The flow of sugar in the phloem is about 10,000 times faster than it would be by diffusion alone, suggesting an active process.
Aphids show that when they are anaesthetised their stylet remains there and the pressure in the phloem is shown as the sap will flow out.
Xerophytes
are plants that live in conditions with low water availability. This means they have to conserve their water resources and reduce the rate of transpiration
Adaptations of xerophytes
A thick waxy cuticle - minimises water loss as their is less evaporation of the waxy cuticle.
Sunken stomata - creates a humid environment in the pits so the air doesn’t move causing the concentration gradient to be low and their will be low levels of transpiration
Reduced number of stomata - less transpiration
Reduced leaves - water loss reduced, some have needles for leaves so less transpiration.
Hairy leaves - create a humid microclimate around stomata - so less transpiration
Curled leaves - creates a humid microenvironment which reduces loss of water.
Succulents - water is stored in plentiful supply and used during a drought.
Leaf loss - some plants lose leaves when water isn’t available
Root adaptations - long deep roots, help them get as much water as possible from the soil.
Avoiding the problem - some plants will become dormant and regrow when water comes
Hydrophytes
plants that actually live in water, they need to float above water which is an issue, water logging is an issue aswell.
Adaptations of hydrophytes
Vern thin or no waxy cuticle - they don’t need to conserve water as there is always plenty availabble
Many always open -stomata on the upper surfaces - open all the time with inactive guard cells
Reduced structure to the plant - the water supports the leaves and flowers so therei is no need for strong supporting structures.
Wide,flat leaves - some hydrophytes including water lilies have wide flat leaves that spread across the surface of the water to capture as much light as possible
small roots - water can diffuse into the stem
large SA - maximises area for photosynthesis and for oxygen to diffuse into submerged plants
Air sacs - some hydrophytes have air sacs to enable the leaves and flowers to float to the surface of the water.
Aerenchyma - specialised parenchyma tissue forms in the leaves, stem and roots of hydrophytes. Functions of it include : 1. making the leaves and stems more buoyant, 2. forming a low resistance internal pathway for the movement of substances such as oxygen to tissues below the water.W
What happens when it gets waterlogged
the roots can become waterlogged. It is air which is in short supply. Special aerial roots called pneumatophores grow upwards into the air. They have many lentiicles which allow entry of air into the woody tissue.
