Transport of substances in plant Flashcards
(23 cards)
How is ions absorbed by plants?
By active transport and diffusion at root hairs
What is mycorrhizae?
Structure created by mutualistic (symbiotic) association between a fungus and a plant root
Majority of land plant
Generally young plant roots are infected and once infection occurs, root hair production ceases or is reduced
How does mycorrhizae function?
Fungi absorbs ions and water and pass to plant and in exchange receive organic compounds and vitamins
Fungi hyphae provides larger surface area
Ectotrophic mycorrhizae?
Form a sheath around the root and
penetrate the air spaces between the cells
in the cortex but not cells
Form extensive intercellular net
Found mainly in forest trees
Involve fungi of the mushroom group and their ‘fruiting bodies’ (mushrooms) are commonly seen near the trees
Endotrophic mycorrhizae?
Occur in virtually all other plants
Form an intercellular network and extend into the soil, but they appear to penetrate cells (although in
fact they do not break through the surface membranes of the root cells)
Pathway for movement of water across root cell?
apoplast, symplast and vacuolar
Apoplast Pathway?
H2O through the cellulose cell walls of adjacent cells and
small intercellular spaces between
The cell walls are fully permeable across the root except for the endodermis
Casparian strip cell walls of the endodermis prevents passage of water and ions via the apoplast
Therefore water and ions are forced to pass through the symplast pathway into the cells
This allows some control of the movement of water into the xylem
Symplast Pathway
This is the movement of water, by osmosis, through the inter-connecting cytoplasm of adjacent cells
The water travels through the plasmodesmata, thin strands of protoplasm linking the cytoplasm of adjacent cells
Structure of xylem?
It consists of xylem vessels and tracheid
These are present in the inner part of each vascular bundle
The xylem tissue is dead, no cell contents
So hollow tubes so minimal resistance for water and ions
Cell wall has been strengthened by lignin
This makes their cell walls more rigid
providing support and is impermeable to water
In tracheids the end walls have partially broken down
Both vessels and tracheids have bordered pits (holes) that allow water and solutes to move laterally to adjacent vessels
Root pressure hypotheses for transpiration?
Root pressure involves a positive hydrostatic pressure
Can be observed when a freshly cut root stump continues to exude sap
Involves active transport of mineral ions by endodermal cells into the xylem of the root, lowering its water potential, so water moves in by osmosis
If the production of ATP via respiration is decreased root pressure should be decreased as it is an active process
Root pressure cannot account for the transport of water to the top of trees
Cohesion-tension hypothesis?
Due to transpiration water leaves leaf
Water evaporates from the mesophyll cells and diffuses out through the stomata into the air
The water potential of these mesophyll cells is reduced compared to inner mesophyll cells
Water moves from adjacent cells along this water potential gradient
Movement of water is by the apoplast and symplast pathways
This water potential gradient extends across the leaf mesophyll cells to the xylem vessels
Water is drawn from the xylem creating a tension in the xylem vessels “pulling up”
the water and dissolved ions
The water column is due to xylem cohesive forces and adhesive forces
The upward movement of the water from the xylem in the roots maintains the water potential
gradient across the root cortex cells, providing the mechanism for water uptake from the soil
Where is organic substances transported?
Respiring cells which do not carry out photosynthesis
Growing areas such as young leaves, shoot tips and roots tips
Storage areas
What is mass flow hypothesis?
Photosynthetic products are produced in the mesophyll cells in the leaves, known as the source
Sugars are actively transported into the sieve tubes by transfer cells
This lowers water potential causing water to enter
This creates a high hydrostatic pressure in the sieve tubes of leaf
Sugars are being used in respiration for growth or stored as starch
Consequently water moves out
These are sinks and have a low hydrostatic pressure
The photosynthetic products are transported along this pressure gradient by mass flow
Ringing experiment?
Remove a complete ring of phloem from a part of a plant
Transport is prevented at this point
A slight swelling would develop above the girdle
This is due to build up of products which
is prevented from being transported due to the removal of the phloem
Hydrophytes?
Freshwater aquatic plants (e.g. Canadian pondweed and the water lily)
Fewer osmoregulatory problems than any other plant types
Plant cells are surrounded by solution of a higher water potential
The cells become turgid and a point is reached where the water potentials of the surrounding water and the plant cells are equal and so no further net
movement of water into the cells
Mesophytes?
Mesophytes occupy habitats with adequate water supplies (not too wet or dry)
Problem of water loss by transpiration from all aerial parts
Reduced water loss by waxy cuticle, protected stomata (whose diameters can be regulated), a variable leaf shape and abscission (leaf fall by deciduous plants)
Halophytes?
Plants inhabiting areas of high salinity
In estuaries and salt marshes salinity is changing and may exceed that of sea water
So must tolerate the increased salinities and also hot
windy periods when the tide is out
So adapted:
-High transpiration rates and a low water potential in root cells which enables water uptake at the roots
-An extensive root network providing anchorage and a large surface area for water and iron uptake (e.g. cord grass)
-Smaller plants are able to store water when it is freely available (e.g. glasswort and seablite)
-Some species (e.g. sea milkwort) are able to regulate their salt content by excreting salt from the margins of leaves
Xerophytes?
Live in habitats where water is in short supply
A thick waxy cuticle provide long diffusion pathway so dec rate of evaporation
Reduced surface area of leaves (e.g. pine needles)
Curled leaves (e.g. marram grass) reduce the surface area for evaporation and increase the humidity in the air around the stomata
Hairs on the leaf surface trap air, which becomes saturated with water vapour
Stomata positioned in ‘epidermal pits’ beneath the leaf surface become saturated with water vapour
Extensive shallow root network and deep tap root to maximise water uptake
Close stomata during the hottest times of the day
Bergman’s rule
The total heat production of endotherms depends upon the volume of the body whilst the rate at which heat is lost depends upon surface area
Volume increases more rapidly than surface area as the size of an animal increases
Animals living in cold regions tend to be large, for example
whilst animals living in hot climates are generally smaller
This phenomenon is known as Bergman’s rule and is observed in many species,
Example is tiger, which decreases in size with distance from the poles
SThe total heat production of endotherms depends upon the volume of the body whilst the rate at which heat is lost depends upon surface area
Volume increases more rapidly than surface area as the size of an animal increases
Animals living in cold regions tend to be large, for example
whilst animals living in hot climates are generally smaller
This phenomenon is known as Bergman’s rule and is observed in many species,
Example is tiger, which decreases in size with distance from the poles
Small mammals in temperate or arctic regions have a large appetite enabling them to maintain a high metabolic rate
They have small extremities, such as small ears, to reduce heat loss and are forced to hibernate in winter
Large mammals living in hot regions, such as the elephant and the hippopotamus, have the opposite problems
The elephant has extremely large ears which are well supplied with blood, and flapping of these ears encourages heat loss by radiation and convection
The hippopotamus lacks sweat glands and adopts a similar behavioural response to temperature as the crocodile, in that it alternates its time between land and water in an
attempt to minimise the effects on its body of changes in temperature
Allen’s rule?
Species living in colder climates have smaller extremities than related species in warmer climates
This rule may be seen in closely related species of, for instance, the fox
Osmoregulation in desert-living rodents?
Animals in hot deserts like the Sahara have adapted to the lack of water and extreme hot temperatures
The Kangaroo rat is a nocturnal mammal that spends the day in deep burrows away from the heat of the sun
Kangaroo rat feeds on dry seeds and other dry plant material and does not drink
It survives on water produced by the chemical reactions of respiration and minute amounts present in its food
Have very long loops of Henle
Therefore, high levels of sodium chloride are maintained so that water can be absorbed from the kidney filtrate
The longer the loop the more water can be
reabsorbed because the concentration gradient of sodium chloride is greater and the filtrate takes longer to pass through the loop
Level of ADH in the blood is exceptionally high, so more water is reabsorbed from the DCT and the collecting ducts
Urine of a Kangaroo rat is twice as concentrated as that of a rabbit
Water loss by evaporation from the lungs is reduced by exhaling air at a temperature below body temperature
As air is inhaled it gains heat from the nasal passages,
which are cooled as a result
During exhalation water vapour in the warm air
condenses on the nasal passages and is conserved (Fig 2)
Depression of freezing points
Lowering of a solution’s freezing point below that of the pure solvent depends on the number of molecules of solute per unit volume of the solution
Animals in arctic conditions cope with the extreme cold
either by manufacturing natural ‘antifreeze’ and staying active, or by allowing themselves to freeze
Some species are able to manufacture proteins to prevent cellular damage as a result of the formation of ice crystals
Antifreeze proteins depress the freezing point of blood and body fluids
These proteins can also bind to the surface of small ice crystals and inhibit their growth
Species of fish from the Arctic and Antarctic oceans are able to survive with very small ice crystals present in their body fluids
AFPs from insects have been found to be much more effective at depressing the freezing point than fish AFPs
Other ‘antifreeze’ which can depress the freezing point of body fluids include glycerol, sorbitol and manitol
The presence of increased concentrations of glycerol in the haemolymph of some invertebrates has been associated with a tolerance of temperatures as low as -40
Migration
Defined as a periodic long-distance movement from one location to another
Birds tend to fly north in the summer to breed and then south in the winter to find food
Some are summer visitors and some are winter visitor
The control of migration is thought to be mostly endogenous (from within the bird’s own body), although exogenous factors such as photoperiod may also play a part
