3.1.3 Transport In Plants Flashcards
Main purpose of transport systems in plants
Moce substances between leaves, stems and roots
Pressure in plant transport systems
2000kPa - in our arteries it is around 16kPa and in steam turbines it is 4000kPa ; in plants this is confined to much smaller spaces
3 main reasons why multicellular plants need transport systems
Metabolic demands
Size
Surface area : Volume
Metabolic demand
Many internal and undergorund parts of the plant do not photosynthesise (although modt of the green psrts are autotrophic) - they need oxygen and glucose transported to rhem and rhen the waster products to be removed. Hormones in one part need to be transported to where rheg take an effect. Mineral ions absorbed need to be transported to all cells to make the proteins for enzymes and the structure of the cell
Size
Plangs comtinue to grow throughout their lives - many perennials (live a long time and reproduce every year) are large ; redwood trees ; therefore plants need very effective transport systems to mlve substances both up and down from the tip of the roots to the leaves
SA:V ratio
Although leaves have a high SA:V ratio - if the stem, roots trunk etc are taken into account they still have a relatively low SA:V therefore they cannot rely on diffusion alone to supply their cells and meet the metabolic demand
What ate dicotyledonous plants?
Dicots make seeds that contain two cotyledons - organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates
Two different types of dictos
Herbaceous - soft tissues and short life cycle (leaves and stems die down at the end of the growing season to the soil level)
Woody (arborescent) - hard, lignified tissues and a long life cycle (hundreds of years)
Which one are we focusing on?
Herbaceous dicots
What is the vascular system?
Series of transport vessels running through the stem, roofs and leaves - in herbsceous dictos this is made up of two main types of transport vessels ; xylem and phloem
How are the transport tissues arranged in herbaceous dictos?
As vascular bundles (phloem + xylem) in leaves, stems and roots
Transections of stem of young herbaceous plant
Vascular bundles are around the edge to provide strength and suppprt
PARENCHYMA tussue for packing and support
Epidermis is final outer layer with cortex slightly within it
Transection of root of young herbaceous plant
Vascular bundles are in the middle to help the plang withstand the tugging strains that result as the stems and leaves are blown in the wind
Cortex again on inside with EXODERMIS total outer layer around epidermis
Transection of lead dicot
Midrib of dicot leaf carrying the vascular bundle - vasuclar tissue also helps support the structure of the leaf and many small branching veins help in support and further transport
Palisade mesophyll tissue on outside - main photosynthetic tissue
Structure & function of xylem
Largely non-living tissue
1) Transport of water and mineral ipms
2) Support
Flow of materials in xylem is up from the roots to shoots and leaves ONE WAY - made up of many types of cells most of which are dead. Xylem vessels are long hollow structures made by several columns of cells fusing together end to end
Tissues associated with xylem in herbaceous dicots
Thicm walled xylem parenchyma packs aroudn the xylem vessels storing food and confainjng tannin deposits - tannin is a bitter astringent tasting chemical that protects plant tissues from attack by herbivores
What to spirals of lignin do in xylem?
Reinforce lumen so that it doesnt collapse under transpiration pull
What are bordered pits?
Small unlignified areas of rhe xylem where water leaves the xylem and moves into other plant cells
Function of phloem?
Living tissue that transporat food in thenform of organic solutes around the pkanr from the leaves (autotrophic) - it supplies the cells with rhe sugars and amino acids needed for cellular respiration and for the synthesis of all other useful molecules. FLOW OF MATERIALS CAN GO BOTH UP AND DOWN
Structure of phloem
Main trabsporting vessels are the sieve tube elements - made up of many cells to form a hollow structure (with no nuclei) BUT UNLIKE XYLEM they are NOT lignified
Areas between the cells, the walls become perforated to form sieve plates which let the phloem contents flow through
What happens as sieve plates are formed?
As large pores appear in these cell walls, the tonoplast (vacuolar membrane), the nuclues and some of the other organelles break down and the phloem is filled with phloem sap (the nature cells have mo nucleys)
What are companion cells?
Cels linked to sieve tube elements - linked to the sieve tube elements by many plasmodesmata (microscopic channels through the cellulose cell walls linking the CYTOPLASM) - they have nuclei and are very active as they act as a life support system for sieve tube cells (which have lost most of their normal cell functions)
Support in phloem?
Supporting tissues like fibres and sclereids - cells with extremely thick cell walls
Reasons why water is needed in a plant?
Hydrostatic/turgor pressure as a result of osmosis in plant cells provides a hydrostatic skeleyom to support the stems and leaves
Turgor drives cell expansion ; allows roots to force their way through concrete etc
Loss of water through transpiration/evaporation keeps plants cool
Mineral ions are transported in aqueous solutions around the plant
Water is a raw material for photosynthesis
Movement of water into the root
Root hair cells are the exchange surface in planhs where water is taken imto the body from the soil - root hair is a long thin extension of a root hair cell (specialsied epidermal cell found near meristem root tip)
How are root hairs adapted?
Microscopic size means they can penetrate easily between soil particles
Large SA:V with there being thousands of them
Thin surface layer through which diffusion and osmosis takes place quickly
Concentration of solutes in cytoplasm mainatains a water potential gradient between cell and soil water
How does concentration of aoil water and cells differ?
Soil water has high water potential (low concentration of dissolved compounds) and cytoplasm and vacuole sap of the root hair cells contain many solutes meaning water potential is lower - water moves into the cells
Cellulose cell wall (thin)
2 different pathways across the root into xylem
Symplast
Apoplast
Symplast pathway
Continuous cytoplasm of living plant cells which is commected through the plasmodesmata - VIA OSMOSIS. Root hair cell has a higher water potential than the next cell along - this is because of water diffusing in from soil which makes the cytoplasm more dilute (higher water potential). This process of osmosis comtinues from cell to cell across root until the xylem is reached
Symplast pathway maintaing conc.gradient
As water leaves root hair cell water potential of cytoplasm in root hair cells drops - low water potential - ensuring that the steep concentration gradient from soil to cells is maintained
Apoplast pathway
Movement of water through cell walls and intercellular spaces - water fills the spaces between the loose, open network of fibres in the cellulose cell wall. As water molecules move into xylem - mlre water molecules wre pulled through ghe apoplast due to cohesive forces ; this pull from water moving into the xylem and up the plant creates a tension that means there is a continuous flow of water through the open structure pf the cellulose wall with little resistance
How does water move across roots and till when?
Apoplast snd symplast pathways until it reaches rhe endodermis - layer of cells surrounding the vascular tissue
What does the casparian strip do?
It is a band of waxy material called suberin that forms a waterproof layer around the endodermal cells. WATER IN APOPLAST (CELL WALL) can go no further and is forced into cytoplasm
Purpose of casparian strip?
This diversion is important as to get there water must pass through selectively permeable membranes - thus filtering out any potentially toxic solutes from reaching living tissues (no carrier proteins to admit them)
Endodermis -> xylem?
Solute concentraion in cytoplasm of endodermal cells much lower rhan in xylem and endodermal cells move mineral ions into the xylem by active transport - therefore water potential of xylem cells is much lower rhan water potential of endodermal - INCREASES RATE OF WATER MOVING INTO XYLEM VIA OSMOSIS DOWN A WATER POTENTIAL GRADIENT (through symplast)
Once inside the vascular bundle?
Water returns to the apoplast pathway to enger the xylem itself and move up the plant - active pumping of mineral ions (to produce movement of water by osmosis) results in root pressure. Root pressure gives water a push up the xylem and is independent from transporation
Evidence of active transport in root pressure? NOTE THAT IT IS NOT MAJOR FACTOR IN MOVEMENT OF WATER (PASSIVE)
1) Poisons like cyanide affect mirochondria and prevent production of A - ic cyanaide is applked to root cells, root pressure disappears
2) Root pressure falls with a fall in temperature and vice versa therefore chemical reaction involved
3) Levels of oxygen/resporatory susbatnces fall then root pressure falls
4) Xylem sap may exude from the cut end of stems at certajn times; forced out of special pores at the ends of leaves in some conditions (overnight - when transpiration is low : guttation)
What balance must land plants strike?
Compromise between gaseous exchange for photosynthesis (taking in CO2) and losinf the water needed for turgir pressure and transport. Large SA:V to capture light but this increases risk of water loss through transpiration
Normal adaptations to conserve water?
Waxy cuticle
Stomata on underside of leaf
Roots that grow down to water in the soil
Why are more adaptations needed?
In habitats where often there is a very dhort supply of water + hot dry and breezy conditions, water will evaporate from leaf surfaces rapidly
What are Xerophytes?
Plants in dry habitats have evolved a wide range of adaptations that enable them to live and reproduce in places with low water availability
Examples of Xerophytes
Conifers, Marram grass (found in sand dunes and coastal areas - dry and salty)
Many plants that survive in very cold and ice conditions also xerophytes - water is not freely available because it is frozen
CACTI
Xerophytes - thick waxy cuticle
Most plants up to 10% is through cuticle - extra thick layer minimises water loss and this is common in evergreen plants which allows them to survive both hot dry summers and cold winters when water can be hard to absorb from ground
Xerophytes - sunken stomata
Stomata located in pits - reduces air movmeent and produces a microclimate of still, humid air = lower water vapour potential gradient thus less transpiration
Xerophytes - reduced number if syomata
Reduces water loss bur also reduces gas exchange
Reduced leaves - Xero
Water loss decreases as leaf area decreases ; also have thin needles to reduce SA:V, minimising water lost in transpiration
Hairy leaves - xerophytes
Microclimate of still, humid air and reduces water vapor potential hrsdient - minimising water loss… marram grass even have microhairs in the sunken stomatal pits
Curled leaves - Xero
Confines all of the stomata within a micro environment of still, humid air to reduce diffusion of water vapour from the stomata. MARRAM GRASS
Succulents - Xero?
Store water in soecialised parenchyma tissue in their stems and roots - swollen/fleshy and water is stored when in plentiful supply + used in times of drought. Aloes are succulants + salicornia
Leaf loss - Xero
Lose their leaves when water is not available - Palo verde sheds ifs leavss and teunk turns green to photosynthesise with minimal water loss
Root Adaptations - Xero
Many xerophytes have root adaptarions thar help them to get as much water as possible from fhe soil ; long tap roots can penetrate several metres so they can access water stored much lower. Mass of widespreadh shallow roots - can absorb kode water before shower evaporated (CACTI HAVE BOTH ADAPTATIONS)
MARRAM = LONG VERTICAL PENETRATION
Additional feature of Marram Grass
Mat of horizontal rhizomes (modified stems) from shich many more roots develop to form an extensive network that enables sand to hold more water
Avoiding the problems - Xero
Plants may lose leaves and become doemant or die completely ; some survive as storage organs such as bulbs, codms or tubers. Some can survive toral dehydration and appear dead but when rain comes the cells recover, become turgid and green again to photosynthesise - DISACCHARIDE TREHALOSE ALLOWS THIS
Hydrophytes
Plants thar are submerged in water (or on surface) - which need special adaptations to cope with growing in water or in permanenlty saturayed soil
Examples of hydrophytes
Water lilliee, water cress, duckweeds and bulrushes/yellow iris
What must the leaves of hydrophytes do?
Float - so they are near the surface to get the light needed for photosynthesis
Major problem for hydrophytes
Warer logging - air soaces need to be full of air and not water to survive
Thin waxy cuticle - Hydro
No need to conserve water as there is always planet available so transpiration is not an issue
Stomata on upper surfaces
Always open stomata on upper surface - maximises gas exchange and no risk to loss of turgor due to abundance of water so guard cells inactive with stomata always open. Floating leaves means stomata must be on top side of leaf in contact with air
Reduced structure of plant - hydro
Water supports leaves and flowere (turgor pressure) - no need for strong supporting structure
Wide flat leaves - hydro
Spread across water to capture as much light as possible
Small roots - hydro
Water can diffuse directly into stem and leaf so less need for uptake theough roots
Large SA of stems and roots underwater
Maximises area for photosynthesise and for oxygen to difffuse into the submerged plant
Air sacs - hydro
Allows them to enable leaves/flowers to float to the surface of the water
Aerenchyma - hydro
Specialised parenchyms packing tisue that forms in stems, leafes and roots : MANY LARGE AIR SPACES FORMED BY APOPTOSIS
Benefits if Aerenchyma?
Makes the leafes/stems more buoyant
Forms a low resistance infernal pathway for movement of O2 to tissues below the water; allows them to cope with anoxic conditions in the mud.
Downside of arrrendhyma
Also provides a low resistande pathway by which methane produced by the rice plants - nothing can be vented into the atmosphere - contributed to greenhouse effect and resulting climate change
Mangrove swamps?
Roote can beckme waterlogged - special arial roots called pneumatophores grow upwards into the air
How are the reactants for photosynthesis found?
Water is transported from the roots and carbon dioxide into the leaf from the air ; diffuses into the leaf down a concentration gradient from air spaces within the leaf. O2 moves out of the lead cells info air spaces by diffusion down s concentration gradient. Water too evaporates into air spaces
How do carbon dioxide and oxygen exchange in leaves?
By diffusion theough stomata - they can be opened and closes by guard cells which surround the stomatal opening
What is transporation?
This loss of water fapour from the leaves and stems of plants ; an inevitable consequence of gaseous exchange
Why are some stomata open the whole time?
During the day a plant needs to take in CO2 for photosynthesis and at night when no oxygen is being produced by photosynthesis, it needs to take in oxygen for cellular respiration therefore some stomata are open at all times
What happens once inside xylem?
Here 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 wallss of the mesophyll into the air spaces. Then it moves down the concentration gradient into the external air through stomata : TRANSPIRATION STREAM
How does passive process of moving water up roots work?
Water molecules evaporate from surface of mesophyll into the air spaces in the lead and move out of the stomata into surrounding air by diffusion down a concentration gradient. Loss of water by evaporation from a mesophyll cell lowers the water potential of the cell so water moved into the cell from an adjacent cell by osmosis along symplast and apoplast pathways. Repeated across leaf to the xylem (ater moves out of xylem through osmosis)
How does cohesion tension theory help?
Water forms hydrogen bonds tih carbohydrates in the walls of the narrow xylem vessels (adhesion). Also form hudrogen bonds eith each other - cohesion. Combined effect results in capillary tube action ; water can rise up a tube agsindt force of gravity (narrow). Water is drawn up in a continuous stream to replace the water lost by evaporation - this is the transpiration pull. Transpiraiton pull also results in tension in xylem which helps to move water across the roots from the soil
Evidence for cohesion tension?
- When transpiration is at highest during the day - pressure in xylem increases thus diameter of treee trunk decreases. At night when transpiration is lowest, the diameter
increases because tension is low - When a xylem vessel is broken ; air is drawn up into xylem rather than water leaking out thus palmt can no longer move water uo the stem because cohesive forces are broken (no continuous stream)
How to measure transpiration
Very difficult to separate water vapour from transpiration and respiration + evaporationg/condensing to collect all water that leaves. Therefore we measure the uptake of water as about 99% of this is lost through transpiration. USE A POTOMETER
Potometer tips
All joints sealed with waterproof jelly to make sure any water loss measured is as a result of transpiration from the stem and leaves
Stem of fresh root is cur under water and transferred to prevent any air bunnles neinf introduced - if cut in the air it would introudce air bubbles cutting off continuous transpiration stream
Rate of uptake?
Distance moved by air bubble/time taken for air bubble to move that distancd
Potometer variables
Indepedent = environmental conditions Dependent = rate of transpiration Control = species of plant tested, amount of time for transpiration, soil they grew in, volume of water,
What controls rate of transpiration?
Opening and closing of stomata - turgor driven process ; when turgor is low the guard cell walls are closed and when conditions are favourable, guard cells pump in solutes to then increase turgor. Inner wall is thicker than thinner outer wall thus cells become bean shaped and open the pore ; hormonal signals can trigger turgor loss from guard cells which close the stomatal pore and conserve water
Factors affecting transpiration
Light Humidity Temperature Air movement Soil water availability
How does light affect transpiration rate?
Light causes stomata to open for gas exchanhe ; increasing number of open stomata = increases rate of water vapour diffusing out and increasing evaporation from surface of the leaf. PROPORTIONAL
Humidity affect transpiration rate?
High humidity lowers transpiration rate because of reduced water vapour potetnial gradient between inside of leaf and outside air. Very dry air has oppsoite effect and increases transpiration rate
How does temperature affect transpiration?
Increases KE of water particles and thus increases rate of evaporation from sppngy mesophyll into air spaces
Increases the amount of water vapour air can hold until it gets saturated so it decreases relative humdity
Both factors increase diffusion gradient inside and outside thus increasing rate of transpiration
Air movement transpiration rate?
Fewtures like hairs decrease air movement close to the leaf - water vapour accumulates here and so water vapour potential around stomata increases thus reducing gradient. Air movement will INCREASE RATE OF TRANSPIRATION and still air will decrease transpiration rate
Soil water availability
If very dry the plant will be under water stress and rate of transpiration will be reduced
How are different substances transported around the plant?
Leaves produce glucose which is then converted to sucrose - when it reaches the cells where it js needed, it is converted back to glucose for respiration or to starch for stirage or used to produce amino acids
Where do plants transport substances from?
Sources to sinks in translocation ; an active process which requires energy to take llace and substances to be transported both up and down the plant
What are assimilates?
The products of photosynthesis fhat are transported
Main assimilate transported
Sucrose - makes up about 30% of the phloem sap content
Sources of assimilates
Green leaves and green stems
Storage organs such as tubers and tap roots that unload their stores
Food stores in seeds when they germinate
Main sinks
Roots that are growing or activeky absorbing mineral ions
Meristems that are active,h dividing
Parts of the plant that are laying down foot stores such as dwveloping seeds, fruite or storage organs
Phloem loading
Solubke products of photosynthesis are moved into phloem from the sources by an active process - sucrose is the main carbohydrate that is transported and is not hsed un. Metabolism as readiy as glucose thus less likely to be metabolised during the transport process
Two makn ways in which plants load assimilates
One is large,y passive and the other is active
Apoplast route
Sucrose travels through cell walls and inter-cell spaces to the companion cells and sieve elements (apoplast route) by diffusion down a concentration gradient, miantained by removal of sucrose intk thr phloem vessels. In companion cells sucrose is moved into cytoplasn across cell membrane in an actibe process - H+ ions are actively pumped out of the companion cell into surrounding tissue using ATP ; these ions then return down the concentration gradient ejth sucrose thus increasing sucrose concentration in companion cells and sieve elements through plasmodesmata
How are companion cells adapted?
Have many infoldings in their cell membranes to give a large SA for active transport of sucrose into cell cytoplasm and also many mitochondris to supply ATP needed for transport pumps
What happens as a result of build up of sucrose in the companjon cells and sieve tube elements?
Water moves into phloem via osmosis - leads to a build up in turgir oresshre due to rigid cells walks andthe water carrying the assimilates moves into the tubes of the sieve elements, reducing pressure in companjon crlls andthus moving up and down the pksnt by mass flow to areas of lower pressure (sinks)
Pressure differences?
Solute accumulatuon in source phloem leads to an increase in turgor pressure fhat forces sap to regions of lower pressure in sinks ; these pressure differences are what allow solutes and water to berapidly transported over many metres
Phloem unloading
Sucrose is unloaded from phloem at any point into cells that need it ; main mechanism is just diffusion of sucrose from phloem into surrounding cells. It moves in rapidly or is converted to another susbtance (such as strach/glucose) to maknfain a concentration gradient between ohloem and surrounding cells
Loss of water from phloem
Leads to a rise in water potetnial of the phloem ; water moves outingo surrounding cells by osmosis and some of that water that carried the soljtes is drawn into the transpiration stream
