Transport in plants and transpiration Flashcards
what is the outer layer of the root called?
-the epidermis
what is the stele?
-it is composed of xylem tissue and a smaller amount of phloem tissue
what is immediately outside the stele?
-a single layer of cells called the endodermis
what is found between the epidermis and endodermis and what is it made of?
-the cortex which is made of undifferentiated cells that have small air spaces between them so the cells may be rich in starch grains
what is the job of the xylem?
-transports water and inorganic ions
what are the xylem made of?
-dead tissue meaning that they have no organelles which means that there is nothing to block the water it is continuous
what is the structure of the xylem vessels?
-the cells end walls have broken down resulting in long hollow tubes
what is formed in the primary cellulose cell wall? what is its job?
-lignin which is impregnated into the secondary cell wall
-it prevents vessels from collapsing when under higher pressure exerted by the transpiration stream and it provides strength
what does the type of vessel and amount of lignin depend on?
-the degree and nature of the cell wall lignin thickening
-lignin can be deposited in a range of patterns which all give structural support and waterproofing the tube
-lignin is impermeable to water
in protoxylem what pattern does the lignin create? why these shapes?
-annular
-spiral
-they allow the vessels to elongate along with other tissues in the growth regions
what is metaxylem?
-xylem vessels that are more mature
what pattern does the lignin create in metaxylem? why these shapes?
-reticulate
-pitted
-reticulated vessels are thickened by interconnecting bars of lignin
-pitted vessels allow water and ions out of the vessels to the surrounding cells
what type of pressure is needed to move water through xylem vessels ?
-less pressure than through living cells where movement would be slowed down by organellesbut still the transpiration stream excerts a high pressure
how might protoxylem and metaxylem work together in new plants?
-in roots protoxylem is pushed to the outer edge of the stele as the metaxylem forms behind it
what are the two main properties of lignin?
-it provides strength the prevents vessels from collapsing when under pressure exerted by the transpiration stream
-waterproof which prevents the leakage of water
what is the job of the pholem?
-transport of sucrose mainly however it also transports amino acids and other organic solutes
what type of tissue is the pholem made up of ?
-living tissues and consist of sieve tube elements and companion cells
how do the phloem differ from the xylem?
-phloem do have end walls but they are perforated with sieve pores to form sieve plates
how are sieve tube elements adapted to maximize the transport of sucrose ?
-they do contain organelles but by the time they are fully grown they have no nuclei and a reduced volume of cytoplasm
-they have microtubules that are thought to be involved in translocation of solutes
what type of cell are linked to the sides of sieve tube elements and how are they linked?
-companion cells and they are linked via the plasmodesmata
what is the job of the companion cells?
-they act as supporting cells, carrying out many metabolic activities for the highly specialized sieve tube elements
how are the vascular tissues arranged in stems ?
-vascular bundles around the outside of the stem
what is the advantage of having the vascular bundles around the outside of the stem?
-provision of greater support necessary in stems to support branches and leaves
what is the vascular bundle called in the leaf?
-the midrib which branches to form smaller veins that are distributed throughout the leaf
how does the arrangement of the vascular bundle differ in woody plants e.g. trees?
-wood is formed of xylem tissue and therefore woody plants cross section is almost all xylem with very small amounts of phloem and other tissues
how do plants lose water?
-by evaporation
what is the definition of transpiration?
-the evaporation of water from the mesophyll surface and the subsequent diffusion of water vapor through the stomata and into the atmosphere
how and what is taken up across the root? how is the root adapted for this?
-water and ions are taken up at the root epidermis where root hair cells greatly increase the surface area
how is water absorbed into the root?
-water is absorbed by osmosis as in normal circumstances water in the soil has a higher water potential than the root hair cells due to the sugars and other compounds present in the cells and ions move by active transport
what are the two pathways that water can travel in the root?
-apoplast (cell wall)
-symplast (cytoplasm)
why does most water move by the apoplast pathway?
-there is a limit to resistance to the water as there are no organelles in the pathway
how do microfibrils help to move water in the apoplast pathway?
-parallel arrangement of microfibrils allow water to pass easily between the different layers rather than through them
how does water move through the symplast pathway?
-the pathway involve water moving by osmosis from cell to cell through the cytoplasm of the cortex cells
how does the cohesive properties help move water through the apoplast pathway?
-as water moves through the wall the cohesive properties of the water which is aided by hydrogen bonding which helps pull the water column along
how is a water potential gradient created and how does it help move water through the symplast pathway?
-as root hair cells take in water its water potential becomes less negative and is higher than the cells in the root cortex which creates a water potential gradient
-water moves from the root hair cell into the cortex cell by osmosis and so on across the cells of the cortex
what structure aids water movement in the symplast pathway?
-water moves from cell to cell through plasmodesmata that link the cells of the cortex together
what are the two ways that ions can enter the root hair cell?
-facilitated diffusion
-active transport
as the intake of ions are usually by active transport what does this cause?
-intake of ions is often by active transport causing the build up of ions in the plant cell which requires ions now to be moved against the concentration gradient, low to high
what is the speed of diffusion in both pathways?
-apoplast pathway has a fast diffusion rate because there are no organelles in the road
-symplast pathway has a slow diffusion rate because there are organelles in the pathway
what is the endodermis ?
-the layer of cells immediately outside the stele and it is waterproof because it is made of suberin
what does the endodermis stop?
-the apoplast pathway
what surrounds the endodermis?
-a waxy material called suberin forms a band of wax called the casparian strip
what is the characteristic of suberin?
-impermeable to water
what happens to the water in the apoplast pathway now that is has been stopped by the casparian strip? what does this mean?
-the water moves to the to join the symplast pathway
-this means that all water moving into the stele is transported by the symplast pathway
-water continues along this pathway until the xylem is reached and the water transport is under metabolic control
how is the water potential lowered in the xylem?
the cytoplasm actively pumps dissolved ions into the xylem which requires energy and lowers the water potential
what are the 5 steps on how root pressure is achieved?
-mineral ions pumped into xylem= water potential
-water enters xylem by osmosis
-volume of water increases
-pressure increases due to extra volume
-water is forced upward
what are the 4 ways water passes up the xylem?
-tension
-adhesion
-cohesion
-root pressure
what does the cohesion tension theory suggest?
-a continuous column of water exists in the xylem from the roots to the tip of the shoot
tension refers to the what?
-transpiration pull
-water in the xylem vessels is being pulled up
tension pull of water can also be described as what?
-negative pressure and a water potential gradient is established through the plant
what is the cohesion of water?
-water sticks to water meaning a continuous stream of water passes up the xylem which is pulled by tension
why do water molecule stick together?
-force of attraction caused by the formation of hydrogen bonds
what is one piece of evidence that supports the cohesion tension theory?
-if the water column is broken an air gap appears meaning the water below the air gap cannot be pulled up
why does the diameter of xylem vessels decrease during the day?
-during the day there is more tension which tends to pull the walls of the xylem inwards too
why is it more obvious when the diameter of a tree trunk changes compared to a herbaceous plant?
-tree trunks are almost all xylem therefore most of it is involved in water uptake
what substance prevents xylem from collapsing under this high pressure and how?
-lignin
-it is waterproof avoiding the loss of water and it is very strong which allows vessels to withstand pressure
what is one structural adaptation which helps water transport?
-missing end walls
what is the adhesion of water?
-the attraction of water to another material
-water sticks to the walls of the xylem
how is a large surface to volume ratio created in the xylem and why is this a good thing?
-the narrowness of the xylem means that there are large amounts of water in contact with the walls
-adhesive forces are stronger than cohesive forces between water molecules so water moves up the xylem
the adhesion of water can also be known as what?
-capillarity
how does water enter the leaf?
-through the midrib which splits into veins that passes water across the leaf
how is water passed to surrounding cells?
-apoplast or symplast pathways
what are the two ways that water is lost from the leaf?
-transpiration
-evaporation
how is the water potential gradient set up that is responsible for the transpiration pull?
-water evaporates from the cell surface membrane of the spongy mesophyll cells into the air spaces of the spongy mesophyll
-this means that the water is diffusing down the concentration gradient and out of the stomata
