Plant Transpiration: Q&A on Water Transport in Plants

#separator:tab #html:true "What is water in a plant absorbed by in the roots ?""In plants water is absorbed by the roots through extensions called root hairs " "Through which vessel is water transported by in flowering plants ?"xylem vessel - hollow, thick-walled tubes  "What is the name of the process which is the main force that pulls water through xylem vessels in the stem of a plant ?""A process called transpiration - evaporation of water from the leaves " "Where is the energy for transpiration supplied from ?""The sun " "What type of process is tranpiration (active or passive) ?""passive " "Where does water start and end up for transpiration ?""- start from the point where water evaporates from the surfaces of cells surrounding the stomatal air space  - end where water vapour diffuses out of the stomatal pore " "How does humidity allow water vapour molecules to diffuse out of the air spaces into the surrounding air  (in steps) ?""1) The humidity of the atmosphere is usually less than that of the air spaces next to the stomata  2) As a result there is a water potential gradient from the air spaces through the stomata to the air  3) provided stomata are open , water molecules diffuse out of the air spaces into the surrounding air " "Where is the water lost from the air spaces replaced from in the cell?""cell walls of surrounding mesophyll cells " "How do plants control the rate of transpiration ?""By changing the size of the stomatal pores , plants can control the rate of tranpiration." "How is water lost from mesophyll cells ?""Water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf " "How is water replaced to the mesophyll cells ?""This is replaced by water reaching the mesophyll cells from the xylem either via cells walls or via the cytoplasm " "Why does the cytoplasmic route occurs (water moving from cytoplasm to mesophyll cells to replace water that has been lost) ?""- mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun " "Outline/explain the process of how water moves throughout the leaf (cytoplasmic route) ?""1) mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun  2) these cells now have a lower potential potential and so water enters by osmosis from neighbouring cells  3) the loss of water from these neighbouring cells lowers their water potential 4) they, in turn , take in water from their neighbours by osmosis " "Summarise how water leaves the cell to the atomosphere  ?""- a water potential is established by transport of water around the cells of a leaf and this pulls water from the xylem  - across the leaf mesophyll  - finally out of the atmosphere " "What is the main factor that is responsible for the movement of water up the xylem roots to the leaves ?""This main factor is cohesion-tension " "What is the process of movement of water up the stem into the xylem ?""1) Water evaporates from mesophyll cells due to the heat from the sun leading to transpiration  2) water molecules form hydrogen bonds between one another and hence tends to stick together - known as cohesion  3) water forms a continuous , unbroken column of across the mesophyll cells and down the xylem  4) As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata , more molecules of water are drawn up behind it as a result of this cohesion  5) A column of water is therefore pulled up the xylem as a result of transpiration - this is called the transpiration pull 6) Transpiration pull puts the xylem under tension , that is there is negative pressure within the xylem , hence the name cohesion-tension theory." "Why do plants need a transport system? ""To ensure that all cells of the plant receive a sufficient amount of nutrients " "What does the xylem transport around the plant ?""xylem tissue enables water and dissolved minerals to travel up the plant in the passive process of transpiration " "What does the phloem tissue do and transport ?""Phloem tissue enables all sugars to reach all parts of the plant in the active process of translocation " "What is the xylem and phloem components structured within ?""Xylem and phloem are components of the vascular bundle" "what does the vascular bundle provide for the components (xylem and phloem) ?""Enable the transport of substances as well as provide structural support " "How are the xylem vessels arranged ?""They are arranged in an X shape in the centre of the vascular bundle." "Why is the xylem vessel arranged as an X shape ? ""This enables the plant to withstand various mechanical forces such as pulling " "The X shape arrangement of xylem vessels is surrounded by ...""an endodermis " "What is the function of the endodermis ?""This is the innermost layer of the cortex; the cells are closely packed and have Casparian strips within their walls (water-impermeable deposits of suberin) which regulate water and mineral uptake by the roots and supply xylem vessels with water " "What is the inner layer of meristem cells known as ?""pericycle " "What is a limitation of the endodermis and how is this overcome ?""- The endodermis acts as a barrier to the free passage of water and ions via the apoplast (cell wall) pathway - Water and ions must therefore cross the membrane of the endodermal cell and here some selection of ions for absorption may take place " "What does the pericycle do ?""It produces lateral roots when cells here divide " "What is conjunctive tissue/cortex ?""It is the tissue betwwen the xylem and phloem ; in dicots, it's made up of parenchyma. " "What is the function of the cortex or parenchyma within ?""This may store starch and can be infected by Rhizobium species to form root nodules on leguminous plants. The intracellular air spaces allows diffusion of gases through the root." "What is the stele ?""All the tissues inner to endodermis constitute stele; here it includes pericycle , vascular bundle ,cambium and xylem ." "What is the piliferous layer (also called epiblema, rhizodermis) ?""- This is an epidermis modified to produce root hairs. - The root hairs are outgrowths of a single cell.  - Layer is only one cell thick , has no protective function hence no cuticle and is soon lost by abrasion - hence root hairs only present on youngest part of root. - Root hairs provide increased surface area for uptake of water and ions in solution  " "What is cambium's function?""It is present in some dicotyledenous species - meristematic and divides to produce new phloem and xylem cells " "What is the exodermis ?""- It protects against dessication and invasion by pathogens  - May replace epidermis in older roots  - may become suberized and thus restrict water uptake to younger regions of the root  -Layer may be several cells thick " "What does the phloem do in the dicotyledonous root ?""- transport of organic solutes  - These solutes are osmotically active so phloem is situated between 'arms' of xylem enabling absorbed water to enter xylem without crossing phloem" "where is the pith (inner cortex)?""It is absent in mature plants but present young ones" "Where is parenchyma found ? ""In the soft parts of the plant - in the cortex " "What type of cell is parenchyma cells ?""unspecialised cells " "What is the cell wall of a parenchyma cell made out of and the rough size of the cell wall ?""The cell wall is made out of cellulose and it is a thin cell wall " "Are there intracellular spaces in parenchyma cells in the cortex ?""Lots of intracellular spaces are present between cells " "Are parenchyma cells living at maturity ?""yes it consists of living cells at maturity." "Summarise the main functions of parenchyma cells ?"- Photosynthesis - Food storage  - Gas exchange  "Where is collenchyma cells found ?""- in the petiole, leaves and young stems  - near the epidermis " "What type of cells are collenchyma cells ?""specialised cells " "What is the collenchyma cell wall made out of and the rough size of it ?""- Made out of extra cellulose and pectin  - and it is an unequally thin cell wall " "How much intracellular space is there in collenchyma cells ?""- little intracellular space " "Does collenchyma cells consist of living cells at maturity ?"Yes  "What is the function of collenchyma cells ?""- It provides mechanical support to the plant  - Especially in young stems as they are living cells and can expand as the stem grows" "Where is Sclerenchyma found ?""- found in mature parts of the plant  - In vascular bundles " "What type of cell is sclerenchyma cells?""specialised cells " "What is the cell wall made out of in Sclerenchyma cells and what is the rough size ?""- Cell wall made of lignin - Thick and rigid cell wall" "Does sclerenchyma cells have any intracellular space ?""No , it does not have any intracellular space" "Does sclerenchyma cells consist of living cells at maturity ?""No , it consists of dead cells at maturity" "What is the function of Sclerenchyma cells ?"- Provides mechanical support  - protection  - Transport substances  "Explain how the structure of Sclerenchyma contributes to why it has no living contents ?""- They are elongated into supporting fibres  - Therefore have heavily lignified cell walls and thus have no living contents." "What does the epidermis do in the herbaceous stem ?""- protects the moist inner tissues of the stem against dessication and invasion of pathogens  - Epidermis also contributes to support by holding in the turgid parenchyma cells of the cortex " "How does the structure of the epidermis in the herbacious stem contribute to another function ?""The epidermis is typically one cell thick and may have extended unicellular or multicellular hairs which (a) reduce air movement and water loss and (b) reflect light and prevent overheating." "What does the cambium do in the herbaceous stem ?""- a lateral meristem  - Simple , non-specialised cells divide and then differentiate into phloem (to outside) and xylem (to inside) " "What is a protoxylem in the herbacious stem ?""- incomplete lignification - may have cytoplasm " "What is the metaxylem in the herbacious stem ?""Heavy lignified secondary cell wall - no living contents " "Water moves through the xylem , which features of the xylem support and aid that ?""- They transport water and minerals , and also serve to provide structural support  - They are long cylinders made of dead tissue with open ends , therefore they can form a continuous column  - xylem vessels also contain pits which enable water to move sideways between the vessels  - They are thickened with a tough substance called lignin, which is deposited in spiral patterns to enable the plant to remain flexible." Describe transpiration briefly ?"Transpiration is the process where plants absorb water from the roots , which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves.Carbon dioxide enters , while water and oxygen exit through a leaf's stomata " "Why is the transpiration stream important which is movement of water up the stem ?""- It enables processes such as photosynthesis, growth and elongation as it supplies the plant with water which is neceasary for all of these processes. - Apart from this, the transpiration stream supplies the plant with the required minerals , whilst enabling it to control its temperature via evaporation of water. " "How does wind speed increase the rate of transpiration ?""- The air is more saturated - steeper concentration gradient  -therefore faster/greater diffusion " "Suggest what the student would need to do in order to get a valid comparison of the rates of transpiration of the two species ?""- same conditions - calculate the rate per unit of time - how ? - draw round leaf on graph paper  - picked at the same time or same degree of turgidity " "Describe how the apparatus of transpiration should be set up to ensure valid meaurements can be obtained ?""- The apparatus is inserted underwater  -There are no EXTRA air bubbles in the apparatus  - use vaseline to ensure air/water-tight joints  - use a healthy leaf  -cut the shoot at a slant underwater  - conditions are the same  - dry off leaves  - time to acclimatise before taking a reading  " "Transpiration itself is not measured by the potometer.State precisely what is measured by using the apparatus ?""- rate of WATER UPTAKE as a result of water loss from the plant " "How does light intensity increase the rate of transpiration ?""- More light means there is more stomata open and and therefore and increased surface area for transpiration  - faster diffusion of water vapour from the stomata " "How does temperature increase the rate of transpiration ?""- higher temperature  - more evaporation (increase water vapour concentration) and therefore more kinetic energy  - Faster diffusion of water vapour from the stomata " "How does humidity increase the rate of ranspiration ?""- less humidity meand less water vapour in the surrounding air and therefore an increase in water vapour concentration gradient  - faster diffusion of water vapour out of the stomatal pore" What is a potometer ?"an apparatus used to measure the rate of transpiration " How does transpiration occur ?"- moist lining of spongy mesophyll cells evaporate forming water vapour  - water vapour builds up in air spaces  - If water vapour concentration is high enough and stomata is open , water vapour diffuses out " "What is the principle of the potometer ?""- as transpiration occurs from the leaves the plant will pull up more water from the potometer by cohesion-tension causing the bubble to move towards the plant  - the more water lots by transpiration , the more water taken up , the further the bubble moves " "Describe the movement of water from the soil to the root hair ?""- uptake of water - water moves into root hairs via osmosis down a water potential gradient (passive)  - root hairs provide a maximum surface area for maximum absorption of water  - the uptake of minerals by active transport takes place at the same time (otherwise it would be less efficient) and reduces the water potential in the cell cytoplasm so water is continuously taken up by osmosis " The epidermis of the roots contains ....root hair cells  Describe the movement of root hair to xylem ?"1) Water taken up by root hairs crosses the root cortex  - water moving through cell walls of neighbouring cells - apoplastic pathway - water moving from plasmodesmata to cytoplasms from cell to cell - symplast and vacuolar pathway ( through plasma membrane and plasmodesmata to cytoplasms from cell to cell , vacuolar is the same , but also through vacuoles) 2) water moves from one cell to another till it reaches the xylem; movement is due to concentration gradient due to concentrated sap vacuole of cells  - down concentration gradient therefore more negative  - water crosses the membrane  - pits in xylem - aquaporins" "Why can the apoplast pathway not be used to reach the parenchyma cells ?""- The apoplastic pathway is stopped at the endodermis due to cells in it haveing a band of suberin forming the casparian strip ( an impenetrable barrier to water)  - suberin deposits increase with age of cells except for certain passage cells (such as vacuolar and symplats pathways)  - water can pass freely through these passage cells " "Summarise what is root pressure ?""- minerals move into the xylem by active transport forcing water into the xylem and pushe sit up to the stem " "Summarise what is transpiration pull ?""- loss of water at the leaves replaced by water moving up the xylem " Summarise what is cohesion-tension theory ?"- cohesion between H2O molecules  - tension in column of water - which is why xylem is strenghtened by lignin so the whole column of water is pulled up in one chain " "summarise what is capillary action ?""adhesion of water molecules to xylem as they are narrow - hydrogen bonding to OTHER  molecules such as the WALLS as well as water molecules " "Summarise the importance of transpiration ?""- support turgid cells  -for cell growth - enables mineral ions to be taken up  -cooling the plant  - for photosynthesis"

Plant Transpiration: Q&A on Water Transport in Plants

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