Transport in Plants - A Level Biology Notes PDF

7 Transport in plants 7.1 Structure of transport tissues Structure of roots (dicot) Types of plant cells Transverse section PARENCHYMA COLLENCHYMA SCLERENCHYM...

7 Transport in plants 7.1 Structure of transport tissues Structure of roots (dicot) Types of plant cells Transverse section PARENCHYMA COLLENCHYMA SCLERENCHYMA Image: https://www.vedantu.com/ Image: http://www.bio.miami.edu/ found in the found in soft found in mature petiole, leaves, parts of the parts of the and young plant plant stems unspecialised specialised cells specialised cells cells cell wall made cell wall made of cell wall made Image: https://www.brainkart.com/ of cellulose and cellulose of lignin pectin piliferous layer (also called epiblema, rhizodermis) – the outermost layer; unicellular root hairs present, unequally thin thick and rigid cuticle and stomata absent thin cell wall cell wall cell wall cortex – it is a multi-layered large zone made of lots of parenchymatous cells with intracellular spaces and intracellular stores food and water little intracellular no intracellular spaces are endodermis – the innermost layer of the cortex; the space space present between cells closely packed and have Casparian strips within cells their walls (water-impermeable deposits of suberin) which regulate water and mineral uptake by the roots consists of consists of living consists of dead pericycle – produces lateral roots when cells here living cells at cells at maturity cells at maturity divide maturity stele – all tissues inner to endodermis constitute stele; provides here it includes pericycle and vascular bundle functions provides mechanical include vascular bundle – xylem and phloem mechanical support, photosynthesis, support to the protection, and conjunctive tissue – the tissue present between xylem food storage, plant transports and phloem; in dicots, it’s made up of parenchyma gas exchange substances pith – absent in mature plants, present in young ones 1 www.alevel-notes.weebly.com Longitudinal section (of roots) Longitudinal section Image: http://mybiologypal.blogspot.com/ epidermis – the outermost layer; made up of a single layer of parenchyma cells and its outer wall is covered Image: https://www.anatomynote.com/ with a cuticle - cuticle prevents infection of the plant by bacteria or fungi Structure of stems (dicot) - also aids in reducing water loss Transverse section cortex – divided into three regions: - hypodermis provides mechanical support - middle cortex is involved in photosynthesis - inner cortex helps in gaseous exchange and stores food materials endodermis – the innermost layer of the cortex, consists of a single layer of cells that contain starch grains pith – large, central, parenchymatous zone with intracellular spaces; helps in storage of food materials Structure of leaves (dicot) Transverse section Image: https://www.brainkart.com/ Image: https://www.brainkart.com/ 2 www.alevel-notes.weebly.com  upper epidermis – the outermost layer made up of a 1) long hollow tubes with no end walls for uninterrupted single layer of parenchyma cells without intracellular transport of water and mineral ions spaces; outer walls have cuticles, stomata are less in 2) no cell contents (no protoplasm) to provide maximum number space and minimum resistance lower epidermis – single layer of parenchymatous 3) their walls are lignified to withstand negative pressure cells with a thin cuticle and provide mechanical support - contains numerous stomata 4) cellulose lining is present for adhesion if water - chloroplasts are only present in guard cells molecules which helps with their movement upwards 5) they have pitted walls (in non-lignified sections) for the - helps in exchange of gases lateral movement of water - loss of water vapour is facilitated through this chamber mesophyll – tissue present between the upper and Phloem lower epidermis, differentiated into palisade transports sucrose and amino acids via mass flow parenchyma and spongy parenchyma (active process) from the source to the sink - palisade parenchyma: tightly packed, elongated bidirectional movement (translocation) cells with lots of chloroplasts (for photosynthesis) mainly composed of sieve tube elements + just below the upper epidermis. companion cells - spongy parenchyma: spherical/oval, irregularly arranged cells with lots of intracellular spaces; helps in gaseous exchange vascular bundles - vascular bundle of midrib is larger - each vascular bundle is surrounded by a sheath of parenchymatous cells called bundle sheath - each vascular bundle consists of xylem lying towards the upper epidermis and phloem towards the lower epidermis For more information about the structure of plant tissues and Image: https://irevise.com/i how to draw them, see Biology Paper 3 Notes. Phloem sieve tube elements Structure of vascular system 1) contains ER, mitochondria, and cytoplasmic stands (cytoplasm reduces friction to facilitate the movement Xylem of the assimilates) transports water and mineral ions via mass flow 2) end-walls modified to sieve plates/perforated plates (passive) which allows for the continuous movement of the unidirectional movement (from roots à rest of the organic compounds plant) 3) phloem tubes are present in a bundle composed of tracheids, vessel elements, xylem fibres, 4) area where sucrose is loaded is source and where it's and xylem parenchyma (all dead except xylem unloaded is sink parenchyma) Companion cells 1) contains organelles such as nucleus, RER, mitochondria, and ribosomes which provides metabolic support to the sieve tube elements and helps with the loading and unloading of the assimilates 2) transport proteins are present in the plasma membrane which move assimilates in and out of the sieve tube elements 3) large numbers of mitochondria present which provide ATP for the active transport of assimilates into or out of the companion cells Image: https://ib.bioninja.com.au/ 3 www.alevel-notes.weebly.com 4) plasmodesmata are present which link the cell to the 2) this gas exchange is required to uptake CO2 for sieve tube elements photosynthesis 3) while stomata are open, water vapour diffuses out Comparing the structures of xylem and phloem Advantages of transpiration tissues 1) helps in plants pulling up water from soil through roots XYLEM PHLOEM (transpiration pull) mainly dead cells 2) helps in sending out excessively absorbed water by (tracheids, vessel living cells (only plants made of elements, xylem phloem fibres are 3) cools the plant (via evaporative cooling) fibres) except for dead) 4) helps in the absorption and transport of mineral salts xylem parenchyma 5) helps in the absorption and distribution of water in cell wall lignin and cellulose cellulose plants material presence of yes (sieve plates no end walls with sieve pores) unidirectional and direction of bidirectional upwards (roots à flow (source à sink) leaves) sucrose, amino substance water and mineral acids, and other transported ions organic compounds does not provide mechanical provides mechanical support mechanical support support 7.2 Transport mechanisms Image: https://alevelbiologystudent.weebly.com/ Plants must take in a constant supply of water and Transpiration pull dissolved minerals to compensate for the continuous Transpiration pull is the force by which water ascends a loss of water via transpiration in the leaves. plant. Transpiration water molecules cling together by hydrogen bonds Transpiration is the loss of water vapour from the aerial between molecules known as cohesive forces parts of the plant. Around 99% of all water absorbed is water molecules experience attraction towards the lost via this process. cellulose in the cell walls of the xylem (adhesion) 1) water evaporates from cell walls of mesophyll cells into air spaces Cohesion-adhesion theory 2) water vapour diffuses (out to atmosphere) water molecules tend to cling to one another via 3) through open stomata (to atmosphere) hydrogen bonds (cohesion) 4) down a water potential gradient when water evaporates from the surfaces of mesophyll cells, a tension is created in the xylem When the following factors increase, transpiration (↑/↓) tissue which is transmitted all the way down the plant due to the cohesiveness of the water molecules 1) humidity (↓) 2) wind speed (↑) the cohesive forces thus produce a continuous column of water (transpiration stream) 3) light intensity (↑) the adhesive force stops the water column from 4) temperature (↑) pulling away from the walls of the xylem vessels, so 5) water supply (↓) water is pulled up the xylem tissue from the roots to replace what was lost in the leaves Transpiration is considered as an inevitable this is known as the cohesion-adhesion theory consequence of gas exchange as – 1) stomata are open for gas exchange 4 www.alevel-notes.weebly.com  Image: By Dylan W. Schwilk - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55396563 this arrangement is thought to: - gives plants control over what mineral ions pass into xylem vessels (everything must cross cell membranes) - may help with generation of root pressure 4) once across endodermis, water moves into xylem through pits in their walls Image: https://www.quora.com/What-is-cohesion-tension-theory Movement of water between plant cells From soil to root hair 1) uptake of water – water moves into root hairs via osmosis down a water potential gradient (passive) 2) root hairs provide max surface area for the max absorption of water 3) the uptake of minerals can be passive or active and occurs by diffusion or active transport respectively From root à stem à leaf via xylem From root hair to xylem 1) removal of water from xylem vessels in leaf reduces 1) water taken up by root hairs crosses the root cortex hydrostatic pressure in xylem water moving thorough cells walls – apoplastic 2) hydrostatic pressure at top of xylem becomes less pathway than bottom water moving through plasmodesmata – 3) pressure difference causes water to move up the symplastic pathway xylem 2) water moves from one cell to another till it reaches Apoplastic pathway xylem; movement is due to concentration gradient due to concentrated sap vacuole of cells most water travels via the apoplastic pathway (when transpiration rates are high) 3) apoplastic pathway is stopped at endodermis due to cells in it having a band of suberin forming the these are the series of spaces running through the Casparian strip (an impenetrable barrier to water) cellulose cell walls, dead cells, and the hollow tubes of the xylem suberin deposits increase with age of the water moves by diffusion as it isn’t crossing a endodermal cells except for certain passage partially permeable membrane cells the water can move from cell wall to cell wall directly water can pass freely through these passage or through the intracellular spaces cells movement of water via this pathway occurs more quickly than in the symplastic pathway 5 www.alevel-notes.weebly.com  this movement is however stopped at the endodermis 2) this creates an excess of H+ ions in the apoplastic due to the presence of the Casparian strip pathway outside the cell 3) H+ ions move back into the cell down their Symplastic pathway concentration gradient back into the cytoplasm of the movement through the cytoplasm, plasmodesmata, or companion cell via a cotransporter protein vacuole of cells (crossing membranes) 4) this cotransporter protein acts as a carrier for both H+ the water moves by osmosis (across partially and sucrose (so sucrose moves into the companion permeable membranes) cell too but against its concentration gradient) 5) sucrose then moves into the sieve tubes via the plasmodesmata from the companion cell - companion cells have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes - many mitochondria also present provide the energy for the proton pump How assimilates that arrive in the phloem sieve Image: Jackacon, vectorised by Smartse - Apoplast and symplast pathways.gif tubes from mesophyll cells (source) can be https://commons.wikimedia.org/w/index.php?curid=12063412 translocated to other parts of the plant (sink) 1) when sucrose is loaded into a sieve tube element, the water potential decreases Movement in the phloem 2) this causes water from surrounding tissue to enter by Mass flow osmosis Movement of fluids under a pressure gradient. 3) this increased volume increases the hydrostatic Concentration gradient does not matter here. pressure at the source compared to the sink 4) assimilates move down a hydrostatic pressure gradient to the sink Sources and sinks the source of the assimilates could be: 1) green leaves and green stem (photosynthesis produces glucose which is transported as sucrose, as sucrose has less of an osmotic effect than glucose) 2) storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period) 3) food stores in seeds (which are germinating) the sinks (where the assimilates are required) could be: 1) meristems (apical or lateral) that are actively dividing 2) roots that are growing and / or actively absorbing mineral ions Image: https://www.researchgate.net/ 3) any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or Sucrose loading into phloem storage organs) This process is not fully understood yet. This is what is thought to happen. 1) H+ ions are pumped out (active process, ATP required) of the cytoplasm of modified companion cells (called transfer cells) 6 www.alevel-notes.weebly.com Xerophytes xerophytes (from the greek xero for ‘dry’) are plants that are adapted to dry and arid conditions xerophytes have physiological and structural (xeromorphic) adaptations to maximise water conservation 7 www.alevel-notes.weebly.com