3.1.3 - Transport in Plants

• why do plants require a transport system? they have large surface area to volume ratios high metabolic rates and they are multicellular.
• what are vascular bundles? vessels within the plant which function to transport water and sugars from one part of the plant to another
• what is found within vascular bundles? xylem and phloem vessels and sclerenchyma fibres
• what is the function of sclerenchyma fibres? to provide support to the stem
• what do xylem vessels do? transport water and mineral ions from the roots to the rest of the plant
• describe the structure of xylem vessels xylem vessels are made up of dead hollow cells with no end cell walls allowing xylem cells to be stacked on top of one another to form one continuous tube
• what do the cell walls of xylem vessels contain? pits which allow water and mineral ions to move in and out of the vessel
• what does lignin do? strengthens the cell walls of the xylem vessel providing structure and support to the plant
• what is the role of phloem vessels? they transport dissolved substances from sources to sinks
• what two types of cell are phloem vessels made up of? sieve tube elements and companion cells
• what are sieve tube elements? living cells joined end to end to form sieve tubes
• describe the structure of sieve tube elements and link to how this allows them to perform their function the ends of each element contain a sieve plate which allows solutes to move from one cell to the next. these cells contain no organelles and very little cytoplasm to allow more space for solutes to be transported
• what are companion cells? cells which are paired with sieve tube elements as these cells cannot survive without organelles
• what do companion cells contain? a nucleus and lots of mitochondria to provide energy for the active loading of sucrose into the sieve tube element
• how are sieve tube elements and companion cells connected? through plasmodesmata
• how does water enter the plant? through the soil where it enters root hair cells by osmosis
• why does water enter the root hair cells? there is always a higher water potential in the soil compared to the plant because water is constantly lost from the leaves in transpiration. this means that water moves down the water potential gradient into the root hair cells
• state the pathway of water after entering from the soil soil - root hair cells - through root cortex and epidermis - xylem
• what is the symplast pathway? when water moves from cell to cell via the cytoplasm as neighbouring cells are connected by plasmodesmata
• what is the apoplast pathway? when water moves from cell to cell through the cell walls
• why do the roots have the casparian strip? usually through the apoplast pathway water and dissolved substances bypass the ccell membrane. the casparian strip is impermeable to water and so forces water to go through a cell membrane that controls what substances can enter. once water has passed through the casparian strip water can reach the xylem
• what is tension? the suction force created after transpiration when more water is pulled into the leaf to replace lost water
• what is cohesion? when water molecules are attracted to one another and strong hydrogen bonds between water molecules causes them to stick together to form a column of water
• when tension pulls water up the xylem the whole column of water moves upwards
• what is adhesion? the attraction of water to non-water molecules which causes water to be attracted to the xylem walls. this facilitates movement of water up the xylem
• what is transpiration? the loss of water vapour through evaporation from a plant's surface
• why do plants close their stomata at night? no gas exchange or photosynthesis takes place so water needs to be conserved. closing stomata minimises transpiration
• list factors which affect the rate of transpiration light intensity temperature wind humidity
• what are xerophytes? plants adapted to living in regions where water is scarce
• list adaptations of xerophytes (there are six!) waxy epidermis layer sunken stomata hairs on epidermis spines rolled leaves closure of stomata in certain conditions
• what are hydrophytes? plants which live on water
• why do hydrophytes need adaptations? oxygen does not dissolve well in water so hydrophytes need adaptations to cope with low oxygen levels
• list adaptations of hydrophytes (there are three!) stomata on the upper surface of floating leaves, air spaces, flexible leaves + stems
• what is translocation? movement of dissolved substances from sources to sinks
• when is translocation impaired? if respiration is reduced or inhibited as translocation is an active process
• when sucrose reaches a sink it is converted into starch for carbohydrate storage this maintains a concentration gradient between the source and sink so that more sucrose moves into the source
• the mass flow hypothesis: 1) sucrose moves from companion cells into the sieve tube elements by active transport 2) this reduces the water potential of the sieve tube element 3) water then moves into the phloem by osmosis which increases the hydrostatic pressure 4) a pressure gradient is formed with high hydrostatic pressure near the source cell and lower hydrostatic pressure near sink cell 5) solutes move down the pressure gradient towards the sink end of the phloem 6) solutes move into the sink cells and are converted into other molecules 7) removing solutes increases water potental at the sink end causing water to move out of the phloem by osmosis 8) this maintains the hydrostatic pressure between the source and the sink
• active loading: 1) the companion cell actively transports H+ ions to the surrounding cells 2) this creates a H+ ion gradient between the surrounding cell and companion cell 3) H+ ions move back into the companion cell down their concentration gradient through a co-transporter protein 4) whenever a H+ ion moves through the co-transporter a sucrose molecule is also transported into the companion cell against its concentration gradient 5) the same process occurs to transport sucrose from the companion cell into the sieve tube element