Transport in Plants Explain the need for transport systems in multicellular plants in terms of size and surface area:volume ratio; Describe, with the aid.
Presentation on theme: "Transport in Plants Objectives:"— Presentation transcript:
1Transport in Plants Objectives: * Explain the need for transport systems in multicellular plants in terms of size and surface area:volume ratio;**Describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants;*** Relate, with the aid of diagrams and photographs, the structure and function of xylem vessels, sieve tube elements and companion cells;
2Transport in PlantsPlants need a transport system so that cells deep within the plants tissues can receive the nutrients they need for cell processesThe problem in plants is that roots can obtain water, but not sugar, and leaves can produce sugar, but can’t get water from the air
3What substances need to be moved? The transport system in plants is called vascular tissueXylem tissue transports water and soluble mineralsPhloem tissue transports sugars and amino acids
4The Vascular TissuesXylem and phloem are found together in vascular bundles, that sometimes contain other tissues that support and strengthen them
5Root vs. stem vs. leafThe vascular bundle differs depending on if it is a root or stem
6Root The vascular bundle is found in the centre There is a large central core of xylem- often in an x-shapeThis arrangement provides strength to withstand the pulling forces to which roots are exposedAround the vascular bundle are cells called the endodermis which help to get water into the xylem vesselsJust inside the endodermis is the periycle which contains meristem cells that can divide (for growth)
7Stem The vascular bundles are found near the outer edge of the stem The xylem is found towards the inside of each vascular bundle, the phloem is found towards the outsideIn between the xylem and phloem is a layer of cambiumCambium is a layer of meristem cells that divide to make new xylem and phloem
8LeafThe vascular bundles (xylem and phloem) form the midrib and veins of the leafA dicotyledon leaf has a branching network of veins that get smaller as they branch away from the midribWithin each vein, the xylem can be seen on top of the phloem
14Structure of XylemUsed to transport water and minerals from roots to leavesConsists of tubes for water, fibres for support and living parenchyma cellsDraw and make annotated labelling from next slide
17Xylem tissue is composed of dead cells joined together to form long empty tubes. Different kinds of cells form wide and narrow tubes, and the end cells walls are either full of holes, or are absent completely. Before death the cells form thick cell walls containing lignin, which is often laid down in rings or helices, giving these cells a very characteristic appearance under the microscope. Lignin makes the xylem vessels very strong, so that they don’t collapse under pressure, and they also make woody stems strong.
18Vessel with annular thickening Xylem vesselsXylem vessels show different patterns of woody thickening (lignification), giving them a function in support as well as water conduction.LSXylem parenchymaFibrePitted vesselVessel with annular thickeningTS
19Structure of xylem Xylem is a compound tissue, consisting of: two types of conducting cell, vessels and tracheidsfibres (thin elongated cells with thick woody walls and no living contents)Xylem parenchyma (living cells with thin cellulose cell walls)
20Vessels and tracheidsVessels are short hollow cells with woody (lignified) cell walls and no living contents at maturity.Their end walls break down, so that water can flow freely from one to the next.Many vessels have pits allowing sideways movement of water from vessel to vessel: this can help by-pass blockages.Tracheids are narrower lignified cells with tapered ends that overlap, transferring water from cell to cell via pits.
21Xylem vessels Obvious in dicotyledonous plants Long cells with thick walls containing ligninLignin waterproofs walls of cells and strengthens themCells die and ends decay forming a long tubeLignin forms spiral, annular rings or broken rings (reticulate)Some lignification is not complete and pores are left called pits or bordered pits, allowing water to move between vessels or into living parts
22Adaptations of Xylem to Function Xylem can carry water and minerals from roots to shoot tips because:Made of dead cells forming continuous columnTubes are narrow so capillary action is effectivePits allow water to move sidewaysLignin is strong and allows for stretchingFlow of water is not impeded as: there are no end walls, no cell contents, no nucleus, lignin prevents tubes collapsing
23Xylem Structure and Function: Tracheids and vessel members specialise in efficient water transport.Long, narrow, dead cells with walls thickened and strengthened with lignin.Tracheids have intact end walls and are tapered at their ends.Vessel members do not have end walls.A series of vessel members forms a long continuous open tube called a xylem vessel.Pits in the thickened walls allow easy water transfer to neighbouring cells.Tracheids and vessel members also give great mechanical support to the plant.
24Structure of PhloemFunction to transport sugars from one part to anotherMade of sieve tube elements and companion cells
25Sieve TubesSieve tube elements not true cells as they have little cytoplasmLined up end to end to form a tubeSucrose is dissolved in water to form a sapTubes (known as sieve tubes) have a few walls across the lumen of the tube with pores (sieve plates)
26Companion cells In between sieve tubes Large nucleus, dense cytoplasm Many mitochondria to load sucrose into sieve tubesMany plasmodesmata (gaps in cell walls between companion cells and sieve tubes) for flow of minerals
27Phloem Structure and Function: Sieve ElementsSpecialises in efficient transport of food.Living cells but do not have a nucleus.Long, narrow, thin walled living cells.End walls are heavily perforated – called a sieve plate.A series of sieve elements is called a sieve tube.Companion CellsAssist the sieve element in food transport.Live narrow cells with a prominent nucleus.Its nucleus also controls the sieve element.Dense cytoplasm particularly rich in mitochondria.
28Movement of Water Objectives: * describe the pathways and explain the mechanisms by which water is transported from soil to xylem and from roots to leaves;**explain the movement of water between plant cells and between them and their environment, in terms of water potential
29How is water transported against gravity from the roots, up the xylem and to the leaves?
30Think Like a ScientistScientists use ‘thought experiments’ to help them solve problems.30
31I wonder where trees get water from? Well, obviously from the ground.What are the processes involved?
32How does water move through the transport system of a plant IF it does not have a heart to act as a pump?How is water lifted against gravity from the ground to the leaves through this transport system?Are the products of photosynthesis also carried in a set of vessels from the leaves to the roots?PAUSE toPONDER32
33Root HairsExchange surfaces in plants responsible for absorption of mineral ions and water.Plants constantly lose water by transpiration, all must be replaced by water absorbed through root hairs.Each root hair is a long, thin extension of a root epidermal cell.Only remain functional for a few weeks.
34Efficient Exchange Surfaces Provide large surface area as are very long extensions and occur in thousands on each root branch.Have thin surface layer (cell surface membrane and cellulose cell wall), across which materials can move easily.
35Root Hairs Arise from epidermal cells behind the tips of young roots. Hairs grow into the spaces around soil particles.In damp conditions, surrounded by soil solution containing small amount mineral ions.Soil solution mostly water so has a high water potential – slightly less than 0.
36Root HairsRoot Hairs and other cells of root have sugars, amino acids and mineral ions dissolved inside them. Therefore they have much lower water potential.Therefore have a much lower water potential.Water moves by osmosis from soil solution into root-hair cells down this water potential gradient.
37Water route between cells Apoplast: between cell walls of neighbouring cellsSymplast: through plasma membrane and plasmodesmata to cytoplasms from cell to cellVacuolar: same as symplast, but also through vacuoles
38Water uptake from the soil Epidermis of roots contain root hair cellsMinerals absorbed by active transport using ATPMinerals reduce the water potential in the cell cytoplasm (more negative) so water is taken up by osmosis
40Movement across the root Active process occurring at the endodermis (layer of cells surrounding the xylem, some containing waterproof strip called casparian strip)Casparian strip blocks the apoplast pathway (between cells) forcing water into the symplast pathway (through the cytoplasm)The endodermis cells move minerals by active transport from the cortex into the xylem, decreasing the water potential (more negative), thus water moves from the cortex through the endodermal cells to the xylem by osmosisA water potential gradient exists across the whole cortex, so water is moved along the symplast pathway (through cytoplasm) from the root hair cells across the cortex and into the xylem
41Passage of water across a root Root hairEpidermisCortexEndodermisPericycleXylem
42Passage of water across a root Some water (blue line) crosses the cell surface membrane into the cytoplasm and passes from cell to cell via plasmodesmata: this is the symplastic pathway.Some water enters the root hair vacuole by osmosis, and travels by osmosis from vacuole to vacuole across the cortex.This is the vacuolar pathway.Most water (red line) does not enter the living cells at all but passes along cells walls and intercellular spaces: this is the apoplastic pathway.The vacuolar pathway presents the most resistance to water flow (because of the number of membranes to be crossed), the apoplastic pathway the least …… but at the endodermis the apoplastic pathway is completely blocked by a strip of corky material (the Casparian strip) around the walls of the endodermal cells.
43Passage of water across a root The Casparian strip completely blocks the apoplast pathway …… so that only the symplast and vacuolar pathways are available.Why is this important?It allows the flow of water and dissolved minerals into the plant to be controlled.
44Movement through the xylem Water enters the xylem because its water potential is reduced by the upward ‘pull’ (tension) on the water column it containsAdhsion of water molecules to the xylem vessel walls also helps maintain the column.
45Casparian Strip Blocks the apoplast pathway (cell walls) Water and dissolved nitrate ions have to pass into the cell cytoplasm through cell membranesThere are transporter proteins in the cell membranes that actively transport nitrate ions into the xylem lowering the water potential (more negative)Water enters xylem down concentration gradient and cannot pass back
46outline the roles of nitrate ions and of magnesium ions in plants TRANSPIRATIONObjectives:*define the term transpiration and explain that it is an inevitable consequence of gas exchange in plants;**describe how to investigate experimentally the factors that affect transpiration rate;***describe how the leaves of xerophytic plants are adapted to reduce water loss by transpiration;H/W : due in on 1/12outline the roles of nitrate ions and of magnesium ions in plants
47Cohesion-Tension Theory Water molecules have dipoles which cause an attraction between them.Water is ‘pulled’ up the xylem vessels by transpiration. When this happens, the pull is transmitted all the way down the water column, pulling all of the water molecules up the vessel.For this to work, the xylem vessel must be a continuous column of water i.e. contain no bubbles.
48Water movement up stemRoot pressure: minerals move into xylem by active transport, forcing water into xylem and pushes it up the stemTranspiration Pull: loss of water at leaves replaced by water moving up xylem. Cohesion-tension theory- cohesion between water molecules and tension in the column of water (which is why xylem is strengthened with lignin) means the whole column of water is pulled up in one chainCapillary action: adhesion of water to xylem vessels as they are narrow
49Give a definition of transpiration, explain why it is inevitable, and list the advantages of transpiration.
50Transpiration Loss of water vapour from upper parts of the plant Water enters leaf from xylem and passes to mesophyll cells by osmosisWater evaporates from surface of mesophyll cells to form water vapour (air spaces allow water vapour to diffuse through leaf tissue)Water vapour potential rises in air spaces, so water molecules diffuse out of the leaf through open stomata
51Transpiration: three processes Osmosis from xylem to mesophyll cellsEvaporation from surface of mesophyll cells into intercellular spacesDiffusion of water vapour from intercellular spaces out through stomata
52How water leaves the leaf Through stomataTiny amount through the waxy cuticleWater evaporates from the cells lining the cavity between the guard cells, lowering water potential and meaning that water enters them by osmosis from neighbouring cells, which is replaced by further neighbouring cells and so on
53Draw a large diagram of vertical section through part of a leaf, adding numbered annotations to show the pathway of water and the sequence of events occurring.
56Water use in plant Photosynthesis Cell growth and elongation Turgidity Carriage of mineralsCools the plant
57Measuring transpiration Potometer is used to estimate water loss
58List as many factors as you can affecting transpiration and explain why they affect transpiration
59Factors affecting transpiration Leaf number: more leaves(more SA), more transpirationNumber, size, position of stomata: more and large, more transpiration, under leaf, less transpirationCuticle: waxy cuticle, less evaporation from leaf surfaceLight: more gas exchange as stomata are openTemperature: high temperature, more evaporation, more diffusion as more kinetic energy, decrease humidity so more diffusion out of leafHumidity: high humidity, less transpirationWind: more wind, more transpirationWater availability: less water in soil, less transpiration (e.g. in winter, plants lose leaves)
60Too much water loss Less turgidity Non-woody plants wilt and die Leaves of woody plants die first then it will die if water loss continues
61Xerophytes Smaller leaves reducing surface area e.g. pine tree Densely packed spongy mesophyll to reduce surface area, so less water evaporating into air spacesThick waxy cuticle e.g. holly leaves to reduce evaporationClosing stomata when water availability is lowHairs on surface of leaf to trap layer of air close to surface which can become saturated with water, reducing diffusionPits containing stomata become saturated with water vapour reducing diffusionRolling the leaves so lower epidermis not exposed to atmosphere also traps air which becomes saturatedMaintain high salt concentration to keep water potential low and prevent water leaving
62Xerophytes Cont. Some xerophytes may have large numbers of stomata. Xerophytes cells may have extra support to prevent cells collapsing when they dry out.Extensive root system.Leaves may have evolved to become spines, with water being stored in the stem e.g. cacti.
63Marram Grass Found on sand dunes. When dry, leaves roll up, so stomata open to an enclosed space.Water vapour accumulates in this space = reduced diffusion gradient.Spines increase width of boundary layer
64Marram Grass Leaf rolled up to trap air inside Thick waxy cuticle to reduce water evaporation from the surfaceTrapped air in the centre with a high water potential (less negative)Hairs on lower surface reduce movement of airStomata in pits to trap air with moisture close to the stomata
65Movement of SugarsTranslocation: movement of assimilates (sugars and other chemicals) through the plantSource: a part of the plant that releases sucrose to the phloem e.g. leafSink: a part of the plantthat removes sucrose fromthe phloem e.g. root
66Sucrose Entering the Phloem Active process (requires energy)Companion cells use ATP to transport hydrogen ions out of their cytoplasmAs hydrogen ions are now at a high concentration outside the companion cells, they are brought back in by diffusion through special co-transporter proteins, which also bring the sucrose in at the same timeAs the concentration of sucrose builds up inside the companion cells, they diffuse into the sieve tubes through the plasmodesmata (gaps between sieve tubes and companion cell walls)
67Sucrose movement through phloem Sucrose entering sieve tube lowers the water potential (more negative) so water moves in by osmosis, increasing the hydrostatic pressure (fluid pushing against the walls) at the sourceSucrose used by cells surrounding phloem and are moved by active transport or diffusion from the sieve tube to the cells. This increases water potential in the sieve tube (makes it less negative) so water moves out by osmosis which lowers the hydrostatic pressure at the sink
68Movement along the phloem Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving at the sink produces a flow of water along the phloem that carries sucrose and other assimilates. This is called mass flow. It can occur either up or down the plant at the same time in different phloem tubes
69Evidence for translocation Radioactively labelled carbon from carbon dioxide can appear in the phloemRinging a tree (removing a ring of bark) results in sugars collecting above the ringAn aphid feeding on the plant stem contains many sugars when dissectedCompanion cells have many mitochondriaTranslocation is stopped when a metabolic poison is added that inhibits ATPpH of companion cells is higher thanthat of surrounding cellsConcentration of sucrose is higher atthe source than the sink
70Evidence against translocation Not all solutes move at the same rateSucrose is moved to parts of the plant at the same rate, rather than going more quickly to places with low concentrationsThe role of sieve plates is unclear