Plant Anatomy and Transport Chapter 35 and 36 Plant Anatomy and Transport 2005-2006

Putting it all together Obtaining raw materials sunlight leaves = solar collectors CO2 stomates = gas exchange H2O uptake from roots nutrients 2005-2006

Plant tissues Dermal Vascular Ground single layer of tightly packed cells that covers & protects plant endodermis, epidermis, guard cells, root hairs Vascular transport materials between roots & shoots xylem & phloem Ground everything else: storage, photosynthetic bulk of plant tissue 3 cell types – parenchyma, collenchyma, sclerenchyma 2005-2006

Plant cell types in tissues Parenchyma “typical” plant cells = least specialized Thin, flexible primary cell walls – no secondary cell wall photosynthetic cells, storage cells tissue of leaves, stem, fruit, storage roots Collenchyma unevenly thickened primary walls; lack secondary cell wall = support Sclerenchyma very thick, “woody” secondary walls = lignin used for support of plant – fibers and sclerids rigid cells that can’t elongate dead at functional maturity 2005-2006

Leaves Function of leaves? photosynthesis energy production sugar production gas exchange transpiration 2005-2006

Stomates Function of stomates? 2005-2006

Control of transpiration Stomate function always a compromise between photosynthesis & transpiration leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis 2005-2006

Regulation of stomates Microfibril mechanism guard cells attached at tips microfibrils in cell walls elongate causing cells to arch open = open stomate shorten = close when water is lost Ion mechanism uptake of K+ ions by guard cells proton pumps water enters by osmosis guard cells become turgid loss of K+ ions by guard cells water leaves by osmosis guard cells become flaccid Leaves generally have broad surface areas and high surface area–to–volume ratios. The broad surface area is a morphological adaptation that enhances the absorption of light needed to drive photosynthesis. The high surface area–to–volume ratio aids in the uptake of carbon dioxide during photosynthesis as well as in the release of oxygen produced as a by–product of photosynthesis. Upon diffusing through the stomata, CO2 enters a honeycomb of air spaces formed by the spongy parenchyma cells. Because of the irregular shape of these cells, the internal surface area of the leaf may be 10 to 30 times greater than the external surface area we see when we look at the leaf. Although broad surface areas and high surface area–to–volume ratios increase photosynthesis, they also have the serious drawback of increasing water loss by way of the stomata. Thus, a plant’s tremendous requirement for water is part of the cost of making food by photosynthesis. By opening and closing the stomata, guard cells help balance the plant’s requirement to conserve water with its requirement for photosynthesis 2005-2006

Regulation of stomates Other cues Increase in K+ in the guard cells - this lowers water potential, which causes water to diffuse in light trigger blue-light receptor in plasma membrane of guard cells triggers ATP-powered proton pumps causing K+ uptake depletion of CO2 CO2 is depleted during photosynthesis (Calvin cycle) circadian rhythm = internal “clock” automatic 24-hour cycle Guard cells arbitrate the photosynthesis–transpiration compromise on a moment–to–moment basis by integrating a variety of internal and external stimuli. Even the passage of a cloud or a transient shaft of sunlight through a forest canopy can affect the rate of transpiration. 2005-2006

Vascular tissue Transports materials in roots, stems & leaves Xylem carry water & minerals up from roots tube-shaped dead cells only their walls provide a system of microscopic water pipes Phloem carry nutrients throughout plant sugars (sucrose), amino acids… tube-shaped living cells 2005-2006

Xylem vessel elements tracheids dead cells  water-conducting cells of xylem 2005-2006

Xylem Dead at functional maturity Cell elongated into tubes tracheids long, thin cells with tapered ends walls reinforced with lignin = support thinner pits in end walls allows water flow vessel elements wider, shorter, thinner walled & less tapered perforated ends walls allows free water flow 2005-2006

Rise of water in a tree by bulk flow Transpiration pull adhesion & cohesion H bonding brings water & minerals to shoot Water potential high in soil  low in leaves Root pressure push due to flow of H2O from soil to root cells upward push of xylem sap The transpiration–cohesion–tension mechanism that transports xylem sap against gravity is an excellent example of how physical principles apply to biological processes. In the long–distance transport of water from roots to leaves by bulk flow, the movement of fluid is driven by a water potential difference at opposite ends of a conduit. In a plant, the conduits are vessels or chains of tracheids. The water potential difference is generated at the leaf end by transpirational pull, which lowers the water potential (increases tension) at the “upstream” end of the xylem. On a smaller scale, water potential gradients drive the osmotic movement of water from cell to cell within root and leaf tissue. Differences in both solute concentration and turgor pressure contribute to this short–distance transport. In contrast, bulk flow depends only on pressure. Another contrast with osmosis, which moves only water, is that bulk flow moves the whole solution, water plus minerals and any other solutes dissolved in the water. The plant expends no energy to lift xylem sap by bulk flow. Instead, the absorption of sunlight drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by lowering the water potential in the air spaces within a leaf. Thus, the ascent of xylem sap is ultimately solar powered. 2005-2006

Phloem: food-conducting cells sieve tube elements & companion cells 2005-2006

Phloem: food-conducting cells sieve tube elements & companion cells 2005-2006

Phloem Living cells at functional maturity Cells lack nucleus, ribosomes & vacuole more room: specialized for liquid food (sucrose) transport Cells sieve tubes end walls, sieve plates, have pores to facilitate flow of fluid between cells companion cells nucleated cells connected to the sieve-tube help sieve tubes 2005-2006

Phloem sieve plate sieve tubes 2005-2006

Transport of sugars in phloem Loading of sucrose into phloem flow through symplast via plasmodesmata active cotransport of sucrose with H+ protons proton pumps 2005-2006

Pressure flow in sieve tubes Water potential gradient “source to sink” flow direction of transport in phloem is variable sucrose flows into phloem sieve tube decreasing H2O potential water flows in from xylem vessels increase in pressure due to increase in H2O causes flow can flow 1m/hr In contrast to the unidirectional transport of xylem sap from roots to leaves, the direction that phloem sap travels is variable. However, sieve tubes always carry sugars from a sugar source to a sugar sink. A sugar source is a plant organ that is a net producer of sugar, by photosynthesis or by breakdown of starch. Mature leaves are the primary sugar sources. A sugar sink is an organ that is a net consumer or storer of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season. When stockpiling carbohydrates in the summer, it is a sugar sink. After breaking dormancy in the spring, it is a source as its starch is broken down to sugar, which is carried to the growing tips of the plant. A sugar sink usually receives sugar from the nearest sources. Upper leaves on a branch may send sugar to the growing shoot tip, whereas lower leaves export sugar to roots. A growing fruit may monopolize sugar sources around it. For each sieve tube, the direction of transport depends on the locations of the source and sink connected by that tube. Therefore, neighboring tubes may carry sap in opposite directions. Direction of flow may also vary by season or developmental stage of the plant. 2005-2006 What plant structures are sources & sinks?

Vascular tissue in herbaceous stems dicot trees & shrubs monocot grasses & lilies 2005-2006

1 Roots Roots anchor plant in soil, absorb minerals & water, & store food fibrous roots (1) mat of thin roots that spread out monocots tap roots (2) 1 large vertical root also produces many small lateral, or branch roots dicots root hairs (3) increase absorptive surface area 2 3 2005-2006

Root structure: dicot phloem xylem 2005-2006

Root structure: monocot 2005-2006

Routes from cell to cell Moving water & solutes between cells transmembrane route repeated crossing of plasma membranes slowest route but offers more control symplast route move from cell to cell within cytosol apoplast route move through connected cell wall without crossing cell membrane fastest route but never enter cell Functions of the Symplast and Apoplast in Transport How do water and solutes move from one location to another within plant tissues and organs? For example, what mechanisms transport water and minerals from the root hairs to the vascular cylinder of the root? Such short–distance transport is sometimes called lateral transport because its usual direction is along the radial axis of plant organs, rather than up and down along the length of the plant. Three routes are available for this transport. By the first route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell in the pathway by the same mechanism. This transmembrane route requires repeated crossings of plasma membranes as the solutes exit one cell and enter the next. The second route, via the symplast, the continuum of cytosol within a plant tissue, requires only one crossing of a plasma membrane. After entering one cell, solutes and water can then move from cell to cell via plasmodesmata. The third route for short–distance transport within a plant tissue or organ is along the apoplast, the pathway consisting of cell walls and extracellular spaces. Without entering a protoplast, water and solutes can move from one location to another within a root or other organ along the byways provided by the continuum of cell walls. 2005-2006

Long distance transport Bulk flow movement of fluid driven by pressure flow in xylem tracheids & vessels negative pressure transpiration creates negative pressure pulling xylem sap upwards from roots flow in phloem sieve tubes positive pressure loading of sugar from photosynthetic leaf cells generates high positive pressure pushing phloem sap through tube Diffusion in a solution is fairly efficient for transport over distances of cellular dimensions (less than 100 μm), but it is much too slow to function in long–distance transport within a plant. For example, diffusion from one end of a cell to the other takes seconds, but diffusion from the roots to the top of a giant redwood would take decades or more. Long–distance transport occurs through bulk flow, the movement of a fluid driven by pressure. In bulk flow, water and solutes move through the tracheids and vessels of the xylem and through the sieve tubes of the phloem. In the phloem, for example, the loading of sugar generates a high positive pressure at one end of a sieve tube, forcing sap to the opposite end of the tube. In xylem, it is actually tension (negative pressure) that drives long–distance transport. Transpiration, the evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots. 2005-2006

Water & mineral uptake by roots Mineral uptake by root hairs dilute solution in soil active transport pumps this concentrates solutes (~100x) in root cells Water uptake by root hairs flow from high H2O potential to low H2O potential creates root pressure The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. If minerals do not enter the symplast of cells in the epidermis or cortex, they must enter endodermal cells or be excluded from the vascular tissue. The endodermis also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution. The structure of the endodermis and its strategic location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem. 2005-2006

Route water takes through root Water uptake by root hairs a lot of flow can be through cell wall route apoplasty The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. If minerals do not enter the symplast of cells in the epidermis or cortex, they must enter endodermal cells or be excluded from the vascular tissue. The endodermis also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution. The structure of the endodermis and its strategic location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem. 2005-2006

Controlling the route of water in root Endodermis cell layer surrounding vascular cylinder of root lined with impervious Casparian strip forces fluid through selective cell membrane & into symplast filtered & forced into xylem vessels 2005-2006

Mycorrhizae increase absorption Symbiotic relationship between fungi & plant symbiotic fungi greatly increases surface area for absorption of water & minerals increases volume of soil reached by plant increases transport to host plant 2005-2006

Meristem Regions of growth perpetually embryonic tissue regenerate new cells apical shoot meristem growth in length primary growth apical root meristem lateral meristem growth in girth secondary growth 2005-2006

Root structure & growth 2005-2006

Growth in woody plants Woody plants grow in height from tip primary growth apical meristem Woody plants grow in diameter from sides secondary growth vascular cambium vascular meristem layer 2005-2006

Growth in woody plants Primary growth shoot root tips of roots & shoots (apical meristem) restricted to youngest parts of plant shoot root 2005-2006

Growth in woody plants Secondary growth thickens & strengthens older part of tree cork cambium makes bark growing ring around tree vascular cambium makes xylem & phloem 2005-2006

Woody stem cork cambium bark phloem late vascular cambium early xylem Phloem produced to the outside Xylem produced to the inside cork cambium bark phloem late vascular cambium early xylem 2005-2006

Woody stem How old is this tree? cork cambium vascular cambium late early xylem phloem bark 2005-2006