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39 Cards in this Set
- Front
- Back
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Adaptations for aquiring resources |
●The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water
●Early nonvascular land plants lived in shallow water and had aerial shoots ●Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport ●Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss |
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Xylem and Phloem |
●Xylem transports water and minerals from roots to shoots
●Phloem transports photosynthetic products from sources to sinks |
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Shoot architecture and light capture |
●Stems serve as conduits for water and nutrients and as supporting structures for leaves
●Shoot length and branching pattern affect light capture ●There is a trade-off between growing tall and branching |
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Phyllotaxy |
●the arrangement of leaves on a stem, is a species-specific trait important for light capture
●Most angiosperms have alternate phyllotaxy with leaves arranged in a spiral ●The angle between leaves is 137.5 and likely minimizes shading of lower leaves |
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Environment and morphology |
●The canopy depth, leafy portion of all the plants in a community, affects the productivity of each plant
●Self-pruning, the shedding of lower shaded leaves, occurs when they respire more than photosynthesize ●Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows |
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Root structure |
●roots branch more in a pocket of high nitrate than low nitrate ●Roots are less competitive with other roots from the same plant than with roots from different plants ●Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae that helped plants colonize land ●Mycorrhizal fungi increase the surface area for absorbing water and minerals, especially phosphate |
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Apoplast |
●The apoplast consists of everything external to the plasma membrane
●It includes cell walls, extracellular spaces, and the interior of vessel elements and tracheids |
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Symplast |
●The symplast consists of the cytosol of all the living cells in a plant, as well as the plasmodesmata
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Three routes for water and solutes |
●The apoplastic route, through cell walls and extracellular spaces
●The symplastic route, through the cytosol ●The transmembrane route, across cell walls |
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Short distance transport of solutes |
●Plasma membrane permeability controls short-distance movement of substances
●Both active and passive transport occur in plants ●In plants, membrane potential is established through pumping H by proton pumps ●In animals, membrane potential is established through pumping Na by sodium-potassium pumps ●Plant cells use the energy of H gradients to cotransport other solutes by active transport ●Plant cell membranes have ion channels that allow only certain ions to pass |
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Short distance travel of water |
●Osmosis is the diffusion of water into or out of a cell
●Water potential is a measurement that combines the effects of solute concentration and pressure which determines the direction of movement of water ●Water flows from higher water potential to low ●Potential refers to water’s capacity to perform work ●Water potential is abbreviated as and measured in a unit of pressure called the megapascal (MPa) ●MPa for pure water at sea level and at room temperature |
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How solutes and pressure affect water potential |
●Both solute concentration and pressure affect water potential
●This is expressed by the water potential equation: S P ●The solute potential (S) of a solution is directly proportional to its molarity ●Solute potential is also called osmotic potential ●Pressure potential (P) is the physical pressure on a solution ●Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast ●The protoplast is the living part of the cell, which also includes the plasma membrane |
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Water movement across cell membranes |
●Water potential affects uptake and loss of water by plant cells ●If a flaccid (limp) cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis ●Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall ●If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid ●Turgor loss in plants causes wilting, which can be reversed when the plant is watered |
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Aquaporins |
●Aquaporins are transport proteins in the cell membrane that facilitate the passage of water
●These affect the rate of water movement across the membrane |
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Long distance transport |
●Efficient long-distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure
●Water and solutes move together through tracheids and vessel elements of xylem, and sieve-tube elements of phloem ●Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few or ganelles in their cytoplasm |
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Absorption of water and minerals from by cells |
●Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water whichaccount for much of the surface area of roots
●After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals ●The concentration of essential minerals is greater in the roots than soil because of active transport |
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Transport of water and minerals into xylem |
●The endodermis, casuarina strip
●It surrounds the vascular cylinder and is thelast checkpoint for selective passage of minerals from the cortex into the vascular tissue ●Water can cross the cortex via the sym our apo ●Water and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder |
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Endodermis |
●the innermost layer of cells in the root cortex which surrounds the vascular cylinder
●The last checkpoint for selective passage of minerals from the cortex into the vascular tissue ●regulates and transports needed minerals from the soil into the xylem ●Water and minerals move from the protoplasts of endodermal cells into their own cell walls ●Diffusion and active transport are involved in this movement from symplast to apoplast |
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Casparian strip |
●The waxy strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder |
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Bulk flow in xylem |
●Xylem sap, water and dissolved minerals, is transported from roots to leaves by bulk flow
● involves transpiration, the evaporation of water from a plant’s surface ●Transpired water is replaced as water travels up from the roots ●Is sap pushed up from the roots, or pulled up by the leaves? |
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Pushing xylem sap |
●At night root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential
●Water flows in from the root cortex, generating root pressure, a push of xylem sap which sometimes results in guttation, the exudation of water droplets on tips or edges of leaves ●Positive root pressure is relatively weak and is a minor mechanism of xylem bulk flow |
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Pulling xylem sap |
●According to the cohesion-tension hypothesis, transpiration and water cohesion pull water from shoots to roots
●Xylem sap is normally under negative pressure, or tension |
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Transpirational pull |
●Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata
●As water evaporates, the air-water interface retreats further into the mesophyll cell walls ●The surface tension of water creates a negative pressure potential ●This negative pressure pulls water in the xylem into the leaf ●The transpirational pull on xylem sap is transmitted from leaves to roots |
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Adhesion and cohesion in xylem sap |
●Water molecules are attracted to cellulose in xylem cell walls through adhesion which helps offset the force of gravity
●Water molecules are attracted to each other through cohesion which makes it possible to pull a column of xylem sap ●Thick secondary walls prevent vessel elements and tracheids from collapsing under negative pressure ●Drought stress or freezing can cause a break in the chain of water molecules through cavitation, the formation of a water vapor pocket |
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Xylem sap ascent by bulk flow |
●The movement of xylem sap against gravity is maintained by the cohesion-tension mechanism
●Bulk flow is driven by a water potential difference at opposite ends of xylem tissue ●Bulk flow is driven by transpiration and does not require energy from the plant; like photosynthesis it is solar powered |
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How build flow differs from diffusion |
●It is driven by differences in pressure potential, not solute potential
●It occurs in hollow dead cells, not across the membranes of living cells ●It moves the entire solution, not just water or solutes and faster |
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Stomata |
90% of water loss ●Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape ●Stomatal density is under genetic and environmental control |
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Stomata like opening and closing |
●Changes in turgor pressure open and close stomata
●When turgid, guard cells bow outward and the pore between them opens ●When flaccid, guard cells become less bowed and the pore closes ●This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells |
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Stimuli for stomata |
●Generally, stomata open during the day and close at night to minimize water loss
●Stomatal opening at dawn is triggered by Light, CO2, and internal clock in guard cells ●Drought, high temperature, and wind can cause stomata to close during the daytime ●The hormone abscisic acid (ABA) is produced in response to water deficiency and causes the closure of stomata |
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Effects of transpiration of leaves |
●large amount of water lost by transpiration and if it is not replaced by sufficient transport of water, the plant will lose water and wilt
●also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes |
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Xerophytes |
●plants adapted to arid climates
●Some desert plants complete their life cycle during the rainy season ●Others have fleshy stems that store water or leaf modifications that reduce the rate of transpiration ●Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night |
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Translocation |
●The products of photosynthesis are transported through phloem by the process of translocation
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Movement from sugar sources to sinks |
●In angiosperms, sieve-tube elements are the conduits for translocation
●Sugar must be loaded into sieve-tube elements before being exported to sinks ●Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways ●Companion cells enhance solute movement between the apoplast and symplast ●In many plants, phloem loading requires active transport ●Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose●At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water |
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Phloem sap |
●Phloem sap is an aqueous solution that is high in sucrose that travels from sugar source to sink
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Sugar source and Sink |
●A sugar source is an organ that is a net producer of sugar, such as mature leaves
●A sugar sink is an organ that is a net consumer or storer of sugar, such as a tuber or bulb ●A storage organ can be both a sugar sink in summer and sugar source in winter |
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Bulk flow by positive pressure |
●Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow
●The pressure flow hypothesis explains why phloem sap always flows from source to sink●Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms ●Self-thinning, the dropping of sugar sinks such as flowers, seeds, or fruits, occurs when there are more sugar sinks than the sources can sup port |
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Symplast |
●The symplast is a living tissue and is responsible for dynamic changes in plant transport processes
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Plasmodesmata number and pore size |
●Plasmodesmata can open or close in response to turgor pressure, cytoplasmic calcium levels, or cytoplasmic pH
●Plant viruses can cause plasmodesmata to dilate, allowing viral RNA to pass between cells |
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Phloem as information highway |
●Phloem is a “superhighway” for systemic transport of macromolecules and viruses
●Systemic communication through the phloem helps integrate functions of the whole plant ●The phloem allows for rapid electrical communication between widely separated organs ●For example, rapid leaf movements in the sensitive plant (Mimosa pudica) |
