2Plan Diagram of Dicot Stem Stem cross section (Helianthus spp)Plan Diagram of Dicot StemCross-section shows:Epidermis – protective, often with waxy cuticleCortex – often cells have thickenings which provide secondary support of plantVascular bundles: contain xylem, phloem and cambiumCambium tissue: lateral meristemwhere cell division produces secondaryxylem and phloemPith – thin-walled cells (fill space andsometimes degenerate if stem is hollow)
5Functions of Leaf Tissues Absorption of light: mostly in palisade layerGas Exchange: mostly through stomata – gases stored and exchanged in spongy layer, usually on bottom of leafSupport: xylem and phloem; veinsWater conservation: waxy cuticle on epidermis and stomata (closing)Transport of water: xylemTransport of Photosynthetic Products: phloemDoes there seem to be a relationship between the placement of these tissues and their function?
7Monocots: Taproot with adventitious extensions Dicots: Tap root with lateral extensions
8Modifications of Roots Tubers: modified roots for storage of photosynthetic productsEg. Potato stores starch (carbohydrate)(eyes are axillary buds)
9Modification of Roots Tap roots can also be modified for other reasons Eg. CarrotGrow in sandy soils – anchor and store water for plant
10Roots Large surface area provided by root hairs and branches .RootsLarge surface area provided by root hairs and branches- The extension of the cell wall increases the surface area for the absorption of water and minerals at the cellular level.The root hair cell provides both an increase in the cell wall (apoplastic pathway) and the cytoplasmic route (symplastic pathway) for the movement of water
11Movement of Minerals to the Root 3 Ways:Diffusion – often not sufficient if minerals are in low concentration in soilFungal hyphae – mycorrhizal relationships – fungi increase surface area for absorption, gain access to plant carbohydratesMass flow…Fungal hyphae on roots
12Mass Flow HypothesisMinerals dissolved in water that the plant is absorbing form hydrogen bonds with water and are thus ‘dragged’ near to plant with the waterThis concentrates the minerals for absorption
13Mineral Absorption in Roots By Active Transport Mineral Absorption in Roots By Active TransportClay particles are negatively charged – attract positively charged ions like potassium, sodium, calciumproton pumps in roots pump protons ( H+) outside of the cell. This creates an electro-negative charge within the cell. 2. When the root cells secrete protons into the surrounding soil water the hydrogen ions displace the mineral ions from the clay particle, freeing them into solution.3. The mineral ions in the soil water are free to be absorbed by various pathways.
14Mineral Absorption in Roots By Active Transport Mineral Absorption in Roots By Active TransportPlants can also directly actively transport minerals into rootsExperiments that metabolically poison the root (stop ATP production) causes all mineral absorption to stop.
15Modifications of Stems :Modifications of StemsBulbs: short vertical underground stemsMany fleshy highly modified leaves for the storage of nutrient.Eg. Onion and LilyNot to be confused withcorms (daffodil, tulip, garlic) which areMore solid in tissue.
16Stem ModificationRunners: eg. Strawberry – allow plant to form new plantlets (asexual reproduction)Tendrils: used by climbing plants for support (eg. Peas)Tendril Animation
17Meristems Are areas of active cell division in plant body Two main types:Apical (up/down/sideways growth)Lateral (growth in girth)Cambium forms secondary growth
21Primary Growth Added by apical meristems Primary GrowthAdded by apical meristemsThis tissue diagram is a cross section of the stem of the primary plant body.This means that there has been no additional secondary thickening of the cell walls.
22Secondary GrowthSecondary growth added by the Lateral meristem (cambium) has two types:1. Vascular cambium that produces secondary xylem and phloem2. Cork cambium produces some of the bark layer of a stem.
23Phototropism Bending of plant toward light Auxins are a group of plant hormonesUnlike animal hormones, plant hormones often have multiple target sites and actionsMost important auxin may be IAA:
24Darwin’s ExperimentsCharles Darwin studies of auxin effects are published a book called, 'The Power of movement'.The cylindrical shoot is enclosed in a sheath of cells called the coleoptile." when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend".
25PhototropismStems may exhibit positive phototropism, roots negative phototropismAuxins cause cell elongation and cell division on ‘dark’ side of plant(a type of auxin was used as a herbicide “Agent Orange” in Viet Nam – dioxins that contaminated it caused many health problems)
26Plant Support Plants can support themselves in 3 ways: Thickening of cellulose wallsLignified xylem vesselsTurgor pressure
27Thickened Cellulose Walls Cells deposit extra cellulose in areas that require more support
28Lignified Xylem Vessels A polymer that binds to cellulose to strengthen cell wallsOften deposited in ringsRhubard with lignin rings
29Turgor PressureThe pressure of a cell’s contents (mostly caused by the vacuole in plant cells) on the interior cell wallsMaintained by osmotic pressureLow turgor pressure causes ‘wilting’
30Stomata Regulate transpiration, thus osmotic pressure in cells Regulated by abscisic acidDehydrated mesophyll cells release this hormone which stimulates stomata to close, no matter how much photosynthesis is inhibitedStomata
31Factors Affecting Transpiration Humidity – high humidity reduces rate of transpiration – low water gradientWind – increases evap, increasing transpirationTemperature – increasing temp. increases transpiration – more evap from leaf surfaceLight – more photosynthesis stimulates the stomata to open, increasing transpiration
32(a) The guard cell absorbs light and produces ATP in the light dependent reaction. (b) The ATP is used to drive proton pumps that pump out H+ . The inside of the cell becomes more negative.(c) Potassium ions enter the cell which increases the solute concentration.(d) Water moves from the surrounding tissue by osmosis.
33Xerophytes Plants adapted to low water conditions Adaptations include: Reduced or rolled leavesSpinesDeep rootsThick cuticleStomata in pits surrounded by hairsWater storage tissueLow growth formCAMC4 physiology
34PhotorespirationMaize: a C4 plantTendency of Rubisco to fix oxygen instead of carbon dioxide when carbon dioxide levels are lowInefficient, but may have a safety function in prevention of oxygen free radical formationPrevented by C4 and CAM
35CAM PlantsCrassulacian Acid Metabolism – plants “hold breath” all day!At night carbon dioxide is combined with phosphoenol pyruvic acid (C3) to form Oxoloacetic acid (C4).Oxoloacetic acid is changed to malic acid or aspartic acid. This stores the carbon dioxide until required for photosynthesis during the dayDuring the day, malic acid converted back to carbon dioxide for use in Ps.
36C4 PlantsEnzyme used to ‘fix’ carbon into a four carbon (instead of three carbon) compoundThis is then is shuttled as oxaloacetate or malate to bundle sheath cells for storage
38Phloem actively translocates sugars and amino acids from their source (usually leaves or storage areas) to the ‘sink’ (fruits, seeds, roots).The source is where food is produced, this would be the leaves. They produce glucose which is then converted to sucrose which enter the phloem. This makes the water potential more negative making water from the surrounding xylem enter.All of this extra material increases the pressure and forces the solution down and through the sieve plate. Then it gets to the sink where the sucrose is moved by active transport into the parenchyma; where it is made into insoluble starch so the water returns to the xylem.Translocation
39TranslocationSugars can move in the opposite direction if no light (at night) when energy must come from storage areas (eg. Tubers)
40Combination of Transpiration/Translocation 1. Source produces organic molecules2. Glucose from photosynthesis produced3.Glucose converted to sucrose for transport4. Companion cell actively loads the sucrose5. Water follows from xylem by osmosis6. Sap volume and pressure increased to give Mass flow7. Unload the organic molecules by the companion cell8. Sucrose stored as the insoluble and unreactive starch9. Water that is released is picked up by the xylem10. water recycles as part of transpiration to re supply the sucrose loadingCombination of Transpiration/Translocation