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What are the three general lengths of transport in plants, with a small description?
- Cellular uptake/unloading of materials: occurs in root cell and soil. Requires ATP to absorb minerals against concentration gradient.
- Short distance transport: within a tissue or organ. (eg moving material within the root or within the leaf).
- Whole plant transport: Systemic transport throughout the entirety of the plant (root -> leaf)
What is the biggest difference between plant and animal circulation?
Animals have a pump to circulate their blood, while plant movement is not circulatory and is performed without a pump.
In what directions is water moved through the plant?
Vertically by tracheids and vessel elements, but also laterally through pits (thin portions of the cell wall w/ high plasmodesmata) in these same structures.
Describe Ψ in detail
- Ψ (water potential) is a combined measurement of both pressure and [solute] used to determine directional flow of water.
- Water flows from high Ψ to low Ψ.
- High Ψ = low [solute], high pressure (turgid cell, water would exit)
- Low Ψ = high [solute], low pressure (limp cell, water would enter)
- The differential created between Ψ is so strong that low Ψ can act as a syringe, pulling water from the roots to the leaves!
How much water do plants used compared to animals? Why?
- Plants use an incredibly large amount of water, much more than animals because plants cannot circulate their water. (A sunflower plant imbibes 17x more water than a human over 24 hours)
- 99% of water taken in with the roots is released as water vapor (not used by the plant), this is called transpiration.
Why does transpiration have to occur? What generic strategies have been developed to limit transpiration?
- The major route for water loss in plants is through the stomata, and there are about 12,000 stomata per square inch of leaf. The stomata cannot remain closed because CO2 is required to perform photosynthesis. Every time CO2 is let in via stomata transpiration occurs, thus transpiration is a necessary evil.
- Plants have developed various strategies to try and deal with transpiration including trichomes, the cuticle, stomatal crypts in xerophytes, CAM plants, and most importantly CLOSING stomata.
Describe how water is moved from roots to leaves in detail (name, sequence of action, properties, etc)
- The TTCA (transpiration-tension cohesion-adhesion) model is used to explain water movement from roots to leaves. Essentially water is pulled (by tension), not pushed, into the upper canopy. Transpiration and the physical properties of water drive xylem contents upwards.
- Sequence of action: transpiration of water from the interstitial spaces of the mesophyll cells occurs at the stomata which creates a drop in pressure (lower Ψ). This lower Ψ pulls water from surrounding cells, which in turn pull water from surrounding cells, and so on until this tension force (negative pressure) reaches veins. Xylem sap (water + ions) is pulled from veins, and the cohesive and adhesive forces of water continue this “pulling” all the way down to the roots (adhesive forces cause walls of xylem to move inward). Water being pulled out of the roots increases [solute] (lower Ψ) and causes a proportional water uptake to the roots.
- Essentially the rate of transpiration is directly related to the rate of water uptake.
- Note – the diameter of a tree may actually decrease during the warm part of a day due to the tension forces.
- Note - Flow rate is significantly higher in xylem than phloem
What is the normal transpiration rate? How do plants quickly alter this rate? Describe the specific way that plants physiologically achieve this regulation in detail.
- A leaf is kept from wilting by having a transpiration stream that flows at a rate of 75cm/min within xylem vessels. Guard cells control the photosynthesis/transpiration compromise (close stomata if rate is too high, open stomata if rate is too low)
- Stomata are controlled through various mechanisms including…
- Osmosis: K+ redistribution and water potential changes cause guard cells to swell and open when gaining water; “shrink” and close when losing water
- Blue light photoreceptor triggers ATP driven proton pumps which trigger the K+ influx as described above.
- Decrease in CO2 within air spaces
- Biological rhythms
What conditions will cause an increase in transpiration rate? A decrease?
- Sunny, warm, dry, and windy days will increase transpiration rate.
- Humid conditions will decrease transpiration rate.
Describe succulents’ and CAM plants’ strategies to minimize transpiration.
- CAM plants: Open stomata at night (when the least amount of transpiration occurs) to take in CO2, and perform photosynthesis during the day WITHOUT opening their stomata.
Describe root pressure in detail.
- Root pressure is a second (lesser) type of force that pushes xylem sap upwards.
- As root cells pick-up solutes from the soil this decreases Ψ, forcing water to enter the root cells. This creates a pressure that pushes water up into the stem.
- The mechanism does not exert enough force to push water up a tall shrub/tree.
- Guttation: droplets of liquid water forced out by root pressure, most evident during the night when the least amount of transpiration occurs. Not the same thing is dew!
What are the two methods of water transportation within a root?
- Apoplastic: transportation through areas outside of the plant cells. Found in areas of the cell walls and dead tissues of the xylem.
- Symplastic: transportation through the cytoplasm of cells via that plant’s plasmodesmata.
Describe how organic molecules are moved throughout a plant in detail.
- The pressure/flow model is used to explain the movement of sugar, amino acids, minerals, and hormones through the plant via phloem (sieve-tube members).
- Phloem transport is directional, from sugar source (any cell that creates/stores glucose) to sugar sink (any cell that needs glucose).
- Phloem loading: requires ATP and active transport. A sucrose cotransporter is used in the process of sucrose loading.
- Sequence of action: Loading of sugar into the sieve tube at the source (an active process) causes a decrease in Ψ (increased [solute]). Because of this, the sieve tube (source) takes in water. This creates a water pressure at the source end and moves water w/material through the phloem. At the sink end, sucrose is unloaded. Removing the solute from the sieve tube increases Ψ (lower [solute]), thus water exits the sieve tube at the sink end relieving the pressure.
- Note - Flow rate is significantly higher in xylem than phloem
Give information about the plant endocrine system, incl. mechanisms of action, types of cellular responses, and major hormone categories.
- Hormones (chemical messengers) function to coordinate growth & development and mediate response to external cues (affect division, elongation, gene expression, and differentiation of cells)
- Hormones are produced in a variety of locations, and are active even in small amounts (not used in their mechanisms, so they can continue to affect another cell)
- Hormones work by signal transduction pathways: ligand (substance that binds to a receptor), target tissue (tissue equipped with receptors specific to a particular ligand)
- Hormone sequence of action: reception, signal transduction, cellular response
- Examples of cellular responses: Altering gene expression (protein synthesis regulation), activating or deactivating enzymes (protein kinase enzymes), changing the properties of membranes (permeability to ions)
- Cellular response may differ based on hormone concentration, developmental state of tissue, interplay of two or more hormones, tissue type
- Major hormone categories: Auxins, Cytokinins, Gibberellins, Ethylene, Abscisic Acid
Auxin – Produced where? Function? Other information?
- Manufactured in apical bud meristem, young leaves, and seed embryo
- Major stimulator of shoot elongation (moves down from shoot apical meristem from cell to cell by way of parenchyma cells)
- Name literally means “to increase”
- Most common naturally occurring auxin is IAA (indoleacetic acid) which was studied by Darwin
- Formation of adventitious roots in stem cuttings and lateral roots from pericycle
- Stimulates vascular cambium (secondary growth)
- Seed embryo secretes auxins to stimulate fruit development (has been used to create seedless fruits; normally for fruit development pollination and fertilization must occur, creating seeds).
- Functions in vascular tissue differentiation
- Apical dominance prevents growth of lateral shoots
- Synthetic auxins: (2,4-D) – herbicidal compound (mechanism unknown). Agent orange – a 50/50 mix of two chemicals (2,4,D and 2,4,5,T) was combined with kerosene or diesel fuel and dispersed to destroy foliage and food suppy in South Vietnam during the war. Earliest concerns about Agent Orange were about the product’s contamination with TCDD, or dioxin.
Cytokinins – Produced where? Function? Other information?
- Produced in actively growing tissues like roots, embryos, and fruits
- Synthesized in the roots and then transported via xylem
- Stimulates axillary bud formation (more roots -> increase cytokinin -> stimulates axillary buds -> more lateral branches)
- Delay leaf senescence (yellowing and eventual death)
- Can produce anti-aging properties, may work by inhibiting protein breakdown, stimulating protein synthesis, etc.
- Zeatin is the most common naturally occurring cytokinin
- Structurally similar to the purine adenine (a nitrogenous base)
- Work by stimulating cell division and regulation of cell differentiation (Promote cytokinesis)
Describe the interplay between auxins and cytokinins
- Auxins promote apical dominance and root growth
- Cytokinins promote axillary bud production and lateral shoot formation
- Callus tissue will develop into roots in an excess of auxin, buds and shoots in an excess of cytokinins, and will remain undeveloped when the ratio is equal between auxin and cytokinins
Gibberellins – Produced where? Function? Other information?
- Produced in root and bud apical meristem, young leaves and embryos
- Functions in shoot elongation through division and cell elongation (causes growth in dwarf mutants)
- Stimulates fruit growth without fertilization (commercial use allows for seedless grapes)
- Signals seeds to break dormancy and to germinate (ex. water soaked into the seed -> release of GA). GA then causes release of enzymes that break down starchy endosperm which supports developing seedlings by making nutrients available to them.
Ethylene – Produced where? Function? Other information?
- Synthesized in most tissues in response to stress (especially in ripening fruits and/or leaf senescence)
- Promotes fruit ripening and abscission of fruit/leaves
- In fruit ripening, seems to play a role in promoting chlorophyll degradation, formation of other pigments, digestion of pectin (middle lamella), synthesis of sugars from a variety of other nutrients
- Leaf, flower, and fruit abscission (Ethylene triggers enzymes that break down cell walls in petiole)
Describe the interplay between auxins and Ethylene
- Ethylene promotes abscission of fruit/leaves
- Auxins inhibit abscission of fruit/leaves (used to prevent pre-harvest drop of fruit)
- Auxin seems to work by making cells in the abscission zone less sensitive to ethylene
Abscisic acid (ABA) – Produced where? Function? Other information?
- Produced by seed early during development, causes an increase in seed storage proteins
- Helps prevent early germination (corn mutants less responsive to ABA germinate while still on the cob)
- The rinsing of ABA from a seed helps trigger germination
- Water stress triggers ABA synthesis in mature leaves – leads to closure of stomata (therefore involved in regulation of transpiration)
What is a tropism? Describe the various tropisms
- Tropisms: growth respons that curves the plant organ toward or away from stimuli (toward = positive, away = negative)
- Mechanism of tropisms: elongation of cells on OPPOSITE side of the organ
- Phototropism: curving of shoot towards light. Sensation of light is tied to the shoot tip, special blue light photoreceptor (cryptochrome). Function via auxin redistribution (migrates from light side to dark side of root tip, then to internode below which causes growth)
- Gravitropism: shoots grow against gravity (negative), roots growth with gravity (positive). Sedimentation of amyloplasts within cells used as an indicator of gravity for cells
- Thigmotropism: response to touch that allows roots to navigate around rocks, and tendrils to wrap around other structures. Cells touching surface slightly shorten, other side elongates.
- Movements in response to a mechanical stimulus: turgor movements (controlled by water efflux following ions), plants drop suddenly after touch. Venus flytrap; scientists unsure of how the mechanism works.
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