Exam 4 Plants Lectures Study Guide
Exam 4 Plants Lectures Study Guide ECOL 182R
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This 22 page Study Guide was uploaded by Camille Hizon on Monday March 21, 2016. The Study Guide belongs to ECOL 182R at University of Arizona taught by Bonine, Hunter, Martinez in Spring 2016. Since its upload, it has received 123 views. For similar materials see Introductory Biology II in Science at University of Arizona.
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ECOL182L Exam 4 Study Guide Spring 2016 Part 1: Plant Structure and Function There are two dominant clades in flowering plants: Monocots and Eudicots (aka Dicots). o These names refer to the first leaves that appear on the plant embryo, cotyledons. The table below compares monocots and dicots. Be sure to memorize these characteristics. They are illustrated in the figure below. Table 1: Monocots vs. Dicots Angiosperm Seed leaves Leaf veins Stems Flowers Roots Secondary Clade Growth? Monocots One Main veins Vascular Floral parts Fibrous absent cotyledon usually in bundles in usually in root parallel complex multiples of system arrangement. three Dicots Two Main veins Vascular Floral parts Taproot Often cotyledons usually bundles usually in usually present branched arranged in multiples of present ring four or five Figure 1: Monocots vs. Dicots, Illustrated Monocots o Examples: orchids, bamboos, palms, lilies, grasses, and grains. o Roots make excellent ground cover that reduces erosion. Dicots o Includes most angiosperms o “True dicots” are all related, other dicots have the same characteristics but evolved independently. ECOL182L Exam 4 Study Guide Spring 2016 o Most true dicots include shrubs and trees (except conifers) and food crops. Evolutionary adaptations that allowed plants to move onto land: o Ability to take up water and minerals from the soil o Ability to absorb light and take in carbon dioxide from the air for photosynthesis o Ability to survive dry conditions Roots and shoots are interdependent. Root system – anchors the plant to the soil, absorbs transport minerals and water, and stores food. o Monocot root system: Provides broad exposure to soil water and minerals Provides firm anchorage o Dicot root system: Many small secondary roots growing out of one large taproot Root hairs grow near root tips in both monocots and dicots. Each root hair is an outgrowth of an epidermal cell. o Increase surface area of the root o Absorb water and minerals Shoot system – made up of stems, leaves, and adaptations for reproduction (i.e. flowers for angiosperms). Stems – parts of the plant that are generally above the ground and that support the leaves and flowers. o Nodes – the points at which leaves re attached o Internodes – the portions of the stem between nodes o Leaves – the main site of photosynthesis in most plants; consist of a flattened blade and a stalk, or petiole, which joins the leaf to the stem. o Terminal bud – the bud at the apex (or tip) of the stem; sometimes produces hormones that inhibits growth of the axillary buds (process called apical dominance) o Axillary buds – buds that are located in each of the angles formed by a leaf and the stem; usually dormant. Apical dominance is an evolutionary adaptation that increases the plant’s exposure to light; typically dominant in areas where vegetation is dense and there is a lot of competition for sunlight. Branching is also important for increasing the exposure of the shoot system to the environment; under certain conditions the axillary buds start growing. Epicormic buds in Eucalyptus o Trees are not killed by many forest fires; instead they are defoliated, and can rapidly regrow foliage from the epicormic buds protected from the heat of the fire by thick bark on the trunk and large branches. Taproot – large roots that store food in the form of carbohydrates such as starch. Plants use the stored sugars during periods of active growth and when producing flowers/fruits. The strawberry plant has a horizontal stem, or runner, that grows along the ground surface. o A runner is a means of asexual reproduction. A new plant can emerge from its tip. Rhizomes – horizontal stems; large, brownish, root-like structures near the soil surface o Store food ECOL182L Exam 4 Study Guide Spring 2016 o Can spread and form new plants because they have buds. The white potato plant has rhizome that end in enlarged structures called tubers (the potatoes we eat) where food is stored in the form of starch. Grasses and most other monocots have long leaves that lack petioles. Some dicots (i.e. celery) have large petioles that contain a lot of water and stored food. Tendril – a modified leaf that helps plants such as a vetch plant climb. Cactus spines are also modified leaves that protect the plant from plant-eating animals. Leaves are often, but not always, oriented to the sun; sometimes leaf moves tracking the sun. Different plants have different arrangements of leaves on the stem. This is a reflection of the evolutionary history of the plant but different arrangements can be beneficial in different environments (examples: opposite leaves, whorled leaves, alternate leaves, rosette) Leaf display (angle to the stem, orientation, and spacing of leaves) is plastic, changing to suit the environment. Figure 2: Sectional view of the cell wall. Pits are located where the cell wall is relatively thin, while the Plasmodesmata are channels between adjacent plant cells that allow for communication and circulation. Cell wall – the most distinctive feature of the plant cell. o Made mainly of cellulose. o Many plant cells have a two-part cell wall; a primary cell wall is laid down first, and then a more rigid secondary cell wall is secreted between the plasma membrane and the primary wall. The middle lamella is a sticky layer that holds the adjacent cells in plant tissues together. Pits – the area where the cell wall is relatively thin; pits allow the contents of adjacent cell walls to lie close together. Plasmodesmata – channels of communication and circulation between adjacent plant cells. Five major types of plant cells: o Parenchyma The most abundant type of cell in most plants. Remain alive when mature and have only primary (often thin) walls. Perform functions including: food storage, photosynthesis, and aerobic respiration. Often multisided. Can divide and differentiate into other types of plant cells, which they may do during repair of an injury. o Collenchyma ECOL182L Exam 4 Study Guide Spring 2016 Resemble parenchyma cells in lacking secondary walls, but they have unevenly thickened primary walls. Expandable primary cell wall. Main function: provide support in parts of plant that are still growing. Young stems have collenchyma cells just below the surface that elongate with the growing stem. o Sclerenchyma Have rigid secondary cell walls hardened with lignin, which is the main chemical component of wood. Mature sclerenchyma cells cannot elongate; they occur only in regions that have stopped growing in length. When mature, most sclerenchyma are dead. Cell walls form a rigid scaffold that supports plant. Two types of Sclerenchyma: Fiber – long and slender; usually occurs in bundles. Sclereid – aka stone cell; shorter than the fiber and has a thick, irregular, and very hard secondary wall. Figure 3: The three basic types of plant tissue: parenchyma, collenchyma, and sclerenchyma. Table 2: Comparison of Parenchyma, Collenchyma, and Sclerenchyma cells Parenchyma Collenchyma Sclerenchyma - Living cell - Living cell - Dead cell - Intercellular space - Little intercellular space - No intercellular - Irregular shape - Palisade mesophyll, spongy space mesophyll, cortex, pith, epidermis - Usually in xylem - Support tissue o Water-conducting cells Characterized by rigid, lignin-containing secondary cell walls. Two types: Tracheids – long cells with tapered ends; spindle-shaped; pits for water movement. ECOL182L Exam 4 Study Guide Spring 2016 Vessel elements – wider, shorter, and less tapered; pits and perforated ends. Chains of tracheids and vessel elements form a system of hollow tubes that transports water from roots to stems and leaves. Both are dead when mature. Water passes through pits in walls of tracheids and vessel elements. Cells function in support. o Food-conducting cells Aka sieve-tube members Arranged end-to-end forming tubes Have thin primary walls and no secondary walls Have cytoplasm but lack nuclei and most organelles. Remain alive at maturity End walls are perforated with large plasmodesmata and form sieve plates Sieve plates –where sugars, other compounds, and some mineral ions move between adjacent food-conducting cells. Each sieve-tube member flanked by companion cell – connected to the sieve- tube member by plasmodesmata. Nucleus and ribosomes of the companion cells make certain proteins for the sieve-tube member, which loses nucleus and ribosomes during development. Simple plant tissues consist of one type of plant cell, meaning parenchyma cells would make up parenchyma tissue. Complex tissue – tissue composed of more than one type of cell. o Vascular tissues – tissue that conduct water and food. o Xylem – type of vascular tissue that contains water-conducting cells that transport water and dissolved minerals upward from the roots. o Phloem – type of vascular tissue that contains sieve-tube members that transport sugars from leaves or storage tissues o Phloem and xylem also have sclerenchyma cells for support and parenchyma cells for storage of materials. Roots, stems, and leaves are made up of three tissue systems: the epidermis, the vascular tissue system, and the ground tissue system. Epidermis (the dermal tissue system) – the “skin” of the plant; covers and protects the leaves, young stems, and young rots. It is the first line of defense against physical damage and infections. o Single layer of tightly packed cells covering the entire root where water and minerals enter the plant through. o Some young epidermal cells grow to form root hairs. On leaves and some stems, epidermal cells secrete a waxy coating called a cuticle that helps the plant retain water. Vascular tissue system – made of xylem and phloem; provides support, transports water and nutrients. Ground tissue system – makes up the bulk of a young plant; fills spaces between the epidermis and vascular tissue system. ECOL182L Exam 4 Study Guide Spring 2016 o Mainly parenchyma, also includes collenchyma and sclerenchyma. o Functions: photosynthesis, storage, support. Ground tissue system of the root forms the cortex, which consists mostly of parenchyma which: o Store food o Take up minerals that entered via epidermis Figure 4: The three tissue systems Table 3: Plant tissue systems, their functions, and their component tissues Tissue system and its functions Component tissues Dermal tissue system - Epidermis - Protection - Periderm (in older stems and roots) - Prevention of water loss Ground tissue system - Photosynthesis - parenchyma tissue - Food storage - collenchyma tissue - Regeneration - sclerenchyma tissue - Support - protection Vascular tissue system - xylem tissue - transport of water and minerals - phloem tissue - transport of food Endodermis – innermost layer of the cortex; thin cylinder one cell thick. o Selective barrier, determines which substances pass between the rest of the cortex and the vascular tissue. Vascular bundles o monocot stem – vascular bundles throughout ground tissue system o dicot stem – distinct ring of vascular bundles and a two-part ground tissue system (made up of the cortex and the pith) Pith – part of the dicot tissue system; fills the center of the stem and is important in food storage. ECOL182L Exam 4 Study Guide Spring 2016 Stomata – pores located in the epidermis of leaves; a gap between two guard cells where gases are exchanged. o Major avenues of water loss from the plant. Guard cells – regulate the size of the stomata, allowing gas exchange between the surrounding air and the photosynthetic cells inside the leaf. Mesophyll – ground tissue system of a leaf o Consists mainly of parenchyma cells equipped with chloroplasts and specialized for photosynthesis. o In dicot leaf, cells in lower mesophyll are loosely arranged with many air spaces → main site of gas exchange Stomata are more numerous in leaf’s lower epidermis which helps minimize water loss; the upper surface of the leaf is more exposed to sunlight and therefore can be dehydrated. Vein – forms a network that makes up the leaf’s vascular system o A vascular bundle composed of xylem and phloem and surrounded by a sheath of parenchyma cells. Figure 5: Tissue systems in young stems Indeterminate growth – plants that continue to grow as long as they live; most species of plants. o This enables plants to increase their exposure to sunlight, air and soil throughout life. Annuals – complete life cycle in a single year or growing season. Biennials – complete life cycle in 2 years. Perennials – plants that live and reproduce for many years. Meristem – localized, unspecialized cells that divide and generate new cells and tissues; present throughout a plant’s life. Apical meristems – meristems located at the tips of roots and in the terminal and axillary buds of shoots. o Cell division in apical meristems produces new cells that enable a plant to grow in length. o Root apical meristem: Replaces cells of the root cap that are scraped away by the soil Produces the cells for primary growth o Sustains growth of the roor by continuously adding cells to the three developing tissue cylinders ECOL182L Exam 4 Study Guide Spring 2016 Primary growth – lengthwise growth produced by apical meristems. o Primary growth helps push root through the soil. Root cap – tip of the root; cone of cells that protect the delicate actively dividing cells of the apical meristem. Cell elongation takes place when cells in the meristem grades upward into a zone of elongation where the cells undergo a tenfold increase in length. o Cells elongate by taking up water which causes the cellulose fibers to separate like an accordion. Epidermis, cortex, and vascular cylinder take shape in the zone of differentiation. Cells of the vascular cylinder differentiate into vascular tissues called primary xylem and primary phloem. Figure 7: Meristem Structure Secondary growth – an increase in a plant’s girth, which is most evident in woody plants (trees, shrubs, and vines). Wood is made up of mature, dead xylem tissue; xylem cells and fibers that have thick walls rich with lignin. Secondary growth involves cell division between two meristems: the vascular cambium and the cork cambium. o Vascular cambium – first appears as a cylinder of actively dividing cells (called a lateral meristem) between the primary xylem and primary phloem. o Secondary growth adds cells on either side of the vascular cambium. o Yearly production of a new layer of secondary xylem accounts for most of the growth in thickness of a perennial plant. When secondary growth begins, the epidermis and cortex start sloughing off. ECOL182L Exam 4 Study Guide Spring 2016 Cork – a new outer layer that replaces the cortex and epidermis after they have sloughed off. Mature cork cells are dead and have thick, waxy walls, which protect the underlying tissues of the stem. Cork is produced by meristematic tissue called cork cambium, which first forms from parenchyma cells in the cortex. Bark – all of the secondary phloem, the cork cambium and the cork. _____________________________________________________________________________________ Part 1 Review Questions: 1. By studying the structure of a plant’s cells and tissues, botanists gain insight into how these structures ______. a. Function 2. The “cot” in monocot and dicot refers to the number of _________ in the seed. a. Cotyledons (seed leaves) 3. Explain why pruning certain types of fruit trees increases future fruit harvest. a. Removal of terminal buds from major branches results in more branching by reducing inhibition of axillary buds. More branching produces more flowers and hence more fruit. 4. The “eyes” of a white potato mark nodes with buds. If those buds break dormancy and the potato “sprouts”, are the resulting appendages root branches or shoot branches? Explain. a. Shoot branches; the potato tuber is a modified stem, part of the shoot system. 5. Which of the following cell types has the potential to give rise to all others on the list: collenchyma, Sclereid, parenchyma, vessel element, companion cell? a. Parenchyma 6. Biologists generally define an “animal tissue” as a group of cells with common structure and function. How does this definition of a tissue contrast with what biologists call a “tissue system” in plants? a. A plant issue system may consist of several types of specialized cells, such as the different cell types of the vascular tissue system. 7. You have cells in lower layers of your skin that continue dividing, replacing dead cells that slough from your surface. Why is it inaccurate to compare such regions of active cell division in your body to a plant meristem? a. Your dividing cells normally are limited in the types of cells they can form. In contrast, the products of cell division in plant meristem differentiate into all the diverse cell types of a plant. 8. What type of plant tissue makes up wood? a. Secondary xylem 9. What is bark? a. All tissues exterior to the vascular cambium – secondary phloem, cork cambium, and cork. 10. The plant shoot system consists of: a. Stems, leaves, and reproductive structures 11. Two major clades of angiosperms, monocots and eudicots, are distinguished by: a. Net-like venation (veins) in eudicots/dicots and parallel venation in monocots ECOL182L Exam 4 Study Guide Spring 2016 12. Of the following, which is not a leaf, leaf part, or leaf derivative? (petiole, tendril, cactus spine, prickly pear disk) a. Prickly pear disk 13. Taproots: a. Sometimes have a storage function 14. Buds on the plant stem: a. Are present as the eyes on potatoes 15. Examples of plant stems as storage organs do not include…(options: carrots, potatoes, the trunk of a saguaro, the rhizome of ginger) a. Carrots 16. Plasmodesmata: a. Connect the endoplasmic reticulum in adjacent cells 17. Vessel elements: a. Form a major component of the xylem in angiosperms 18. Plants grow by: a. Division of cells in specialized tissues called meristems 19. Stomata: a. Are openings in the leaf cuticle and epidermis that allow gas exchange _____________________________________________________________________________________ Part 2: Plant nutrition and growth Transport in Plants o Water and dissolved minerals move from the soil through the xylem to all parts of the plant. o Transpiration - the process by which water exits a plant by evaporating through the stomata. o Water, carbohydrates, and minerals are transported by the phloem to where they are needed in the plant. o Water moves from areas of higher concentration to areas of lower concentration. o Water potential: solute potential + water potential o Solute Potential (aka osmotic potential) 1. Solute potential - the tendency of water to move by osmosis 2. In an isotonic solution, solute potential inside the cell and in surrounding solution is the same. No net movement of water. 3. In a hypotonic solution, a cell is placed in pure water and its solute potential is low relative to its surroundings. Water moves into the cell via osmosis. a) Turgor pressure generated inside the cell increases pressure potential, halting the flow of water. o Pressure Potential 1. Pressure potential - the tendency of water to move in response to pressure. 2. Turgor pressure is a major source of water pressure in cells. ECOL182L Exam 4 Study Guide Spring 2016 Figure 7: Solute potential in U-tube Figure 7: Solute potential of flaccid cell vs. turgid cell Figure 8: Plant and Animal Cells ECOL182L Exam 4 Study Guide Spring 2016 Pressure potential in plant cells helps the plant keep its shape. If the pressure is too low, the plant wilts. Potential Gradient in Plant and Soil o Low water potential in the atmosphere: -95.2 MPa o Leaf: -0.8 MPa Depends on transpiration rate; low when stomata are open o Root: -0.6 MPa Medium-high Water gets into the plant by going through the cortex to the xylem in the stele (all the tissue inside endodermis of the root). Membrane water channels – aquaporin proteins. Apoplast: cell walls and aqueous spaces outside the plasma membranes of cells. Symplast: the continuous cytoplasm connected by plasmodesmata of living cells. Casparian strips ensure that cell membranes, in conjunction with the cytoplasm, can regulate the entry of water and solutes to the xylem: it is impossible to get all the way from the soil to the xylem via the apoplast alone. o The Casparian strip blocks the apoplastic route at the endodermis. o Casparian strip has cell walls impregnated with wax Three forces that moves water up through the xylem: o Surface tension – downward pull of water molecules at air-water interface, forming meniscus. o Adhesion – water molecules that adhere to the glass resist the downward pull of cohesion. o Cohesion – water molecules cohere to each other. In the plant, the meniscus is not in the xylem, but in air spaces connected to stomata in the leaf. Transport in xylem: o Water is pulled up the xylem under tension o Negative pressure potential, from surface tension. o Energy input (heat) evaporates water from the leaves. o Cohesion between water molecules and adhesion to the xylem walls maintains continuity. o Water and minerals rise to the leaves. o Water and minerals (along with sugars and other substances) redistributed through phloem) More detailed explanation: o Inside a leaf, the area not occupied by cells is filled with moist air .Water diffuses from the inside of the leaf to the atmosphere. o As water exists the leaf, the humidity of the species inside the leaf drops, causing water to evaporate from the menisci that exist at eh air-water interfaces. o The resulting tension created at the menisci pulls water from the surrounding mesophyll cells, which in turn pulls water out of the xylem. ECOL182L Exam 4 Study Guide Spring 2016 o Tension is transmitted from water in leaf xylem through stem all the way to root xylem by cohesion (continuous hydrogen bonding). o Tension pulls water from root cortex cells into root xylem. o Tension pulls water from soil into roots. How researchers measure water potential in a living plant: o By applying just enough pressure… o So that xylem sap is pushed back to the cut surface of a plant sample… o A scientist can determine the tension on the sap in the living plant. Root pressure through an osmotic gradient can lead to transport under limited conditions. Stomatal functioning o Guard cells open to admit CO2 o Guard cells close to stop water loss Too much water loss means wilting and death for a plant. o In most plants, stomata remain closed a night because they cannot photosynthesize then. Adaptations to conserve water: o Stomata in crypts, e.g. oleander, along with thick waxy cuticle. o Cacti: Night - stomata are open. CO2 is stored as malic acid. Day – malic acid → CO2 → sugar using light energy to fuel the Calvin cycle. o C4 plants: CO2 concentrated deep within the leaf in bundle-sheath cells. More efficient CO2 uptake with steeper gradient across stomata. Stomata smaller, and open less. Transport take 2: translocation, the movement of sugars around the plant body. Sources and sinks: o Sugar enters the phloem at sources and exists the phloem at sinks. o Within the phloem, sugars travel from areas of high osmotic concentration and high water pressure, called sources, to regions of low osmotic concentration and low water pressure, called sinks. o Sugar passes vertically through pores in the wall between sieve-tube members. Sources: o Sources are nutrient-rich regions that supply sugars for the rest of the plant. o Include leaves, where sugar is generated through photosynthesis. o When leaves are high in supplies, the nutrient storage areas like roots and stems can also function as sources. o In the sources, sugar is moved into the phloem by active transport, in which the movement of substances across cell membranes requires energy expenditure on part of the cell. Sinks: o Sinks are areas in need of nutrients, such as growing tissues. o When they are low in supply, roots and stems can function as sinks. ECOL182L Exam 4 Study Guide Spring 2016 o The contents of the phloem tubes flow from the sources to the sinks, where the sugar molecules are taken out of the phloem by active transport. o Sink unloading: varies both between species and between sinks within a species. o When sugar is quickly used in the sink, passive transport along a concentration gradient is sufficient. o When sugar is stored in a sink, active transport is necessary at some point. Pressure flow model o Pressure flow – the mechanism by which sugars are transported through the phloem, from sources to sinks. o Transport of sucrose into phloem reduces solute potential (makes it a larger negative number) and pulls in water from the xylem, building up turgor pressure) o This leads to bulk flow to sinks where turgor pressure is lower because osmotic potential is a smaller negative number. o Note that the bulk flow follows a pressure gradient, which would include solute potential as well as pressure potential. ECOL182L Exam 4 Study Guide Spring 2016 How is sugar actively transported in phloem? o Sucrose is transported actively into the companion cell to high concentration → phloem passively via plasmodesmata. What do plants need to grow? o Light o Water o CO2 o O2 o Mineral Nutrients: macro and micro Macro nutrients and where needed: N: proteins, nucleic acids, hormones, coenzymes S: proteins and coenzymes P: nucleic acids, ATP, phospholipids, some coenzymes Ca: cytoskeleton, membranes, regulatory and signaling functions… Mg: chlorophyll, etc. K: cofactor for enzymes, osmotic adjustment, synthesis of organic molecules. Macronutrients (at least one g per kg) Micronutrients (less than 100 mg per kg), e.g. chlorine, iron, manganese, zinc, boron, copper, nickel, molybdenum. Uptake by plants requires the nutrient to be present in particular ionic forms. When there is an iron deficit in plants, plants grow a lot shorter and leaves are smaller, less green. Plants obtain their mineral nutrients from the soil. ECOL182L Exam 4 Study Guide Spring 2016 Decomposition of organic matter releases soluble N. Soils contain both living and nonliving components. Soil structure is important to its use by plants. Soils have a layered structure, reflecting their origin from weathering of rock, leaching from the topsoil A horizon, and buildup of organic matter there. Most rots are in the horizon. Humus contains a high density of diverse organisms, dead and alive. Clay particles and roots: o A clay particle, which is negatively charged, binds cations. o The cations are exchanged for hydrogen ions obtained from carbonic acid (H2CO3) or from the plant itself. o Mineral cations are released into the soil solution. Anions (NO3-, SO42-) leach from the A horizon (topsoil layer). Cations often interact with negative charges on the surface of clay. Anions usually dissolve in soil water, they are readily available for absorption by root hairs. Getting water and mineral ions into the xylem across cell membranes in the symplast of the root. o Passive flow of water – facilitated by membrane proteins called aquaporins. Can regulate the rate of osmosis but not the direction of movement. o Mineral ion uptake – electrically charged. ECOL182L Exam 4 Study Guide Spring 2016 Require transport proteins. Passive transport along concentration gradient. Active transport via proton pump. How the proton pump sets up an electrochemical gradient across cell membranes. Powered by ATP. Cations move with the gradient of positive charge outside the cell to negative charge inside. Anions enter the cells of root hairs via cotransporters; cotransporters couple the movement of negative and positive ions into the cell. o Anions “hitch a ride” with H+ moving back into the cells of the root hairs. Mutualistic associations of plants with mycorrhizal fungi transport essential nutrients into plant roots. o In return, the plants feed the fungi (aka mutualism, because both benefit). Plants also need to keep ions that are harmful, and restrict entry of ions that are harmful in high concentrations. o Exclusion of undesirable ions by the endodermis while allowing passage of desirable ions. If a plant cannot keep excess quantities of ion from entering, it can put it somewhere where it can’t do too much damage; example: stashing Na+ in vacuoles to deal with salinity. o In the tonoplast, antiporters send H+ out and Na+ in. Plants with H+/Na+ antiporters tolerate salt. o Experiment showing how ability to use antiporters to stash Na+ in vacuoles allows plants to tolerate saline soils. Nitrogen is really important for plant growth. Plants take up N in two forms. o NO3- and NH4+. The air is 80% N2 and needs to be converted so plants can use it. Some nitrogen comes from chemical interactions in the atmosphere, powered by lightning, forest fires, and volcanic eruptions. Most supplied by bacteria. Some free-living in the soil, others in association with plant roots. Various species of bacteria and plants are involved in these often mutualistic associations. ECOL182L Exam 4 Study Guide Spring 2016 Figure 8: Nitrogen cycles between the atmosphere and the soil. Symbiotic bacteria, Rhizobium, fix nitrogen in special nodules on the roots of legumes. Formation of a rhizobium nodule; note that the plant receives N in the form of ammonia from the rhizobium, and the rhizobium gains carbohydrate and protection from the plant. o Root hairs release a flavonoid that attracts rhizobia. Rhizobia move into hairs. o Rhizobia proliferate inside root hair and cause an infection thread to form. o Infection thread grows into the cortex of the root. o Nodule forms from rapidly dividing cortex cells. Other adaptations to obtaining nutrients: epiphytes do not grow and so much traps nutrients from water, dust, and organic debris. o Bromeliads trap materials in water holding tanks and absorbing nutrients through their leaves. o Parasitic plants such as mistletoe obtain water and mineral nutrients from a host plant; mistletoe haustoria penetrate host xylem and extract water and ions. o Some plants get nitrogen by eating animals. _____________________________________________________________________________________ Part 3: Regulation of plant growth Plants sense their environment and respond to it. o Ex: bending towards light Plants need to transmit information from sensing cells to responding cells. Plants transmit information by means of hormones What does transduce mean? Process: Steps in information processing ○ 1. Receptor cell transduces signal (Internal signal) ○ 2. Hormone released (cell-cell signal) ○ 3. Change of activity occurs in responder cell (internal signal) Sending information on signal striking receptor protein, changes it shape and leads to either a phosphorylation cascade or the release of a second messenger. ECOL182L Exam 4 Study Guide Spring 2016 ○ May lead to a release of a hormone to pass the information on, or some other change such as change in ion flow across a cell membrane ○ Cells only respond to a signal, be that hormonal or physical such as light, if they have a receptor for that particular signal. Light as a signal- it comes in different wavelengths. Different plant pigments respond to different wavelengths of light. ○ Phototropism involves blue light responses. ○ Receptors that respond to blue light and cause bending are called phototropins. ○ Phototropins are also involved in chloroplast movements and stomatal opening. How does the blue light receptor PHOT1 work? ○ A shape change occurs, allowing it to accept a P from a passing ATP, i.e. leading to phosphorylation. Auxin is a phototropic hormone. ○ The tip is where auxin is produced. ○ It travels down the opposite side of the stem. ○ Causes the cells to elongate and bend. ○ Note: auxin is released on the light side of the tip, and travels to the dark side ○ Steps: ■ 1. The phototropic signal is a chemical (chemical diffuses through agar) ■ 2. The hormone can cause bending in darkness ■ 3. The hormone causes bending by elongating cells; cells on the shaded side elongate in response to the hormone Red and far-red light: switch plants between physiological states The receptor molecule phytochrome is switched from one state to another by red and far-red light. ○ Phytochrome changes in form in response to different wavelengths of red light ○ Red light: r → Pfr ○ Far-red light:fr → Pr Photoperiodic responses ○ Different plants flower at different times of the year; they ‘tell time’ by measuring the length of the night ○ Phytochrome helps them measure the length of the night. ○ Switch-like response to red and far-red light suggests the involvement of phytochrome (a far-red flash cancels out a red flash before it, and makes the plant sense the night as long) Gravitropism - growth relative to gravity. ○ The plant has to work out which way is up: root tip cells sense gravity ○ Auxin helps the root grow downwards by inhibiting cell elongation on the lower side ○ Steps of auxin as the gravitropic signal: ■ 1. Normal distribution of auxin in vertical root prior to disturbance ■ 2. Root tip moved into horizontal position ECOL182L Exam 4 Study Guide Spring 2016 ■ Gravity-sensing cells actively redistribute the auxin - more goes to bottom side ■ Asymmetric auxin distribution inhibits cell growth on lower side and stimulates growth on upper side, leading to bending Apical dominance - an active apical meristem inhibits buds lower on the stem ○ Auxin travels down the stem inhibiting lateral buds ○ Auxin has many different roles in different parts of the plant. ○ Does not act alone: interacts with other hormones like cytokinins which promote cell division. Growth of the whole plant ○ Major stages in plant development: breaking dormancy, germination, shoot and root development, flowering, fruiting, senescing. Stages of Germination ○ Imbibition - uptake of water ■ Activation of metabolism ■ Expansion of the cells of the embryo ○ Mobilization of reserves ■ From the endosperm ■ From the cotyledons ○ Penetration of the seed coat ○ Growth of the embryo into a young seedling. Example: germination in cereal grasses; hormones called gibberellins have an important role Gibberellins (GAs) activate the production of a-amylase ○ 1. The seed absorbs water, and germination begins ○ 2.Gibberellins diffuse from the embryo to the aleurone layer ○ 3.Cells in the aleurone layer respond by releasing digestive enzymes, such as a-amylase. ○ 4.The enzymes digest starch, releasing sugars and other molecules to the growing plant. - Other hormones o Cytokinins Role in germination of seeds that require light. Inhibit stem elongation, and promote lateral swelling. Stimulate axial buds. Delay senescence Interact with auxins to promote cell division. o Ethylene Hastens senescence and fruit ripening, among other roles. o Abscisic acid: many effects… Storage protein accumulation in developing seeds. Role in seed dormancy. Inhibits stem elongation. Closure of stomata under water stress. o Brassinosteroids ECOL182L Exam 4 Study Guide Spring 2016 Promotes elongation for plants grown in the dark. - Flowers in plants are for sexual reproduction. - Liverworts and angiosperms and all other land plants show variations on a common theme. - Liverworts – gametophytes are large and long lived; the sporophyte is small and short-lived. - Angiosperms – sporophyte is large and long-lived; gametophytes are small (microscopic) and short-lived. o Pollen grains produce sperm (n) by mitosis. o Developing sporophyte (2n). o Note that the female gametophyte is inside the ovule, inside the ovary, which is part of the flower – sporophyte tissue. - Formation of female gametophyte o Female gametophyte is just a seven-cell structure. - Pollen: the male gametophytes in seed plants o Spread by wind or by animals. o Pollen tubes grow down the style. - The anther is composed of microsporangia; individual microspores divide to form a two-celled male gametophyte. o The male gametophyte grows further when the pollen grain lands on a stigma. - Plant stigmas can exercise mate selection: pollen with an S allele found in the stigma will not grow. ECOL182L Exam 4 Study Guide Spring 2016 - Note that two sperms are released, one fertilizes the egg while the other unites with the two polar nuclei to give a triploid cell, which then divides to form endosperm. - In some legumes, such as beans and peas, the cotyledons absorb all the endosperm: the cotyledons themselves are the nutrient stores as well as the first leaves of the new seedling upon germination.
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