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1 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights DISTINGUISHING CHARACTERISTICS OF PHYLA The four main groups of land plants are the bryophytes, pteridophytes, gymnosperms, and angiosperms. Bryophytes are different from algae in that they have several evolutionary adaptations to living on land, most of which are reproductive. This group includes liverworts, hornworts, and mosses. The group of vascular plants consists of plants with vascular tissue, tissue in which cells are joined into tubes that transport water and nutrients throughout the plant body. The pteridophytes, pteridophytes the group containing lycophytes, ferns, and horsetails, are sometimes referred to as seedless plants because they have no seed stage in their life cycle. In contrast, the gymnosperms and angiosperms are seed plants. Seeds are plant embryos packed with a food supply into a protective coat. Examples of gymnosperms are ginkgos, cycads, and conifers. Most modern-day plant species are angiosperms, also known as flowering plants. ADAPTATIONS OF LAND PLANTS Apical Meristems Terrestrial plants need light and carbon dioxide from the air, while also requiring water and mineral nutrients in the soil. As a result, most plants have roots and leaf-bearing shoots to allow for maximum exposure to environmental resources. Growth in length is sustained by the activity of apical meristems, meristems localized regions of cell division at tips of shoots and roots. The cells produced by these meristems differentiate into a plant’s various tissues and also generate leaves in most plants. Multicellular, Dependent Embryos Zygotes that are kept within tissues of the female parent give rise to multicellular plant embryos, which are provided with nutrients from the parental tissues. The embryo has placental transfer cells that enhance this transfer of nutrients. This is similar to the system in placental mammals. Land plants are also known as embryophytes, embryophytes a distinction recognizing multicellular, dependent embryos as a derived characteristic common to the land plant clade. Alternation of Generations The gametophyte and sporophyte generations are the two multicellular body forms that alternate in the life cycle of land plants. Cells of the gametophyte are haploid and produce gametes. Fusion of eggs and sperm during fertilization results in diploid zygotes. Mitotic division of the zygote results in the multicellular, diploid sporophyte. sporophyte Meiosis in a mature sporophyte will result in haploid reproductive cells called spores. A spore is a reproductive cell that can develop into a new organism without fusing with another cell. Mitotic division then produces new multicellular gametophytes. 2 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights Remember – alternation of generations is a special type of haploid ↔ diploid sexual cycle because both stages can be multicellular. Walled Spores Produced in Sporangia Plant spores are haploid reproductive cells with the potential to grow into multicellular, haploid gametophytes by mitosis. A polymer called sporopollenin, sporopollenin the most durable organic material known, makes the walls of plants spores strong and protects from harsh environments. Multicellular organs called sporangia found on the sporophyte generation of a plant, produce spores. Within a sporangium, spore mother cells undergo mitosis and generate the haploid spores. The outer tissues of the sporangium protect developing spores until they are ready to be released into the air. Multicellular Gametangia The gametophyte forms of bryophytes, pteridophytes, and gymnosperms all produce their gametes in multicellular organs called gametangia. gametangia The female gametangia are called archegonia, archegonia with each archegonium producing a single egg cell and retaining it within the base of the organ. Male gametangia, called antheridia, antheridia produce many sperm cells that are released to the environment upon maturity. The sperm cells bear flagella and swim through water droplets or films to eggs. Fertilization takes place within archegonia, where the zygote will begin developing into an embryo. Other Terrestrial Adaptations Common to Many Land Plants I n order to conserve water, the epidermis of leaves and other aerial parts of most land plants is coated with a cuticle, cuticle a layer consisting of polymers called polyesters and waxes. The cuticle helps protect the plant from microbial attack and the waxy nature prevents excessive water loss. The epidermis of leaves and other photosynthetic organs have pores called stomata that allow exchange of gases between the outside air and leaf interior. Changes in the shapes of the cells bordering the stomata can close the pores to minimize water loss. Excluding bryophytes, land plants have true roots, stems, and leaves, which are defined by the presence of vascular tissue. The two types of tissue are xylem and phloem, phloem with xylem carrying water and minerals up from the roots, and phloem distributing nutrients (sugars, amino acids, and other organic products) throughout the plant. Land plants also produce secondary compounds such as alkaloids, terpenes, tannins, and phenolics. Some of these compounds have bitter tastes, strong odors, or toxic effects that help defend land plants against herbivores. Adiditonally, Flavonoids absorb harmful UV radiation while some phenolics deter attack by pathogenic microbes. 3 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights ALTERNATION OF GENERATIONS Bryophytes Gametophytes are the most conspicuous, dominant phase of life history, with sporophytes being typically smaller and present only part of the time. If bryophyte spores are dispersed to a favorable habitat, they may germinate and grow into gametophytes by mitosis. Germinating moss spores typically produce a mass of green, branched, one-cell-thick filaments known as protonema. protonema They have a large surface area in order to easily absorb water and minerals. Once sufficient resources are available, a protonema will produce buds with tissue-producing meristems. These meristems generate the mature, gamete-producing structure known as a gametophore. gametophore Remember: protonema + gametophore = gametophyte (moss) Bryophyte gametophytes are generally only one to a few cells thick in order to place all cells close to water and dissolved minerals. Most are only a few centimeters tall, growing close to the ground, anchored by rhizoids, rhizoids which are long, tubular single cells or filaments of cells. They differ from the roots of vascular plants because they are not composed of tissue, lack specialized conducting cells, and do not play a primary role in water and mineral absorption. Because the structures of bryophytes do not have lignin-coated vascular cells, they are not true stems and leaves. Bryophyte sporophytes disperse enormous amounts. These spores stay attached to the female gametophytes throughout their life, continuing to be dependent on the gametophyte for sugars, amino acids, minerals, and water. Moss sporophytes consist of a foot, foot an elongated stalk known as a seta, seta and a spore-producing organ known as the sporangium or capsule. capsule An immature capsule is covered with the calyptras, calyptras a protective cap of gametophyte tissue, lost when the capsule is ready to release spores. The upper part of the capsule, known as a peristome, peristome is often specialized for gradual spore discharge. 4 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights Pteridophytes: Seedless Vascular Plants The first instance of a sporophyte-dominant life cycle can be seen in seedless vascular plants. The sporophyte of a homosporous plant produces one type of spore. Each spore will develop into a bisexual gametophyte having both female and male sex organs, with the gametangia being called archegonia and antheridia. In contrast, sporophytes of heterosporous plant produce megaspores (female gametophytes with archegonia) and microspores (male gametophytes with antheridia). Most ferns are heterosporous. It is also important to note that the sperm cells of ferns and all other seedless vascular plants are flagellated and must swim through a film of water to reach eggs, as in bryophytes. Seed plant evolution resulted in the gymnosperms and angiosperms. The three most important reproductive adaptations are: 1. Continued reduction of the gametophyte: In bryophytes, the sporophyte was dependent on the gametophyte for nutrients. In pteridophytes (ferns), the large sporophyte and the small gametophytes were independent of each other. However, in seed plants, the gametophyte is dependent on the sporophyte and derives nutrition from the sporophyte. 2. Advent of the seed: Spores were previously used by plants to spread themselves across the Earth. A seed consists of a sporophyte embryo packaged along with a food supply in a protective coat. Seed plants have two different types of sporangia that produce different spores – megasporangia produce megaspores, which give rise to female gametophytes (egg-containing) and microsporangia produce microspores, which give rise to male gametophytes (sperm-containing). 3. Evolution of pollen: Microspores develop into pollen grains, protected by tough sporopollenincontaining coats. The use of resistant, far-traveling, airborne pollen to bring gametes together is an adaptation that contributed to the success and diversity of land plants. Gymnosperms The four phyla of gymnosperms are ginkgo, cycads, gnetophytes, and conifers. Conifers are the largest group and have cones for reproductive structures. The pine tree is a sporophyte, with its sporangia located on the cones (sporophylls). The female gametophyte generation develops from haploid spores retained within the sporangia. Small pollen cones produce microspores that develop into the male gametophytes, or pollen grains. Larger cones make megaspores that develop into female gametophytes. 5 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights Angiosperms The flower of the sporophyte produces microspores that form male gametophytes and megaspores that form female gametophytes. Immature male gametophytes are contained in pollen grains, grains which develop in the anthers of stamens. Each pollen grain has two haploid cells. Ovules, Ovules which develop in the ovary, contain the female gametophyte, also known as the embryo sac. sac After it is released from the anther, the pollen is carried to the sticky stigma at the tip of a carpel. Although some flowers self-pollinate, most have mechanisms that ensure crosscross- pollination. pollination In some cases, the stamens and carpels of a single flower may mature at different times, or the organs may be so arranged within the flower that self-pollination is unlikely. The pollen grain germinates after it adheres to the stigma of a carpel. The pollen grain, now containing a mature male gametophyte, extends a tube that grows down within the style of the carpel. After it reaches the ovary, it penetrates the micropyle (pore in the integuments of the ovule), discharging two sperm cells into the female gametophyte. Double fertilization takes place, as one sperm fuses with the egg to form a diploid zygote and the other sperm fuses with two nuclei in the large center cell. The ovule matures into a seed after double fertilization. The zygote develops into a sporophyte embryo with a rudimentary root and either one or two cotyledons. cotyledons The triploid nucleus in the center divides repeatedly, resulting in in the endosperm, endosperm a tissue rich in starch and other food reserves. 6 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights THE BIOLOGY OF ANGIOSPERMS DICOTS VERSUS MONOCOTS The two main groups of angiosperms are the monocots and an dicots. Monocots Dicots One cotyledon Two cotyledons Leaves with parallel veins Netlike veins in leaves Complex arrangment of vascular bundles Vascular bundles arranged in a ring Fibrous root system Taproot usually present Floral parts in multiples of three Floral parts in multiples of four or five THE THREE BASIC PLANT ORGANS The Root System Roots are responsible for anchoring the plant in the soil, absorbing minerals and water, and storing food. Monocots generally have fibrous root systems consisting of a mass of thin roots that tha spread out below the soil surface to extend the plant’s plant exposure to soil,, water, and minerals. Many dicots have a taproot system, stem, consisting of one large, vertical root that produces many smaller lateral roots. Taproots firmly anchor the plant in the soil and store food. The plant consumes the food reserves during flowering and fruit production. 7 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights Most absorption of water and minerals in both monocots and dicots occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root enormously. These hairs are extensions of individual epidermal cells on the root surface. Some plants have adventitious roots rising aboveground from stems or leaves. These roots can help support tall stems. The Shoot System: Stems and Leaves A stem is an alternating system of nodes (points where leaves are attached) and internodes, internodes the stem segments between nodes. In the angle (axil) formed by each leaf and the stem is an axillary bud, bud a structure with the potential to form a vegetative branch. Most axillary buds of a young shoot are dormant, with growth mainly concentrated at the terminal bud. bud The presence of the terminal bud is partly responsible for inhibiting the growth of the axillary buds, a phenomenon called apical dominance. dominance Concentrating resources on growing taller allows the plant to have increased exposure to sunlight. However, under certain conditions, the axillary buds can break dormancy and form vegetative branches. Leaves are the main photosynthetic organs of most plants, although green stems can also perform photosynthesis. They vary extensively in form, but generally consist of a flattened blade and a stalk, the petiole, petiole which joins the leaf to a node of the stem. A simple leaf has a single, undivided blade. The blade of a compound leaf is divided into several leaflets, which are themselves divided in a doubly compound leaf. Remember, there is only one axillary bud per leaf. TISSUE SYSTEMS The dermal tissue, tissue or epidermis, epidermis is generally a single layer of tightly packed cells that covers and protects all young parts of the plant. The epidermis has more specialized characteristics depending on where it located on the plant. For example, the epidermis of leaves and most stems secrete a waxy coating called a cuticle to help the plant retain water. Vascular tissue is continuous throughout the plant and involved in the transport of materials between roots and shoots. Xylem moves water and dissolved minerals from the roots to the shoots, while phloem moves sugar from leaves and tubers to “sugar sinks” such as roots and fruits. The waterconducting elements of xylem, the tracheids and vessel elements are elongated cells that are dead at functional maturity. When the living interior of a tracheid or vessel element disintegrates, the cell’s thickened walls remain behind, forming a nonliving conduit for water flow. Tracheids and vessel elements form in parts of the plant that are no longer elongating. Their secondary walls are interrupted by pits, pits thinner regions where only primary walls are present. Tracheids are long, thin cells with tapered ends that function in support as well as water transport. Vessel elements are generally wider, shorter, thinner walled, and less tapered than tracheids. Vessel elements are aligned end to end, 8 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights forming xylem vessels – the end walls are perforated, allowing for the free movement of water through them. In phloem, sucrose and other compounds are transported through tubes formed by chains of sievesievetube members, members cells lacking organelles such as the nucleus, ribosomes, and a distinct vacuole. In angiosperms, the end walls between sieve-tube members, called sieve plates, plates have pores that presumably facilitate the flow of fluid from cell to cell along the sieve tube. A companion cell is located next to each sieve-tube member and the two are connected by the plasmodesmata. The nucleus and ribosomes of the companion cell serve the adjacent sieve-tube member as well as the companion cell itself. In some plants, the companion cells in leaves help move sugar produced in the leaf to the sieve-tube members. Ground tissue is tissue that is neither dermal nor vascular and makes up most of the plant. In dicot stems, ground tissue is divided into pith, pith internal to the vascular tissue, and cortex, cortex external to the vascular tissue. THREE BASIC CELL TYPES Parenchyma Cells Mature parenchyma cells have primary walls that are relatively thin and flexible, and most lack secondary walls. The protoplast generally has a large central vacuole. They are generally the least specialized cell, but there are exceptions, such as the sieve-tube members. Parenchyma cells perform most metabolic functions in a plant, such as photosynthesis. Developing plant cells of all types are parenchyma cells before specializing further in structure and function. Cells that continue to stay less specialized and become mature parenchyma cells generally do not undergo cell division. Most, however, retain the ability to divide and differentiate into other types of plant cells under special conditions. Collenchyma Cells Collenchyma cells have unevenly thickened primary walls and are used to help support the young parts of the plant shoot. Young stems and petioles often have a cylinder of collenchymas just below their surface. They lack secondary walls and do not have lignin, thus allowing for support with unrestrained growth. Functioning collenchymas cells are living and flexible, elongating with the stems and leaves they support. Sclerenchyma Cells Sclerenchyma cells are much more rigid than collenchyma cells, cannot elongate, and are found in regions of the plant that have stopped growing. There are two types of sclerenchyma cells called fibers and sclereids that specialize entirely in support. Fibers usually occur in groups, while sclereids 9 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights are shorter than fibers and irregular in shape. Sclereids are responsible for the hardness in nutshells and seed coats. PRIMARY GROWTH Primary Growth of Roots The root tip is covered by the root cap, cap which physically protects the delicate meristems as the root pushes through the soil in primary growth. From the root tip upward, the successive stages of primary growth are the zone of cell division, the zone of elongation, and the zone of maturation. The zone of cell division includes the apical meristems and primary meristems. Near the center of the apical meristems is the quiescent center, center a population of cells that divide much more slowly than the other meristematic cells and are relatively resistant to damage from radiation and toxic chemicals. Above the apical meristem are three concentric cylinders of cells that continue to divide for some time. The protoderm, protoderm procambium procambium, ambium and ground meristem will produce the dermal, vascular, and ground tissue of the root. The zone of cell division blends into the zone of elongation, elongation where cells elongate sometimes to more than ten times their original length. This zone is responsible for pushing the root tip ahead. Cells of the root begin specializing in structure and function in the zone of maturation. maturation Primary Growth of Shoots The apical meristem of a shoot is a dome-shaped mass of dividing cells at the tip of the terminal bud that will alter result in the primary meristems. Leaves arise as leaf primordial while axillary buds develop from islands of meristematic cells. Axillary buds can later form branches of the shoot system at a later time. In stems, vascular tissue runs the length of the stem in strands called vascular bundles. bundles This arrangement contrasts with that of the root, where the vascular tissue forms a vascular cylinder in the center of the root. Each vascular bundle is surrounded by ground tissue. Leaf epidermis consists of cells tightly locked together like pieces of a puzzle. The epidermal barrier is interrupted only by the stomata, stomata tiny pores flanked by specialized epidermal cells called guard cells. cells The stomata allow gas exchange between the surrounding air and the photosynthetic cells inside the leaf. Ground tissue of a leaf is sandwiched between the upper and lower epidermis in the region called mesophyll, mesophyll which consists of parenchyma cells specialized for photosynthesis. SECONDARY GROWTH The secondary plant body consists of the tissues produced during this secondary growth in diameter. Two lateral meristems function in secondary growth: the vascular cambium, cambium which produces 10 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights secondary xylem (wood) and secondary phloem, and the cork cambium, cambium which produces a tough, thick covering for stems and roots that replaces the epidermis. Secondary Growth of Stems The vascular cambium is a cylinder of meristematic cells that forms secondary vascular tissue. It produces secondary xylem to its interior and secondary phloem to its exterior. There are alternating regions of cambium cells called ray initials and fusiform initials. The ray initials are cambium cells that produce radial files of parenchyma cells known as xylem rays and phloem rays. The cambium cells within the vascular bundles are the fusiform initials initial s, which produce new vascular tissue. Secondary xylem accumulates to produce the tissue commonly referred to as wood, which consists mostly of tracheids, vessel elements, and fibers. These cells, dead at functional maturity, have thick, lignified walls that give wood its hardness and strength. During the early stages of secondary growth, epidermis falls off the stem and is replaced with cork cambium, a cylinder of meristematic cells that first form in the outer cortex and then later in the secondary phloem. Cork cambium produces cork cells, which accumulate to create cork tissue. This tissue results in a barrier that helps protect the stem from physical damage and pathogens. Together, the layers of cork and the cork cambium make up the periderm. periderm Lenticels Lenticel s are regions where the periderm splits open, allowing gas exchange for cellular respiration. Bark refers to all tissues external to the vascular cambium - secondary phloem, cork cambium, and cork. Secondary Growth of Roots The two lateral meristems also develop and produce secondary growth in roots. The vascular cambium forms within the stele and produces secondary xylem to its inside and secondary phloem to its outside. Periderm is impermeable to water – thus, the older parts of the roots function mainly to anchor the plant and to transport water and solutes, with only the younger roots absorbing water and minerals from the soil. AN OVERVIEW OF TRANSPORT MECHANISMS IN PLANTS The most important active transporter in the plasma membrane of plant cells is the proton pump, pump which hydrolyzes ATP and uses the released energy to pump hydrogen ions out of the cell. This results in a protein gradient with a higher proton concentration outside the cell than inside. Since the proton pump moves positive charge, in the form of protons, out of the cell, the pump also generations a membrane potential, making the inside of the plant cell negative in charge relative to the outside. This stored energy can be used to transport many different solutes. The membrane potential can contribute to the uptake of potassium ions by root cells (K+). In cotransport, cotransport a transport protein couples the downhill passage of one solute to the uphill passage of another, which works for both anions and neutral solutes. 11 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights Differences in water potential also drive water transport in plant cells. Water potential is affected by solute concentration and pressure. Water will move across a membrane from the solution with the higher water potential to the solution with the lower water potential. Addition of solutes lowers the water potential because water molecules that form shells around a solute have less freedom to move than they do in pure water. Increasing pressure raises water potential. Physical pressure will cause water to escape via any available exit. It is also possible to create a negative pressure, or tension, tension on water or solutions. Aquaporins are specific channels for passive traffic of water in the form of transport proteins. They affect the rate at which water diffuses down its water potential gradient. Aquaporins may form gated channels that open and close in response to variables such as turgor pressure of the cell. Plasmodesmata connect the cytosolic compartments of neighboring cells, forming a continuous pathway for transport of certain molecules between cells. This cytoplasmic continuum is called the symplast. symplast The walls of adjacent plant cells are also in contact, forming a continuum of cell walls called the apoplast apo plast. plast The transmembrane route moves substances out of one cell, across the cell wall, and into the neighboring cell and requires repeated crossings of plasma membranes. The symplastic route requires only one crossing of a plasma membrane via plasmodesmata. The apoplastic route uses the extracellular pathway of cell walls and extracellular spaces. Water and solutes move through xylem vessels and sieve tubes by bulk flow, flow the movement of a fluid driven by pressure. In phloem, for example, hydrostatic pressure is generated at one end of a sieve tube, forcing sap to the opposite end of the tube. ABSORPTION OF WATER AND MINERALS BY ROOTS Most absorption of water and minerals occurs near root tips, where the root hairs are located. Soil particles adhere tightly to the hairs and the soil solution passes along the apoplast into the root cortex. As the soil solution moves into the roots, cells of the epidermis and cortex take up water and certain solutes into the symplast. Symbiotic fungi infect roots, forming mycorrhizae, mycorrhizae which are symbiotic structures consisting of the plant’s root sand the hyphae of the fungi. The hyphae absorb water and selected minerals, transferring much of these resources to the host plant. The endodermis surrounds the stele and functions as a last checkpoint for the selective passage of minerals from the cortex into the vascular tissue. Minerals already in the symplast when they reach the endodermis continue through the plasmodesmata of the endodermal cells and pass into the stele, having already been screened by the selective membranes. In the wall of each endodermal cell is the Casparian strip, strip a belt made of suberin. Water and minerals cannot cross the endodermis and enter vascular tissue via the apoplast – they must cross the plasma membrane of an endodermal cell and 12 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights enter the stele via the symplast. Thus, no minerals can reach the vascular tissue without passing through a selectively permeable plasma membrane. Water and minerals can now enter the tracheids and vessel elements of xylem to be transported. TRANSPORT OF XYLEM SAP Root Pressure: Pushing Xylem Sap At night, when transpiration is very low or zero, the root cells are still expending energy to pump mineral ions into the xylem. Accumulation of minerals in the stele lowers water potential there, causing water to flow in from the root cortex and generate a positive pressure. This upward push of xylem sap is called root pressure, pressure which causes guttation, guttation the exudation of water droplets on leaf margins. In most plants, root pressure is not the major mechanism driving the ascent of xylem sap. The Transpiration-Cohesion-Tension Mechanism: Pulling Xylem Sap On most days, the air outside the leaf is drier than the air inside the leaf that is saturated with water vapor. Thus, gaseous water, diffusing down its concentration gradient, exits the leaf via the stomata. As the water evaporates, the remaining film of liquid water retreats into the pores of the cell walls, attracted by adhesion to the hydrophilic walls. At the same time, cohesive forces in the water resist an increase in the surface area of the film. This results in the water forming a meniscus. Since the water film at the surface of the leaf cells has a negative pressure, it draws water out of the leaf xylem. Thus, mesophyll cells will lose water to the surface film lining air spaces, which in turn loses water by transpiration. Transpirational pull-cohesion tension theory states that for each molecule of water that evaporates from a leaf by transpiration, another molecule of water is drawn in at the root to replace it. Cohesion and adhesion of water due to hydrogen bonding allow water molecules to be pulled against the downward force of gravity. The small diameter of tracheids and vessel elements allow a majority of the water to be exposed to the hydrophilic walls, thus enhancing adhesion. TRANSLOCATION OF PHLOEM SAP The transport of food in the plant is called translocation. translocation Phloem sap is an aqueous solution that differs markedly in composition from xylem sap. Phloem sap is mostly sugar and can also contain minerals, amino acids, and hormones. A sugar source is a plant organ in which sugar is being produced by either photosynthesis or the breakdown of starch, with mature leaves being the major sugar sources. A sugar sink is an organ that is a net consumer or storer of sugar, such as growing roots, shoot tips, stems, and fruit. A storage organ can be either a source or sink, depending on whether the organism is currently using the supply or building it up. A sugar sink usually receives its sugar from the sources nearest to it. 13 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights In some species, sucrose travels from mesophyll cells to sieve-tube members via the symplast, while in other species, it can reach sieve-tube members in a combination of symplastic and apoplastic pathways. Companion cells pass the sugar they receive to the sieve-tube members through plasmodesmata. In some plants, the companion cells have numerous ingrowths in their walls that increases the cells’ surface area, enhancing the transfer of solutes between apoplast and symplast – they are called transfer cells. cells Once the sugar reaches a sugar sink, the sugar molecules will diffuse from the phloem into the sink tissues and water will follow by osmosis. Phloem sap moves by bulk flow, which is driven by pressure. Phloem loading results in a high solute concentration a the source end of a sieve tube, which lowers the water potential and causes water to flow into the tube. Hydrostatic pressure will develop within the sieve tube, with the greatest pressure at the source end. At the sink end, the pressure is relieved by the loss of water. Thus, water will flow from source to sink and carry the sugar along with it. Water is recycled by xylem vessels. MACRONUTRIENTS AND MICRONUTRIENTS Mineral nutrients are essential chemical elements absorbed from the soil in the form of inorganic ions. Water is also considered a nutrient because it supplies most of the hydrogen atoms and some of the oxygen atoms incorporated into organic compounds during photosynthesis. A particular chemical element is considered an essential nutrient if it is needed for a plant to grow from a seed and complete the life cycle, producing another generation of seeds. Elements required by plants in relatively large amounts are called macronutrients - carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, and magnesium. Elements needed in small amounts are called micronutrients – iron, chlorine, copper, manganese, zinc, molybdenum, boron, and nickel. They generally function as cofactors of enzymatic reactions. SEXUAL REPRODUCTION The life cycles of angiosperms and other plants are characterized by an alternation of generations, generations in which haploid and diploid generations take turns producing each other. The diploid plant, called the sporophyte, sporophyte produces haploid spores by meiosis. These spores divide by mitosis, giving rise to multicellular male and female haploid plants – the gametophytes. gametophytes These gametophytes develop and produce gametes – sperm and eggs. Flowers are the reproductive shoots of the angiosperm sporophyte. The four kinds of floral organs are the sepals, sepals, petals, stamens, stamens and carpels. carpels Stamens and carpels are the male and female reproductive organs, respectively, while sepals and petals are nonreproductive organs. Sepals enclose and protect the floral bud before it opens, while the petals are brightly colored to advertise the flower to pollinators. A stamen consists of a stalk called the filament and a terminal structure called the anther; anther within which the pollen is produced (in pollen sacs). A carpel has an ovary at its base and a slender neck 14 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights called the style, which is topped by a sticky structure called the stigma. Inside the ovary are one or more ovules. ovules The stamen and carpels of flowers contain the sporangia, the structures where first the spores and then the gametophytes develop. The male gametophytes are sperm-producing structures called pollen grains, grains which form within the pollen sacs of anthers. The female gametophytes are egg-producing structures called embryo sacs, sacs which form within the ovules in ovaries. Pollination results when a pollen grain lands on a stigma. Each pollen grain produces a structure called a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac, resulting in fertilization of the egg. The zygote gives rise to an embryo, and the ovule containing it will develop into a seed. The entire ovary will then develop into a fruit containing one or more seeds. A pollen grain consists of a generative cell (sperm) and a tube cell (pollen tube) – this is an immature male gametophyte. When the generative cell divides by mitosis to form two sperm cells, it becomes a mature male gametophyte. Ovules, each containing a single sporangium, form within the chambers of the ovary. One cell in the sporangium of each ovule, the megasporocyte, will grow and go through meiosis to produce four haploid megaspores. megaspores Generally, only one survives. This megaspore continues to grow and its nucleus divides by mitosis three times, resulting in one large cell with eight haploid nuclei. Membranes partition the mass into a multicellular female gametophyte – the embryo sac, which has the egg cell, two synergids, three antipodal cells, and two polar nuclei. Self-fertilization is prevented in order to create genetic variety. In some plants, the stamens and carpels mature at different times or are structurally arranged in such a way that it is unlikely an animal pollinator could transfer pollen from the anthers to the stigma of the same flower. The most common way of preventing “selfing” is selfself -incompatibility, incompatibility which is the ability of a plant to reject its own pollen and the pollen of closely related individuals. Double fertilization gives rise to the zygote and endosperm; this is a distinctive feature of the angiosperm life cycle. One sperm fertilizes the egg to form a diploid zygote. The other sperm combines with two polar nuclei to form a triploid nucleus that will give rise to the endosperm, endosperm a food-storing tissue of the seed. OVARY TO FRUIT As the seeds develop from ovules, the ovary of the flower develops into a f ruit, ruit which protects the enclosed seeds and aids in their dispersal by wind or animals. Pollination triggers hormonal changes that cause the ovary to begin its transformation into a fruit. The wall of the ovary becomes the pericarp, pericarp the thickened wall of the fruit. Other floral parts can contribute to a fruit depending on the specific plant. 15 http://guidesbyjulie.blogspot.com/ AP Biology Plant Highlights FROM SEED TO SEEDLING Germination of seeds depends on imbitition, the uptake of water due to the low water potential of the dry seed. Seeds, in order to germinate, need water, oxygen, and the correct temperature. Imbibing water causes the seed to expand, rupturing its coat. Enzymes will be triggered that will break down starch into sugar, allowing growth of organs such as the radicle, which will become the embryonic root. In monocots, the cotyledons protect the foliage leaves. In dicots, the cotyledons stay below ground. PLANT RESPONSES TO HORMONES Hor Ho rmones are chemical signals that coordinate all parts of the organism. Any growth response that results in curvatures of whole plant organs toward or away from stimuli is called a tropism tropism . The growth of a shoot toward light is called positive phototropism. phototropism It is important to remember that plant hormones work cooperatively in affecting the development of a plant – response to a hormone is not as dependent on absolute amount of that hormone as on its relative concentration compared to other hormones. Auxin: Auxin any chemical substance that promotes the elongation and growth. It is found in the embryos of seeds, meristems of apical buds, and young leaves. Auxin functions in fruit development, cell differentiation, apical dominance, and functions in phototropism and gravitropism. Cytokinins: Cytokinins stimulate cell division and differentiation. They are produced in actively growing tissues and work with auxin to stimulate cell division and influence the pathway of differentiation. Both cytokinins and auxin are factors in the control of apical dominance. They have anti-aging effects. Gibberellins: Gibberellins promote seed and bud germination, stem elongation, and leaf growth, stimulate flowering and development of fruit; affect root growth and differentiation. They are found in meristems of apical buds and roots, young leaves, and embryos. Abscisic acid: acid generally slows down growth, keeps seeds dormant, closes stomata during stress conditions E t hylene: hylene promotes fruit ripening, results in aging REMEMBER TO STUDY PLANT RESPONSES TO LIGHT AND TROPISMS! Additionally, review the differences between C3, C4, and CAM plants.