Introduction to the Plant Kingdom PDF

Introduction to the Plant Kingdom copyright cmassengale 1 Early Ancestors Aquatic to Terrestrial Life copyright cmassengale 2  Aquatic Ancestor ◼ Closest living species to a possible land plant ancestor ◼ Group of green algae ◼ Called Charyophyceans...

Introduction to the Plant Kingdom copyright cmassengale 1 Early Ancestors Aquatic to Terrestrial Life copyright cmassengale 2  Aquatic Ancestor ◼ Closest living species to a possible land plant ancestor ◼ Group of green algae ◼ Called Charyophyceans Chara copyright cmassengale 3  Algae & Land Plant Similarities ◼ Both contain chlorophylls a and b ◼ Have chloroplasts with stacks of thylakoids ◼ Store starch in plastids ◼ Cellulose in cell walls ◼ Go through Alternation of Generations life Cycle copyright cmassengale 4  Aquatic Habitat Terrestrial Habitat copyright cmassengale 5 Living in Aquatic Environments ◼ Plants surrounded by water so don’t dry out ◼ Sperm swims to egg ◼ Water supports plant ◼ Plants stay in upper surface near light ◼ Absorb nutrients from the H2O copyright cmassengale 6  Plant Adaptations to Land Problems: Solutions: ◼ Need minerals ◼ Roots absorb H2O & minerals ◼ Gravity ◼ Lignin & cellulose in cell ◼ Increase in walls Height for Light ◼ Vascular Transport ◼ Adaptations for System Drier ◼ Waxy cuticle & environment stomata with guard ◼ Reproduction cells ◼ Pollen containing sperm copyright cmassengale 7 How Are Plants All Alike? copyright cmassengale 8  Plant Characteristics ◼ Multicellular ◼ Autotrophic (photosynthesis) ◼ Chlorophylls a and b in thylakoid membranes ◼ Surrounded by cell walls containing cellulose (polysaccharide) ◼ Store reserve food as amylose (starch) copyright cmassengale 9  Plant Reproduction ◼ Alternation of generations life cycle ◼ Diploid (2n) sporophyte stage ◼ Haploid (1n) gametophyte stage ◼ Produce multicellular embryo protected inside multicellular haploid (gametophyte egg sac) tissue copyright cmassengale 10  Plant Reproduction ◼ Diploid (2n) sporophyte stage produces haploid spores by meiosis ◼ Haploid spores undergo mitosis to produce gametophyte stage ◼ Gametophyte makes gametes (eggs and sperm) by meiosis ◼ Zygote (2n) produces the new sporophyte copyright cmassengale 11 Alternation of Generations Gametophyte 2n Sporophyte 2n gametophyte 1n pollen 2n seed with plant embryo Ovary with 1n ovules (eggs) Sporophyte copyright cmassengale 12  Plant Divisions copyright cmassengale 13  Taxonomy ◼ Plants are divided into two groups ◼ Based on the presence or Vascular absence of an Bundles internal transport system for water and dissolved materials ◼ Called Vascular System copyright cmassengale 14  Vascular System ◼ Xylem tissue carries water and minerals upward from the roots ◼ Phloem tissue carries sugars made by photosynthesis from the leaves to where they will be stored or used ◼ Sap is the fluid carried inside the xylem or phloem copyright cmassengale 15  Nonvascular Plants ◼ Do not have vascular tissue Sporophyte stage for support or conduction of materials ◼ Called Bryophytes ◼ Require a Gametophyte Stage constantly moist environment Moss Gametophytes & copyright cmassengale Sporophytes 16  Nonvascular Plants ◼ Plants can’t grow as tall ◼ Cells must be in direct contact with moisture ◼ Materials move by diffusion cell-to-cell ◼ Sperm must swim to egg through water droplets copyright cmassengale 17  Nonvascular Plants ◼ Includes mosses (Bryophyta), liverworts (Hepatophyta), and hornworts (Antherophyta) Liverworts copyright cmassengale Hornworts 18  Main Parts of Vascular Plants ◼ Shoots -Found above ground -Have leaves attached - Photosynthetic part of plant ◼ Roots -Found below ground -Absorb water & minerals -Anchor the plant copyright cmassengale 19  Vascular Plants ◼ Also called Tracheophytes ◼ Subdivided into two groups -- Seedless vascular plants and Seed- bearing vascular plants copyright cmassengale Club Moss 20  Seedless Vascular Plants ◼ Includes club moss (Lycophyta), horsetails (Sphenophyta), whisk ferns (Psilophyta), and ferns (Pterophyta) Whisk ferns copyright cmassengale Horsetails 21  Seed-Producing Vascular Plants ◼ Includes two groups – Gymnosperms and Angiosperms ◼ Gymnosperms have naked seeds in cones ◼ Angiosperms have flowers that produce seeds to attract pollinators and produce seeds copyright cmassengale 22  Gymnosperms ◼ Coniferophyta are known as conifers ◼ Includes pine, cedar, spruce, and fir ◼ Cycadophyta – cycads Cycad ◼ Ginkgophyta - ginkgo Ginkgo copyright cmassengale 23  Gymnosperms ◼ Contains the oldest living plant – Bristle cone pine ◼ Contains the tallest living plant – Sequoia or redwood copyright cmassengale 24  Angiosperms ◼ Flowering plants ◼ Seeds are formed when an egg or ovule is fertilized by pollen in the ovary ◼ Ovary is within a flower ◼ Flower contains the male (stamen) and/or female (ovaries) parts of the plant ◼ Fruits are frequently produced from these ripened ovaries (help disperse seeds) copyright cmassengale 25  Angiosperms ◼ Division Anthophyta ◼ Subdivided into two groups – Monocots and Dicots ◼ Monocots have a single seed cotyledon ◼ Dicots have two seed cotyledons copyright cmassengale 26  Monocots ◼ Parallel venation in leaves ◼ Flower parts in multiples of 3 ◼ Vascular tissue scattered in cross section of stem copyright cmassengale 27  Dicots ◼ Net venation in leaves ◼ Flower parts in multiples of 4 or 5 ◼ Vascular tissue in rings in cross section of stem copyright cmassengale 28 Plant Uses copyright cmassengale 29  Why We Can’t do Without Plants! ◼ Produce oxygen for the atmosphere ◼ Produce lumber for building ◼ Provide homes and food for many organisms ◼ Prevent erosion ◼ Used for food copyright cmassengale 30  More Reasons We Can’t do Without Plants! ◼ Produce wood pulp for paper products ◼ Source of many medicines ◼ Ornamental and shade for yards ◼ Fibers such as cotton for fabric ◼ Dyes copyright cmassengale 31 Tissues and Primary Growth of Stems  The Plant Body: Stems FUNCTION OF STEMS Stems support leaves and branches. Stems transport water and solutes between roots and leaves. Stems in some plants are photosynthetic. Stems may store materials necessary for life (e.g., water, starch, sugar). In some plants, stems have become adapted for specialized functions. Used with permission from http://education-portal.com – May be vegetative (leaf bearing) or reproductive (flower bearing). – Node- area of stem where leaf is born – Internodes- stem area between nodes – Buds: Stem elongation. Embryonic tissue of leaves and stem (not flower bud) – Terminal bud- Located at tip of stems or branches. – Axillary bud- Gives rise to branches – Apical Dominance: Prevention of branch formation by terminal bud The stem  Parts of the Stem – Xylem Water and minerals travel up to other plant parts – Phloem Manufactured food travels down to other plant parts – Cambium – Separates xylem and phloem  Plant Tissues 1) Dermal Tissue System Outer covering Protection 2) Vascular Tissue System “Vessels” throughout plant Transport materials 3) Ground Tissue System “Body” of plant Photosynthesis; storage; support Used with permission from http://education-portal.com  Stems – Structure and Development Stems have all three types of plant tissue Grow by division at meristems – Develop into leaves, other shoots, and even flowers Leaves may be arranged in one of three ways Rhizomes Stems – Many Plants Have Modified Stems Bulbs Storage leaves Stem Stolons Stolon Tubers  Modified shoots with diverse functions have evolved in many plants. – These shoots, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots. – Stolons, such as the “runners” of strawberry plants, grow on the surface and enable a plant to colonize large areas asexually when a parent plant fragments into many smaller offspring. Fig. 35.4a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  – Rhizomes, like those of ginger, are horizontal stems that grow underground. – Tubers, including potatoes, are the swollen ends of rhizomes specialized for food storage. – Bulbs, such as onions, are vertical, underground shoots consisting mostly of the swollen bases of leaves that store food. Fig. 35.5b-d Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Plant Tissues – Ground Tissue Some major types of plant cells: – Parenchyma – Collenchyma – Sclerenchyma Tissues that are neither dermal nor vascular are ground tissue Ground tissue internal to the vascular tissue is pith; ground tissue external to the vascular tissue is cortex Ground tissue includes cells specialized for storage, photosynthesis, and support  Parenchyma Characteristics – least specialized cell type – only thin primary cell wall is present – possess large central vacuole – generally alive at functional maturity Functions – make up most of the ground tissues of the plant – storage – photosynthesis – can help repair and replace damaged organs by proliferation and specialization into other cells  Collenchyma Characteristics – possess thicker primary cell walls the that of parenchyma – no secondary cell wall present – generally alive at functional maturity Functions – provide support without restraining growth  Sclerenchyma Characteristics – have secondary cell walls strengthened by lignin – often are dead at functional maturity – two forms: fibers and sclereids Functions – rigid cells providing support and strength to tissues  Sclerenchyma – Fibers are long, slender and tapered, and usually occur in groups. Those from hemp fibers are used for making rope and those from flax for weaving into linen. – Sclereids, shorter than fibers and irregular in shape impart the hardness to nutshells and seed coats and the gritty texture to pear fruits.  Vascular Tissue Vascular tissue: Runs continuous throughout the plant transports materials between roots and shoots. – Xylem transports water and dissolved minerals upward from roots into the shoots. (water the xylem) – Phloem transports food from the leaves to the roots and to non-photosynthetic parts of the shoot system. (feed the phloem)  Overview of Plant Structure Xylem: – Main water-conducting tissue of vascular plants. – arise from individual cylindrical cells oriented end to end. – At maturity the end walls of these cells dissolve away and the cytoplasmic contents die. – The result is the xylem vessel, a continuous nonliving duct. – carry water and some dissolved solutes, such as inorganic ions, up the plant  Overview of Plant Structure Phloem: – The main components of phloem are sieve elements companion cells. – Sieve elements have no nucleus and only a sparse collection of other organelles. Companion cell provides energy – so-named because end walls are perforated - allows cytoplasmic connections between vertically-stacked cells. – conducts sugars and amino acids - from the leaves, to the rest of the plant  Phloem transport requires specialized, living cells Sieve tubes elements join to form continuous tube Pores in sieve plate between sieve tube elements are open channels for transport Each sieve tube element is associated with one or more companion cells. – Many plasmodesmata penetrate walls between sieve tube elements and companion cells – Close relationship, have a ready exchange of solutes between the two cells  Phloem transport requires specialized, living cells Companion cells: – Role in transport of photosynthesis products from producing cells in mature leaves to sieve plates of the small vein of the leaf – Synthesis of the various proteins used in the phloem – Contain many, many mitochondria for cellular respiration to provide the cellular energy required for active transport – There ate three types Ordinary companion cells Transfer cells Intermediary cells  Plant Classification – Monocots vs. Dicots Basic categories of plants based on structure and function Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Plant Classification – Monocots vs. Dicots 1 cotyledon 2 cotyledons 4 or 5 floral 3 floral parts parts Parallel veins Netlike veins 1 pore in pollen 3 pores in pollen Stem vascular Stem vascular bundles bundles in ring dispersed Remember Plant Tissues? 1) Dermal Tissue System Outer covering Protection 2) Vascular Tissue System “Vessels” throughout plant Transport materials 3) Ground Tissue System “Body” of plant Photosynthesis; storage; support Used with permission from http://education-portal.com  Vasculature - Comparisons In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring as with Dicots Phloem Xylem Sclerenchyma Ground Ground tissue (fiber cells) tissue connecting pith to cortex Pith Epidermis Key to labels Epidermis Cortex Vascular Dermal bundles Vascular bundle Ground 1 mm Vascular 1 mm (a) Cross section of stem with vascular bundles forming (b) Cross section of stem with scattered vascular bundles a ring (typical of dicots) (typical of monocots) Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  epidermis Dicot Stem Anatomy phloem cortex vascular bundle pith vascular cambium xylem Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Monocot Stem phloem Anatomy epidermis vascular bundles ground tissue xylem Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Plant stem growth ▪ Vegetative development is based on meristems, in which cell division occurs throughout life, producing cells that go on to differentiate. ▪ When a meristem is converted from vegetative to reproductive development, regulatory transcription factors are activated that control the identity and position of floral organs.  Plant Growth 1) Primary Growth: Apical Meristems: Mitotic cells at “tips” of roots / stems length 1) Increased length 2) Specialized structures (e.g. fruits) 2) Secondary Growth: girth Lateral Meristems: Mitotic cells “hips” of plant Responsible for increases in stem/root diameter  Plant Growth 1) Indeterminate: Grow throughout life 2) Growth at “tips” (length) and at “hips” (girth) Growth patterns in plant: 1) Meristem Cells: Dividing Cells 2) Differentiated Cells: Cells specialized in structure & role Form stable, permanent part of plant  Plant Growth Shoot apical meristem Leaf primordia Young leaf Developing vascular strand Axillary bud meristems Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Meristems The tissue in most plants consisting of undifferentiated cells (meristematic cells), found in zones of the plant where growth can take place. Meristematic cells are analogous in function to stem cells in animals, are incompletely or not differentiated, and are capable of continued cellular division. Furthermore, the cells are small and protoplasm fills the cell completely. Tunica-Corpus model of the apical meristem (growing tip). The epidermal (L1) and subepidermal (L2) layers The vacuoles are extremely small. form The cytoplasm does not contain the outer layers called the tunica. chloroplasts although they are present The inner L3 layer is called the in rudimentary form (proplastids). corpus. Cells in the L1 and L2 layers divide in Meristematic cells are packed closely a sideways fashion which keeps these together without intercellular cavities. layers distinct, while the L3 layer divides in a more random fashion.  Plant Growth Two lateral meristems: vascular cambium and cork cambium Primary growth in stems Epidermis Cortex Shoot tip (shoot Primary phloem apical meristem and young leaves) Primary xylem Pith Lateral meristems: Vascular cambium Secondary growth in stems Cork cambium Axillary bud Periderm meristem Cork cambium Cortex Pith Primary phloem Primary Root apical xylem Secondary meristems Secondary phloem xylem Vascular cambium Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Plant Growth Stem – Secondary Growth: primary phloem thicker, stronger stems vascular cambium Vascular Cambium: between primary xylem primary xylem and phloem epidermis Produces inside stem: pith A) Secondary xylem cortex - moves H2O, inward B) Secondary phloem primary xylem - moves sugars, outward dividing vascular cambium primary phloem Vascular cambium Is a lateral meristem in the vascular tissue of plants. It is a cylinder of unspecialized meristematic cells that divide to give rise to cells that further divide, differentiate and specialize to form the secondary vascular tissues. The vascular cambium is the source of both the secondary xylem (inwards, towards the pith) And the secondary phloem – (outwards), And is located between these tissues in the stem and root. Vascular cambium Made from, procambium that remains undifferentiated between the primary xylem and primary phloem. Upon maturity, this region is known as the fascicular cambium, and the area of cells between the vascular bundles (fascicles) called pith rays becomes what is called the interfascicular cambium. The fascicular and inter-fascicular cambiums, therefore, represent a continuous ring which bisects the primary xylem and primary phloem. The vascular cambium then produces secondary xylem on the inside of the ring, and secondary phloem on the outside, pushing the primary xylem and phloem apart. Vascular cambium The vascular cambium usually consists of two types of cells: – Fusiform initials (tall cells, axially orientated. – Ray initials (almost isodiametric cells - smaller and round to angular in shape). Remember: The vascular cambium is a type of meristem - tissue consisting of embryonic (incompletely differentiated) cells from which other (more differentiated) plant tissues originate. Primary meristems are the apical meristems on root tips and shoot tips. Vascular Cambium: Plant Growth primary xylem Secondary growth new secondary secondary phloem xylem dividing primary phloem vascular cambium primary xylem secondary xylem new secondary primary vascular cambium phloem phloem pith cortex Vascular cambium Growth Vascular X X C P P cambium Secondary Secondary X X C P phloem xylem X C P C X C C After one year After two years C of growth of growth Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Production of Secondary Xylem and Phloem – The accumulation of this tissue over the years accounts for most of the increase in diameter of a woody plant. – Secondary xylem forms to the interior and secondary phloem to the exterior of the vascular cambium. C=cambium cell X=2o xylem P=2o phloem D=derivative  Cork cambium Another lateral meristem is the cork cambium, which produces cork, part of the bark. Growth Together, the secondary vascular ring tissues (produced by the vascular cambium) and periderm (formed by Vascular the cork cambium) makes up the secondary plant body. ray Heartwood Vascular cambia are found in dicots and gymnosperms but Secondary not monocots, which usually lack secondary growth. xylem Sapwood In wood, the vascular cambium is the obvious line separating the bark and wood. Vascular cambium Secondary phloem Capon, Brian (2005). Botany for Gardeners (2nd ed.). Portland, OR: Bark Timber Publishing Layers of periderm  Cork Cambium The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems. It is found in woody and many herbaceous dicots, gymnosperms an d some monocots, which usually lack secondary growth. Growth and development of cork cambium is very variable between different species, and is also highly dependent on age, growth conditions, etc. as can be observed from the different surfaces of bark: – smooth, fissured, tesselated, scaly, flaking off, etc.  Plant Growth Stem – Secondary Growth: heartwood (xylem) sapwood (xylem) vascular cambium phloem annual ring Sapwood = Young xylem, water late Heartwood = Old xylem, support xylem Seasonal Growth = annual rings early Secondary phloem = grows outward xylem older phloem crushed Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Secondary Growth of a Stem Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  ANY QUESTIONS? Leaves  Leaf morphology Arrangement (look for bud) Alternate 1 leaf for each node Betula spp., Cercis spp., Quercus spp., Gleditsia spp. Ulmus spp. Leaf morphology (arrangement)  Leaf morphology Arrangement (look for bud) Opposite 2 leaves for each node Acer spp., Cornus spp., Fraxinus spp., Viburnum spp. Leaf morphology (arrangement)  Leaf morphology Arrangement (look for bud) Subopposite 2 leaves for each node Lagerstroemia indica Leaf morphology (arrangement)  Leaf morphology Arrangement (look for bud) Whorled 3 or more leaves for each node Catalpa spp., Hydrangea spp., Nerium oleander Leaf morphology (arrangement)  Leaf morphology Leaf types Angiosperm Simple Bud in leaf axil of single leaf Acer spp., Betula spp., Cornus spp., Quercus spp., Ulmus spp.  Leaf morphology (angiosperm leaf types)  Leaf morphology Leaf types Angiosperm Compound Bud in leaf axil of more than one leaflet Aesculus spp., Fraxinus spp., Gleditsia spp., Pistacia spp.  Leaf morphology (Angiosperm leaf types)  Leaf morphology (Angiosperm leaf types)  Leaf morphology (Angiosperm leaf types)  Leaf morphology (Angiosperm leaf types)  Leaf morphology Leaf types Gymnosperm Needle-like Prickly to touch Juniperus spp.(adult foliage)  Leaf morphology (Gymnosperm leaf types)  Leaf morphology Leaf types Gymnosperm Scale-like Overlaps like shingles or fish scales Soft to touch Cupressus spp., Thuja spp., Juniperus spp. (juvenile foliage)  Leaf morphology (Gymnosperm leaf types)  Leaf morphology Leaf types Gymnosperm Needle-like Pinus spp. Combined in fascicles of 1, 2, 3, 4, or 5 needles  Leaf morphology (Gymnosperm leaf types)  Leaf morphology Leaf types Gymnosperm Abies spp., Cedrus spp., Picea spp., Taxus spp. Singly or in clusters on stem  Leaf morphology (Gymnosperm leaf types) Spruce Foliage  Leaf morphology Leaf shapes Angiosperm Ovate  Leaf morphology Leaf shapes Angiosperm Obovate  Leaf morphology Leaf shapes Angiosperm Cordate  Leaf morphology Leaf shapes Angiosperm Lanceolate  Leaf morphology Leaf shapes Angiosperm Linear  Leaf morphology Leaf shapes Angiosperm Deltoid  Leaf morphology Leaf margins Angiosperm Entire  Leaf morphology Leaf margins Angiosperm Serrate  Leaf morphology Leaf margins Angiosperm Sinuate  Leaf morphology Leaf margins Angiosperm Lobed  Leaf morphology Leaf margins Angiosperm Crenate The End Roots and Structure  Roots!!!!!!! The main driving forces for water flow from the soil through the plant to the atmosphere include: Differences in: [H2O vapor] Hydrostatic pressure Water potential All of these act to allow the movement of water into the plant. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Roots: Function Roots anchor the plant in the substratum or soil. Roots absorb water and dissolved nutrients or solutes (nitrogen, phosphorous, magnesium, boron, etc.) needed for normal growth, development, photosynthesis, and reproduction. In some plants, roots have become adapted for specialized functions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Plant Root Zones Meristematic zone – Cells divide both in direction of root base to form cells that will become the functional root and in the direction of the root apex to form the root cap Elongation zone – Cells elongate rapidly, undergo final round of divisions to form the endodermis. Some cells thicken to form casparian strip Maturation zone – Fully formed root with xylem and phloem – root hairs first appear here  Primary Growth in Roots Root Cap: covers root tip & protects the meristem as the root pushes through the abrasive soil during primary growth. – The cap also secretes a lubricating slime. Growth in length is concentrated near the root’s tip, where three zones of cells at successive stages of primary growth are located. – zone of cell division – zone of elongation – zone of maturation Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  thimble-shaped mass of parenchyma cells at the tip Root Cap of each root protects the root from mechanical injury Golgi bodies release a mucilaginous lubricant (mucigel) cells lasts less than a week, then these die possibly important in perception of gravity (i.e., geotropism or gravitropism) amyloplasts (also called statoliths) appear to accumulate at the bottom of cells  Primary Growth in Roots The zone of cell division includes the apical meristem and its derivatives, primary meristems. Near the middle is the quiescent center (or radicle) – This forms the primary root of a young plant. cells that divide more slowly than other meristematic cells. – These cells are relatively resistant to damage from radiation and toxic chemicals. – They may act as a reserve that can restore the meristem if it becomes damaged. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Primary Growth in Roots The zone of cell division blends into the zone of elongation where cells elongate, sometimes to more than ten times their original length. – It is this elongation of cells that is mainly responsible for pushing the root tip, including the meristem, ahead. – The meristem sustains growth by continuously adding cells to the youngest end of the zone of elongation. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  Primary Growth in Roots In the zone of maturation, cells begin to specialize in structure and function. – In this root region, the three tissue systems produced by primary growth complete their differentiation, their cells becoming functionally mature Three primary meristems give rise to the three primary tissues of roots. – The epidermis develops from the dermal tissues. – The ground tissue produces the endodermis and cortex. – The vascular tissue produces the stele, the pericycle, pith, xylem, and phloem. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Primary Growth in Roots The protoderm, the outermost primary meristem, produces the single cell layer of the epidermis Water and minerals absorbed by the plant must enter through the epidermis. Root hairs enhance absorption by greatly increasing the surface area. The procambium gives rise to the stele, which in roots is a central cylinder of vascular tissue where both xylem and phloem develop. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  The procambium gives rise to the stele, which in roots is a central cylinder of vascular tissue where both xylem and phloem develop. Stele Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Primary Growth in Roots The ground tissue between the protoderm and procambium gives rise to the ground tissue system. These are mostly parenchyma cells between the stele and epidermis. They store food and are active in the uptake of minerals that enter the root with the soil solution. The innermost layer of the cortex, the endodermis, is a cylinder one cell thick that forms a boundary between the cortex and stele. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  cortex Dicot Root endodermis pericycle Anatomy epidermis cortex stele xylem phloem  Monocot Root epidermis Anatomy cortex endodermis cortex pericycle stele pith phloem xylem pith Dicot Monocot  Mycorrhizal associations Not unusual – 83% of dicots, 79% of monocots and all gymnosperms Ectotrophic Mycorrhizal fungi – Form a thick sheath around root. Some mycelium penetrates the cortex cells of the root – Root cortex cells are not penetrated, surrounded by a zone of hyphae called Hartig net – The capacity of the root system to absorb nutrients improved by this association – the fungal hyphae are finer than root hairs and can reach beyond nutrient-depleted zones in the soil near the root  Water transport processes Moves from soil, through plant, and to atmosphere by a variety of mediums – Cell wall – Cytoplasm – Plasma membranes – Air spaces How water moves depends on what it is passing through  Water absorption from soil Water clings to the surface of soil particles. As soil dries out, water moves first from the center of the largest spaces between particles. Water then moves to smaller spaces between soil particles. Root hairs make intimate contact with soil particles – amplify the surface area for water absorption by the plant.  Water across plant membranes There is some diffusion of water directly across the bi- lipid membrane. Auqaporins: Integral membrane proteins that form water selective channels – allows water to diffuse faster – Facilitates water movement in plants Alters the rate of water flow across the plant cell membrane – NOT direction  Osmosis and Tonicity Osmosis is the Tonicity is the diffusion of water osmolarity of a solution- across a plasma -the amount of solute in membrane. a solution. Osmosis occurs when Solute--dissolved there is an unequal substances like sugars concentration of water and salts. on either side of the Tonicity is always in selectively permeable comparison to a cell. plasma membrane. The cell has a specific Remember, H2O amount of sugar and CAN cross the plasma salt. membrane.  Tonic Solutions A Hypertonic solution has more solute than the cell. A cell placed in this solution will give up water (osmosis) and shrink. A Hypotonic solution has less solute than the cell. A cell placed in this solution will take up water (osmosis) and become turgid. An Isotonic solution has just the right amount of solute for the cell. A cell placed in this solution will stay the same.  Plant cell in hypotonic solution Flaccid cell in 0.1M sucrose solution. Water moves from sucrose solution to cell – swells up –becomes turgid This is a Hypotonic solution - has less solute than the cell. So higher water conc. Pressure increases on the cell wall as cell expands to equilibrium  Plant cell in hypertonic solution Turgid cell in 0.3M sucrose solution Water movers from cell to sucrose solution A Hypertonic solution has more solute than the cell. So lower water conc Turgor pressure reduced and protoplast pulls away from the cell wall  Plant cell in Isotonic solution Water is the same inside the cell and outside An Isotonic solution has the same solute than the cell. So no osmotic flow Turgor pressure and osmotic pressure are the same This is of NO real benefit to a plant cell and it enters a state known as Flacid. Chemical reactions will still occur, BUT the plant cell can’t help support the structure of the entire plant.  Water uptake in the roots Root hairs increase surface area of root to maximize water absorption. From the epidermis to the endodermis there are three pathways in which water can flow: 1: Apoplast pathway: Water moves exclusively through cell walls without crossing any membranes – The apoplast is a continuous system of cell walls and intercellular air spaces in plant tissue  Water uptake in the roots 2: Transmembrane pathway: Water sequentially enters a cell on one side, exits the cell on the other side, enters the next cell, and so on. 3: Symplast pathway: Water travels from one cell to the next via plasmodesmata. – The symplast consist of the entire network of cell cytoplasm interconnected by plasmodesmata  Water uptake in the roots At the endodermis: Water movement through the apoplast pathway is stopped by the Casparian Strip – Band of radial cell walls containing suberin , a wax- like water-resistant material The casparian strip breaks continuity of the apoplast and forces water and solutes to cross the endodermis through the plasma membrane – So all water movement across the endodermis occurs through the symplast  Water transport to the xylem After water moves through the Casparian Strip via the symplast pathway – Water traveling from one cell to the next via plasmodesmata It enters the xylem system Compared with water movement across root tissue the xylem is a simple pathway of low resistance Water movement through xylem needs less pressure than movement through living cells. Cohesion-tension theory:  Water transport to the xylem Cohesion-tension theory: Relies on the fact that water is a polar molecule Water is constantly lost by transpiration in the leaf. When one water molecule is lost another is pulled along. Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants.  Comparison of Root Systems Types of roots: – Taproot - A thick primary root that grows long and is found mainly in dicots – Adventitious (Fibrous) roots – branch extensively and are found mainly in monocots From the wikimedia free licensed media file repository  The Tap root A taproot is a very large, somewhat straight to tapering plant root that grows downward. It forms a center from which other roots sprout laterally Plants with taproots are difficult to transplant. The presence of a taproot is why weeds are hard to uproot—the top is pulled, but the long taproot stays in the ground, and resprouts Dicots, one of the two divisions of angiosperms, start with a taproot, which is one main root forming from the enlarging radicle of the seed. The tap root can be persistent throughout the life of the plant but is most often replaced later in the plant's development by a fibrous root system From the wikimedia free licensed media file repository  Adventitious (fibrous) Roots: Roots that arise from anything other than the radicle A fibrous root system is universal in monocotyledonous plants and ferns Adventitious rooting may be a stress- avoidance acclimation for some species, driven by such inputs as: hypoxia/anoxia or nutrient deficiency. Arise out-of-sequence from the more usual root formation of branches of a primary root, and instead originate from the stem, branches, leaves, or old woody roots. Adventitious roots dry out quicker, thus cannot tolerate drought Better able to hold within soil, giving plant better stability From the wikimedia free licensed media file repository  Modified Roots Food storage – swollen with nutrients and water to prepare for unfavorable conditions. – Some are swollen main roots - carrots. – Others are swollen branched roots or advertitious – sweet potato Photosynthetic roots Parasitic roots – Some plants live on other plants and get food materials from their hosts. Parasitic roots are used to absorb food materials form their hosts. From the wikimedia free licensed media file repository  Modified Roots Buttress roots – large roots on all sides of a tall or shallowly rooted tree. – Typically they are found in rain forests where soils are poor so roots don't go deep. – They prevent the tree from falling over and help gather more nutrients. – They are there to anchor the tree and soak minerals and nutrients from the ground, a function that would prove difficult if the tree was unsoundly rooted. – Examples: silk cotton From the wikimedia free licensed media file repository  Modified Roots Aerial roots – roots above the ground. – They are almost always adventitious. – They can absorb water from the air. – They are also used to hold on to their support. – They are found in diverse plant species which include: – the orchids – tropical coastal swamp trees such as mangroves. From the wikimedia free licensed media file repository  Modified Roots Respiratory roots – Some plants in swampy areas have branch roots that grow upwards, through the mud and into the air. – The exposed parts of the roots are spongy and they take in air for respiration - red mangrove Clasping roots – grow from the nodes of the soft stem to cling on to other plants. Examples: pepper, ivy From the wikimedia free licensed media file repository  Symbiotic Roots Legumes – Legumes seedlings germinate without any association to rhizobia – Under nitrogen limiting conditions, the plant and the bacteria seek each other out by an elaborate exchange of signals – The first stage of the association is the migration of the bacteria through the soil towards the host plant From the wikimedia free licensed media file repository  Symbiotic Roots Nodule formation results a finely tuned interaction between the bacteria and the host plant – Involves the recognition of specific signals between the symbiotic bacteria and the host plant The bacteria forms NH3 which can be used directly by the plant The plant gives the bacteria organic nutrients.  Nodule formation During root nodule formation, two process occur simultaneously Infection and Nodule Organogenesis – (A) Rhizobia attach to the root hairs and release nod factors that produce a pronounced curling of the root hair cell – (B) Rhizobia get caught and curl, degrade the root hair cell wall allowing the bacterial cells direct access to the outer surface of the plant plasma membrane  Nodule formation (C) Then the infection thread forms – Formed from Golgi depositing material at the tip at the site of infection. Local degradation of root hair cell wall also occurs (D) Infection thread reaches the end of the cell, and thread plasma membrane fuses with plasma membrane of root hair cell – Then bacterial cells are released into the fused plasma membranes  Nodule formation (E) Rhizobia are released into the apoplast and enter the middle lamella, – This leads to the formation of a new infection thread, which forms an open channel with the first (F) Infection thread expands and branches until it reaches target cells – Vesicles composed of plant membrane enclose bacterial cells and they are released into the cytoplasm  Movement of pathogens from cell to cell Fungi, Bacteria, and Viruses all move through the plant in the same when following a successful penetration. Movement proteins (MP) are proteins dedicated to enlarging the pore size of plasmodesmata and actively transporting the pathogen into the adjacent cell. Thereby allowing local and systemic spread of pathogen in plants.  Movement of pathogens from cell So, from the entry point (1) to cell the pathogen moves from cell to cell via the plasmodesmata (2). As a pathogen travels it also reproduces. Some of the pathogen can exit the infected plant by stomata and infect nearby plants (3). If the pathogen gets to the bundle sheath it can rapidly be transported through the plant by the xylem and phloem (4) The End. Any Questions?  LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Meiosis and Sexual Life Cycles Lectures by Erin Barley Kathleen Fitzpatrick Inheritance of Genes  Genes are the units of heredity, and are made up of segments of DNA  Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs)  Each gene has a specific location called a locus on a certain chromosome  Most DNA is packaged into chromosomes © 2011 Pearson Education, Inc. Comparison of Asexual and Sexual Reproduction  In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes  A clone is a group of genetically identical individuals from the same parent  In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents © 2011 Pearson Education, Inc. Figure 13.2 0.5 mm Parent Bud (a) Hydra (b) Redwoods Figure 13.2a 0.5 mm Parent Bud Figure 13.2b (b) Redwoods Sets of Chromosomes in Human Cells  Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes  A karyotype is an ordered display of the pairs of chromosomes from a cell  The two chromosomes in each pair are called homologous chromosomes, or homologs  Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters © 2011 Pearson Education, Inc. Figure 13.3 APPLICATION TECHNIQUE Pair of homologous 5 m duplicated chromosomes Centromere Sister chromatids Metaphase chromosome Figure 13.3a Figure 13.3b Pair of homologous 5 m duplicated chromosomes Centromere Sister chromatids Metaphase chromosome Figure 13.3c 5 m   The sex chromosomes, which determine the sex of the individual, are called X and Y  Human females have a homologous pair of X chromosomes (XX)  Human males have one X and one Y chromosome  The remaining 22 pairs of chromosomes are called autosomes © 2011 Pearson Education, Inc.   Each pair of homologous chromosomes includes one chromosome from each parent  The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father  A diploid cell (2n) has two sets of chromosomes  For humans, the diploid number is 46 (2n = 46) © 2011 Pearson Education, Inc.   In a cell in which DNA synthesis has occurred, each chromosome is replicated  Each replicated chromosome consists of two identical sister chromatids © 2011 Pearson Education, Inc. Figure 13.4 Key Maternal set of 2n = 6 chromosomes (n = 3) Paternal set of chromosomes (n = 3) Sister chromatids of one duplicated chromosome Centromere Two nonsister Pair of homologous chromatids in chromosomes a homologous pair (one from each set)   A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)  For humans, the haploid number is 23 (n = 23)  Each set of 23 consists of 22 autosomes and a single sex chromosome  In an unfertilized egg (ovum), the sex chromosome is X  In a sperm cell, the sex chromosome may be either X or Y © 2011 Pearson Education, Inc. The Variety of Sexual Life Cycles  The alternation of meiosis and fertilization is common to all organisms that reproduce sexually  The three main types of sexual life cycles differ in the timing of meiosis and fertilization © 2011 Pearson Education, Inc.   Gametes are the only haploid cells in animals  They are produces by meiosis and undergo no further cell division before fertilization  Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism © 2011 Pearson Education, Inc. Figure 13.6 Key Haploid (n) Haploid unicellular or Haploid multi- Diploid (2n) cellular organism multicellular organism (gametophyte) n Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n MEIOSIS FERTILIZATION Spores n n Gametes n Gametes MEIOSIS FERTILIZATION Zygote 2n MEIOSIS FERTILIZATION 2n 2n Diploid 2n Zygote Diploid multicellular 2n multicellular Mitosis Mitosis organism Zygote organism (sporophyte) (a) Animals (b) Plants and some algae (c) Most fungi and some protists   Plants and some algae exhibit an alternation of generations  This life cycle includes both a diploid and haploid multicellular stage  The diploid organism, called the sporophyte, makes haploid spores by meiosis © 2011 Pearson Education, Inc.   Each spore grows by mitosis into a haploid organism called a gametophyte  A gametophyte makes haploid gametes by mitosis  Fertilization of gametes results in a diploid sporophyte © 2011 Pearson Education, Inc. Figure 13.6b Key Haploid (n) Diploid (2n) Haploid multi- cellular organism (gametophyte) Mitosis n Mitosis n n n n Spores Gametes MEIOSIS FERTILIZATION 2n Diploid 2n Zygote multicellular organism Mitosis (sporophyte) (b) Plants and some algae   In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage  The zygote produces haploid cells by meiosis  Each haploid cell grows by mitosis into a haploid multicellular organism  The haploid adult produces gametes by mitosis © 2011 Pearson Education, Inc. Figure 13.6c Key Haploid (n) Diploid (2n) Haploid unicellular or multicellular organism Mitosis n Mitosis n n n Gametes n MEIOSIS FERTILIZATION 2n Zygote (c) Most fungi and some protists   Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis  However, only diploid cells can undergo meiosis  In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring © 2011 Pearson Education, Inc. Meiosis reduces the number of chromosome sets from diploid to haploid  Like mitosis, meiosis is preceded by the replication of chromosomes  Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II  The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis  Each daughter cell has only half as many chromosomes as the parent cell © 2011 Pearson Education, Inc. The Stages of Meiosis  After chromosomes duplicate, two divisions follow  Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes  Meiosis II (equational division) sister chromatids separate  The result is four haploid daughter cells with unreplicated chromosomes © 2011 Pearson Education, Inc. Figure 13.7-1 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair Chromosomes of homologous duplicate chromosomes Sister Diploid cell with chromatids duplicated chromosomes Figure 13.7-2 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair Chromosomes of homologous duplicate chromosomes Sister Diploid cell with chromatids duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Figure 13.7-3 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair Chromosomes of homologous duplicate chromosomes Sister Diploid cell with chromatids duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes   Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids  The sister chromatids are genetically identical and joined at the centromere  The single centrosome replicates, forming two centrosomes © 2011 Pearson Education, Inc.   Division in meiosis I occurs in four phases – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis © 2011 Pearson Education, Inc. Figure 13.8 MEIOSIS I: Separates homologous chromosomes MEIOSIS I: Separates sister chromatids Telophase I and Telophase II and Prophase I Metaphase I Ana phase I Prophase II Metaphase II Ana phase II Cytokines is Cytokines is Centrosome Sister chromatids (with centriole pair) remain attached Sister Chiasmata Centromere chr om atids (with kinetochor e) Spindle Metaphase plate During another r ound of cell division, the sister chromatids finally separate; Cleavage four haploid daughter cells r esult, containing unduplicated chromosomes. furrow Homologous Sister chromatids Haploid daughter Homologous Fragm ents chr om osomes separ ate cells forming chr om osomes of nuclear separ ate envelope Micr otubule Each pair of homologous Two haploid cells attached to chr om osomes separ ates. form; each chromosome kinetochore still consists of two Chromosomes line up sister chromatids. Duplicated homologous chr om osomes (red and blue) by homologous pairs. pair and exchange segments; 2n = 6 in this example. Figure 13.8a Metaphase I Anaphase I Telophase I and Prophase I Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Sister Chiasmata Centromere chromatids (with kinetochore) Spindle Metaphase plate Cleavage furrow Homologous Homologous Fragments chromosomes chromosomes of nuclear separate envelope Microtubule Each pair of homologous Two haploid attached to chromosomes separates. cells form; each kinetochore chromosome Duplicated homologous Chromosomes line up still consists chromosomes (red and blue) by homologous pairs. of two sister pair and exchange segments; chromatids. 2n = 6 in this example.  Prophase I  Prophase I typically occupies more than 90% of the time required for meiosis  Chromosomes begin to condense  In synapsis, homologous chromosomes loosely pair up, aligned gene by gene © 2011 Pearson Education, Inc.   In crossing over, nonsister chromatids exchange DNA segments  Each pair of chromosomes forms a tetrad, a group of four chromatids  Each tetrad usually has one or more chiasmata, X- shaped regions where crossing over occurred © 2011 Pearson Education, Inc.  Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome © 2011 Pearson Education, Inc.  Anaphase I  In anaphase I, pairs of homologous chromosomes separate  One chromosome moves toward each pole, guided by the spindle apparatus  Sister chromatids remain attached at the centromere and move as one unit toward the pole © 2011 Pearson Education, Inc.  Telophase I and Cytokinesis  In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids  Cytokinesis usually occurs simultaneously, forming two haploid daughter cells © 2011 Pearson Education, Inc.   In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms  No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated © 2011 Pearson Education, Inc.   Division in meiosis II also occurs in four phases  Prophase II  Metaphase II  Anaphase II  Telophase II and cytokinesis  Meiosis II is very similar to mitosis © 2011 Pearson Education, Inc. Figure 13.8b Telophase II and Prophase II Metaphase II Anaphase II Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids Haploid daughter separate cells forming  Prophase II  In prophase II, a spindle apparatus forms  In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate © 2011 Pearson Education, Inc.  Metaphase II  In metaphase II, the sister chromatids are arranged at the metaphase plate  Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical  The kinetochores of sister chromatids attach to microtubules extending from opposite poles © 2011 Pearson Education, Inc.  Anaphase II  In anaphase II, the sister chromatids separate  The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles © 2011 Pearson Education, Inc.  Telophase II and Cytokinesis  In telophase II, the chromosomes arrive at opposite poles  Nuclei form, and the chromosomes begin decondensing © 2011 Pearson Education, Inc.   Cytokinesis separates the cytoplasm  At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes  Each daughter cell is genetically distinct from the others and from the parent cell © 2011 Pearson Education, Inc. A Comparison of Mitosis and Meiosis  Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell  Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell © 2011 Pearson Education, Inc. Figure 13.9 MITOSIS MEIOSIS Parent cell MEIOSIS I Chiasma Prophase Prophase I Chromosome Chromosome Duplicated Homologous duplication duplication chromosome 2n = 6 chromosome pair Metaphase Metaphase I Anaphase Anaphase I Telophase Telophase I Haploid n=3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells n n n n of mitosis Daughter cells of meiosis II SUMMARY Property Mitosis Meiosis DNA Occurs during interphase before Occurs during interphase before meiosis I begins replication mitosis begins Number of One, including prophase, metaphase, Two, each including prophase, metaphase, anaphase, divisions anaphase, and telophase and telophase Synapsis of Does not occur Occurs during prophase I along with crossing over homologous between nonsister chromatids; resulting chiasmata chromosomes hold pairs together due to sister chromatid cohesion Number of Two, each diploid (2n) and genetically Four, each haploid (n), containing half as many daughter cells identical to the parent cell chromosomes as the parent cell; genetically different and genetic from the parent cell and from each other composition Role in the Enables multicellular adult to arise from Produces gametes; reduces number of chromosomes animal body zygote; produces cells for growth, repair, by half and introduces genetic variability among the and, in some species, asexual reproduction gametes Figure 13.9a MITOSIS MEIOSIS Parent cell MEIOSIS I Chiasma Prophase Prophase I Chromosome Chromosome Duplicated duplication Homologous duplication chromosome 2n = 6 chromosome pair Metaphase Metaphase I Anaphase Anaphase I Telophase Daughter Telophase I cells of Haploid meiosis I n=3 2n 2n MEIOSIS II Daughter cells n n n n of mitosis Daughter cells of meiosis II Figure 13.9b SUMMARY Property Mitosis Meiosis DNA Occurs during interphase before Occurs during interphase before meiosis I begins replication mitosis begins Number of One, including prophase, metaphase, Two, each including prophase, metaphase, anaphase, divisions anaphase, and telophase and telophase Synapsis of Does not occur Occurs during prophase I along with crossing over homologous between nonsister chromatids; resulting chiasmata chromosomes hold pairs together due to sister chromatid cohesion Number of Two, each diploid (2n) and genetically Four, each haploid (n), containing half as many daughter cells identical to the parent cell chromosomes as the parent cell; genetically different and genetic from the parent cell and from each other composition Role in the Enables multicellular adult to arise from Produces gametes; reduces number of chromosomes animal body zygote; produces cells for growth, repair, by half and introduces genetic variability among the and, in some species, asexual reproduction gametes   Three events are unique to meiosis, and all three occur in meiosis l  Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information  At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes  At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate © 2011 Pearson Education, Inc.   Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I  Protein complexes called cohesins are responsible for this cohesion  In mitosis, cohesins are cleaved at the end of metaphase  In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids) © 2011 Pearson Education, Inc.  Genetic variation produced in sexual life cycles contributes to evolution  Mutations (changes in an organism’s DNA) are the original source of genetic diversity  Mutations create different versions of genes called alleles  Reshuffling of alleles during sexual reproduction produces genetic variation © 2011 Pearson Education, Inc.  Origins of Genetic Variation Among Offspring  The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation  Three mechanisms contribute to genetic variation  Independent assortment of chromosomes  Crossing over  Random fertilization © 2011 Pearson Education, Inc.  Independent Assortment of Chromosomes  Homologous pairs of chromosomes orient randomly at metaphase I of meiosis  In independent assortment, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs © 2011 Pearson Education, Inc.   The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number  For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes © 2011 Pearson Education, Inc. Figure 13.10-1 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure 13.10-2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.10-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Crossing Over  Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent  Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene © 2011 Pearson Education, Inc.   In crossing over, homologous portions of two nonsister chromatids trade places  Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome © 2011 Pearson Education, Inc. Figure 13.11-1 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs Figure 13.11-2 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.11-3 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11-4 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Figure 13.11-5 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes Figure 13.11a Chiasma Centromere TEM Random Fertilization  Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)  The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations © 2011 Pearson Education, Inc.   Crossing over adds even more variation  Each zygote has a unique genetic identity © 2011 Pearson Education, Inc. The Evolutionary Significance of Genetic Variation Within Populations  Natural selection results in the accumulation of genetic variations favored by the environment  Sexual reproduction contributes to the genetic variation in a population, which originates from mutations © 2011 Pearson Education, Inc. Figure 13.12 200 m Figure 13.UN01 Prophase I: Each homologous pair undergoes synapsis and crossing over between nonsister chromatids with the subsequent appearance of chiasmata. Metaphase I: Chromosomes line up as homologous pairs on the metaphase plate. Anaphase I: Homologs separate from each other; sister chromatids remain joined at the centromere. Figure 13.UN02 F H Figure 13.UN03 Figure 13.UN04 Cone and Flower Structure Conifer life cycle: This image shows the life cycle of a conifer. Pollen from male cones blows up into upper branches, where it fertilizes female cones. Examples are shown for female and male cones. Conifers are monoecious plants that produce both male and female cones, each making the necessary gametes used for fertilization. Pine trees are conifers (cone bearing) and carry both male and female sporophylls on the same mature sporophyte. Male and female gametophytes: These series of micrographs shows male and female gymnosperm gametophytes. (a) This male cone, shown in cross section, has approximately 20 microsporophylls, each of which produces hundreds of male gametophytes (pollen grains). (b) Pollen grains are visible in this single microsporophyll. (c) This micrograph shows an individual pollen grain. (d) This cross section of a female cone shows portions of about 15 megasporophylls. (e) The ovule can be seen in this single megasporophyll. (f) Within this single ovule are the megaspore mother cell (MMC), micropyle, and a pollen grain. Structures of the flower: The four main parts of the flower are the calyx, corolla, androecium, and gynoecium. The androecium is the sum of all the male reproductive organs, and the gynoecium is the sum of the female reproductive organs. Staminate and carpellate flowers: The corn plant has both staminate (male) and carpellate (female) flowers. Staminate flowers, which are clustered in the tassel at the tip of the stem, produce pollen grains. Carpellate flower are clustered in the immature ears. Each strand of silk is a stigma. The corn kernels are seeds that develop on the ear after fertilization. Also shown is the lower stem and root. Testa – an outer seed coat that protects the embryonic plant Micropyle – a small pore in the outer covering of the seed, that allows for the passage of water Cotyledon – contains the food stores for the seed and forms the embryonic leaves Plumule – the embryonic shoot (also called the epicotyl) Radicle – the embryonic root During the beginning stage of germination, the seeds take up water rapidly and this results in swelling and softening of the seed coat at an optimum temperature. This stage is referred to as Imbibition. It starts the growth process by activation of enzymes. The seed activates its internal physiology and starts to respire and produce proteins and metabolizes the stored food. This is a lag phase of seed germination. By rupturing of the seed coat, radicle emerges to form a primary root. The seed starts absorbing underground water. After the emerging of the radicle and the plumule, shoot starts growing upwards. In the final stage of seed germination, the cell of the seeds become metabolically active, elongates and divides to give rise to the seedling.