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Download Topic 9 Plant Biology
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Topic # 9: Plant Biology I. Transport in the Xylem of plants A. Transpiration 1. Transpiration is the inevitable consequence of gas exchange in the leaf 2. Plant leaves are the primary organ of photosynthesis a. CO2 is vital to this process b. O2 is produced as well 3. Exchange of CO2 and O2 must occur in order to sustain photosynthesis 4. Absorption of CO2 is essential for photosynthesis and the waxy cuticle of the top of the leaf has low permeability to it 5. Pores in the epidermis are needed - stomata I. Transport in the Xylem of plants 6. If stomata are open to absorb CO2, then water will be lost from the leaf to the atmosphere 7. This is a problem 8. Transpiration: Loss of water vapor from the leaves of a plant 9. Guard cells minimize water loss 10. Open and close based on the needs of the plant I. Transport in the Xylem of plants B. Xylem structure helps withstand low pressure 1. The cohesive property of water and the structure of the xylem vessels allow transport under tension 2. Xylem vessels are long, continuous tubes with thickened cell walls 3. Lignin is within the cell walls to help with this thickening 3. Helps to strengthen the walls so that they will not collapse under low pressure 4. Xylem vessels are formed from files of cells – arranged end to end 5. When mature, the xylem cells are nonliving I. Transport in the Xylem of plants a. Water must move as a passive process b. Water molecules are polar and the partial negative oxygen atom in one molecule attracts the hydrogen atom in another = cohesion c. Water is also attracted to hydrophilic parts of the cell walls of the xylem = adhesion d. Water moves up the plant as a continuous stream as the result of the interaction of cohesive and adhesive forces C. Tension in leaf cell walls maintains the transpiration stream 1. The adhesive property of water and evaporation generate tension forces in leaf cell walls I. Transport in the Xylem of plants 2. When water evaporates from the surface of the wall in a leaf, adhesion causes water to be drawn through the cell wall from the nearest available supply to replace the water lost by evaporation 3. Even if the pressure in the xylem is low, the force of adhesion between water and the cell walls is enough to suck water out of the xylem 4. The low pressure generates a pulling force that is transmitted through the water in the xylem vessels a. Down through the stems b. All the way to the roots I. Transport in the Xylem of plants 5. Transpiration pull a. Strong enough to move water against the force of gravity b. Trees are kind of a big deal…they defy gravity 6. It’s a passive process a. The energy needed comes from the thermal energy that causes the original evaporation b. The pulling of the water in the xylem depends on the cohesion between water molecules 7. cavitation – low pressure in the xylem tubes causing the liquid column to break a. Usually doesn’t happen with water I. Transport in the Xylem of plants b. Even though water is a liquid, it can transmit pulling forces the same way a solid length of rope does D. Active Transport of mineral ions in the roots 1. Active uptake of mineral ions in the roots causes absorption of water by osmosis 2. Water is absorbed into root cells by osmosis 3. Solute concentration in root cells is greater than that in water in the soil 4. The solutes in question are mineral ions 5. Mineral ion concentrations inside the root cells can be 100X higher than those in the soil sounds like a little active transport to me I. Transport in the Xylem of plants 6. There are separate pumps for each type of ion needed by the plant 7. Active transport for mineral ions can only happen if the ions come in contact with the appropriate membrane protein 8. Mineral ions can also move in through diffusion and with mass flow when water carrying the ions drains through the soil 9. Plant-Fungi relationship a. Some ions move too slowly b. Ions are bound to the surface of soil particles c. Certain plants have a relationship with a fungus I. Transport in the Xylem of plants d. The fungus grows on the surface of the roots e. Thread-like hyphae grow out into the soil and absorb mineral ions from the surface of soil particles f. The ions are supplied to the roots g. Found in many trees, members of the heather family and orchids Heather Orchid I. Transport in the Xylem of plants E. Replacing losses from transpiration 1. Plants transport water from roots to leaves to replace losses from transpiration a. Water leaving through the stomata is replaced by water from the xylem b. Water in the xylem climbs the stem through the pull of transpiration i. Adhesion ii. Cohesion c. Water moves from the soil into the roots by osmosis due to active transport of minerals into the roots d. Once water is in the root, it travels through the xylem through cell walls (apoplast pathway) and through the cytoplasm (symplast pathway) II. Transport in the phloem of plants A. Translocation occurs from source to sink 1. Plants transport organic compounds from sources to sinks 2. Phloem tissue is found throughout plants a. Stems b. Roots c. Leaves 3. Phloem is composed of sieve tubes 4. Sieve tubes are composed of columns of specialized cells called sieve tube cells 5. Sieve tube cells are separated by sieve plates II. Transport in the phloem of plants 6. Translocation – transport of organic compounds throughout the plant 7. Links parts of the plant that need sugars and amino acids to parts that have a surplus 8. Sometimes sinks turn into sources and vice versa a. Phloem can transport compounds in either direction b. Depends on fluid flow because of pressure gradients c. Energy is needed to generate the pressures so it is an active process Sources Sinks Photosynthetic Tissues Roots that are growing or absorbing mineral ions using energy from cell respiration Mature green leaves Parts of the plant that are growing or developing food stores: Green Stems Developing fruits Storage tissues in germinating seeds Developing seeds Tap roots or tubers at the start of the growth season Growing leaves Developing tap roots or tubers II. Transport in the phloem of plants B. Phloem Loading 1. Active transport is used to load organic compounds into phloem sieve tubes at the source 2. Sucrose is the most prevalent solute in phloem sap 3. Sucrose is a disaccharide that can flow through the phloem without being metabolized in cell respiration…like if it was glucose 4. Phloem loading a. Apoplast route: sucrose transport proteins actively transports sugar in from mesophyll cells cell walls of companion cells sieve cells II. Transport in the phloem of plants b. Concentration gradient of sucrose is established by active transport c. Uses ATP as an energy source to push H+ out of the companion cells from surrounding tissues d. The build up of H+ flows down a concentration gradient through a co-transport protein e. The energy released is used to carry sucrose into the companion cell-sieve tube complex b. Symplast route: sucrose travels between cells through connections called plasmodesmata (singular plasmodesma) a. Once the sucrose enters the companion cell II. Transport in the phloem of plants it is converted to an oligosaccharide b. Maintains the sucrose concentration gradient C. Pressure and water potential differences play a role in translocation 1. Incompressibility of water allows transport by hydrostatic pressure gradients 2. The build up of sucrose and other carbohydrates draws water into the companion cells through osmosis 3. The rigid cell wall combined with the incompressibility of water result in a build-up of pressure II. Transport in the phloem of plants 4. Water will flow from this area of high pressure to an area of low pressure 5. At the sink end, sucrose is withdrawn from the phloem and either utilized as an energy source or converted to starch 6. In either case, the loss of solute causes a reduction in osmotic pressure a. The water that carried the solute to the sink is then drawn back into the transpiration stream in the xylem Stem in cross-section Leaf in Cross Section Root in Cross Section III. Growth in plants A. Growth in plants 1. Undifferentiated cells in the meristems of plants allow indeterminate growth 2. Plants have indeterminate growth a. The cells will continue to divide so long as conditions are right b. Many plant cells have the capacity to generate whole plants (cuttings) c. Cells are totipotent sets plants apart from animals 3. Meristem tissue a. Composed of undifferentiated cells that are undergoing active cell division Shoot apical meristem Developing flower bud on shoot apical meristem III. Growth in plants b. Apical meristems: a type of primary meristem found at the tips of stems and roots i. The root apical meristem is responsible for the growth of the root ii. The shoot apical meristem is at the tip of the stem c. Lateral meristems: developed by many dicotyledonous plants B. Role of mitosis in stem extension and leaf development 1. Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves III. Growth in plants 2. Meristem cells are small and go through the cell cycle repeatedly to produce more cells 3. Root apical meristem is responsible for the growth of the root. period. Like, that’s it…roots beget roots 4. Shoot apical meristem is more complex a. It sends off the cells needed for growth of the stem b. Also produces groups of cells that grow and develop into leaves and flowers c. With each cell division, one cell remains in the meristem while the other increases in size and differentiates as it is pushed away from the meristem region III. Growth in plants 5. Each apical meristem can give rise to additional meristems a. Protoderm gives rise to epidermis b. Procambium gives rise to vascular tissue c. Ground meristem gives rise to pith 6. Chemical influences also play a large role in determining which type of specialized tissue arises from unspecialized plant cells 7. Young leaves are produced at the sides of the shoot apical meristem – they appear as small bumps known as leaf primordia let’s look at my plant and find some! III. Growth in plants C. Plant hormones affect shoot growth 1. Plant hormones control growth in the shoot apex 2. A hormone is a chemical message that is produced and released in one part of an organism to have an effect in another part of an organism 3. Auxins a. Initiating growth of roots b. Influencing the development of fruits c. Regulating leaf development d. IAA indole-3-acetic acid is the most abundant type of auxin and controls growth of the shoot apex III. Growth in plants i. IAA promotes the elongation of cells in stems ii. IAA is synthesized in the apical meristem and is transported down the stem to stimulated growth iii. At high concentrations, IAA can inhibit growth 4. Axillary buds shoots that form at the junction or node of the stem and at the base of a leaf 5. As the shoot meristem grows and forms leaves, regions of meristem are left behind at the node III. Growth in plants 6. Growth at these regions are inhibited by auxin termed apical dominance 7. The further distant a node is from the shoot apical meristem… a. The lower the concentration of auxin b. The less likely that growth in the axillary bud will be inhibited by by auxin 8. Cytokinins – hormones produced in the root a. Promote axillary bud growth b. The ratio of cytokinins and auxins determine whether the axillary bud will develop 9. Gibberellins – another hormone group that contribute to stem elongation III. Growth in plants D. Plant tropisms 1. Plants respond to the environments by tropisms 2. Phototropism a. Growth toward light b. Auxin accumulates near the shady side of the stem c. Causing elongation of the cells on the shady side 3. Gravitropism a. Growth in response to gravitational force E. Auxin influences gene expression 1. Auxin influences cell growth rates by changing the pattern of gene expression III. Growth in plants 2. The first stage in phototropism is the absorption of light by photoreceptors 3. Phototropins have this role a. They absorb light of a specific wavelength b. Their conformation changes c. They bind to receptors within the cell which control the transcription of specific genes d. The genes involved likely code for a group of glycoproteins that transport the auxin from cell to cell PIN3 proteins F. Intracellular pumps 1. Auxin efflux pumps can set up concentration gradients of auxin in plant tissue III. Growth in plants 2. The position and type of PIN3 proteins can be varied to transport auxin to where growth is needed 3. If phototropins in the tip detect a greater intensity of light on one side than the other, auxin will be transported laterally from the side with the brighter light to the more shaded side 4. Gravitropism is also auxin dependent a. Upward growth of shoots and downward growth of roots b. If a root is placed on its side, gravity causes cellular organelles called statoliths to accumulate on the lower side of cells III. Growth in plants c. This leads to the distribution of PIN3 transporter proteins that direct auxin transport to the bottom of the cells d. High concentration of auxin inhibit root cell elongation so the top cells elongate at a higher rate than the bottom cells e. Causes the root to bend downward f. The pattern of auxin effect is opposite in the root as compared to its effect in the shoot IV. Reproduction in plants A. Flowering and gene expression 1. Flowering involves a change in gene expression in the shoot apex 2. Vegetative phase when a seed germinates, the young plant grows roots, stems and leaves a. This can last weeks, months or years b. A trigger will cause the plant to change into the reproductive phase c. Happens when flowers are produced from meristem instead of leaves 3. Flowers are reproductive organs for a plant 4. Temperature can play a role, but day length is the main trigger IV. Reproduction in plants 5. More precisely, the length of the dark period 6. Some plants are categorized as short-day plants because they flower when the dark period becomes longer than a critical length poinsettias 7. Other plants are long-day plants because they flower during the long days of early summer when nights are short red clover 8. Light plays a role in the production of either inhibitors or activators of genes that control flowering 9. phytochrome pigment IV. Reproduction in plants a. Long-day plants will transcribe the genes that cause flowering when phytochrome is active (FT gene) b. The FT mRNA is then transported in the phloem to the shoot apical meristem c. There it is translated into FT protein d. FT protein binds to a transcription factor e. This leads to the activation of many flowering genes which transform the leaf meristem into a reproductive meristem B. Photoperiods and flowering 1. The switch to flowering is a response to the length of light and dark periods in many plants IV. Reproduction in plants 2. Long-day plants flower in summer when the nights are short 3. Short-day plants flower in the autumn when the nights have become long enough 4. It’s the length of darkness that matters not the length of daylight 5. The pigment that measures the length of dark periods – called phytochrome – and can switch from one form to another. PR and PFR a. When PR absorbs red light (660nm) it is converted to PFR b. When PFR absorbs far-red light(730nm), it is converted to PR IV. Reproduction in plants i. not of great importance bc sunlight contains more wavelength of 660nm than 730nm ii. In normal sunlight, phytochrome is converted rapidly to PFR c. However, PR is more stable than PFR, so in darkness PFR very gradually changes into PR 6. Further experiments have shown that PFR is the active form of phytochrome 7. receptor proteins are present in the cytoplasm to which PFR but not Pr binds a. In long-day plants, large enough amounts of PFR remain at the end of short nights to bind the receptor IV. Reproduction in plants b. Promotes transcription of genes needed for flowering c. In short-day plants, the receptor inhibits the transcription of the genes needed for flowering when PFR binds to it i. At the end of long nights, very little PFR remains ii. Inhibition fails iii. Plant flowers C. Mutualism between flowers and pollinators 1. Most flowering plants use mutualistic relationships with pollinators in sexual reproduction Animal Pollinated Flower Diagram IV. Reproduction in plants 2. Sexual reproduction in flowering plants depends on the transfer of pollen from the stamen to the stigma of another plant 3. Pollen is transferred via a number of strategies a. Wind b. Water c. Animals i. Birds ii. Bats iii. Insects flies, butterflies and bees 4. Mutualism is a close association between two organisms in which both organisms benefit from the relationship IV. Reproduction in plants a. Pollinators gain food in the form of nectar b. Plant gains a means of transfer of pollen to another plant C. Pollination, fertilization and seed dispersal 1. Success in plant reproduction depends on pollination, fertilization and seed dispersal 2. Fertilization: after pollination a. Actual joining of sperm with the egg b. Each pollen grain on the stigma grows a tube down the style to the ovary c. The sperm swim down the tube to fertilize the eggs d. Fertilized egg develops into a seed and the ovary develops into fruit Bee with pollen sacs – cross pollinating IV. Reproduction in plants 3. Seed dispersal a. Seeds cannot move themselves b. Seeds need to travel long distances away from their parent plant c. Reduces competition between offspring and parent plant d. Helps to spread the species e. Depends on the structure of the fruit i. dry and explosive ii. Fleshy and delicious iii. Feathery or winged iv. Covered in hooks IV. Reproduction in plants 3. Seed dispersal a. Seeds cannot move themselves b. Seeds need to travel long distances away from their parent plant c. Reduces competition between offspring and parent plant d. Helps to spread the species e. Depends on the structure of the fruit i. dry and explosive ii. Fleshy and delicious iii. Feathery or winged iv. Covered in hooks IV. Reproduction in plants D. The structure of seeds 1. embryo root – becomes the root 2. embryo shoot – becomes the stem and leaves 3. cotyledons a. Monocotyledon - one b. Dicotyledon – two c. Provides food for the seed while it germinates 4. testa a. Seed coat b. Protects the seed 5. micropyle a. Only part of the seed permeable to water b. Looks like the seed’s belly button IV. Reproduction in plants E. Germination of seeds 1. The early growth of seeds before they sprout leaves and begin photosynthesis 2. All seeds need water for germination 3. Water stimulates the release of gibberellins 4. Gibberellins stimulate the translation of enzymes: amylase and maltase 5. Amylase breaks starch in the cotyledon down to maltose 6. Maltase breaks maltose down into glucose 7. Glucose fuels cell respiration which produces ATP seed can germinate and grow