IB Biology Topic 9: Plant Biology HL

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1.1 Transport in the xylem of plants
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Xylem
Xylem
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part of vascular bundle that is responsible for the transports water throughout the plant – no membrane or organelles are present.
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Pores
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are present allowing water to leave the xylem to cells.
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Lignen
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is laid down in rings around the cellulose cell wall. this adds strength (to resist pressure to the xylem)
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Lumen
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is filled with sap (water + minerals).
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Roots
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roots anchor the plants in the ground, and absorb minerals (by active transport) and water (osmosis) roots provide a large surface area due to branching and root hairs (extension of cells)
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Absorption of minerals (roots)
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-there is usually a higher concentration of minerals within root cells than in the soil. – root cells use active transport to move minerals from the soil into the root cells. -there are lots of mitochondria in the root cells to produce ATP for active transport. – specific protein pumps are present in the plasma membrane of root cells, which actively transport the minerals into the cytoplasm.
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Absorption of water
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-there is a high concentration of minerals in the cytoplasm of root cells due to active transport – water moves from the low solute concentration in the soil, through plasma membranes of root cells to the high solute concentration in the root cells’s cytoplasm, by osmosis. – the water (and minerals) moves to the root xylem.
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(Evapo) Transpiration
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the loss of water from leaves and other organs of the plant in the xylem
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Stomata
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pores at the bottom surface of leaves which allow water to vaporise out of the leaf
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The process of transpiration
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1) water evaporates from cells within the leaf. the water vapour diffuses out of the lead through stomata. 2) the leaf cells replace the water lost by taking water from the xylem. this reduces the water pressure in the xylem (creating a partial vacuum). 3) water is \”pulled\” up along the xylem from high pressure in the roots. This is known as transpiration pull. 4) the flow of water in the xylem ( transpiration stream( continues to flow because: a) water is cohesive (water is a polar molecule which forms hydrogen bonds) b) water adheres to the cellulose walls of the xylem. c) the lignified xylem walls are able to withstand pressure.
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Abiotic factors affecting transpiration
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1) Temperature: as temp increases, the water particles gain kinetic energy and diffuse faster. 2) Humidity (concentration of water in the air): If humidity is low, the water diffuses quickly out of the leaf. If humidity is high, water diffuses slower. 3) Wind: increasing wind usually increases the rate of transpiration.
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Xerophytes
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are plants that have been adapted to tolerate dry conditions (eg. cacti)
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Xerophyte adaptations
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– water storage tissue – the leaves are modified into needles, decreasing surface area for transpiration – reduces number of stomata – vertical stems to allow them to absorb maximum sunlight during morning and evening. – thick waxy cuticles to prevent evaporation of water. – CAM physiology: a version of photosynthesis where the stomata open at night (to get CO2) but stay closed during the day.
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Halophytes
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are plants adapted to survive in saline (high salt) conditions.
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Halophyte adaptations
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9.2 Transport in the phloem of plants
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Phloem
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part of vascular bundle that transports the products of photosynthesis throughout the plant
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Phloem structure
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photo ! – sieve tubes – sieve plates – companion cells – plasmodesmata
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Translocation
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organic compounds (eg sugars + amino acids- are transported in plants from sources to sinks through the phloem.
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Sieve plates
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between the sieve tube cells, which makes it easier for fluids to flow through the phloem
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Plasmodesmata
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narrow companion cells and sieve tubes.
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Companion cells
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– maintain the sieve tubes – have many mitochondria to produce ATP for active transport of organic compounds into sieve tubes.
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Sources
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can be photosynthesising leaves, or storage organs (such as roots) releasing their store of nutrients.
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Sink
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can be storage organs where nutrients are stored, growing leaves, flower, fruit.
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Sieve tubes
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– form a column of cells – have a plasma membrane so that sugar concetration can be controlled. – does not have a nucleus and not many organelles, so that fluid can flow easily.
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Process of translocation
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1) sucrose is produced in the leave (a source) by photosynthesis. The sucrose is actively transported into sieve tubes by companion cells. 2) water from the xylem moves into the phloem by osmosis (due to the high sucrose concentration, creating high pressure) 3) the water moving into the phloem;s sieve tubes pushes the water (containing sucrose) towards the sink. This occurs de to water being incompressible. 4)the fluid in the sieve tubes is pushed towards the sink. 5) the sucrose is then actively transported from the sieve tube into the source cells (eg. storage cells in the root) 6) the sucrose can be used for growth, converted to starch for storage, or respiration.
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9.3 Growth in plants
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Tropism
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directional growth or movement of a plant towards or away from stimulus
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Phototropism
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Directional growth of a plant towards or away from light.
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Auxin
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plant hormone that promotes growth in the shoot of plants. auxin is produced by the shoot tips.
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Process of photo tropism
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1) light is detected by pigments found in plants including phototropins. When these detect differences in (blue) light in the short tip, they trigger the movement of auxin by active transport. 2) Auxin efflux pumps are located in the plasma membrane of cells in the shoot. The auxin efflux pumps are used to actively transport auxin through cell from the light side of a plant to the shaded side. NOTE: auxin efflux pumps are moved to ensure that auxin is moved towards the shaded side 3) Auxin receptors (proteins) are found within the plasma membrane of cells. Auxin binds to the receptor. This causes changes within the cell that causes certain growth genes to be expressed. This results in the release of H+ into the cell walls. This loosens connections between cellulose, allowing the cells to grow.
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Meristem
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– tissue in a plant consisting of undifferentiated cells which can differentiate into other plant cells, and undergo mitosis and cell division. – responsible for growth of a plant
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Lateral meristem
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– tissue in a plant responsible for growth in width – found between xylem and phloem
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Micropropagation of plants
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produces many genetically identical plants in a short period of time.
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Micropropagation process
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9.4 Reproduction of plants
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Flower structure
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Female [carpel] – anther – filament – petals Male [stamen] – stigma – style – ovary – sepals
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carpel
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female reproductive organs of an angiosperm
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anther
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male reproductive organs of a plant that produces pollen
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filament
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stalk of stamen that holds up the anther
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petals
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attract pollinators for pollination with colours
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stamen
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???
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sepal
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protect the developing flower while in the bud
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style
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structure of carpel that holds up the stigma
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ovary
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base of carpel in which the female sex cells develop
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stigma
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sticky top of carpel on which pollen lands
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Pollination
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– the transfer of pollen grains from the male anther to the female stigma. – the pollen contains sperm – often facilitated by animals, wind or water movement.
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Fertilisation
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– fusion of the male gamete(sperm) with the female gamete(egg) to form a zygote. – In plants, pollen lands on the stigma. It germinates and grows down through the style to the ovary. – the zygote develops into a seed.
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Seed dispersal
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when a seed, formed from a fertilised ovule, is moved away from the parental plant to reduce competition for resources.
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Short day plants
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Germination of seeds
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all seeds require oxygen (for aerobic respiration), water (most metabolic reactions occur in water) and a suitable temperature (required for the enzyme involved in germination) some seeds require other conditions such as light, fire, frost before they will germinate.

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