IB topic 9 biology HL

• Root Uptake of Minerals:

  1. Minerals are bound to soil particles

  2. Minerals dissolve in water, allowing them to move towards roots by:

  a. Diffusing through soil: minerals move down the concentration gradient towards roots

  b. Mass flow of water: minerals move with water through soil towards roots

  3. Minerals (e.g., Iron, magnesium, calcium, potassium) enter the plant through the roots by active transport:

  a. H+ ions are pumped out into the soil, displacing mineral ions on clay particles for diffusion into root cells

  b. Negative minerals cross membranes with H+ ions moving back into roots via active transport
• Uptake of Water:

  1. Active uptake of mineral ions in roots creates a low solute concentration around roots, allowing water to enter via osmosis

  2. Water moves to the xylem via symplastic/apoplastic pathways:

  a. Symplastic: through cytoplasm

  b. Apoplastic: through cell walls, transferring into endodermis cytoplasm
• Maximizing Root Uptake:

  • Increase surface area by branching roots and having root hairs
  • Hyphae of mutualistic fungi on roots enhance surface area for absorption
• Xylem Transport:

  • Transpiration creates negative pressure at the top of xylem, pulling water up (transpiration pull)
  • Adhesion of water to cellulose generates tension forces, drawing water out
  • Water moves up the xylem in a continuous column by capillary action due to cohesion and adhesion
• Cohesion:

  • Attraction between water molecules held by weak hydrogen bonds

  Adhesion:

  • Water attracted to other polar substances, forming hydrogen bonds
• Models of Transpiration:

  1. Porous pots: allow water loss, creating negative pressure

  2. Capillary tubing: capillary action against gravity due to adhesion and cohesion

  3. Blotting/filter paper: water rises due to adhesion and cohesion
• Apparatus for Measuring Transpiration:

  Potometer measures water uptake rate by tracking air bubble movement in a capillary tube
• Factors Influencing Transpiration Rates:

  1. Temperature: higher temp increases transpiration

  2. Humidity: high humidity decreases transpiration

  3. Wind: air currents increase transpiration

  4. Light: high light increases transpiration
• Experiments on Transpiration Rates:

  • Temperature: use heaters or water baths
  • Humidity: vary levels in a plastic bag
• Xerophytes:

  • Adapted for desert environments with high transpiration rates
  • Less and smaller leaves, rolled leaves, hairy stomata, thick waxy cuticle, water storage, CAM physiology
• Halophytes:

  • Adapted for saline soils with high water loss
  • Reduced leaves, thick cuticles, sunken stomata, long roots, salt excretion structures
• Purpose of the phloem in plants:

  • Phloem transports organic compounds from sources (e.g., leaves) to sinks (e.g., fruits) in a process called translocation
  • Sugars, mainly transported as sucrose, are a key component
  • Phloem can transport organic compounds in either direction
  • The fluid in the phloem is referred to as plant sap
• Structure of the phloem:

  • Living cells located next to the xylem
  • Movement of nutrients is directional
  • Sieve tubes made by sieve tube elements joined in a column
  • Sieve tube elements lack nuclei and have fewer organelles to maximize translocation
  • Rigid cell walls of sieve tube elements withstand hydrostatic pressure
  • Sieve tube elements connected end to end by sieve plates that are perforated to enable flow
• Function and characteristics of companion cells:

  • Associated with sieve tube elements
  • Plasmodesmata connect their cytoplasm to facilitate metabolite transfer
  • Companion cells have infolding plasma membranes to increase surface area for material exchange
  • Transport proteins in companion cell membranes move materials in/out of sieve tubes
  • Many mitochondria in companion cells supply ATP for active transport
• Process of phloem loading:

  1. H+ actively transported out of the phloem by proton pumps from companion cells, creating a proton gradient

  2. H+ passively diffuse back into sieve tube/companion cell via co-transport protein, requiring sucrose

  3. Sucrose diffuses into the phloem via plasmodesmata due to concentration differences
• Transport through the phloem:

  At the source:

  • Increasing sucrose concentrations in phloem make sap hypertonic, drawing in water from xylem via osmosis
  • Incompressibility of water and rigid cell walls lead to increased hydrostatic pressure at the source, causing solutes to move towards areas of lower pressure (sink)

  
  

  At the sink:

  • Solutes are unloaded by companion cells, decreasing solute concentration and making sap hypotonic, leading water to return to xylem via osmosis
• Experiment to test phloem transport rate:

  1. Plant grown in radioactive CO2 to produce radioactively labeled carbohydrates

  2. Aphids feed on phloem sap, and their stylet is severed to analyze sap for radioactively labeled carbohydrates

  3. Translocation rate calculated based on the time for labeled sugars to reach different plant positions
• Factors affecting phloem transport rate:

  • Rate of photosynthesis affecting sugar concentration in phloem, influenced by light intensity, CO2 concentration, and temperature
  • Rate of cellular respiration (ATP production for phloem loading), impacted by physical stress on the plant
  • Rate of transpiration affecting water content in the phloem
  • Diameter of sieve tubes influencing hydrostatic pressure
• Contrast and identify xylem and phloem in microscope images:

  • Xylem generally larger than phloem
  • Xylem typically located more internally
  • Monocots show a circular arrangement with xylem more inside, while dicots display an X-shaped xylem arrangement with phloem in surrounding gaps
  • In stems, monocots have scattered phloem more towards the outside, while dicots have a circular arrangement with xylem more inside and outside
• Meristems consist of undifferentiated cells that allow for indeterminate growth in plants
• Meristems are totipotent, allowing the plant to regrow structures or even new plants
• Apical Meristems are found at the root & stem tips and are used for primary growth to increase plant length and produce leaves and flowers
• Shoot apical meristem has regions for various types of growth
• Root apical meristem (root tip) is responsible for root growth
• Lateral Meristem occurs at the cambium and is responsible for widening and thickening of the stem
• Growth in plants occurs through cell enlargements and an increase in cell numbers through mitotic division
• Stem growth occurs in sections called nodes, with one cell remaining in the apical meristem to extend the stem and the other forming an inactive auxiliary bud
• Cells in meristems progress through the cell cycle faster for repeated cell division and grow by absorbing nutrients and water
• Auxin (IAA) is a group of hormones produced at the shoot and root apical meristems that control and stimulate growth
• High concentrations of Auxin in the shoot apical meristem promote shoot growth by promoting cell division and cell elongation
• Apical dominance occurs when the meristematic tissue at the top inhibits the growth at the auxiliary buds
• Auxin efflux pumps set up concentration gradients of auxin in plant tissue to change its distribution and control the direction of plant growth
• Auxin enters the cell through diffusion and influx transporter proteins
• Auxin moves out of the cell through efflux transporters
• Auxin from the root apical meristem inhibits cell elongation and high concentrations limit growth
• Tropisms are the growth or turning of a plant in response to directional external stimulus
• Phototropism is where the plant grows towards the brightest light source
• Geotropism is where plants grow upwards against gravity
• Micropropagation is a technique to produce large numbers of identical plants from a stock plant
• Applications of micropropagation include producing virus-free strains, rapid bulking, and propagation of rare species