MOD 3

• Terrestrial plants


  • Maintaining water and nutrient balance
  • Providing sufficient structural support for upright growth
• Vascular system


  Transports water, minerals, and organic molecules over great distances
• Secondary growth of vascular tissue

  Allows trees to achieve great heights
• Water balance


  Keeps herbaceous plants upright
• Animal systems


  • Respiratory system obtains oxygen
  • Digestive system processes nutrients
  • Circulatory system transports materials and regulates bodily fluids
• Transport in plants


  1. Water moves through cell-wall spaces, plasmodesmata, plasma membranes, and interconnected conducting elements

  2. Xylem transports water and minerals from roots to shoots

  3. Phloem transports products of photosynthesis
• Xylem


  Transports water and minerals from roots to shoots
• Phloem


  Transports products of photosynthesis from where they are made or stored to where they are needed
• Transpiration


  Causes pulling force that moves water in xylem
• Cohesion and adhesion


  Result in stable column of liquid reaching great heights in xylem
• Apoplast


  Everything external to the plasma membranes of living cells, including cell walls, extracellular spaces, and interior of dead cells
• Symplast


  Entire mass of cytosol of all the living cells in a plant, as well as the plasmodesmata
• Transport routes in plants

  • Apoplastic
  • Symplastic
  • Transmembrane
• Apoplastic route


  Water and solutes move along the continuum of cell walls and extracellular spaces
• Symplastic route


  Water and solutes move along the continuum of cytosol, requiring substances to cross a plasma membrane once
• Transmembrane route


  Water and solutes move out of one cell, across the cell wall, and into the neighboring cell, requiring repeated crossings of plasma membranes
• Plasma membrane in plant cells

  • Selective permeability controls short-distance movement of substances
  • Has active and passive transport mechanisms
  • Has pumps, transport proteins, and cotransporters
• Hydrogen ions (H+)


  Play primary role in basic transport processes in plant cells, rather than sodium ions (Na+)
• Membrane potential in plant cells

  Established mainly through pumping of H+ by proton pumps, rather than pumping of Na+ by sodium-potassium pumps
• Cotransport in plant cells

  H+ is most often cotransported, whereas Na+ is typically cotransported in animals
• H+/sucrose cotransporter


  Couples movement of sucrose against its concentration gradient with movement of H+ down its electrochemical gradient
• Ion channels in plant cell membranes

  • Allow only certain ions to pass, and are often gated in response to stimuli
  • Involved in producing electrical signals analogous to action potentials in animals, but 1,000 times slower and using Ca2+-activated anion channels rather than Na+ ion channels
• Osmosis


  The diffusion of water
• Water potential


  A physical property that predicts the direction in which water will flow, including the effects of solute concentration and physical pressure
• Water moves from regions of higher water potential


  To regions of lower water potential if there is no barrier to its flow
• Water potential


  Water's potential energy - water's capacity to perform work when it moves from a region of higher water potential to a region of lower water potential
• Expansion of plant cells and seeds


  • Breaking concrete sidewalks
  • Swelling of wet grain seeds within the holds of damaged ships causing hull failure and sinking
• Diffusion


  An effective transport mechanism over the spatial scales typically found at the cellular level, but too slow for long-distance transport within a plant
• Bulk flow


  The movement of liquid in response to a pressure gradient, occurring from higher to lower pressure, independent of solute concentration
• Conducting cells in xylem and phloem

  • Facilitate bulk flow
  • Mature tracheids and vessel elements are dead cells with no cytoplasm
  • Sieve-tube elements have almost devoid of internal organelles
• Transport of carbohydrates through phloem

  1. Carbohydrates manufactured in leaves and other green parts are distributed through the phloem to the rest of the plant

  2. Carbohydrates concentrated in storage organs are converted into transportable molecules and moved through the phloem
• Pressure-flow hypothesis


  Model of carbohydrate transport through the phloem, where dissolved carbohydrates flow from a source to a sink, where they are unloaded and used
• Phloem loading


  1. Carbohydrates (mostly sucrose) enter the sieve tubes in the smallest veins at the source

  2. Much of the sucrose arrives at the sieve cell through apoplastic transport and is moved across the membrane via a sucrose and H+ symporter

  3. Companion cells and parenchyma cells adjacent to the sieve tubes provide the ATP energy to drive this transport
• Unlike vessels and tracheids, sieve cells must be alive to participate in active transport</b>
• Difference in water potential between sieve tubes and nearby xylem cells

  Water flows into the sieve tubes by osmosis, increasing turgor pressure and driving the fluid throughout the plant's system of sieve tubes
• Transport of sucrose and other carbohydrates within sieve tubes

  Does not require energy, but the pressure needed to drive the movement is created through energy-dependent loading and unloading of these substances from the sieve tubes
• Circulatory system


  Has three basic components: a circulatory fluid, a set of interconnecting vessels, and a muscular pump, the heart
• How the heart powers circulation

  1. Uses metabolic energy to elevate the circulatory fluid's hydrostatic pressure

  2. The fluid then flows through the vessels and back to the heart
• Function of the circulatory system

  Connects the aqueous environment of the body cells to the organs that exchange gases, absorb nutrients, and dispose of wastes
• In mammals, O2 from inhaled air diffuses across only two layers of cells in the lungs before reaching the blood