General Biology 2

• Xylem


  Plant tissue that transport water and various ions in plants from roots to growing stem in an upward direction without metabolic energy

  
• Phloem


  The food or photosynthates of the plants move to all parts of the plants through the phloem tissue in upward, downward, and lateral direction (multidirectional transport)

  
• Substances transported within a plant

  • Mineral nutrients
  • Organic nutrients
  • Water
  • Plant growth regulators
• Angiosperms (flowering plants) consist of a well organized, complex, continuous traffic of various substances moving in different directions in order to maintain their integrity
• Diffusion


  A physical process where solids, liquids or gasses move passively (without energy expenditure) from a region of higher concentration to a region with a lower concentration through the available space
• Diffusion


  • Substances in plants move over short distances by the process of diffusion
  • Diffusion is a slow process
  • The diffusing particles move slowly across a concentration gradient in a random fashion without energy expenditure
  • Different particles can diffuse simultaneously in the same space
  • The gaseous movement of carbon dioxide and oxygen within the plant parts and between the plant and its environment occurs solely by diffusion
• Rate of diffusion


  Affected by the concentration gradient of the diffusing particles, the membrane permeability, size of the diffusing particles, temperature, and pressure
• Facilitated diffusion


  The hydrophobic substances can easily diffuse through a cell membrane along an already existing concentration gradient, while the hydrophilic substances cannot pass through a biological membrane due to the polar nature of water, thus their movement along a concentration gradient is facilitated by certain membrane proteins

  
• Facilitated diffusion


  • Occurs through special proteins present in the plant cell membrane without expenditure of ATP energy
  • Membrane proteins are highly specific for a specific module to be diffused
  • Shows a saturation effect if the transport proteins are all used up
  • Sensitive to inhibitors
  • Porins are large membrane proteins present in the outer membrane of the plastids
  • Aquaporins or water channels are membrane proteins for passive transport of water
• Carrier proteins


  Bind to a specific diffusing particle and deliver it to the other side of the membrane
• Channel proteins


  Allow passive diffusion of specific particles of an appropriate size through them
• Uniport


  Carrier proteins allow the movement of a molecule across a membrane independent of other molecules
• Cotransport


  Two types of molecules are allowed to move together through carrier proteins simultaneously
• Symport


  Both molecules cross the membrane in the same direction at the same time
• Antiport


  Both molecules cross the membrane in opposite directions at the same time
• Active transport


  Substances are pumped across a biological membrane through carrier membrane proteins by utilizing energy from ATP against a concentration gradient
• Pumps (in active transport)

  • Highly specific and sensitive to inhibitors that react with protein side chains
  • Inhibits saturation effect
  • Allows transport from lower to higher concentration (uphill transport)
• Water


  Major component of a living cell, essential for all physiological activities of the plant
• The water content in a watermelon is over 92 percent. Most herbaceous plants have about 85 to 90 percent water in them (approximately 10 to 15 percent of their fresh weight is dry matter)
• Water potential


  Measure of energy possessed by the water molecules, greater concentration of water in a system = greater water potential, pure water has the greatest water potential compared to a solution, denoted by the Greek symbol Psi or Ψ (Ψw) and is expressed in pascals (Pa), bars or atmospheres, water potential of pure water at normal pressure and temperature is zero, water potential of a solution will always be negative
• Solute potential/osmotic potential


  Decrease in the water potential of pure water due to the addition of solute particles
• Pressure potential/hydrostatic potential


  Pressure that develops in a plant cell due to entry or exit of water from it, due to diffusion, increased pressure potential or the pressure that the water builds up against the cell wall makes the cell turgid, denoted by Ψp and is usually positive

  
• Osmosis


  The movement or diffusion of water molecules from a region of their higher chemical potential or water potential or concentration (as present in pure state of water or a dilute solution) to its lower chemical potential (as present in a solution or a stronger solution) without diffusion of any solute particle, by means of a differentially or selectively permeable membrane

  
• The net direction and rate of osmosis are affected by both the pressure gradient and concentration gradient, water will move from its region of higher chemical potential (or concentration) to its region of lower chemical potential until equilibrium is reached

  
• A plant cell controls movement of different molecules in and out of it because of the presence of a selectively permeable cell membrane as the cell wall of a plant cell is freely permeable to water and ions, inside a plant cell, a large central vacuole is present which contains the vacuolar sap or cell sap

  
• Tonoplast


  Movement of molecules in and out of the central vacuole is controlled by its membrane
• Osmotic pressure


  Pressure required to completely prevent the entry of water into an osmotically active solution across a selectively permeable membrane, denoted by π (pi) and is measured in pascals, bars, or atmospheres, greater the solute concentration, greater will be the pressure required to prevent water from diffusing in, osmotic pressure = positive value, osmotic potential = negative value

  
• Plasmolysis


  Process of shrinkage of the protoplast or the living part of a plant cell (nucleus and the cytoplasm, surrounded by a cell membrane) from its cell wall due to diffusion of water out of the plant cell when it is placed in a hypertonic solution, the process is usually reversible

  
• The solution surrounding a plant cell can be hypotonic, isotonic, or hypertonic in comparison to the protoplasm of the plant cell

  
• Turgor pressure (pressure potential, Ψp)


  Pressure that builds up in the cytoplasm of a plant cell due to the increase in water, the cell is said to be turgid

  
• Imbibition


  A special type of diffusion in which water moves from a region of higher water potential and is absorbed by the solid particles of an adsorbent having a lower water potential, causing it to enormously increase in volume without forming a solution

  
• Translocation


  The movement of substances through the vascular tissues (xylem and phloem) of plants in a bulk flow or mass flow

  
• Xylem


  Translocates water, mineral salts, some organic nitrogen, and hormones, from roots to the aerial parts of the plants

  
• Phloem


  Helps in translocation of a variety of organic and inorganic solutes, mainly from the leaves to other parts of the plants
• Absorption of Water by Plants

  1. Water and minerals present in between the soil particles initially enters in a plant root through millions of root hairs by simple diffusion

  2. After entering in the root hairs, it moves deeper into root layers by apoplast pathway and symplast pathway

  
• Apoplast pathway


  The water moves along a system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots

  
• Symplast pathway


  Water moves through a system of protoplasts (intercellular movement) of various neighbouring cells which are connected through plasmodesmata

  
• Plasmodesmata


  Fine cytoplasmic channels between neighboring plant cells which pass through the intervening cell walls and allow cell to cell communication
• Root hair


  Thin-walled slender extensions of some epidermal cells of plant roots which greatly increase the surface area for absorption of water and minerals
• The water moves through the loosely packed cortical cells via apoplast pathway, as the water reaches the endodermis, it travels via symplast pathway due to the presence of an impermeable band of suberised matrix called the casparian strip in the endodermis of roots, the water finally moves in the xylem through symplast pathway