3.3 - transport in plants

• Xerophytes are plants adapted to dry conditions
• Hydrophytes are adapted to wet conditions
• The most known xerophyte is the cactus
• Xerophytes have shallow root systems for rapid absorption of water
• Xerophytes have large, thin walled cells in stems and roots for maximum water storage
• Xerophytes have small surface area to volume ratio for a slow diffusion rate
• Xerophytes have roots which quickly grow new root hairs after rainfall
• Xerophytes have high concentration on pentose compounds which increase water holding capacity of cells
• Features of xerophytes include:

  pale, waxy, hairy or scaled leaves reduce water loss

  thickened epidermis to reduce cuticular transpiration

  reduced leaf area

  reduced number and size of stomata

  sunken stomata

  developed mechanical tissue to prevent plant from wilting
• Hydrophytes are found in water, which supports plants so they need little mechanical support
• Hydrophytes have parenchyma With many air spaces called aerenchyma
• Hydrophytes often have:

  large thin leaves

  epidermal cells which contain chloroplasts

  much aerenchyma for rapid gas exchange

  Thin cuticle
• Water logging is a major problem for all hydrophytes

  the air spaces in the plants need to be full of air rather than water.
• The stomata on the upper surfaces of hydrophytes are always open to maximise gaseous exchange. This also means the guard cells are inactive.

  Stomata need to be on upper surface of leaves so they are in contact with air and sun
• Hydrophytes have a reduced structure as the water supports the leaves and flowers
• Hydrophytes have large surface areas of stems and roots under water which maximises area for photosynthesis and for oxygen to diffuse into submerged plants.
• Plants need a transport system to ensure all cells receive a strong supply of nutrients they require.

  Especially important as plants need to transport substances against gravity up the stem
• Xylem is long

  made up of continuous columns made of dead tissue allowing transport of water for the whole plant
• Xylem contains pits

  allowing water to move sideways between vessels
• Xylem is thickened with lignin

  tough substance that provides structural support
• Phloem has sieve tube elements

  used to transport sugars around the plant
• Phloem has companion cells

  designed for active loading of sugars into the tubes
• Phloem has plasmodesmata

  allows flow of substances between cytoplasm of different cells
• Vascular system in the ROOTS:

  Consists of xylem and phloem

  xylem arranged in X shape - provides resistance against force

  phloem are dotted around the inner regions of the X

  vascular bundle surrounded by endodermis as a water supply
• vascular system in STEM:

  consists of xylem and phloem

  xylem are on the inside of the bundle to provide support and flexibility

  phloem are on the outside

  layer of meristem cells inbetween xylem and phloem to produce new tissue when needed
• Vascular system in LEAVES

  consists of xylem and phloem

  forms midrib and veins

  involved in transport and support
• Transpiration is the evapouration of water from the leaves of a plant.

  it is the consequence of gaseous exchange and occurs when the plant opens stomata to exchange oxygen and co2
• Factors that affect rate of transpiration

  • Increased light intensity = increased transpiration
  • increased temperature = increased transpiration
  • increased humidity = decreased transpiration
  • increased wind = increased transpiration
  • waxy cuticle = prevents transpiration
• We can measure transpiration rate using a potometer
• Potometers work by putting a plant cutting in a water filled tube that contains an air bubble.

  Rate of transpiration is calculated by measuring the movement of air bubble over time
• Water potential is the tendency of water to move by osmosis from high water potential to low.
• There are three pathways water can take when moving through plant cells

  • Apoplast pathway
  • symplast pathway
  • vacuolar pathway
• The apoplastic pathway is A method of osmosis where water moves through the cell walls and intercellular spaces

  pathway can only be used until water reaches the casparian strip
• The symplastic pathway is a method of osmosis where water moves through the cytoplasm via plasmodesmata.

  To begin the pathway water must be actively transported into cells
• The cohesion tension theory explains that water molecules form hydrogen bonds with eachother causing them to stick together (cohesion)

  surface tension of water also creates sticking effect so sticks to surfaces of xylem to be drawn upwards
• Mesophytes are plants that live in conditions that are both dry and wet
• Translocation is the movement of dissolved assimilates such as sucrose up and down a plant
• Translocation:

  sucrose produced in leaves is actively loaded into sieve tubes via active transport

  lowers water potential causing water to move in from xylem

  increased water will increase pressure so assimilates move along the sieve tube Towards areas of lower hydrostatic pressure (sink).

  Sucrose diffuses into surrounding cells where it is needed
• Water enters the plant as there is a lower water potential in the root hair cells due to a higher concentration of solutes. So water moves in by osmosis.
• Casparian strip is a waterproof strip in cell wall that forces water from apoplast pathway into the xylem cells