3.3.4.2 Mass Transport in plants

• The evaporation of water from leaves is known as TRANSPIRATION.
• Cohesion is when water molecules form hydrogen bonds between each other and stick together.
• Adhesion is when water molecules stick to the walls of the walls of a tube as they are attracted to it.
• Transpiration pull is the pull of water in a continuous stream under tension (as a result of transpiration).
• Cohesion-tension theory describes water being pulled under tension (because of transpiration) and in a continuous stream due to water molecules attraction for each other. 
• Higher temperature means faster rate of transpiration because water vapour particles will have a higher kinetic energy, causing the particles to spread out. This reduces the water vapour concentration outside the leaf and sets up a diffusion gradient for water to move down a diffusion gradient, out of the leaf.
• High wind speeds means a faster rate of transpiration because it causes water vapour to move away from the stomata. This reduces the water vapour concentration outside the stomata and therefore creates a water potential gradient.

  This causes water to diffuse down a water potential gradient out of the leaf.
• Higher stomatal density means faster transpiration rate because it means more stomata open at the same time, meaning more water can be lost through the stomata.
• Investigating transpiration rate

  • The potometer is filled with water and a cutting of a shoot is placed inside.
  • The shoot must be cut at a slant and placed into the potometer while underwater to ensure no air enters the xylem.
  • Remove the end of the capillary tube from the water beaker.
  • Wait for a bubble of air to form in the capillary tube.
  • Place the capillary tube back into the water.
  • Mark the starting position of the air bubble.
  • Use a stopwatch to record the distance moved by the air bubble in a given time period.
• Phloem

  • Sieve tube elements are the cells that make up the phloem vessel.
  • Sieve tube elements are living cells that contain a cytoplasm but no nucleus.
  • The walls of sieve tube elements are made of cellulose.
• Phloem

  • Companion cells are connected to sieve tube elements via plasmodesmata.
  • Plasmodesmata allows the cytoplasm to be shared between companion cells and sieve tube elements.
  • Companion cells have a nucleus.
• Phloem

  • At either end of the sieve tube elements are sieve plates.
  • Sieve plates have large pores that allow sap to move through the sieve tube elements.
  • Sieve plates allow sugars to be transported through the phloem.
• Translocation is the process where sugars produced in photosynthesis are transported from the leaves to other parts of a plant.
• The site where sugar is produced in photosynthesis in a plant is known as a source. An example is the leaf.
• The site in a plant where sugar made in photosynthesis is transported to, to be used is called a sink. An example of this is the roots.
• A source is an area of a plant where sucrose produced in photosynthesis is loaded into the phloem e.g. a leaf
• A sink is an area where sucrose is removed from the phloem to be stored as starch e.g. roots.