3.3.4.2 Mass Transport in plants

• Suggest reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration

  • water used for support/turgidity
  • water used in photosynthesis
  • water used in hydrolysis
  • water produced during respiration
• Describe how water is moved through a plant according to the cohesion-tension hypothesis (4)

  • water evaporates from leaves
  • reduces water potential in cell
  • water is drawn out of xylem
  • creates tension
  • cohesive forces
  • water pulled up as a column
• Use your knowledge of leaf structure to explain why less water is lost through the upper surface of leaves than is lost through lower surface (2)

  • more stomata on lower surface
  • thicker waxy cuticle on upper surface
• Describe the cohesion-tension theory of water transport in the xylem (5)

  • water is lost from leaf because of transpiration
  • lowers water potential of mesophyll/leaf cells
  • water pulled up xylem
  • water molecules cohere/‘stick’ together by hydrogen bonds
  • this forms a continuous water column
  • adhesion of water molecules to walls of xylem
• Describe and explain the relationship between humidity and transpiration rate (3)

  • high humidity leads to decreased transpiration
  • high humidity means increased water potential
  • reduces water potential gradient
  • less evaporation
• Why is it important that the xylem tissue contains hollow tubes? (1)

  maintains transpiration stream
• Why is lignin present in xylem cell walls? (1)

  provides support
• The student cut the shoot and put it into the photometer under water. Explain why (1)

  prevent air entering
• The student wanted to calculate the rate of water uptake by the shoot in cm3 per minute. What measurements did she need to make? (2)

  • distance and time
  • radius of capillary tube
• Suggest how a reservoir allows repeat measurements to be made (1)

  returns bubble to start
• If the enzymes of the mitochondria of the root cells are inhibited, eg with cyanide, then root pressure drops. 

  Suggest why this happens. (6 marks)

  • Aerobic respiration occurs in mitochondria
  • Supplies root cells with energy
  • Mitochondria cannot produce ATP if enzymes inhibited
  • Energy required for active transport of ions into the xylem
  • If no ions actively pumped into xylem then water potential stays high
  • No/reduced water potential gradient for water to move by osmosis into root cells
  • Less water means lower hydrostatic pressure
• The evaporation of water from leaves is known as TRANSPIRATION.
• Why is transpiration important in a plant?

  • It provides a means of cooling the plant via evaporation
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells provides support to leaves and the stem of non-woody plants
• 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.
• The mass flow hypothesis is used to explain the movement of substances through phloem.

  Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube (3)

  • sucrose actively transported into phloem
  • lowering the water potential
  • water moves into phloem by osmosis from xylem
• One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots.

  Describe how a high pressure is produced in the leaves (3)

  • water potential becomes lower as sugar enters phloem
  • water enters phloem by osmosis
  • increased volume of water = increased pressure
• Describe the mass flow hypothesis for the mechanism of translocation in plants (4)

  • in source/leaf sugars actively transported into phloem
  • by companion cells
  • lowers water potential of sieve tube and water enters by osmosis
  • increase in pressure causes mass movement towards sink/root
  • sugars used in root for respiration
• 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.
• The diameter of a tree is less during the day, when the tree is transpiring, than it is at night. Explain how this supports the cohesion-tension theory (2)

  • evaporation from leaves during daytime mainly
  • tension pressure on water in xylem creates inward pull on walls of xylem vessel
• Describe one other piece of evidence that supports the cohesion-tension theory and explain how it supports this theory (2)

  • break in xylem vessel (introducing air) prevents water movement
  • because the continuous water column is broken
• Describe the processes involved in the transport of sugars in plant stems (5)

  • (At source) sucrose is actively (transported) into the phloem
  • By companion cells
  • Lowers water potential in phloem and water enters by osmosis
  • (Produces) high (hydrostatic) pressure
  • Mass flow/transport towards sink/roots
  • At sink/roots sugars are unloaded