Transport in plants

• Where are meristems found?

  Found in the pericycle and the cambium
• Where are the phloem and xylem found?

  In vascular plants, the phloem is found on the outer side of the vascular bundle, while the xylem is found on the inner side.
• What are properties of xylem?

  • transports water and minerals
  • Provides structural support
  • Made up of dead cells therefore forms a continuous column.
  • Contains pits which enable water to move sideways between the vessels
  • Thickened with lignin to enable plants to remain flexible.
  • 1 way flow
• What are properties of the phloem?

  • tubes made up of living cells
  • involved in translocation, which is the movement of nutrients to storage
  • consist of sieve tubes elements and companion cells
  • sieve tube elements form a tube to transport sugar, such as sucrose dissolved form of sap
  • transported up-and-down
• Transport in plants

  Plants require a transport system to ensure that all the cells of a plant receive a sufficient amount of nutrients
• Transport in plants

  1. Xylem tissue enables water and dissolved minerals to travel up the plant in the passive process of transpiration

  2. Phloem tissue enables sugars to reach all parts of the plant in the active process of translocation
• Vascular bundle

  • Xylem and phloem are components
  • Xylem vessels are arranged in an X shape in the centre
  • Enables the plant to withstand various mechanical forces
  • Endodermis supplies xylem vessels with water
  • Pericycle is an inner layer of meristem cells
• Vascular bundle in stem

  • Xylem is located on the inside in non-wooded plants to provide support and flexibility
  • Phloem is found on the outside
  • Cambium is a layer of meristem cells involved in production of new xylem and phloem tissue
• Vascular bundle in leaf

  • Dicotyledonous leaves have a network of veins starting at the midrib and spreading outwards which are involved in transport and support
• Xylem vessels

  • Transport water and minerals, and also serve to provide structural support
  • Long cylinders made of dead tissue with open ends
  • Contain pits which enable water to move sideways
  • Thickened with lignin to enable flexibility
  • Water can only flow upwards
• Phloem vessels

  • Tubes made of living cells
  • Involved in translocation which is the movement of nutrients to storage organs and growing parts
  • Consist of sieve tube elements and companion cells
  • Sieve tube elements transport sugars in the dissolved form of sap
  • Companion cells are involved in ATP production for active processes
• Transpiration

  1. Plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves

  2. Enables processes such as photosynthesis, growth and elongation by supplying water and minerals

  3. Involves osmosis, evaporation and diffusion
• Potometer

  • Used to investigate the rate of transpiration by measuring the movement of the meniscus as water vapour lost by the leaf is replaced
• Factors affecting rate of transpiration

  • Number of leaves
  • Number/size or position of stomata
  • Presence of waxy cuticle
  • Amount of light
  • Temperature
  • Humidity
  • Air movement
  • Water availability
• Xerophytes

  • Plants adapted to living in dry conditions
  • Smaller leaves to reduce surface area for water loss
  • Densely packed mesophyll and thick waxy cuticles prevent water loss
  • Respond to low water availability by closing stomata
  • Contain hairs and pits to trap moist air
  • Roll leaves to reduce exposure of lower epidermis
• Hydrophytes

  • Plants that live in water
  • Thin or absent waxy cuticle as they don't need to conserve water
  • Constantly open stomata on upper leaf surfaces to maximise gas exchange
  • Wide, flat leaves for light absorption
  • Air sacs and large air spaces to enable buoyancy
• Movement of water in the root

  1. Water enters through root hair cells and moves into the xylem tissue

  2. Occurs due to a water potential gradient

  3. Root hair cells provide a large surface area for water movement

  4. Minerals absorbed through root hair cells by active transport

  5. Water can move via symplast or apoplast pathways

  6. Endodermis with Casparian strip forces water to enter symplast pathway
• Movement of water in the xylem up the stem

  1. Water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient

  2. Root pressure pushes water into the xylem by osmosis

  3. Cohesion and surface tension (tension-cohesion theory) and capillary action maintain the flow of water
• Translocation

  1. Energy requiring process to transport assimilates like sucrose from sources (leaves) to sinks (roots, meristem)

  2. Sucrose enters phloem by active loading in companion cells

  3. Sucrose diffuses into sieve tube elements, reducing water potential and causing water to enter by osmosis

  4. Increased hydrostatic pressure drives mass flow of water and sucrose from source to sink
• What are the three pathways taken by water?

  The apoplast pathway

  The symplast pathway

  the vacuolar pathways
• what Is the apoplast pathway?

  Water passes through the spaces in the cell walls and between the cells. Water moves by mass flow rather than by osmosis. Also, dissolved mineral ions and salts can be carried with the water. 
• What is the symplast pathway?

  Water enters the cell cytoplasm through the plasma membrane. It can then pass through the plasmodesmata from one cell to the next.
• what is plasmodesmata?

  gaps in the cell wall containing cytoplasm that connects two cells. 
• What is the vacuolar pathway?

  This is similar to the symplast pathway, but the water is not confined. It is able to enter and pass through the vacuoles as well. 
• What is water potential?

  Water potential is a measure of the tendency of water molecules to move from one place to another. Water always moves from a region of higher water potential to a region of lower water potential. The water potential of pure water is zero.
• Describe the movement of water in plants?

  When plant cells are touching each other, water molecules can pass from one cell to another. The water molecules will move from the cell with the less negative (higher) water potential to the cell with the more negative (lower) water potential. This is osmosis.
• What is transpiration?

  The evaporation of water from the stomata
• Active loading

  1. Sucrose is loaded into the sieve tube

  2. Energy from ATP in the companion cells is used to actively transport hydrogen ions (H+) out of the companion cells

  3. This increases the concentration of hydrogen ions outside the cells and decreases their concentration inside the companion cells

  4. A concentration gradient is created

  5. Hydrogen ions diffuse back into the companion cells through special cotransporter proteins

  6. These proteins only allow the movement of the hydrogen ions into the cell if they are accompanied by sucrose molecules

  7. This is known as cotransport or secondary active transport

  8. As the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube
• How is sucrose moved?

  Movement of sucrose along the phloem is by mass flow.
• What is a source?

  
  

   is any part of the plant that loads sucrose into the sieve tube.
• What is the role of the sink?

  A sink is anywhere that removes sucrose from the phloem sieve tubes
• what is sap?

  A solution of sucrose, amino acids and other assimilates flows along the tube
• What is adhesion?

   the attraction between water molecules and the walls of the xylem vessel
• What is cohesion?

  the attraction between water molecules caused by hydrogen bonds.
• How to make results valid in potometer?

  1.Set it up under water to make sure there are no air bubbles inside the apparatus. 

  2. Ensure that the shoot is healthy. 

  3. Cut the stem under water to prevent air entering the xylem. 

  4. Cut the stem at an angle to provide a large surface area in contact with the water.

  5. Dry the leaves. 
• The importance of transpiration?

  • Transports useful mineral ions up the plant
  • maintains cell turgidity
  • supplies water for growth, cell elongation and photosynthesis
  • supplies water that as it evaporates can keep the plant cool on a hot day.
• What are the factors affecting transpiration?

  Humidity, temperature, air movement, light intensity, water availability
• How does temperature increase transpiration?

  A higher temperature will increase the rate of transpiration in three ways. It will:

  1. increase the rate of evaporation from the cell surfaces so that the water-vapour potential in the leaf rises

  2. increase the rate of diffusion through the stomata because the water molecules have more kinetic energy

  3. decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf. 
• How does humidity affect transpiration?

  Higher relative humidity in the air will decrease the rate of water loss. This is because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside.
• How does air movement affect transpiration?

  Air moving outside the leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a high water vapour potential gradient.