sieve plates (end walls of sieve tube elements) is perforated with many tiny pores

provide sieve tube with nutrients and energy to aid sieve tube element to carry out metabolic processes

• absorption of water takes pace through the root hairs
  

1. root hairs grow between the soil particles, and they are in close contact with the surrounding soil particles

   
   

2. each soil particle has a thin film of liquid surrounding it. Soil solution is a dilute solution of mineral salts

   
   

3. cell sap in the root hair is more concentrated due to the presence of sugars (from the leaves) and mineral salts (from active transport) Thus it has a lower water potential than the soil solution and thus the water enters the root hair via osmosis

   
   

4. entry of water dilutes the root hair's cell sap and thus sap of the root hair has a higher water potential than that of the neighbouring cell.

   
   

5. Water thus passes by osmosis from the root hair cells to the inner cells of the cortex, and up until the water enters the xylem vessels and moves up the plant

• entry of minerals salts into roots
  Root hair cells can absorb dissolved mineral salts by active transport or diffusion depending on the reletive concentration of a type of a mineral salt ion

• Root pressure is pressure resulting from the constant entry of water into the roots

  
  

• cells that surround the xylem vessels in the root actively pump mineral salts into it

  
  

• water potential in the xylem vessel decreases, creating a water potential gradient

  
  

• causing water to enter it by osmosis, and this pressure pushes the water column up the xylem

  
  

• however it is minor as it could not bring water all the way to the tops of tall trees

  
  

• however in small plants it can causes guttation, forcing after through special openings in the leaves of some plants, in the night and early morning when transpiration is low and the moisture is high

• capillary action is the tendency of water to move up narrow tubes due to:

  
  

• cohesion: attraction of water molecules to one another

• adhesion: attraction of water molecules to the walls of the narrow tube (xylem vessel)

  
  

• the narrower the tube, the higher the water rises

loss of water vapour through the aerial parts of the plant, mainly through the stomata of the leaves

transpiration is a consequence of gaseous exchange in plants. how so?

stomata need to open for gaseous exchange, allowing for water to evaporate from the surfaces of the spongy mesophyll cells.

water vapour is lost to the atmosphere due to the lower concentration of water vapour in the atmosphere

1. Water in xylem vessel enters a mesophyll cell and moves from cell to cell via osmosis

   
   

2. Water exits mesophyll cells to form a thin film of moisture over the cell surfaces

   
   

3. Water exits mesophyll cells to form water vapour to form water vapour in the intercellular air spaces

   
   

4. Water vapour accumulates in the intercellular air spaves

   
   

5. Water vapour from air spaces diffuses through the stomata into the drier air outside the leaf, in a process known as transpiration

   
   
   

[continued above]

6. Movement of water out of the cells to replace the thin film of moisture that has evaporated decreases the mesophyll cell's sap's water potential

   
   

7. The mesophyll cells absorb water via osmosis from the cells deeper in the leaf

   
   

8. These cells absorb water from the xylem vessels

   
   

9. resulting in a production of a suction force or tension that pulls the column of water in the xylem vessels up, in a process known as transpiration pull.

   
   
   

[continued below at 1]

• Transpiration pull is the pulling force caused by transpiration as illustrated below

  
  

• water movement through the plant begins with the loss from the leaves and is completed with water absorption from the soil through the roots

  
  

• water is pulled up the xylem in a continuous unbroken column called a transpiration stream

  
  

• how is it possible for water to be pulled up in an unbroken column?

  
  

• cohesion prevents water column from breaking apart and adhesion prevents water from slipping back down

• mass Potometer (loss in mass/time taken)

• bubble potometer (loss in volume/time taken)

  
  

• Note amt of water lost through transpiration ≠ amt of water absorbed by the plant (i.e. the reading on the potometer)

• as some of the water absorbed is used up in photosynthesis

  
  

• but it is assumed that rate of transpiration ∝ rate of water absorbed as little of water is used for photosynthesis

• Cools the plant by removing latent heat for vaporisation
• brings water to leaves for photosynthesis
• delivers mineral salts along with the water in transpiration stream from roots to other parts of the plant
• Brings water to various plant parts to maintain cell turgidity
• TURGOR PRESSURE allows non-woody plant parts to maintain shape and upright position

• excessive transpiration
• plant cells becomes flaccid
• Leaves often wilt first
• Guards cells are flaccid, thus stomata close and the rate of transpiration is reduced
• REDUCING WATER LOSS obv

• Advantages
• Reduces rate of transpiration and thus prevents excessive water loss
• Disadvantages
• Stomata close, decreasing intake of CO2
• Leaves droop and decrease the absorption of sunlight
• decreasing the rate of photosynthesis

• As temperature increases, water molecules gain kinetic energy and move faster
• Thus rate of evaporation of water from mesophyll cells and the rate of diffusion of water vapour out through the stomata increases --> Rate of transpiration increases

• Water vapour that diffuses our of the stomata tend to accumulate in a layer around the leaf
• Wind blows away at this layer of water capour maintaining the water vapour concentration gradient the air spaces in the leaf and the atmospheric air
• Rate of transpiration increases as wind speed increases

• Intercellular air spaces are very saturated with water vapour
• Thus air humidity plays a part in regulating the concentration gradient
• In dry air, the humidity is low, and thus there is a steeper gradient in the concentration gradient between the air spaces in the leaf and the surrounding air, thus the rate of transpiration is high

• In the day, the stomata open, thus rate of transpiration increases
• At night, the stomata close, thus rate of transpiration decreases

Impermeable to water which reduces rate of transpiration from cuticle surface (i.e. cuticular respiration)

Traps water vapour closer to the leaf, increasing the humidity around the stomata, thus reducing the transpiration rate

1. Dissolved sucrose is moved from sugar source (leaf cell into a companion cell)
2. Companion cell loads sucrose into a sieve tube element by active transport
3. Concentration of sucrose in the sieve tube element increases, thus water potential decreases. Water moves by osmosis from nearby xylem vessels into sieve tubes, thus hydrostatic pressure increases
4. High turgor pressure pushes the cell sap down the sieve tube
5. The companion cell diffuses sucrose from the sieve tube element or active transport to the root cell
6. Concentration of sucrose in the sieve tube element decreases, thus water potential increases. Water moves by osmosis out of the sieve tubes into the surrounding cells, thus the hydrostatic pressure decreases
7. Mist of this water enters the xylem vessels and is transported back up