Transpiration and translocation Notes

What is Transpiration?

1. Transpiration is the loss of water vapor from a plant’s leaves through the stomata.
2. This loss creates a continuous pulling force that draws water upward from the roots through the xylem, forming the transpiration stream.

How Does Transpiration Occur Inside a Leaf?

1. Transpiration occurs when water evaporates from mesophyll cells and diffuses out through the stomata.
2. Water enters the leaf in the xylem, moves into mesophyll cells, and then evaporates into the air spaces.
3. Water vapor exits through stomata whenever they are open.
4. Sequence:
   1. Roots absorb water → enters xylem.
   2. Water travels upward to the leaves.
   3. Mesophyll cells use some water for photosynthesis and turgor.
   4. Remaining water evaporates inside the leaf.
   5. Water vapor diffuses out through stomata.

Why Can Water Move Up the Xylem Without a Pump?

1. Water moves upward because cohesion and tension produce a pulling force throughout the xylem.
2. When water evaporates from the leaf, it pulls on the water molecules below, drawing more water upward from the roots.
3. Key mechanisms
   1. Cohesion: water molecules stick together, forming an unbroken column.
   2. Tension: evaporation at the leaf surface pulls this column upward.

What Factors Affect The Rate of Transpiration?

The rate of transpiration is influenced by several environmental factors:

1. Temperature
   1. Increases in temperature speed up the rate of evaporation, leading to a higher rate of transpiration.
   2. Heat increases the kinetic energy of water molecules, allowing them to evaporate more easily.
2. Humidity
   1. The drier the air, the steeper the concentration gradient between the leaf and the atmosphere, leading to faster transpiration.
   2. Conversely, high humidity slows down transpiration.
3. Wind Speed
   1. Wind removes the moist air surrounding the leaf, increasing the concentration gradient and speeding up transpiration.
4. Light Intensity
   1. Bright light triggers the opening of stomata, allowing more water vapor to escape and increasing transpiration rates.

Why Is Transpiration Important for Plants?

1. Transpiration maintains water flow, cools the plant, and supplies water for photosynthesis.
2. Roles
   1. Maintains the transpiration stream
      1. transports minerals from soil
      2. supplies water for photosynthesis
      3. keeps cells turgid so leaves remain firm
   2. Cools the plant
      1. evaporation removes heat from leaf surfaces
   3. Supports photosynthesis
      1. water is a raw material delivered to the leaves

What Is Translocation?

1. In contrast to transpiration, translocation involves the movement of dissolved sugars, such as sucrose, from the leaves to other parts of the plant.
2. It distributes sucrose from where it is produced or released to where it is needed for growth, storage, or energy use.

How Does Translocation Work in the Phloem?

1. Translocation moves sugars by creating pressure differences between sources and sinks.
2. Sucrose enters the phloem at sources, increasing pressure and pushing the solution toward sinks, where sugars are removed and used.
3. Structure–function points
   1. Phloem cells are alive, allowing active loading of sugars.
   2. Movement can be upward or downward depending on plant needs.
   3. Transport requires energy, unlike transpiration.

How Do Transpiration and Translocation Differ?

They differ in their direction, purpose, transport structures, and energy use.