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CIE Categories Archives: 8. Transport in Plants

8.4) Translocation

 8.4) Translocation

 

Translocation: is the movement of sucrose and amino acids in the phloem, from regions of production (the ‘source’) to regions of storage or to regions where they are used in respiration or growth  (the ‘sink’).

  • This is the movement of sucrose and amino acids in the phloem tubes of the plant.
  • Glucose is very important as it makes many other important nutrients.
  • For Example, Glucose is used to make sucrose.
  • Sucrose then enters the phloem
  • The phloem then transports the sucrose all across the leaf where it can be made used of.

 

Some parts of a plant can act as a source and a sink at different times during the life of a plant:

E.g. while a bud containing new leaves is forming it would require nutrients and therefore act as a sink.

However, once the bud has burst and the leaves are photosynthesising, the region would act as a source, sending newly synthesised sugars and amino acids to other parts of the plant.

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8.3) Transpiration

8.3) Transpiration

 

Transpiration: is the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by the diffusion of water vapour through the stomata.

 

  • The main force that draws water from the soil and through the plant is caused by this.
  • Water evaporates from the leaves and causes a kind of ‘suction’, which pulls water up the stem.
  • The water travels up the xylem vessels in the vascular bundles and this flow of water is called the transpiration stream.
  • Root → Stem → Leaf

 

Factors affecting the rate of transpiration:

Describe how water vapour loss is related to cell surfaces, air spaces and stomata:

  • Transpiration is the loss of water vapour from the leaf;
  • Water in the mesophyll cells form a thin layer on their surfaces;
  • The water evaporates into the air spaces in the spongy mesophyll;
  • This creates a high concentration of water molecules in the air spaces.
  • Water vapour diffuses out of the leaf into the surrounding air, through the stomata, by diffusion.

 

The mechanism of water uptake and movement in terms of transpiration producing a tension (“pull”) from above, creating a water potential gradient in the xylem, drawing cohesive water molecules up the plant:

Mechanism of water uptake:

  1. Water enters root hair cells by osmosis (as the water potential in the soil surrounding the root is higher than in the cell);
  2. As the water enters the cell, its water potential becomes higher than in the cell next to it, e.g. in the cortex;
  3. So the water moves by osmosis, into the next cell;
  4. This process is repeated until water reaches the xylem.

 

Mechanism of water movement through a plant:

  1. Transpiration continuously removes water from the leaf;
  2. Thus water is constantly being taken from the top of the xylem vessels, to supply the cells in the leaves;
  3. This reduces the effective pressure at the top of the xylem vessels;
  4. This creates a transpiration stream or ‘pull’, pulling water up;
  5. Water molecules have a strong tendency to stick together. This is called cohesion;
  6. When the water is ‘pulled’ up the xylem vessels, the whole column of water stays together;
  7. Roots also produce a root pressure, forcing water up the xylem vessels.

 

Wilting:

Occurs when the transpiration rate is faster than the rate of water absorption. The amount of water in the plant keeps on decreasing. The water content of cells decreases and cells turn from turgid to flaccid. The leaves shrink and the plant will eventually die.

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8.2) Water uptake

8.2) Water uptake

 

Root hair cells:

The root hairs are where most water absorption happens. They are long and thin so they can penetrate between soil particles, and they have a large surface area for absorption of water.

Water passes from the soil water to the root hair cell’s cytoplasm by osmosis. This happens because the soil water has a higher water potential than the root hair cell cytoplasm:

Osmosis causes water to pass into the root hair cells, through the root cortex and into the xylem vessels

 

The large surface area of root hairs increases the rate of the absorption of water by osmosis and ions by active transport

 

The elongated section of the root hair, basically provides a large surface area for the absorption of water and inorganic ions.

Additionally, the membrane of the root hair cell is semi-permeable. What that means is basically only minerals and water can go through the membrane, but not necessarily go back out.

Investigate, using a suitable stain, the pathway of water through the aboveground parts of a plant.

  • Cut the base, non-leafy end, of a fresh stalk of celery underwater.
  • Place the cut end into a beaker of water stained with red food dye.
  • Leave the celery stalk in bright light at room temperature in a breeze.
  • Note the red lines moving up the stalk and then along the veins of the leafy parts.
  • Cut across the stalk and note the curve of red dots close to the outer edge.
  • If a thin section is examined under the microscope it is the xylem which has been stained red in colour.
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8.1) Transport in plants

8.1) Transport in plants

 

Xylem vessels: transport water and dissolved minerals from the root up to all the other parts of the plant.

Phloem Vessels: The function of Phloem Vessels is to transport food nutrients such as glucose from the leave to other parts of the plant.

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