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Transport in Plants


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9. Transport in Plants


Summary

Features and adaptations of the root hair cell

 Feature                Adaptation
 Long and narrow
Increases surface area to volume ratio, thus increasing rate of absorption of water and mineral salts
 Has cell surface membrane    
Maintains high concentration of sugars, amino acids and salts in cell sap --> results in lower water potential than soil solution so water can enter by osmosis
 LivingCan provide energy for active transport of ions into cell

Features and adaptations of the xylem

 Feature                Adaptation
 Empty lumen without protoplasm or cross walls
Reduces resistance to water flow, allowing for rapid transport of water
Walls thickened with lignin
Lignin is hard and rigid --> provides mechanical support, prevents collapse of vessel

Features and adaptations of the phloem

 Feature                Adaptation
Companion cells have many mitochondria
Provides energy for companion cell to load sugars from mesophyll cells into sieve tubes by active transport
Sieve plates have holes   
Allows rapid flow of manufactured food substances through sieve tubes

Cross section of a stem


Cross section of a root


Absorption of water in the roots

Transpiration Pull


Structured Question Worked Solutions

1. The diagram below shows a xylem vessel.
Are xylem vessels living or dead structures? Give a reason.

Solution

They are dead structures as they do not contain protoplasm.


2. What are the main functions of xylem vessels?

Solution

To transport water and dissolved mineral salts from the roots to the stems and leaves, and to provide mechanical support to the plant.


3. State three ways in which xylem vessels are adapted to their functions.


Solution
  • The xylem does not have any cross walls and protoplasm - this enables water to move easily through the lumen
  • Lignin deposited on the walls helps strengthen the walls and prevent the vessel from collapsing
  • When bundled together, the xylem vessels provide mechanical support to the plant.

4. Explain the processes involved in the movement of water from the soil into the conducting tissue of a plant root.


Solution
  • The absorption of water takes place through the root hairs.
  • The root hairs grow between the soil particles and are in close contact with the water surrounding them. The sap in the root hair cell is a relatively concentrated solution of sugars and various salts.
  • Thus, the sap has a lower water potential than the soil solution.
  • These two solutions are separated by the partially permeable cell surface membrane of the root hair cells. Water therefore enters the root hairs by osmosis.
  • The entry of water dilutes the sap.
  • The sap of the root hair cell becomes more diluted than that of the next cell.
  • Therefore, water passes by osmosis from the root hair cells into the other inner cells of the cortex. This process continues until the water enters the xylem vessels and moves up the plant.
  • The living cells around the xylem vessels in the root use active transport to pump the mineral salts or ions into the vessels. This lowers water potential in the xylem vessels.
  • Water therefore passes from living cells into the xylem vessels by osmosis and flows upwards.

5. Products of photosynthesis, such as sucrose, are translocated via the phloem from sites of production in green leaves to places where they are utilised, such as seeds, fruits, roots, and new leaves. These places are known as 'sinks'

a. Describe a pathway by which a sucrose molecule is transported from the leaf to a sink such as a fruit.
b. Describe an experiment that can demonstrate the process described in (a).
c. Suggest and explain one reason why a sucrose molecule may be transported to a particular sink and not to other sinks.


Solution

a. A pathway is the route taken by the sucrose molecule from the cells in the leaf to the fruit. The sucrose molecule moves from the mesophyll cells in the leaf to the phloem in a vascular bundle of the leaf. This is followed by moving to the phloem in a vascular bundle of the fruit and finally to the cells of the fruit.

b. step 1: cut off a complete ring of bark including the phloem and cambium from the main stem of a woody twig. This will leave the xylem exposed. Place the twig in water with the ring immersed.
step 2: prepare another twig that has a cut ring above the water level.
step 3: set up a control using an unringed twig
step 4: observe the twigs daily. Note where roots or swellings appear. Make drawings of observations.

c. Sucrose can be converted into glucose. Glucose is required for tissue respiration. Sucrose will first be transported to sinks which have higher rates of metabolic activity, such as growing points (shoots and root tips)


6. Describe how water from the capillary tube enters the shoot to reach one of the leaves.


Solution

Transpiration by the leaves creates a transpiration pull, which sucks water up the xylem vessels to the leaves, water in the container enters the shoot to replace that lost in transpiration. Capillary action, whereby water tends to move up very narrow tubes, also allows water to move through the capillary tube and through the xylem vessels in the stem to reach the leaves.
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