NCERT Class 11-Biology: Chapter –11 Transport in Plants Part 9

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Question 23:

Explain the mass flow hypothesis of transport in phloem.


The mass flow hypothesis of transport in phloem is as follows:

Mass Flow Hypothesis proposed by Munch in 1930 explains the movement of sap via the phloem When there is a high concentration of sugar present in the source (which is where the food is prepared) then a diffusion gradient gets created between the sugar source and the sugar shrink where the sugar is stored. This is responsible for drawing water into the cells from the neighbouring xylem. When this happens, turgor pressure gets created (bidirectional movement).

A part of the glucose produced by photosynthesis is converted into sucrose. This is then transported actively to the companion cells and then to the sieve tubes via diffusion. The osmotic pressure moves the sucrose through the sieve tube towards the sink. When the sucrose reaches the sink, it is removed into apoplast. Water moves out of the sieve tube by osmosis establishing a pressure gradient. A low turgor pressure is created at the sink to maintain this gradient. Water passes back into the xylem later again.

Thus the mass flow hypothesis’ mechanism is dependent on the turgor pressure as well as the osmotic pressure gradient between the source and the sink.

Question 24:

Observe the diagram and answer the following;

Chapter 11 - Question 24 Image of Observe the Diagram

Chapter 11 - Question 24 Image of Observe the Diagram

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a. Are these types of guard cells found in monocots or dicots?

b. Which of these shows a higher water content (i) or (ii)?

c. Which element plays an important role in the opening and closing of stomata?


(a) These types of guard cells are found in dicots because as we know the shape is bean shaped. Whereas for monocots, it is usually dumble-shaped.

(b) Higher water content is shown in figure (i) because as we can see in that figure the guard cells are showing turgidity and the stoma is open carrying out transpiration as well as photosynthesis.

(c) The element that plays an important role in the opening and closing of stomata is potassium ion. Potassium is responsible for the k+ or malate ion hypothesis. In short in this theory, the change in turgor pressure of the guard cells which is responsible for opening and closing of the stomata results in absorption andloss of K+ ions by guard cells.

Question 25:

Define Uniport, Symport and Antiport m. Do they require energy?


Uniport, Symport and antiport are kind of facilitated diffusion where the diffusion is facilitated by carrier proteins and channel proteins along a concentration gradient so energy is not required directly. Symporters and antiporters though are also involved in active transport where they require energy.

Long Answer Type Questions

Question 1:

Minerals are present in the soil in sufficient amounts. Do plants need to adjust the types of solutes that reach the xylem? Which molecules help to adjust this? How do plants regulate the type and quantity of solutes that reach xylem?


Yes, plants need to adjust the type and quantity of solutes that reach the xylem. The transport proteins of end dermal cell help in maintaining and adjusting solute movement. As the minerals are” present in soil as charged particles with a very low concentration as compared of roots, they, all cannot be completely passively transported across cell membranes of root hairs. Thus, minerals are transported both by active and passive processes, to the xylem. Upon reaching xylem, they are further transported, i.e., upwards to sinks through transpiration stream. At the sink regions mineral ions are unloaded through diffusion and active uptake by receptor cells. Some of the mineral ions moving frequently through xylem are

(i) Nitrogen travels in plants as inorganic ions, but much of the nitrogen moves in the form of amino acids and related organic compounds.

(ii)Sulphur and Phosphorus small amount of these two nutrients are carried in organic forms.

(iii) Mineral ions are frequently remobilised particularly from older senescing parts. Older dying leaves export much of their mineral content to younger leaves. Similarly, before leaf fall in decidous plants, minerals are removed to other parts. Elements most readily mobilised are phosphorus, sulphur, nitrogen and potassium. Some elements that are structural components like calcium are not remobilised.

Question 2:

Plants show temporary and permanent wilting. Differentiate between the two. Do any of them indicate the water status of the soil?


Difference between permanent and temporary wilting is as follows:

In temporary wilting, plants lose turgidity when rate of transpiration is more than rate of water absorption from the soil but in temporary wilting, wilting of the plant occurs and is a permanent phenomenon as the soil is unable to meet water requirement of the plant.

In temporary wilting, the plant regains turgidity when water requirement is met whereas in permanent wilting, the damage occurred is permanent causing eventual death of the plant.