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Class 12 Bio

1. Water Relation Syllabus

Unit 2: Water Relation Notes

2. Plant Physiology
2.1 Water relation: Introduction and significance of diffusion, osmosis, and plasmolysis, ascent of sap, transpiration and guttation.
2.2 Photosynthesis: Introduction and significance, photosynthetic pigments, mechanism (photochemical phase and Calvin-Benson cycle), C₃ and C₄ plants, photorespiration, factors affecting photosynthesis.
2.3 Respiration: Introduction and significance, types, mechanism (glycolysis, Kreb cycle, electron transport system), factors affecting respiration.
2.4 Plant hormones: Introduction, physiological effects of auxins, gibberellins and Cytokinins.
2.5 Plant growth and movement: Concept on seed germination, dormancy, photoperiodism, vernalization, senescence; plant movements (tropic and nastic).

These water relation notes follow the NEB Class 12 Biology syllabus (+2 Biology, Nepal), chapter 2.1 of Unit 2 Plant Physiology. Practise the multiple choice questions first, then work through the solved short and long answer questions with their diagrams, and finish with the PDF notes for quick revision.

Micro syllabus: 2. Plant Physiology

Describe diffusion, osmosis, plasmolysis, ascent of sap, transpiration, and guttation. (this chapter)
Define photosynthesis and explain its mechanism, pigments, C₃ and C₄ pathways, and influencing factors.
Explain respiration, its types, mechanism, and influencing factors.
Define phytohormones and describe the physiological roles of auxins, gibberellins, and cytokinins.
Describe seed germination, dormancy, photoperiodism, vernalization, senescence, and plant movements.

Read more about plant physiology, osmosis and transpiration on Wikipedia. More notes: Class 12 Biology notes, Class 12 Chemistry notes and Class 12 Physics notes.

2. Water Relation Notes: Multiple Choice Questions

Question 1. What is the cause of diffusion?

  • a) Sunlight
  • b) Kinetic energy
  • c) Chemical energy
  • d) Potential energy

Correct Answer: b) Kinetic energy

Reason: Diffusion occurs because of the free (kinetic) energy of the moving molecules, ions or particles.

Question 2. What is the medium, where diffusion occurs, called?

  • a) Water
  • b) Diffusant
  • c) Diffusate
  • d) Imbibant

Correct Answer: b) Diffusant

Reason: The medium in which particles spread is the diffusion medium or diffusant. The spreading particles are the diffusate.

Question 3. What is the direction of diffusion of molecules?

  • a) From higher concentration to lower concentration
  • b) From lower concentration to higher concentration
  • c) It has no fixed direction
  • d) Against the concentration gradient

Correct Answer: a) From higher concentration to lower concentration

Reason: Diffusion is a passive process that goes down the concentration gradient until equilibrium.

Question 4. What is true about transpiration through stomata?

  • a) It is not diffusion process.
  • b) Diffusion of water droplets from leaf to air.
  • c) Diffusion of water vapours from leaf interior to air.
  • d) Diffusion of water vapours from air to leaf interior.

Correct Answer: c) Diffusion of water vapours from leaf interior to air.

Reason: Vapour pressure is higher in the sub-stomatal cavity than outside, so vapour diffuses out through the stoma.

Question 5. Which of the following acts as semi-permeable membrane?

  • a) Primary wall
  • b) Lignified wall
  • c) Suberized wall
  • d) Cell membrane

Correct Answer: d) Cell membrane

Reason: The plasma membrane lets water pass but restricts most solutes. Cell walls are freely permeable or waterproof.

Question 6. When does endosmosis occur?

  • a) When any cell is placed in hypotonic solution
  • b) When a living cell is placed in hypotonic solution
  • c) When any cell is placed in hypertonic solution
  • d) When a living cell is placed in hypotonic solution

Correct Answer: b) When a living cell is placed in hypotonic solution

Reason: Water enters a living cell by osmosis when the outside solution is hypotonic. Only a living cell has a semi-permeable membrane.

Question 7. A plant cell has undergone exosmosis. Which statement is correct?

  • a) Its turgor pressure goes on increasing.
  • b) Cell accumulates more water and becomes turgid.
  • c) Protoplasm gets dehydrated and cell becomes flaccid.
  • d) Cell neither becomes turgid nor becomes flaccid and remains in normal state

Correct Answer: c) Protoplasm gets dehydrated and cell becomes flaccid.

Reason: Exosmosis is the loss of water from the cell, so the protoplasm shrinks and turgor falls.

Question 8. What is true about osmosis?

  • a) It occurs through any membrane.
  • b) It occurs through semi-permeable membrane.
  • c) Solvent molecules move from hypertonic solution to hypotonic solution.
  • d) Solute molecules move from hypertonic solution to hypotonic solution.

Correct Answer: b) It occurs through semi-permeable membrane.

Reason: Osmosis is the movement of water through a semi-permeable membrane, from hypotonic to hypertonic solution.

Question 9. What is the significance of plasmolysis?

  • a) Seed swells when soaked in water
  • b) Foul smell of dumping site spreads in surrounding.
  • c) Bacteria can not survive in pickles/ jams.
  • d) Cell bursts due to expansion of protoplasm.

Correct Answer: c) Bacteria can not survive in pickles/ jams.

Reason: Salt and sugar make a hypertonic medium, so bacteria lose water and are plasmolysed.

Question 10. Which of following material shows imbibition?

  • a) Tin
  • b) Plastic
  • c) Rock
  • d) Wood

Correct Answer: d) Wood

Reason: Imbibition is the absorption of water by hydrophilic colloids like wood, seeds and gum. Tin, plastic and rock do not absorb water.

Question 11. What is true about imbibition?

  • a) It is endothermic process.
  • b) It causes swelling (increase in volume).
  • c) It is due to presence of hydrophobic molecules.
  • d) It occurs in living materials only.

Correct Answer: b) It causes swelling (increase in volume).

Reason: Imbibants take up water and swell. The process gives out heat, and it occurs in dead materials too.

Question 12. The path of ascent of sap is tracheary elements of xylem. Which of following are tracheary elements?

  • a) Vessels and xylem fibres
  • b) Vessels and xylem parenchyma
  • c) Vessels and tracheids
  • d) Tracheids and xylem fibres

Correct Answer: c) Vessels and tracheids

Reason: Tracheids and vessels are the water-conducting elements of xylem.

Question 13. What is the cause of ascent of sap?

  • a) Transpiration pull
  • b) Root pressure
  • c) Atmospheric pressure
  • d) Imbibition

Correct Answer: a) Transpiration pull

Reason: Loss of water from the leaves creates a suction (tension) that pulls the water column up through the xylem.

Question 14. A plant excretes water droplets from leaf margins, mainly during humid nights or mornings. Which of the following plants exhibits such phenomenon?

  • a) Cactus
  • b) Pinus
  • c) Mango
  • d) Colocasia

Correct Answer: d) Colocasia

Reason: Guttation is common in herbaceous plants such as Colocasia, tomato, balsam and grasses.

Question 15. Which part of leaf is responsible for guttation?

  • a) Stomata
  • b) Lamina
  • c) Cuticle
  • d) Hydathode

Correct Answer: d) Hydathode

Reason: Hydathodes are special pores at leaf tips and margins that exude water under root pressure.

3. Water Relation Notes: Short and Long Answer Questions

Short Answer Questions

Q.1 Observe the figure below and answer the questions.
Class 12 Biology Unit 2 Water relation notes - diffusion of dye molecules in water

Figure: Water relation notes: diffusion of dye molecules in water

a) What process does the figure represent?

The figure shows diffusion. The dye molecules spread from the place of high concentration to the place of low concentration until they are evenly mixed (equilibrium).

b) What causes the spreading of dye molecules in water?

The spreading is caused by the free (kinetic) energy of the dye and water molecules. They keep moving at random, and the dye moves down its concentration gradient.

c) In this process, what do water molecules act as?

Water acts as the diffusion medium (diffusant), and the dye molecules are the diffusing particles (diffusate).

d) What effect would be seen if temperature of water is increased?

The kinetic energy of the molecules rises, so the dye spreads faster. The rate of diffusion increases with temperature.
Q.2 Define osmosis. Write its significance in plants.

Osmosis is the movement of water molecules from their higher concentration to their lower concentration through a semi-permeable membrane. A solution with a higher concentration of water is called hypotonic, and one with a lower concentration of water is called hypertonic. So water moves from the hypotonic solution to the hypertonic solution.

Significance of osmosis in plants

Plant roots absorb water from the soil by osmosis.
Water moves from cell to cell in the plant parts by osmosis.
Osmosis makes the cells turgid or flaccid.
The opening and closing of stomata is due to the osmosis of the guard cells.
Osmosis plays a vital role in the dehiscence of anther, sporangium and fruit.
The growth of radicle and plumule during seed germination is due to the force developed by osmosis.
Q.3 Mention the two conditions necessary for osmosis. Does osmosis take place in non living cell/object? Why?

Conditions necessary for osmosis

The two solutions must have unequal concentration of water, that is, one is hypotonic and the other is hypertonic.
The two solutions must be separated by a semi-permeable membrane.

Osmosis in a non-living cell or object. It does not take place in a dead (non-living) cell. When a cell dies, its plasma membrane loses the semi-permeable nature and becomes freely permeable, so the water cannot be held by a membrane and no net osmosis occurs. In a non-living object osmosis can take place only if it has a semi-permeable membrane, for example an egg membrane or a cellophane bag in an osmometer.

Q.4 What is endosmosis? What is its effect on plant cell?

When a living plant cell is placed in water or a hypotonic solution, water enters the cell by osmosis. This inward movement of water is called endosmosis. It happens because the cell sap is hypertonic and the surrounding solution is hypotonic.

Effect on the cell: water collects in the vacuole, so the vacuole grows and the protoplast expands. The protoplast presses on the cell wall, which gives the turgor pressure (TP), and the wall presses back with an equal wall pressure (WP). At the end the cell becomes fully expanded and turgid. During endosmosis the osmotic pressure (OP) of the cell falls and the TP rises. In a fully turgid cell OP = TP, so the diffusion pressure deficit (DPD) is zero and no more water enters.

Class 12 Biology Unit 2 Water relation notes - Endomosis

Figure: Water relation notes: endosmosis and exosmosis in a plant cell

Q.5 In the given figure, a cell has undergone successive changes, when it is placed in a solution. Answer the following questions.
Class 12 Biology Unit 2 Water relation notes - turgid cell to plasmolysed cell

Figure: Water relation notes: successive stages of a plant cell from turgid to plasmolysed

a) Is the solution hypotonic or hypertonic?

The solution is hypertonic. The cell is losing water, so the outside solution has more solute and less water than the cell sap.

b) What type of osmosis is it?

It is exosmosis, the movement of water out of the cell.

c) The protoplast of cell has shrunken to its minimum volume. What is this phenomenon called?

This is plasmolysis. In cell d the protoplast has shrunk away from the cell wall, so the cell is fully plasmolysed.

d) What happens if the cell ‘d’ is placed in water. What is it called?

Water enters the cell by endosmosis, the protoplast swells and regains its original shape, and the cell becomes turgid again. This recovery is called deplasmolysis.
Q.6 Write significance of plasmolysis.
Class 12 Biology Unit 2 Water relation notes - turgid cell to plasmolysed cell

Figure: Water relation notes: successive stages of a plant cell from turgid to plasmolysed

It helps to find out whether a cell is living or dead, because plasmolysis takes place only in living cells.
It demonstrates exosmosis in plant cells.
Salting of pickles, meat and fish, and sweetening with jam and jellies, prevent microbial infection, because the bacteria are plasmolysed and cannot survive.
Grasses (weeds) can be killed by spraying excess salt on the ground, because of plasmolysis in their root cells.
It proves that the cell wall is permeable in nature.
Q.7 Define ascent of sap. Mention three important points which make ascent of sap possible in plants.
Class 12 Biology Unit 2 Water relation notes - balsam twig in eosin dye

Figure: Water relation notes: balsam twig placed in eosin dye (ascent of sap)

The upward movement of water and minerals from the roots to the leaves and other parts of the plant, against gravity, is called ascent of sap.

Path: epiblema, cortex, endodermis, pericycle, protoxylem, metaxylem, xylem of stem, xylem of branch, xylem of leaf petiole, xylem of leaf (midrib, vein, veinlet) and then the mesophyll cells of the leaf.

Three important points that make it possible

Unbroken channel of xylem. The tracheary elements (tracheids and vessels) of xylem make a continuous channel of sap from the root to the leaf.
Cohesion and adhesion. Water molecules attract each other (cohesion) and also stick to the walls of the xylem (adhesion). So the water column can bear tension and does not break.
Transpiration pull. The loss of water from the leaf surface creates a suction force in the leaf xylem. This tension is passed down to the root xylem, and the sap rises continuously.
Q.8 Observe the figure given below and answer the questions.
Class 12 Biology Unit 2 Water relation notes - balsam twig in eosin dye

Figure: Water relation notes: balsam twig placed in eosin dye (ascent of sap)

a) Why do white flowers of the twig turn into red?

The twig takes up the red eosin dye with water. The dye is carried upward through the xylem by the ascent of sap and reaches the flowers, so the white flowers turn red.

b) Can colour change be seen in other parts too? If yes, in which part?

Yes. The red colour can be seen along the stem and in the veins (midrib and veinlets) of the leaves, because the dye passes through them with the water.

c) Transverse section of the twig under the microscope was observed as in above figure. What can be concluded from it?

In the transverse section only the xylem is stained red. So it is concluded that water and minerals are conducted upward through the xylem, and xylem is the path of the ascent of sap.
Q.9 Define transpiration and mention its types.

The loss of water in the form of vapour from the aerial parts of a plant is called transpiration. The water is released in the purest state through stomata, cuticle or lenticels by diffusion.

Types of transpiration

Stomatal transpiration: evaporation of water through the stomata. It is about 80 to 90 % of the total transpiration.
Cuticular transpiration: loss of water through the cuticle of the leaf, about 3 to 10 %. It increases when stomata are closed in dry conditions, and is more in herbaceous, shade-loving plants with a thin cuticle.
Lenticular transpiration: loss of water through lenticels of the bark of woody plants and twigs. It is very small, about 0.01 %.
Q.10 Write about the role of stomata in transpiration.

Stomata are minute pores in the epidermis of the aerial parts, and each is guarded by two guard cells. They are the main path of transpiration (80 to 90 %), so they control the amount of water lost by the plant.

Opening: when water enters the guard cells, turgor pressure increases. The thin outer wall stretches more outward and pulls the thick inner wall with it, so the pore opens.
Closing: when water leaves the guard cells, they become flaccid and the inner walls come together, so the pore closes.
Stomata generally open during the day and close at night.
Guard cells are kidney-shaped in dicots and dumb-bell shaped in monocots.
Dicot leaves have more stomata on the lower surface, and monocot leaves have them almost equally on both surfaces.
Vapour passes out through the open stoma, so the stomata also allow the exchange of gases.
Class 12 Biology Unit 2 Water relation notes - Water relation notes: opening and closing of a stoma by the guard cells

Figure: Water relation notes: opening and closing of a stoma by the guard cells

Q.11 How do diffusion and osmosis play vital role in stomatal transpiration? Give your views.

Stomatal transpiration is carried out in two steps, and both processes help in it.

Osmosis. Water from the xylem of the leaf vein moves into the neighbouring mesophyll cells and then from cell to cell by osmosis. This keeps the cell walls of the mesophyll wet. Osmosis in the guard cells also opens and closes the stoma by changing their turgor.
Evaporation and diffusion. Water evaporates from the wet walls of the mesophyll cells into the intercellular spaces and the sub-stomatal cavity. The water vapour pressure there becomes higher than in the outer air, so the vapour diffuses out through the open stoma.

So osmosis supplies the water and keeps the stomata working, and diffusion carries the vapour out of the leaf.

Q.12 Describe the role of light and temperature in transpiration.

Light. Light makes the stomata open, so transpiration is high in the day and low at night. High light intensity also raises the temperature of the leaf, which increases evaporation. The quality of light matters too: stomata do not open in green, UV and infra-red light.

Temperature. A rise in temperature increases the kinetic energy of water molecules, so evaporation from the mesophyll cells increases, and the air can hold more vapour. So the rate of transpiration rises with temperature. But if the temperature is too high (about 30 to 35 °C), the stomata close and the rate of transpiration falls.

Q.13 List out the advantages and disadvantages of transpiration.

Transpiration has many uses but it can also harm the plant, so it is called a necessary evil.

Advantages

It helps in the conduction of water and minerals to different parts.
It maintains the water balance of the plant, because water is continuously removed.
It maintains osmosis and keeps the cells rigid (turgid).
It creates a suction force (transpiration pull) for the ascent of sap.
Certain hydrophilic salts accumulate on the leaf surface and keep the leaves moist.
It helps in cell division by maintaining turgidity, and optimum transpiration helps in proper growth.
The cooling effect of a tree is due to the evaporation of water from its leaves.

Disadvantages

If the transpired water is not made up by absorption from the soil, transpiration slows down, and the plant may wilt.
A lot of energy is released (lost) during transpiration.
Plenty of unnecessary water is absorbed by the plant in the process.
Q.14 Write differences between transpiration and guttation.
TranspirationGuttation
Takes place through stomata, cuticle and lenticels.Takes place through hydathodes.
Water is lost as vapour.Water is lost as liquid droplets.
Takes place mainly in the daytime.Takes place generally in the early morning or at night.
Prevalent at high temperature.More common at cooler temperature.
Favoured by dry conditions.Hindered by dry conditions and increases in humid conditions.
Involves simple diffusion.Does not involve diffusion.
A controlled process, regulated by the opening and closing of stomata.An uncontrolled process, because hydathodes do not open and close.
Does not need root pressure.Is driven by root pressure.
Only pure water evaporates.Sugars, salts and amino acids may be lost with water.
Gives a cooling effect to the plant.Does not give a cooling effect.

Long Answer Questions

Q.1 Define transpiration. Discuss the various factors affecting transpiration in plants. (1+7)

Transpiration is the loss of water in the form of vapour from the aerial parts of plants, mainly through the stomata, cuticle and lenticels.

Factors affecting transpiration

A. External factors

1. Relative humidity: when the relative humidity of the air increases, the rate of transpiration decreases, and the reverse.
2. Carbon dioxide concentration: for the stomata to open, the CO₂ concentration should be below 300 ppm. A higher concentration closes the stomata.
3. Temperature: transpiration increases with the rise in temperature. But when the temperature is too high (30 to 35 °C), the stomata close and transpiration falls.
4. Light: light opens the stomata, and high light intensity raises the temperature, which increases transpiration. Stomata do not open in green, UV and infra-red light.
5. Wind velocity: a wind of 20 to 30 km per hour favours transpiration, because it removes the vapour from near the leaf. At 40 to 50 km per hour the stomata close and the rate decreases.
6. Atmospheric pressure: an increase in atmospheric pressure decreases the rate of transpiration.
7. Water availability: the rate of transpiration is directly proportional to the water available in the soil. More soil water gives more transpiration, and less water gives less.

B. Internal factors

1. Leaf area: a larger leaf area has more stomata, so transpiration is more. In xerophytes the leaf area is reduced, the leaves fall early, or they are changed into spines.
2. Leaf structure: a thick cuticle reduces transpiration. More stomata give a high rate. Stomata present only on the lower surface reduce the rate. In xerophytes such as Pinus and Nerium the stomata are sunken in small cavities with hairs, which lower the rate.
3. Leaf modifications: some plants have modified leaves to reduce water loss, for example phylloclade in Opuntia, cladode in Asparagus and phyllode in Acacia melanoxylon.

4. Water Relation Notes PDF

PDF Notes 1: Plant Physiology (Unit 2)

PDF Notes 2: Water Relation (Chapter 2.1)

5. Water Relation: Frequently Asked Questions

What is water relation in plant physiology? (Water relation notes)
Water relation in plant physiology is the study of how plants take in, move and lose water. It includes diffusion, osmosis, plasmolysis, imbibition, ascent of sap, transpiration and guttation. Stephen Hales (1677-1761) is called the father of plant physiology for his work on transpiration and the movement of fluids in plants.
What are diffusion, osmosis and plasmolysis?
Diffusion is the random movement of particles from a region of higher concentration to a region of lower concentration, caused by kinetic energy. Osmosis is the movement of water from a hypotonic to a hypertonic solution through a semi-permeable membrane. Plasmolysis is the shrinking of the protoplast of a living cell when it loses water by exosmosis in a hypertonic solution, and the reverse process is deplasmolysis.
What is ascent of sap and what causes it?
Ascent of sap is the upward movement of water and minerals from the root to the leaves through the tracheary elements (tracheids and vessels) of xylem. It is caused by the transpiration pull, with the help of the cohesion and adhesion of water molecules that keep the water column unbroken.
What are the types of transpiration?
The types are stomatal transpiration (80 to 90 percent), cuticular transpiration (3 to 10 percent) and lenticular transpiration (about 0.01 percent). Stomatal transpiration happens through the stomata, which are opened and closed by the guard cells.
What is the difference between transpiration and guttation?
Transpiration is the loss of water as vapour, mainly in the daytime through stomata, cuticle and lenticels, and it is controlled by the guard cells. Guttation is the loss of liquid water droplets through the hydathodes at leaf margins, mainly at humid nights and early mornings, and it is driven by root pressure.
Where can I get plant physiology class 12 pdf, plant physiology class 12 notes and plant physiology class 12 notes pdf download?
This page has two PDF notes that you can read online in the PDF section: the PDF notes of Unit 2 Plant Physiology and the PDF notes of the chapter Water Relation. They work as plant physiology class 12 pdf and plant physiology class 12 notes pdf. If a download option is shown in the viewer, you can use it to download the plant physiology class 12 pdf free of cost. The other chapters of the unit (photosynthesis, respiration, plant hormones, plant growth and movement) are listed in the syllabus.
Where can I get water relation pdf, plant water relations PDF and water relation ppt?
The water relation pdf of the chapter is in the PDF section of this page, where you can read the plant water relations PDF online. For a water relation ppt, the figures and answers on this page can be used to make your own slides for study.
Where can I get a plant water relation assignment and plant water relation theory?
The solved questions on this page cover the plant water relation theory of the NEB syllabus: diffusion, osmosis, plasmolysis, ascent of sap, transpiration and guttation. You can use these answers and diagrams to prepare a plant water relation assignment, and the textbook project work on guttation (listing ten plants that show guttation) can be added to it.
Where can I get Class 12 Biology Book PDF Nepal and Class 12 Biology Book PDF Asmita Publication?
The Class 12 Biology book of Asmita Publication is a textbook of its publisher, and this page does not claim to host it. These notes follow the NEB Class 12 Biology syllabus of Unit 2 Plant Physiology, solve the exercise of the chapter Water Relation and add diagrams, so you can use them together with the textbook.
Where can I get Class 12 biology notes, Class 12 biology pdf and Class 12 Biology Notes NEB for this chapter?
This page gives the Class 12 biology notes NEB of the chapter Water Relation (chapter 1 of Unit 2): syllabus, micro syllabus, solved multiple choice questions, short and long answers and the notes PDF. The Class 12 biology notes pdf of the other units are on the Class 12 Biology page of the website.
Where can I get Class 12 Biology notes in the style of Note Mandir?
Students who look for notes in the style of Note Mandir will find this chapter in the same way: syllabus, micro syllabus, solved exercise with diagrams and PDF notes. These are NEB Class 12 Biology notes for Unit 2, chapter Water Relation.

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