Transport in Plants


Question#1): What is the primary function of transport system?

Answer: The primary function of transport system is to maintain a link between all cells of the body and the external environment. It transports the nutrients and water to the points where they are to be used.


Question#2) Describe the importance of water for plants and other organisms?

Answer: Water plays an important role in our daily life:

  1. ) It is an important active constituent of living matter, the protoplasm.
  2. ) It serves as a medium in many physiological processes.

3) It is a universal solvent and a medium of transport.

Question#3) Write a short note on transport of water and food in plants?



Transport of water: Water is vital to plant life. It is necessary not just for photosynthesis and turgor, but much of the cellular activities occur in the presence of water molecules. Internal temperature of plant body is also regulated by water. Land plants get water and minerals from soil. After absorption by roots, water and minerals have to be transported to the aerial parts of body.

Transport of food: food is manufactured in leaves (by photosynthesis). This food is transported to other parts of body for utilization and storage. All land plants (except for mosses and liverworts), have developed complex vascular tissues (xylem and phloem) that move water and food throughout plant body.


Recalling: Cells are the primary sites for metabolic processes. That is why they are regarded as the units of life.


Question#4) How water, ion and water uptake occurs in plants?  “OR”  How would you relate the internal structure of root with the uptake of water and salts?

Answer: Plants have two types of vascular tissues called xylem and phloem. Xylem transport water and minerals from roots to leaves. Most the gymnosperms lack these vessels and they have tracheids. Each tracheid is an elongated, dead cell with lignified walls, and intact end walls.

Water travels upward through two paths: –

  1. i) Along the cell wall.
  2. ii) Through the cells.

Uptake of water and minerals by Root:

  • First of all root hairs absorbs water and minerals from soil due to their spongy nature. Root hairs provide large surface area for absorption.
  • Water from soil, move towards root through osmosis.
  • The entry of water into the root hairs dilutes its sap.
  • Now sap of root hair is more than next cell. So water from hairs move into the

root cells.

  • The water from the epidermal cells then passes into the xylem vessel and

ascends up the stem.

  • This upward movement of water and salts is called ascent of sap.


For your information: Water always moves from an area of higher water potential to an area of lower water potential. The relationship between the concentration of solute and water potential is inverse. When there is a lot of solute ( i.e. hypertonic solution), the water potential is low and vice versa.


For your information: Plants also form beneficial relationships with soil bacteria and fungi in order to increase absorption of minerals.


Question#5) Define transpiration and relate it with cell surface and with stomatal opening and closing? 3. How do different factors affect the rate of transpiration?

Answer: Transpiration: • The loss of water in the form of vapour from aerial parts of the plants, especially

through stomata is called transpiration.

  • Potometer is used to measure the rate of transpiration.
  • The difference of osmotic pressure is present between lower and upper parts of

a plant. This difference produces a pull or tension, which pull the water form high

pressure to lower pressure. This pull or suction force is produced in xylem tissue

and is called transpiration pull.

  • Due to transpiration pull of water molecules, water moves up in xylem as an

unbroken channel called transpiration stream.

Common types of transpiration: Most common types of transpiration are; 1) Stomatal transpiration: Transpiration through stomata is called Stomatal transpiration. 2)cuticular transpiration: Transpiration takes place through cuticle is called cuticular transpiration. 3) Lenticular transpiration: Transpiration through lenticles is called lenticular transpiration.


Factors affecting the transpiration:

Temperature: –

  1. i) High temperature increases the rate of transpiration.
  2. ii) Low temperature decreases the rate of transpiration.

Humidity: –

  1. i) In dry conditions or low humidity the rate of transpiration increases.
  2. ii) In wet conditions or high humidity the rate of transpiration decreases.

Light: –

  1. i) In day time, in presence of light stomata remains open. They cause

increase in rate of transpiration.

  1. ii) In night time, in darkness stomata remain closed. They cause decrease in

rate of transpiration.

Air movement:

Wind (air in motion) carries away the evaporated water from leaves and it causes an increase in the rate of evaporation from the surfaces of mesophyll. When air is still, the rate of transpiration is reduced.


Atmospheric Pressure: –

  1. i) Low atmospheric pressure increases the rate of transpiration.
  2. ii) High atmospheric pressure decreases the rate of transpiration.

Leaf surface area:

The rate of transpiration also depends upon the surface area of leaf. More surface area provides more stomata and there is more transpiration .

Question#6) Transpiration is a necessary evil. Give comments? OR  What are the advantages or importance of transpiration?


Answer: Importance of transpiration:

  • Transpiration is very important in the plant life as it provides the forces to pull the

sap up in the xylem vessels form root to the leaves.

  • Transpiration also keeps the cell surface moist to facilitate the exchange of gases

in dissolved state.

  • Tiny openings present on the lower side of leaves are called stomata. Each stoma

is surrounded by sausage shaped cells called guard cells.

  1. i) Stomata remain open when guard cells are turgid.
  2. ii) Stomata remain close when the guard cells loose their turgidity.


For your information: Roughly 90% of the water that enters a plant is lost via transpiration.


Question#7) How opening and closing of stomata regulates transpiration?


Opening and closing of stomata and Transpiration: The stomata regulate the transpiration by the action of guard cells. he two guard cells of a stoma are attached to each other at their ends. The inner concave sides of guard cells that enclose a stoma are thicker than the outer convex sides.

When guard cells get water and become turgid, their shapes are like two beans and the stoma between them opens. When guard cells loose water and become flaccid, their inner sides touch each other and stoma closes.

The concentration of solutes (glucose) in guard cells is responsible for the opening and closing of stomata. Recent studies have revealed that light causes the movement of potassium ions from epidermal cells into guard cells. Water follows these ions and enters guard cells. Thus their turgidity increases and stoma opens. As the day progresses, guard cells make glucose i.e. become hypertonic.

So water stays in them. At the end of the day, potassium ions flow back from guard cells to epidermal cells and the concentration of glucose also falls. Due to it, water moves to epidermal cells and guard cells loose turgor. It causes the closure of stoma.


For your information: There is strong evidence that even mild water stress results in reduced growth rate in plants.


Question#8) Explain the movement of water in terms of transpirational pull?


According to cohesion tention theory, the force which carries water (and dissolved materials) upward through the xylem is transpirational pull. The pulling force created by the transpiration of water is called transpirational pull. Transpiration creates a pressure difference that pulls water and salts up from roots.

When a leaf transpires (loses water), the water concentration of its mesophyll cells drops. This drop causes water to move by osmosis from the xylem of leaf into mesophyll cells. When one water molecule moves up in the xylem of the leaf, it creates a pulling force that continues all the way to root.


Transpirational pull causes water to move transversely (from root epidermis to cortex and pericycle). Following are the reasons for the creation of transpirational pull.

  • Water is held in a tube (xylem) that has small diameter.
  • Water molecules adhere to the walls of xylem tube (adhesion). The forces of attraction between unlike molecules is called adhesive forces. Or the forces of attraction between liquid molecules and the wall of materials in which it was placed is called adhesive forces(adhesion).
  • Water molecules cohere to each other (cohesion). The attractive forces between same liquid substances that resist separation between molecules of liquids is called cohesive forces.

These attractions make an overall tension among water molecules. This tension forms ‘columns’ of water. The columns of water move from root to shoot and the water content of the soil enters in these ‘columns’.


Question#9)Describe the theory of pressure flow mechanism to explain the translocation of food in plants?

Answer: TRANSPORT OF FOOD : The transport of food in plants is called translocation.

Tissues responsible for translocation: Phloem is responsible for transporting food substance throughout plant body.

Sucrose formation : •The glucose formed during photosynthesis in mesophyll cells, is used in respiration and the excess of it is converted into sucrose. In most plants, food is transported in the form of sucrose.

Pressure flow mechanism (Hypothesis): Transport of food is through pressure-flow mechanism. This theory propose that water containing food molecules flows under pressure through the phloem.

In pressure-flow mechanism, food is moved from sources to sinks. Sources include the exporting organs, typically a mature leaf or storage organ. Sinks are the areas of active metabolism or storage e.g. roots, tubers, developing fruits and leaves, and growing regions.

A storage organ is capable of storing food and exporting the stored materials. For example; root of beet is a sink in first growing season, but becomes source in next growing season, when sugars are utilized in the growth of new shoots.

At source, food (sugars) is moved by active transport into the sieve tubes of phloem. Due to the presence of sugar in sieve tubes, their solute concentration increases and water enters them from xylem (via osmosis). This results in higher pressure of water in these tubes, which drives the solution of food towards sink.

At the sink end, food is unloaded by active transport. Water also exits from the sieve tubes. The exit of water decreases pressure in sieve tubes, which causes a mass flow from the higher pressure at the source to the now lowered pressure at the sink.


For your information: Xylem is a one way street from roots to leaves for water and salts. Phloem is a two way street for food. The direction of the movement of food is decided by supply and demand in sources and sinks

For your information: Plants need a lot of water. Young Brassica plants take up an amount of water equal to their shoot weight in about 5 hours – if that applied to us, we would have to drink 3 gallons of water an hour to stay alive!

Recalling: Some invertebrates like arthropods have open circulatory system.

Question: What  What are lenticels and where are they found in plant body?

Answer: pores on the stem of a woody plant that allows gas exchanges between the atmosphere and the internal tissues. These are located on woody parts of the plant such as stem and branches.

Q)What is the role of potassium ions in the opening of stomata?

Answer: Recent studies have revealed that light causes the movement of potassium ions from epidermal cells into guard cells. Water follows these ions and enters guard cells.

Thus their turgidity increases and stoma opens. As the day progresses, guard cells make glucose i.e. become hypertonic. So water stays in them. At the end of the day, potassium ions flow back from guard cells to epidermal cells and the concentration of glucose also falls. Due to it, water moves to epidermal cells and guard cells loose turgor. It causes the closure of stoma.

Question) Define the cohesion-tension theory?

Answer: Cohesion tention theory explains how water move from roots to the tip of the plant. According to this theory the main mechanism involve in this process is transpiration pull. That is developed due to the attractive forces between the molecules.

Question) What do you mean by sources and sinks according to the pressure flow mechanism?

Answer: Sources are parts of plants where fixation of carbon dioxide occurs and food is formed. Sinks are the parts where the foods are stored and used in plants.

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