GSEB Class 11 Biology Solutions Chapter 11 Transport in Plants

Question 1. What are the factors affecting the rate of diffusion?
Answer: Factors that influence the speed of diffusion include:

• Concentration gradient.
• Membrane's ability to let substances pass through.
• Temperature.
• Pressure.

Exam Tip: When discussing diffusion, remember to include key physical factors like concentration, membrane properties, and environmental conditions to ensure a complete answer.

Question 2. What are porins? What role do they play in diffusion?
Answer: Porins are proteins which create large openings in the outer membranes of plastids, mitochondria, and some bacteria, letting molecules, even those as big as small proteins, move across. When an external molecule binds to the transport protein, this protein then rotates to release the molecule inside the cell. For instance, water channels consist of 8 different aquaporin types.
In simple words: Porins are special proteins in cell walls that act like big gates. They let small molecules and even tiny proteins pass into or out of cells, helping with movement of substances.

Exam Tip: When defining porins, remember to mention their protein nature, pore-forming ability, location (outer membranes), and the size of molecules they permit to pass.

Question 3. Describe the role played by protein pumps during active transport in plants.
Answer: Active transport requires energy to move molecules contrary to their concentration gradient. Active transport is performed by membrane proteins. Hence, different proteins in the membrane have an important function in both active as well as passive transport. Pumps are proteins that utilize energy to transport materials across the cell membrane.
These pumps are able to move materials from a region of low concentration to one of high concentration. The transport rate achieves its highest rate once all protein transporters are occupied or fully loaded. Like enzymes, the carrier protein is highly particular about the substances it moves over the membrane. These proteins are affected by inhibitors that interact with protein side chains.
In simple words: Protein pumps help plants move things inside them, even when there's already a lot of that thing. They need energy to do this, and they can only work so fast before they get full.

Exam Tip: For active transport questions, highlight the requirement of energy, movement against a concentration gradient, and the involvement of specific protein pumps that can become saturated.

Question 4. Explain why pure water has the maximum water potential.
Answer: Water molecules have kinetic energy. In liquid and gaseous form they move randomly, quickly, and constantly. When a system contains more water, its kinetic energy or its 'water potential' will be higher. Therefore, it is clear that pure water will exhibit the highest water potential. The water potential of pure water at standard temperatures, which is not under any pressure, is considered to be zero.
In simple words: Pure water has the most energy to move because it's not mixed with anything. This means it has the highest 'water potential', which we set as zero for comparison.

Exam Tip: When explaining pure water's maximum water potential, emphasize its high kinetic energy and the absence of solutes, which by convention, sets its water potential to zero.

Question 5. Differentiate between the following:
(a) Diffusion and Osmosis
Answer: Diffusion is a type of passive movement that can happen anywhere, and substances move from a high concentration to a lower one. Osmosis occurs through a semi-permeable membrane. A semi-permeable membrane is not a requirement for diffusion.
In simple words: Diffusion is when things spread out on their own, anywhere. Osmosis is similar but it specifically means water moving through a special filter-like wall.

Exam Tip: When differentiating diffusion and osmosis, clearly state that diffusion is for any substance and can occur anywhere, while osmosis specifically involves water movement across a semi-permeable membrane.

Question 5. (b) Transpiration and Evaporation
Answer: Transpiration is the loss of water through evaporation from the plant's aerial sections, mainly from leaves, but also from stems, flowers, and roots. Leaf surfaces have tiny openings, called stomata, surrounded by guard cells. Together, these structures are known as stomata. Leaf transpiration occurs through stomata and is considered a necessary trade-off for opening stomata to permit carbon dioxide gas to move in from the air for photosynthesis. Transpiration also cools plants and allows for the bulk movement of mineral nutrients and water from roots to shoots.
In simple words: Transpiration is when plants sweat out water vapor, mostly from their leaves. Evaporation is just any water turning into vapor.

Exam Tip: To distinguish between transpiration and evaporation, emphasize that transpiration is a biological process in plants involving stomata, while evaporation is a general physical process of water turning into vapor.

Question 5. (c) Osmotic Pressure and Osmotic Potential
Answer: The osmotic potential is described as a solution's ability to draw in water if divided from another solution by a semi-permeable membrane. It is always a negative number. A more negative osmotic potential value for a solution means it will draw in more water.
The terms isotonic, hypotonic and hypertonic characterize the variation in osmotic pressure among solutions with a particular osmotic potential. Solutions are isotonic if their osmotic potentials are the same. If they differ, the solution with higher potential will exert less pressure, and the one with lower potential will have greater pressure.
In simple words: Osmotic potential shows how much a liquid wants to pull water into itself. Osmotic pressure is the force needed to stop that pulling. They are related to how much stuff is dissolved in the water.

Exam Tip: Clearly define osmotic potential as the tendency to absorb water and osmotic pressure as the opposing force. Remember that osmotic potential is typically negative.

Question 5. (d) Imbibition and Diffusion
Answer: Imbibition is a unique kind of diffusion where water is taken up by solid colloids, making them greatly expand in size. Common instances of imbibition include seeds and dry wood absorbing water. Imbibition can also be seen as diffusion because water moves down a concentration gradient; seeds and similar materials contain very little water, so they readily take it up. A water potential difference between the absorbing material and the liquid it absorbs is crucial for imbibition. Moreover, for any substance to absorb a liquid, a natural attraction between the solid and the liquid is also necessary.
In simple words: Imbibition is a special kind of diffusion where solid things soak up water and swell up, like seeds. It's still diffusion because water moves where there's less of it, but it needs an attraction between the solid and the liquid.

Exam Tip: When comparing imbibition and diffusion, emphasize that imbibition is a specific case of diffusion involving water uptake by solids, leading to a significant increase in volume.

Question 5. (e) Apoplast and Symplast pathways of movement of water in plants.
Answer: Within a plant, the apoplast represents the open space for diffusion located outside the plasma membrane. It is broken by the Casparian strip in roots, air gaps among plant cells, and the plant's cuticle. In terms of structure, the apoplast consists of the continuous cell walls of neighboring cells and the areas outside the cells, creating a tissue-level section similar to the symplast. The apoplastic route helps in moving water and dissolved substances throughout a tissue or organ. This process is known as apoplastic transport.
A plant's symplast is the area inside the plasma membrane where water can easily diffuse. Plasmodesmata permit the direct passage of small molecules like sugar, amino acids, and ions between cells. Larger molecules, including transcription factors and plant viruses, can also be moved with assistance from actin structures.
In simple words: Apoplast is like the 'outside' path for water in a plant, moving through cell walls and spaces. Symplast is the 'inside' path, where water moves through the living parts of cells, connected by tiny channels.

Exam Tip: When explaining apoplast and symplast pathways, define apoplast as the extracellular route through cell walls and symplast as the intracellular route through cytoplasm connected by plasmodesmata.

Question 5. (f) Guttation and Transpiration.
Answer: Guttation is the phenomenon of water drops appearing on vascular plants, like grasses. During the night, transpiration typically does not happen as most plants keep their stomata shut. If the soil has a lot of moisture, water will move into the plant roots due to the roots' water potential being less than that of the soil solution. Water will build up inside the plant, causing a small amount of root pressure. This root pressure pushes some water out through specific leaf tips or edges, called hydathodes, forming visible drops. Root pressure gives the driving force for this movement. Transpiration, conversely, occurs due to a transpiration pull.
In simple words: Guttation is when plants push out small drops of water from their leaves, often at night, due to root pressure. Transpiration is the normal process of water evaporating from leaves during the day.

Exam Tip: Differentiate guttation and transpiration by noting that guttation involves liquid water exudation due to root pressure, primarily at night, while transpiration is water vapor loss due to a transpiration pull, mainly during the day.

Question 5. (g) Briefly describe water potential. What are the factors affecting it?
Answer: Water potential is the stored energy of water compared to pure free water under standard conditions. It measures how likely water is to move from one place to another because of osmosis, gravity, physical pressure, or surface tension effects. Water potential is given in pressure units and is often shown using the Greek letter Psi.
In simple words: Water potential is like the 'energy' water has to move. It tells us which way water will flow, affected by things like dissolved stuff, pressure, and gravity.

Exam Tip: When defining water potential, remember to mention it as a measure of water's potential energy and its tendency to move, influenced by solutes, pressure, and gravity.

Question 6. Briefly describe water potential. What are the factors affecting it?
Answer: Water potential \( (\Psi_w) \) is a key idea for grasping water movement. It shows the stored energy of water compared to pure water in standard situations. Pure water holds the greatest water potential, typically set as zero. The key elements shaping water potential are solute potential \( (\Psi_s) \) and pressure potential \( (\Psi_p) \). Solute potential lowers water potential, making it more negative, as dissolved substances lessen the available energy of water molecules. Pressure potential, which is generally positive, boosts water potential, for example, the turgor pressure found in plant cells. The total water potential is determined by \( \Psi_w = \Psi_s + \Psi_p \).
In simple words: Water potential explains how water moves. It's highest in pure water (which is zero) and gets lower when you add dissolved stuff or if there's less pressure. It's figured out by combining the 'solute potential' and 'pressure potential'.

Exam Tip: When describing water potential, always define it and identify solute potential and pressure potential as its primary components, explaining how each affects the overall value.

Question 7. What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Answer: If an increased pressure, above atmospheric levels, is put to pure water or a solution, its water potential goes up.
In simple words: When you push on pure water or a solution with more force than the air usually does, the water's potential to move will rise.

Exam Tip: Remember that applying pressure above atmospheric levels always raises the water potential, making water more likely to move out of that system.

Question 8. (a) With the help of well-labeled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.
(b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.
Answer:
(a) Plasmolysis happens when water exits the cell, causing the plant cell's membrane to pull away from its cell wall. This condition arises when a cell or tissue is put into a hypertonic solution, meaning it has more dissolved substances than the protoplasm. Water exits, first from the cytoplasm, and then from the vacuole. When water leaves the cell by diffusion into the fluid outside the cell, the protoplast pulls back from the walls. The cell becomes plasmolyzed. Water moved through the membrane from a region of high water potential (the cell) to an area of lower water potential (outside the cell). If the cell is in an isotonic solution, there is no overall movement of water into or out of it. If the liquid outside balances the cytoplasm's osmotic pressure, it is called isotonic. When water entry and exit are balanced, the cells are termed flaccid.
(b) When cells are put into a hypotonic solution (one with higher water potential or a weaker solution than the cytoplasm), water moves into the cell, making the cytoplasm push against the wall, which is known as turgor pressure. The force produced by the protoplasts because water enters and pushes against the stiff walls is called pressure potential \( \Psi_p \). Due to the stiff nature of the cell wall, the cell does not burst. This turgor pressure is what eventually drives the cell's expansion and growth.
In simple words: When a plant cell is in a liquid with more water, water rushes into the cell. This makes the cell swell up and press against its wall, creating 'turgor pressure,' which helps the plant grow and stops the cell from bursting.

Exam Tip: When describing plasmolysis, focus on water loss in hypertonic solutions and the cell membrane pulling away. For hypotonic solutions, explain water intake, turgor pressure, and how the cell wall prevents bursting.

Question 9. How is mycorrhizal association helpful in the absorption of water and minerals in plants?
Answer: Some plants have extra structures linked to them that aid in absorbing water and minerals. Mycorrhiza represents a cooperative relationship between a fungus and a plant's root system. The fungal threads create a web around the young root or even go inside the root cells. The fungal hyphae offer a huge surface area, taking in mineral ions and water from a much bigger soil volume than a plant root alone could. The fungus gives minerals and water to the roots, and in return, the roots supply sugars and nitrogen-rich compounds to the mycorrhizae.
In simple words: Mycorrhiza is a team-up between a fungus and plant roots. The fungus helps the plant get more water and minerals from the soil because its tiny threads spread out widely. In return, the plant gives the fungus food.

Exam Tip: When explaining mycorrhizal association, emphasize the symbiotic nature, the fungus's role in increasing absorption surface area, and the mutual exchange of nutrients (water/minerals for fungus, sugars/nitrogen for plant).

Question 10. What role does root pressure play in water movement in plants?
Answer: When different ions from the soil are actively moved into the root's vascular tissue, water follows its potential gradient, increasing the pressure within the xylem. This positive force is termed root pressure and can cause water to rise to low levels in the stem. Root pressure offers only a small force in the general process of water transport. It clearly does not significantly contribute to water moving up tall trees. The most important role of root pressure might be to restore the unbroken water columns in the xylem, which frequently rupture due to the strong pulling forces from transpiration. Root pressure is not responsible for most water transport; most plants get what they need through transpiration pull.
In simple words: Root pressure is a weak push that helps water move up a little in plants, usually at night. It forms when roots take in minerals and water, increasing pressure inside. It's not strong enough for tall trees, but it helps fix broken water columns.

Exam Tip: Explain root pressure as a positive pressure generated in roots, capable of pushing water to small heights. Clarify its limited role in overall water transport and its importance in re-establishing water columns.

Question 11. Describe the transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Answer: Transpiration is the loss of water through evaporation from plants. This mostly happens through tiny openings called stomata on the leaves. Oxygen and carbon dioxide also move in and out of the leaf through these stomata. Typically, stomata stay open during the day and shut at night. This is caused by changes in the guard cells' turgidity (swelling). The guard cell's inner wall, which faces the pore, is thick and flexible. As turgidity rises within the two guard cells surrounding each stomatal opening, their thin outer walls expand, pushing the inner walls into a curved form. The stomatal opening also benefits from how the microfibrils are arranged in the guard cell walls. Elements influencing Transpiration include temperature, light, humidity, and wind speed. The significance of Transpiration is that it helps in the movement of liquids and minerals.
In simple words: Transpiration pull is how plants draw water up from roots to leaves, like sipping through a straw, as water evaporates from tiny leaf pores (stomata). Factors like heat, light, air moisture, and wind affect how much water evaporates. It helps plants get water and minerals.

Exam Tip: When describing the transpiration pull model, explain how water loss from leaves creates a continuous pull. List key environmental factors (temperature, light, humidity, wind) and mention its dual role in water transport and cooling.

Question 12. Discuss the factors responsible for the ascent of xylem sap in plants.
Answer: The transpiration driven ascent of xylem sap relies mainly on these physical characteristics of water:
1. Cohesion – the way water molecules attract each other.
2. Adhesion – water molecules being drawn to charged surfaces, like those of tracheary elements.
3. Surface Tension – water molecules sticking together more strongly in liquid form than in gas form.
These features give water strong tensile strength, meaning it can withstand a pulling force, and high capillarity, which is its capacity to climb in narrow tubes. In plants, the small width of tracheary elements, such as tracheids and vessel elements, assists capillarity. Photosynthesis needs water. Xylem vessels, running from root to leaf vein, can deliver the necessary water. However, what power does a plant employ to shift water molecules into the leaf parenchyma cells where they are required? When water evaporates from the stomata, the continuous thin layer of water on the cells causes water to be drawn, molecule by molecule, into the leaf from the xylem. Additionally, since there is less water vapor in the atmosphere compared to the spaces inside the leaf, water moves into the air. This generates a 'pull'. Studies show that the forces produced by transpiration are powerful enough to raise a column of water, similar to what's in xylem, over 130 meters high.
In simple words: Water climbs plants through xylem thanks to three water properties: 'cohesion' (water molecules stick together), 'adhesion' (water sticks to tube walls), and 'surface tension'. These make water strong and able to rise in tiny tubes. The main driver is the 'transpiration pull,' where water evaporating from leaves literally pulls more water up from the roots, strong enough for very tall plants.

Exam Tip: When discussing xylem sap ascent, always explain the cohesion-tension theory, detailing the roles of cohesion, adhesion, and surface tension, and how transpiration creates the essential pulling force.

Question 13. What essential role does the root endodermis play during mineral absorption in plants?
Answer: Unlike water, not all minerals can be taken up passively by the roots. Consequently, most minerals have to enter the root through active uptake into the epidermal cell cytoplasm. This process requires energy in the form of ATP. The active absorption of ions contributes to the water potential gradient in roots, thus aiding water uptake by osmosis. A few ions also passively move into the epidermal cells. Specialized proteins in root hair cell membranes actively push ions from the soil into the epidermal cell cytoplasms. Transport proteins in endodermal cells act as control points, allowing a plant to regulate the amount and kinds of dissolved substances that get to the xylem. It is crucial to remember that the root endodermis, thanks to its suberin layer, can actively move ions in just one direction.
In simple words: The root endodermis acts like a gatekeeper. It controls which minerals get into the plant's xylem. Because of a special layer called suberin, it makes sure minerals only move in one direction, often actively pumping them and needing energy to do so, while also affecting water movement.

Exam Tip: Focus on the endodermis as a control point for mineral entry into the xylem. Emphasize the role of active transport, the Casparian strip (implicitly by suberin), and the unidirectional movement of ions.

Question 14. Explain why xylem transport is unidirectional and phloem transport bi-directional.
Answer: Because the source-sink connection changes, materials in the phloem can move either up or down, making it bi-directional. This differs from the xylem, where movement is always one-way, meaning upwards. Therefore, unlike water's single-direction flow in transpiration, food in phloem sap can be moved in any necessary direction, as long as there's a sugar source and a destination (sink) to use, store, or get rid of the sugar. Phloem sap mainly contains water and sucrose, but other sugars, hormones, and amino acids are also carried through the phloem.
In simple words: Xylem moves water only upwards, from roots to leaves, like a one-way street. Phloem, however, moves food (sugars) both up and down, depending on where the plant needs it, like a two-way street, because food sources and needs change.

Exam Tip: Explain xylem's unidirectional flow by linking it to water absorption by roots and transpiration pull. For phloem, emphasize its bidirectional nature, driven by the variable 'source-sink' relationship for sugars and nutrients.

Question 15. Explain the pressure-flow hypothesis of translocation of sugars in plants?
Answer: The widely accepted method for moving sugars from where they are made (source) to where they are needed (sink) is known as the pressure-flow hypothesis. When glucose is made at the source, like through photosynthesis, it transforms into sucrose, a type of sugar. This sucrose then moves into companion cells and further into the living phloem sieve tube cells through active transport. This loading action at the source creates a hypertonic state within the phloem. Water from the nearby xylem shifts into the phloem due to osmosis. As osmotic pressure increases, the phloem sap will travel towards regions with less pressure. Active transport is once more needed to move sucrose out of the phloem sap and into cells that will convert the sugar into energy, starch, or cellulose. When sugars are taken away, the osmotic pressure drops, and water leaves the phloem. In summary, sugars start moving in the phloem from the source, where they are actively loaded into a sieve tube. This loading in the phloem establishes a water potential difference that promotes the bulk flow within the phloem.
Phloem tissue consists of sieve tube cells, forming extended columns with openings in their end walls, known as sieve plates. Cytoplasmic threads extend through these sieve plate holes, creating unbroken strands. When hydrostatic pressure inside the phloem sieve tube rises, pressure-driven flow starts, and the sap travels through the phloem. At the sink, incoming sugars are actively moved out of the phloem and changed into complex carbohydrates. The removal of dissolved substances results in a high water potential in the phloem, and water then exits, eventually returning to the xylem.

Exam Tip: When explaining the pressure-flow hypothesis, clearly outline the process: sugar loading at the source creates high osmotic pressure, water moves in from xylem, generating turgor pressure that drives mass flow, and sugar unloading at the sink reduces pressure, causing water to return to xylem.

Question 16. What causes the opening and closing of guard cells of stomata during transpiration?
Answer: Typically, stomata stay open during the day and shut at night. The primary reason stomata open or close is a shift in the guard cells' turgidity (swelling). Each guard cell's inner wall, which faces the pore, is thick and flexible. As turgidity rises within the two guard cells surrounding each stomatal opening, their thin outer walls expand, pushing the inner walls into a curved form. The stomatal opening also benefits from how the microfibrils are arranged in the guard cell walls. Cellulose microfibrils are arranged outwards from the center, not lengthwise, which helps the stoma open more easily. When guard cells lose their turgor because of water loss or stress, the flexible inner walls return to their first shape, making the guard cells flaccid and closing the stoma.
In simple words: Stomata open and close because of changes in 'turgor' (water pressure) inside the guard cells. When guard cells swell with water, they curve outwards, opening the stomata. When they lose water, they become floppy, and the stomata close. The special shape and fibers in the guard cell walls help this movement.

Exam Tip: Explain that stomatal opening and closing are controlled by guard cell turgidity. When guard cells gain water, they swell and open the pore; when they lose water, they become flaccid and close it, aided by specific wall structures.