8.2 Transport in Plants

Short Introduction You have learnt that plants absorb nutrients from the soil which are then utilized in the leaves to prepare organic nutrients by photosynthetic activity. These organic solutes are also to be moved to different parts of the plant body. When organic solutes are utilized in plant body, some waste compounds are also produced which are then removed out of the plant body. All these movements of materials are generally referred to as transport in plants.

Main Idea

generated by DeepSeek to PDF

Transport in plants occurs at three different levels: (1) from outside to inside the plant body (absorption), (2) within the plant body from one place to another (transport), and (3) from inside to outside the plant body (removal of waste). The root is the main organ for absorbing water and minerals. Roots possess root hairs to increase surface area. Water and minerals are taken up by active and passive processes. Water is lost through transpiration, and food is transported through the phloem via translocation. The upward movement of water and minerals through xylem is called ascent of sap, explained by the TACT theory.

Core Concept

Need of Transport In a plant body, transport occurs at three different levels: 1. First level (Outside to inside): Materials are moved from outside to inside the plant body. Plants need mineral nutrients and water to maintain life activities, which are absorbed from the soil. Gases like CO₂ and O₂ are obtained from air. 2. Second level (Within the plant body): Materials are transported from one place to another. Inorganic nutrients must be transported to the leaves to prepare organic solutes by photosynthesis. These organic solutes are also to be moved to different parts of the plant body for growth and metabolism. 3. Third level (Inside to outside): Materials are moved from inside to outside the plant body. When organic solutes are utilized, some waste compounds are produced which are removed out of the plant body. Structure of Root and Root Hairs Root is an underground part of the plant, which serves as the organ of uptake of mineral nutrients and water. It possesses a large surface area due to an extensive branching system and root hairs, which make them favourable for the organ of uptake. Dicot plants have a tap root which consists of a thick primary root that bears several lateral roots. Monocot plants have a fibrous adventitious root system. Root hairs are the primary sites for the absorption of water and minerals. They are extensions of epidermal cells that increase the surface area for absorption. Uptake of Mineral Salts and Water Plant roots serve two vital functions: anchoring the plant in soil and absorbing essential minerals and water. To support photosynthesis, plants require carbon dioxide, water, minerals, and light. Most minerals enter the root hairs or epidermal cells through bulk flow with water. Some are absorbed through diffusion, facilitated diffusion, or active transport. Passive uptake: Happens through diffusion as minerals move along their concentration gradient through plasmodesmata to cells in the root's cortex, endodermis, and pericycle before reaching the xylem cells. They are then pulled up by transpiration. Facilitated diffusion: Certain nutrients are transported from the soil into the epidermal cells of roots through their cell membranes by carrier proteins. Active transport: Requires energy (ATP) and allows plants to take in minerals even when their concentration inside the root cells is higher than in the soil (against the concentration gradient). It is selective and relies on respiration. Mechanism of Water Absorption by Root generated by DeepSeek to PDF

Absorption of water by roots occurs by means of a passive transport mechanism called osmosis. Osmosis is the movement of water molecules through a partially permeable membrane from an area where they are more concentrated to an area where they are less concentrated. Transpiration Plants absorb water from the soil by the roots. This absorbed water moves to the aerial parts of the plant, from where most of this water (approx. 99%) is lost in the form of vapours into the atmosphere. This loss is called transpiration. 90% of this loss occurs through the stomata present mainly in the leaves and some other aerial parts. This is called stomatal transpiration. Some other amount (7–9%) is lost from the general body surface through the cuticle, called cuticular transpiration. A very little amount (less than 3%) is lost through lenticels (scar-like regions with ruptured epidermis and loosely packed underlying cortical cells) present in the stem. This is called lenticular transpiration. Relation of transpiration with leaf surface area and stomatal opening and closing: Plants with a large surface area of leaves having more stomata show a greater rate of transpiration compared to plants with narrow and reduced leaf surfaces having fewer stomata. More transpiration is generally observed in daytime than at night, as stomata are widely open during the day and nearly closed at night. Factors affecting the rate of transpiration: Temperature: On a sunny day, the air temperature rises, lowering humidity, which increases evaporation from mesophyll cells and raises the transpiration rate. For every 10°C increase in temperature, the rate of transpiration roughly doubles. However, very high temperatures (around 40–45°C) cause stomata to close to conserve water. If high temperatures persist and soil lacks water, plants may wilt and die. Wind: Wind is moving air that speeds up the diffusion of water molecules, leading to a higher rate of evaporation from mesophyll cells and increased transpiration. In calm, still air, diffusion slows down, decreasing the transpiration rate. Humidity: (The text mentions humidity as a factor but the detailed explanation is cut off. In general, high humidity decreases the rate of transpiration, while low humidity increases it.) Ascent of Sap (Transport of Water and Salts) Root epidermal cells absorb water and minerals from the soil. These substances are then transported from the roots to the leaves. This upward movement of water and dissolved minerals through the xylem tissue is known as the "ascent of sap". This process requires a passage and a significant amount of force because it occurs against the force of gravity, especially in tall plants. The passage of ascent of sap is the xylem tissue. The most widely accepted theory that explains the force responsible for ascent of sap is the "TACT theory." Passage of Ascent of Sap: Xylem is a complex and permanent tissue which acts as a passage for ascent of sap. Two types of xylem cells are very important: Tracheids (long, dead cells with pointed ends) and Xylem vessels (long, dead, thick-walled tube-like structures formed by the fusion of vessel cells placed end to end). generated by DeepSeek to PDF

Both are essential for water and mineral transport as well as structural support. TACT Theory: According to the TACT theory, four factors work together to make a force that moves water and minerals up a plant: 1. Transpiration Pull (T): When stomata on leaves are open, water evaporates from the leaves into the air, creating a pulling force known as transpiration. This pull makes water move from the roots to the leaves through the xylem tissue. 2. Adhesion (A): Adhesion is the attraction between water molecules and other substances. Water is strongly attracted to the walls of the xylem cells because both water and cellulose (in cell walls) are polar molecules. This adhesion helps water move upward against gravity and keeps water in the xylem when transpiration is not happening. 3. Cohesion (C): Cohesion is the attraction between nearby water molecules, possible because water is a polar molecule. This force helps hold the water molecules together in a continuous column within the xylem. 4. Tension (T): Tension refers to the tension created in the water column. This force helps hold the water molecules together in a continuous column within the xylem. How TACT Force Works: Imagine the column of water in the xylem as strong as a steel wire. Hydrogen bonds among water molecules create cohesion, holding the water molecules together like a string. As long as transpiration continues (water evaporating from leaves), the string of water remains tense and is pulled upward as a single unit. This bulk flow of water to the top of the plant is driven by solar energy, as it's the evaporation from leaves that causes the transpiration pull. Transport of Organic Solutes (Translocation) The transport of prepared food (organic solutes) to different parts of the plant through the phloem tissue is called translocation. Passage of Translocation: Translocation occurs through phloem tissues, which are complex and permanent plant tissues. Sieve tube elements and companion cells are important components of phloem. Sieve tubes are long tube-like structures that transport organic solutes (like sucrose) throughout the plant. Since sieve tube elements lack nuclei, companion cells next to them control their functions. Together, sieve elements and companion cells act as conducting channels for moving organic solutes. Direction of Translocation: This movement is not just up or down; it goes in various directions. Translocation goes from a "source" to a "sink." A source supplies food (like leaves making food), and a sink uses food (like roots or storage organs). Leaves are sources, fruits are sinks, and stems and roots can act as both. Composition of Translocating Fluid (Phloem Sap): Phloem sap contains about 10–25% dry matter. Most of this dry matter is sucrose (a kind of sugar), with some other organic compounds. Mechanism of Translocation (Pressure Flow or Mass Flow Theory): The most accepted idea for how organic solutes move in plants is the "pressure flow" or "mass flow" theory: generated by DeepSeek to PDF

• Sugars produced in source regions (like leaves or storage areas) are loaded into the phloem's sieve
• This active transport raises the sugar concentration and reduces the water concentration in the phloem,
• Water moves into the phloem through osmosis from nearby xylem cells, increasing hydrostatic pressure
• The pressure difference from the high-pressure source to the lower-pressure sink causes translocation,
• When the solution reaches a sink (like the roots), root cells actively absorb the organic solutes, making the
• The loss of solutes in phloem sap increases solute potential and water potential, causing water to flow

Detailed Explanation

Transport in plants is essential for survival. Plants need to move water, minerals, and organic nutrients throughout their bodies. 1. Three Levels of Transport: Transport occurs at three levels: (1) absorption from the environment into the plant (water, minerals, CO₂), (2) distribution within the plant (water and minerals to leaves; organic solutes to all parts), and (3) removal of waste products. 2. Root Structure and Absorption: The root is adapted for absorption with its extensive branching and root hairs, which increase surface area. Water is absorbed by osmosis (passive transport) — movement from a region of higher water concentration in the soil to a lower concentration in the root cells. Minerals are absorbed by diffusion, facilitated diffusion, or active transport. Active transport requires ATP and allows plants to accumulate minerals against a concentration gradient. 3. Transpiration: Transpiration is the loss of water vapor from aerial parts of the plant. About 90% occurs through stomata (stomatal transpiration), 7–9% through the cuticle (cuticular transpiration), and less than 3% through lenticels (lenticular transpiration). Transpiration is driven by evaporation and is influenced by temperature, wind, and humidity. It creates a pull that draws water up the xylem. 4. Ascent of Sap (TACT Theory): The upward movement of water and minerals through xylem is called the ascent of sap. The TACT theory explains this: Transpiration pull (created by evaporation from leaves), Adhesion (water molecules attracted to xylem walls), Cohesion (water molecules attracted to each other), and Tension (the tension in the water column). These forces work together to pull water up the plant, even against gravity. 5. Translocation (Pressure Flow Theory): The transport of organic solutes through the phloem is called translocation. It moves from a source (where sugar is produced or stored) to a sink (where sugar is used or stored). The pressure flow (mass flow) theory explains this: sugars are actively loaded into sieve tubes at the source, water follows by osmosis, creating high hydrostatic pressure. At the sink, sugars are unloaded, water leaves, and the pressure difference drives the flow.

Intermediate Level

Transport in plants occurs at three levels: absorption from the environment, distribution within the plant, and removal of waste. The root is adapted for absorption with root hairs that increase surface area. Water is absorbed by osmosis, while minerals are absorbed by diffusion, facilitated diffusion, or active transport (requiring ATP). Transpiration is the loss of water vapor from aerial parts of the plant. About 90% occurs through stomata (stomatal transpiration), 7–9% through the cuticle (cuticular transpiration), and less than 3% through lenticels (lenticular transpiration). The rate of transpiration is influenced by temperature (doubles for every 10°C rise, up to a point), wind (increases rate), and humidity (high humidity decreases rate). The ascent of sap is the upward movement of water and minerals through xylem. The TACT theory explains this: Transpiration pull, Adhesion (water to xylem walls), Cohesion (water to water), and Tension (tension in the water column). This creates a continuous column of water pulled up by evaporation from the leaves. Translocation is the transport of organic solutes through phloem. It moves from source to sink. The pressure flow (mass flow) theory explains it: sugars are loaded into sieve tubes at the source, water follows by osmosis, creating high hydrostatic pressure. At the sink, sugars are unloaded, water leaves, and the pressure difference drives flow.

Important Points

• 1. Transport in plants occurs at three levels: outside to inside, within the plant, and inside to outside.
• 2. The root is the organ of uptake, adapted with a large surface area and root hairs.
• 3. Dicots have a tap root; monocots have a fibrous adventitious root system.
• 4. Minerals are absorbed by diffusion, facilitated diffusion, or active transport.
• 5. Water is absorbed by osmosis (passive transport).
• 6. Transpiration is the loss of water vapor from aerial parts of the plant (approx. 99% of absorbed water).
• 7. Types of transpiration: stomatal (90%), cuticular (7–9%), lenticular (<3%).
• 8. Factors affecting transpiration: temperature (doubles per 10°C rise), wind (increases rate), humidity
• (high humidity decreases rate).
• 9. Ascent of sap = upward movement of water and minerals through xylem.
• 10. TACT theory: Transpiration pull, Adhesion, Cohesion, Tension.
• 11. Translocation = transport of organic solutes through phloem.
• 12. Direction: source (leaves) → sink (roots, fruits).
• 13. Pressure flow (mass flow) theory: Sugars loaded at source → water enters by osmosis → high
• hydrostatic pressure → pressure difference drives flow to sink.
• 14. Phloem sap composition: 10–25% dry matter, mostly sucrose.

Remember

• Transport levels: Outside → inside, within plant, inside → outside.
• Root hairs = increase surface area for absorption.
• Water absorption = osmosis.
• Mineral absorption = diffusion, facilitated diffusion, active transport.
• Transpiration = loss of water vapor (99% of absorbed water).
• Stomatal transpiration = 90%; cuticular = 7–9%; lenticular = <3%.
• TACT theory: Transpiration pull, Adhesion, Cohesion, Tension.
• Ascent of sap = water/minerals through xylem.
• Translocation = food through phloem (source → sink).
• Pressure flow theory = sugars loaded at source → water follows → pressure drives flow to sink.
• Phloem sap = 10–25% dry matter, mostly sucrose.

Common Mistakes

• 1. Thinking transpiration is only through stomata: Transpiration also occurs through the cuticle and
• lenticels.
• 2. Confusing ascent of sap and translocation: Ascent of sap is water/minerals through xylem;
• translocation is organic solutes through phloem.
• 3. Forgetting the direction of translocation: Translocation goes from source to sink, not just upward.
• 4. Thinking water absorption is active: Water absorption is passive (osmosis).
• 5. Mixing up TACT factors: T = Transpiration pull, A = Adhesion, C = Cohesion, T = Tension.
• 6. Thinking xylem cells are living: Xylem vessels and tracheids are dead cells.
• generated by DeepSeek to PDF
• 7. Thinking phloem sieve tubes have nuclei: Sieve tube elements lack nuclei and are controlled by
• companion cells.
• 8. Forgetting the effect of temperature on transpiration: Rate doubles for every 10°C rise, but very high
• temperatures (40–45°C) close stomata.

Quick Revision

• Transport in plants occurs at three levels: absorption from the environment, distribution within the plant, and
• removal of waste. The root absorbs water by osmosis and minerals by diffusion, facilitated diffusion, or active
• transport. Root hairs increase surface area for absorption. Transpiration is the loss of water vapor from aerial parts of
• the plant: 90% through stomata, 7–9% through the cuticle, and less than 3% through lenticels. The rate of
• transpiration is affected by temperature, wind, and humidity. The ascent of sap (upward movement of water and
• minerals through xylem) is explained by the TACT theory: Transpiration pull, Adhesion, Cohesion, and Tension.
• Translocation (transport of organic solutes through phloem) moves from source to sink, explained by the pressure
• flow (mass flow) theory: sugars are loaded into sieve tubes at the source, water follows by osmosis, creating high
• hydrostatic pressure, which drives the flow to the sink where sugars are unloaded.

60-Second Revision

• Transport levels: Outside → inside, within plant, inside → outside.
• Root hairs = increase surface area for absorption.
• Water absorption = osmosis.
• Mineral absorption = diffusion, facilitated diffusion, active transport.
• Transpiration = loss of water vapor (99% of absorbed water).
• Stomatal transpiration = 90%; cuticular = 7–9%; lenticular = <3%.
• TACT theory: Transpiration pull, Adhesion, Cohesion, Tension.
• Ascent of sap = water/minerals through xylem.
• Translocation = food through phloem (source → sink).
• Pressure flow theory = sugars loaded at source → water follows → pressure drives flow to sink.
• Phloem sap = 10–25% dry matter, mostly sucrose.
• Frequently Asked Questions (FAQs)
• Q1. What is transport in plants? Answer: Transport in plants refers to the movement of materials — water,
• minerals, organic solutes, and waste products — within the plant body.
• generated by DeepSeek to PDF

• Q2. At what three levels does transport occur in plants? Answer: Transport occurs at three levels: (1) from outside
• to inside the plant body (absorption), (2) within the plant body from one place to another (distribution), and (3) from
• inside to outside the plant body (removal of waste).
• Q3. What is the structure of the root and root hairs? Answer: The root is an underground part of the plant that
• serves as the organ of uptake of mineral nutrients and water. It possesses a large surface area due to an extensive
• branching system and root hairs. Dicot plants have a tap root (thick primary root with lateral roots); monocot plants
• have a fibrous adventitious root system. Root hairs are the primary sites for absorption.
• Q4. How do roots take up water and minerals? Answer: Water is absorbed by osmosis (passive transport).
• Minerals are absorbed by diffusion, facilitated diffusion, or active transport (which requires energy/ATP).
• Q5. What is transpiration? What are its types? Answer: Transpiration is the loss of water vapor from the aerial
• parts of the plant (about 99% of absorbed water). Types: stomatal transpiration (90%, through stomata), cuticular
• transpiration (7–9%, through the cuticle), and lenticular transpiration (less than 3%, through lenticels).
• Q6. What factors affect the rate of transpiration? Answer: Temperature (rate roughly doubles for every 10°C rise,
• but very high temperatures close stomata), wind (increases rate), and humidity (high humidity decreases rate).
• Q7. What is the ascent of sap? Answer: The ascent of sap is the upward movement of water and dissolved minerals
• through the xylem tissue from roots to leaves.
• Q8. Explain the TACT theory. Answer: The TACT theory explains the ascent of sap through four factors:
• Transpiration pull (evaporation from leaves creates a pulling force), Adhesion (water molecules attracted to xylem
• walls), Cohesion (water molecules attracted to each other), and Tension (tension in the water column). These forces
• work together to move water up the plant.
• Q9. What is translocation? Answer: Translocation is the transport of prepared food (organic solutes) to different
• parts of the plant through the phloem tissue.
• Q10. Explain the pressure flow theory. Answer: The pressure flow (mass flow) theory explains translocation: sugars
• are loaded into sieve tubes at the source, water follows by osmosis, creating high hydrostatic pressure. At the sink,
• sugars are unloaded, water leaves, and the pressure difference drives the flow of phloem sap from source to sink.
• Multiple Choice Questions (MCQs)
• Q1. Transport in plants occurs at how many levels? A. One B. Two C. Three D. Four Correct Answer: C Explanation:
• Transport in plants occurs at three levels: outside to inside, within the plant, and inside to outside.
• Q2. Water is absorbed by roots through: A. Active transport B. Osmosis C. Facilitated diffusion D. Bulk flow Correct
• Answer: B Explanation: Water is absorbed by osmosis, a passive transport mechanism.
• Q3. Which of the following is the primary site for absorption of water and minerals? A. Root cap B. Root hairs C.
• Xylem D. Phloem Correct Answer: B Explanation: Root hairs are the primary sites for absorption of water and
• minerals.
• Q4. Approximately what percentage of absorbed water is lost through transpiration? A. 50% B. 75% C. 90% D. 99%
• Correct Answer: D Explanation: About 99% of absorbed water is lost through transpiration.
• Q5. Most transpiration occurs through: A. Lenticels B. Cuticle C. Stomata D. Root hairs Correct Answer: C Explanation:
• About 90% of transpiration occurs through stomata (stomatal transpiration).
• generated by DeepSeek to PDF

• Q6. The TACT theory explains: A. Translocation B. Ascent of sap C. Photosynthesis D. Respiration Correct Answer: B
• Explanation: The TACT theory explains the ascent of sap.
• Q7. In the TACT theory, "A" stands for: A. Absorption B. Adhesion C. Active transport D. Ascent Correct Answer: B
• Explanation: A stands for Adhesion (attraction between water molecules and xylem walls).
• Q8. Translocation occurs through: A. Xylem B. Phloem C. Cambium D. Epidermis Correct Answer: B Explanation:
• Translocation is the transport of organic solutes through phloem.
• Q9. In translocation, food moves from: A. Sink to source B. Source to sink C. Root to leaf only D. Leaf to root only
• Correct Answer: B Explanation: Translocation moves from a source (where food is produced) to a sink (where food is
• used or stored).
• Q10. The pressure flow theory explains: A. Water absorption B. Transpiration C. Translocation D. Ascent of sap
• Correct Answer: C Explanation: The pressure flow (mass flow) theory explains translocation in the phloem.
• Subtopic 8.2 complete. Reply "next" to approve and I will produce Subtopic 3: 8.3 Gas Exchange in Plants.