Transport in Plants

Means Of Transport

Plants need to transport various substances, including water, mineral nutrients, organic nutrients (like sugars), and plant growth regulators, over varying distances. This transport can be short-distance, occurring within a cell, across cell membranes, or over short distances within tissues. Long-distance transport occurs through the vascular tissues, xylem and phloem.

Diffusion

Diffusion is the movement of substances from a region of higher concentration to a region of lower concentration. It is a passive process, meaning it does not require the expenditure of metabolic energy by the cell.

Characteristics of Diffusion:

• Occurs along a concentration gradient (downhill movement).
• It is a slow process and is not dependent on a living system.
• Can occur across membranes or across any living or non-living system.
• Movement is random, resulting in a net movement from high to low concentration until equilibrium is reached.

Factors Affecting Diffusion Rate:

• Concentration gradient: The greater the concentration difference, the faster the rate of diffusion.
• Permeability of the membrane: Diffusion is faster across a more permeable membrane.
• Temperature: Higher temperature increases the kinetic energy of molecules, increasing the diffusion rate.
• Pressure: Higher pressure generally increases the diffusion rate.
• Size of the diffusing substance: Smaller molecules generally diffuse faster.
• Solubility in lipids: Substances soluble in membrane lipids diffuse faster across the membrane.

Facilitated Diffusion

Diffusion of substances across a membrane can also be facilitated by membrane proteins. This is called facilitated diffusion.

Characteristics of Facilitated Diffusion:

• Movement occurs along a concentration gradient (passive process, no energy required).
• Requires membrane proteins (transport proteins or carriers) to move the substance across the membrane.
• The transport rate reaches a maximum when all the protein carriers are being used (saturation).
• It is specific to the type of substance being transported, as each protein carrier is specific for a particular molecule or ion.
• Inhibited by substances that bind to the protein carriers.

The transport proteins provide hydrophilic channels or binding sites that make it easier for specific substances to cross the hydrophobic lipid bilayer. No ATP energy is consumed in this process because the movement is still driven by the concentration gradient.

*(Image shows a lipid bilayer with simple diffusion of small nonpolar molecules and facilitated diffusion of a specific molecule through a channel or carrier protein)*

Passive Symports And Antiports

Facilitated diffusion can involve the transport of two different molecules simultaneously across the membrane by the same carrier protein. This is a form of co-transport.

• Symport: Both molecules move in the same direction across the membrane.

  *(Image shows a membrane with a carrier protein transporting two different substances in the same direction)*

• Antiport: Both molecules move in opposite directions across the membrane.

  *(Image shows a membrane with a carrier protein transporting two different substances in opposite directions)*

• Uniport: When a carrier protein transports a single molecule across the membrane (this is just basic facilitated diffusion).

The prompt refers to "Passive Symports and Antiports", implying that these transport mechanisms are driven by the concentration gradient of at least one of the transported substances, without the direct use of ATP by the carrier itself. This aligns with the concept of facilitated diffusion or secondary active transport where the energy comes from a pre-existing gradient established by active transport elsewhere.

Active Transport

Active transport is the movement of substances across a membrane against a concentration gradient (from a region of lower concentration to a region of higher concentration). This process requires the expenditure of metabolic energy, usually in the form of ATP.

Characteristics of Active Transport:

• Occurs against a concentration gradient (uphill movement).
• Requires metabolic energy (ATP).
• Requires membrane proteins (pumps) to move the substance across the membrane.
• The transport rate reaches a maximum when all protein pumps are in use (saturation).
• It is specific to the type of substance being transported.
• Inhibited by substances that interfere with protein pumps or energy production.

Pumps are membrane proteins that use energy (ATP) to change shape and move substances across the membrane against their concentration gradient. This is crucial for accumulating ions or molecules inside or outside the cell even when their concentration is already high there.

*(Image shows a membrane with a pump protein binding a substance, ATP hydrolysis providing energy, the protein changing shape to move the substance across the membrane against its gradient)*

Comparison Of Different Transport Processes

Here's a comparison of the key features of Diffusion, Facilitated Diffusion, and Active Transport:

Plant-Water Relations

Water is essential for all physiological activities of plants. It acts as a solvent, provides turgor pressure necessary for rigidity and growth, is a reactant in photosynthesis, and is the medium for transport of substances. Understanding how plants absorb, transport, and lose water is crucial.

Water Potential

Water moves from a region of higher water potential to a region of lower water potential. Water potential ($\Psi_w$) is a concept fundamental to understanding water movement. It is the potential energy of water per unit volume relative to pure water in reference conditions.

Key Concepts:

• Pure water has the maximum water potential. By convention, the water potential of pure water at standard temperature and atmospheric pressure is taken as zero ($0 \: \text{bars}$ or $0 \: \text{Mega Pascals}$).
• Adding solutes to pure water reduces its free energy and thus reduces its water potential. Solute potential is always negative.
• Pressure applied to water increases its water potential.
• Water potential is measured in pressure units, such as Pascals (Pa) or megapascals (MPa). $1 \: \text{bar} \approx 0.1 \: \text{MPa}$.

Components of Water Potential:

Water potential ($\Psi_w$) is influenced by two main factors:

1. Solute Potential ($\Psi_s$): This is due to the presence of dissolved solutes. Solute potential is always negative. The more solutes present, the more negative the solute potential. It is also called Osmotic Potential.
2. Pressure Potential ($\Psi_p$): This is the pressure exerted on water. It can be positive or negative.
   • In a plant cell, the rigid cell wall exerts a pressure on the cell contents, which is a positive pressure potential (turgor pressure). Turgor pressure is responsible for the rigidity of the cell.
   • Negative pressure potential (tension) occurs in the xylem vessels of plants during transpiration.

The relationship between water potential, solute potential, and pressure potential is given by the formula:

$ \Psi_w = \Psi_s + \Psi_p $

In a living plant cell, water potential is affected by both the solute concentration inside the cell and the pressure exerted by the cell wall.

Answer:

Osmosis

Osmosis is a special type of diffusion of water across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Key Concepts:

• Movement of water is driven by the difference in water potential.
• The selectively permeable membrane allows the passage of water molecules but restricts the passage of solute molecules.
• Osmotic Pressure: It is the pressure required to prevent water from diffusing into a solution across a semipermeable membrane. Osmotic pressure is a positive pressure. Osmotic pressure is numerically equal to the osmotic potential ($\Psi_s$), but with a positive sign. Higher the solute concentration, higher the osmotic pressure.
• Osmotic potential ($\Psi_s$) is always negative. So, $\text{Osmotic Pressure} = - \Psi_s$.

What happens when a plant cell is placed in solutions of different concentrations:

• Isotonic solution: The external solution has the same osmotic concentration (solute potential) as the cell cytoplasm. There is no net movement of water. The cell remains flaccid or turgid depending on its initial state.
• Hypotonic solution: The external solution has a lower osmotic concentration (higher water potential) than the cell cytoplasm. Water diffuses into the cell. The cell swells and becomes turgid due to the pressure exerted by the influx of water against the cell wall. The rigid cell wall prevents the cell from bursting.
• Hypertonic solution: The external solution has a higher osmotic concentration (lower water potential) than the cell cytoplasm. Water diffuses out of the cell. The cell shrinks, and the plasma membrane pulls away from the cell wall. This phenomenon is called plasmolysis.

*(Image shows a plant cell in isotonic (flaccid), hypotonic (turgid), and hypertonic (plasmolysed) solutions, illustrating water movement direction and cell appearance)*

Plasmolysis

Plasmolysis is the process where the protoplast of a plant cell shrinks away from its cell wall due to the loss of water by osmosis when the cell is placed in a hypertonic solution.

Stages of Plasmolysis:

• Deplasmolysis: If a plasmolysed cell is placed in a hypotonic solution, water re-enters the cell by osmosis, causing the protoplast to swell and return to its original shape, pressing against the cell wall. The cell becomes turgid again.

Imbibition

Imbibition is a special type of diffusion where water is absorbed by solid colloids, causing them to increase in volume.

Characteristics of Imbibition:

• It is a type of diffusion because water moves along a potential gradient (from the region of high water potential, e.g., liquid water, to the region of low water potential in the dry solid).
• Requires an affinity between the adsorbent (solid) and the liquid (water).
• Results in a large amount of heat release (heat of hydration).

Example: Absorption of water by dry seeds or dry wood. Seeds swell when soaked in water due to imbibition.

Importance: Imbibition is essential for the absorption of water by dry seeds, which initiates the process of seed germination. Swelling of wood due to imbibition can also exert powerful pressure (e.g., used by early humans to split rocks).

Long Distance Transport Of Water

Water absorbed by the roots needs to be transported upwards to the leaves and other parts of the plant, sometimes over considerable heights (e.g., in tall trees). This long-distance transport occurs through the xylem vessels and tracheids. This upward movement of water is called the ascent of sap.

How Do Plants Absorb Water?

Plants absorb water and mineral salts from the soil primarily through the roots, specifically through the root hairs located in the region of maturation.

Pathways of Water Movement in the Root:

Water can move from the root hair cell into the xylem vessels in two main pathways:

1. Apoplast Pathway: Water moves exclusively through the cell walls and intercellular spaces of the cortical cells. It does not cross any cell membrane, except potentially the plasma membrane of the root hair cell itself. This pathway is faster and offers little resistance to water movement. However, this pathway is blocked at the endodermis by the Casparian strips.
2. Symplast Pathway: Water moves through the cytoplasm of adjacent cells, connected by plasmodesmata (cytoplasmic bridges). Water enters a cell through the plasma membrane and then moves from cell to cell through the symplast. This pathway is slower as water has to cross cell membranes multiple times.

At the endodermis, the Casparian strips (made of suberin, impermeable to water) force water to enter the cytoplasm of the endodermal cells (symplast pathway) before entering the xylem. This ensures that water and dissolved minerals are filtered and transported selectively.

Once inside the xylem vessels (which are non-living hollow tubes), water moves freely upwards.

*(Image shows a cross-section of root cortex and endodermis, illustrating water movement through cell walls (apoplast) and through cytoplasm connected by plasmodesmata (symplast), showing the barrier function of the Casparian strip in the endodermis)*

Water Movement Up A Plant

Several forces are involved in the ascent of sap. The two main ones are Root Pressure and Transpiration Pull.

Root Pressure:

• This is a positive hydrostatic pressure that develops in the xylem sap of the root when the rate of water absorption is greater than the rate of transpiration (e.g., at night or in conditions of high humidity).
• Active absorption of ions by root cells leads to an increase in the solute concentration within the xylem, causing water to move into the xylem by osmosis, generating a positive pressure (root pressure).
• Root pressure can push water up to a small height. It can be observed as guttation (exudation of water droplets from leaf margins in some herbaceous plants in the morning).
• Root pressure is not sufficient to account for water transport in tall trees.

Transpiration Pull

The most widely accepted theory for the ascent of sap in tall plants is the Cohesion-Tension-Transpiration Pull theory. This theory proposes that water is primarily pulled up the xylem from the roots to the leaves due to the tension created by transpiration.

Mechanism of Transpiration Pull:

1. Transpiration at the leaf surface: Water evaporates from the surface of mesophyll cells into the intercellular spaces and then diffuses as water vapour out through the stomata. This creates a negative pressure (tension) in the leaf xylem.

2. Cohesion and Adhesion of water molecules: Water molecules have unique properties due to hydrogen bonding:

• Cohesion: Mutual attraction between water molecules, which holds them together as a continuous column in the xylem vessels.
• Adhesion: Attraction of water molecules to the polar surfaces (like the walls of xylem vessels - made of cellulose and lignin). This helps prevent the water column from breaking.

3. Transpiration Pull: The negative pressure (tension) created by transpiration in the leaf xylem extends down through the continuous column of water in the xylem vessels all the way to the roots. This tension 'pulls' the water upwards from the roots.

4. Water absorption from soil: As water is pulled out of the root xylem, the water potential in the root decreases, causing water to move from the soil into the root xylem by osmosis.

This continuous column of water is maintained by cohesion and adhesion, and the upward pull is driven by the negative pressure (tension) generated by transpiration. This transpiration pull is a very strong force, capable of lifting water to the top of the tallest trees.

*(Image shows a diagram of a tree illustrating water absorption by roots, upward movement through xylem, and transpiration from leaves, highlighting the continuous water column and forces of cohesion and adhesion)*

Transpiration

Transpiration is the process of loss of water in the form of water vapour from the aerial parts of the plant, primarily through the stomata in the leaves.

About 99% of the water absorbed by a plant is lost through transpiration; only a small amount is used in photosynthesis and other metabolic activities.

Types of Transpiration:

• Stomatal Transpiration: Occurs through the stomata. Most significant (80-90%).
• Cuticular Transpiration: Occurs directly through the cuticle of leaves and stems. Less significant (up to 10%).
• Lenticular Transpiration: Occurs through lenticels in woody stems. Very negligible.

Importance (Advantages) of Transpiration:

• Provides the transpiration pull for the ascent of sap, which is essential for transport of water and minerals from roots to leaves.
• Helps in the absorption and transport of mineral salts from the soil.
• Provides a cooling effect to the plant surface (evaporation of water requires energy, thus lowering leaf temperature). This can prevent heat damage in intense sunlight.
• Maintains the shape and size of cells by keeping them turgid.

Disadvantages of Transpiration:

• Results in the loss of a large amount of water, which can be a limiting factor for plant growth, especially in dry conditions. It is often called a 'necessary evil'.

Factors Affecting Transpiration:

• External factors:
  • Light: Stomata generally open in light (for photosynthesis), increasing transpiration.
  • Temperature: Higher temperature increases evaporation rate, increasing transpiration.
  • Humidity: Higher humidity in the air reduces the water potential gradient between the leaf and the air, decreasing transpiration.
  • Wind speed: Moderate wind increases transpiration by removing water vapour from the leaf surface. High wind may decrease it by causing stomatal closure.
  • Atmospheric pressure: Lower pressure increases transpiration.
• Internal factors:
  • Number and distribution of stomata: More stomata lead to higher transpiration.
  • Percentage of open stomata: Regulated by guard cells.
  • Water status of the plant: If the plant is under water stress, stomata may close, reducing transpiration.
  • Canopy structure: Size and arrangement of leaves.

Transpiration And Photosynthesis – A Compromise

Photosynthesis requires carbon dioxide ($CO_2$), which enters the leaf through the stomata. However, opening the stomata for $CO_2$ uptake also leads to the loss of water vapour through transpiration. This presents a fundamental conflict for plants.

Plants need to balance the need for $CO_2$ for photosynthesis (requiring open stomata) with the need to conserve water (requiring closed stomata). This balance is crucial for survival, especially in dry environments.

The Compromise:

• Most plants keep their stomata open during the day (when light is available for photosynthesis) and closed at night.
• In dry conditions, plants may partially or fully close their stomata even during the day to reduce water loss, which in turn reduces $CO_2$ uptake and thus limits photosynthesis.
• Some plants (like CAM plants) have evolved alternative photosynthetic pathways (e.g., Crassulacean Acid Metabolism) that allow them to open stomata and fix $CO_2$ during the night (reducing water loss) and then use the stored $CO_2$ for photosynthesis during the day with stomata closed.

The transpiration ratio is the ratio of the mass of water transpired to the mass of $CO_2$ assimilated during photosynthesis. This ratio is very high (hundreds of grams of water lost for every gram of $CO_2$ fixed), indicating that water conservation is a major challenge for terrestrial plants.

$ \text{Transpiration Ratio} = \frac{\text{Mass of water transpired}}{\text{Mass of } CO_2 \text{ assimilated}} $

The structure of the leaf (cuticle, sunken stomata, etc.) is often adapted to reduce transpiration while allowing sufficient $CO_2$ uptake for photosynthesis.

Uptake And Transport Of Mineral Nutrients

Plants require mineral nutrients for their growth and development. These nutrients are absorbed from the soil, primarily as inorganic ions. Unlike water, which moves passively up the xylem due to transpiration pull, the uptake of mineral ions often involves active processes.

Uptake Of Mineral Ions

Mineral ions are absorbed by the roots.

Mechanism of Ion Uptake:

• Unlike water which moves passively, the concentration of mineral ions in the soil is often much lower than the concentration inside the root cells. Therefore, uptake against the concentration gradient often requires active transport.
• Uptake of ions is mediated by membrane proteins present in the plasma membranes of root hair cells and other root cells. These proteins act as carriers or pumps.
• Active uptake: Involves specific pumps that use ATP energy to move ions across the membrane, even against a concentration gradient. This is crucial for accumulating ions inside the cell.
• Passive uptake: Some ions may move passively through ion channels or along an electrochemical gradient (facilitated diffusion). However, accumulation often requires active transport.
• Specificity: Transport proteins are highly specific for particular ions.

The endodermis, with its Casparian strips, plays a critical role in controlling which minerals enter the xylem. The transport proteins of the endodermal cells are selective and regulate the passage of ions into the vascular cylinder. Active transport is involved at this layer.

*(Image shows a root cell membrane with a carrier protein/pump binding a mineral ion, showing ATP being used to transport it into the cell against a gradient)*

Translocation Of Mineral Ions

After being absorbed by the root, mineral ions are transported upwards to other parts of the plant.

Pathway of Translocation:

• Mineral ions are primarily transported through the xylem vessels along with the ascent of water (transpiration stream).
• The movement of mineral ions in the xylem is essentially unidirectional, from roots to the aerial parts (stems, leaves, flowers, fruits).
• While transport in xylem is generally passive (carried by the water stream), the loading of ions into the xylem vessels in the root is an active process involving specific transport proteins.

Fate of Minerals:

• Mineral ions reach the parts of the plant where they are needed, such as growing regions (apical and lateral meristems, young leaves, developing flowers and fruits, storage organs).
• Minerals are unloaded at these 'sinks' through passive or active transport.
• Remobilisation: Elements that are relatively mobile in the plant (e.g., Phosphorus, Sulphur, Nitrogen, Potassium) can be transported from older, senescing leaves to younger growing parts before the old leaves fall. Elements like Calcium are relatively immobile and are not easily remobilised from older tissues.
• Small amounts of mineral elements are also transported in the phloem as organic compounds (e.g., nitrogen as amino acids).

Answer:

Phloem Transport: Flow From Source To Sink

Phloem is the vascular tissue responsible for the translocation of organic nutrients, primarily sugars produced during photosynthesis, from the leaves (source) to other parts of the plant where they are needed or stored (sink).

Phloem Sap Composition:

Phloem sap mainly consists of water and sucrose, but also contains other sugars, hormones, amino acids, and other organic solutes.

Direction of Transport:

Phloem transport is generally bidirectional (upwards or downwards). This is unlike xylem transport which is mostly unidirectional (upwards).

• The direction of movement in the phloem depends on the relative positions of the source and the sink.
• Source: The region where food is synthesised (e.g., mature leaves) or stored food is mobilised (e.g., storage organs during regrowth).
• Sink: The region where food is utilised (e.g., growing roots, young leaves, shoot apices, developing fruits, storage organs).

The source-sink relationship can be variable. For example, roots can be a sink for sugars during growth but can become a source in spring when stored sugars are mobilised to support new shoot growth.

The Pressure Flow Or Mass Flow Hypothesis

The most accepted mechanism for the translocation of sugars in the phloem is the Pressure Flow Hypothesis (also known as the Mass Flow Hypothesis), proposed by Ernst Münch.

Mechanism of Pressure Flow:

This hypothesis describes how a pressure gradient drives the movement of sap in the sieve tubes of the phloem.

1. Loading at the Source:

• Sugars (primarily sucrose) are transported from the mesophyll cells of the leaf (where they are synthesised) into the sieve tube elements of the phloem.
• This loading process is typically an active process, requiring energy (ATP) and involves specific transport proteins.
• Loading of sugars into the sieve tube increases the solute concentration within the sieve tube elements at the source.

2. Osmotic Influx of Water:

• Due to the increased solute concentration in the sieve tube elements at the source, their water potential ($\Psi_w$) decreases.
• Water moves from the adjacent xylem (which has higher water potential) into the sieve tube elements by osmosis.
• This osmotic influx of water increases the hydrostatic pressure (turgor pressure) within the sieve tube elements at the source.

3. Creation of Pressure Gradient (Mass Flow):

• The high hydrostatic pressure developed at the source creates a pressure gradient between the source (high pressure) and the sink (low pressure).
• The phloem sap (water containing dissolved sugars) flows from the region of higher pressure (source) to the region of lower pressure (sink) through the sieve tubes. This bulk movement is called mass flow or pressure flow.
• This flow is driven by the difference in turgor pressure, not by diffusion or active transport of individual molecules along the length of the sieve tube.

4. Unloading at the Sink:

• At the sink (e.g., root cells, fruit cells), sugars are actively transported out of the sieve tube elements and into the sink cells, where they are either used for metabolism (e.g., respiration) or stored (e.g., as starch, sucrose, or cellulose).
• This unloading process is also typically an active process, requiring energy.
• Unloading of sugars decreases the solute concentration in the sieve tube elements at the sink.

5. Osmotic Efflux of Water:

• Due to the decreased solute concentration in the sieve tube elements at the sink, their water potential ($\Psi_w$) increases.
• Water moves out of the sieve tube elements back into the adjacent xylem by osmosis.
• This loss of water decreases the hydrostatic pressure (turgor pressure) within the sieve tube elements at the sink, maintaining the pressure gradient between the source and the sink.

*(Image shows a simplified diagram of source leaf connected to sink root via phloem and xylem, illustrating sucrose loading at source, water influx from xylem, pressure build-up, mass flow through phloem, sugar unloading at sink, and water efflux back to xylem)*

The continuous loading of sugars at the source and unloading at the sink maintains the pressure gradient that drives the bulk flow of phloem sap. This mechanism allows efficient transport of food to different parts of the plant.