|
TRANSPORT IN PLANTS: In a flowering plant the substances that would need to be transported are water, mineral nutrients, organic nutrients and plant growth regulators. Over small distances substances move by diffusion and by cytoplasmic streaming supplemented by active transport. Transport over longer distances proceeds through the vascular system (the xylem and the phloem) and is called translocation.
In rooted plants, transport in xylem (of water and minerals) is essentially unidirectional, from roots to the stems. Organic and mineral nutrients however, undergo multidirectional transport. |
Means of Transport:
There are three means of transport in plants: (i) Diffusion, (ii) Facilitated Diffusion, and (iii) Active transport.
Differences between the three means of transport is given in the table below.
Differences between the three means of transport is given in the table below.
(i) Simple diffusion:
- Diffusion is important to plants since it is the only way for gaseous movement within the plant body.
- Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature and pressure.
(ii) Facilitated diffusion:
- Membrane protein is required on top of concentration gradient for facilitated diffusion. This is because membrane proteins provide sites through which large and hydrophilic molecules can cross the hydrophobic cell membrane (see Figure 1 below).
- Diffusion is important to plants since it is the only way for gaseous movement within the plant body.
- Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature and pressure.
(ii) Facilitated diffusion:
- Membrane protein is required on top of concentration gradient for facilitated diffusion. This is because membrane proteins provide sites through which large and hydrophilic molecules can cross the hydrophobic cell membrane (see Figure 1 below).
- Since membrane proteins are limited in amount, transport rate reaches a maximum when all of the protein transporters are being used (saturation).
- Some channels are always open; others can be controlled.
- porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through. - Some carrier or transport proteins allow diffusion only if two types of molecules move together (Figure 2). Symport - both molecules cross the membrane in the same direction. Antiport - Both molecules cross the membrane in the opposite direction. Uniport - movement of molecules across the membrane is independent of one another. |
Osmosis:
- Osmosis is the movement of water from its higher concentration (hypotonic solution) to the lower concentration (hypertonic solution)
through a semi-permeable membrane.
- The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient.
- Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
- Osmotic potential has negative sign while osmotic pressure has positive sign.
through a semi-permeable membrane.
- The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient.
- Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
- Osmotic potential has negative sign while osmotic pressure has positive sign.
Plasmolysis:
Isotonic solution is an external solution that balances the osmotic pressure of the cytoplasm.
When the cell (or tissue) is placed in an isotonic solution, there is no net flow of water towards the inside or outside. Such cells are called flaccid.
Hypotonic solution is an external solution that is more dilute (higher osmotic pressure) than the cytoplasm.
When the cells are placed in a hypotonic solution (more dilute, high osmotic pressure), water diffuses into the cell causing the cell to become turgid. The pressure exerted by cytoplasm to the cell wall is called the turgor pressure.
Hypertonic solution is an external solution is more concentrated (lower osmotic pressure) than the cytoplasm.
When the cell is placed in a solution that is hypertonic (more concentrated, low osmotic pressure), water diffuses out of the cell and the cell shrinks. This outward movement water from the cell leading to the shrinkage of cell membrane is called plasmolysis (Figure 3).
When the cell (or tissue) is placed in an isotonic solution, there is no net flow of water towards the inside or outside. Such cells are called flaccid.
Hypotonic solution is an external solution that is more dilute (higher osmotic pressure) than the cytoplasm.
When the cells are placed in a hypotonic solution (more dilute, high osmotic pressure), water diffuses into the cell causing the cell to become turgid. The pressure exerted by cytoplasm to the cell wall is called the turgor pressure.
Hypertonic solution is an external solution is more concentrated (lower osmotic pressure) than the cytoplasm.
When the cell is placed in a solution that is hypertonic (more concentrated, low osmotic pressure), water diffuses out of the cell and the cell shrinks. This outward movement water from the cell leading to the shrinkage of cell membrane is called plasmolysis (Figure 3).
Imbibition:
Imbibition is the diffusion in which water is absorbed by solids, causing them to enormously increase in volume.
Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition.
Examples of imbibition are absorption of water by seeds and dry wood
Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition.
Examples of imbibition are absorption of water by seeds and dry wood
Long Distance Transport of Water:
Long distance transport of substances within a plant cannot be by diffusion alone because (i) diffusion is a slow process and
(ii) diffusion accounts for only short distance movement of molecules.
In large plants, water, minerals, and food are generally moved by a mass or bulk flow system. Mass or bulk flow is the movement of substances in bulk from one point to another as a result of pressure differences between the two points. The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
The higher plants have highly specialised vascular tissues – (i) xylem and (ii) phloem.
(i) Xylem translocates substances (mainly water, mineral salts, some organic nitrogen and hormones) from roots to the aerial parts of the plants.
(ii) Phloem translocates substances (variety of organic and inorganic solutes) from the leaves to other parts of the plants.
(ii) diffusion accounts for only short distance movement of molecules.
In large plants, water, minerals, and food are generally moved by a mass or bulk flow system. Mass or bulk flow is the movement of substances in bulk from one point to another as a result of pressure differences between the two points. The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
The higher plants have highly specialised vascular tissues – (i) xylem and (ii) phloem.
(i) Xylem translocates substances (mainly water, mineral salts, some organic nitrogen and hormones) from roots to the aerial parts of the plants.
(ii) Phloem translocates substances (variety of organic and inorganic solutes) from the leaves to other parts of the plants.
How do Plants Absorb Water?
Water is absorbed along with mineral solutes by the root hairs through diffusion. Root hairs are extensions of root epidermal cells that greatly increase the surface area for absorption of water.
Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct pathways:
(i) apoplast pathway and (ii) symplast pathway.
(i) Apoplast pathway: In apoplast pathway, water travels through the intercellular spaces (between the cell walls) without entering the cell.
(ii) Symplast pathway: In symplast pathway, water enters the cell through cell membrane and travels inter-cellularly with the help of plasmodesmata. At the casparian strip, water cannot cross through the cell walls, therefore, the water cross the cells by enter the cell (symplast pathway).
Difference between apoplast and symplast pathways:
Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct pathways:
(i) apoplast pathway and (ii) symplast pathway.
(i) Apoplast pathway: In apoplast pathway, water travels through the intercellular spaces (between the cell walls) without entering the cell.
(ii) Symplast pathway: In symplast pathway, water enters the cell through cell membrane and travels inter-cellularly with the help of plasmodesmata. At the casparian strip, water cannot cross through the cell walls, therefore, the water cross the cells by enter the cell (symplast pathway).
Difference between apoplast and symplast pathways:
At the casparian strip, water cannot cross through the cell walls, therefore, the water cross the cells by enter the cell (symplast pathway) as shown in the figure 4 below.
Root Pressure:
Root pressure is the positive pressure created inside the xylem due to absorption of water into the vascular tissues of the roots.
Guttation is the loss of water in its liquid phase from openings near the tip of grass blades and leaves of herbaceous plants.
Guttation is the loss of water in its liquid phase from openings near the tip of grass blades and leaves of herbaceous plants.
Transpiration and transpiration pull:
Transpiration is the process of loss of water in the form water vapor through the stomata in the leaves.
- Transpiration accounts for the loss of 99 percent of water absorbed by the plants.
- The upward pull of water generated by the transpiration is the main driving force for the water to move upward in the plants.
- The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
• Cohesion – mutual attraction between water molecules.
• Adhesion – attraction of water molecules to polar surfaces (such as the surface of tracheary elements).
• Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
Cohesion-tension-transpiration pull mod el of water transport: water (H20) is a polar molecule that forms hydrogen bonds with other water molecules. This force between the water (polar) molecules is called the cohesive force. Cohesive force is responsible for the holding the water molecules together and therefore creating surface tension. The attraction of water molecules to other polar surface is called the adhesion. This cohesive and adhesive force of water along with the upward pull generated by loss of water (transpiration) through the stomata leads to the upward movement of water in the plants.
- Transpiration accounts for the loss of 99 percent of water absorbed by the plants.
- The upward pull of water generated by the transpiration is the main driving force for the water to move upward in the plants.
- The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
• Cohesion – mutual attraction between water molecules.
• Adhesion – attraction of water molecules to polar surfaces (such as the surface of tracheary elements).
• Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
Cohesion-tension-transpiration pull mod el of water transport: water (H20) is a polar molecule that forms hydrogen bonds with other water molecules. This force between the water (polar) molecules is called the cohesive force. Cohesive force is responsible for the holding the water molecules together and therefore creating surface tension. The attraction of water molecules to other polar surface is called the adhesion. This cohesive and adhesive force of water along with the upward pull generated by loss of water (transpiration) through the stomata leads to the upward movement of water in the plants.
Stomata:
Stomata open in the day time and close during the night. The opening and closing of stomata is controlled by the turgidity of guard cells. When water enters the guard cells, the turgidity increases and the pore (stomatal aperture) opens (Figure 5). At night, guard cells lose turgor due to lack of water and the stomata closes.
Transpiration and Photosynthesis – a Compromise:
Transpiration and Photosynthesis – a Compromise
Transpiration has many functions:
• creates transpiration pull for absorption and transport of plants
• supplies water for photosynthesis
• transports minerals from the soil to all parts of the plant
• cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling
• maintains the shape and structure of the plants by keeping cells turgid
However, photosynthesis is limited by available water which can be swiftly depleted by transpiration.
Transpiration has many functions:
• creates transpiration pull for absorption and transport of plants
• supplies water for photosynthesis
• transports minerals from the soil to all parts of the plant
• cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling
• maintains the shape and structure of the plants by keeping cells turgid
However, photosynthesis is limited by available water which can be swiftly depleted by transpiration.
Uptake of Mineral Ions:
Ions are absorbed from the soil by both passive and active transport.
However, most mineral ions cannot be passively absorbed by the roots because of two main reasons: (i) minerals are present in the soil as charged particles (ions) which cannot move across cell membranes and (ii) the concentration of minerals in the soil is usually lower than the concentration of minerals in the root. Therefore, most mineral ions must enter the root by active absorption into the cytoplasm of epidermal cells with the help of transport proteins.
However, most mineral ions cannot be passively absorbed by the roots because of two main reasons: (i) minerals are present in the soil as charged particles (ions) which cannot move across cell membranes and (ii) the concentration of minerals in the soil is usually lower than the concentration of minerals in the root. Therefore, most mineral ions must enter the root by active absorption into the cytoplasm of epidermal cells with the help of transport proteins.
Translocation of Mineral Ions:
- After the mineral ions have entered the xylem, they are further transported up the stem by transpirational pull described before.
- Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
- Mineral ions such as phosphorus, sulphur, nitrogen and potassium are remobilized from old senescing parts. Some elements that are structural components like calcium are not remobilised.
- Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
- Mineral ions such as phosphorus, sulphur, nitrogen and potassium are remobilized from old senescing parts. Some elements that are structural components like calcium are not remobilised.
PHLOEM TRANSPORT: FLOW FROM SOURCE TO SINK
Food, primarily sucrose, is transported by the vascular tissue phloem from a source to a sink. Source is the part of a plant that synthesizes food (leaf) and the sink is the site that stores the food. But, the source-sink relationship is be upwards or downwards, i.e., bi-directional depending on the need of a plant. This contrasts with that of the xylem where the movement is always unidirectional, i.e., upwards.
Therefore, in phloem the movement of mineral ions is bidirectional while in xylem, the movement of is unidirectional.
Therefore, in phloem the movement of mineral ions is bidirectional while in xylem, the movement of is unidirectional.
The Pressure Flow or Mass Flow Hypothesis:
The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis. (see Figure ). As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a disaccharide). The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport. This process of loading at the source produces a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up the phloem sap will move to areas of lower pressure. At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar – converting it into energy, starch, or cellulose. As sugars are removed, the osmotic pressure decreases and water moves out of the phloem. as complex carbohydrates. The loss of solute produces a high water potential in the phloem, and water passes out, returning eventually to xylem.
To summarise, the movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem.
Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments. As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins, and the sap moves through the phloem. Meanwhile, at the sink, incoming sugars are actively transported out of the phloem and removed.
To summarise, the movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem.
Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments. As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins, and the sap moves through the phloem. Meanwhile, at the sink, incoming sugars are actively transported out of the phloem and removed.
HTML Comment Box is loading comments...
|
|