Key Notes: Transport in Plants - NEET PDF Download

1. Transport over longer distances proceeds through the vascular system (the xylem and the phloem) and is called TRANSLOCATION.

2. Transport of water and minerals in xylem is essentially UNIDIRECTIONAL, from roots to the stems.

3. Organic and mineral nutrients undergo MULTIDIRECTIONAL TRANSPORT.

Diffusion

4. It is passive (no requirement of energy) transportation over short distance.

5. Substances moving from regions of higher concentration to regions of lower concentration.

6. Gaseous movement within plant take place through diffusion.

7. Diffusion rates are affected by → he gradient of concentration, o the permeability of the membrane separating, o temperature and pressure.

8. Facilitated Diffusion

• If Substances has a concentration gradient and  has a hydrophilic moiety (half part), need facilitated transportation  and membrane proteins provide sites at which such molecules cross the membrane.
• THIS DIFFUSION WITH THE HELP OF PROTEIN IS CALLED FACILITATED TRANSPORTATION.
• There is NO REQUIREMENT OF ENERGY EXPENDITURE.
• Transport rate reaches a maximum when all of the protein transporters are being used (saturation).
• FACILITATED DIFFUSION IS VERY SPECIFIC: it allows cell to select substances for uptake.
• It is sensitive to inhibitors which react with protein side chains.
• The proteins form channels in the membrane for molecules to pass through.
• Some channels are always open; others can be controlled.
• Some protein channel are large, allowing a variety of molecules to cross.
• The PORINS are proteins that form large pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through.
• Water- channels transport protein made up of eight different types of aquaporins.

9. When only one molecule moves across a membrane independent of other molecules, the process is called UNIPORT.

10. In a SYMPORT passive transport, two molecules cross the membrane in the same direction.

11. In an ANTIPORT passive transport, two molecules move in opposite directions.

ACTIVE TRANSPORT

12. Active transport uses energy to transport and pump molecules against a concentration gradient.

13. Active transport is carried out by specific membrane-proteins.

14. Pumps are proteins that use energy to carry substances across the cell membrane.

15. These pumps can transport substances from a low concentration to a high concentration called UPHILL TRANSPORT.

16. Transport rate reaches a maximum when all the protein transporters are being used or are saturated

17. Pump protein is very specific in what it carries across the membrane.

18. These proteins are sensitive to inhibitors that react with protein side chains.

19. Facilitated transport and active transport need special membrane protein for transportation and highly selective for molecules to be transported and they are under hormonal control.

Plant Water Relations

20. Water Potential

• Water potential (Ψw) is a concept fundamental to understanding water movement.
• Solute potential (Ψs) and pressure potential (Ψp) are the two main components that determine water potential.
• Water molecules possess kinetic energy (water potential) which is responsible for random motion in liquid and gaseous state.
• PURE WATER HAS THE GREATEST WATER POTENTIAL (IT’S ZERO)
• The greater the concentration of water in a system, the greater is its kinetic energy or ‘water potential’.
• If two systems containing water are in contact, random movement of water molecules will result in NET MOVEMENT OF WATER MOLECULES FROM THE SYSTEM WITH HIGHER WATER POTENTIAL TO THE ONE WITH LOWER WATER POTENTIAL.
• This process of movement of substances down a gradient of free energy is called DIFFUSION.
• Water potential is denoted by the Greek symbol Psi or Ψ and is expressed in pressure units such as pascals (Pa).
• The water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.
• If there is some solutes in pure water, the solution has fewer free water molecules and the concentration (free energy) of water decreases, reducing its water potential. Therefore, all solutions have a lower water potential than pure water.
• The magnitude of this lowering due to dissolution of a solute is called solute potential or Ψs. Ψs is always negative.
• The more the solute molecules, the lower (more negative) is the Ψs .
• For a solution at atmospheric pressure-
  (water potential) Ψw = (solute potential) Ψs.
• If pressure applied in water/solution is greater than atmospheric pressure then water potential of water/solution increases.
• Pressure potential is usually positive, though in plants negative potential or tension in the water column in the xylem plays a major role in water transport up a stem.
• Pressure potential is denoted as Ψ p.
• Water potential of a cell is affected by both solute and pressure potential.  The relationship between them is as follows:
  Ψw = Ψs + Ψp

Osmosis

21. In plant cells, the cell membrane and the membrane of the vacuole - THE TONOPLAST, together are important determinants of movement of molecules in or out of the cell.

22. In plant cell, the vacuolar sap, contribute to the solute potential of the cell.

23. In osmosis movement of Water molecules from its region of higher chemical potential (or concentration) to its region of lower chemical potential across the selective /differentially permeable membrane until equilibrium is reached.

24. At equilibrium the two chambers should have nearly the same water potential. 25. Osmosis is property of living cell.

26. The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient.

27. The solution with lower water potential is called HYPERTONIC SOLUTION (having higher solute potential).

28. The solution with higher water potential is called HYPOTONIC SOLUTION (having lower solute potential).

29. Diffusion of water occur from hypotonic solution to hypertonic solution.

30. We can demonstrate osmosis by raw potato osmometer or by using unboiled egg membrane.

31. In an experiment of osmosis, an osmometer filled with hypertonic solution is placed in a container having hypotonic solution, the water molecules start diffuse into hypertonic solution. External pressure can be applied that no water diffuses into the hypertonic solution through the membrane.

32. This pressure required to prevent water from diffusing is the osmotic pressure and this is the function of the solute concentration; more the solute concentration, greater will be the pressure required to prevent water from diffusing in.

33. Numerically osmotic pressure is equivalent to the osmotic potential, but the sign is opposite.

34. Osmotic pressure is the positive pressure applied, while osmotic potential is negative.

Plasmolysis

35. When a cell is placed in hypertonic solution, the water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall. This is called PLASMOLYSIS.

36. Water first lost from the cytoplasm and then from the vacuole.

37. The water when drawn out of the cell through diffusion into the extracellular (outside cell) fluid causes the protoplast to shrink away from the walls.

38. Outer solution (as cell wall is permeable for all) occupies the space between the cell wall and the shrunken protoplast in the plasmolysed cell.

39. When the cell (or tissue) is placed in an isotonic solution, there is no net flow of water towards the inside or outside.  If the external solution balances the osmotic pressure of the cytoplasm it is said to be isotonic.

40. The process of plasmolysis is usually reversible. When the cells are placed in a hypotonic solution water diffuses into the cell causing the cytoplasm to build up a pressure against the wall, that is called turgor pressure.

41. The pressure exerted by the protoplasts due to entry of water against the rigid walls is called pressure potential Ψp.

42. Because of the rigidity of the cell wall, the cell does not rupture.

43. The turgor pressure is ultimately responsible for enlargement and extension growth of cells.

Imbibition

44. Imbibition is a special type of diffusion when water is absorbed by solids – colloids – causing them to increase in volume.

45. The classical examples of imbibition are absorption of water by seeds and dry wood.

46. The pressure that is produced by the swelling of wood had been used by prehistoric man to split rocks and boulders.

47. Emerging of seedling open in to soil is also because of imbibition.

48. Imbibition is also diffusion since water movement is along a concentration gradient.

49. For any substance to imbibe any liquid, affinity between the adsorbent and the liquid is also a pre-requisite.

Long distance transport of water

• Diffusion can account for only short distance movement of molecules. It is very slow process. For example, the movement of a molecule across a typical plant cell (about 50 µm) takes approximately 2.5 s.
• Special long-distance transport systems become necessary to move substances across long distances and at a much faster rate.
• Water and minerals, and food are generally moved by a mass or bulk flow system.
• Mass flow is the movement of substances in bulk or en masse from one point to another as a result of pressure differences between the two points.
• It is a characteristic of mass flow that substances, whether in solution or in suspension, are swept along at the same pace.
• Bulk flow can be achieved either through a positive hydrostatic pressure gradient (e.g., a garden hose) or a negative hydrostatic pressure gradient (e.g., suction through a straw).
• The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
• Xylem is associated with translocation of mainly water, mineral salts, some organic nitrogen and hormones, from roots to the aerial parts of the plants.
• The phloem translocates a variety of organic and inorganic solutes, mainly from the leaves to other parts of the plants.

Water Absorption

• The responsibility of absorption of water and minerals is more specifically the function of the root hairs.
• Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.
• Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion.
• Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct pathways:
  - APOPLAST PATHWAY
  - SYMPLAST PATHWAY.
• The apoplast(non-living) is the system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots.
• The apoplastic movement of water occurs exclusively through the intercellular spaces and the walls of the cells.
• Movement through the apoplast does not involve crossing the cell membrane. This movement is dependent on the gradient. The apoplast does not provide any barrier to mass flow of water.
• As water evaporates into the intercellular spaces or the atmosphere, tension develop in the continuous stream of water in the apoplast so mass flow of water occurs due to the adhesive and cohesive properties of water.
• The symplastic system is the system of interconnected protoplasts. Neighbouring cells are connected through cytoplasmic strands i.e. plasmodesmata.
• During symplastic movement, the water travels through the cells – their cytoplasm; intercellular movement is through the plasmodesmata.
• Water movement through symplast is slow as water has to enter the cells through the cell membrane.
• Movement is again down a potential gradient.
• Symplastic movement may be aided by cytoplasmic streaming.
• Most of the water flow in the roots occurs via the apoplast because the cortical cells are loosely packed, and there is no resistance to water movement (but symplast also contributes little).
• Innermost layer of cortex is endodermis which has suberin deposition in radial and transverse wall. This deposition is called CASPERIAN STRIP which prevent the apoplatic movement of water beyond cortex.
• Water cross the endodermis through symplast then enter to xylem.
• Once inside the xylem, water is again free to move between cells as well as through them.
• In young roots, water enters directly into the xylem vessels and/or tracheids.
• Xylem except xylem parenchyma is part of apoplast as they are non-living.
• In mycorrhiza, fungal filaments form a network around the young root or they penetrate the root cells.  The hyphae have a very large surface area that absorb mineral ions and water from the soil from a much larger volume of soil that perhaps a root cannot do.
• Pinus seeds cannot germinate and establish without the presence of mycorrhizae. (obligate association – Mycorrhiza).

Upside Movement of Water in Plants

Root Pressure

• Ions are actively transported into the vascular tissues of the roots, from the soil and water follows its potential gradient and increases the pressure inside the xylem.
• This positive pressure is called root pressure and can be responsible for pushing up water to small heights in the stem.
• GUTTATION is due to root pressure when rate of transpiration is low i.e. at night and in early morning.
• Guttation is loss of water (in liquid form) and in this process excess water collects in the form of droplets around special openings of veins(hydathode) near the tip of grass blades, and leaves of many herbaceous parts.
• Root pressure do not play a major role in water movement up tall trees.
• The greatest contribution of root pressure may be to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.

Transpiration Pull

• Transpiration pull model is the most acceptable to explain the transport of water.
• Generally, speed of water movement in upside direction is 15mt/hour against the gravity.
• Water is mainly ‘pulled’ through the plant, and that the driving force for this process is transpiration from the leaves.
• THIS IS REFERRED TO AS THE COHESION-TENSION-TRANSPIRATION PULL MODEL OF WATER TRANSPORTATION.
• 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 (attraction of water molecule with lignified wall of xylem-which maintain the water column in xylem element).
  - Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
• These properties give water high tensile strength, i.e., an ability to resist a pulling force, and high capillarity, i.e., the ability to rise in thin tubes.
• In plants capillarity is aided by
  - The small diameter of the tracheary elements – the tracheids and vessel elements promotes the capillarity.
• The process of photosynthesis in leaf requires water.
• The system of xylem vessels from the root to the leaf vein can supply the water.
• There are two causes which generate PULL FORCE FOR ASCENT OF XYLEM SAP.
  - First, As water evaporates through the stomata,  the thin film of water over the cells is continuous, it results in pulling of  water, molecule by molecule(cohesive property of water molecule), into the leaf from the xylem.
  - Second, there is concentration gradient of water vapour between atmosphere and sub-stomatal cavity. Therefore because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. This creates a ‘pull’.
• The forces generated by transpiration (transpiration pull) can create pressures sufficient to lift a xylem sized column of water over 130mt high.

Transpiration

• Transpiration is the evaporative loss of water by plants.
• It occurs mainly through stomata.
• Exchange of oxygen and carbon dioxide in the leaf also occurs through these stomata.
• Normally stomata are open in the day time and close during the night.
• The immediate cause of the opening or closing of stomata is a change in the turgidity of the guard cells.
• The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape.
• In open stoma, Cellulose microfibrils in cell wall of guard cell are oriented radially (in turgid guard cell).
• In close stoma, the orientation of cellulose microfibril  in cell wall of guard cell is longitudinal(here guard cell is in flaccid state).
• Usually the lower surface of a dorsiventral (often dicotyledonous) leaf has a greater number of stomata while in an isobilateral (often monocotyledonous) leaf they are about equal on both surfaces.
• Transpiration is affected by several external factors: temperature, light, humidity, wind speed.
• Plant factors that affect transpiration include number and distribution of stomata, per cent of open stomata, water status of the plant, canopy structure etc.
• Less than 1 per cent of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost through the transpiration.

Importance of Transpiration

• 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.
• The humidity of rainforests is largely due to this vast cycling of water from root to leaf to atmosphere and back to the soil(via rain).

Uptake and Transport of Mineral Nutrients

• Some of the minerals absorb passively but some require active transportation because:
  - minerals are present in the soil as charged particles (ions) which cannot move across cell membranes are transported actively.
  - the concentration of minerals in the soil is usually lower than the concentration of minerals in the root therefore active transportation is needed.
• Most minerals must enter the root by active absorption into the cytoplasm of epidermal cells. This needs energy in the form of ATP.
• The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis.
• Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of the epidermal cells.
• Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem.
• The root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.
• After endodermis, the ions have reached xylem either active or passive uptake, or a combination of the two, their further transport up the stem to all parts of the plant is through the transpiration stream.
• The growing regions of the plant, such as the apical and lateral meristems, young leaves, developing flowers, fruits and seeds, and the storage organs are the chief sink of mineral ions.
• Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
• Remobilization of minerals from old, senescing part to developing young growing part take place.
• Elements most readily mobilised are phosphorus, sulphur, nitrogen and potassium.
• Some elements that are structural components like calcium are not remobilised.
• From analysis of xylem exudate, , small amount of exchange of materials does take place between xylem and phloem. Hence, it is not that we can clearly make a distinction and say categorically that xylem transports only inorganic nutrients while phloem transports only organic materials, as was traditionally believed.
• Nitrogen, sulphur and phosphorus are carried in inorganic form as well as organic compound.
• N₂ mostly carried as organic compound whereas Sulphur and phosphorus mostly travel as inorganic form.

Phloem Transport: Flow from Source to Sink

• FOOD, PRIMARILY SUCROSE, IS TRANSPORTED BY THE VASCULAR TISSUE PHLOEM FROM A SOURCE TO A SINK.
• Normally the leaf, is the SOURCE as it perform photosynthesis and sink is the part that needs or stores the food.
• But depending on the season or the plant’s needs, Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees act as sink as they need energy for growth and development of the photosynthetic apparatus.
• The direction of movement in the phloem can be upwards or downwards,
  - i.e., bi-directional: the source-sink relationship is variable.
• Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported or translocated through phloem.
• The translocation in phloem is explained by the pressure flow hypothesis.
• Glucose is prepared through photosynthesis in leaves then it is converted into sucrose then this sucrose move into the companion cells and then into the living phloem sieve tube cells by active transport.This is called loading at source point.
• Loading in sieve tubes leads to hypertonic solution in phloem so water from neighbouring xylem cells enter into phloem by osmosis.
• Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem.
• As osmotic pressure builds up the phloem sap will move to areas of lower pressure.
• At the sink osmotic pressure must be reduced.
• At sink, sucrose moves out from the phloem sap by active transport i. e. unloading is also energy expenditure process.
• Source will use the sugar, either converting it into energy, starch, or cellulose.
• As sucrose moves out the phloem, the osmotic pressure decreases and water moves out of the phloem.
• sieve tube cells form long columns with holes in their end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates and forming continuous filaments.
• As hydrostatic pressure in the sieve tube of phloem 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 as complex carbohydrates.
• Girdling experiment was used to identify the tissues through which food is transported.
• On the trunk of a tree a ring of bark up to a depth of the phloem layer, can be carefully removed. In the absence of downward movement of food the portion of the bark above the ring on the stem becomes swollen after a few weeks(because of removal of phloem food get deposited above girdling).