Transportation In Plants: Concepts, Active and Passive Transport, Xylem, Phloem

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What is Transportation in Plants ?

Transportation is a vital process in plants. Trees transport all the nutrients and water it needs for survival from its roots to the tips of the leaves.

In the case of transportation in plants, the biggest constraint is water as it ends up being a limiting factor in growth. To overcome this problem, trees and other plants have the perfect system for the absorption and translocation of water.

“Transportation is the process that involves the movement of water and necessary nutrients to all parts of the plant for its survival.

Translocation – Bulk or mass transport over long distances through vascular bundles.

Xylem is unidirectional from roots to stems.

Phloem is multidirectional from leaves to root, growing part.

Means of Transport –

Diffusion-

Passive process, no energy expenditure, slow
Takes place in liquid, gas, solids.
It can be from one part of the cell to other or cell to cell.
From higher concentration to lower concentration.
Depends on gradient of concentration, permeability of Membrane, temperature, pressure, size of substance, solubility in Lipids.

Facilitated diffusion –

Substances soluble in lipids diffuse through the Membrane faster.
Hydrophilic substances find difficult to pass, so they need some proteins to facilitate diffusion.
No expenditure of ATP energy.
It can’t cause net transport from low to high concentration.
The transport rate reaches the maximum when all protein transporters are being used(saturation).
Allows cells to select substances sensitive to inhibitors that react with protein.
Some channels are open, and some are controlled.

Porins – Proteins that form a large pore in the outer Membrane of plastid, mitochondria and some bacteria allowing the molecule to pass.

Aquaporins – Water channels made of 8 different aquaporins. The molecule binds to transport protein; it rotates and releases it inside the cell.

Passive symports and antiport –

Symport – Both molecules cross the Membrane in the same direction.

Antiport – Move in the opposite direction.

Uniport – Move independently of another molecule.

Active transport –

Use energy, against the concentration gradient, by specific proteins
Pumps are proteins that use energy to carry substances.

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Plant – Water Relations –

Water provides a medium to dissolve
Protoplasm is water in which different molecules are suspended.
Watermelon – 92% water; herbaceous- 85 – 90 % water.
Loss of water through evaporation from leaves – transpiration.
Mature corn plant – 3 litres of water/day
Mustard plants absorb water equal to their own weight in 5 hours.
Mustard plants absorb water equal to their own weight in 5 hours.

Water Potential – (Ψ w)

Water molecules possess Kinetic energy; the concentration of water is proportional to Kinetic energy.
Move along a concentration gradient (from high Ψ w to low Ψ w).
Pure water has high water potential, i.e.- 0.
Unit-pascals (Pa). It has two main components-

(a). Solute potential – (Ψ s) – When some solute is dissolved in pure water, the concentration of water is low, low Ψ w.

All solution has low Ψ w than pure water.

Magnitude of lowering due to dissolution of solute.

It is always negative, low solute molecule – less (more negative) Ψ s.

For solution at atmospheric pressure à Ψ w = Ψ s.

(b). Pressure potential – (Ψ p) – When pressure greater than atmospheric pressure is applied, Ψ w more.

It is also a built-in cell during diffusion, which puts pressure on the cell wall that makes it turgid. So, more Ψp.

Usually positive, in plants negative potential in xylem plays an important role. Ψ w = Ψ s + Ψp

Osmosis –

Diffusion of water through PM.
Depends on concentration gradient (higher to lower), pressure gradient.

Osmotic pressure – Pressure required to prevent water from diffusing more solute concentration. High pressure to prevent water from diffusing.

Osmotic pressure = – osmotic potential.

Plasmolysis –

Isotonic – External solution balance osmotic pressure of cytoplasm. No net flow of water, water flow inside and outside is in equilibrium, flaccid cell.
Hypotonic – External solution is more dilute than the cytoplasm, more Ψ w, water diffuse into the cell, pressure by cytoplasm against a wall is turgor pressure, pressure by protoplast due to entry of water against the wall is pressure potential Ψ p, responsible for engagement and growth of the cell, the cell swells.
Hypertonic – External solution is more concentrated, cell shrinks, more solutes, plasmolysis, water moves out first from cytoplasm then vacuole, reversible process.

Imbibition –

Water is absorbed by solids, colloids to increase volume. E.g.- Absorption of water by seeds & dry wood.
Along concentration gradient (high to low).
Water potential gradient and affinity between absorbent & liquid is required.

Long Distance Transport of Water –

Water & minerals, food is moved by mass or bulk flow (because of pressure difference).
Substances, whether in solution or suspension, are swept at the same pace.
It can be achieved either through hydrostatic pressure (garden haze) or negative hydrostatic pressure (suction through a straw).

How do plants absorb water?

By diffusion in root hair.
Root hair increases the surface of absorption. It can be by two ways.

(a). Apoplast – Most common, system of the adjacent cell wall that is continuous except Casparian strips that are suberized (impermeable for water), so move through a region not suberized.

Doesn’t involve crossing cell membrane.

Dependent on a gradient, no barrier, by mass flow.

Common as cortical cells are loosely packed, so no resistance to water.

(b). Symplastic – System of interconnected protoplast, neighbouring cells are connected through cytoplasmic strands through plasmodesmata, slow, down potential gradient, cytoplasmic streaming. E.g., in Hydrilla leaf.

After endodermis, water moves by symplast to reach the xylem.

In young roots, water enters indirectly into xylem vessel & tracheid (non-living conduits)- apoplast.
Some have additional structures to help water absorption – mycorrhiza, a symbiotic association of a fungus with a root system. Hyphae have a layer surface area to absorb ions. So, provide minerals & water to roots and in return, roots provide Sugar and N- containing compounds.
E.g., Pinus seeds can’t germinate & establish without mycorrhizae.

Water movement up a plant –

Root pressure –

When ions from the soil are actively transported into vascular tissue(roots), water follows, high pressure inside the xylem.
The pressure is root pressure, responsible for pushing up water in small heights, with no role in tall trees.
Observable at night when evaporation is low.
Guttation – Excess water collects in the form of droplets around the special opening of veins near the tip of grass and monocots. Such water loss in a liquid phase is guttation.

Transpiration pulls –

Water can be transported up at 15 m/hr.
Water is mainly pulled through plants; a driving force is transpiration is called the cohesion tension transpiration pull model of water transport.
Less than 1% of water reaching leaves is using in photosynthesis. It is lost through stomata in leaves. It can be studied by using cobalt chloride paper that turns colour on absorbing water.

Transpiration

Mainly day, no root pressure, through stomata.
Occurs through stomata, exchange of O2 & Co2 takes place.
Stomata open during the day & close during the night.
Opening & closing is a change in the turgidity of guard cells.
Its inner wall is thick & elastic.
When turgidity increases, a thin outer wall bulges and force the inner wall into a crescent shape.
Cellulose microfibril is oriented radially rather longitudinally, making it easier for stomata to open.
When they lose turgor, regain shape, guard cells become flaccid.
The lower surface of the dicot leaf has more stomata, and both surfaces have equal stomata in monocot.

Factors –

Temperature, light, humidity, wind, no. of stomata, water status, canopy structure, % of open stomata, distribution of stomata.

Physical properties of water on which transpiration depend.

Cohesion – Attraction between water molecules.

Adhesion – Attraction of water molecule to the polar molecule (treachery elements).

Surface tension– Water molecule is attracted to each other in liquid phase more than to water in the gas phase.

This gives high tensile strength (ability to resist pulling force), high capillary (ability to rise in thin tubes.)
A lower concentration of vapour in the atmosphere creates a pull for transpiration.
Forces by transpiration can create pressure sufficient to the left xylem size column of water over 130 m high.
Transport minerals cool the leaf surface by 10-150, maintains the shape of the plant by keeping cells turgid.

Photosynthesis – Humidity of rainforest is due to vast cycling of water from root →Leaf atmosphere →back of soil.

The evolution of C4 photosynthesis maximize the availability of Co2 & minimize water loss. C4 plants are twice as efficient as C3 in fixing Co2(makes Sugar). C4 lose half as much water as C3 for the same amount of Co2 fixed.

Uptake and transport of mineral nutrients –

Uptake of mineral ions – They can’t be passively absorbed as they are present as a charged particle in soil and the concentration of mineral in the soil is less than in root. Some ions move passively.

So, they need to enter by active absorption, partly responsible for potential water gradient in roots.
Some proteins in PM pump ions actively from the soil into the cytoplasm.
Endodermal cells have transport proteins that help in the transport of solutes.
They are control points where plants adjust the quantity & types of solutes in the xylem.
Root endodermis because of suberin transports in 1 direction only.

Translocation of Mineral Ions –

Chief sinks for mineral elements are growing regions of plants (apical, lateral meristem)
Unloading of mineral ions occurs at fine vein endings through diffusion and active uptake of the cell.
They are frequently mobilized, from older dying leaves to young.
Elements most readings mobilized – P, S, N, K.
Structural component- Ca (not mobilized).
Nitrogen travels as inorganic but can be organic like amino acid. Similarly, P & S are carried as organic. So, we can’t distinguish the xylem transport only inorganic nutrient or phloem organic.

Phloem transport: Flow from source to sink.

Food (sucrose) from source to sink. The source is part that synthesize food and sink where it stores food.
It can be reversed according to season. E.g., Sugar in roots is mobilized to become a source in early spring when buds act as a sink (as they need energy for growth and development of photosynthetic apparatus).
Phloem sap is mainly water & sucrose.

The pressure-flow or mass flow hypothesis –

Mechanism for translocation of Sugar from source to sink.
Glucose is prepared at source à converted to sucrose à Companion cells à Sieve tube cells by active transport.
Produce hypertonic conditions in the phloem. Water in the adjacent xylem moves in phloem by osmosis. So, osmotic pressure increases.
Sap moves to the area of low pressure. At the sink, OP must be reduced. Again, active transport is required to move sucrose out of phloem à into cellà used the sugar à convert in energy, starch, cellulose.
As sugars are removes, OP decreases water moves out of phloem.
Phloem contains sieve tube elements that have a hole in sieve plates. Cytoplasmic strands pass-through holes, so they form continuous filaments. So, hydrostatic pressure increases, pressure flow begins, sap moves through the phloem. At the sink, incoming Sugar is actively transported out of phloem and removed as a complex carbohydrate. Loss of solute increases Ψ w, water passes out, returning to the xylem.

Girdling experiment –

On the trunk, a ring of bark up to the depth of phloem is removed. In its absence, downward movement of food was blocked, so its swelled after few weeks.
It shows phloem is responsible for the translocation of food. It takes place in one direction (towards roots).

Citation

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