Transportation in plants is a fundamental process that is necessary for its survival. Plants make their food for the process of photosynthesis. This process requires water which is transported from the roots which further absorb the water from the soil. Plants need food for their growth. The flowering and fruit formation also require energy which gets transported from the leaves to the different parts of the plant.
Role of Transportation in Plants
Plants lack both interstitial fluid as well as a regular circulation system. They must transport various types of substances, not only over short distances but also over very long distances. Substances move over short distances through diffusion and active transport is boosted via cytoplasmic streaming. Long-distance transport occurs through vascular systems, xylem, and phloem. This transport of substances over longer distances through the vascular tissue is called translocation.
Transportation in Plants
The two complex tissues involved with transportation in plants are:
- Xylem: Xylem is responsible for conducting water from the roots to the stems and leaves. It is made up of four elements- tracheids, vessels, Xylem fibers and Xylem Parenchyma. The movement through Xylem occurs through osmosis. A giant tree contains multiple xylem tissues. The transport of water occurs throughout the plant body.
- Phloem: Food is synthesised in the leaves and transported throughout the plant body through phloem.Phloem is further divided into Sieve tube elements, Companion cells, Phloem parenchyma and Phloem fibers. The carbohydrates and sugars manufactured are sent across the plant from the leaves. The direction of translocation is basically unidirectional in the case of water(from root to stem, leaves, flowers, and fruits). It is multidirectional in the case of minerals and organic solutes.
Water Absorbetin in Plants
Absorption in plants occurs via two pathways:-
- Apoplast Pathway: This pathway uses the gradient to transport water though the intercellular spaces and walls of the cells. No movement of water occurs across the cell membrane. The water moves smoothly as a result of cohesive and adhesive forces produced due to transpiration. This is also called as passive absorption.
- Symplast Pathway: This pathway uses Plasmodesmata to move water across neighbouring cells. The movement of water is slower in the Symplast pathway. It is also aided by cytoplasmic streaming.
Means of Transport
Transport in plants occurs via two methods- active and passive transport. Passive transport includes diffusion and facilitated diffusion. Below is a diagram showing the different means of transport in plants.
Diffusion
Movement by diffusion is passive and slow and it occurs along the concentration gradient. In diffusion, molecules move in a random fashion, the net result being substances moving from regions of higher concentration to regions of lower concentration. Diffusion does not require any energy. Diffusion acts as the primary method for gaseous exchange within the plant organism. The rate of diffusion is influenced by several factors, including the concentration gradient, the permeability of the intervening membrane, temperature, and pressure.
Facilitated Diffusion
The cell membrane is made up of lipids. Particles that are lipid soluble can easily pass directly through the cell membrane as it is almost entirely made of it. The hydrophilic solutes, find it difficult to pass through the membrane. Their movement has to be facilitated. The membrane proteins help in transporting these molecules, hence the term 'Facilitated Diffusion'.
Aquaporins are membrane proteins responsible for the passive transport of water-soluble substances. They do not set up a concentration gradient. Porins form large pores in the outer membrane of cell organelles like plastids and mitochondria.This diffusion is very specific as it allows cells to select substances for uptake. It is sensitive to inhibitors, and furthermore, shows a saturation effect.
Two major types of transport proteins are known viz., Carrier Proteins (also called carriers, transporters) and Channel Proteins. Carrier proteins bind the particular solute to be transported and deliver the same to the other side of the membrane. Channel proteins are usually gated, i.e., they may be open or closed. When the 'gate' is open, the solute of an appropriate size may diffuse.
Active Transport
Pumps are proteins that use energy to carry substances across the cell membrane. These pumps can transport substances from low attention to high attention ('uphill' transport). The transport rate reaches a maximum when all the protein transporters are being used or are saturated. Like enzymes, the carrier protein is very specific in what it carries across the membrane. These proteins are sensitive to impediments that inhibit protein side chains. Active transport is faster than passive transport.
Some carrier proteins allow transport only if two types of molecules move together. This is called Cotransport. It is of two types. In the symport system of cotransport, both molecules cross the membrane in the same direction at the same time. In the antiport system of cotransport, both molecules move in the opposite directions. When a molecule moves across a membrane independent of another molecule, the process is called uniport.
Given below is an image depicting the different types of active transport in plants.
Forces Responsible for Transportation in Plants
Transportation is plants occurs as a result of the following forces:
This is responsible for moving or bumping water through very short distances. It allows the formation of a continuous water column which sometimes breaks due to the tension created by transpiration.
Transpiration Pull
Cohesion-tension- transpiration pull model of water transport is applicable in case of long trees and plants. Water is usually lost from the leaves through the stomata. This process is called Transpiration. Transpiration results in driving the xylem sap upwards towards the highest points of a tall tree. When water evaporates due to transpiration, a suctional pull is created which creates a continuous column of water. Few properties of water allows this process to happen like Cohesion, Adhesion and Surface Tension.
- Cohesion: This is the property by which water molecules stick to each other.
- Adhesion: This is the property by which water molecules attach to the wall of the polar surfaces.
- Surface Tension: This property makes sure that the water molecules are attracted to each other in liquid phase more than to water in the gas phase.
All these properties allow the water to rise in the form of small tubes which is known as the capillary action. The small diameter of tracheids and vessels aids in capillarity.