Active transport

Introduction

Transport is a crucial component of plant physiology and involves moving organic nutrients and water throughout the plant body. Food is produced by photosynthesis in the leaves which is transported to other parts of the plant and water is absorbed from the soil through the roots and then to various aerial regions of the plant. In higher plants, the transport of food and water takes place through specialized structures known as the xylem and phloem. An elaborate root system helps these plants to absorb water from the soil.  However, primitive plants perform the function of absorption of water through simple structures such as pores, or the external body surface.

For more help, you can Refer to our video in Class 7 Science in Lesson no 11. Check out the video Lesson for a better understanding.

Transportation in plants-

The main 3 means of transport in plants are via diffusion, Facilitated diffusion, and Active transport. Vascular bundles assist in the movement of water and carbohydrates throughout the entire plant body.

Simple Diffusion- Diffusion is the process of movement of a molecule from a location of higher concentration to a lower concentration i.e. along the concentration gradient. It is a spontaneous process and doesn’t require any energy. Here no special membrane proteins are required and this method only allows the transportation of hydrophobic molecules as the cell membrane is made up of a lipid bilayer.

• Facilitated diffusion- Facilitated diffusion is a type of diffusion facilitated by some proteins which assist in the movement of various metabolites through the cell membranes. These are majorly used for the transportation of water, ions, and other hydrophilic molecules which cannot pass through the lipid bilayer. The channel proteins and the carrier proteins are the two different types of membrane transport proteins.
• Channel proteins- Pores in the plasma membrane are created by channel proteins. These proteins are highly selective and hence allow the transportation of specific molecules only. Eg-Aquaporins only permit water movement and Aquaglyceroporins facilitate only the movement of glycerol and water.
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• Carrier proteins- These are special types of proteins that undergo conformational changes after binding to a particular solute. Carrier proteins help in the transportation of ions eg- chloride-bicarbonate exchanger, also known as the anion exchange protein, which facilitates the simultaneous transit of HCO3- and Cl–. It also helps in the transport of glucose via the glucose transporter(GLUT).
• Active transport- Active transport facilitates uphill solute movement throughout the cell by functioning against a concentration gradient. Here the molecules move from an area of lower concentration to a higher concentration. Hence, some kind of energy is required. There are two ways to supply energy:

• The energy produced by ATP hydrolysis is known as primary or direct active transport. For instance, the energy is given through the Na+-K+ ATPase (electrogenic pumps). 

• The energy supplied through the electrochemical gradient is enabled by the Symport pumps (such as the Na+-Glucose symporter, lactose permeases, etc.) and Antiport pumps (such as the Na+-Ca2+ antiporter) These are often referred to as secondary or indirect active transport.

Transport of water from roots

Long-distance transportation of water and nutrients in plants takes place through the xylem and phloem. The soil-plant-atmosphere continuum (SPAC) is a pathway that discusses the movement of water from the soil, its transportation to plant parts, and 

the expulsion of water from the plant. Absorption of water in plants occurs through-Passive absorption- when water is absorbed from a higher water potential (in soil) to a lower water potential (in root cells). Active absorption-which occurs due to transpiration.

Ascent of sap

The term “ascent of sap” refers to the movement of water and minerals from the soil to aerial portions like leaves and stems. The cohesion-tension theory, also known as transpirational pull, explains this (Dixon and Jolly, 1894). According to this idea, water from locations with higher water potential (such as the roots) is drawn up, to areas with lower water potential (the leaves)  due to the tension (negative hydrostatic pressure) which is generated by the leaves. There is low water potential in the leaves because they lose water due to the process of transpiration. The water column is kept from collapsing by the cohesive forces of attraction between the water molecules. As a result, water absorption happens to make up for the transpirational loss.

Summary

Intricate transportation networks are needed by plants to enable the interchange of materials and nutrients. The passive process of moving molecules from higher to lower concentrations is known as diffusion. It could be simple (independent) or facilitated transport. Energy is spent during active transport, which is carried out by membrane proteins called transporters and channel proteins. The transpirational pull/cohesion-adhesion principle is used by the roots to absorb water. 

Frequently Asked Questions

1. What do you understand about water potential?
Ans: The potential energy of water per unit volume is referred to as water potential. It describes the amount of water in the atmosphere, plants, and soil.

It is shown as the sum of Osmotic potential, matrix potential, hydrostatic potential, and gravitational potential. 𝜳w = 𝜳𝛑 (osmotic potential)   + 𝜳m (matrix potential)  +𝜳p (hydrostatic potential)  +  𝜳g (gravitational potential).

2. What is the symplastic pathway of water absorption?
Ans: The continuous network of cell cytoplasms is known as the symplastic route.

Through the plasmodesmata connections present between two cells, this pathway allows absorbed water to flow from cell to cell.

3. Which method of water absorption is quick?
Ans: The apoplastic pathway will move more quickly since there won’t be as many obstacles in its path as there are in the symplastic pathway, which is obstructed by Casparian strips and must travel through the protoplasm, which slows down movement.