Transport in Plants: Overview, Topics, Mechanism, Books, Tips

What is Transport in Plants?

Transport in plants is the process by which water, minerals and other substances are transported with the help of the xylem and phloem. This process is essential for the growth, development, and survival of plants. Plants have specialized tissues like xylem and phloem to carry out the long-distance transport of water, minerals, and food.

The flowering plants—angiosperms have a highly coordinated transport system that supports their structure and functions efficiently. The transport in plants involves various transport mechanisms, like diffusion, facilitated diffusion, active transport, and osmosis.

Types of Transport Systems in Plants

Plants use a highly organized transport system to move water, minerals, and nutrients internally. It involves transport across cell membrane in plants that occurs at three levels:

Cell-to-Cell Movement

The substances are moved from one cell to another through:

1. Diffusion

2. Facilitated diffusion

3. Active transport

4. Plasmodesmata (cytoplasmic connections)

Long-Distance Transport (Mass Flow or Bulk Flow)

Ascent of sap is the upward transport of water and minerals through xylem via mass flow or bulk flow. Phloem conducts the bidirectional transport of organic nutrients, also by bulk flow, based on the pressure flow hypothesis.

Uptake and Release Mechanisms

Single cells absorb water and discharge solutes. Excretion or secretion of substances into surrounding tissues or the environment and carried out by osmosis, active transport, or endocytosis/exocytosis.

Methods of Transportation in Plants

Plants perform their transport procedures through three major types of transportation. Plants transport various substances at the cellular level, mainly through cell membranes, involving passive and active processes.

Diffusion

Diffusion in plants is the most common transport mechanism. This is because it does not involve the use of energy; molecules are simply transferred from a region of high concentration to a region of low concentration.

The factors that affect this passive process include the concentration gradient, temperature, and pressure. It plays a very significant or crucial role in the flow of gases, oxygen, carbon dioxide, and small solutes within the plant tissues.

Facilitated Diffusion

Facilitated diffusion refers to the movement of molecules across the cell membrane through particular transport proteins.

Source-to-Sink Model quantifies nutrient flow from source to sink—for instance, roots, and fruits—where they are either utilized or stored.

Mass Flow Hypothesis is used to explain the phloem sap flow due to the pressure difference between source and sink areas. This result follows because of the osmotic pressure and active transport mechanisms.

Key Proteins Involved:

• Porins form pores in cell organelles like mitochondria and plastids that allow specific molecules to pass through.

• Aquaporins are specialized channels allowing the movement of water molecules.

Active Transport

Active transport is the process by which molecules move against their concentration gradient, thus requiring energy, which is typically derived from ATP. It primarily occurs during the absorption of indispensable minerals and nutrients from the soil, generally done to ensure there is ion balance within the plant cell.

This process is important in mineral and nutrient uptake, where they are available in a lower concentration outside the plant. It is also a factor contributing to the maintenance of different cellular functions and general health conditions of a plant.

Vascular Tissue System

The vascular tissue system The xylem and phloem are the channels for moving substances in the plants. The xylem carries the water and minerals from the roots to the leaves, while the phloem carries the sugars and nutrients produced in the leaves to the rest of the plant. It thus forms like a spider's web, moving from the roots up through the trunk into the leaves.

Structure and Function of Xylem

• The xylem is a long, hollow tube running from the roots up to the leaves. It includes tracheids, vessel elements, fibres, and parenchyma cells.

• Water in the soil is absorbed by root hairs and moved through cells by osmosis before entering the xylem. The process of water and mineral transport through this tissue in a plant is considered to be one of its most important features.

• Water and minerals are carried upwards, from the roots to all parts of the plant, through the xylem.

• Meanwhile, sucrose and amino acids are carried throughout the leaves through the phloem.

Types of Xylem Cells

• Tracheids: Long, thin cells with tapered ends; found in all vascular plants

• Vessel Elements: Shorter, wider cells with perforated end walls; primarily in angiosperms

Structure and Function of Phloem

• The phloem serves to bring nutrients and sugars manufactured by the leaves to where they are needed most.

• The phloem is made up of living cells, which have small pores termed sieve plates for transport.

• It comprises sieve tubes, companion cells, fibers, and parenchyma cells. It conducts organic nutrients, mainly sucrose, produced by photosynthesis from leaves to other parts of the plant.

Types of Phloem Cells

• Sieve Tubes: The long tubes formed by sieve tube elements joined end-to-end conduct nutrient transport

• Companion Cells: The cells that assist in conducting the process by providing pressure to the sieve tubes and giving them metabolic support

Difference Between Xylem and Phloem (Table)

The difference between xylem and phloem includes:

Transport of Water in Plants

The transport of water in plants takes place through various mechanisms like:

Cohesion-Tension Theory

Water molecules exhibit cohesion, thereby sticking to one another, and adhesion, thereby sticking to xylem walls, forming a continuous column of water moving upwards due to tension created by transpiration.

Capillary Action

The rise of water up through the narrow xylem vessels is occasioned by the combined effects of cohesion and adhesion.

Transpiration Pull

In transpiration, the water evaporates through the stomata of the leaf. This creates negative pressure that pulls more water up from the roots through the xylem.

Root Absorption in Plants

Water is essential for all physiological activities in plants. Water absorption by roots and movement through the plant involve both passive and active mechanisms. Water absorption takes place through two basic mechanisms:

Active Absorption

Active absorption of water happens in the symplast with the help of the Diffusion Pressure Deficit. It is relatively slow, and both osmotic and non-osmotic forces play a part. The cells of the root generate the force required to absorb the water; this can be affected by temperature and humidity.

Passive Absorption

Passive absorption of water takes place through osmosis, and there is no energy involved. It occurs via the apoplast pathway (through cell walls). It is a dominant method under transpiring conditions.

Stomata and Transpiration

Transpiration is the process of loss of water vapor from the aerial parts of plants, mainly through small pores on the leaf surface called stomata. The small leaf surface pores controlled by guard cells regulate gas exchange and the loss of water vapors.

Factors Affecting Transpiration

Light intensity, temperature, humidity, and wind. Generally, light intensity increases the temperature, which raises the rate of transpiration, and high humidity with low wind reduces it.

Role in Water Regulation

It regulates the water balance of plants, cools the plant, and the suction force that it pulls helps in the upward flow of nutrients by pulling up water through the xylem.

Transport in Plants NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Types of Transport System

• Vascular Tissue system

Practice Questions for NEET

Q1. Diffusion is the result of concentration gradient which refers to

1. Change of concentration with respect to time

2. Change of concentration with respect to space

3. Change of concentration with respect to temperature

4. Change of concentration with respect to gravity

Correct answer: 2) Change of concentration with respect to space

Explanation:

Movement of all substances from the area of their higher free energy to the area of their lower free energy along the concentration gradient. Neutral solutes move across the membranes by diffusion. The net direction of diffusion of particles depends on their concentration in a particular region or space.

Hence, the correct answer is option 2) Change of concentration with respect to space.

Q2. Why does an increase in temperature increase the rate of diffusion?

1. The increased thermal energy of the medium reduces the kinetic energy of particles

2. The decreased thermal energy of the medium increases the kinetic energy of particles

3. The increased thermal energy of the medium does not change the kinetic energy of particles

4. The increased thermal energy of the medium increases the kinetic energy of particles

Correct answer: 4) The increased thermal energy of the medium increases the kinetic energy of particles

Explanation:

Higher the thermal energy in a medium: this raises the kinetic energy of its particles resulting in faster diffusion. The more heat there is, the more violent motion the particles perform the more collisions they experience, and the better the diffusion becomes as particles spread out and mix with others. Such a process plays a very significant role in biological and chemical systems, for example, in the transport of molecules, gases, and ions that determine the rate of chemical reactions and dynamics of systems.

Hence, the correct answer is option 4) The increased thermal energy of the medium increases the kinetic energy of particles.

Q3. Which of the following has the maximum diffusion pressure?

1. Cell sap

2. Rainwater

3. Ice caps

4. All of these

Correct answer: 2) Rainwater

Explanation:

Pure water has a maximum diffusion pressure of 1236 atm, meaning it has the highest tendency to diffuse and move freely. When solutes are added, the diffusion pressure decreases, leading to a reduction in water potential. This works on the principle that during osmosis, the water moves from a region of higher diffusion pressure (pure water) to a region of lower diffusion pressure (solution).

Hence, the correct answer is option 2) Rainwater.

Also Read:

• MCQs on Diffusion

• Transport in Plants

• Reverse Osmosis

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