Transport System in Plants Class 11 Biology

Plants obtain a variety of inorganic elements (ions) and salts from their surroundings, especially from water and soil. The movement of these nutrients from the environment into the plant as well as from one plant cell to another plant cell essentially involves movement across a cell membrane. Transport across cell membranes can be through diffusion, facilitated transport, or active transport. Xylem and phloem are the two components by which transportation takes place in a plant. Xylem transports water and minerals in plants. It is a vascular tissue. Whereas, phloem transports food prepared by the leaves to the other parts of the plant.

Plants lack both interstitial fluid as well as a regular circulation system. Even then they have to move various types of substances not only to short distances but also to very long distances.

Means of Transport

The passage of materials into and out of the cells is carried out by a number of methods- diffusion, facilitated diffusion, and active transport.

Diffusion

Diffusion is the movement of molecules such as gases, liquids, and solutes from the region of higher concentration to the region of lower concentration. 

The difference in the concentration of matter i.e. gas, liquid, or solution at different points decides the rate and direction of diffusion. The diffusion of one matter is interdependent on the other. So, mostly gases and solutes move simultaneously and independently at different rates in different directions at some place and time, without collision. From soil, water and minerals move into plants, through root cells, by a process called diffusion. Water and solutes move with the help of dead and living vessels and also from cell to cell by diffusion.

1. Diffusion Pressure gradient: A gradient is necessary for diffusion to occur. The greater the gradient pressure faster will be the rate of diffusion.
2. Size of substances: Smaller the substance, the diffusion rate is faster.
3. Solubility of lipids: Greater the solubility, the greater will be the rate of diffusion. 

Importance

• Photosynthesis and respiration take place through the process of diffusion.
• Transpiration of water vapors involves the process of diffusion.
• The aroma of flowers also involves the process of diffusion. Due to the diffusion of volatile aromatic compounds to attract pollinators.
• In the process of passive salt uptake, the ions are absorbed due to diffusion.
• Translocation of food material is done by diffusion.

Facilitated Diffusion 

👁 Facilitated Diffusion

In this diffusion, substances move beyond the membrane through their concentration gradient in presence of certain carrier protein molecules, called permeases, present in the membrane.

Those substances, which are hydrophilic have difficulty passing beyond the membrane. The movement of such substances makes easier by some proteins. These proteins provide sites for molecules to pass through the membrane. Proteins develop a channel that is present on membranes, through which molecules pass.

Characteristics

1. It is passive transport because it is not dependent on energy.
2. On a membrane, some channels are always open and others can be closed sometimes. Some channels are large to allow a variety of molecules to pass.
3. In the outer membranes of plastids, mitochondria, and a few bacteria, large pores are formed due to the presence of porin protein. Eight different types of aquaporins formed water channels.
4. Same to diffusion, a concentration gradient is the main component of it. 
5. In this process, permeases are involved which show substrate specificity.
6. It transports so faster than simple diffusion.

Significance

Facilitated diffusion helps in the transport of glucose in liver cells and RBCs through their cell membrane. It also helps in the absorption of fructose and nucleotides in the small intestine.

Passive Symports and Antiports 

👁 Passive and Antiport Transport

Some transport proteins allow diffusion only when two types of molecules move together.

1. Symport: In symport, both molecules pass the membrane together and in the same particles.
2. Antiport: In antiport, both molecules pass the membrane together but in opposite directions.
3. Uniport: In uniport, molecules pass a membrane without any support from other molecules.

Active Transport 

👁 Active Transport

When the cell needs to move substances against a concentration gradient it will need energy. Some intrinsic protein molecules work as molecular pumps. They allow cells for active transport to gather glucose or ions against the concentration gradient. The cell undergoes active transport and produces abundant mitochondria to provide ATP needed for active transport.

Features of Active Transport

1. In this process, movement occurs from lower concentration to higher concentration.
2. Carrier proteins are very specific such as enzymes for substances to be carried across the membrane.
3. Inhibitors can hold back the process by reacting with protein side chains.
4. When whole protein carriers are in use with the substance to be carried, the transport rate reaches its maximum.

Active Transport helps in

• Absorption of amino acids from the gut.
• Absorption of mineral ions by plant roots 
• Excretion of urea and hydrogen ions by the mammalian kidney 
• Exchange of sodium and potassium ions in nerve cells 
• Loading of sugar from the leaf into the phloem in the plant
• Filling of the contractile vacuole in amoeba 
• The phloem is rich in sugars because of the high rate of active transport.

Permeability

The degree of diffusion of gases, liquids, and dissolved substances through a membrane is called permeability. The diffusion of water and solutes into plant cells from the environment is carried by a membrane by enclosing the protoplasts. There are three types of membrane:

1. Permeable: Permeable membranes permit free motion of water and solutes into the interior and exterior of the cell, eg. Cell walls in plant cells.
2. Impermeable: In process of diffusion, it prevents the flow of water and solutes into the protoplasm of the cell eg .  Suberized walls of cork cells, and the cuticle layer of the leaf. 
3. Semipermeable: They are selective in nature. These membranes allow solvent on one side to move freely but at the same time do not allow movement of solutes. Eg Fish and animal bladders, egg membranes, and plasma membranes of cells are all semi-permeable membranes.

Plant-Water Relations 

During a plant's biological clock, water in large amount is absorbed continuously from the soil and move via the plant. Water is necessary for all physical activities of plants. Water acts as a good solvent. Land plants get their almost water from the soil. Water is the most important component of plants. The configuration of plants attests mostly herbaceous plants constructed of 70 to 85 % water and the woody part of plants contains about 50 % of water. In addition, plants lose water in large amounts by transpiration. Without constant water, plants cannot do physical activities, such as photosynthesis, respiration, and growth. 

Water Potential 

In a chemical system, any substance of free energy per mole is chemical potential. Thus, the chemical potential of a substance depends on the number of molecules of a substance under constant pressure and temperature. The potential energy of water is called water potential.

Osmosis

👁 Osmosis and Reverse Osmosis

Osmosis is a diffusion, in which liquid moves from a higher concentration to a lower concentration through a semipermeable membrane.

Types of Osmosis 

1. Exosmosis: Due to a hypertonic solution (of higher concentration), water moves out of the cell. The process is called exosmosis.
2. Endosmosis: Due to hypotonic solution (of lower concentration), water enters the cell. The process is called endosmosis.

Plasmolysis 

In plasmolysis, a plant cell is dipped in a higher solute concentration solution than the cell sap and contraction of protoplasm begins. It can be seen in the amount of the spirogyra filament in a strong hypertonic solution.

Imbibition 

Absorption of liquid without setting up a solution, this event is called imbibition. Many imbibants are present in plant cells such as protein, cellulose, starches, etc. Solid substances are called imbibing. The liquid is known as imbibate.

Long-distance Transport of Water  

Through diffusion, long distances of substances are not covered as it is a very slow process. By mass or bulk flow system material moves in a large multicellular complex organism.

Mechanism of Water Transport

Water is absorbed by two methods:

1. Active Absorption: In active absorption, energy is required, pressure on the xylem is positive, and it takes place in the season when the rate of transpiration is low, living cells are required, and metabolic inhibitors stop the process.
2. Passive Absorption: In passive absorption, energy is not required, pressure on the xylem is negative, it takes place in all seasons, living cells are not required, and metabolic inhibitors show no effect.

Water Movement Up a Plant 

When water moves from the root towards the upward direction of the plant is called the ascent of sap. The upward transport of water in plants which are in some cases as tall as 400ft. Posses a serious problem.

Root pressure 

When a plant that is well watered is cut near the base, through the cut end xylem sap flows out with pressure. This process is mostly seen in herbaceous plants. The water flows slowly out through a small opening due to bleeding or exudation. This exudation of water is due to hydrostatic pressure inside the root hence, known as root pressure. If roots are provided with hypertonic or isotonic solution root pressure disappears. Stephen Hales (1727) coined the term root pressure.

Transpiration Pull 

The theory was originally proposed by Dixon and Joly (1894) and supported by Renner (1911,1915), Curtis and Clark (1951 ), Bonner and Golston (1952), and Kramer and Kozlowski (1960).

A transpiration pull is a process in which, inside the xylem tissue the force of pulling is produced. This force helps water to move in an upward direction into the xylem vessels. In this process, the loss of water in the form of vapors takes place.

Transpiration 

It is the evaporative loss of water by plants. It occurs mainly through the stomata in the leaves. Mostly, from the aerial parts of the plant water is absorbed and lost into the air by the process of transpiration. Only less than 2% of water is used in most metabolic activities in the plant.

👁 Transpiration

Uptake and Transport of Mineral Nutrients

From the soil, plants absorb the minerals and transfer them to other parts of the body. Soil is the main source of mineral salts in which clay crystals with a central nucleus are called micelle.

• Uptake of Mineral Ions- Mineral elements can be absorbed in the form of molecules or as ions. When a positively charged ion as K⁺  is absorbed by the cell, either a positively charged ion as H⁺ is removed from the cell. 
• Translocation of Mineral Ions- The direction of transport in phloem changes during the developing period of the plant. Young seedlings move food in the upward direction from seed to young leaves until they start to form food. Food moves from leaves to roots and fruits move in a downward direction for storage. Thus food migrates from the parts where it is in plenty to those where it is required to sink. 

Phloem Transport: Flow From Source to Sink 

The metabolites can flow only in sieve elements of phloem as they resemble the hollow pipelines called cellular channels. The sieve tubes are arranged end to end to constitute a continuous system throughout the plant. Thus, the structure of the phloem clearly suggests that it is well adapted for the purpose of translocation of food.

Question 1: If a cell is placed in a hypotonic solution, what will happen? 

Answer:

The cell enters water and becomes turgid . This process is called endosmosis.

Question 2: What is the difference between diffusion and osmosis?

Answer:

Diffusion is the movement of molecules, ions of solid , gases and liquid from a region of higher concentration to lower concentration whereas Osmosis is the movement of only liquid molecules from a region of higher concentration to the region of lower concentration through a semi permeable membrane. 

Question 3: What are the factors affecting the rate of diffusion? 

Answer:

Factors affecting rate of diffusion are: 

• Diffusion Pressure gradient: A gradient is necessary for diffusion to occur. Greater the gradient pressure, faster will be the rate of diffusion. 
• Size of substance: Smaller the substance, diffusion rate is faster. 
• Solubility of lipids: Greater the solubility, greater will ne rate of diffusion.

Question 4: What are porins? 

Answer:

In outer membranes of plastids, mitochondria and few bacteria , large pores are formed due to presence of porin proteins.

Question 5: Describe the role played by protein pumps during active transport in plants. 

Answer:

When the cell needs to move substances against concentration gradient it will need energy. Some intrinsic protein molecules works as protein pumps. They allow cell for active transport to gather glucose or ions against concentration gradient. Cell undergoes active transport and produce abundant mitochondria to provide ATP needs for active transport.