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 

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 

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 

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.

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.