Cell to Cell Transport-Active Transport

Introduction

Cell to cell transport of substances is considered as the small distance transportation. This small distance transportation occurs by diffusion and cytoplasmic streaming. Diffusion is a passive process that means it does not require energy whereas the cytoplasmic streaming is an active process that needs energy for transportation.

Body 

Like all other living cells, plant cells are also surrounded by a semipermeable membrane that must be crossed by any particle moving into or out of the cell. Diffusion, facilitated diffusion, and active transport are the three basic processes by which particles move across plant cell membranes.

Active transport necessitates the direct application of energy to move particles across the cell membrane. Diffusion, facilitated diffusion and osmosis are the three types of passive transport which can occur without the use of cellular energy.

Diffusion

Diffusion is the primary mode of transport in plants. It is a crucial photosynthesis process in which carbon dioxide diffuses from the stomata into the leaves and then into the cells. Furthermore, water and oxygen diffuse from the leaves into the environment during transpiration.

It refers to the movement of particles in a medium from a region of higher concentration to a region of lower concentration without the use of energy. This is a slow process that occurs mostly in the gases and liquids. Temperature and pressure, concentration gradient, permeability of the separating membrane, and other factors all influence the rate of diffusion.

Diffusion is possible because of continuous movement of molecules in a liquid or gaseous phase. They are constantly moving due to kinetic energy, which exists at temperatures higher than absolute zero. A substance’s free energy increases in proportion to its concentration. When molecules move, they exchange kinetic energy by colliding with one another, and there is a random but progressive movement from high free energy (high concentration) regions to low free energy regions (low concentration). Diffusion can happen quickly over short distances but take a long time over long distances.

Water is an essential component of all cells, and it enters plant cells via osmosis. Water diffusion across a semipermeable membrane is referred to as osmosis. Many plant nutrients reach the root surface via soil solution diffusion.

Some nutrients diffuse across cell membranes of the root epidermal cells into their cytosol (cell sap or cytoplasm) or from endodermal cell cytosol into xylem tissue. Carbon dioxide diffuses from the atmosphere into the air spaces of leaves via the stomata. Water vapor evaporates from a leaf’s surface via diffusion through open stomata.

There are two types of diffusion that take place in the plants:

1. Simple diffusion 
2. Facilitated diffusion

Passive diffusion is defined by the downhill movement of solutes in particular. Simple and facilitated diffusion mechanisms are both types of passive transport in which solute molecules move from a higher concentration region to a lower concentration region. The difference is that in simple diffusion, molecules move without the assistance of membrane proteins, whereas in facilitated diffusion, membrane proteins assist molecules in moving downhill.

Simple Diffusion

Diffusion is also involved in the transport of photosynthetic products such as sugars through the plant’s phloem. Because cellular membranes are made up of a lipid bilayer, lipid-soluble materials cross the membrane surface via simple diffusion.

Simple diffusion is exemplified in biological systems by the simple transport of molecules at the cellular level. The cell membrane’s bilipid membrane feature prevents all molecules from entering and exiting. Not all substances can diffuse freely through simple diffusion. Small and nonpolar molecules, such as oxygen, carbon dioxide, and ethanol, are examples of such molecules. These molecules can easily pass through the membrane. Because they are hydrophobic, they can easily diffuse through the cell membrane’s hydrophobic lipid bilayer. Within the cell, simple diffusion also occurs.

Facilitated Diffusion

Facilitated diffusion allows substances with low lipid solubility to move across membranes. The substance binds to a transporter molecule, commonly referred to as an ionophore, which transports the substance across the membrane and down the concentration gradient.

There is a distinction between channels (also known as pores) and transporters in terms of material movement through membrane proteins. Channels, for the most part, provide openings with some specificity, and molecules pass through them at a rate close to that of diffusion. They are usually associated with the movement of water or ions. Examples include sodium and potassium channels in nerve cells. Transporters are highly specific and have transfer rates that are orders of magnitude slower. The sodium-potassium pump, the sodium-calcium exchanger, and other transport proteins are examples.

There are three types of transporter proteins as concerned with the facilitated diffusion pathways: Uniport, Symport and Antiport.

A uniport is a protein that only moves one molecule across a membrane. Symports are proteins that move two molecules across the membrane in the same direction. An antiport protein is one that moves two molecules in opposite directions across a bilayer.

Active Transport

Active transport is a type of cellular transport in which substances move in the opposite direction of a concentration gradient. This means that the direction is from a lower concentration area to a higher concentration area. As a result, this process will necessitate the expenditure of energy as well as the assistance of membrane proteins such as carrier proteins.

Active transport can be either primary or secondary. A primary active transport uses chemical energy in the form of ATP, whereas a secondary active transport uses potential energy, which is often derived from an electrochemical potential difference. There is a direct coupling of energy, such as ATP, in primary active transport. Membrane protein transporters in primary active transport include ion pumps, ion channels, and ATPases.

There is no direct ATP coupling in secondary active transport. Rather, the energy from electrochemical potential difference as the ions are pumped into and out of the cell powers the transport. Secondary active transport allows one ion to move down an electrochemical gradient. As a result, entropy increases, which can be used as a source of energy. The antiporters and symporters are the transporters in secondary active transport. Coupled transport or cotransport are other terms for secondary active transport. The simultaneous transport of two substances across a biological membrane is referred to as coupled transport. Depending on the direction of movement of the two substances, it may be a symport or an antiport. A symport type of coupled transport occurs when both move in the same direction. It is called antiport if their movements are in opposite directions.

Active transport examples in plants can be seen in the following cases:

• Ions move from the soil to the roots of plants
• Chloride and nitrate transport from the cytosol to the vacuole
• Photosynthesis sugars move from leaves to fruit
• Calcium moves between cells with the help of ATP energy
• Minerals move through a stem to different parts of the plant
• Water moves from plant roots to other plant cells as a result of root pressure

Conclusion 

The cellular movements and transportations occurring inside the plant body can be passive or active. The solutes particles can be transported upto very small distances by either simple or facilitated diffusion whereas the solvent molecules move through osmosis. However, the transportation that occurs by the expenditure of energy is the active transport of molecules.