Chapter 2: Substance Transport (The membrane model and substance passage)

II.1.Introduction

Membrane transport is defined as passage of a substance across a membrane. The cell membrane is selectively permeable, the phospholipid bilayer allows some molecules to easily pass across the membrane and does not allow other molecules to easily pass across the membrane.

There are two types of membrane transport; active transport and passive transport. Passive transport is movement of a substance across a membrane from an area of high concentration to an area of low concentration. There is no energy or ATP required. The goal of passive transport is equilibrium across the membrane.

II.2.Transport Across Cell Membrane

Transport across the cell membrane is a special process that occurs via the cell membrane. The cell membrane keeps the internal and external environment separate and allows only specific molecules to transport across it. Transportation of molecules occurs via a specialized process known as active and passive transport. The selection of a molecule transporter depends on the molecule to be transported.

Figure:20 Types of Membrane Transport

II.3.Types and Mechanisms of Transport Across Cell Membrane

The mechanism of transport across the cell membrane occurs via two main processes, but there are some other processes as well, which are described below.

1.    Active Transport

2.    Passive Transport

3.    Bulk Transport Across

Figure 21: Mechanisms of Transport Across Cell Membrane

II.4.Passive Transport

Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable—they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells; they must have a way of obtaining these materials from extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate (ATP),

The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a range of concentrations of a single substance is said to have a concentration gradient.

II.4.1. Diffusion

Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle; its lowest concentration is at the edges of the room.

 The ammonia vapor will diffuse, or spread away, from the bottle, and gradually, more and more people will smell the ammonia as it spreads. Materials move within the cell’s cytosol by diffusion, and certain materials move through the plasma membrane by diffusion. Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, dissipated as the gradient is eliminated.

Figure 22 : Diffusion through a permeable membrane moves a substance from an area of high concentration down its concentration gradient.

II.4.2. Facilitated diffusion

In facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are ions or polar molecules that are repelled by the hydrophobic parts of the cell membrane. Facilitated diffusion proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell. These proteins are called transportproteins and can be channels or carrier proteins.

A-Channels

Channelproteins are transmembrane proteins that fold in such as way as to form a channel or pore through the membrane. Each channel is specific for one particular substance. Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids. In addition, they have a hydrophilic channel through their core that provides a hydrated opening through the membrane layers. Passage through the channel allows polar compounds to avoid the nonpolar central layer of the plasma membrane that would otherwise slow or prevent their entry into the cell. Aquaporins are channel proteins that allow water to pass through the membrane at a very high rate.

Figure 23 : Facilitated transport moves substances down their concentration gradients. Theymay cross the plasma membrane with the aid of channel proteins.

 B-Carrier Proteins

Another type of transmembrane transporter protein is a carrierprotein. Like channels, carrier proteins are usually specific for particular molecules. A carrier proteins binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule across the membrane. Carrier proteins are used to transport molecules that are too large to pass through channels, such as amino acids and glucose.

 

Figure24 : Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins.

 II.4.3. Osmosis

Osmosis is the diffusion of water across a semipermeable membrane. Since it is diffusion, it depends on the concentration gradient, or the amount of water on each side of the membrane. The amount of water in a solute is inversely proportional to the concentration of solutes. In other words, the higher the concentration of water, the lower the concentration of solutes, and vice versa. Water can move readily across most membranes, due in part to the presence of aquaporins; however, the membrane limits the diffusion of solutes in the water.

A-Mechanism of Osmosis

Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane). If the volume of the solution on both sides of the membrane is the same, but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane.

                

Figure 25: In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can.

B-Tonicity

Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution’s tonicity often directly correlates with the osmolarity of the solution. Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.

 In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity (and more water) to the side with higher osmolarity (and less water). This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient.

Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid. In living systems, the point of reference is always the cytoplasm, so the prefix hypo- (“lower”) means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances.

                       

Figure 26 : Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions

Hypotonic Solutions

In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell. The extracellular fluid has a higher concentration of water than does the cell and water will move down its concentration gradient and enter the cell.

Hypertonic Solutions

In a hypertonic solution (hyper- = “more”), the extracellular fluid has a higher osmolarity than the cell’s cytoplasm. The fluid contains less water than the cell does, so water will leave the cell.

Isotonic Solutions

In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. There is no net movement of water into or out of the cell (although water will still move in and out).