There is a point where in the mystery of existence contradictions meet; where movement is not all movement and stillness is not all stillness; where the idea and the form, the within and the without, are united; where infinite becomes finite, yet not
—Rabindranath Tagore (Indian poet, 1861–1941)
Oftentimes in cellular life, the concentration of a critical molecule is greater outside than within the confines of the plasma membrane. Ions and nutrients are often able to cross the barrier, nourishing and providing the cell with essential constituents. Such molecules generally move along or down the concentration gradient of the molecule in a passive manner. In the first chapter of this unit, we will consider basic concepts of transport, mainly focusing on passive transport processes. Additionally, we will consider osmosis, or movement of water. Since cells exist within an aqueous environment, water, the fluid of life, continuously flows into and out of cells via protein channels in the membrane.
While water transport is constant and involves large volumes over time, there is no net movement of water when isotonic conditions exist. The vast movement appears to be negligible when it is not! At times, a cell requires a crucial molecule that is already present in higher concentration within than outside its boundaries. Contrary to what may be viewed as prudent use of energy currency, a cell may use an active transport process, powered by ATP to pump in the molecule, which is then sometimes used to generate more energy for the cell. Active transport is the topic of the second chapter in this unit.
For our third chapter, we discuss in detail the transport of glucose into cells as cells have an almost insatiable need for this carbohydrate. Disruptions of glucose transport are seen in diabetes mellitus. The final chapter in this unit concerns transport of drugs. These synthetic molecules can exploit the transport mechanisms that have evolved to meet normal cellular needs. With them, we have treatments for many illnesses and perhaps even the potential for extending the finite existence of our cells and our species.
Certain ions and molecules must enter and exit cells. Cells must maintain their volume and the necessary balance of ions for normal processes of cellular life to occur. Molecules that will serve as cellular food must also enter cells so that they can be broken down for energy. The plasma membrane protects and isolates the cytoplasm from the environment and in so doing, it presents a barrier to the entry of molecules into the cell. This barrier is selectively permeable, allowing physiologically important molecules to enter and exit cells, while excluding other molecules (Figure 13.1). Some drugs can also often gain access to the interior of the cell.
FIGURE 13.1.Selective permeability of the plasma membrane.
Proteins embedded in the plasma membrane are important in facilitating transport of ions and other molecules into cells (Figure 13.2). These proteins are ion channels, transporters, and pumps. Individual membrane proteins specifically bind to certain ligands (e.g., to chloride, to glucose, or to sodium and potassium) and facilitate their movement across the plasma membrane.
FIGURE 13.2.Membrane proteins facilitate transport.
In most membrane transport, passive transport is used where molecules are moved across plasma membrane along, down, or with their concentration gradients (Figure 13.3). The molecule flows from where it is at higher concentration to where it is at a lower concentration. In contrast, in active transport, molecules are moved against their concentration gradient in energy-requiring processes (see also Chapter 14).
FIGURE 13.3.Concentration gradients of solutes to be transported.
Knowledge of diffusion is useful in describing the movement of molecules from higher concentration to lower concentration. Diffusion is powered by the random movement of molecules in a solution, where molecules spread out until they are equally distributed within the space they occupy (Figure 13.4). The process can continue to occur at the same rate as long as the concentration gradient exists. Diffusion of one substance does not interfere with the diffusion of another substance within the same solution. The net movement or flux of a substance diffusing across a barrier depends on several criteria. First is the concentration gradient, next is the size, and then the permeability of the substance in the barrier across which it will diffuse. Consequently, ordinary diffusion across cell membranes does not normally occur.
FIGURE 13.4.Distribution of particles in solution via diffusion.
Permeability in a membrane is an important consideration if a molecule is to cross it by diffusion. Phospholipids that constitute the plasma membrane are amphipathic in nature, containing both hydrophilic (water loving) and hydrophobic (water fearing) components (see also Chapter 3). Hydrophobic molecules such as steroid hormones may be able to dissolve in the interior hydrophobic core of a plasma membrane. However, the hydrophilic head groups of the phospholipids present an obstacle at the interface with the environment and with the cytosol (Figure 13.5).
FIGURE 13.5.Amphipathic phospholipids as barriers to membrane diffusion.
Diffusion and plasma membranes
In sheets of cells in tissues, some molecules may diffuse through tight junctions between neighboring cells (see also Chapter 2). In this way, some molecules diffuse through the layer of cells, but not into the cytoplasm of cells. The plasma membrane is always a barrier to diffusion and impedes the flow of materials through it. Movement of molecules along their concentration gradient into the cytoplasm of cells occurs through membrane proteins that form channels within the plasma membrane. Some ions and small molecules (generally with a molecular weight of <80 daltons) may gain access to the cytoplasm of cells through water pores or channels. Larger molecules require specific membrane transport proteins. Water entry into cells by osmosis has been described as diffusion, but it too occurs via membrane channels.
Osmosis is the transfer of liquid solvent through a semipermeable membrane that does not permit dissolved solutes to pass. Water is the most important physiological solvent. By osmosis, water passes through a plasma membrane from an area of high concentration of water to an area of lower concentration of water (Figure 13.6). Plasma membranes are permeable to water but not to certain solutes in solution in the water. Protein channels known as aquaporins enable water to pass through the hydrophobic core of the phospholipids of the membrane. Movement of water via osmosis occurs from the region of high water concentration toward the region with lower water concentration.
Net movement of water
Water constantly enters and exits our cells by osmosis. However, net movement of water is usually negligible. Rates of water entry and exit are usually equal. In erythrocytes, water equal to approximately 250 times the volume of the cell enters and exits the cell each second, but with no net movement of water! In the small intestine though, there is a net movement of water across sheets of cells as water is absorbed and secreted. Also, net movement of water via osmosis occurs to concentrate the urine. There, water moves via osmosis into the blood from the filtrate that will form urine across a layer of epithelial cells lining the kidney tubules.
Different solute concentrations result in different concentrations of free water. In a solution with a high concentration of particles or solute, there is less free water than in a solution with low solute concentration (Figure 13.7). Where there is more free water, the water molecules will strike the water channels in the membrane with greater frequency. The result will be more water leaving the area of high free water concentration. The result is net movement of water. The size or molecular weight of the solutes in water does not influence the net movement of water. When sufficient water has crossed the barrier to equalize solute concentrations on both sides, then movement of water will stop.
FIGURE 13.7.Concentrations of free water determine the direction of water movement in osmosis.
Differences in solute concentration on opposing sides of a barrier such as a plasma membrane generate an osmotic pressure. If the pressure is raised on the side of the barrier into which water is flowing, movement of water will stop (Figure 13.8). This is referred to as the hydrostatic or water stopping pressure.
FIGURE 13.8.Osmotic pressure generated by differences in solute concentration on different sides of a barrier.
When the osmotic pressure is the same both inside and outside the cell, the external solution surrounding the cell is said to be isotonic (same). Cells maintain their volume in isotonic solutions. While osmosis occurs in both directions, in and out of the cell, there is no net movement of water when osmotic pressure is equal on both sides of the membrane (Figure 13.9). A solution with less osmotic pressure than the cytosol is hypotonic. Cell volume increases in hypotonic solutions. This occurs because free water is at higher concentration outside the cell. Water moves with or along its concentration gradient and there is a net movement of water into cells. If cells are instead placed in a solution with a higher osmotic pressure than their cytosol, the solution is hypertonic. Water will exit the cells. In this attempt to equalize osmotic pressure, cells will shrink.
FIGURE 13.9.Cell volume changes as a consequence of osmosis.
Based on their size, charge, and low solubility in phospholipids, many biologically important molecules such as ions, sugars, and amino acids would be predicted to enter into cells very slowly. However, their uptake into cells occurs at fairly rapid rates (Figure 13.10). This can occur because ion channels or membrane transport proteins facilitate the entry or exit of specific molecules into and out of cells. This process is also called catalyzed transport and sometimes referred to as facilitated diffusion. No direct source of energy is required to power the process.
FIGURE 13.10.Rates of transport into cells.
Transport through ion channels
Ions gain entry into cells through ion channels, complexes of transmembrane proteins within the plasma membrane that provide a hydrophilic route for an ion to pass through the hydrophobic core of the plasma membrane (Figure 13.11). Ion channels are selective, allowing only ions of certain size and charge to enter. For example, calcium channels are specific for calcium and sodium channels for sodium. Many types of potassium channels (at least 16 types) and chloride channels (at least 13 types) exist in plasma membranes but all are specific for uptake of either potassium or chloride.
Ions move through an ion channel from their region of higher concentration to a region of lower ion concentration. Ion channels may be open or closed. The opening and closing of gated channels is regulated by physical or chemical stimuli. Ligand-gated channels are regulated by neurotransmitters. Voltage-gated channels are regulated by an electric field and are particularly important in the nervous system (see also LIR Physiology). Some other channels are regulated by G proteins (see also Chapter 17). Others respond to osmotic pressure and some are not gated. Ion channels also have properties in common with transporters, described below.
FIGURE 13.11.Ion channels.
Transport through transporters
Transporters are transmembrane proteins that catalyze migration of molecules into or out of cells. Uniport is a term used to describe facilitated transport of one molecule by a transporter. Transporters or uniporters are also sometimes called permeases because of their enzyme-like function in catalyzing movement of their ligand across the plasma membrane barrier. Individual transporters are able to bind to and interact with only certain molecules. Their specificity is similar to an enzyme’s specificity for its substrate (see also LIR Biochemistry, Chapter 5 for a discussion of enzymes). For example, the major physiological form of glucose, d-glucose, has a much higher affinity for its transport protein than does l-glucose, a stereoisomer of glucose (see also Chapter 15).
Cystic fibrosis and defective chloride ion transport
Cystic fibrosis (CF) is the most common lethal genetic disease in Caucasians, with a prevalence of about 1:2,500 births. CF is also common in the Ashkenazi Jewish population but is rare in African and Asian populations. Certain mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) cause the disease. CF is inherited as an autosomal recessive trait, with approximately 1 in 25 individuals of Caucasian descent being a carrier and having one copy of a mutant CFTR gene. Inheritance of two mutant CFTR genes is necessary for a person to have the disease. While over 1,500 mutations are known in CFTR, most do not cause disease. Some disease-causing mutations result in more severe forms of disease than others.
Normal CFTR functions as a chloride channel in epithelial cells. Defective chloride ion transport occurs when two copies of disease-causing mutations in CFTR are present. Having a “salty brow” was an early diagnostic test for CF. The sweat of affected individuals contains more salt than normal, as a result of inappropriate chloride ion transport. Chloride sweat tests have been used in diagnosis of CF.
Defective CFTR has much more serious consequences than causing salty sweat. Inability to transport chloride across epithelial cells leads to decreased secretion of chloride and increased reabsorption of sodium and water, resulting in the production of thick sticky secretions in the lungs and increased susceptibility to infections. The leading cause of death of persons with CF is respiratory failure often during their 20 s or 30 s. In the pancreas, thick mucus often prevents pancreatic enzymes from reaching the intestine to aid in digestion of dietary lipids. Most males with CF are sterile. While they make normal sperm, mutation of CFTR also usually results in the absence of the vas deferens which is needed for release of the sperm.
Treatments include percussion therapy to loosen respiratory mucus, antibiotics for infections, and pancreatic enzyme replacement therapy. With improved treatments, affected individuals may survive into their 40 s or 50 s, but no cure is available at present. Gene therapy continues to be a future possibility.
Catalyzed transport via transporters (and ion channels) requires a concentration gradient of the solute or the substrate being transported. Molecules bind to their specific transport protein with a certain affinity, represented as the Km (Figure 13.12) (see also LIR Biochemistry, Chapter 5 for discussion of affinity of enzyme for substrate, Km, which is analogous to affinity of transporter and solute). Numerically, Km is equal to the concentration of solute that yields half the maximal velocity of transport. This maximal velocity (Vmax) of transport will be achieved when all available transporter proteins are bound by their specific solute. After that point, the addition of more solute will not result in an increased rate of uptake into the cells. Therefore, membrane transport via transporters (and via ion channels) is a saturable process (Figure 13.13).
FIGURE 13.12.Transport proteins.
FIGURE 13.13.Characteristics of transport catalyzed by transporters.
- Selective permeability of the plasma membrane allows certain materials only to enter and exit cells.
- Diffusion of particles is powered by movement of the particles within a solution and results in a distribution of particles that move from their area of higher concentration to an area of lower concentration.
- Many molecules enter (or exit) cells by moving from an area of high concentration to low concentration.
- The plasma membrane is always a barrier to access the cytoplasm and entry into cells requires membrane proteins.
- Water enters and exits cells through osmosis and requires water channel proteins.
- Ion channels facilitate movement specific ions into or out of cells, along the concentration gradient of the ions.
- Transport proteins catalyze the movement of specific solutes across plasma membranes by a process called passive transport or catalyzed transport (and sometimes called facilitated diffusion).
- Channels and transport proteins bind their solute with a certain affinity and catalyze the movement of their solute across the membrane in a saturable manner.