Membrane Transport

Learning Objectives:

  • Compare and contrast passive and active transport
  • Describe the process of diffusion, osmosis, and facilitated diffusion
  • Describe the process of the sodium/potassium pump and endo/exocytosis
  • Describe the three types of endocytosis

Transport across the Cell Membrane

One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca++, Na+, K+, and Cl; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2), which must leave the cell. The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer.

All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).

Passive Transport

In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A concentration gradient is the difference in concentration of a substance across a space. Molecules (or ions) will spread/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move down their concentration gradient.) Three common types of passive transport include simple diffusion, osmosis, and facilitated diffusion.

 

Simple Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept. Imagine being inside a closed bathroom. If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the bathroom, and this diffusion would go on until no more concentration gradient remains. Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures. Having an internal body temperature around 98.6° F thus also aids in diffusion of particles within the body.

Visit this link to see diffusion and how it is propelled by the kinetic energy of molecules in solution. How does temperature affect diffusion rate, and why?

Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the plasma membrane, any substance that can move down its concentration gradient across the membrane will do so. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2. O2 generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them. Neither of these examples requires any energy on the part of the cell, and therefore they use passive transport to move across the membrane. Before moving on, you need to review the gases that can diffuse across a cell membrane. Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O2 inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO2 as a byproduct of metabolism, CO2 concentrations rise within the cytoplasm; therefore, CO2 will move from the cell through the lipid bilayer and into the interstitial fluid, where its concentration is lower. This mechanism of molecules spreading from where they are more concentrated to where they are less concentration is a form of passive transport called simple diffusion (Figure 3.15).

This figure shows the simple diffusion of small non-polar molecules across the plasma membrane. A red horizontal arrow pointing towards the right indicates the progress of time. The nonpolar molecules are shown in blue and are present in higher numbers in the extracellular fluid. There are a few nonpolar molecules in the cytoplasm and their number increases with time.
Figure 3.15. Simple Diffusion across the Cell (Plasma) Membrane
The structure of the lipid bilayer allows only small, non-polar substances such as oxygen and carbon dioxide to pass through the cell membrane, down their concentration gradient, by simple diffusion.

 

Osmosis is the diffusion of water through a semipermeable membrane (Figure 3.16). Water can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself.  However, it is concentration of solutes within the water that determine whether or not water will be moving into the cell, out of the cell, or both.

This figure shows the diffusion of water through osmosis. The left panel shows a beaker with water and different solute concentrations. A semipermeable membrane is present in the middle of the beaker. In the right panel, the water concentration is higher to the right of the semipermeable membrane.
Figure 3.16. Osmosis
Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic.

Solutes within a solution create osmotic pressure, a pressure that pulls water.  Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell.  The more solute a solution contains, the greater the osmotic pressure that solution will have. A solution that has a higher concentration of solutes than another solution is said to be hypertonic. Water molecules tend to diffuse into a hypertonic solution because the higher osmotic pressure pulls water (Figure 3.17). If a cell is placed in a hypertonic solution, the cells will shrivel or crenate as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting, a process called lysis. A critical aspect of homeostasis in living things is to create an internal environment in which all of the body’s cells are in an isotonic solution, an environment in which two solutions have the same concentration of solutes (equal osmotic pressure). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, so water flows both in and out and the cells maintain their normal shape (and function). Various organ systems, particularly the kidneys, work to maintain this homeostasis.

This image shows how a red blood cell responds to the tonicity of solution. The left panel shows the hypertonic case, the middle panel shows the isotonic case and the right panel shows the hypotonic case.
Figure 3.17. Concentration of Solutions
A hypertonic solution has a solute concentration higher than another solution. An isotonic solution has a solute concentration equal to another solution. A hypotonic solution has a solute concentration lower than another solution.

 

Facilitated diffusion is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size and/or polarity (Figure 3.18). A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.  There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell.

This diagram shows the different means of facilitated diffusion across the plasma membrane. In the top panel, a channel protein is shown to allow the transport of solutes across the membrane. In the bottom panel, the membrane contains carrier proteins in addition to channel proteins.
Figure 3.18. Facilitated Diffusion
(a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.

Active Transport

For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually against its concentration gradient. One of the most common types of active transport involves proteins that serve as pumps. The word “pump” probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration). The sodium-potassium pump, which is also called N+/K+ ATPase, transports sodium out of a cell while moving potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP molecule that is used (Figure 3.19). This process is so important for nerve cells that it accounts for the majority of their ATP usage.

This diagram shows many sodium potassium pumps embedded in the membrane. Potassium is pumped into the cytoplasm and sodium is pumped out of the cytoplasm.
Figure 3.19. Sodium-Potassium Pump
The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.

Other forms of active transport do not involve membrane carriers. Endocytosis (bringing “into the cell”) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane (Figure 3.20). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested. Phagocytosis (“cell eating”) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. In contrast to phagocytosis, pinocytosis (“cell drinking”) brings fluid containing dissolved substances into a cell through membrane vesicles. Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis.Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way.

 

This image shows the three different types of endocytosis. The left panel shows phagocytosis, where a large particle is seen to be engulfed by the membrane into a vacuole. In the middle panel, pinocytosis is shown, where a small particle is engulfed into a vesicle. In the right panel, receptor-mediated endocytosis is shown; the ligand binds to the receptor and is then engulfed into a coated vesicle.
Figure 3.20. Three Forms of Endocytosis
Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.

 

In contrast with endocytosis, exocytosis (taking “out of the cell”) is the process of a cell exporting material using vesicular transport (Figure 3.21). Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis (Figure 3.22). Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.

This figure shows the process of exocytosis. A vesicle is shown fusing with the membrane and then releasing its contents into the extracellular fluid.
Figure 3.21. Exocytosis
Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.
 
This micrograph shows the structure of a pancreatic acinar cell and the location of secretory vesicles.
Figure 3.22. Pancreatic Cells’ Enzyme Products
The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will be exported from the cells via exocytosis. LM × 2900. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)