How do ions cross the cell membrane

Ernest Z. Jun 27, Explanation: Passive Transport In passive transport , transmembrane proteins create a water-filled pore through which ions can pass by diffusion down a concentration gradient. This is simple diffusion and requires no energy. Active Transport In active transport , specific carrier proteins use the energy of ATP to force ions through the membrane against a concentration gradient. Some ion pumps involve the simultaneous transport of two different ions.

Related questions How is the cell membrane affected by temperature? How does the cell membrane change shape? How does a cell membrane affect water movement? 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.

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. When molecules move in this way, they are said to move down their concentration gradient, from high concentration to low concentration. 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 room. 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 room, and this diffusion would go on until the molecules were equally distributed in the room. 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.

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 cell membranes, any substance that can move down its concentration gradient across the membrane will do so. If the substances can move across the cell membrane without the cell expending energy, the movement of molecules is called passive transport.

Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen O 2 and carbon dioxide CO 2. These small, fat soluble gasses and other small lipid soluble molecules can dissolve in the membrane and enter or exit the cell following their concentration gradient. This mechanism of molecules moving across a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion.

O 2 generally diffuses into cells because it is more concentrated outside of them, and CO 2 typically diffuses out of cells because it is more concentrated inside of them. Before moving on, it is important to realize that the concentration gradients for oxygen and carbon dioxide will always exist across a living cell and never reach equal distribution. This is because cells rapidly use up oxygen during metabolism and so, there is typically a lower concentration of O 2 inside the cell than outside.

As a result, oxygen will diffuse from outside the cell directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO 2 as a byproduct of metabolism, CO 2 concentrations rise within the cytoplasm; therefore, CO 2 will move from the cell through the lipid bilayer and into the extracellular fluid, where its concentration is lower.

Figure 3. Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane.

A common example of facilitated diffusion using a carrier protein 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, and therefore, repelled by the phospholipid membrane. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.

The difference between a channel and a carrier is that the carrier usually changes shape during the diffusion process, while the channel does not. 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.

A specialized example of facilitated transport is water moving across the cell membrane of all cells, through protein channels known as aquaporins. Osmosis is the diffusion of water through a semipermeable membrane from where there is more relative water to where there is less relative water down its water concentration gradient Figure 3. On their own, cells cannot regulate the movement of water molecules across their membrane, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells in the extracellular fluid is equal to the concentration of solutes inside the cells in the cytoplasm.

Two solutions that have the same concentration of solutes are said to be isotonic equal tension. When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape and function. Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell.

A solution that has a higher concentration of solutes than another solution is said to be hypertonic , and water molecules tend to diffuse into a hypertonic solution Figure 3.

Cells in a hypertonic solution will shrivel 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 , and water molecules tend to diffuse out of a hypotonic solution.

Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. Various organ systems, particularly the kidneys, work to maintain this homeostasis. 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 primary active transport, ATP is required to move a substance across a membrane, with the help of membrane protein, and against its concentration gradient.

One of the most common types of active transport involves proteins that serve as pumps. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, against their concentration gradients from an area of low concentration to an area of high concentration. The activity of these pumps in nerve cells is so great that it accounts for the majority of their ATP usage.

Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell.

In this way, the action of an active transport pump the sodium-potassium pump powers the passive transport of sodium ions by creating a concentration gradient. Describe how a cell moves sodium and potassium out of and into the cell against its electrochemical gradient. The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur.

The secondary transport method is still considered active because it depends on the use of energy as does primary transport. Active Transport of Sodium and Potassium : Primary active transport moves ions across a membrane, creating an electrochemical gradient electrogenic transport.

The process consists of the following six steps:. Several things have happened as a result of this process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior.

This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump a pump that creates a charge imbalance , creating an electrical imbalance across the membrane and contributing to the membrane potential.

Define an electrochemical gradient and describe how a cell moves substances against this gradient. Electrochemical Gradient : Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. Simple concentration gradients are differential concentrations of a substance across a space or a membrane, but in living systems, gradients are more complex.

Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane.

The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed. The situation is more complex, however, for other elements such as potassium. The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient. To move substances against a concentration or electrochemical gradient, the cell must use energy.

Active transport mechanisms, collectively called pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements.

Two mechanisms exist for the transport of small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement. There are three types of these proteins or transporters: uniporters, symporters, and antiporters. A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction.

An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose.

These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process. Both of these are antiporter carrier proteins. Uniporters, Symporters, and Antiporters : A uniporter carries one molecule or ion.

A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions. In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient. Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest.

Instead, another molecule is moved up its concentration gradient, which generates an electrochemical gradient. The molecule of interest is then transported down the electrochemical gradient. While this process still consumes ATP to generate that gradient, the energy is not directly used to move the molecule across the membrane, hence it is known as secondary active transport.

Both antiporters and symporters are used in secondary active transport. Co-transporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions across the cell membrane.

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane.

Many amino acids, as well as glucose, enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP. Secondary Active Transport : An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport.

Endocytosis takes up particles into the cell by invaginating the cell membrane, resulting in the release of the material inside of the cell.

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: the plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane. Phagocytosis : In phagocytosis, the cell membrane surrounds the particle and engulfs it.

For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil.

In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly-formed compartment endosome.

When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly-formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane. Pinocytosis : In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. A variation of endocytosis is called pinocytosis.

In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome. Potocytosis, a variant of pinocytosis, is a process that uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin.

The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin.

The vacuoles or vesicles formed in caveolae singular caveola are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis. Receptor-Mediated Endocytosis : In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane.

A targeted variation of endocytosis, known as receptor-mediated endocytosis, employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances. In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood.

Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the failure of receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood. Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site.

Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells. Privacy Policy. Skip to main content.

Organization at the Cellular Level. Search for:. Transport Across Membranes. Diffusion Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration. Learning Objectives Describe diffusion and the factors that affect how materials move across the cell membrane. Key Takeaways Key Points Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients.

After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another, a state known as dynamic equilibrium. Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled.

Key Terms diffusion : The passive movement of a solute across a permeable membrane concentration gradient : A concentration gradient is present when a membrane separates two different concentrations of molecules. Osmosis Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration.

Learning Objectives Describe the process of osmosis and explain how concentration gradient affects osmosis. Key Takeaways Key Points Osmosis occurs according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes.

Osmosis occurs until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. Osmosis occurs when there is a concentration gradient of a solute within a solution, but the membrane does not allow diffusion of the solute.

Key Terms solute : Any substance that is dissolved in a liquid solvent to create a solution osmosis : The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane semipermeable membrane : A type of biological membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally by specialized facilitated diffusion.

Tonicity Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow.