Concentration gradients lead to what kind of movements

Substances diffuse from areas of high concentration to areas of lower concentration; this process continues until the substance is evenly distributed in a system.

In solutions containing more than one substance, each type of molecule diffuses according to its own concentration gradient, independent of the diffusion of other substances. Many factors can affect the rate of diffusion, including, but not limited to, concentration gradient, size of the particles that are diffusing, and temperature of the system.

In living systems, diffusion of substances in and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane, but others are hindered; their passage is made possible by specialized proteins, such as channels and transporters. The chemistry of living things occurs in aqueous solutions; balancing the concentrations of those solutions is an ongoing problem.

In living systems, diffusion of some substances would be slow or difficult without membrane proteins that facilitate transport. The hydrophobic and hydrophilic regions of plasma membranes aid the diffusion of some molecules and hinder the diffusion of others.

Describe how membrane permeability, concentration gradient, and molecular properties affect biological diffusion rates. Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the membrane.

In fact, there is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane. On the interior of the membrane, some proteins serve to anchor the membrane to fibers of the cytoskeleton.

There are peripheral proteins on the exterior of the membrane that bind elements of the extracellular matrix. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane.

These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes.

Asymmetry in Plasma Membranes : The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane. Recall that plasma membranes are amphiphilic; that is, they have hydrophilic and hydrophobic regions. This characteristic helps the movement of some materials through the membrane and hinders the movement of others.

Lipid-soluble material with a low molecular weight can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues.

Molecules of oxygen and carbon dioxide have no charge and so pass through membranes by simple diffusion. Polar substances present problems for the membrane.

While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, while small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so.

Ions such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes, achieved by various transmembrane proteins channels. Diffusion across a semipermeable membrane : This interactive shows that smaller molecules have an easier time making it across a semipermeable membrane.

Adjust the pore size so the larger molecules can make it through! Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration. 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; gradually, more and more people will smell the ammonia as it spreads. Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, dissipated as the gradient is eliminated.

Diffusion : Diffusion through a permeable membrane moves a substance from an area of high concentration extracellular fluid, in this case down its concentration gradient into the cytoplasm. Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient independent of the concentration gradients of other materials.

In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium. Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature.

This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. 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.

This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:.

A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly. This occurs in the kidney where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules.

The rate of diffusion in this instance is almost totally dependent on pressure. Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins. Facilitated transport is a type of passive transport. Unlike simple diffusion where materials pass through a membrane without the help of proteins, in facilitated transport, also called 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 transport proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell.

The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane. This allows the material that is needed by the cell to be removed from the extracellular fluid.

The substances are then passed to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of beta-pleated sheets that form a channel through the phospholipid bilayer.

Others are carrier proteins which bind with the substance and aid its diffusion through the membrane. The integral proteins involved in facilitated transport are collectively referred to as transport proteins; they function as either channels for the material or carriers. In both cases, they are transmembrane proteins. Channels are specific for the substance that is being transported.

Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids; they additionally 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. Channel Proteins in Facilitated Transport : Facilitated transport moves substances down their concentration gradients. They may cross the plasma membrane with the aid of channel proteins.

The attachment of a particular ion to the channel protein may control the opening or other mechanisms or substances may be involved. In some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues, a gate must be opened to allow passage.

An example of this occurs in the kidney, where both forms of channels are found in different parts of the renal tubules. Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes.

Opening and closing of these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting in the facilitation of electrical transmission along membranes in the case of nerve cells or in muscle contraction in the case of muscle cells. Another type of protein embedded in the plasma membrane is a carrier protein.

This protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior; depending on the gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. This adds to the overall selectivity of the plasma membrane.

The exact mechanism for the change of shape is poorly understood. Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. When the flood gates open, water will flow through from the side with lots of water to the side with less water until the water on both sides of the dam is level.

As the water flows through the dam, it can be used to turn a turbine like a water wheel to generate electricity. When we look back to cell membranes again, movement "down" the gradient can be used to help store more energy from the gradient in molecules like ATP.

As an ion passes through the membrane, it usually goes through a channel or transporter made by a protein. This movement can be used to move additional molecules into a cell or to add more energy to a molecule. Joshua Haussler, Karla Moeller. Concentration Gradients. By volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program.

If you are interested in helping with the website we have a Volunteers page to get the process started. Digging Deeper. Digging Deeper: Depression and the Past. Digging Deeper: Germs and Disease. Digging Deeper: Milk and Immunity. How Do We See? How Do We Sense Smell? How Do We Sense Taste? How Do We Sense Touch? What is Evolutionary Medicine? What's a Biologist? 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.

Privacy Policy. Skip to main content. Structure and Function of Plasma Membranes. Search for:. Active Transport. Learning Objectives Define an electrochemical gradient and describe how a cell moves substances against this gradient. Key Takeaways Key Points The electrical and concentration gradients of a membrane tend to drive sodium into and potassium out of the cell, and active transport works against these gradients.

To move substances against a concentration or electrochemical gradient, the cell must utilize energy in the form of ATP during active transport. Primary active transport, which is directly dependent on ATP, moves ions across a membrane and creates a difference in charge across that membrane. Secondary active transport, created by primary active transport, is the transport of a solute in the direction of its electrochemical gradient and does not directly require ATP.

Primary Active Transport The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell. Learning Objectives Describe how a cell moves sodium and potassium out of and into the cell against its electrochemical gradient. When the sodium-potassium- ATPase enzyme points into the cell, it has a high affinity for sodium ions and binds three of them, hydrolyzing ATP and changing shape.

As the enzyme changes shape, it reorients itself towards the outside of the cell, and the three sodium ions are released. The enzyme changes shape again, releasing the potassium ions into the cell. After potassium is released into the cell, the enzyme binds three sodium ions, which starts the process over again. Key Terms electrogenic pump : An ion pump that generates a net charge flow as a result of its activity.