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. Learning Objectives Describe diffusion and the factors that affect how materials move across the cell membrane. 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. Diffusion Diffusion is a passive process of transport. Factors That Affect Diffusion 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.
While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion: Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.
The kinetic energy of the two gases will be the same if the gases are at the same temperature. So at equal temperatures, a lighter particle B must have a greater velocity speed and a heavier particle A must have a lower speed.
Here is a video which gives further explanation of this topic. It discusses effusion, which is basically the diffusion of particles through a small opening in a container. How does molecular weight affect the rate of diffusion? Chemistry Matter Atomic Mass. 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.
This can cause problems in transporting enough of the material for the cell to function properly. Carrier Proteins : Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane. An example of this process occurs in the kidney. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney.
This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported; it is excreted from the body in the urine. Channel and carrier proteins transport material at different rates. Diffusion is an important mechanism by which essential molecules are transported in biological systems.
Several factors affect the rate of diffusion of particles, including molecular weight, concentration gradient, pressure, temperature and type of solute. The rate of diffusion has a direct relationship with the concentration gradient, pressure and temperature. The higher the difference between the two concentrations, the faster the rate of diffusion.
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