Why substances move in the process of diffusion

In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can. Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of passing through the membrane will diffuse through it.

In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated.

This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. In the beaker example, this means that the level of fluid in the side with a higher solute concentration will go up. Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow.

Tonicity is the reason why salt water fish cannot live in fresh water and vice versa. If you place a salt water fish in fresh water, which has a low osmolarity, water in the environment will flow into the cells of the fish, eventually causing them to burst and killing the fish. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis.

Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.

In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water. This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient.

An important distinction that concerns living systems is that osmolarity measures the number of particles which may be molecules in a solution. Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear if the second solution contains more dissolved molecules than there are cells.

Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells. In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. In living systems, the point of reference is always the cytoplasm, so the prefix hypo- means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm.

It also means that the extracellular fluid has a higher concentration of water in the solution than does the cell. In this situation, water will follow its concentration gradient and enter the cell, causing the cell to expand. Changes in Cell Shape Due to Dissolved Solutes : Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions.

Because the cell has a relatively higher concentration of water, water will leave the cell, and the cell will shrink. In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances.

Cells in an isotonic solution retain their shape. Cells in a hypotonic solution swell as water enters the cell, and may burst if the concentration gradient is large enough between the inside and outside of the cell. Cells in a hypertonic solution shrink as water exits the cell, becoming shriveled. Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.

Channel-mediated facilitated diffusion functions much like a bridge over a river that must raise and lower in order to allow boats to pass. When the bridge is lowered, boats cannot pass through to the other side of the river. Similarly, a gated channel protein often remains closed, not allowing substances into the cell until it receives a signal like the binding of an ion to open.

When this signal is received, the bridge gate opens, allowing the boats substance to pass through the bridge and into the other side of the river cell.

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. 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 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. 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.

This is primary active transport. In the picture on the right side, substance S, already at higher concentration in the cell, is brought into the cell with substance X. Since S is being transported without the direct use of ATP, the transport of S is an example of secondary active transport. For substance X primary active transport of X is occurring. The high concentration of X outside the cell is being used to bring in substance S against its concentration gradient.

It is possible for large molecules to enter a cell by a process called endocytosis, where a small piece of the cell membrane wraps around the particle and is brought into the cell. If the particle is solid, endocytosis is also called phagocytosis.

If fluid droplets are taken in, the processes is called pinocytosis. Illustration of endocytosis. Note that the particle entered the cell surrounded by a piece of cell membrane. The opposite of endocytosis is exocytosis. Cells use exocytosis to secrete molecules too large to pass through the cell membrane by any other mechanism.

Click on the button above to open a problem solver to help you practice your understanding of membrane transport with the following examples:. A white blood cell engulfs a bacterium as you fight off an infection. The crystals begin to diffuse into the water to give layers of different concentrations. After one hour, equilibrium has been reached which means the potassium permanganate crystals are at an equal concentration throughout the water. Diffusion has stopped. Diffusion Molecules can move into or out of cells by the process of diffusion.

Substances also need to enter or leave whole organisms and this often requires diffusion too. Some substances move into and out of living cells by diffusion. Carbon dioxide is needed in the leaf for photosynthesis, and so diffuses into the leaf from a region of higher concentration in the air.

Oxygen from the air passes into the bloodstream in the lungs through structures called alveoli. It diffuses to a region of lower concentration in the bloodstream.