Diffusion and Transportation within a Cell

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This presentation outlines the process of diffusion as well as active transport and fascilitated diffusion within cells

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The Cell and It’s Environment:

The Cell and It’s Environment Passive Transportation

Objectives:

Objectives Relate concentration gradients, diffusion, and equilibrium. Predict the direction of water movement into and out of cells. Describe the importance of ion channels in passive transport. Identify the role of carrier proteins in facilitated diffusion

Diffusion:

Diffusion One way cells maintain homeostasis is by controlling what enters and leaves the cell. Cells must use energy to move some things in and out past the cell membrane. Other molecules can move across the membrane without help.

Random Motion and Concentration:

Random Motion and Concentration Movement across a cell membrane that does not require energy (in the form of ATP) is called passive transport. An example of passive transport: Two rooms side by side separated by a wall with a closed door. Suppose we release several rubber balls into one room. The balls move randomly through out the room by bouncing off the walls, floor, and each other. Suppose we open the door. Some balls will bounce from the first room into the second. Eventually the number of balls in the second room will be great enough that statistically some balls will bounce back. When the number of balls leaving the first room and the number of balls entering the first room are equal, we have reached equilibrium.

A Gradient: Where is the Green Concentrated at?:

A Gradient: Where is the Green Concentrated at?

Concentration Gradient:

Concentration Gradient Concentration Gradient: The difference between the number of rubber balls in the first room and the number in the second room forms a concentration gradient. Diffusion works by moving particles from areas of great concentration to an area of less concentration. As more balls enter the second room in our example, the pressure for balls to leave the room is reduced. The concentration gradient is reduced until it approaches equilibrium.

From Great Concentration to Less Concentration until Equilibrium is Reached:

From Great Concentration to Less Concentration until Equilibrium is Reached

Diffusion:

Diffusion The passive movement of particles form an area of great concentration down the concentration gradient, to an area of less concentration. Many substances dissolved in the cytoplasm or the interstitial fluid outside the cell, move back and forth across the cell membrane by diffusion.

The Cell Membrane:

The Cell Membrane The nonpolar interior of the membrane repels most ions and polar molecules. These substances are prevented from diffusing across the membrane and must move by some other means. In contrast, molecules that are nonpolar, or that are very small , can move across the membrane by simple random diffusion.

Osmosis:

Osmosis Osmosis: The diffusion of water in response to the presence of solute in a solution across a semi permeable membrane. If we have salt water on a container on one side of a membrane and pure water on the other side of the membrane in the same container, we find that the water moves from the pure side to the salty side in an effort to dilute the salt concentration and equalize the solution pressure. Osmosis can cause cells to shrink or expand based on the surrounding solution.

Solutions:

Solutions Water moves out of the cell: When water diffuses out of a cell the concentration of solute inside the cell is less than that outside the cell. This causes the cell to shrink in size as water leaves it. This is called a hypertonic solution. Water moves into a cell: When water diffuses into a cell the concentration of solute inside the cell is greater than the concentration outside. This causes the cell to expand as it takes in water. This is called a hypotonic solution. No Net Water Movement: If water does not appear to move from one side of a membrane to another we may assume that the concentration of solute is equal on both sides. This is called an isotonic solution.

Electrical Charge & Ion Transport:

Electrical Charge & Ion Transport The rate of movement across the membrane is determined by the concentration gradient. If the difference between concentrations across the membrane is large, the particles will move faster to reach equilibrium. Electrical charge can also affect transport speed. The outside of a cell tends to be negatively charged. Positively charged ions will be attracted to the surface of the cell and will tend to move through a bit easier. Negatively charged ions will be slightly repulsed (like charges repulse and opposites attract) and so will be slower to move.

Facilitated Diffusion:

Facilitated Diffusion Many cells have a specific type of transport protein that can bind to a specific substance on one side of the membrane, carry it across, and release it on the other side. When these carrier proteins are used to transport substances like amino acids and large sugar molecules down their concentration gradients, the process is called Facilitated Diffusion. Facilitated Diffusion is still a passive transport mode (DOWN the concentration gradient). It does not require ATP to power the process.

Assignment:

Assignment Read Ch 4, pp. 81 - 86 Terms and Questions Due Tomorrow (Thursday ) 10/4/2012 Be prepared to discuss in class tomorrow.

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