Transport of Materials

The red blood cells found in many animals are very important to the survival of the organism because they transport oxygen from the gills, skin, or lungs to the cells of the various tissues (muscles, nerves, etc.). These blood cells are transported in a fluid (serum) that has approximately the same salt content as sea water. Wanna know much more about it? Now...let’s get started! Next>> Next>>Introduction

Content Diffusion, Osmosis and Cell MembraneDiffusion, Osmosis and Cell Membrane Active Transport Diffusion Osmosis Concentration Gradient Water Potential

All living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases (usually CO2 and O2), taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell. This membrane is a complex structure that is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell, and for interacting with the environment surrounding the cell. There are two ways that the molecules move through the membrane: passive transport and active transport. Diffusion, Osmosis and Cell Membrane

Active Transport It requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. Passive transport does not require such an energy expenditure, and occurs spontaneously. It needs a selectively permeable membrane, which the particles in it goes from Low Concentration to High Concentration. And it is the only condition which requires energy from respiration with active process.

Diffusion It is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout the system. Diffusion can still occur even if there is no membrane. It does not require any energy and it is a Passive Process.

(continue) Diffusion can occur through a cell membrane though. The membrane allows small molecules like water (H2O), oxygen (O2), carbon dioxide (CO2), and others to pass through easily. It is said to be permeable to these molecules. If a cell is floating in a water solution (like the ocean) that has some oxygen dissolved in it, the oxygen molecules will move into the cell. If the cell uses some of the oxygen as it comes into the cell, more oxygen will move into the cell than out of the cell. So the oxygen effectively moves from a region of high concentration (the seawater) to a region of low concentration (the cell), and diffusion occurs. Now let us see an example. Next !

Copper Sulphate Crystal 1. Diffusion Next!

Crystal dissolved in water 1. Diffusion Next!

Spreading (diffusion) 1. Diffusion Lower concentration Higher concentration

Even spread of copper sulphate. And so, diffusion stops. All blue

Osmosis Osmosis is a special case of diffusion. In this case, a large molecule like starch is dissolved in water. The starch molecule is too large to pass through the pores in the cell membrane, so it cannot diffuse from one side of the membrane to the other. The water molecules can, and do, pass through the membrane. Hence the membrane is said to be semipermeable, since it allows some molecules to pass through but not others. However, on the side of the membrane with the starch, the starch molecules interfere with the movement of the water molecules, preventing them from leaving as rapidly as they enter. Thus, more water flows into the side with the starch than flows out, and the starch gets diluted. Next

(continue) If the starch (or some other large molecule like a protein) is in a cell, the water moves into the cell faster than it leaves, and the cell swells. The cell membrane acts somewhat like a balloon, and if too much water enters the cell, the cell can burst, which kills the cell. So cells usually have some kind of mechanism for preventing too much water from entering or pumping the water out or simply making a tough outer coat that will not rupture. Things are more difficult when the starch or other large molecule is on the outside of the cell. Then the cell loses water faster than it comes in, and the cell shrinks, which might not be too bad except that the cell needs the water for the chemical reactions that take place inside that keep it alive. In fact, this principle is used in food preservation. Foods that are packed in salt or sugar prevent bacterial growth by essentially sucking the water out of the bacterial cells (or, more properly, preventing water from entering the cells) and preventing their growth.

High Concentration Low Concentration This is a diagram showing the net movement of water, in which the molecules in high concentration go to low concentration. Pore Water molecule Sugar molecule (solute molecule) The sugar molecules are too big, they cannot pass through the pores A selectively permeable membrane

Water Potential It is used to describe the total free energy of water molecules in a system. Water potential of a cell refers to the tendency of water to diffuse out of the cell, or, the ability of the cell to give out water. The water potential of a solution will decrease the water potential of pure water is zero. Thus, the water potential of a solution is less than zero or negative. When a cell absorbs water, its pressure will increase and so as the water potential. But when a cell loses water, its pressure and water potential will decrease.

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