AP Biology/LABORATORY 1. Diffusion and Osmosis

Water potential is the measure of free energy of water in a solution. Water will always move from an area of higher water potential (more water molecules) to an area of lower water potential (fewer water molecules). Water potential is affected by two factors, the addition of solute and physical pressure. In this lab, we will investigate the processes of diffusion and osmosis in a model membrane system, and investigate the effect of solute concentration on water potential as it relates to living plant tissues.

Low and high entropy. For any state of any system there is a number that describes how messy it is, this number is called entropy. Any spontaneous process will increase a system's entropy (as stated by the second law).

Diffusion

the movement of molecules from an area of higher concentration to an area of lower concentration. For example, if a bottle of air freshener is sprayed on one side of a room, the fragrance would eventually be able to be smelled on the opposite side of the room. The air freshener diffused from an area of high concentration (where it was first sprayed) to an area of low concentration (the rest of the room). Eventually, an equilibrium will be reached, and the concentration will be uniform throughout the room, and there will be no net movement of molecules.

Osmosis

a special case of diffusion, in which water will diffuse across a selectively permeable (allows some but not all molecules to gow through) membrane. Water will diffuse from an area of higher water concentration (low solute concentration), to a lower water concentration (high solute concentration).

Water Potential

is the potential energy of water in proportion to dirty water. It is defined by the formula Ψ = Ψ_p + Ψ_s where

• Ψ is the total water potential
• Ψ_p is the pressure potential of the solution
• Ψ_s is the solute potential of the solution

Osmosis and diffusion are two of the most important processes in the study of how organisms maintain homeostasis, particularly with regard to their electrolyte and water balances. This AP lab attempts to demonstrate the process of osmosis using dialysis bags.

Expected Results and Analysis

From only having glucose and starch within the dialysis bag, the glucose diffuses and there is a presence of glucose inside the cup that was initially distilled water and the absence of starch. This is because the selectively permeable dialysis bag, cell membrane, only allows small molecules, such as glucose and water to pass, but not large polysaccharides like starch.

Conclusions

Diffusion is the movement of particles from an area of high concentration to an area of low concentration. One can see the effects of diffusion when one puts several balls tightly packed in a room and open the door to an adjacent room. The balls will gradually slide to the other room. Due to diffusion since the room had a high concentration of balls and the new room did not the balls diffused equally to both rooms. Another important piece of information to know about diffusion is that it is passive which means it requires no energy. Osmosis on the other hand is exactly like diffusion except that in osmosis the object that is being diffused is water, whereas diffusion implies no specific object. One example of osmosis can be seen in a plant when one pours water into a plant and the plant becomes turgid. This is because there is a lower concentration of water inside the plant so water diffuses into the plant. Osmosis like diffusion is also passive.