Question

1. Using your understanding of plant cell structure, explain what you are seeing in the difference between Elodea cells placed in pond water compared with cells that have been submerged in 5% NaCl.

2. Briefly explain how kinetic energy contributes to Brownian motion (the visible or measurable physical movement of all molecules and compounds), and how this helps explain diffusion and osmosis.

Answer 1

1. Elodea is a freshwater angiosperm plants native to North America. The leaves of this plant are only two cells thick and its epithelial cells carryout photosynthesis unlike the other plants in which photosynthesis takes place in internal mesophyll cells.

Pond water is hypotonic as it contains fewer solutes compared to other solutions. So when the Elodea cells are placed in hypotonic solution, the solution has the more water than the cells and water has the tendency to move into the cells resulting in the swelling of the cells.

5% Nacl is hypertonic solution. So when the elodea cells are placed in 5% Nacl the vacuoles disappear and the protoplasm comes away from the cell wall making the organelles appear to be clumped in the middle of the cell. Such cells are said to be plasmolyzed. This happens as the water potential inside the cells was greater than the salt solution, as a result water moves out of the cells into the salt solution and cell membrane shranks away from the cell wall.

2. Brownian motion is the continous random movement of microscopic particles in a fluid as a result of collision with the fast moving particles in the fluid. According to Einstein's theory of Brownian motion, the temperature of a substance is proportional to the average kinetic energy with which molecules of the substance are moving.

Energy for any movement is in the form of kinetic energy.The rate of Diffusion increases as the kinetic energy increases because when the heat energy is added to a system the molecular kinetic energy increases and the molecules begin to move more quickly, hence the molecules with the greater kinetic energy diffuse faster. Osmosis is a specialized type of diffusion. If there is no kinetic energy in a system then there will be no diffusion or osmosis