Experiment 3 - Percentage of water absorbed by raisins

Learning Objective:

To show experimentally that carbon dioxide is given out during respiration

Materials Required:

Safety Precautions:

1. HANDLE GLASSWARE WITH CARE AS IF BROKEN, SHARP SHARDS CAN CAUSE INJURIES

Experimental Accuracy:

1. Ensure all masses measured are accurate (and within the limits of experimental accuracy)

2. If you are using an electrical balance, ensure it is tared

3. Ensure the raisins have their stalks intact, and are fully immersed in the water

Processes:

1. Osmosis: the passive transport of water from a region of higher concentration to lower concentration across a semipermeable membrane. Endosmosis is a subclass of osmosis in which the solvent flows through the membrane into the cell(the opposite of exosmosis).

2. Imbibition: the absorption of water(or other adsorbents) by colloids that does not result in the formation of a solution. Imbibition results in an increase in volume of the imbibant(the colloid through which the absorbent is absorbed).

Methodology:

1. Weigh the raisins using the balance. Denote this value as W1

2. Place the raisins in 200ml of water, ensuring they are completely immersed, for 2 hours.

3. Cover a watch glass with filter/blotting paper and use forceps to place the wet raisins on the paper. Ensure that the surfaces of the raisins are dry by wiping them with the blotting/filter paper. Measure the mass of the wet raisins and denote this value as W2

4. We now have the values of the weights of the dry raisins, and the wet raisins. We can find the percentage of water absorbed by the raisins by finding the percentage change in their respective weights:
(W2 - W1)/ W1 = Y
Y × 100 = percentage of water absorbed

Observation

The raisins will appear to be swollen after being immersed in the water. This is as the water would be a hypotonic solution (due to its lack of solute relative to the raisins’ interior) and generate a concentration gradient along which osmosis can occur.

Experiment in Context:

The transport of materials within organisms is an important process. In plants, it occurs primarily through two major processes: translocation and transpiration; which occur in the vascular bundles. Osmosis is the first step in the larger process of transpiration, where root hair cells absorb water from the soil passively/without expending energy as they have a lower water potential than the soil. It must be noted however that minerals and ions are not able to enter root hair cells passively, and instead use protein pumps and active transport to enter roots. This is why waterlogged soils are not suitable for plants, as despite having an excess of water, it will deprive the plant of oxygen for aerobic respiration and hence reduce its ability to perform active transport.

Roots in plants are continuously growing and maintaining their underground architecture(both laterally and longitudinally) due to the presence of auxin, a growth hormone. As a result, they constantly face abrasion, due to their friction with the soil. This is why the root hair cell’s cap is a sight of constant cell division: to continually renew cell presence and replace worn away cells.