Water is important in organisms as it is essential for:
- Digestion
- Excretion
- Transport

Role of water as solvent in organisms:
- Water is the main componentof blood plasma.
  - Blood plasma is primarily made up of water. Almost 92% of blood plasma is water.
  - Blood plasma is the liquid component of blood which carries the blood cells, proteins, nutrients, waste products, and dissolved gases within our body.
- Similarly, in the case of plants, water is a medium that helps to transport nutrients from the roots to their leaves.
- It also plays an important role in removing waste products from the human body. For instance, removal of urea from our body is done through urine and sweat, both in the form of water.
- Additionally, water also has an important role in digestion as it helps to break down and dissolve food particles. However, plants use water and carbon dioxide to make food for themselves.

Water potential:
- Water potential is the concentration of water molecules in a solution.
- In the image below, it can be noticed that there are more water molecules on the left side than on the right. This means that the left side has a higher water potential.
- So, when there is a higher water potential outside the cell, the water will enter the cells that have a lower water potential.
- This causes the cells to swell up and the cells may burst.

Osmosis:
- Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
- The presence of a partially permeable membrane is one of the conditions for osmosis to happen.
- Water diffuses through partially permeable membranes by osmosis.
- In other words, water moves in and out of the cells by osmosis through the cell membrane.
- Osmosis occurs in both animal cells and plant cells. However, it will be slightly different for plant cells when osmosis takes place.
- In animal cells, there is no cell wall. Hence, the cells will burst once too much water gets into the cells.
- However for plant cells, there are cell walls. As cell walls give the cells strength and support, plant cells will not burst, they simply become turgid. In other words, the cells become swollen.
- When the water potential inside the cell is higher than the water potential outside the cell, water will leave the cell through osmosis.
- When too much water is lost from the cell, it starts to shrink and the cell membrane gets pulled away from the cell wall. This process is known as plasmolysis and the cell is said to be plasmolysed.
- Meanwhile, when too much water leaves the animal cell, the cell shrinks and becomes crenated or deformed.

Investigation of osmosis using dialysis tubing:
- A visking tubing is a partially permeable membrane made up of cellulose that allows molecules smaller than the membrane pores to pass through.
- There are several types of visking tubing with varying pore sizes. An example would be one that allows water and glucose to pass through, but not sucrose or starch, as they are larger.
- For an experiment, we take a visking tubing attached to a capillary tube.
  - Next, the tubing is filled with the sucrose solution and the liquid level is marked.
  - Then, a beaker is taken in which water is added, and the visking tubing is placed in the beaker. Finally, the water level in the beakeris marked.
  - The concentration of solutes inside the visking tubing is higher, which means that there is a lower water potential.
  - Since, during osmosis, water molecules move from a region of higher water potential to another region of lower water potential, the water molecules in the beaker will enter the tubing.
  - Then the water level in the beaker will drop, but conversely, the water level in the capillary tube will rise.

Effect of hypertonic solution and hypotonic solution in a plant cell:
- When a plant cell is placed inside a hypertonic solution (solution with low water potential), water starts to flow out of the cell causing the cell to shrink/ plasmolysed.
- Conversely, when the plant cell is placed inside a hypotonic solution (solution having a higher water potential), water will flow from the solution into the cell.
- When too much water moves into the cell, the vacuole becomes bigger, pushing the cell membrane against the cell wall. The force of this increases the turgor pressure within the cell, making it turgid.
- The cell wall prevents the cell from bursting.
- On the other hand, if the plant cell does not receive enough water, the water potential inside the cell is now higher than the surrounding. Thus, water will leave the cell. The cell will become flaccid and the plant will wilt.
- In animal cells, when too much water is leaving the cells, the process is called crenation.
- Conversely, when too much water enters the animal cell, the cell may burst.
- A solution which has a water potential similar to the cell or the surroundings is what we called isotonic solution.
- If the water potential is too high, cells will burst/become swollen, if the water potential is too low, cells become crenated/plasmolysed.
- That’s why maintenance of water potential is so important in both animal and plant cells.

3.02 - Osmosis

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