3.3 - Movement of Substances across a Plasma Membrane in Living Organisms

Now, we will look at some of the active and passive transports that happen in animals and plants.

Figure 10 - Gaseous exchange (oxygen and carbon dioxide) at the alveolus happens through simple diffusion. Oxygen coming from the lungs is higher in concentration than carbon dioxide. Meanwhile, carbon dioxide coming from the body tissues is higher in concentration compared to oxygen. Therefore, oxygen diffuses into the bloodstream and carbon dioxide diffuses out.

Figure 11 - The sodium-potassium (Na⁺-K⁺) pump requires ATP to function. In each cycle, 2 K⁺ ions are pumped into the cell and 3 Na⁺ ions are pumped out of the cell.

Isotonic, hypotonic and hypertonic solutions

• Isotonic : The two solutions have the same concentration of solutes.
• Hypotonic : The solution has a lower concentration of solutes.
• Hypertonic : The solution has a higher concentration of solutes.

Figure 12 - Both solution A and solution B (Top) are isotonic, which means they contain the same concentration of solutes. Therefore, no net movement of water across the semi-permeable membrane.

Solution A (middle) is hypotonic compared to solution B, which means the concentration of solute is lower than in solution B. Therefore, the net movement of water moves from solution A to solution B until an equilibrium is achieved.

Solution A (bottom) is hypertonic in comparison to solution B, which means the concentration of solutes is higher than in solution B. Hence, the net movement of water travels from solution B to solution A until an equilibrium is reached.

The effects of isotonic, hypotonic and hypertonic solutions on animal and plant cells

The process of osmosis is constantly happening in both animal and plant cells. Intracellular fluid refers to the fluid found inside the cell while the extracellular fluid is the fluid that surrounds the outside of a cell.

Figure 13 - Effect of Isotonic Solution

Figure 14 - Effect of Hypotonic Solution

Figure 15 - Effect of Hypertonic Solution

• When the cell is placed in a hypertonic solution, water diffuses out of the cell by osmosis at a higher rate.
• The net movement of water is out of the cell. This causes the cell to shrink.
• Crenation refers to the shrinking of red blood cells.
• In plant cells, the process is called plasmolysis. The plant is said to have wilted. The interesting thing is that the cells can still regain its turgidity if they are placed in a hypotonic solution quickly. When they do, the plant is said to have deplasmolysed. The process is called deplasmolysis.