Cell Physiology and Diffusion | Form One - High School Biology

- A membrane is a surface structure that encloses the cell and cell organelles.
- The membranes include the cell membrane, tonoplasts, nuclei membrane, mitochondrial membrane and chloroplast membrane.
- The membranes have a common basic structure which regulates the movement of materials in and out of the cells.
- The cell membrane is made up of a phospholipid layer sandwiched by two protein layer (it is a lipoprotein layer) the overall thickness of the cell membrane is about 7.5 nm thick.
- The membrane is perforated by small pores that allow the passage of substances in and out of the cells.

1. The cell membrane is semi permeable- The pores that occur on the cell membrane allows the passage of the small size molecules but does not allow the passage of the large sized molecules. Such a membrane is said to be selectively permeable or semi-permeable. In particular, when a cell is surrounded by a dilute sugar solution, the small sized water molecules will enter the cell but the larger sugar molecules will not pass through the cell membrane. In contrast, the cell wall is permeable as it allows both sugar and water molecules to pass through it; it has larger pores. This property of selectively permeability enables the cell membrane to select what enters and leaves the cell.
2. The cell membrane is sensitive to changes in temperature and pH- Cell membranes are made up of protein. Proteins are adversely affected by extreme changes in temperature and pH. Changes in temperature and pH will alter the structure of the cell membrane thereby hindering the normal functioning of the cell membrane. High temperature denatures (destroys) the proteins thereby impairing the functions of the cell membrane.
3. The cell membrane possesses electric charges- The cell membrane has both positive and negative charges. These charges affect the manner in which substances move in and out of the ells. The charges also enable the cell to detect changes in the environment.

- In this section, we discuss the various physiological processes through which materials move in and out of the cells across the cell membrane.
- Materials move in and out of the cells through three main physiological processes:

• Diffusion
• Osmosis
• Transport

- From kinetic theory, matter is made up of particles that are in continuous random motion.
- In solids, the particles are at fixed positions and can only vibrate at these fixed positions.
- In liquids and gases, the particles are loosely held and are free to move from one region to another randomly.

- This movement of gas or liquid particles is observed to be from regions of high concentration to a region of low concentration
- The process by which particles move from a region of high concentration to a region of low concentration is known as diffusion.

- In particular, the scent of a flower or perfume experienced by an individual is as a result of the flower scent particles or perfume particles move from a region of high concentration.
- Diffusion occurs until the regions have an even concentration of the liquid or gas particles.
- The difference in concentration of particles between the region of high concentration and region of low concentration is known as the diffusion gradient/concentration gradient.

Experiment on Diffusion using Potassium Manganate (VII)

Requirements are; potassium manganate (VII) crystals, glass tubing, 100 cm3 beaker and water.
The following procedure should be followed.

• Hold the glass tubing vertically in a beaker so that one end of the tubing rests on the bottom of the beaker.
• Cautiously and quickly drop a crystal of potassium manganate (VII) through the upper opening of the glass tubing.
• Close the upper hand of the glass tubing with the thumb.
• Half fill the beaker with water.
• Carefully withdraw vertically the glass tubing so that the crystal is left undisturbed at the bottom of the beaker.
• Record your observations for the first 15 minutes.
• Explain your observations.

Expected Observations:After some time, the purple colour of the potassium manganate (VII) spread throughout the water and eventually all the water turned purple.
Explanation:The crystals of potassium manganate (VII) are highly concentrated with the potassium manganate (VII) particles. The potassium manganate (VII) particles break away from the crystals, dissolve in water and then diffuse through the water until they are evenly distributed.

In Plants

1. It helps in absorption of mineral salts from the soil to the plant. Most salts dissolve in soil water. For those salts whose concentration in soil water is higher that their concentration in the cell sap of root hair cells, they move into the root hair cells through diffusion. Plants require mineral salts for numerous life processes.
2. Diffusion plays a role in gaseous exchange in plants. The respiratory gases (oxygen and carbon (IV) oxide) diffuse across the stomata and lenticels of plants.
3. Diffusion also contributes to the transportation of manufactured food materials from the leaves to other parts of the plant.

In Animals

1. It helps in the absorption of digested food materials in the alimentary canal. End products of digestion such as amino acids and glucose diffuse across the wall of the ileum into the blood for transport to other parts of the animal body.
2. Diffusion also plays a significant role in gaseous exchange in animals. In animals, gaseous exchange occurs at certain structures known as respiratory surfaces. These include the skin, gills, lungs, tracheal system and the cell membrane (in unicellular organisms). Gaseous exchange at these surfaces occurs through the process of diffusion.
3. Diffusion is important in excretion of nitrogenous wastes especially in unicellular animals.

1. Diffusion gradient - A greater diffusion gradient between two points increases the rate of diffusion. Increasing the concentration of diffusing molecules also increases diffusion gradient with corresponding regions hence increases the rate of diffusion.
2. Surface area to volume ratio - - Rate of diffusion directly depends on the surface area to volume ratio. The greater the surface area to volume ratio, the greater the rate of diffusion will be. Conversely, low surface area to volume ratio results in a low diffusion rate.
   - This implies that diffusion rate is greater in small organisms than the large organisms.
   - This is because the small organisms have a large surface area to volume ratio. As a result, most of their body parts are closer to the external surrounding leading to faster diffusion.
   - Small organisms can, therefore, depend on diffusion alone as a means of transporting foods, respiratory gases and waste products.
   - To large organisms, diffusion alone is inadequate as a means of transport of foods and excretion. They have an additional transport system.
   - Organisms always lose heat to the surrounding through diffusion. This implies that small animals lose a lot of heat to the surrounding compared to the large animals.
3. Thickness of membranes and tissues - The thicker the membrane or tissue, the lower the rate of diffusion. This is because the distance covered by the diffusing molecules is greater through the thicker membranes.
   - The rate of diffusion is higher in thinner membranes.
4. Size of molecules - Small and light molecules diffuse much faster than the heavy and large sized particles.
5. Temperature - An increase in temperature increases the energy content of the diffusing particles; thereby causing them to move faster, this implies that the rate of diffusion increases with increase in temperature.