3.1 Diffusion
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Cells need to take in certain substances that the cell desires from their surroundings, like glucose and oxygen, and get rid of undesired substances, like carbon dioxide. In addition to holding the cell together, the cell membrane controls what substances enter and leave a cell. Only very small molecules can pass through a cell membrane. Oxygen, glucose, amino acids and water can pass through a cell membrane. However, larger molecules like starch and proteins are unable to pass through the membrane as they are too large. The cell membrane is partially permeable.
Substances can enter or leave a cell in three different ways. These are diffusion, osmosis and active transport. In this section, we are going to be looking at diffusion. By substances, we mean molecules and ions.
Substances can enter or leave a cell in three different ways. These are diffusion, osmosis and active transport. In this section, we are going to be looking at diffusion. By substances, we mean molecules and ions.
Diffusion
Diffusion is the net movement of particles from a place where there is a high concentration of them to a place where there is a lower concentration of them. It is the net movement down a concentration gradient. It occurs because of the kinetic energy of particles. It happens in both liquids and gases, as the molecules in these two states move around randomly (whereas in solids the particles are in a fixed position and are therefore not able to move).
An example of diffusion would be someone spraying deodorant or perfume in a room. After the perfume has been sprayed, the perfume particles will be concentrated around where it was sprayed and there would be no perfume particles on the other side of the room. However, over time, the perfume particles will diffuse through the air in the room and there will be an equal concentration of perfume particles around the room.
Diffusion also occurs through the partially permeable membrane in cells. When the cell is respiring, it is using glucose and oxygen to create energy, water and carbon dioxide. The concentration of oxygen in the cell will be lower than outside of the cell because the cell is turning oxygen and glucose into carbon dioxide and water. All oxygen particles are moving around in random directions because of the kinetic particles energy that they possess. Oxygen is able to pass both ways through the cell membrane. As there is a greater concentration of oxygen outside the cell compared with inside the cell, more oxygen travels into to the cell than out of the cell. We say that there is a net inflow of oxygen from outside the cell to inside the cell by diffusion.
Diffusion is the net movement of particles from a place where there is a high concentration of them to a place where there is a lower concentration of them. It is the net movement down a concentration gradient. It occurs because of the kinetic energy of particles. It happens in both liquids and gases, as the molecules in these two states move around randomly (whereas in solids the particles are in a fixed position and are therefore not able to move).
An example of diffusion would be someone spraying deodorant or perfume in a room. After the perfume has been sprayed, the perfume particles will be concentrated around where it was sprayed and there would be no perfume particles on the other side of the room. However, over time, the perfume particles will diffuse through the air in the room and there will be an equal concentration of perfume particles around the room.
Diffusion also occurs through the partially permeable membrane in cells. When the cell is respiring, it is using glucose and oxygen to create energy, water and carbon dioxide. The concentration of oxygen in the cell will be lower than outside of the cell because the cell is turning oxygen and glucose into carbon dioxide and water. All oxygen particles are moving around in random directions because of the kinetic particles energy that they possess. Oxygen is able to pass both ways through the cell membrane. As there is a greater concentration of oxygen outside the cell compared with inside the cell, more oxygen travels into to the cell than out of the cell. We say that there is a net inflow of oxygen from outside the cell to inside the cell by diffusion.
Similarly but on the contrary, the concentration of carbon dioxide inside the cell will be greater than the concentration outside the cell. This is because carbon dioxide is produced inside the cell during respiration. The carbon dioxide molecules move around randomly and carbon dioxide is able to pass through the cell membrane in both directions. As there is a greater concentration of carbon dioxide inside the cell compared with outside the cell, more carbon dioxide molecules will move out of the cell than into the cell. This results in a net movement of carbon dioxide molecules outside of the cell.
Rate of Diffusion
The rate of diffusion depends on a variety of different factors.
The rate of diffusion depends on a variety of different factors.
- One factor is temperature. The higher the temperature is, the faster the rate of diffusion. This is because the diffusing particles have more kinetic energy at higher temperatures and are therefore moving round quicker. This means that the diffusing substance reaches an equal concentration in a shorter time.
- The steeper the concentration gradient, the greater the rate of diffusion. This is because more particles from the high concentration part/ section will be moving to the low concentration part/ section and fewer particles will be moving in the opposite direction. For example, if the concentration of oxygen was very high outside the cell and very low inside the cell, many oxygen particles will move into the cell and few particles will move out, resulting in a high rate of diffusion.
- The rate of diffusion is also dependant of the surface area to volume ratio. The larger the surface area ratio to volume, the greater the rate of diffusion.