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B1 I) Diffusion
B1 I) Diffusion
Cells need to take in certain substances that they desire 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 cells. The cell membrane is partially permeable, which means that certain substances can pass through the membrane and other substances cannot. Only very small molecules can pass through a cell membrane like oxygen, glucose, amino acids and water. Larger molecules like starch and proteins are unable to pass through the membrane as they are too large.
Substances can enter or leave cells in three different ways; diffusion, osmosis and active transport. In this section, we are going to look at diffusion, and we will look at osmosis and active transport in later sections.
Substances can enter or leave cells in three different ways; diffusion, osmosis and active transport. In this section, we are going to look at diffusion, and we will look at osmosis and active transport in later sections.
Diffusion
The definition of diffusion is shown below.
The definition of diffusion is shown below.
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration
Diffusion can also be defined as the net movement of particles down a concentration gradient (“down a concentration gradient” just means from high concentration to low concentration). There is a diagram below of a membrane with some particles on either side of the membrane.
All of the particles in the above diagram are moving around in random directions. From looking at the diagram, we can see that there is a higher concentration of particles on the left side compared to the right. This means that the net movement of particles due to diffusion will be towards the right (net movement means overall movement. Some particles will go from right to left, but more particles will go from left to right, thus meaning that the overall net movement will be from left to right).
An example of diffusion would be someone spraying perfume in a room. After the perfume has been sprayed, the perfume particles will be in a high concentration around where the perfume has been sprayed, and in a very low concentration around the rest of the room. Over time, the perfume particles will diffuse from an area of high concentration (where the perfume was sprayed) to an area of low concentration (the rest of the room). Diffusion will result in there being an equal concentration of perfume around the whole room.
Diffusion only happens in gases and liquids because the particles are able to move in these states. Diffusion cannot happen in solids because in solids, the particles are in fixed positions and therefore cannot move.
An example of diffusion would be someone spraying perfume in a room. After the perfume has been sprayed, the perfume particles will be in a high concentration around where the perfume has been sprayed, and in a very low concentration around the rest of the room. Over time, the perfume particles will diffuse from an area of high concentration (where the perfume was sprayed) to an area of low concentration (the rest of the room). Diffusion will result in there being an equal concentration of perfume around the whole room.
Diffusion only happens in gases and liquids because the particles are able to move in these states. Diffusion cannot happen in solids because in solids, the particles are in fixed positions and therefore cannot move.
Diffusion in Cells
There are many different substances that diffuse into or out of cells. An example of a substance that diffuses into cells is oxygen; I am going to use the example of a muscle cell. Oxygen is carried to all body cells through the bloodstream. The bloodstream will have a higher concentration of oxygen than the muscle cell. This means that the net movement of oxygen will be from the bloodstream (where oxygen is in high concentration) to the muscle cell (where oxygen is in low concentration). The muscle cell will use the oxygen for respiration reactions, which gives the cell energy.
There are many different substances that diffuse into or out of cells. An example of a substance that diffuses into cells is oxygen; I am going to use the example of a muscle cell. Oxygen is carried to all body cells through the bloodstream. The bloodstream will have a higher concentration of oxygen than the muscle cell. This means that the net movement of oxygen will be from the bloodstream (where oxygen is in high concentration) to the muscle cell (where oxygen is in low concentration). The muscle cell will use the oxygen for respiration reactions, which gives the cell energy.
Respiration reactions produce carbon dioxide. This means that carbon dioxide will be in a higher concentration inside the muscle cell compared to the bloodstream, which results in carbon dioxide diffusing from the muscle cell into the bloodstream.
More information about the diffusion of oxygen and carbon dioxide into and out of cells can be found in the circulatory section.
More information about the diffusion of oxygen and carbon dioxide into and out of cells can be found in the circulatory section.
Rate of Diffusion
The rate of diffusion depends on three main factors:
The rate of diffusion depends on three main factors:
- Temperature – a higher temperature results in a greater rate of diffusion. This is because a higher temperature gives the particles more kinetic energy, which means that they move around faster, thus resulting in a greater rate of diffusion.
- Concentration gradient – this refers to the difference in concentrations between the area of high concentration and the area of low concentration. The greater the concentration gradient, the faster the rate of diffusion. This is because more particles from the high concentration area will be moving to the low concentration area, and fewer particles will be moving in the opposite direction (low to high); this results in the net movement being higher/ a greater rate of diffusion.
- Surface area – the greater the surface area, the greater the rate of diffusion because a greater surface area means that more particles can pass through a membrane at once.