Simple diffusion The term simple diffusion refers to a process whereby a substance passes through a membrane without the aid of an intermediary such as a integral membrane protein. The force that drives the substance from one side of the membrane to the other is the force of diffusion. In order for substances to pass through a cell membrane by simple diffusion it must penetrate the hydrophobic core of the phosphoric belayed. The types of molecules that can do this are themselves substantially hydrophobic in nature such as carbon dioxide, oxygen or ethanol.
In the figure below the green triangle indicates a concentration gradient of carbon dioxide. The blue arrow indicates the direction of net flow of carbon dioxide. The carbon dioxide penetrates the phosphoric belayed without the aid of an intermediary molecule. You should be aware that the relative sizes of the molecules in this figure are not correct. The carbon dioxide molecules are much smaller than the phosphoric. Osmosis Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/ incinerated solution) through a partially permeable membrane.
The Effects of 1 . If the water concentration of the cells cytoplasm is lower then that of the medium (I. E. The medium is a hypotonic solution) surrounding the cell then osmosis will result in the cell gaining water. The water molecules are free to pass across the cell membrane in both directions, but more water molecules will enter the cell than will diffuse out with the result that water enters the cell, which will then swell up and could possibly burst. 2. If the water concentration inside the cell is the same as that n the surrounding medium (I. E. He medium is a isotonic solution) there will exist a dynamic equilibrium between the number of molecules of water entering and leaving the cell and so the cell will retain its original size. For example, the red blood cell in the blood plasma retains its shape because of the isotonic nature of the plasma. 3. If the water concentration inside the cell is higher then that of the medium (I. E. The medium is a hypersonic solution) the number of water molecules diffusing out will be more than that entering and the cell will shrink and shrivel due o osmosis.
Osmosis in plant cells Osmosis is the way in which plants take up water. This is how. Root hairs of the plant take in water from the soil by osmosis. The cell membrane of the root hair cell acts a partially permeable membrane (the cell wall is fully permeable) and because the cell sap inside the vacuole is a strong solution (low water concentration) water passes from the soil (high water concentration) into the root hair cell by osmosis. The concentration of the sap in the vacuole is now weaker as there is a high concentration tot water.
Water will now pass trot this area tot high concentration to the next cell which has a low water concentration by osmosis. In this way water continues to move along the cells of the root up the xylem to the leaf. All the time water is moving to areas of lower water concentration As water enters plant cells it makes the cell swell up. The water moves into the plant cell vacuole and pushes against the cell wall. Eventually the cell contains as much water as it can hold. The strong cell wall stops the cell bursting. We say that cell is turgid.
Turgid cells are useful implants as the give the plant support as they keep the stems of plants upright. When plants are placed into a strong sugar or salts solution water will pass out of the cells by osmosis. As water passes out, the sap vacuole starts to shrink. These cells are no longer firm they are limp. We say that they are flaccid and the plant will wilt. If a lot of water leaves the cells then the cytoplasm starts to peel away from the cell wall. We say that the cell has undergone polynomials. Osmosis in animal cells
As you will remember animal cells have partially permeable cell membrane. This means that if they are placed in pure water because their cytoplasm is a stronger solution than the pure water, water will pass into the cells by osmosis. The cells will therefore swell up. However animal cells have no cell wall to stop them swelling too much so they keep swelling until they burst! We call this homeostasis. Facilitated diffusion Facilitated diffusion is a process whereby a substance passes through a membrane with a aid of an intermediary or a facilitator.
The facilitator is an integral membrane rotten that spans the width of the membrane. The force that drives the molecule from one side of the membrane to the other is the force of diffusion. In the figure below K+ ions are passing through a membrane using a potassium transport protein. The ions are moving down a concentration gradient so the process is a diffusion of K ions. Transport Proteins Transport proteins are very specific with respect to the kinds of molecules or ions that will be transported. For example a An+ transport protein transports Aka+ ions much better than it will transport K+ ions.