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Transcript
Cell Transport, Diffusion, and Osmosis Reading Diffusion Many times this year, you can walk down the hallway and already know that we are doing a lab. I hear from the hallway, “Smells like science!”. The smells that come from my classroom spread into the hallways quickly. This is an example of diffusion. Diffusion is when molecules move from an area of high concentration to an area where there is low concentration. If there are a lot of molecules in my classroom that have a strong scent, that means there is a strong concentration. If the molecules in the hallway do not have a scent, that means there is a low concentration. Concentration means the number of particles of a substance in a particular volume. In many things in science, reach an equilibrium, or reach stability. That means that the scent spreads until there is an even amount of it everywhere. As a result, in many real world phenomena, things will move from a high to a low concentration. The bigger the difference in concentration, the faster the molecules spread out. In science, when molecules move from high to low concentrations it is referred to as molecules moving across the concentration gradient. Diffusion occurs in cells too. Diffusion is essential in order for a cell to survive. Important molecules needed for important processes and reactions that take place in cells must enter and exit through the cell membrane. For example, oxygen that is needed for cells to go through cellular respiration enters the cell through diffusion. On the “flip side”, carbon dioxide leaves the cell through diffusion as a product of cellular respiration. As you know, cell membranes are semipermeable; they let some things in and out. These membranes have tiny gaps in them that materials can move through. Take a look at the diagram below to see one process that uses diffusion: Plant cells go through photosynthesis, they produce oxygen. Now, there are equal amounts of O2 inside and outside of the cell. O2 O O2 2 O2 O2 There are more O2 molecules inside the cell than outside the cell. That means there is a higher concentration. O2 O2 O2 O2 molecules start to move from high to low concentrations. They diffuse through gaps in the cell membrane out of the cell. O2 O2 O2 O2 Osmosis Water makes up 2/3 of the cell. It is an extremely important molecule for cell function. Thus, it is important that it can diffuse in and out of the cell when it needs to. Because water is so important, scientists have given a special name for water diffusing in and out of a cell. Osmosis is the name for water moving in and out of cells from high to low concentrations. Passive Transport Diffusion and osmosis are examples of passive transport. Passive transport occurs when the cell does not need to use its own energy to move a molecule through the cell membrane. Active Transport Active transport is the process of using energy to move important molecules across a cell membrane. This is much different than diffusion, osmosis, and passive transport. Cells require many materials that cannot simply pass through the cell membrane. Molecules such as salt need to be able to pass in and out of a cell but require energy in order to do so. Endocytosis One example of active transport is endocytosis. Endocytosis occurs when molecules are too large to fit through the cell membrane and into the cell. Notice the prefix “endo”. This means “in”. These molecules that are too large to fit through the membrane are often called “packages”. When this occurs, the cell membrane wraps itself around the “package” and breaks off from the cell membrane. The cell membrane reseals itself and the package enters the cell completely wrapped up by the cell membrane. Look at the diagram below to see what endocytosis looks like. Cell membrane Package Exocytosis Exocytosis begins with the prefix “exo”. This means out. Exocytosis is simply the opposite of endocytosis. It is a form of active transport that allows things or “packages” that are too large to fit through the membrane. Cells often use exocytosis when they are trying to remove waste from the cell. Look at the diagram below to see what exocytosis looks like. Cell membrane Package Surface Area to Volume Ratio Surface area is the amount of surface covering the outside of an object. As the cell membrane is the organelle that surrounds the cell, when we talk about surface area, we are often talking about how large the surface of the cell membrane is. Volume is the amount of space an object takes up. In a cell, this often refers to how big the cell is. The cell’s ability to transport objects across the membrane has to do with the surface area to volume ratio of the cell. This means it is important to recognize the relationship between how large the cell membrane is compared to how much space the cell takes up. The higher the surface to volume ratio, the better the cell can be at transporting substances in and out of the cell, and, in turn, the better the cell can perform its functions. How do you calculate surface area to volume ratio? Let’s say the surface area of a large cell is 96 cm2 and the volume is 64 cm3. To create the ratio, we write the surface area, then a colon, and then the volume. The surface area always has to go first! So, we would write 96:64. Then, we see if we can reduce. Are there any numbers that go into both 96 and 64? Yes, 32! If we divide both numbers by 32, we get a surface area to volume ratio of 3:2. In order for cells to be effective at transport, we want the first number (the surface area) to be higher than the second number (the volume). What affects surface area to volume ratio? The size of a cell affects its surface area to volume ratio. In order to be effective at transport, cells need to remain small. As a cell grows in size, the volume of the cell increases a lot more than the surface area of the cell does. This means the surface area to volume ratio goes down and transport of important materials becomes difficult. Smaller cells have a higher surface area to volume ratio. If a cell grows and begins to get too big, the big cell divides into two smaller cells to keep the surface area to volume ratio high! When the cell divides, it means there is more membrane available for transport. It also means there is a greater surface area. With more surface area to the membrane, important molecules that need to get in and out of the cell to carry out cell functions have more places to enter and exit. The shape of the cell affects its surface area to volume ratio. Some cells are shaped differently than others. If a specialized cell requires a lot of transport, it is extremely important that it has a higher surface area to volume ratio. For example, single-celled organisms are frequently thin and flat to help with this. In our bodies, nerve cells and muscle cells are long and skinny in order to have a higher ratio for better transport of signals and molecules. Thus, the structure of the cells helps with its function! Surface area to volume ratio does not only affect the entire cell, it affects organelles as well. Organelles inside cells that require a lot of transport of materials have unique shapes that increase their ratio. Think of the endoplasmic reticulum and golgi apparatus (pictured below). These organelles are folded up and “smooshed” which increases their surface area to volume ratio.