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Honors Biology. Module 6: The Cell Part 2 November 7, 2013. Class Challenge. Flexibility. Was the Website Helpful?. To learn the cell parts for the Quiz: Let’s see how much you learned !. How Substances travel in and out of Cells.
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Honors Biology Module 6: The Cell Part 2 November 7, 2013
Class Challenge Flexibility
Was the Website Helpful? To learn the cell parts for the Quiz: Let’s see how much you learned !
How Substances travel in and out of Cells A plasma membrane is responsible for substances travelling in and out of a cell. A plasma membrane is made up of: • Proteins • Cholesterol • Phospholipids: is a lipid in which one of the fatty acid molecules has been replaced by a molecule that contains a phosphate group.
Lipids are molecules that contain three fatty acids linked together on a glycerol molecule. A phospholipid has, instead of the third fatty acid molecule, a small molecule that contains a phosphate group, which is composed of phosphorus, hydrogen, and oxygen. See figure 6.6 (Symbol for lipid)
The interesting thing about a phospholipid is that the phosphate group gives the molecule a slight affinity to water. Since the two fatty acid components are so long and since the molecule containing the phosphate group is so short, the affinity to water exists only on one end of the molecule – the end that contains the gylcerol. The other end of the molecule has no affinity for water.
Phospholipid has a hydrophilic end (water-loving) And a hydrophobic end (water-fearing) The symbol: The top part of the molecule (end with the circle) is attracted to water, The bottom part (the wavy lines) is repelled by water.
The Plasma Membrane The Phospholipids are arranged in a double layer called phospholipid bilayer. The hydorphobic layers are pointing towards each other and the hydrophilic ends point out towards the surroundings and other cells. See figure 6.7
Proteins and cholesterol are interspersed throughout the phospholipid bilayer. Some proteins are near the top, and others are near the bottom. Some proteins span the entire width of the plasma membrane, leaving no room for phospholipds. This protein “bridge” can be an active transport site.
Active Transport There are two basic ways that substances enter or exit a cell: • Passive Transport: Movement of molecules through the plasma membrane according to the dictates of osmosis or diffusion. • Active Transport: Movement of molecules through the plasma membrane (typically opposite the dictates of osmosis or diffusion) aided by a process that requires energy.
In passive transport, no cellular energy is expended as a result of the substance passing through the plasma membrane. Water for example, is a very small molecule and thus passes easily through the spaces between the proteins in the plasma membrane. It will travel according to the dictates of osmosis. If the concentration of solutes in the solution surrounding the cell is essentially equal to the concentration of solutes inside the cell, water will aimlessly wander back and forth across the plasma membrane, resulting in no net charge in the amount of water inside or outside the cell.
Isotonic Solution A solution in which the concentration of solutes is essentially equal to that of the cell which resides in the solution. The red blood cells in your bloodstream are an in an isotonic solution. Your kidneys get rid of excess solutes in the blood t make sure that the bloodstream stays isotonic with your red blood cells.
Hypertonic Solutions Solutions that have a higher concentration of solute than the inside of a cell hare called hypertonic solutions. Although a hypertonic solution has the potential to kill any cell, the implosion that results from placing cells that have cell walls in a hypertonic solution is called plasmolysis.
Plasmolysis Is the collapse of a walled cell’s cytoplasm due to a lack of water. That is why you should not over fertilize a plant. Too much fertilizer in the ground near a plant, when the fertilizer mixes with the water in the soil, a hypertonic solution could be formed. This will result in plasmolysis of the plant’s cells, killing the plant you were trying to fertilize.
Osmosis can also cause cytolysis, when a cell is placed in a hypotonic solution. Cytolysis: the rupturing of a cell due to excess internal pressure. Hypotonic solution: A solution in which the concentration of solutes is less than that of the cell that resides in the solution.
In a hypotonic solution, there is greater solute concentration on the inside of the cell than on the outside. As a result, water flows by osmosis into the cell until the cell bursts from too much water pressure.
Organisms that live in fresh water A lot of cells must deal with living in hypotonic solutions. Fresh water has few solutes dissolved in it, therefore it is a hypotonic solution for cells. Water flows into a a plant cell, it collects in the central vacuole. This expands the vacuole, pushing the cytoplasm against the rigid cell wall. This increases the pressure, called turgor pressure.
Once the pressure gets large enough, it counteracts the effect of osmosis, and no more water can get into the cell. Even though the water “wants” to get into the cell due to osmosis, the turgor pressure keeps the water out. Turgor pressure counteracts the effects of osmosis, keeping the cell from rupturing. See page 180 .
Mr. Anderson: Cellular Respiration:http://youtu.be/Gh2P5CmCC0M
Mr. Anderson and the Cell Membrane http://youtu.be/y31DlJ6uGgE
The Plasma Membrane • http://youtu.be/moPJkCbKjBs
Water does not easily pass through the membrane by simple diffusion. • Proteins called aquaporin facilitate water going in/out of the cell. • Ion channels facilitate ion diffusion. This is a very important point not made clear in this video. • You can not be 'selectively permeable' and allow things to just go in and out by simple diffusion alone! • Point being: Water and ions do not cross the plasma membrane by themselves, proteins are needed.
Experiment 6.2Cell Structure II Object: To see how cells vary in structure and to observe plasmolysis. • Study of a living leaf • Study of banana cells
Homework • Complete OYO questions for Module 6 • Complete Study Guide question for Module 6 • Complete Experiment 6.2 in Lab book • Review Website posts • Take Module 6 Test • Begin reading Module 7 pps. 195-211 • Class challenge: • Quiz:
